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Title:
ALGORITHM FOR REAL-TIME PROCESS CONTROL OF ELECTRO-POLISHING
Document Type and Number:
WIPO Patent Application WO/2004/111314
Kind Code:
A2
Abstract:
Method and apparatus for process control of electro-processes. The method includes electro-processing a wafer by the application of two or more biases and determining an amount of charge removed as a result of each bias, separately. In one embodiment, an endpoint is determined for each bias when the amount of charge removed for a bias substantially equals a respective target charge calculated for the bias.

Inventors:
Manens, Antoine P. (530 Kendall Ave, #7 Palo Alto, CA, 94306, US)
Duboust, Alain (668 Bryan Avenue, Sunnyvale, CA, 94086, US)
Neo, Siew S. (1556 Halford Ave, Suite 292 Santa Clara, CA, 95051, US)
Chen, Liang-yuh (1400 Melbourne Street, Foster City, CA, 94404, US)
Tsai, Stan D. (5444 Dekker Terrace, Fremont, CA, 94555, US)
Application Number:
PCT/US2004/007501
Publication Date:
December 23, 2004
Filing Date:
March 12, 2004
Export Citation:
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Assignee:
APPLIED MATERIALS, INC. (3050 Bowers Avenue, Santa Clara, CA, 95054, US)
Manens, Antoine P. (530 Kendall Ave, #7 Palo Alto, CA, 94306, US)
Duboust, Alain (668 Bryan Avenue, Sunnyvale, CA, 94086, US)
Neo, Siew S. (1556 Halford Ave, Suite 292 Santa Clara, CA, 95051, US)
Chen, Liang-yuh (1400 Melbourne Street, Foster City, CA, 94404, US)
Tsai, Stan D. (5444 Dekker Terrace, Fremont, CA, 94555, US)
International Classes:
B23H5/06; B23H5/08; B24B37/013; B24B37/04; B24B49/04; B24B49/16; C25F3/02; C25F3/22; C25F3/30; C25F7/00; G01B7/06; H04N1/00; (IPC1-7): C25F/
Attorney, Agent or Firm:
Patterson, Todd B. (Moser, Patterson & Sheridan LLP, 3040 Post Oak Blvd., Suite 150, Houston TX, 77056, US)
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Claims:
AMENDED CLAIMS {Receded by the International Bureau on 23 May 2005 (23. 05. 2005) : original claims 1-35 replaced by amended claims 1-341
1. A method for determining an amount of material removed from a substrate, comprising: electropolishing one or more conductive materials on the substrate, wherein the electropolishing one or more conductive materials on the substrate comprises contacting the substrate with a conductive polishing pad; determining a total charge removed from the substrate during the electropolishing of the substrate; and correlating the total charge removed to a thickness of material removed from the substrate.
2. The method of claim 1, wherein determining the total charge removed from the substrate comprises measuring a total current provided to the substrate over time.
3. The method of claim 1, wherein determining the total charge removed from the substrate comprises: periodically measuring a total current provided to the substrate over time; subtracting, from each measured total current value, a leakage current to determine a plurality of effective current values ; and summing the plurality of effective current values.
4. The method of claim 1, wherein correlating the total charge removed to the thickness of material removed from the substrate comprises accessing a data structure containing a predetermined relationship between the total charge value and the thickness of material removed.
5. The method of claim 1, wherein electropolishing comprises delivering an electrical signal through an electrolyte between a first electrode and a second electrode, wherein only the first electrode is positionable in direct physical contact with the substrate.
6. The method of claim 1, further comprising: determining an endpoint of the polishing cycle for the substrate by determining whether a premeasured initial thickness of the substrate, less the thickness of material removed, is equal to or less than a selected target thickness of the substrate.
7. The method of claim 1, wherein the electropolishing step further comprises producing relative motion between the substrate and the conductive polishing pad.
8. The method of claim 1, further comprising: determining, for the substrate, a removal profile in order to obtain a desired profile; and terminating the electroprocessing when the desired profile is substantially achieved according to the determined amount of charge removed; wherein the step of electropolishing further comprises: contacting the substrate with an electrolyte solution; applying a first bias to the substrate by a first electrode ; applying a second bias to the substrate by a second electrode, the second bias being different from the first bias; and electroprocessing the substrate under the influence of the first and second bias to selectively change a thickness of a conductive material of the substrate; and wherein the step of determining the total charge removed from the substrate further comprises: determining an amount of charge removed from the substrate by each electrode.
9. The method of claim 8, wherein determining the removal profile comprises: determining a preprocessing profile of the substrate; providing a target profile; and determining a difference between the preprocessing profile and the target profile.
10. The method of claim 8, wherein terminating the electroprocessing comprises terminating the first and second biases when a target amount of charge is removed for each respective electrode.
11. The method of claim 8, wherein determining the amount of charge removed from the substrate by each electrode comprises integrating, with respect to time, a respective current provided to each electrode.
12. The method of claim 8, further comprising, prior to the electroprocessing, determining a target amount of charge to be removed for each of the first and second biases, and wherein terminating the electroprocessing comprises terminating the first and second biases when the respective target amount of charge is removed for each bias.
13. The method of claim 12, wherein determining the target amount of charge to be removed for each of the first and second biases comprises accounting for an amount of time a given location of the substrate is affected by each electrode.
14. A method for removing a desired amount of material from a substrate by electropolishing, comprising: determining, for the substrate, a desired removal profile in order to obtain a desired profile; electropolishing the substrate to selectively remove different amounts of conductive material from the substrate according to the desired removal profile, wherein the electropolishing the substrate comprises contacting the substrate with a conductive polishing pad; determining an amount of charge removed from a plurality of locations on the substrate; determining a thickness of material removed from the substrate for each of the plurality of locations on the basis of each amount of charge removed at each of the plurality of locations; and terminating the electropolishing when the desired removal profile is substantially achieved.
15. The method of claim 14, wherein determining the desired removal profile comprises: determining a preprocessing profile of the substrate; providing a target profile; and determining a difference between the preprocessing profile and the target profile.
16. The method of claim 14, wherein determining the amount of charge removed from the plurality of locations comprises: determining, for each zone defined by at least two electrodes independently biasable relative to a third electrode in contact with the substrate, a total amount of charge removed from the substrate as a result of the bias for that zone; and determining an amount of time each of the plurality of locations spends being affected by each zone.
17. The method of claim 16, wherein determining the amount of charge removed from the plurality of locations further comprises: calculating a weighted average for each of the plurality of locations on the basis of the total amount of charge removed from the substrate and the amount of time being affected.
18. A method for determining an endpoint of a polishing cycle for a substrate, comprising: providing an electrolyte in contact with a substrate; providing at least a first electrode and a second electrode; positioning a substrate in contact with a conductive polishing pad; electropolishing one or more conductive materials on the substrate by biasing each of the first and second electrodes at different bias levels to effect different removal rates of material from the substrate; and determining whether an endpoint of the polishing cycle for the substrate is reached, the determining comprising: determining an amount of charge removed from the substrate by each electrode ; and determining whether each amount of charge removed from the substrate by each electrode equals a target charge value for the respective electrode.
19. The method of claim 18, wherein determining the amount of charge removed from the substrate by each electrode comprises measuring a total current provided by the respective electrode to the substrate over time.
20. The method of claim 18, wherein the target charge value for each electrode is determined at least in part according to a predetermined relationship between charge and a thickness of material removed.
21. A method of determining removal of material from a plurality of points on a substrate, the removal being effected at least in part by electrochemical activity, the method comprising: determining trajectories for a plurality of points on a substrate relative to a plurality of electrodes each capable of being separately biased at different bias levels during electrochemical processing; based on the trajectories, determining an amount of time each of the plurality of points spends being affected by each of the electrodes; based on the trajectories and a predetermined chargeremoval relationship relating charge removed to amount of material removed from the substrate, determining an average amount of material removed from the substrate as effected by each of the electrodes; and for each point of the plurality of points on the substrate, calculating an amount of material removed at the point as the sum of the average amounts of material removed from the substrate as effected by each of the electrodes weighted by the corresponding amount of time spent being affected by the respective electrode.
22. A method of electroprocessing a substrate, comprising: providing an electrode assembly comprising at least two separately biased electrodes ; providing a conductive polishing pad in contact with the substrate; providing a process model which relates a charge for each of the electrodes to an amount of material removed at each of a plurality of points on the substrate on the basis of a predetermined trajectory of the plurality of points relative to the electrodes and a predetermined chargeremoval relationship which relates charge removed to amount of material removed from the substrate; for a specified desired profile of the substrate, determining a target charge for each electrode according to the process model ; moving the substrate relative to the electrodes according to the predetermined trajectory of the plurality of points; and terminating each bias to the respective electrode upon reaching the respective target charge.
23. The method of claim 22, wherein the polishing pad comprises contact elements defining a counterelectrode to the at least two electrodes and wherein the process model accounts for a shape of the electrodes and a location of contact elements.
24. An electroprocessing system, comprising: an electrode assembly comprising at least two electrodes; at least two corresponding power supplies each electrically connected to one of the at least two electrodes to separately bias the respective electrode ; a conductive polishing pad connected to the corresponding power supplies; a charge determination unit configured to at least periodically determine an amount of charge removed from the a substrate by each of the electrodes based on the current supplied to each of the electrodes over time; and a process control unit configured to detect an endpoint for each of the electrodes, separately, based on whether the amount of charge removed by a given electrode equals a calculated target charge for the given electrode.
25. The system of claim 24, wherein the process control unit is configured to detect the endpoint for each of the electrodes by performing an operation while each of the electrodes is biased, the operation comprising: determining the amount of charge removed from the substrate, as effected by each of the electrodes ; determining whether the amount of charge removed by each of the electrodes is equal to the calculated target charge for the respective electrode ; and if so, terminating a bias to the respective electrode.
26. An electroprocessing, comprising: an electrode assembly comprising at least two electrodes; at least two corresponding power supplies each electrically connected to one of the at least two electrodes to separately bias the respective electrode ; a conductive polishing pad connected to the corresponding power supplies ; a charge determination unit configured to at least periodically determine an amount of charge removed from the substrate by each of the electrodes based on the current supplied to each of the electrodes over time; and a process control unit configured to determine when a desired profile is substantially achieved according to the determined amount of charge removed by each of the electrodes.
27. The system of claim 27, wherein the process control unit comprises an endpoint detection unit to detect an endpoint for each of the electrodes, separately, based on whether the amount of charge removed by a given electrode equals a calculated target charge for the given electrode.
28. The system of claim 27, wherein the process control unit is configured to determine when the desired profile is substantially achieved by, at least, determining a difference between a preprocessing profile of the substrate and the desired profile to establish a removal profile representing a profile of material to be removed from the substrate.
29. The system of claim 27, wherein the at least two corresponding power supplies are configured to separately bias their respective electrodes at different bias levels during processing.
30. The system of claim 27, wherein the at least two electrodes are concentrically disposed relative to each other.
31. The system of claim 27, wherein the process control unit is configured to terminate the biases when a target amount of charge is removed for each respective electrode.
32. An electrochemical mechanical polishing system, comprising: a conductive polishing pad; a power supply configured to supply an electrical signal to electrolyte in contact with the conductive polishing pad; and an endpoint detector configured to detect a polishing endpoint by performing an operation comprising: determining a total charge removed from the substrate; correlating the total charge removed to a thickness of material removed from the substrate; and determining whether a premeasured initial thickness of the substrate, less the thickness of material removed, is equal to or less than a selected target thickness of the substrate.
33. An electroprocessing system, comprising: an electrode assembly comprising at least two electrodes ; at least two corresponding power supplies each electrically connected to one of the at least two electrodes to separately bias the respective electrode ; a conductive polishing pad for processing a substrate thereon, wherein the polishing pad is connected to the corresponding power supplies; a charge determination unit configured to at least periodically determine an amount of charge removed from the wafer by each of the electrodes based on the current supplied to each of the electrodes over time ; and a process control unit configured to detect an endpoint for each of the electrodes, separately, based on whether the amount of charge removed by a given electrode equals a calculated target charge for the given electrode.
Description:
ALGORITHM FOR REAL-TIME PROCESS CONTROL OF ELECTRO-POLISHING BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention generally relates to polishing, planarization, plating and combinations thereof. More particularly, the invention relates to the monitoring and control of electro-processing, polishing and plating.

Description of the Related Art [0003] Sub-quarter micron multi-level metallization is one of the key technologies for the next generation of ultra large-scale integration (ULSI). The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio apertures, including contacts, vias, trenches and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual wafers and die.

[0004] In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting, and dielectric materials are deposited on or removed from a surface of a wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electro-chemical plating (ECP).

[0005] As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer may become non-planar across its surface and require planarization. An example of a non-planar process is the deposition of copper films with the ECP process in which the copper topography simply follows the already existing non-planar topography of the wafer surface, especially for lines wider than 10 microns. Planarizing a surface, or"polishing"a surface, is a process where material is removed from the surface of the wafer to form a generally even, planar surface. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials.

