TECHNICAL FIELD

[0001] This specification relates to chemical mechanical polishing applications using cerium oxide slurries.

BACKGROUND

[0002] An integrated circuit is typically formed on a silicon wafer by sequential deposition of conductive, semi conductive or insulative layers. One fabrication step involves depositing a layer over a non-planar surface and planarizing the layer. For some applications, the layer is planarized until the top surface of the patterned underlying layer is exposed. For other applications, the layer is planarized until a predetermined thickness is left over the underlying layer.

[0003] Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method mounts the substrate on a carrier head and the surface of the substrate is placed against the surface of a rotating polishing pad. A polishing liquid, such as an abrasive slurry, is dispensed onto the rotating polishing pad, thereby polishing the layer on the substrate through mechanical and chemical means. The abrasive particles in the slurry can be silicon oxide and cerium oxide.

SUMMARY

[0004] In one aspect, a method of polishing includes dispensing a polishing slurry containing negatively charged ceria oxide onto a polishing pad, contacting a surface of a substrate to the polishing pad in the presence of the slurry, generating relative motion between the substrate and the polishing pad to polish the surface of the substrate, measuring at a removal rate for the substrate, determining that the measured removal rate is less than a target removal rate, and in response to determining that the measured removal rate is less than the target removal rate, decreasing a temperature of an interface between the polishing pad and the substrate.

[0005] In another aspect, a method of polishing includes dispensing a polishing slurry containing negatively charged ceria oxide onto a polishing pad, contacting a surface of a substrate to the polishing pad in the presence of the slurry, generating relative motion between the substrate and the polishing pad to polish the surface of the substrate, measuring a removal rate for the substrate, determining that the measured removal rate is greater than a target removal rate, and in response to determining that the measured removal rate is greater than the target removal rate increasing a temperature of an interface between the polishing pad and the substrate. The abrasive agent may include cerium oxide particles.

[0006] In another aspect, a method for removing material from a substrate includes dispensing a slurry that includes a carrier liquid and an abrasive agent on a surface of a polishing pad, storing an indication of a relative charge on the abrasive agent, contacting a surface of a substrate to the polishing pad in the presence of the slurry, generating relative motion between the substrate and the polishing pad to polish the surface of the substrate, measuring a removal rate for the substrate, comparing a the measured removal rate to a target removal rate and determining whether to increase or decrease the removal rate based on the comparison, determining whether to increase or decrease a temperature of an interface between the polishing pad and the substrate based on the indication of the relative charge of the abrasive agent and on whether to increase or decrease the removal rate, and controlling a temperature of the interface as determined to modify the removal rate.

[0007] In another aspect, a method of polishing includes polishing a layer on a substrate by dispensing a polishing slurry onto a polishing pad, contacting a surface of the layer on the substrate to the polishing pad in the presence of the slurry, and generating relative motion between the substrate and the polishing pad, for an initial portion of polishing of the layer controlling temperature of the polishing to be within a first temperature range, obtaining a temperature transition time that is before an endpoint time, upon determining that the temperature transition time is reached, lowering the temperature of the polishing to be within a lower second temperature range that is lower than the first temperature range, and for a subsequent portion of polishing of the same layer controlling temperature of the polishing to be within the second temperature range until the estimated endpoint time.

[0008] In another aspect, a method of polishing includes polishing a layer on a substrate by dispensing a polishing slurry onto a polishing pad, contacting a surface of the layer on a substrate to the polishing pad in the presence of the slurry, and generating relative motion between the substrate and the polishing pad, for an initial portion of polishing of the layer controlling temperature of the polishing to be within a first temperature range, determining a temperature transition time that is before an endpoint time, upon determining that the temperature transition time is reached increasing pressure on the substrate while increasing coolant flow to continue to maintain the temperature of the polishing to be within the first temperature range, and for a subsequent portion of polishing of the same layer maintaining the increased pressure and controlling temperature of the polishing to be within the first temperature range until the estimated endpoint time.

