CHEN, Gim-syang (192 Windermere Ave, Allentown, PA, 18104, US)
KASHKOUSH, Ismail (5919 Ricky Ridge Trail, Orefield, PA, 18069, US)
CHEN, Gim-syang (192 Windermere Ave, Allentown, PA, 18104, US)
Claims
What is claimed is:
1. A method of processing a substrate comprising:
a) supporting a substrate having a hydrophilic surface in a substantially horizontal orientation;
b) rotating the substrate;
c) applying a film of an aqueous solution of HF to the hydrophilic surface of the substrate for a period of time sufficient to convert the hydrophilic surface into a hydrophobic surface,, wherein the concentration of HF is between about 0.1 % to about 0.5 % by weight of HF in water and the period of time is between about 100 and about 300 seconds;
d) applying DI water to the hydrophobic surface of the substrate; and
e) applying a drying fluid to the hydrophobic surface of the substrate so as to substantially dry the hydrophobic surface,
2. The method of claim 1 wherein the substrate is rotated at a substantially constant rotational speed during completion of steps c) through e), wherein the rotational speed is less than 400 rpm.
3. The method of claim 2 wherein the rotational speed is less than about 200 rpm.
4. The method of claim 1, wherein the DI water is degassed.
5. The method of claim ! wherein step d) comprises applying the film of DI water to the hydrophobic surface of the substrate for between about 5 to about 20 seconds.
6. The method of claim 1 wherein the drying fluid is Ni/iPA vapor,
7. The method of claim 6 wherein the drying fluid is coupled to a drying source comprising a iϊrsi bubbler and a second bubbler, wherein the second bubbler is sequentially and operabiy aligned to the first bubbler, the first bubbler generating N^/IPA vapor having a first TPA concentration and coupled to the second bubbler, the second bubbler generating isyiPA vapor having sn elevated IPλ concentration greater than the first !PA concentration, and wherein the drying fluid comprises the elevated IFA concentration.
8. The method of claim 1 wherein the chamber comprises an exhaust positioned tn a substantially tangential position relative to a horizontal edge of the substrate.
9. The method of claim 8 wherein airflow velocity within the chamber is between about 250 and about 280 au.
10. A method of processing a substrate comprising:
a) supporting a substrate having a hydrophilic surface in a substantially horizontal orientation;
b) rotating the substrate about a center point at a rotational speed selected to πύnimi/.e particle addition on the substrate;
c) applying a film of an aqueous solution of HF having a concentration of HF to the hydrophobic surface of the substrate for a period of time, wherein the concentration of HF ami the period of time are selected so that the hydrophilic surface is converted ύUo a hydrophobic surface;
ά) applying Dl water to the hydrophobic surface of the substrate; and
e) applying a drying fluid to the hydrophobic surface of the substrate so as to substantia! Iy dry the hydrophobic surface, the drying fluid coupled to a drying source comprising a ftrst bubbler and a second bubbler, wherein ihe second babbler is sequentially and αperabiy aligned to the first bubbler, the first bubbler generating NVlPA vapor having a first iPλ concentration and coupled to the second babbler, the second bubbler generating NyIPA vapor having an. elevated IPA concentration greater tivm the Sm IPλ concentration, and wherein the drying fluid comprises the elevated IPA concentration.
11. The method of claim 10 wherein the substrate is rotated at a substantially constant rotational speed during completion of steps c) through c). and wherein the rotational speed is less than 400 rpm.
12. The method of claim H) wherein the period oi time is between about 100 and about 300 seconds.
13. The method of claim 10 wherein the concentratioβ of HF is between about 0.1 % to about .0.5 % by weight of HF in water.
14. The method of. claim 10 wherein the DI water is degassed DI water and wherein step d) comprises applying the film of Dl water to the hydrophobic surface of the substrate for between about 5 to about 20 seconds.
15. The method of claim 10 wherein the chamber comprises an exhaust positioned in a substantially tangential position relative to a horizontal edge of the substrate, the exhaust configured to maintain a substantially even distribution of airflow within the chamber, where airflow velocity is between about 250 and about 280 au.