Planarization is also useful in forming features on a wafer by removing excess deposited material used to fill the features and to provide an even surface for subsequent levels of metallization and processing.

Chemical Mechanical Planarization, or Chemical Mechanical Polishing (CMP), is a common technique used to planarize wafers. CMP utilizes a chemical composition, typically a slurry or other fluid medium, for selective removal of materials from wafers. In conventional CMP techniques, a wafer carrier or polishing head is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the wafer, thereby pressing the wafer against the polishing pad. The pad is moved relative to the wafer by an external driving force. The CMP apparatus affects polishing or rubbing movements between the surface of the wafer and the polishing pad while dispersing a polishing composition to affect chemical activities and/or mechanical activities and consequential removal of materials from the surface of the wafer.

[0007] Another planarization technique is Electro Chemical Mechanical Polishing (ECMP). ECMP techniques remove conductive materials from a wafer surface by electrochemical dissolution while concurrently polishing the wafer with reduced mechanical abrasion compared to conventional CMP processes. The electrochemical dissolution is performed by applying a bias between a cathode and a wafer surface to remove conductive materials from the wafer surface into a surrounding electrolyte. Typically, the bias is applied by a ring of conductive contacts to the wafer surface in a wafer support device, such as a wafer carrier head. Mechanical abrasion is performed by positioning the wafer in contact with conventional polishing pads and providing relative motion there between.

[0008] An objective of polishing is to remove a predictable amount of material to achieve a desired profile. Accordingly, any polishing technique requires an endpoint detection to determine when the appropriate amount of material has been removed for various regions of the wafer. However, the progress of the polishing operation is not easily viewable because of the contact between the wafer and the pad.

[0009] In addition, variations in the polishing conditions impede an accurate determination of the polishing endpoint. Variations in the slurry/electrolyte composition, pad condition, relative speed between the pad and the wafer, and the load of the wafer on the pad, etc, cause variations in the material removal rate, which change the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be estimated merely as a function of polishing time.

[0010] One approach to predict the polishing endpoint is to remove the wafer from the polishing apparatus and measure the thickness of the remaining film on the wafer. Doing so periodically during polishing, the quantity of material being removed from the wafer may be determined. As such, a linear approximation of the material removal rate may be used to determine the polishing endpoint.

However, this method is time consuming, and does not account for sudden changes in the removal rate that may occur between measurement intervals.

[0011] Several non-invasive methods of endpoint detection are known. One type of endpoint detection typically requires access to at least a portion of the wafer surface being polished, such as by sliding a portion of the wafer over the edge of the polishing pad or through a window in the pad, and simultaneously analyzing the exposed portion of the wafer. For example, where polishing is used to expose metal lines embedded in a dielectric layer, the overall or composite reflectivity of the surface being polished changes as the lines are exposed. By monitoring the reflectivity of the polished surface or the wavelength of light reflected from the surface, the exposure of the lines through the dielectric layer, and thus the polishing endpoint, can be detected. However, this method does not provide a way of determining the polishing endpoint unless an underlying layer is exposed during polishing. Additionally, this approach is somewhat erratic in predicting the polishing endpoint unless all of the underlying lines are simultaneously exposed. Furthermore, the detection apparatus is delicate and subject to frequent breakdown caused by the exposure of the measuring or detecting apparatus to the slurry or electrolytic fluid.

[0012] A second type of method for determining the polishing endpoint monitors various process parameters, and indicates an endpoint when one or more of the parameters abruptly change. For example, the coefficient of friction at the interface of the polishing pad and the wafer is a function of the surface condition of the wafer. Where an underlying material different from the film being polished is exposed, the coefficient of friction will change also. This affects the torque necessary to provide the desired polishing pad speed. By monitoring this change, the endpoint may be detected.

[0013] In an ideal system, where no parameter other than the wafer surface changes, process parameter endpoint detection is acceptable. However, as the wafer is being polished, the pad condition and the slurry/electrolyte composition at the pad-wafer interface also change. Such changes may mask the exposure of the underlying metal layer, or they may imitate an endpoint condition, leading to a premature stop of polishing.

[0014] Finally, ECMP presents a chemically, electrically and physically unique environment, with respect to conventional CMP. Thus, while the endpoint detection techniques (including those described above) exist for CMP, the techniques may not be readily extendible to ECMP. Even where the techniques are extendible to ECMP, doing so may require retrofitting existing processing systems with expensive equipment. A preferred approach would mitigate or avoid the challenges with retrofitting existing systems.

[0015] Therefore, there is a need for polishing control, and in particular there is a need for endpoint detection, which accurately and reliably determines when to cease polishing during electro-processing.

SUMMARY OF THE INVENTION [0016] In general, embodiments are relating to process control of electro- processing. Some aspects of the invention are generally directed to determining removal of material from a wafer during polishing and to determining an endpoint of a polishing cycle. Other aspects are directed to determining target charge values for a plurality of electrode zones (e. g. , two or more) which can be independently biased.

One embodiment provides a method for producing a desired profile on a wafer by electro-processing. The method includes determining, for the wafer, a removal profile in order to obtain the desired profile; applying a first bias to the wafer by a first electrode ; applying a second bias to the wafer by a second electrode, the second bias being different from the first bias; electro-processing the wafer under the influence of the first and second bias to selectively change a thickness of a conductive material of the wafer; determining an amount of charge removed from the wafer by each electrode ; and terminating the electro-processing when the desired profile is substantially achieved according to the determined amount of charge removed.

Another embodiment provides a method for removing a desired amount of material from a wafer by electropolishing. The method includes determining, for the wafer, a desired removal profile in order to obtain a desired profile; electropolishing the wafer to selectively remove different amounts of conductive material from the wafer according to the desired removal profile ; determining an amount of charge removed from a plurality of locations on the wafer; determining a thickness of material removed from the wafer for each of the plurality of locations on the basis of each amount of charge removed at each of the plurality of locations; and terminating the electropolishing when the desired removal profile is substantially achieved.

[0019] Another embodiment provides a method for determining an endpoint of a polishing cycle for a wafer being processed in a cell body defining an electrolyte- containing volume, wherein the electrolyte-containing volume contains at least electrolyte. The method includes providing at least a first electrode and a second electrode ; positioning a wafer in contact with a polishing pad at least partially submersed in the electrolyte ; electropolishing one or more conductive materials on the wafer by biasing each of the first and second electrodes at different bias levels to effect different removal rates of material from the wafer; and determining whether the endpoint of the polishing cycle for the wafer is reached. In one embodiment the determining includes determining an amount of charge removed from the wafer by each electrode ; and determining whether each amount of charge removed from the wafer by each electrode equals a target charge value for the respective electrode.

[0020] Yet another embodiment provides a method of determining removal of material from a plurality of points on a wafer, the removal being effected at least in part by electro-chemical activity. The method includes determining trajectories for a plurality of points on a wafer relative to a plurality of electrodes each capable of being separately biased at different bias levels during electro-chemical processing; based on the trajectories, determining an amount of time each of the plurality of points spends being affected by each of the electrodes; based on the trajectories and a predetermined charge-removal relationship relating charge removed to amount of material removed from the wafer, determining an average amount of material removed from the wafer as effected by each of the electrodes; and for each point of the plurality of points on the wafer, calculating an amount of material removed at the point as the sum of the average amounts of material removed from the wafer as effected by each of the electrodes weighted by the corresponding amount of time spent being affected by the respective electrode.

[0021] Yet another embodiment provides a method of electro-processing a wafer, the method including providing an electrode assembly comprising at least two separately biased electrodes ; providing a process model which relates a charge for each of the electrodes to an amount of material removed at each of a plurality of points on a wafer on the basis of a predetermined trajectory of the plurality of points relative to the electrodes and a predetermined charge-removal relationship which relates charge removed to amount of material removed from the wafer; for a specified desired profile of the wafer, determining a target charge for each electrode according to the process model ; moving the wafer relative to the electrodes according to the predetermined trajectory of the plurality of points; and terminating each bias to the respective electrode upon reaching the respective target charge.

[0022] Still another embodiment provides a computer readable medium containing a program which, when executed, performs an operation during an electropolishing process occurring for a wafer in contact with a polishing pad at least partially submersed in electrolyte. The operation includes applying a first bias to the wafer by a first electrode ; applying a second bias to the wafer by a second electrode, the second bias being different from the first bias; electro- processing the wafer under the influence of the first and second bias to selectively change a thickness of a conductive material of the wafer; determining an amount of charge removed from the wafer by each electrode ; and terminating the electro- processing when a desired profile is substantially achieved according to the determined amount of charge removed.

[0023] Still another embodiment provides an electro-processing system including a cell body defining an electrolyte-containing volume ; an electrode assembly comprising at least two electrodes; at least two corresponding power supplies each electrically connected to one of the at least two electrodes to separately bias the respective electrode ; a charge determination unit configured to at least periodically determine an amount of charge removed from the wafer by each of the electrodes based on the current supplied to each of the electrodes over time; and a process control unit configured to detect an endpoint for each of the electrodes, separately, based on whether the amount of charge removed by a given electrode equals a calculated target charge for the given electrode.

[0024] Still another embodiment provides an electro-processing system including a cell body defining an electrolyte-containing volume; an electrode assembly comprising at least two electrodes; at least two corresponding power supplies each electrically connected to one of the at least two electrodes to separately bias the respective electrode ; a charge determination unit configured to at least periodically determine an amount of charge removed from the wafer by each of the electrodes based on the current supplied to each of the electrodes over time; and a process control unit configured to determine when a desired profile is substantially achieved according to the determined amount of charge removed by each of the electrodes.

[0025] Another embodiment provides a method for determining an amount of material removed from a substrate. The method comprises electropolishing one or more conductive materials on a substrate; determining a total charge removed from the substrate during the course of polishing the substrate; and correlating the total charge removed to a thickness of material removed from the substrate.

[0026] Another method provides for determining an endpoint of a polishing cycle for a substrate. The method comprises providing a cell body defining an electrolyte-containing volume, wherein the electrolyte-containing volume contains at least electrolyte ; positioning a substrate in contact with a polishing pad at least partially submersed in the electrolyte ; electropolishing one or more conductive materials on the substrate; and determining the endpoint of the polishing cycle for the substrate based on the total charge removed from the substrate. Determining the endpoint comprises determining the total charge removed from the substrate; correlating the total charge removed to a thickness of material removed from the substrate; determining whether a pre-measured initial thickness of the substrate, less the thickness of material removed, is equal to or less than a selected target thickness of the substrate.

[0027] Yet another embodiment provides a computer readable medium containing a program which, when executed, performs an operation during an electropolishing process occurring for a substrate in contact with a polishing pad at least partially submersed in electrolyte. The operation comprises summing a plurality of measured current values of an electrical signal provided to the substrate to calculate a total charge value, wherein the measured current values correspond to measurements made periodically since initiating a polishing cycle for the substrate; and using the total charge value to determine a thickness of material removed from the substrate during the polishing cycle.

[0028] Still another embodiment provides an electro-chemical mechanical polishing system, comprising: a cell body defining an electrolyte-containing volume ; a polishing pad disposed in the electrolyte-containing volume ; a power supply configured to supply an electrical signal to electrolyte contained in the electrolyte-containing volume ; and an endpoint detector configured to detect a polishing endpoint based on the total charge removed from the substrate. To this end, the endpoint detector is configured to perform an operation comprising determining a total charge removed from the substrate; correlating the total charge removed to a thickness of material removed from the substrate; determining whether a pre-measured initial thickness of the substrate, less the thickness of material removed, is equal to or less than a selected target thickness of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS [0029] So that the manner in which the above recited embodiments are attained and can be understood in detail, a more particular description may be had by reference to the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0030] FIGURE 1 is a perspective view of one embodiment of a processing environment having a profile measurement station, an electro-processing platform and a computer for controlling processes performed on the platform.

[0031] FIGURE 2 is a side cross-sectional view of an electro-polishing station.

[0032] FIGURE 3 is a top view of one embodiment of a polishing pad having contact elements disposed therein.

[0033] FIGURE 4 is top view of one embodiment of an electrode assembly defining a plurality of zones.

[0034] FIGURE 5 is a side cross-sectional, exploded view of a polishing assembly which includes the polishing pad of FIGURE 3 and the electrode assembly of FIGURE 4.

[0035] FIGURE 6 is a block diagram of a process control unit configured for process monitoring and endpoint detection based on charge removed by various zones of the electrode assembly of FIGURE 4.

FIGURES 7-1 0 are graphs illustrating removal profiles for various charge values.

[0037] FIGURE 11 shows one example of an empirically determined curve relating total charge and material removed.

[0038] FIGURES 12-13 are top perspective views of a substrate having a material layer thereon, wherein the material layer may be polished in order to develop a relationship between removal and charge.

[0039] FIGURE 14 are representative traces of wafer point trajectories relative to electrode zones.

[0040] FIGURE 15 are representative curves illustrating the percent of time various points on a wafer spent in three separate electrode zones.

FIGURE 16 is a side cross-sectional view of electropolishing system configured with a controller and endpoint detector.