[0009] Implementation may include one or more of the following features. Dispensing the coolant fluid may include spraying the coolant fluid through a convergent-divergent nozzle. Measuring the removal rate may include monitoring the substrate during polishing with an in-situ optical monitoring system. The surface of the substrate may include oxide layer, e.g., silicon oxide. Controlling the temperature of the interface may include increasing the polishing rate by increasing the temperature if the indication is positively charged, increasing the polishing rate by decreasing the temperature if the indication is negatively charged, decreasing the polishing rate by decreasing the temperature if the indication is positively charged, or decreasing the polishing rate by increasing the temperature if the indication is negatively charged,.

[0010] Advantages can include, but are not limited to, one or more of the following. A CMP system can achieve a high polishing rate to match customer production demands. The method described herein improves the throughput of a system further by reducing the time needed to polish each substrate. This leads to increased substrate output and lowered consumable materials cost per substrate. Optimization of the CMP process temperatures in conjunction with charged ceria applications also allows increased polishing pad lifespan, decreasing costs for customers.

[0011] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic cross-sectional view of a chemical mechanical polishing system.

[0013] FIG. 2 is a flow chart of a method of polishing.

[0014] FIG. 3 is a flow chart of another implementation of a method of polishing.

[0015] FIG. 4 is a flow chart of yet another implementation of a method of polishing.

[0016] Like reference numbers and designations in the various drawings indicate like elements. DETAILED DESCRIPTION

[0017] The material removal rate of a CMP process depends on selection of the abrasive and other components of the polishing fluid, pressure applied to the substrate, relative velocity between the polishing pad and substrate, and temperature at the interface between the substrate and polishing pad. Conventionally, chemically reactive processes, e.g., a polishing process, increase with temperature. Thus, increasing temperature can be one technique to increase the removal rate.

[0018] However, the actual dependency of polishing rate on temperature can be a more complex interaction between the effect of temperature on the polishing pad, e.g., the elastic modulus of the polishing pad, as well as reaction rates driven by temperature. Moreover, for some polishing processes, the electrostatic potential of the abrasive particles is a component in this interaction.

[0019] Cerium oxide (e.g., ceria) is an abrasive material in the polishing liquid for some polishing processes. In the polishing liquid, the surface of the abrasive ceria particles can have a positive electrostatic potential, a negative electrostatic potential, or a negligible electrostatic potential on the surface of the abrasive particles. This potential might depend on synthetizing techniques. Polishing processes that use polishing liquids with ceria particles exhibit a polishing rate that responds to temperature differently depending on the positive or negative potential at the surface of particles in the slurry.

[0020] This application describes techniques to perform temperature control based on the charge properties of the abrasive particles. The CMP system includes a heater or a cooler to control the temperature at the interface of the substrate and polishing pad. By changing the temperature of the polishing process, the material removal rate increases or decreases depending on the surface charge of the ceria suspended in the slurry. For negatively charged ceria slurries, cooling can increase the polishing rate and improve topography. Without being limited to any particular theory, cooling using negatively charged ceria can improves the material removal rate by increasing the hardness and modifying the asperity structure of the top surface of the pad. For some positively charged ceria slurries, a combination of heating and cooling can improve the polishing rate by heating at the beginning of the polishing process and cooling near the polishing endpoint improve topography. For other positively charged ceria slurries, a combination of cooling and increased pressure is used to improve the polishing rate; increased pressure increases the polishing rate and cooling prevents pad overheating and maintains topography. [0021] FIG. 1 illustrates an example of a polishing system 20. The polishing system 20 can include a rotatable disk-shaped platen 22 on which a polishing pad 30 is situated. The platen is operable to rotate about an axis 23. For example, a motor 24 can turn a drive shaft 26 to rotate the platen 22. The polishing pad 30 can be detachably secured to the platen 22, for example, by a layer of adhesive. The polishing pad 30 can be a two-layer polishing pad with an outer polishing layer 32 and a softer backing layer 34.