16. The method of claim 10 further comprising positioning an assembly comprising a first dispenser, a second dispenser, and a third dispenser above the surface of the substrate, the first dispenser operabiy coupled to a source of DI water and the second and third dispensers ©perably coupled to a source of drying fluid, the second and third dispensers positioned on the assembly adjacent one another and spaced from the first dispenser, the second dispenser having a larger opening than the third dispenser, and the second dispenser being located between the third dispenser and the first dispenser.
17. Au apparatus for processing a substrate comprising:
a chamber having at least one wall;
a rotary support member located within the chamber for supporting the substrate in a substantially horizontal position and adapted to rotate the substrate; and
a first exhaust exit located within the at least one wall, wherein the first exhaust exit is tangential to a rotational direction of the substrate,
18. The apparatus of claim 17 further comprising a second exhaust exit, wherein the second exhaust exit is parallel to the axis of rotation of the substrate.
19. The apparatus of claim 17 further comprising a second exhaust exit, wherein the first exhaust exit and the second exhaust exit are tangential to the rotational direction of the substrate.
20. The apparatus of claim 17 wherein the first exhaust exit is free of baffles. |
APPARATUS AND METHOD FOR PROCESSING A HYDROPHOBIC SURFACE OK A
S ORSTRATE
Crβss-Reference Io Related Patent Applications j0001| This application claims the benefit of U.S. Provisional Application No. 60/809,656 filed on May 30, 2006; U.S. Provisional Application No. 60/83 J, 793 filed July 59, 2006; and U.S. Provisional Application No. 60/844,859 filed September 15, 2006, the entireties of which are hereby incorporated by reference.
Field of Invention
[0002J The present invention relates generally to the field of processing substrates, ant! specifically to methods and apparatus for rinsing and/or drying hydrophobic surfaces of semiconductor wafers.
Background of the Invention
{00031 ' fhe importance of clean semiconductor wafer surfaces in the fabrication of semiconductor devices has been recognized since the beginning of the industry. Fail ing to removes trace impurities, isuch as sodium ions, metals, and particles, from a semiconductor wafer surface is known to be especially detrimental during high-temperature processing because the impurities tend to spread out and diffuse into the semiconductor wafer, thereby altering the electrical characteristics of the semiconductor devices formed in the wafer. Altering a semiconductor device's electrical characteristics causes the device to fail and, therefore, subtracts from a wafer's yield. The inadequate and/or improper drying of a semiconductor wafer surface is also known to negatively affect a wafer's yield. Over time, as VLSI and ULSl silicon circuit technology has developed, the cleaning and drying processes have become particularly critical steps in the semiconductor fabrication process. [Oϋϋ4j In order to minimize device failure, semiconductor wafers are typically subjected to a multitude of intermediate cleaning and drying steps between the various manufacturing steps required for semiconductor device fabrication. Thus, the integrity, efficiency and .ffectiveness of the cleaning {and subsequent drying) step has become βλtreraely important for tire successful manufacture of semiconductor devices. iOOOSj Of particular importance are cleaning and drying applications that are per formed atler the application of hydrogen fluoride or hydrofluoric acid {HF) to the surface of a wafer. Traditionally, cleaning and drying processes performed subsequent to HF-!ast processes result in less than optimal particle removal and the creation of watermarks on the wafer surface. For example, post-clean light-point defects (LPD) have been observed totaling 20
LPD at >0.12 μm and 9QOX low temperature bake posi-epiiaxiat LPD have bscn observed totaling 15S LPD at > 0.12 μm.