[0042] FIGURE 17 is a top view of an exemplary polishing pad which allows for zone control.

FIGURE 18 is a top view of another exemplary polishing pad which allows for zone control.

[0044] FIGURES 19A-C are a series of side cross-sectional views of a substrate and a polishing pad illustrating a polishing cycle.

[0045] FIGURE 20 is a graph showing that the area under a current signal curve represents the material removed by electrochemical polishing and any leakage current.

FIGURE 21 is a generic graphical representation of a relationship between total charge removed and total material removed by electrochemical polishing.

[0047] FIGURE 22 is a graphical illustration of the current/removal rate relationship, where the y-axis is the removal rate of copper and the x-axis is the total current (offset due to leakage current).

[0048] FIGURE 23 is a calibration curve for a series of wafers processed at different conditions relating average current to average removal rate.

[0049] FIGURE 24 shows one example of an empirically determined curve relating total charge and material removed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0050] The present invention provides systems and methods for electro- processing wafers. Generally, electro-processing is performed to remove or deposit material on the wafer. Embodiments are provided for determining a profile for a wafer, calculating charge values and detecting an endpoint of an electro- processing cycle. In one embodiment, an electro-processing system is provided with a plurality of power supplies each connected to a different zone of an electrode. The power supplies are independently controlled to provide a desired voltage/current signal to a respective zone. The current is monitored and related to the total removal/deposition of material from a wafer to determine the end point of the process.

[0051] The words and phrases used herein should be given their ordinary and customary meaning in the art by one skilled in the art unless otherwise further defined. A wafer refers to a workpiece which is typically disc-shaped, but my be otherwise (e. g. , square). The wafer is typically a substrate having one or more layers formed thereon, at least one of which is conductive (typically the upper- most layer). The layer (s) of the wafer may or may not be patterned. Chemical- mechanical polishing should be broadly construed and includes, but is not limited to, abrading a wafer surface (e. g. , an upper-most conductive layer) by chemical activities, mechanical activities, or a combination of both chemical and mechanical activities. Electro-processing refers to any process using electrical activity to remove or deposit material on a wafer. Particular examples of electro-processing for removing material from a wafer include electro-polishing, electrochemical mechanical polishing, and electrochemical mechanical plating process. Electro- polishing should be broadly construed and includes, but is not limited to, planarizing a wafer by the application of electrical and/or electrochemical activity and a particular example includes electrochemical mechanical polishing.

Electrochemical mechanical polishing (ECMP) should be broadly construed and includes, but is not limited to, planarizing a wafer by the application of electrochemical activity, mechanical activity, or a combination of both electrochemical and mechanical activity to remove materials from a wafer surface.

Electrochemical mechanical plating process (ECMPP) should be broadly construed and includes, but is not limited to, electrochemically depositing material on a wafer and concurrently planarizing the deposited material by the application of electrochemical activity, mechanical activity, or a combination of both electrochemical and mechanical activity.

[0052] Anodic dissolution should be broadly construed and includes, but is not limited to, the application of an anodic bias to a substrate directly or indirectly which results in the removal of conductive material from a substrate surface and into a surrounding electrolyte solution. l0053] Embodiments of the invention broadly provide for real-time removal monitoring and endpoint detection in a electro-processing system. In general, any of the above-defined techniques may be used, individually or in combination.

Further, it is contemplated that polishing and plating may occur simultaneously or alternately. The foregoing embodiments are broadly, collectively characterized as electro-processing. While embodiments are described primarily with reference to a form of electro-polishing, it is understood that the invention is not so limited.

Rather, embodiments include any form of the electro-processing. Thus, any reference to particular embodiments of electro-polishing are merely for purposes of illustration.

[0054] FIGURE 1 depicts wafer processing environment 100. The processing environment 100 includes a profile measurement station 102, an electrochemical mechanical polishing (ECMP) platform 104, a computer 106 and a plurality of power supplies 108A, 108B,... 108N (collectively referred to as the power supplies 108). The profile measurement station 102 is generally any device or devices configured to measure profiles of wafers. Exemplary non-contact devices which may be used include iScan and iMap, which scan and map the wafer, respectively. In the illustrative embodiment of FIGURE 1, the profile measurement station 102 represents an ex-situ device; that is, profile measurement is performed at a location different from where polishing is performed. However, in situ profile measurement (e. g. , on the polishing platform 104) is also contemplated. In either case, the profile measurement station 102 measures a profile of a wafer (referred to as the"initial profile") and provides this measurement to the computer 106.

[0055] The polishing platform 104 may be any apparatus adapted for polishing wafers by electro-processes. One polishing platform that may be adapted to benefit from the invention is a REFLEXION chemical mechanical polisher available from Applied Materials, Inc. located in Santa Clara, California. loos6] Illustratively, the polishing platform 104 includes a plurality of polishing stations 110A-C. By way of example three polishing stations (P1, P2, P3) are shown. More generally, the polishing platform 104 may include any number of polishing stations (i. e. , one or more). The polishing platform 104 may also include other process stations, such as rinse stations.

[0057] The processes performed on wafers by the polishing platform 104 are generally controlled by the computer 106. The computer 106 may be representative of, or include, any programmable device configured to carry out embodiments of the invention. Thus, in one embodiment, the computer 106 is representative of a controller or a plurality of controllers configured with code which, when executed, performs polishing operations, In other embodiments, the computer 106 may be characterized as a client computer, a server computer, a portable computer, an embedded controller, a PC-based server, a minicomputer, a midrange computer, a mainframe computer, and other computers adapted to support the methods, apparatus, and article of manufacture of the invention.

[0058] To implement desired polishing processes the computer 106 is configured with a process control unit 112. In one embodiment, the process control unit 112 is implemented in software, but may also be implemented in hardware. In either case, the code of the process control unit 112 defines functions of the preferred embodiment and can be contained on a variety of signal-bearing media (or computer readable media). The code (or the instructions implemented by such code) includes, but is not limited to, (i) information permanently stored on non-writable storage media, (e. g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive); (ii) alterable information stored on writable storage media {e. g. , floppy disks within a diskette drive or hard-disk drive); or (iii) information conveyed to a computer by a communications medium, such as through a computer or telephone network, including wireless communications. The latter embodiment specifically includes information downloaded from the Internet and other networks. Such signal- bearing media, when carrying computer-readable instructions that direct the functions of the present invention, represent embodiments of the present invention.

[0059] In various embodiments, the process control unit 112 is capable of profile determination, real-time process monitoring and end point detection.

These functions may be performed on the basis of various input, including the initial profile received from the profile measurement station 102. The process control unit 112 then causes the computer 106 to issue control signals 114 to each of the power supplies 108. In response to the control signals 114, the power supplies 108 each issue a separate electrical signal, collectively referred to as the electrical signals 116. Each of the electrical signals 116 provides a current/voltage to a separate one of a plurality of same-polarity electrodes (e. g., two or more) of a given one of the polishing stations 110A-C. The individual electrodes receiving the control signals 114 are also referred to herein as"zones" of a single electrode. Accordingly, in one embodiment, each electrode/zone has an associated one of the power supplies 108. It is noted, however, that while separate power supplies may be used, a single unit capable of producing multiple electrical signals of different voltage/current may also be used. Therefore, the term"power supplies"as used herein does not necessarily denote physically separate power supplies. Also, the number of power supplies and respective electrodes/zones may be varied.

In one embodiment, during processing of a given wafer on one of the polishing stations 110A-C, removal of material from the wafer is measured as charge. In one embodiment, charge is calculated by integrating current with respect to time. As such, each of the power supplies 108 may be configured with an integration unit 118 which integrates the current being provided by the respective power supply to electrode/zone of the polishing station. The output 120 (i. e. , the charge per electrode/zone) of the integration units 118 is provided to the process control unit 112. Although shown as part of the power supplies 108, the integration units 118 may alternatively be part of the process control unit 112, or be separate units altogether. Each of the foregoing aspects will be described in more detail below.

[0061] Referring now FIGURE 2, one embodiment of a process cell 200 is shown in side cross-section. The process cell 200 may be representative of a cell located at any one or all of the polishing stations 110A-C shown in FIGURE 1. In particular, the polishing station 200 is a"face-down"polishing apparatus.

However, embodiments using"face-up"polishing apparatus are also contemplated.

[0062] The process cell 200 generally includes a basin 204 and a polishing head 202. A substrate 208 is retained in the polishing head 202 and lowered into the basin 204 during processing in a face-down (e. g. , backside up) orientation.

An electrolyte is flowed into the basin 204 and in contact with the substrate's surface while the polishing head 202 places the substrate 208 in contact with a polishing article 203. In one embodiment, the substrate 208 and the polishing article 203 disposed in the basin 204 are moved relative to each other to provide a polishing motion (or motion that enhances plating uniformity). The polishing motion generally comprises at least one motion defined by an orbital, rotary, linear or curvilinear (e. g. , sweep) motion, or combinations thereof, among other motions.

The polishing motion may be achieved by moving either or both of the polishing head 202 and the basin 204. The polishing head 202 may be stationary or driven to provide at least a portion of the relative motion between the basin 204 and the substrate 208 held by the polishing head 202. In the embodiment depicted in Figure 2, the polishing head 202 is coupled to a drive system 210. The drive system 210 moves the polishing head 202 with orbital, rotary, linear, or curvilinear (e. g. , sweep) motions, or combinations thereof.

[0063l The polishing head 202 generally retains the substrate 208 during processing. In one embodiment, the polishing head 202 includes a housing 214 enclosing a bladder 216. The bladder 216 may be deflated when contacting the substrate to create a vacuum therebetween, thus securing the substrate to the polishing head 202. The bladder 216 may additionally be inflated to press the substrate in contact with the polishing article 203. A retaining ring 238 is coupled to the housing 214 and circumscribes the substrate 208 to prevent the substrate from slipping out from the polishing head 202 while processing. One polishing head that may be adapted to benefit from the invention is a TITAN HEADY carrier head available from Applied Materials, Inc., located in Santa Clara, California.

[0064] The basin 204 is generally fabricated from a plastic such as fluoropolymers, TEFLON polymers, perfluoroalkoxy resin, PFA, polyethylene- based plastics, PE, sulfonate polyphenylether sulfones, PES, or other materials that are compatible or non-reactive with electrolyte compositions that may be used in electroplating or electropolishing. The basin 204 includes a bottom 244 and sidewalls 246. The sidewalls 246 of the basin 204 include a port 218 formed therethrough to allow removal of electrolyte from the basin 204. The port 218 is coupled to a valve 220 to selectively drain or retain the electrolyte in the basin 204.

[0065] The basin 204 is rotationally supported above a base 206 by bearings 234. A drive system 236 is coupled to the basin 204 and rotates the basin 204 during processing. A catch basin 228 is disposed on the base 206 and circumscribes the basin 204 to collect processing fluids, such as an electrolyte, that flow out of port 218 disposed through the basin 204 during and/or after processing.

[0066] The basin 204 defines housing for the polishing article 203 and an electrode assembly 209 which is disposed between the bottom 244 and the polishing article 203. In one embodiment, the polishing article 203 and the electrode 209 may be secured together forming a unitary body that facilitates removal and replacement. In embodiment, the polishing article 203 and the electrode 209 secured to one another by permanent means such as bonding.

Alternatively, the polishing article 203 and the electrode 209 may be coupled by other techniques or combination thereof, including sewing, binding, heat staking, riveting, screwing and clamping among others. In still another embodiment the polishing article 203 is secured to a belt assembly which dispenses new (i. e., clean) pad material and takes up old (i. e. , worn or contaminated) pad material.

[0067] As will be described in more detail with reference to FIGURE 5, the electrode assembly 209 generally includes a plurality (e. g. , at least two) of electrodes or zones. Each electrode is coupled to a lead 212A of one of the power supplies 108. The other lead 212B of each of the power supplies 108 is coupled to the polishing article 203, which is itself an electrode. For simplicity only one set of leads (positive and negative) is shown. Illustratively, the leads 212A-B are routed through a slip ring 226 disposed below the basin 204. The slip ring 226 facilitates continuous electrical connection between the power supplies 108 and the polishing article and electrode assembly 209 as the basin 204 rotates. The leads 212A-B may be wires, tapes or other conductors compatible with process fluids or having a covering or coating that protects the leads 212A-B from the process fluids. Examples of materials that may be utilized in the leads 212A-B include insulated copper, graphite, titanium, platinum, gold, and HASTELOYE materials among other materials. Coatings disposed around the leads 212A-B may include polymers such as fluorocarbons, PVC, polyamide, and the like.

[0068] In order to operate as electrode, the polishing article 203 is made at least partially conductive by the provision of conductive material disposed in the polishing article 203. In general, the base material of the polishing article 203 may itself be made conductive, or conductive elements may be disposed in a non- conductive base material such as polyurethane. Particular embodiments of the polishing article 203 are described below.