[0022] The polishing system 20 can include a polishing liquid supply port 40 to dispense a polishing liquid 42, such as an abrasive slurry, onto the polishing pad 30. The polishing system 20 can also include a polishing pad conditioner to abrade the polishing pad 30 to maintain the polishing pad 30 in a consistent abrasive state.

[0023] A carrier head 50 is operable to hold a substrate 10 against the polishing pad 30. Each carrier head 50 also includes a plurality of independently controllable pressurizable chambers, e.g., three chambers 52a-52c, which can apply independently controllable pressurizes to associated zones on the substrate 10. The chambers 52a-52c can be defined by a flexible membrane 54 having a bottom surface to which the substrate 10 is mounted. The carrier head 50 can also include a retaining ring 56 to retain the substrate 10 below the flexible membrane 54. Although only three chambers are illustrated in FIGS. 1 and 2 for ease of illustration, there could be two chambers, or four or more chambers, e.g., five chambers. In addition, other mechanisms to adjust the pressure applied to the substrate, e.g., piezoelectric actuators, could be used in the carrier head 50.

[0024] Each carrier head 50 is suspended from a support structure 60, e.g., a carousel or track, and is connected by a drive shaft 62 to a carrier head rotation motor 64 so that the carrier head can rotate about an axis 51. Optionally each carrier head 50 can oscillate laterally, e.g., on sliders on the carousel, by motion along or track; or by rotational oscillation of the carousel itself. In operation, the platen 22 is rotated about its central axis 23, and the carrier head 50 is rotated about its central axis 51 and translated laterally across the top surface of the polishing pad 30.

[0025] The polishing system also includes an in-situ monitoring system 70, which can be used to control the polishing parameters, e.g., the applied pressure in one or more of the chambers 52a-52c. The in-situ monitoring system 70 can be an optical monitoring system, e.g, a spectrographic monitoring system, particularly for polishing of oxide layers on the substrate. Alternatively the in-situ monitoring system 70 can be an eddy current monitoring system, particularly for polishing of metal layers on the substrate. [0026] As an optical monitoring system, the in-situ monitoring system 70 can include a light source 72, a light detector 74, and circuitry 76 for sending and receiving signals between a controller 90, e.g., a computer, and the light source 72 and light detector 74. One or more optical fibers 78 can be used to transmit the light from the light source 72 to a window 36 in the polishing pad 30, and to transmit light reflected from the substrate 10 to the detector 74. As a spectrographic system, then the light source 72 can be operable to emit white light and the detector 74 can be a spectrometer. The measured spectrum can be converted into a characteristic value indicative of the thickness of the layer being polished in each of the zones.

[0027] The output of the circuitry 76 can be a digital electronic signal that passes through a rotary coupler 28, e.g., a slip ring, in the drive shaft 26 to the controller 90. Alternatively, the circuitry 76 could communicate with the controller 90 by a wireless signal. The controller 90 can be a computing device that includes a microprocessor, memory and input/output circuitry, e.g., a programmable computer. Although illustrated with a single block, the controller 90 can be a networked system with functions distributed across multiple computers.

[0028] The polishing system 20 includes a temperature sensor 80 to monitor a temperature of the polishing process, e.g., the temperature of the polishing pad 30 and/or polishing liquid 42 on the polishing pad, or of the substrate. For example, the temperature sensor 80 could be an infrared (IR) sensor, e.g., an IR camera, positioned above the polishing pad 30 and configured to measure the temperature of the polishing pad 30 and/or polishing liquid 42 on the polishing pad. In particular, the temperature sensor 64 can be configured to measure the temperature at multiple points along the radius of the polishing pad 30 in order to generate a radial temperature profile. For example, the IR camera can have a field of view that spans the radius of the polishing pad 30.

[0029] In some implementations, the temperature sensor is a contact sensor rather than a non-contact sensor. For example, the temperature sensor 64 can be thermocouple or IR thermometer positioned on or in the platen 24. In addition, the temperature sensor 64 can be in direct contact with the polishing pad.