JOOOOi It is believed that the difficulties with cleaning and drying semiconductor wafers after HF-lasi processes results from the surface of the semiconductor wafers becoming hydrophobic in nature from the application of HF. Specifically, ii is the transition of the wafer surface from hydrophilic to hydrophobic in nature that causes the undesired particle addition and the creation of watermarks on the surface. J " he application of HF, however, is necessary to prepare the surface of the semiconductor wafer to certain manufacturing steps, such as thin film deposition processes (e.g., the deposition of epitaxial .silicon). Proper pre~ epitaxial cleaning and drying processes am also critical in that they remove unwanted oxides from the surfaces of wafers prior to film deposition. The problems experienced from inadequately cleaning and drying of the surfaces of semiconductor wafers subjected to an HF-last process have become even more exasperated by the transition of the semiconductor industry from batch immersion platforms to single-wafer spin processing platforms. (0QO7J Single-wafer cleaning and drying technology has gained increasing attention in the semiconductor manufacturing industry due to its advantages in cycle time, flexibility, and cost-of-cwttership in fabrication operations. An example of such a system is disclosed m United States Patent No. 6,039,059 to Bran, the entirety of which is herein incorporated by reference. While many of the theories and fundamental concepts for the wet processing of wafers remain similar for both platforms, the change from batch imrnersion platforms to single-wafer spin processing tools has fed tu challenges in some applications. One such application is that of the cleaning and drying of semiconductor wafers subjected to an HF-iasi process, in fact, the subsequent cleaning and drying of wafers subjected to HF last process, has proven u> he one of the most problematic areas for single-wafer spin processing tools, often resulting Jn high particle counts and the creation of watermarks on die wafer. Thus there is a need for an improved cleaning and/or drying process that can be performed on a single-wafer spin processing tool for semiconductor wafer surfaces that have been subjected to an HF-last process.
Summary of the Invention fOOflSJ These problems and others arc solved by the present invention which in one aspect is a method of processing a substrate comprising a) supporting a substrate having a hydrophthc surisce is a substantially horizontal orientation, b) rotating the substrate, c) ajψlymg a film of an aqueous solution of HF to the hydrophilic surface of the substrate for a period of time
sufficient to convert the hydrophiiiυ surface into a hydrophobic surface, whereirs the* concentration of I IF is between about 0.1 % to about 0.5 % by weight of HF in water and the period of time is between about 100 and about 300 seconds, d} applying DI water to the hydrophobic surface of the substrate, and e) applying a drying fluid to the hydrophobic surface of the substrate so as Co substantially dry the hydrophobic surface. |θO09] A further aspect of the invention can be a method of processing a substrate comprising a) supporting a substrate having a hydronlϊilie surface in a substantially bomoαtai orientation, b) rotating the substrate about a center point at a rotational speed selected to minimize particle addition oa the substrate, c) applying a film of an aqueous solution of HF having a concentration of KF to the hydrophobic surface of the substrate for a period of time, wherein the concentration of HF and the period of time are selected so that the hydrophilic surface is converted, into a hydrophobic surface, d) applying DI water to the hydrophobic surface of the substrate, and e) applying a drying fluid to the hydrophobic surface of the substrate so as to substantially dry the hydrophobic surface, the drying fluid coupled to a drying source comprising a first bubbler and a second bubbler, wherein the second bubbler is sequentially and operabiy aligned io the first bubbler, the first bubbler generating hWIFA vapor having a first IPA concentration and coupled to the second bubbler. the second bubbler generating NvTPA vapor having an elevated ϊP A concentration greater than the first IPA concentration, and wherein the drying fluid comprises the elevated IPA concentration. {OθiOj Yet another aspect of ihe invention can. be an apparatus for processing a substrate comprising a chamber having at least one wall; a rotary support member located within the chamber for supporting the substrate in a substantially horizontal position and adapted to rotate the substrate; and a first exhausx exit located within the at least one wall, wherein the first exhaust exit is tangential to a rotational direction of the substrate.
Brief Description of Drawings fOOll} FIG. 1 is a schematic of the cleaning and processing system according to an embodiment of the present invention.
[00J2J FIG. 2 is a chart illustrating the effect of etch time at different HF concentrations <m substrate particle addition,
[0013| PlG- 3 is & graph of etch lime v. normalized oxide thickness for she application of two different aqueous solutions having different HF concentrations to a silicon wafer.
(θ014J FlG, 4 is a chart showing d*e effect of etch time on substrate particle addition as well m native oxide thickness.
|ti()J5] HG. 5 is a chart illustrating the effect of gases-aerated DfW on substrate particie addition. jθtilόj FTG. 6 is a chart showing the effect of varying rotational speed with a 60 second rinse on substrate particle addition for multiple embodiments of the present invention.
[θOI7| FiG. 7 is a graph comparing the addition of particles on a wafer surface when subjected to a DiW rinse as opposed to an aqueous HF solution rinse.