[00691 In one embodiment, the electrode assembly 209 communicates electrically with the substrate 208 via perforations 205 formed in the polishing article 203. Thus, in one embodiment, the ability of the electrode assembly 209 to affect electro-processing is determined, in part, by the presence and location of the perforations 205. Specifically, it is contemplated that, in at least one embodiment, electro-processing is effected substantially only in those locations of the wafer 208"visible"to the electrode assembly 209 through the perforations 205. Relatedly, it is contemplated that the proximity of the electrode assembly 209 to the wafer 208 affects processing. Generally, a closer proximity is preferred. Accordingly, in the illustrative embodiment of FIGURE 2, the electrode assembly 209 is shown directly interfacing with the polishing article 203.

However, the provision of intermediate materials between the electrode assembly 209 and the polishing article 203 is also contemplated.

An electrolyte delivery system 232 is generally disposed adjacent the basin 204. The electrolyte delivery system 232 includes a nozzle or outlet 230 coupled to an electrolyte source 242. The outlet 230 flows electrolyte or other processing fluid from the electrolyte source 242 to into the basin 204. During processing, the electrolyte generally provides an electrical path for biasing the substrate 208 and driving an electro-chemical process to remove and/or deposit material on the substrate 208. Alternatively, the electrolyte delivery system may provide electrolyte through the bottom 244 of the process cell and flow electrolyte through the electrode assembly 209 and the perforations 205 of the polishing article 203 into contact with the substrate 208.

[00711 In operation, electrolyte is flowed into the basin 204 from the electrolyte delivery system 232. The electrolyte fills the basin 204 and is thus brought into contact with the wafer 208 and polishing article 203. To initiate electrochemical mechanical processing, potential differences are applied between the electrode assembly 209 and the conductive portion of the polishing article 203. The wafer 208 being in direct contact with the conductive portion of the polishing article 203 will then be at the same potential as the conductive portion. The current loop is then completed in the polishing station by transforming atomic wafer materials into ions in the electrolyte. Concurrent mechanical polishing of the wafer 208 is achieved by relative movement between the wafer and the polishing article 203.

In one embodiment, polishing continues until reaching an endpoint, as determined by the process control unit 112. In at least one embodiment, "endpoint"refers to a point in time during a polishing cycle at which sufficient bulk metal has been removed from a wafer. Following detection of the endpoint, it may be necessary to continue polishing for a period of time in order to remove residual metal.

[0072] As noted above, the polishing article 203 is at least partially conductive.

The conductivity of the polishing article may be accomplished in a variety of ways.

Referring now to FIGURE 3, one embodiment of the polishing article 203 is shown. In general, the polishing article 203 is a disk-shaped polishing pad 302 having a plurality of conductive elements 304 disposed thereon. For simplicity and clarity, the perforations 205 described above with reference to FIGURE 2 are not shown. The polishing pad 302 may be made of polymeric materials, such as polyurethane, polycarbonate, polyphenylene sulfide (PPS), or combinations thereof, and other polishing materials used in polishing wafer surfaces. An exemplary conventional material includes those found in the IC series of polishing media, for example polyurethane and polyurethane mixed with fillers, commercially available from Rodel Inc., of Phoenix, Arizona. The invention further contemplates the use of other conventional polishing materials, such as a layer of compressible material. The compressible material may include a conventional soft material, such as compressed felt fibers leached with urethane. Further, the use of abrasive materials embedded in the polishing pad is contemplated. The fixed abrasive particles may include conductive abrasive materials and/or nonconductive abrasive materials.

[0073] The conductive elements 304 are shown disposed in a symmetrical pattern at a central region of the polishing pad 302. The pattern shown in FIGURE 3 is merely illustrative and other patterns may be used. In addition, the number of conductive elements 304 may be varied. In one embodiment, the conductive elements 304 are rigidly fixed in place. In another embodiment, the conductive elements 304 are rotatably disposed relative to the polishing pad 302 so that the conductive elements 304 may rotate over the surface of a wafer brought into contact with the conductive elements 304. A variety of embodiments are described in U. S. Patent Application Serial No. 10/210,972, filed August 2, 2002, entitled CONTACTS FOR ELECTROCHEMICAL PROCESSING, herein incorporated by reference in its entirety.

Alternatively or additionally, the polishing article 203 is made conductive by making the polishing material itself of conductive material. The conductive polishing material may include conductive polymers, polymer composites with conductive materials, conductive metals, conductive fillers or conductive doping materials, or combinations thereof. Conductive polymers include polymeric materials that are intrinsically conductive, such as polyacetylene, polyethylenedioxythiophene (PEDT), polyaniline, polypyrrole, and combinations thereof. The polymer composites with conductive materials may include polymer- noble metal hybrid materials. Polymer-noble metal hybrid materials that may be used as the conductive polishing material described herein are preferably chemically inert with a surrounding electrolyte, such as those with noble metals that are resistant to oxidation. An example of a polymer-noble metal hybrid material is a platinum-polymer hybrid material. The invention also contemplates the use of polymer-noble metal hybrid materials, which are chemically reactive with a surrounding electrolyte, when the polymer-noble metal hybrid material is insulated from a surrounding electrolyte by another material. Conductive metals that may be used as the polishing material are those metals that are preferably relatively inert to chemical reactions with the surrounding electrolyte. Platinum is an example of a conductive metal that may be used as the polishing material.

The conductive metals may form a portion or the entire polishing surface of the polishing material. When forming a portion of the polishing surface, the conductive metals are typically disposed in a conventional polishing material.

[0075] The conductive polishing materials may further include conductive fillers or conductive doping materials disposed in a binder material, such as the conductive polymers described above or a conventional polishing material.

Examples of conductive fillers include carbon powder, carbon fibers, carbon nanotubes, carbon nanofoam, carbon aerogels, and combinations thereof. Carbon nanotubes are conductive hollow filaments of carbon material having a diameter in the nanometer size range. The conductive fillers or conductive doping materials are disposed in the binding material in an amount sufficient to provide a polishing medium having a desired conductivity. The binder material is typically a conventional polishing material.

[0076] Alternatively or additionally, the polishing article 203 may comprise a metal mesh disposed in the polishing material. The metal mesh may comprise a chemically inert conductive material, such as platinum. The metal mesh may also include materials that have been observed to react with the surrounding electrolyte, such as copper, if the metal mesh is chemically insulated from the electrolyte such as by a conformal layer of conventional material.

[0077] Other embodiments of conductive polishing pads which may be used to advantage are further described in United States Patent Applications Serial No.

10/033, 732, filed December 27,2001, entitled CONDUCTIVE POLISHING ARTICLE FOR ELECTROCHEMICAL MECHANICAL POLISHING and Serial No.

10/140,010, filed May 7,2002, entitled CONDUCTIVE POLISHING ARTICLE FOR ELECTROCHEMICAL MECHANICAL POLISHING, each of which is hereby incorporated by reference in their entireties.

[0078] In any case, where the polishing article 203 is at least partially conductive, the polishing article 203 acts as an electrode in combination with a wafer during electro-processes. The electrode assembly 209 provides plurality of counter-electrodes to the polishing article 203. One embodiment of the electrode assembly 209 is shown in FIGURE 4, now described.

[0079] FIGURE 4 shows a top view of the electrode assembly 209. The electrode assembly 209 generally includes a plurality of electrodes. Specifically, four electrodes 402A-D (collectively, electrodes 402) are shown, although the electrode assembly 209 may include any number of electrodes. Illustratively, the electrodes 402 are annular members. In particular, an innermost electrode 402A is a disk, while the other electrodes 402B-D are rings. However, the electrodes may more generally be any geometry. Further, the electrodes 402 are conductive member made of, for example, a metal. For electrochemical removal processes, such as anodic dissolution, the electrodes 402 may include a non-consumable material other than the deposited material, such as platinum for copper dissolution. However, the electrodes 402 can also be made of copper for copper polishing, if preferred. Anodic dissolution should be broadly construed and includes, but is not limited to, the application of an anodic bias to a wafer directly or indirectly which results in the removal of conductive material from a wafer surface and into a surrounding electrolyte solution.

[0080] It is noted that for simplicity some features of embodiments of the electrode assembly 209 are not shown. For example, in one embodiment, the contact elements 304 are disposed through the innermost electrode 402A. The openings to receive the contact elements 304 are not shown in FIGURE 4. The electrodes are separated from one another by annular gaps 404A-D (collectively, gaps 404). In one embodiment, insulating material may be disposed in the annular gaps 404. In illustrative configuration, each electrode 402 defines a zone, which can be separately biased by respective power supply 108. Additional aspects in this regard may be described with reference to FIGURE 5.

[0081] FIGURE 5 shows a side cross-sectional exploded view of the electrode assembly 209 and the polishing article 203. For purposes of illustration the polishing head 202 is also shown carrying a wafer positioned in contact with the upper polishing surface of the polishing pad 302. To facilitate illustration, the individual electrodes 402 of the electrode assembly 209 are shown exploded relative to the polishing pad 302 and relative to one another (i. e. , the size of the gaps 404 between electrodes 402 have been exaggerated). FIGURE 5 shows a representative configuration of three power supplies 108A-C with respect to the electrodes 402. In general, the individual electrodes 402 of the electrode assembly 209 may be anodes or cathodes depending upon the positive bias (anode) or negative bias (cathode) applied between the electrodes 402 and the polishing article 203. For example, depositing material from an electrolyte on the wafer surface, the electrodes 402 act as anodes and the wafer surface and/or polishing article 203 acts as a cathode. When removing material from the wafer 208, such as by dissolution from an applied bias, the electrodes 402 function as a cathode and the wafer surface and/or polishing article 203 may act as an anode for the dissolution process. In the illustrated embodiment, the electrodes 402 are connected to the negative leads 512A of the power supplies 108A-C, while the positive lead 512B are connected to the contact elements 304. (While the positive lead 512B is shown connected to only one of the contact elements 304 for simplicity, is understood that each of the contact elements 304 may be commonly connected to the positive lead 512B. ) The embodiment of FIGURE 5 illustrates that two or more electrodes 402 may be connected to the same power supply, and therefore be maintained at the same bias during processing. In the present illustration, the innermost electrode 402A and the outer, middle electrode 402C are connected to the same power supply 108A, while the inner, middle electrode 402B and the outer electrode 402D are connected to their own respective power supplies 108B and 108C, respectively. In this respect, the four member electrode assembly 209 may be considered a three zone assembly.

Referring now to FIGURE 6, one embodiment of the process control unit 112 is shown. Generally, the process control unit 112 includes a process model 602 and a control algorithm 604. In general, the process model 602 describes a process environment. In operation, the process model 602 receives various model inputs 606. Based on the various model inputs 606, the process model 602 generates a target charge value for each zone defined by the electrode assembly 209 (the target charge values are collectively shown in FIGURE 6 as the target charge values 618). In one embodiment, the model inputs 606 include a desired removal profile 608, a charge-removal relationship 610 and operations/cell parameters 612. The desired removal profile 608 describes a profile of the material to be removed from a wafer. In one embodiment, the desired removal profile is the simple arithmetic difference between a measured profile 614 (e. g. , measured by the profile measurement station 102 shown in FIGURE 1) and a desired profile 616, which may be user selected. The charge- removal relationship 610 describes a relationship between the amount of charge removed from a wafer and the thickness of material removed. The charge- removal relationship 610 is particular to a process and wafer type. Thus, a plurality of such relationships may be stored in a storage device and retrieved as needed. The operations/cell parameters 612 describe various aspects of the manner in which a wafer is processed and the cell in which the wafer is being processed and which may have an impact on the resulting removal profile of the wafer. Illustrative operations/cell parameters 612 include process parameters, electrode geometry, and polishing article construction. Process parameters may include, for example, platen rotation speed (i. e. , rotation of the basin 204 in FIGURE 2), head rotation speed (i. e. , rotation speed of the polishing head 202 in FIGURE 2), and sweep range and frequency. "Sweep"refers to the translation motion of the head along a radius of the platen (i. e. , the bottom of the basin 204.

With reference to construction of the electrode assembly 209 of FIGURE 4, for example, electrode geometry refers to the geometry of the various electrodes 402 which make up the electrode assembly 209. Parameters pertaining to the polishing article construction include, for example, the geometry of the polishing pad (such as the polishing pad 302 shown in FIGURE 3) and the placement of the contact elements (such as the contact elements 304 shown in FIGURE 3).

Additional aspects of the process model 602 and its inputs will be described in more detail below.

[0083] The target charge values 618 calculated by the process model 602 are provided as input to the control algorithm 604. In one embodiment, the control algorithm 604 receives additional control inputs 620, which may be operator selected. Illustrative control inputs include the total process time, the number of process steps and the like. The control algorithm 604 then generates a control signal (collectively, control signals 114) for each zone defined by the electrode assembly 209. The control signals 114 are provided to the respective one of the power supplies 108. The control signals 114 may be configured to, for example, control current or voltage for the respective zone. Accordingly, the power supplies 108 produce corresponding electrical signals 622 for the respective zones. In addition, the charge removed per zone is calculated by each integration unit 118 of the power supplies 108. The calculated charge per zone values 624 are fed back to the control algorithm 604. The control algorithm 604 then compares the calculated charge per zone values 624 to the respective target charge values 618.