[0030] In some implementations, multiple temperature sensors could be spaced at different radial positions across the polishing pad 30 in order to provide the temperature at multiple points along the radius of the polishing pad 30. This technique could be use in the alternative or in addition to an IR camera. [0031] Although illustrated in FIG. 1 as positioned to monitor the temperature of the polishing pad 30 and/or polishing liquid 42 on the pad 30, the temperature sensor 64 could be positioned inside the carrier head 50 to measure the temperature of the substrate 10. The temperature sensor 64 can be in direct contact (i.e., a contacting sensor) with the semiconductor wafer of the substrate 10. In some implementations, multiple temperature sensors are included in the polishing system 20, e.g., to measure temperatures of different components.

[0032] The polishing system 20 also includes a temperature control system 100 to control the temperature of the polishing pad 30 and/or polishing liquid 42 on the polishing pad. The temperature control system 100 include a cooling system and/or a heating system. In some implementations both the cooling system and/or heating system operate by delivering a temperature-controlled medium, e.g., a liquid, vapor or spray, onto the polishing surface 36 of the polishing pad 30 (or onto a polishing liquid that is already present on the polishing pad).

[0033] As shown in FIG. 1, an example temperature control system 100 includes an arm 110 that extends over the platen 22 and polishing pad 30. Multiple nozzles 120 are suspended from the arm 110, and each nozzle 120 is configured to spray a temperature control fluid onto the polishing pad. The arm 110 can be supported by a base 112 so that the nozzles 120 are separated from the polishing pad 30 by a gap 126. Each nozzle 120 can be configured to start and stop fluid flow through each nozzle 120, e.g., using the controller 12. Each nozzle 120 can be configured to direct aerosolized water in a spray 122 toward the polishing pad 30.

[0034] To operate as a cooling system, the temperature control fluid is a coolant. The coolant be a gas, e.g., air, or a liquid, e.g., water. The coolant can be at room temperature or chilled below room temperature, e.g., at 5-15 °C. In some implementations, the cooling system uses a spray of air and liquid, e.g., an aerosolized spray of liquid, e.g., water. In particular, the cooling system can have nozzles that generate an aerosolized spray of water that is chilled below room temperature. In some implementations, solid material can be mixed with the gas and/or liquid. The solid material can be a chilled material, e.g., ice, or a material that absorbs heat, e.g., by chemical reaction, when dissolved in water. When dispensed, this coolant can be below room temperature, e.g., from -100 to 20°C, e.g., below 0°C.

[0035] To operate as a heating system, the temperature control fluid is a heated fluid.

The heating fluid can be a gas, e.g., steam or heated air, or a liquid, e.g., heated water, or a combination of gas and liquid. The heating fluid is above room temperature, e.g., at 40-120 °C, e.g., at 90-110 °C. The fluid can be water, such as substantially pure de-ionized water, or water that includes additives or chemicals. In some implementations, the heating system uses a spray of steam. The steam can includes additives or chemicals.

[0036] The temperature control system 100 can include a single arm to dispense either a coolant or a heating fluid, or two dedicated arms to dispense the coolant and the heating fluid, respectively.

[0037] Other techniques can be used by the temperature control system 100, in the alternative or in addition, to control the temperature of the polishing process. For example, heated or cooled fluid, e.g., steam or cold water, can be injected into the polishing liquid 42 (e.g., slurry) to raise or lower the temperature of the polishing liquid 42 before the polishing liquid 42 is dispensed. As another example, resistive heaters could be supported in the platen 22 to heat the polishing pad 30, and/or in the carrier head 50 to heat the substrate 10.

[0038] Moderating the temperature of the slurry and polishing pad during polishing of a layer allows for increased interaction between charge-carrying abrasives such as cerium oxide. By using temperature control, the material rate of removal can be beneficially increased by both modulating the physical parameters of the polishing pad as well as altering the chemical interaction characteristics between the charged ceria and filler layer.