{0018] FIG. S is s chart illustrating the effect of varying rotational speed on substrate particle addition.
[00J 9] FIG. 9 is a chart illustrating the effect of DlW rinse time at 60 seconds and 5 seconds on substrate panic Ie addition.
{0020] FIG. 10 is a chart illustrating the effect of a substrate final spin on particle addition. the final spin conducted within a process chamber wilh a standard exhaust system.
[00211 FlG. H is a chart illustrating the effect of the lack of a substrate final spin on particle addition, the substrate contained within a process chamber with a standard exhaust system.
JO022J FIG. 12a is a simplified top view of the process chamber with connected standard exhaust.
[0823 j FIG 12b is a simplified side view of the process chamber with connected standard exhaust. j ( M)24| FFG. Ba is a simplified side view of " the process chamber with a tangential exhaust exit.
[0025} FIG. 13b is a simplified top view of the process chamber with a tangential exhaust exit and a standard exhaust exit.
J0026] FIG. 13c is a simplified top view of the process chamber with two tangential exhaust exits.
(0027] FIG. 34 is a chart illustrating the effect of a suhstrate final spin on particle additions, the final spin conducted within a process chamber with a tangential exhaust exii and a standard exhaust exit.
{0028) FIG. 15 is a chart ϋimixating the effect of a substrate final spin on particle additions, the final spin conducted within a process chamber with two tangential exhaust exits.
J0029J FlG, 16 is a chart illustrating ά\s effect of a substrate final spin on particle additions, the final, spin conducted within a process chamber wilh a -0.5 in standard exhaust without baffles and a reference air velocity of approximately 206 ipm.
{0030] FiG. 17 is a chart illustrating the effect of a substrate final spin on particle additions, the final spin conducted within a process chamber with a - 1.04 in standard exhaust without baffles aαd a reference air velocity of approximately 150 fprø.
{003J I FIG. 18 is a graph of the airflow velocity achieved by chamber exhaust v. particle addition for HF-last processes.
(0032] FlG. 19 is a chart illustrating the effect of IPA concentration using a single canister versus a dual canister ibr varying etch times on substrate particle addition.
Detailed Description
{0033] The preferred embodiments will he illustrated with reference to the drawings. Various other embodiments should become readily apparent from this description to those skilled in the art.
|0034| The present invention generally relates to HF-iast cleaning processes, which can he used in many Applications including but not limited to Pre Gate, pre EPiASiGe, pre-metai deposition aαd the like. Li such applications, it is important to minimize impurities and contaminants deposited on the surface of the substrate or svafer, which alter the electrical characteristics of a wafer and can lower a water's yield. High counts of particles and watermarks arc typically seen using the HF-iast process when implemented with single wafer spin applications. Accordingly, it has been discovered rhat the environment ix> which the wafers are processed has shown to be the key factor in preventing watermarks and particle addition in single wafer spin applications.
(0035) For example, it has been discovered that low oxygen content on the substrate surface as well as uniform etching is important to the prevention of cleaning detects (which can include but are not limited to particles added to the substrate surface during cleaning). There is a positive correlation between the number of defects on the substrate surface during the cleaning process and the number of defects <?n the substrate post deposition. Low or no metal comamiαauors on the substrate surface is also important in preventing cleaning and post deposition defects.
(0036) Experiments were conducted on a one-chamber skgle wafer module Io determine the key factors of the processing environment for controlling particulate contamination on HF- processed wafers. The test module was capable of processing 200 and 300 ram wafers with variable rotational speeds, chemical concentrations and spin or IPA vapor drying. fftO37] in processing wafers, variables such as rotational ipeed, chemical concentration, and IPA (isopropy. alcohol) vapor concentration, among others, can be adjusted as desired.