That is, the control algorithm 604 compares a calculated charge value for a particular zone to the corresponding target charge value for that zone. When the calculated charge value exceeds the corresponding target charge value, the bias for that zone is terminated. When each of the biases has been terminated, the electro-process is complete. Thus, in one aspect, the process control unit 112 provides for endpoint detection. In another aspect, the calculated charge values for each zone can be used to generate and display the profile of a wafer in real time.

[0084] It is noted that the control algorithm 604 can implement the control signals 114 according to any variety of functions depending, at least in part, on the control inputs 620. In one embodiment, the voltage in each zone is fixed. The voltage in each zone is then independently terminated once the target charge is achieved. The voltage in each zone may be selected according to parameters which include removal rate, planarization capability, surface roughness, defect count, etc. In another embodiment, the polishing time is fixed. In this case, the voltage for each zone is modulated (i. e. changed with respect to time) so that the target charge is met for each zone when the predefined polishing time expires.

While the former embodiment provides simplicity, the latter embodiment allows better control of the step time. In some embodiments, a zone voltage may be zero or negative, while other contemporaneous zone voltages are positive. It is also contemplated that the voltages may be applied prior to any mechanical polishing, during mechanical polishing and/or after mechanical polishing. Persons skilled in the art will recognize other embodiments.

[0085] By way of example only, FIGURES 7-10 show illustrative incoming profiles 700,800, 900, and 1000, respectively. The profiles are plotted on an x-y graph, where the x-axis is the wafer radius in inches and the y-axis is thickness in angstroms. The removal profiles should match these profiles in order to achieve the desired target profile. The charge values for each of the removal profiles are given in corresponding tables 702,802, 902 and 1002. In particular, charge values are given for an outer zone, a middle zone and a center zone. The cathode geometry used was substantially similar to that shown in FIGURE 4, in which zone 1 (defined by the innermost electrode 402A) and zone 3 (defined by the outer middle electrode 402C) were held at the same voltage. Accordingly, zone 1 and zone 3 are collectively referred to in FIGURES 7-10 as the"middle" zone. The"outer"zone corresponds to zone 4 and the"center"zone corresponds to zone 2 of the electrode assembly 209 shown in FIGURE 4.

CHARGE-REMOVAL RELATIONSHIP [0086] As noted above, the process model 602 receives as an input a charge- removal relationship. In one embodiment, the charge-removal relationship is determined empirically, by periodically measuring (e. g. , by sheet resistance measurements) the amount of material removed from a wafer being polished.

Alternatively, the current may be measured for a series of wafers processed at different conditions (e. g., slightly different polishing times, voltage biases, etc.), In this manner, a calibration curve can be acquired. One such calibration curve 1100 is shown in FIGURE 11. In this case, 9 wafers (1 per data point and 3 per zone in a three-zone electrode arrangement) were polished under different conditions and the average current was recorded. The thicknesses of the wafers were measured before and after the polishing cycle to determine the average amount of material removed. The calibration curve (expressed as y = 348.7x + 192.35) exhibits a linear relationship between the Removal (given in angstroms and shown on the y-axis) and the Charge (given in amperes*minutes and shown on the x-axis) and allows for the prediction (by extrapolation) of removal for a given current. Additional embodiments for determining such a relationship are described in U. S. Patent Application Serial No. 10/391, 324, filed March 18,2003, entitled PROCESS CONTROL IN ELECTRO-CHEMICAL MECHANICAL POLISHING, which is hereby incorporated by reference in its entirety, to the extent not inconsistent with the present embodiments.

[0087] In a particular embodiment, a charge-removal relationship may be determined by polishing a plurality of test material layers using a process cell such as the process cell 200 of FIGURE 2. The test material layers may be polished according to a specific set of instructions that may include a relative motion between a polishing pad and a wafer. The relative motion may be, for example, linear, curvilinear, rotational, orbital, or combinations thereof. A test bias, Vt is then applied between the test material layer and the counter electrodes of the electrode assembly 209. The test bias, Vt, may be applied such that a substantially uniform potential is generated across the counter electrodes with respect to the surface to be polished. The bias may be applied to the test material layer using, for example, a pad such as the pad 302 described above in FIGURE 3.

[0088] Referring now to FIGURE 12, a top perspective view of a substrate 1204 shows a first test material layer 1205 formed thereon. Similarly, Figure 13 shows a second substrate 1304 having a second test material layer 1305 formed thereon. The first test material layer 1205 is polished by applying a first test bias such as a uniform test bias across the test material layer 1205 relative to the counter electrodes of the electrode assembly 209.

[0089] After polishing the test material layer 1205 for a pre-determined period of time (a first polishing time), the substrate 1204 is, for example, removed from the process cell 100 and an amount of material removed from the test material layer 1205 is then measured. The amount of material removed may be determined, for example, using conventional methods of measuring layer thicknesses, such as sheet resistance (Rs) measurements. Alternatively, the amount of material removed may be measured using electron microscopy, or similar methods for analyzing thickness and composition of material layers. The material removal may be determined by measuring a thickness 1280 of the test material layer 1205 before polishing and the thickness 1280 after polishing. The thickness 1280 may be measured at a first point 1220. Additional thickness measurements of the first test material layer 1205 may be taken at one or more additional points 122 in order to obtain a statistically representative value for material removal. Alternatively, a property other than thickness may be measured. For example, a mass of material removed or a material removal rate may be measured directly or indirectly. The one or more additional points 1222 on the test material layer 1205 may be chosen such that the points lie within a region or zone of the test material layer 1205 that experiences a relatively uniform rate of polishing (material removal). For example, the first point 1220 and the additional points 122 may be chosen such that they all lie in an intermediate region 1216 of the test material layer 1205. Alternatively, the first point 1220 and the additional points 122 may be chosen such that they each are a distance from a center 1230 of the test material layer 1205 that is substantially the same. A first rate of material removal may be determined by, for example, dividing mass or thickness of the material removed by the first polishing time.

[0090] The second test material layer 1305 may be polished using the same geometry and configuration of the process cell as for the polishing of the first test material layer 1205. The second test material layer 1305 may be polished to a thickness 1380 by applying a second bias applied to the second test material layer 1305. Thereafter, the step of determining material removal may be performed for one or more points 1320 on the second test material layer 1305. Furthermore, the process of determining removal rate may be repeated for additional test material layers (not shown), if desired.

[0091] The one or more points 1320 on the test material layer 1305 may lie, for example, within an intermediate region 1316. The intermediate region 1316 may have a similar shape and define a similar range of distances from a center 1330 of the material layer 1305 as is defined by the intermediate region 1216 with respect to the center 1230.

[0092] By matching the material removal from each test material layer 1205, 1305 with the corresponding charge (current integrated over time) removed from the test material layer, a relationship between material removed and charge may be determined. The relationship thus determined may be relevant for a specific configuration of the process cell, including a specific polishing composition as well as specific composition of material layer. The relationship may be a linear relationship, an exponential relationship, or any other relationship. Further, it is recognized that blanket wafers will exhibit a different charge-removal relationship than patterned wafers.

TRAJECTORY SIMULATION [0093] As noted above, the process model 602 also receives as input, operations/cell parameters 612, which may include process parameters, electrode geometry (i. e. , the geometry of the various electrodes 402 which make up the electrode assembly 209 as illustrated in FIGURE 4), and polishing article construction. In general, the operations/cell parameters 612 are those parameters necessary to simulate a trajectory of points on a wafer. For each point of the wafer, the amount of time affected by each of the electrodes of the electrode assembly 209 can be calculated. In one aspect, a point is affected by an electrode while in direct facing relationship with the electrode via a perforation in the polishing pad (e. g. , one of the perforations 205 of the polishing pad 203 shown in FIGURE 2. However, the presence of a contact element on the polishing pad (e. g. one of the contact element 304 shown in FIGURE 3) may interfere with the interaction between electrode/zone and the point on the wafer. Hence, a point is considered to be affected by an electrode when the electrode is"visible"from the perspective of the point via a perforation.

[0094] Referring now to FIGURE 14, one illustration of simulated point trajectories is shown. Specifically, FIGURE 14 shows the electrode assembly 209 FIGURE 4 and three traces that represent the trajectories of three separate points on a wafer. Only three points are shown for simplicity. In practice, however, the trajectories for a plurality of points would be measured. A first trace 1402 corresponds to a center point on the wafer, a second trace 1404 corresponds to a middle point on the wafer and a third trace 1406 corresponds to an edge point on the wafer. The traces are computer-generated according to a simulation based on the shape of the zones and the relative movement between the wafer and the zones. In addition to the actual path of a wafer point relative to the electrode zones, the time spent affected by each zone is determined according to the simulation by accounting for the relative rotation speeds of the platen and head, the sweep range and frequency and the location of obstructions (e. g. , contact element) between the wafer points and the electrodes. The fraction of time spent by a point, j, in front of a counter-electrode, i, is denoted p, j.

[0095] One graphical representation of the time spent in the various zones is given in FIGURE 15. In particular, three curves represent the time spent by various points on a diameter of the wafer in three separate zones: a central zone, a middle zone, and an edge zone. The zones were defined by concentric electrodes arranged similarly to those shown in FIGURE 4. The wafer points are represented on the x-axis (in this illustration the wafer was a 300mm wafer) and the percent of the total time a point spent in a given zone is represented on the y- axis. A first curve 1500 represents the time wafer points spent in the central zone, a second curve 1502 represents the time wafer points spent in'the middle zone and a third curve 1504 represents the time wafer points spent in the edge zone.

Note that although points in a central part of the wafer (e. g., between about-50 mm and +50 mm) may spend their entire time in the center zone, the first curve (corresponding to the center zone) does not reach 100 percent. This characteristic is due to obstructions to the points'view of the electrode which defines the center zone. As noted above, a point may only experience a bias when disposed over a perforation in the polishing pad, thereby giving the point a view to the electrode. Further, a point's view may be periodically obstructed by a contact element on the polishing pad.

PROCESS MODEL GENERATION [0096] The empirically determined charge-removal relationship and the trajectory simulation are combined to relate to charge in each zone to the removal at each wafer point. Based on the trajectory simulation, the average area of the wafer, A ;, affected by the ith counter-electrode can be determined. Assuming that the current density is uniform across a zone, i. e. , that the voltage drop across the wafer is negligible, the charge-removal relationship in any given zone, Rj, can be computed as follows : Ri = a*Aw/A, *Cj + b (Equation 1) In Equation 1, a is the slope the mathematical expression of the relationship between charge and removal, Aw is the plated area of the wafer, Ci is the charge of the ith electrode and b is the intercept of the mathematical expression of the relationship between charge and removal. l0097] The removal at each point becomes the average of the removal of each zone weighted by the amount of time the point spent in front of each zone, as given by: Rj = sum (i) (pzjRj) (Equation 2) l0098] The process model described above can be summarized in the form of a transfer function taking as input the charge in each zone, C, and generating as output the removal profile R (i. e. , the profile of the material to be removed). The charge in each zone, C, is represented by a vector with N components, where N is the number of zones. The removal profile, R, is represented by a vector with n components, where n is the number of sample points chosen on the wafer. Based on the process model described above, the transfer function is essentially linear and can be written as: R = AC + B, (Equation 3) where A is a nxN matrix and Ajj = pgj (a*Aw*Ai), and B is a (nx1) vector and Bi = b.

Thus, the transfer function relates the removal profile to the charge per zone (i. e., the charge read on each channel of the power supply).

[0099] It is noted that the process model described above is a"physical"model in that the model accounts for physical parameters in a simulation. However, the process model 602 may be generated in a variety of ways. For example, the model 602 may be a statistical model based on calibration wafers (i. e. , the best fit based on x charges, where x is the number of zones).

[00100] In any case, the process model 602 calculates a target charge per zone. If Pin is the incoming profile (provided by the profile measurement station 102) and the Pdes is the desired profile, the process model 602 can implement an optimization with respect to the achievable profile. In one embodiment the optimization is a least square interpolation. Although the optimization may not be uniquely described, specific implementations are contemplated. For example, one optimization is to minimize the standard deviation between the final profile, Pfin, and the desired profile, Pdes. This optimization may be described as: minimize stddev (Pin- (AC+B)-Pdes). Another optimization is to minimize the maximum deviation between Pfin and Pdes. This optimization may be described as minimize maxdevPin- (AC+B)-Pdes). Using such approaches (and others which will be recognized by those skilled in the art) the optimal values for the target charges per zone can be determined prior to processing a wafer. The power supplies for the respective zones are then controlled to ensure that the target charges are reached.