[0039] In some implementations, a temperature sensor measures the temperature of the polishing process, e.g., of the polishing pad or polishing liquid on the polishing pad or the substrate, and the controller 90 executes a closed loop control algorithm to control the temperature control system, e.g., the flow rate or temperature of the coolant or heating fluid relative, so as to maintain the polishing process at a desired temperature.

[0040] In some implementations, the in-situ monitoring system measures the polishing rate for the substrate, and the controller 90 executes a closed loop control algorithm to control the temperature control system, e.g., the flow rate or temperature of the coolant or heating fluid relative, so as to maintain the polishing rate at a desired rate.

[0041] FIG. 2 illustrates a method to carry out this technique, which is applicable for charged ceria slurries. Optionally, the controller 90 stores an indication of the whether the slurry being used contains negatively charged abrasive ceria particles or positively charged abrasive ceria particles (202). Polishing is performed, with the slurry with abrasive ceria particles dispensed onto the polishing pad (204). The controller 90 can store a desired temperature or temperature range, e.g., as part of a polishing recipe. So during polishing, the controller 90 can operate to maintain the temperature of the polishing process at the desired temperature or temperature range, e.g., using an open-loop or closed loop algorithm (206). During polishing, the polishing process is monitored by an in-situ monitoring system, and the removal rate is calculated from the acquired data (208). Due to a variety of causes, the removal rate may depart from a desired polishing rate (210). For example, the controller 90 can detect whether the removal rate varies from a target polishing rate by more than a threshold amount. If this occurs, the controller 90 can cause the temperature control system to modify the process temperature so as to compensate and bring the removal rate back toward the desired polishing rate. However, what action should be taken can depend on the charge of the abrasive ceria particles.

[0042] In particular, referring to Table 1, for a negatively charged abrasive ceria particle slurry, if the removal rate is lower than the desired polishing rate then the temperature can be lowered to increase the polishing rate, whereas below if the removal rate is higher than the desired polishing rate then the temperature can be raised to decrease the polishing rate. In contrast, for a positively charged abrasive ceria particle slurry, if the removal rate is lower than the desired polishing rate then the temperature can be raised to increase the polishing rate, whereas below if the removal rate is higher than the desired polishing rate then the temperature can be lowered to decrease the polishing rate.

Table 1

[0043] This data can be stored and accessed by the controller 90, e.g., as control logic or a lookup table, in order to determine how to adjust the temperature if the removal rate deviates from the desired polishing rate (212). Alternatively, the decision process on whether to increase or decrease the temperature could be embedded in a process recipe associated with a particular slurry that is loaded by the controller.

[0044] The controller 90 then causes the temperature control system to adjust the temperature, e.g., by increasing or decreasing the temperature and/or flow rate of the temperature control fluid to modify the process temperature, e.g., the pad temperature (214). [0045] With respect to increasing the processing temperature, the maximum desirable temperature depends on the glass transition temperature for the polishing pad. If the pad becomes too hot, it can become too viscoelastic, and the polishing process may not proceed as expected, e.g., polishing rate may drop or defects may increase. In general, the controller can be configured to keep the temperature below 2/3 of the melting point (compared to 0 C) of the polishing layer.

[0046] Separate from the issue of the impact of electrostatic charge on the dependence of polishing rate on temperature, for many polishing applications it is useful to reduce the polishing rate as the polishing process approaches the polishing endpoint in order to avoid overpolishing and reduce non-uniformity. On the other hand, keeping the polishing rate high during bulk polishing of thick layers is beneficial. One approach to reducing the polishing rate that has been proposed is to reduce the pressure on the substrate. However, this may not be practical in some applications, e.g., where the carrier head is already operating at a low applied pressure, such as for polishing of fragile layers.

[0047] An approach that could be used instead of or in addition to reduction of the applied pressure near the polishing endpoint is to modify the process temperature to reduce the polishing rate. For example, for traditional silica slurries or for positively charged ceria slurries, the temperature can be reduced before the polishing endpoint to reduce the polishing rate.

[0048] FIG. 3 illustrates a method to carry out this technique. For an initial portion of polishing of the layer, the temperature of the polishing process is controller to be within a first temperature range (302). The initial portion can run from the beginning of the polishing process.