Wafers were conditioned in a standard cleaning step ("SCl step") first before running the processes; of the present invention, including the IlF-iast process. Such a standard cleaning step is not necessary, however, and the processes of the present invention can be implemented absent such a SCl step. Wafers were processed in the single wafer tool and the following parameters, among others, were investigated: HF concentration, etching lirøc at a given concentration of HF, Df (de-ionized) water rinse tisne, IPA vapor concentration, rotational speed during rinse and dry, airflow characteristics and gas content in the rinse water. Before running ihe experiments, P-type bare silicon wafers were first conditioned with SCl megasonic cleaning in a batch immersion bench. An Applied Materials Excite system for particle evaluation was used to inspect the wafers before and after testing. Typical pre-cϋimfe: tvf the testing wafers were less than 20 particles for 100 mm wafers (less than 50 for 300mm wafers) at greater than or equal ϊo lOQrnn with 3mm edge exclusion. De-ionized water ("DfW" or "Dl water") was degasified with a membrane, degasitler operated without N ? sweeping. Dissolved oxygen, solids and TOC levels in the OIW were generally kept below I ppb. The N 2 and COj content in the DlW was 3 ppm and 0 pprø, respectively, as measured by an Orbisphere 3620 gas analyzer. Daring the experiments evaluating dissolved gas effects, a membrane aerator was used to deliver (he gas of interest into the DfW before the single wafer module. During some experiments, chemical f IF was drawn iksm a reservoir and injected into the DlW supply stream and blended by an in-line static mixer. {0038| The concentration of HF and IPA vapor was discovered rø be key factors in achieving satisfactory particle performance. The degree of hydrophobicity, as measured by the combination of HF concentration and etch time was found to be a factor for producing fow particle counts on wafers. The experiments showed that the rotational speed during the rinsing step also has significant effects cm particle results. Finally, it was also discovered thai excessive dissolved gases in rinse water and improper chamber airflow negatively impact paniculate performance for lIF4ast processes. The experimentation of each of the aforementioned factors will be discussed in greater detail below. J0θ39] The system in which the process of the present invention is utilized, however, will now be described. Referring to FIG. 1, the system of the present invention comprises a process chamber 10, a rotary support 12, a DlW source t4, and aa IPA source 18. in sojne preferred embodiments, the system of the present invention can further comprise a jritr-ogeπ reservoir 16, which can be used to supply nitrogen gas to the chamber 10, Nitrogen gas can also be supplied into and mixed with IPA from an IPA source 18. The system of the present invention can also further comprise an assembly 20 positioned above the substrate 22. The
rotary support 12 is positioned within the process chamber lθ and is adapted to support the substrate 22 in a substantially horizontal orientation. Preferably, the rotary support 12 contacts and engages only the perimeter of the substrate in performing its support function The rotary support \2 is operabiy coupled to a motor 24 to facilitate rotation of the substrate wϊthin the horizontal plane of support The motor 24 is preferably a variable speed mourr thai can rotate the support 12 at any desired rotational speed. Optionally, the motor 24 is electrically and operabiy coupled to the controller (not shown). The controller controls the operation of the motor 24, ensuring that the desired rotational speed and desired duration of rotation arc achieved,
|0040| In a preferred embodiment, the assembly 20 is mounted within the process chamber 10 so as to be positioned closely to and above the surface of the substrate 22 positioned on the support 12. The assembly 20 can comprise a housing 26 that holds a DlW dispensing nozzle 30, a first IPA dispensing nozzle 32, and a second IPA dispensing nozzle 34. Optionally, Ike IPA dispensing nozzles 32 » 34 can be Ny [PA dispensing nozzles. The DfW dispensing nozzle 30 and the IPA dispensing nozzles 32, 34 are operabiy and fhiidly coupled to the DlW source 14 and the iPA source 18, respectively. In another embodiment, a rinse dispensing nozzle (not shown) can be fluidly and operabiy coupled to the DiW source. The rinse dispensing nozzle need not he connected to the assembly 20 and may be separate from the assembly 2ft.