EXAMPLE [00101] A counter-electrode assembly, such as the counter-electrode assembly 209 was divided into five zones: an inner zone, an inner-central zone, a central zone, an outer-central zone and an outer zone (Z1, Z2, Z3, Z4 and Z5) respectively. The zones were arranged in a concentric circular manner similar to the zones depicted for the electrode assembly 209 shown in FIGURE 4. Each of the zones was capable of receiving a separate bias with respect to a material layer of a wafer to be polished. One hundred twenty one points, representing a broad sampling of various locations on the material layer were selected. A pre- determined set of instructions (i. e., a polishing program) that encoded a sequence of relative motion between the counter-electrode (as well as the polishing article) and the material layer of a wafer to be processed was provided to a computer (e. g. , the computer 106). An algorithm based on the polishing program was used to determine the sequence of relative positions between the material layer and the individual counter-electrodes of the electrode assembly as a function of time throughout the polishing process. The algorithm calculated the location of each point relative to the five zones of the counter-electrodes for each of a total of 2400 instants-in time (time steps). The algorithm also calculated the number of time steps each point was associated with each of the five zones (e. g. , the number of times the point would be facing or under each of the zones). For a first order approximation, it is assumed that for embodiments in which the process cell comprises a pad, a point on the material layer only experiences a bias when facing a perforation in the pad. It is also, assumed that a point does not experience a bias when directly below a contact element on the pad (i. e. , when the point's view of an opposing electrode is obscured by a contact element).

[00102] Based upon the program to be used to polish the material layer, the algorithm determined that a first point in the center of the material layer was associated with Z2 for 1080 time steps (i. e. 45% of the total number of time steps), associated with Z1, Z3, Z4, Z5 for 0 time steps, and associated with none of the zones (i. e., the point was not under a perforation in the pad and/or the point was under a contact element, and therefore zero bias was experienced by the point) for the remaining 1320 time steps. Therefore, for 45% of time, point A was associated with Z2, and the expected removal would be 0.45 X R2, where R2 is the total removal contributed by Z2.

[00103] From the algorithm it was further determined that a second point B, away from the center of the material layer) was associated with Z2 for 570 time steps (or 23.75% of the total number of time steps), associated with Z3 for 774 time steps (or 32.35% of the total number of time steps), and associated with no zones for 1056 time steps. The expected removal for point B is therefore given by an average of the removals for Z1, Z2, Z3, Z4, and Z5, weighted by the percentage of the time spent in each zone. Expressed in mathematical terms, the expected removal for point B is given by the mathematical expression, [. 2375 x R2] + [. 3235 x R3], where R3 is the total removal contributed by Z3.

[00104] The algorithm further calculated the expected removal rate for the remainder of the 121 points on the material layer in a similar manner. Specifically, for each point an expected removal rate was calculated as [P1 X R1] + [P2 X R2] + [P3 X R3] + [P4 X R4] + [P5 X R5]. P1, P2, P3, P4, and P5 are the percentages of time that the particular point was associated with the zones Z1, Z2, Z3, Z4, and Z5 respectively.

[00105] The material layer to be polished had a non-uniform initial profile, Pin.

A desired removal profile, Rdes, was calculated as Pin-Pdes, where Pdes is the selected desired profile. A least-squares regression was performed to optimize the values for R1, R2, R3, R4, and R5 such that the actual removal profile, Ract, of the material layer after polishing would closely match the desired removal profile, Rdes. The optimal biases to be applied to each of the zones was then determined using a pre-determined (linear) relationship between removal and charge. The resultant removal profile was similar to the desired removal profile.

CONCLUSION [00106] In various embodiments, advantages in electro-processing may be achieved. In some cases, these advantages may be substantial improvements over the prior art. Some of these advantages are now described. However, whether or not an embodiment achieves an advantage and whether or not such an advantage may be considered a substantial improvement is not limiting of the invention. Therefore, the advantages described below do not define or limit the invention, which is limited only by the claims that follow.

[00107] In one aspect, calibration requirements are eliminated. Conventionally, thickness measurement is performed by eddy current probes. The eddy current probe measures a current and then correlates the measured value to thickness.

In order to ensure accurate measurements the probe must be calibrated for pad wear and changes between different wafer types. Such calibration requirements may be eliminated by some embodiments of the invention. As a result, successful process monitoring and endpoint detection according to embodiments of the invention are independent of pad wear.

[00108] In another aspect, good spatial resolution is achieved. Spatial resolution using eddy current probes is limited to the width of the probe (e. g. , 1/4 inch). Because embodiments of the invention do not rely on such intrusive devices, superior resolution is achieved.

In another aspect, embodiments of the invention are not restricted to wafers having a minimal thickness. In contrast, eddy current probes are limited in their ability to provide accurate measurements for thicknesses less than, for example, 2000 angstroms.

[oono] in another aspect, thickness measurements are not corrupted by interference from other metal levels on a wafer. In contrast, adjacent metal levels can interfere with measurements taken by eddy current probes.

[00111] Yet another aspect of the present invention is described with respect to FIGURE 16. FIGURE 16 depicts an electrochemical mechanical polishing (ECMP) station 1600, which may be a component of a larger platform or tool.

One polishing tool that may be adapted to benefit from the invention is a REFLEXION chemical mechanical polisher available from Applied Materials, Inc. located in Santa Clara, California.

[00112] Generally, the electrochemical mechanical polishing (ECMP) station 1600 comprises a polishing head 1630 adapted to retain the substrate 1613. The polishing head 1630 may be mounted to a carousel (not shown) which operates to rotate the polishing head 1630 to a position over various stations, including the ECMP station 1600. Examples of embodiments of polishing heads 1630 that may be used with the polishing apparatus 1600 described herein are described in United States Patent No. 6,024, 630, issued February 25,2000 to Shendon, et a/.

One particular polishing head that may be adapted to be used is a TITAN HEAD wafer carrier, manufactured by Applied Materials, Inc., located in Santa Clara, California.

[00113] The ECMP station 1600 further includes a basin 1602, an electrode 1604, polishing article 1605 and a pad support disc 1606. The basin 1602 generally defines a container or electrolyte-containing volume 1632 in which a conductive fluid such as an electrolyte 1620 can be confined and in which the electrode 1604, polishing article 1605, and disc 1606 are generally housed. The electrolyte 1620 used in processing the substrate 1613 can electrochemically remove metals such as copper, aluminum, tungsten, gold, silver or other conductive materials. Accordingly, the basin 1602 can be a bowl-shaped member made of a plastic such as fluoropolymers, TEFLON@, PFA, PE, PES, or other materials that are compatible with electroplating and electropolishing chemistries.

[00114] The basin 1602 has a bottom 1610 that includes an aperture 1616 generally disposed in the center of the bottom 1610 and allows a shaft 1612 to pass therethrough. A seal 1618 is disposed between the aperture 1616 and the shaft 1612 and allows the shaft 1612 to rotate while preventing fluids disposed in the basin 1602 from passing through the aperture 1616. Rotation is imparted to the shaft 1612 by a motor (not shown) connected to a lower end of the shaft 1612.

The motor may be an actuator capable of rotating the shaft at a predefined speed or speeds.

[00115] At an upper end, the shaft 1612 carries the pad support disc 1606. The pad support disc 1606 provides a mounting surface for the polishing article 1605, which may be secured to the disc 1606 by a clamping mechanism or an adhesive (such as a pressure sensitive adhesive). Although shown connected to the shaft 1612, in another embodiment, the disc 1606 can be secured in the basin 1602 using fasteners such as screws or other fastening means, thereby eliminating the need for the shaft 1612. The disc 1606 can be spaced from the electrode 1604 to provide a better electrolyte recirculation.

[00116] In one embodiment, the disc 1606 may be made from a material compatible with the electrolyte 1620 which would not detrimentally affect polishing. Illustratively, the disc 1606 may be fabricated from a polymer, for example fluoropolymers, PE, TEFLON@, PFA, PES, HDPE, UHMW or the like. In one embodiment, the disc 1606 includes a plurality of perforations or channels formed therein. The perforations are coupled to the perforations of the polishing article 1605 which, cooperatively, define channels 1622 extending from a lower surface of the disc 1606 to an upper surface of the polishing article 1605. The provision of the channels 1622 makes the disc 1606 and the polishing article 1605 generally permeable to the electrolyte 1620. The perforation size and density is selected to provide uniform distribution of the electrolyte 1620 through the disc 1606 to the substrate 1613.

[00117] The polishing article 1605 can be a pad, a web or a belt of material, which is compatible with the fluid environment and the processing specifications.

The polishing article 1605 is positioned at an upper end of the basin 1602 and supported on its lower surface by the disc 1606. Preferably, the polishing article 1605 includes polishing material to effect mechanical polishing of the substrate 1613, although mechanical polishing may also be achieved by the addition of an abrasive to the electrolyte 1620. In still other embodiments, the ECMP station 1600 is operated to provide electropolishing, with or without mechanical polishing.

As such, a current must be provided to the substrate surface. To this end, the ECMP station 1600 is equipped with a power supply 1634 electrically coupled to a pair of electrodes : the substrate 1613 and the counter-electrode 1604. The manner in which a terminal of the power supply 1634 is coupled to the substrate 1613 may be any of a variety, and is not limiting of the invention. In one particular embodiment, the polishing article 1605 is made conductive and coupled to the power supply 1634. In this manner, a current may be provided through the substrate 1613 by contacting a conductive surface of the substrate 1613 with the conductive polishing article 1605.

[00118] As such, in one embodiment, the polishing article 1605 includes at least a partially conductive surface for contact with the substrate surface during processing. Generally, the polishing article 1605 may be a conductive polishing material or a composite of a conductive material disposed in a polishing material (where, in some cases, the conductive material may also be responsible for performing mechanical polishing). The conductive material may also be inserted between the disc 1606 and polishing material 1605 with some conductive ends for contact with the substrate 1613 during polishing. The conductive materials and the polishing materials preferably have mechanical properties which do not degrade under sustained electric fields and are resistant to degradation in acidic or basic electrolytes.

[0o11s] The conductive polishing material may include conductive polymers, polymer composites with conductive materials, conductive metals, conductive fillers or conductive doping materials, or combinations thereof. Conductive polymers include polymeric materials that are intrinsically conductive, such as polyacetylene, poiyethylenedioxythiophene (PEDT), which is commercially available under the trade name Baytron, polyaniline, polypyrrole, and combinations thereof.

[00120] The polymer composites with conductive materials may include polymer-noble metal hybrid materials. Polymer-noble metal hybrid materials that may be used as the conductive polishing material described herein are preferably chemically inert with a surrounding electrolyte, such as those with noble metals that are resistant to oxidation. An example of a polymer-noble metal hybrid material is a platinum-polymer hybrid material. The invention also contemplates the use of polymer-noble metal hybrid materials, which are chemically reactive with a surrounding electrolyte, when the polymer-noble metal hybrid material is insulated from a surrounding electrolyte by another material.

[00121] Conductive metals that may be used as the polishing material are those metals that are preferably relatively inert to chemical reactions with the surrounding electrolyte. Platinum is an example of a conductive metal that may be used as the polishing material. The conductive metals may form a portion or the entire polishing surface of the polishing material. When forming a portion of the polishing surface, the conductive metals are typically disposed in a conventional polishing material.

[00122] The conductive polishing materials may further include conductive fillers or conductive doping materials disposed in a binder material, such as the conductive polymers described above or a conventional polishing material.

Examples of conductive fillers include carbon powder, carbon fibers, carbon nanotubes, carbon nanofoam, carbon aerogels, and combinations thereof. Carbon nanotubes are conductive hollow filaments of carbon material having a diameter in the nanometer size range. The conductive fillers or conductive doping materials are disposed in the binding material in an amount sufficient to provide a polishing medium having a desired conductivity. The binder material is typically a conventional polishing material.

[00123] Polishing materials may include polymeric materials, such as polyurethane, polycarbonate, polyphenylene sulfide (PPS), or combinations thereof, and other polishing materials used in polishing substrate surfaces. An exemplary conventional material includes those found in the IC series of polishing media, for example polyurethane and polyurethane mixed with fillers, commercially available from Rodel Inc., of Phoenix, Arizona. The invention further contemplates the use of other conventional polishing materials, such as a layer of compressible material. The compressible material includes a conventional soft material, such as compressed felt fibers leached with urethane.

[00124] Further, the invention contemplates the use of abrasive materials embedded in the conventional polishing material. The fixed abrasive particles may include conductive abrasive materials and/or nonconductive abrasive materials.

[00125] Alternatively, the polishing article 1605 may comprise a metal mesh disposed in the polishing material. The metal mesh may comprise a chemically inert conductive material, such as platinum. The metal mesh may also include materials that have been observed to react with the surrounding electrolyte 1620, such as copper, if the metal mesh is chemically insulated from the electrolyte such as by a conforma layer of conventional material.