[0049] Control can be performed by the controller 90 using a feedback loop that receives temperature measurements from the sensor 60 and adjusts operation of the temperature control system 100. It may be understood that the temperature of the polishing pad at a particular location, or of the slurry, or the substrate, can be a stand-in for the temperature of the polishing process.

[0050] Either before or after polishing begins, a temperature transition time is determined that is before an expected endpoint time (304). The temperature transition time can be a preset value based on a recipe; in this case the temperature transition time can determined by the user before polishing begins. Alternatively, the polishing process can be monitored by the in-situ monitoring system. The in-situ monitoring system can project an estimated endpoint time based on measured polishing rate of the substrate, and the transition time can be calculated based on the estimated endpoint time, e.g., a preset time, e.g., 10 seconds, or a percentage, e.g., 5-10% of the total polishing time, before the estimated endpoint time.

[0051] Once the temperature transition time is reached, the controller 90 causes the temperature control system to lower the temperature of the polishing to be within a lower second temperature range that is lower than the first temperature range (306). The lower second temperature range can be non-overlapping with the first temperature range, or can overlap no more than 25% of the first temperature range. The midpoint of the second temperature range can be 20-40°C lower than the midpoint of the first temperature range. In some embodiments, the temperature of the polishing surface 36 can be lowered to at or below 30°C, e.g., at or below 20°C.

[0052] Once the temperature of the polishing process has reached the second temperature range, for a subsequent portion of polishing process of the same layer, the controller 90 causes the temperature control system 100 to maintain the temperature of the polishing process within the second temperature range (308). The subsequent portion of the polishing process can last until the estimated endpoint time for the layer.

[0053] Another approach that could be used instead reduction of the applied pressure near the polishing endpoint is to increase the pressure on the substrate so as to reduce non uniformity, while also increasing temperature control flow so that the temperature control system maintains the desired temperature. For example, for traditional silica slurries or for positively charged ceria slurries, the pressure on the substrate and/or the rotation rate of the platen can be increased, and the flow rate of the coolant can be increased before the polishing endpoint to achieve higher non-uniformity without sacrificing polishing rate.

[0054] FIG. 4 illustrates a method to carry out this technique. For an initial portion of polishing of the layer, the temperature of the polishing process is controlled to be within a first temperature range (402). Either before or after polishing begins, a temperature transition time is determined that is before an expected endpoint time (404). These two steps can be performed by as discussed above for step 302 and 304.

[0055] Once the temperature transition time is reached, the controller 90 adjust the pressure in one or more chambers in the carrier head 50 to increase pressure on the substrate (406). In conjunction, the controller 90 causes the temperature control system to increase the flow rate of the temperature control fluid, e.g., the coolant for silica slurries or for positively charged ceria slurries, so that the temperature is maintained within the first temperature range (408). The subsequent portion of the polishing process can last until the estimated endpoint time for the layer.

[0056] With respect to increasing the processing temperature, the maximum desirable temperature depends on the glass transition temperature for the polishing pad. If the pad becomes too hot, it can become too viscoelastic, and the polishing process may not proceed as expected, e.g., polishing rate may drop or defects may increase. In general, the controller can be configured to keep the temperature below 2/3 of the melting point (compared to 0 C) of the polishing layer.

[0057] More generally, for traditional silica slurries or for positively charged ceria slurries, in order to maximize polishing rate, it may be desirable to run the polishing process at the maximum possible temperature before polishing degrades due to viscoelasticity of the polishing pad. Thus, rather than letting the temperature ramp up due to friction between the substrate and polishing pad, the temperature may be driven to a desired temperature at the beginning of the polishing process by the temperature control system. The temperature can then be maintained within a desired temperature range, e.g., at a temperature of about 50-66% of the melting point (compared to 0 C) of the polishing layer.

[0058] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially be claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

[0059] Similarly, while operations are depicted in the drawings and recited in the claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.

[0060] Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results.