{00413 The housing 26 can be mounted above the substrate in a variety of ways, none of which are limiting of the present invention. The assembly 20 can be translated/moved above the substrate 22 in a generally horizontal direction so that the DlW dispensing nozzle 30 and the IPA dispensing nozzles 32, 34 can be moved from a position above the center of the substrate 22 to a position beyond the edge of the substrate 21, as more fully disclosed in United States Patent Application No. 11/624,445 entitled "System and Methods for Drying a Rotating Substrate," foe teachings of which are hereby incorporated by reference. [0042} In applying the HF*!asi cleaning process according to one embodiment of the present invention, the substrate 22 is first supported in a substantially horizontal position and then rotated about a rotational center point, while housed within the processing chamber 10. h% one embodiment, the processing chamber 10 substantially coma ins nitrogen gas, meaning the processing chamber IO is a nitrogen-rich chamber. The effect of a nitrogen-rich chamber is to prevent oxygen or oxygen radicals from oxidizing silicone, which would have the effect of leaving watermarks, particles and the like on the substrate surface.
(0043 j The substrate is rotated at a constant speed selected to minimize particle addition ts.; the substrate surface. A film of an aqueous solution of diluted hydrofluoric acid can then be applied to the substrate 22 to etch the substrate 22 surface. The diluted hydrofluoric acid solution can be of varying concentrations. The application of a film of diluted hydrofluoric acid is then followed by applying a fihn of DlW to generally rinse the etching chemicals and/or contaminants from the wafer surface. The fihrs of DIW can be applied to the substrate surface for any desired time period that would minimize particle addition. Although the DIW need not necessarily be degassed in order to practice the present invention, in one embodiment- the DIW is degassed prior to applying the DIW to the substrate surface. The DIW can be degassed at any desired point on the DlW supply line 38 at the DIW source 14. The substrate surface is then dried using a drying fluid used in conjunction with the assembly 2«. j 0ft44j Thus, for single wafer spin applications utilizing cleaning and processing steps as described above, it has been discovered that environment factors through which the wafers are processed is important in preventing watermarks and particle addition. Such environmental factors, as described in greater detail below, can he implemented independently or in combination with one or mow other environmental factors to minimize particle addition.
(004S] Referring to HG. 2, experiments show the effect of etch, time at different HF concentrations (at approximately 2S degrees Celsius, with soft DIW rinse and utilizing dual IVA bubblers) on substrate particle addition greater than 100 am. With respect HF-processed wafers for a single wafer spin, the concentration of MF is important in achieving low particle counts on the substrate surface. Specifically, lower concentrations of HF used in the HF-last process in creating a substantially hydrophobic surface produced wafer surfaces with relatively low particle counts. To achieve such a result, it is preferred thai (he HF solution have a concentration of about.0.1 % to 5 % by weight of HF in water. More preferably, the HF solution should have a concentration of about 0.1 % to 0.5 % by weight of HF in water, it was observed that a 30 second etch time produced widely varying results, where an HF concentration of 200:1 ( " 0.4 % by weight of HF in water) produced a panicle addition of -15 particles, while rø IiF concentration of 100:1 (-0.5 % by weight of HF in water) produced a particle addition of 20 particles. Orrihe contrary, it was observed that for a 300 second etch time, the particle addition dispersion did not fluctuate as wildly as with shorter etch times. The particle addition at 300 seconds remained at a consistently low level. Specifically, at 300 seconds, an HF concentration of 100:1 (-0.5 % by weight of HF in water) produced a particle
addition of -1 particle, an HF concentration of 200: 1 (-0.4 % by weight of HF in water) produced a panicle addition of 6 particles, and an HF concentration of 500: ! (-0. i % by weight of HF in water) produced a particle addition of 4 particles.