[00126] In some cases (where the polishing article 1605 is conductive) it may be desirable to restrict current flow to those areas of the polishing article 1605 which are in contact with the substrate 1613. One embodiment suitable for providing power to selected areas of the polishing article 1605 is described with reference to FIGURE 17 which shows a top view of the polishing article 1605. A power strip 1702 and conducting members 1704 (eight shown) are used to provide a current to the polishing article 1605. The power strip 1702 and the conducting members 1704 may be of any sufficiently conductive material, such as copper. The power strip 1702 is connected at one end to the power supply 1634 and at another end to an anchor 1705. The anchor may be any insulated member. Illustratively, a portion of the power strip (indicated by arc length 1706) is wrapped around a circumferential edge of the polishing article 1605. The conducting members 1704 are generally radially disposed from a center 1708 of the polishing article 1605 to the edge of the conductive polishing article 1605. The terminal ends of the conducting members 1704 at the edge of the conductive polish article 1605 are sufficiently exposed to make electrical contact with the conducting strip 1702. To this end, the conducting members 1704 may extend slightly beyond the edge of the polishing article 1605. In this manner, the power strip 1702 is capable of providing a current to any conductive element (s) 1704 of the polishing article 1605 with which it comes into contact with.

[00127] At least in one embodiment, the conducting members are electrically isolated from one another (note that in FIGURE 17 the conducting members to not touch at the center of the polishing article 1605). This allows, at any given time, some of the conducting members to be positively biased by contact with the power strip, while other conducting members are not biased. The fact is that of a "zone switch"which allows current to be selectively delivered depending on the rotation of the polishing article 1605.

[00128]) n operation, a substrate 1613 is brought into contacting with the upper polishing surface of the polishing article 1605 while the polishing medium is rotated about its center axis. In some embodiments, the substrate may be moved relative to the polishing article 1605 by action of the polishing head 1630.

However, in order to ensure anodic dissolution, the substrate's position should be constrained to a region in which contact with one of the energized conducting members (that is, one of the conducting members currently in contact with the power strip) is made. Those conducting members not in direct contact with the power strip, or indirect contact with the power strip via the substrate, will not experience a positive bias, or at least a relatively smaller bias. As a result, electrochemical attack and damage of the conducting members and any conductive elements electrically connected thereto is reduced. In one embodiment, any one of the conductive members and power strip are electrically connected for between about 20% and 60% of the rotation period of the polishing article 1605.

[00129] Typically, the highest potential on the energized conducting members is closest to the power strip and the lowest potential is at the end of the conducting members closest to the center of the polishing article 1605. Accordingly, in order to ensure an equipotential surface along the length of the conducting members, and therefore over the surface of the contacting substrate, the conducting members may be of increasing conductivity from the edge of the polishing article 1605 to the center 1708. In some cases, substrate rotation relative to the polishing pad will equalize or average out the potential imparted to the substrate surface to provide for more uniform material deposition rate or removal rate.

[00130] Over time, the power strip contacting the edge of the polishing article 1605 may become worn. Accordingly, it may be necessary to replace or recondition the power strip. To this end, the anchor 1705 may be equipped with a power strip dispenser. A mechanism may be provided on the other end of the power strip to take up slack when a length of power strip is dispensed from the power strip dispenser.

[00131] FIGURE 18 shows a top view of another embodiment of the polishing article 1605 in which power is coupled from the power supply 1634 to a substrate (not shown) being polished. As in the embodiment described above with reference to FIGURE 2, the polishing article 1605 includes a plurality of electrically isolated conducting members 1704. A plurality of conductive ring portions 1802A- D (four shown, by way of illustration) are disposed at the perimeter of the polishing area of the polishing article 1605. The ring portions 1802 are in electrical communication with the one or more conducting members 1704. However, the ring portions 1802 are isolated from one another by gaps 1804A-D. Although not shown, in one embodiment insulating material is disposed within the gaps to prevent arcing between the ring portions. Periodic electrical contact is made between the ring portions 1802 and the power supply 1634 via a contact finger 1806 connected to a flexible arm 1808. The flexible arm 1808 provides a mechanical bias against the ring portions 1802 to ensure adequate contact. In one embodiment, the conductive ring portions are embedded within the perimeter edge of the polishing article 1605, but exposed to allow contact with the contact finger 1806.

[00132] In operation, a substrate is placed into contact with the upper polishing surface of the rotating polishing article 1605. At any given time, a selective positive bias is provided to one or more of the conductive ring portions 1802, and therefore, the associated (i. e. electrically connected) conducting members 1704.

To ensure anodic dissolution, the substrate is positioned to be in electrical contact with the appropriate conducting members 1704. During rotation of the polishing article 1605, one or more of the conducting members 1704 will be positively biased, while the other conducting members will be unbiased. In this manner, electrochemical damage to any of the conducting portions of the polishing article 1605 is reduced.

[00133] In any case (referring again to FIGURE 16), where the polishing article 1605 is at least partially conductive, the polishing article 1605 acts as an electrode in combination with the substrate 1613 during electrochemical processes. The electrode 1604 is a counter-electrode to the polishing article 1605 contacting a substrate surface. The electrode 1604 may be an anode or cathode depending upon the positive bias (anode) or negative bias (cathode) applied between the electrode 1604 and polishing article 1605.

[00134] For example, depositing material from an electrolyte on the substrate surface, the electrode 1604 acts as an anode and the substrate surface and/or polishing article 1605 acts as a cathode. When removing material from a substrate surface, such as by dissolution from an applied bias, the electrode 1604 functions as a cathode and the substrate surface and/or polishing article 1605 may act as an anode for the dissolution process.

[00135] The electrode 1604 is generally positioned between the disc 1606 and the bottom 1610 of the basin 1602 where it may be immersed in the electrolyte 1620. The electrode 1604 can be a plate-like member, a plate having multiple holes formed therethrough or a plurality of electrode pieces disposed in a permeable membrane or container. A permeable membrane (not shown) may be disposed between the disc 1606 and the electrode 1604 to prevent particles or sludge from being released from the electrode 1604 into the electrolyte. The permeable membrane may also act as a filter and prevent gas evolution from the counter electrode from reaching the substrate during processing. Pores size and density of the permeable membrane are defined in a way to optimize the process performances.

[00136] For electrochemical removal processes, such as anodic dissolution, the electrode 1604 may include a non-consumable electrode of a material other than the deposited material, such as platinum for copper dissolution. However, the electrode 1604 can also be made of copper for copper polishing, if preferred.

[00137] In one embodiment, the polishing station 1600 includes a reference electrode. For example, a reference electrode 1624A may be disposed between the disc 1606 and the counter electrode 1604. More generally, a reference electrode may be at any location in the basin as long as the reference electrode is submerged within the electrolyte 1620. For example, a reference electrode 1624B is shown suspended between a sidewall of the basin 1602 and the polishing article 1605. The reference electrode acts to maintain a constant electrochemical potential on the substrate. Accordingly, the provision of the reference electrode makes the removal rate independent from the changes in the conductivity in the current loop, which may caused by the deposition of loose copper on the counter electrode 1604 for instance. The reference electrode may be made of a very thin metal wire, such as a wire made of platinum.

[00138] The polishing station 300 is energized by one or more power supplies, such as power supply 1634. In one embodiment, the power supply 1634 is a direct current (DC) power supply. However, the power supply 1634 may also be an alternating current (AC) power supply. In one aspect, a DC power supply is preferred to avoid alternately removing and depositing material on a substrate. In general, the power supply 1634 is capable of providing power between about 0 Watts and 1600 Watts, a voltage between about 0V and 10V, and a current between about Oamps and about 10amps. However, the particular operating specifications of the power supply 1634 may vary according to application.

[00139] The power supply 1634 is particularly adapted to provide a current through the electrolyte 1620. To this end, the power supply 1634 is connected by a positive (+) terminal to a first electrode and by a negative (-) terminal to a second electrode. In one embodiment, the first electrode is a conducting portion of the polishing article 1605, such as the conducting members 1704. As a result, the first electrode is in direct contact with a substrate disposed on the polishing article 1605, at least during part of a polishing cycle. The second electrode is the counter electrode 1604 disposed on a floor of the basin 1602, for example. In contrast to the first electrode, the second electrode need not be in direct physical contact with the substrate. In one embodiment, the power supply 1634 is also connected a reference electrode 31 OA or 31 OB.

[00140] A plating or polishing process is performed by flowing a current from the power supply 1634 through electrolyte 1620. The electrolyte 1620 may be a solution which includes commercially available electrolytes. For example, in copper containing material removal, the electrolyte may include sulfuric acid, sulfuric acid salt based electrolytes or phosphoric acid, phosphoric acid salt based electrolytes, such as potassium phosphate (K3P04), (NH4) H2PO4, (NH4) 2HPO4, or combinations thereof. The electrolyte may also contain derivatives of sulfuric acid based electrolytes, such as copper sulfate, and derivatives of phosphoric acid based electrolytes, such as copper phosphate. Electrolytes having perchloric acid- acetic acid solutions and derivatives thereof may also be used. Additionally, the invention contemplates using electrolyte compositions conventionally used in electroplating or electropolishing processes, including conventionally used electroplating or electropolishing additives, such as brighteners, chelating agents, and levelers among others. In one aspect of the electrolyte solution, the electrolyte may have a concentration between about 0.2 and about 1.2 Molar of the solution. Preferably, the electrolyte is selected to react with metal but not with the underlying materials, such as the dielectric.

[00141] The operation of the polishing system 1600 is controlled by a control system 1640. In one embodiment, the control system 1640 includes one or more controllers (represented as controller 1642) and an endpoint detector 1644. The controller 1642 is operably connected to one or more of the devices of the polishing system 1600 such as, for example, the power supply 1634, a fluid delivery system (not shown), a motor (not shown) for rotating the support disc 1606, and the carrier head 1630. The endpoint detector 1644 is configured to monitor the current value samples 1646 taken by an amp meter 1638 of the power supply 1634. The frequency with which samples are taken is determined by a sampler 136 of the power supply 1634. In one embodiment, the sampler 136 may be user-configured with a desired frequency value. Alternatively, the current may be monitored as a continuous signal, rather than as discrete values.

[00142] The current value measurements taken from the meter 1638 may then be used by the endpoint detector 1644 to determine whether an endpoint has been reached based on a target quantity of current value 1648, where the quantity of current is the total charge of the material removed from the substrate 1613. In one embodiment, the target quantity of current value 1648 is input to the endpoint detector 1644 by an operator. If a polishing endpoint has been reached, the endpoint detector 1644 may notify the controller 1642, which may then issue one or more control signals (e. g., control signals 1650 to the power supply 1634) to initiate additional steps and/or halt the polishing of the substrate. Alternatively, the input to the endpoint detector 1644 may be a target thickness a pre-measured initial thickness of a wafer to be polished until reaching the target thickness.

[00143] In operation, electrolyte 1620 is flowed into the volume 1632 of the basin 1602. The electrolyte 1620 fills the volume 1632 and is thus brought into contact with the substrate 1613 and polishing article 1605. Thus, the substrate 1613 maintains contact with the electrolyte 1620 through the complete range of relative spacing between the cover 108 and the disc 1606.

[00144] To initiate electrochemical mechanical processing, a potential difference is applied between two electrodes e. g., electrode 1604 and the conductive portion of the polishing article 1605. The substrate 1613 being in direct contact with the conductive portion of the polishing article 1605 will then be at the same potential as the conductive portion. The current loop is then completed in the polishing station by transforming atomic substrate materials into ions in the electrolyte 1620.

Concurrent mechanical polishing of the substrate 1613 is achieved by relative movement between the substrate and the polishing article 1605. Polishing continues until reaching an endpoint, as determined by the endpoint detector 1644. In at least one embodiment, "endpoint"refers to a point in time during a polishing cycle at which sufficient bulk metal has been removed from a substrate.

Following detection of the endpoint, it may be necessary to continue polishing for a period of time in order to remove residual metal.

[00145] A polishing operation will now be described with reference to FIGURES 19A-C. However, it is understood that the following polishing operation is merely illustrative and not limiting of the invention since aspects of the invention may be applied to advantage regardless of the particular operation or material removal mechanism during electro-polishing of a substrate. Referring first FIGURE 19A a side view of the substrate 1613 and the polishing article 1605 is shown. The polishing article 1605 is shown submerged in the electrolyte 1620 which is made an ionic conductor by application of a current from the power supply 1634. The substrate 1613 is shown located over the electrolyte 1620 and moving downward toward the polishing article 1605. In general, the substrate 1613 includes a base material 1902 (typically made of silicon) having features formed therein. The base material 1902 may be covered by multiple layers of dielectric materials, semiconducting materials and conducting materials. The outermost metal layer 1906 has been previously deposited in the features 1904 and over the previous dielectric, semiconducting and conductive layers. Illustratively, the metal layer 1906 is copper.

The polishing which occurs in electrochemical mechanical polishing is a combination of mechanical polishing (as a result of relative movement between the substrate 1613 and the polishing article 1605) and anodic dissolution (as a result of chemical interaction between the substrate 1613 and the electrolyte 1620). Since is desirable to selectively polish the peaks of the metal layer 1906, a passivation layer 1908 formed over the metal layer 1906 is selected to ensure that polishing occurs primarily where contact is made with the polishing article 1605.

Passivation agents which are part of the conductive electrolyte will passivate the recessed areas of the incoming metal layer to be polished, and thereby prevent anodic dissolution in those areas. Thus, copper lines (i. e. , the copper in the features 1904 are not being polished due to the fact that they are protected by the passivation layer 1908 and they are not in contact with the polishing article 1605.

Once the polishing article 1605 removes the passivation layer 1908 to expose the metal layer 1906, both mechanical polishing and anodic dissolution can occur on the exposed metal layer 1906. Accordingly, as shown in FIGURE 19B, the passivation layer 1908 is not present at the interface of the polishing article 1605 and the metal layer 1906. Illustrative passivation agents include BTA, TTA, etc.

Polishing continues until the excess bulk metal has been removed, at which time the endpoint detector 1644 indicates to the controller 1642 that a polishing endpoint has been reached. FIGURE 19C is illustrative of a surface condition of the substrate at a polishing endpoint. Copper lines (i. e. , the copper in the features 1904) are not being polished due to the fact that they are protected by the passivation agent and they are not in contact with the polishing article 1605. In one embodiment, polishing is allowed to continue for some period of time in order to ensure adequate removal of metal residues. This phase of polishing is referred to herein as"over polishing". Because the polishing endpoint has already been detected, over polishing can be carefully timed and controlled to minimize copper dishing and optimize wafer throughput.

[00148] To determine the occurrence of an endpoint of a polishing cycle, the endpoint detector 1644 shown in FIGURE 16 is configured with an endpoint detection algorithm 1652. The endpoint detection algorithm 1652 (and, more generally, the endpoint detector 1644 itself) may be implemented by hardware, software, or a combination of both. Accordingly, one embodiment of the invention is implemented as a program product for use with a computer system such as, for example, the system 1640 shown in FIGURE 16. In this regard, the system 1640 may be representative of, or include, a computer, computer system or other programmable electronic device, including a client computer, a server computer, a portable computer, an embedded controller, a PC-based server, a minicomputer, a midrange computer, a mainframe computer, and other computers adapted to support the methods, apparatus, and article of manufacture of the invention. The program (s) of the program product defines functions of the preferred embodiment and can be contained on a variety of signal-bearing media (or computer readable media), which include, but are not limited to, (i) information permanently stored on non-writable storage media, (e. g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive); (ii) alterable information stored on writable storage media (e. g., floppy disks within a diskette drive or hard- disk drive); or (iii) information conveyed to a computer by a communications medium, such as through a computer or telephone network, including wireless communications. The latter embodiment specifically includes information downloaded from the Internet and other networks. Such signal-bearing media, when carrying computer-readable instructions that direct the functions of the present invention, represent embodiments of the present invention.

[00149] In a particular embodiment, the endpoint detection algorithm 1652 calculates the total charge removed from a wafer. This is done by integrating, with respect to time, the total current signal provided to the cell/wafer for polishing (herein referred to as the total current signal or cell current signal). Recognizing that the area under the cell current signal (given by the integral of the cell current signal) gives the material removed and any leakage current (as represented by FIGURE 20), a polishing endpoint can be determined by relating the total charge removed (the integral of the cell current signal corrected for any leakage) to the total material removed. A graphical representation of such a relationship is generically described by the plot line of FIGURE 21. As will be described in more detail below, this charge/removal relationship may be theoretically or empirically determined. Detection of the endpoint for a polishing cycle of a wafer then requires knowing only the initial thickness of the wafer prior to polishing. The wafer is polished until the difference between the initial wafer thickness and the removed thickness is equal to the desired thickness: Target Thickness = Initial Thickness-Thickness of Removed Material (Equation 1) Where the"Thickness of Removed Material"is a function of the quantity of current, which is given by the integral of the current (which may be corrected for any leakage current, as described below). Where the current signal is only periodically sampled (i. e. , where the endpoint detection algorithm 1652 uses the current value samples 1646 received from the meter 1638 of the power supply 1634), the integral is approximated as a sum: Quantity of Current (t) = integral (0, t) [I (t) dt] {sum (0, t) fl (t) * (sampling period) ] (Equation 2) Having calculated the sum of the current values over time (i. e., total charge removed), the"Thickness of Removed Material"can be determined with reference to the predetermined the charge/removal relationship. In one embodiment, the charge/removal relationship is stored in data structures such as lookup tables 1654. Thus, the lookup tables 1654 relate a value calculated by the endpoint detection algorithm 1652 (i. e. , total charge removed determined by summation, as described above with reference to Equation 2) to the total material removed from a substrate. A separate lookup table 1654 may be provided for each of a variety of processing conditions and substrate types. For example, separate lookup tables 1654 may be provided for patterned and unpatterned substrates. The lookup tables 1654 may be further characterized by electrolyte composition, the type of material to be removed, etc. In this manner, the endpoint detector 1644 is adaptable for detecting the endpoint of a polishing cycle for a variety of processing conditions and substrate types.

[00151] Thus, in one embodiment, the endpoint detection algorithm 1652 calculates the integral of the current signal and then accesses the appropriate lookup table 1654 to determine the corresponding amount of material removed.

[00152] While aspects of the invention are described with respect to lookup tables 1654, it should be understood that such tables are merely representative of one embodiment. More generally, any technique whereby a calculated total charge value is correlated to total material removed may be used to advantage.

[00153] In general, the lookup tables 1654 may be populated using theoretically derived information or empirically derived information. In either case, in electrochemical mechanical polishing, the current measured in the cell (i. e. , the total/cell current) is related to the metal (e. g. , copper) removal. The cell (total) current is composed of: (i) leakage current; and (ii) metal removal on the wafer (i. e. , the actual removal process which may be, for example, Cu+/Cu++/CuComplex removal). Leakage current may generally be characterized as the noneffective current and its sources include the chemical reaction of contact members (e. g. , contact members 1704) and the different oxidations which may occur. The first type of leakage current can be substantially eliminated where a zone switch is used (such as the ones described above with reference to FIGURES 17 and 18). The significance of the second type of leakage current is dependent upon the particular polishing reaction. In the case of copper polishing, the potential for copper oxidation is greater then the potential of the reaction of oxygen evolution. Thus, when there is competition between these two oxidation reactions, most of the current goes to copper oxidation.

Accordingly, leakage current due to oxygen evolution on the wafer can be neglected where copper is being polished. In any case, the leakage current can be calibrated easily using a silicon wafer. This will give a relationship between the leakage current and the voltage applied to the cell.

[00154] Having determined (or neglecting) the leakage current, the total current can be known by determining the contribution of metal removal on the wafer.

Then, the total current over time and the removal rate over time can be expressed as total charge removed and total material removed for a point in time. This can be done theoretically or empirically, as will now be described. By way of illustration only, the material to be polished is assumed to be copper. It is understood that the invention may be used to advantage with any other conducting material.

Theoretically Derived Current/Removal Rate and Chame/Removat Relationship [00155] The current/removal rate relationship can be described as follows : Current-> Charge per unit time-> Atoms removed per unit time-> removal rate on the wafer.

[00156] The current/removal rate relationship may be different depending on whether the wafer is a blanket wafer or a patterned wafer and depending on chemistry. In any case, the current/removal rate relationship can be obtained theoretically if the reaction scheme is known. For example, assume that it is known that only Cu++ (and not Cu+) is removed for a given process. Assume further that a uniform removal rate of 16000 A/min over a 200 mm wafer (surface area = 314 cm2) has been measured for a wafer. It is known that for a copper crystal : a=b=c=361.49pm = 3. 6149A. Thus, the volume of a unit cell is 47.23 A.

Since there are four (4) atoms in a unit cell and two (2) charges per unit cell, the total charge per unit cell removed is: 4atoms*2charges*1. 6e~19 C. Further, since the volume of 16000A is 314erz A3, the number of unit cells per 16000A is 314*e19/47. 23-6. 64e19. The total charge removed is therefore 6. 64e19 * (4atoms*2charges*1. 6e-19 C) = 85 C/min. Accordingly, a removal rate of 16000min corresponds to 1.42 Amps of Cu++ current. For a 200mm wafer, then, the current/removal relationship is 1.4Amps per kAngstrom/min. In this manner, the current/removal rate relationship may be determined for a desired range of current values and removal rates.

[00157] An illustrative current/removal rate relationship is expressed by the linear plot line 700 of FIGURE 22, where the y-axis is the removal rate of copper and the x-axis is the total current (offset due to leakage current). Note that the relationship is substantially linear. Thus, the plot line 700 is described by y = mx +b, where m is the slope of the line. The slope of the line depends on the oxidation processes taking place. For example, where Cu+ and Cu++ removal occur, the slope depends on the Cu+/Cu++ removal ratio. By way of illustration, recall that it was calculated above that where removal is purely Cu++, the current is 1.42 Amps per kAngstrom/min. It can similarly be calculated that where the removal is purely Cu+ the current is 0.71 Amps per kAngstrom/min. Thus, if the removal involves both Cu+ and Cu++, the current/removal rate relationship will be between 0.71 and 1.42 Amps per kAngstrom/min. Ultimately, the effective current will depend upon the particular chemistry being employed.

[00158] Having established the relationship between cell current and removal rate, it remains only to establish a meaningful application of this relationship for determining the endpoint of a polishing cycle. As described above, the endpoint is determined by calculating the total charge removed from measured current values. Accordingly, what is needed is to relate the total charge removed to the total material removed at a given time. Since a theoretical relationship between cell current and removal rate has been established according to the method described above, the derived relationship can be readily expressed as total charge removed and total material removed, as illustrated generically by FIGURE 21. The relationship between the total charge removed and total material removed is then used to populate the lookup tables 1654, which are used by the endpoint detection algorithm 1652 to determine the amount of material removed at a given point in time and, hence, the endpoint of a polishing cycle. More specifically, the thickness can be known by measuring current values over time and solving the following equation (which is a particular instance of Equation 1): Thickness (t) = Initial Thickness-Sum (O, t) [ (Current (t)- Leakage (V (t))) *Current_To_Removal_Coefficient] (Equation 3), where the Current To Removal Coefficient is the slope ("m") of the current/removal rate curve (e. g., plot 700 of FIGURE 22) and leakage is a constant ("b"). Thus, the values"m"and"b"for different processes/wafers can be stored in lookup tables, which are then accessed for a given process to retrieve the appropriate values for"m"and"b".

[00159] If the wafer is patterned, a density coefficient is needed to take into account that if the selectivity is 1600%, removal occurs only in the elevated areas.

The density coefficient can be a function of the copper thickness for a variety of reasons including the initial height differences between all the protrusions and the different speeds at which the features are planarized. Further, since the elevated areas diminish with polishing the density approaches 1 with time as the profile is flattened. Thus, the coefficient 1/Density (Thickness (t) ) can be used to correct the thickness estimation of a patterned wafer: Thickness (t) = Initial Thickness-Sum (O, t) [ (Current (t) - Leakage (V (t))) *Current_To_Removal_CoefficienVDensity (Thickness (t) )] (Equation 4) [00160] Thus, it can be seen that Equation 4 is a more general equation than Equation 3, since the coefficient 1/Density (Thickness (t) ) goes to one (1) for a blanket wafer, in which case Equation 3 and Equation 4 are equal. Again, the coefficients and the leakage current needed to solve for thickness using Equation 4 can be stored in the lookup tables 1654.

Empirically Derived Current/Removal Rate and Charne/Removal Relationship [001611 In an alternative embodiment, the current/removal rate and charge/removal relationship is determined empirically. For example, the amount of material removed from a wafer can be measured periodically (e. g. , by sheet resistivity measurements) while operating the power supply 1634 in the current mode.

[00162] Alternatively, the current may be measured for a series of wafers processed at different conditions (e. g., slightly different polishing times, voltage biases, etc.). In this manner, a calibration curve can be acquired. One such calibration curve 800 is shown in FIGURE 23. In this case, 20 wafers were polished under different conditions and the average current was recorded. The thicknesses of the wafers were measured before and after the polishing cycle to determine the average removal rate. The calibration curve (expressed as y = 1. 1185x + 1.2512) exhibits a linear relationship between the average current and the average removal rate and allows for the prediction (by extrapolation) of a removal rate for a given current.

[00163] Having established a relationship between current and removal rate, it now remains to establish a relationship between the total charge removed (given by the sum of measured current values, as described with reference to Equation 2) and the thickness of the material removed. FIGURE 24 shows one example of an empirically determined curve relating total charge (referred to Quantity of Current (A*sec) on the x-axis) and material removed (referred to as Removal on the y-axis). The information of FIGURE 24 is used to populate a lookup table 1654.

[00164] In operation, a total charge value is calculated using the periodically measured current values. The appropriate lookup table 1654 is then accessed to determine the thickness of removed material. The endpoint detection algorithm then determines whether a target thickness (or a target total charge) has been reached. If so, the polishing process is halted and the substrate is removed from the cell.

[00165] Persons skilled in the art will recognize that the foregoing embodiments are merely illustrative. The invention contemplates and admits of many other embodiments. For example, a number of the foregoing embodiments described a face down electropolishing technique. That is, the substrate to be processed is in a face down orientation relative to the polishing pad. However, in other embodiments, face up electropolishing techniques are employed. These and other embodiments are considered within the scope of the invention.

[00166] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.