[0046J ' Referring to FlG. 3, the residual oxide thickness on hare Si is shown as a function of etch time. Two aqueous solutions having different HF concentrations " were used. The more eotfceπlratcd aqueous HF solution, 100:1 (DIW: HF) is shown by the triangle shaped data points. The other aqueous HF solution, 500: 1 , is depicted by the solid diamond shaped data points. In the early stage of etching, the wafers are hydrophilic so that HF solution can fully cover the entire surface. When ihe waters become hydrophobic, HF converts into discrete liquid drops that roll on the surface of the wafer. The HF droplets -cannot provide full coverage of the wafer. As shown in FIG. 3, applying a more concentrated aqueous HF solution apparently transitions a wafer from the hydrophilic state to the hydrophobic state faster than the less concentrated aqueous HF solution. The remaining oxide, however, seems to increase as the exposure of the Si substrate to the atmosphere is prolonged. fQθ47) Referring again to FIG. 3, the effect of etching time in f(F on resultant particle addition is represented by the solid diamond data points. There has been discovered a range of etch time in which thϋ wafer is most vulnerable to particular!}' high particle addition. St has been reported that a minimum time is required io remove the native oxide and a monolayer of Si to render the wafer completely hydrophobic. It has been discovered that panicle count will depend on the degree of hydrophobicity, as measured by the combination of HF concentration and etch time. The discovery is as follows: For short etch tiroes shown by the 30 and SO second processes in FIG. 2, the wafer is still hydrophilic and particle addition is not significant due to the repulsive forces between particles and the native oxide surface. For longer etch times, shown at 120 seconds, the surface state is in the transition from hydrophilic to hydrophobic. Some parts of the wafer surface become hydrophobic before the others because the etching is not uniform. Higher panicle deposition depends on the location of the etch by-product particles on the wafer. If die environment and subsequent rinse and dry cycles are controlled properly, the panicle addition will decrease when the wafer is over- etched and turns completely hydrophobic. However, prolonged over-etching has been discovered to be undesirable because the chance of particle coniarainalion may increase again.
{0048] Referring also to FlG. 4, experiments similarly show the effect of «tch time on substrate particle addition and native oxide thickness. The experiments in FlG. 4 were conducted with a MF concentration of 500: ! HF at 25 degrees Celsius, with an SOU rpin DIW
rinse followed by spin dry. The conversion time at 25 degrees Celsius to make the surface substantially hydrophobic is as follows: for HF concentration of 100: 1 the surface Conversion time is greaser than 45 seconds, for HF concentration of 200: 1 the surface conversion time is greater than 90 seconds; for HF concentration of 500: 1 the surface conversion time is greater than 200 seconds.
(0049) Used in connection with HF concentrations as described herein, it has been observed that a minimum time is required to remove the native oxide and a monolayer of Si to fully reader the wafer completely hydrophobic, which assists in the prevention of particle addition. Thus, a longer etch time correlates to lower the particle addition, the long etch tirae sufficient to cause a substantially hydrophobic substrate surface. In other words, the lower particle counts depend on the degree of "hydrophobictty" of the substrate surface. It believed that such a mechanism works as follows. Wafers are typically hydrophilic prior to the cleaning or processing methods of the present invention. Thus, the wafer surface is negatively charged, which will repwlse similarly charged objects, particles or the like. ψor short etch times, low pjtrticle addition is observed on a substrate surface due to the repulsive forces (negative) between negatively-charged particles and wafers surface.
J9050J For longer etch tunes, generally greater than 60 second*, the surface state transitions from hydrophilic to hydrophobic. Thus, a hydrophobic substrate surface is one that is positively charged, As the surface transitions from hydrophilic to hydrophobic, some parts of the wafer surface become hydrophobic while other parts of the wafer surface remain hydrophilic (due to etch non-uniformity). This transition thereby increases particle counts on the substrate surface because instead of the negatively charged particles being repulsed by a uniformly and negatively charged surface, those negatively charged particles deposit on the positively charged portions of the substrate surface. Likewise, instead of the positively charged particles being repulsed by a uniformly and positively charged surface, tlio.se positively charged particle deposit on the negatively charged portions of the substrate surface. Since etch by-products are a mix of negatively charged particles (e.g., SiOj) and positively charged particles (e.g., Si), the result of such a transition is that the particle count increases on the substrate surface in the interim. As can be observed in FlG. 4, high panicle deposition typically takes place depending where these particles arc on die wafer, [0051} Once the wafers are over-etched and turn completely hydrophobic, the wafers become positively charged and repel any positively charged particles during the rinse cycle. If the environment is kept so that no aerosols deposit on the wafers during the drying cycle, HF- typicaiJy yields very low particle addition. This can be accomplished in a variety of ways,
one of which is io maintain a substantially NT rich chamber. Referring hack to FlG. 2, only 6 particles v.r less were added at any HF concentration for a long etch time (about 300 seconds). When the environment is not substantially isolated fkwn outside aerosols, however, it has been generally observed that the longer the substrate is processed, the more particles are deposited on the substrate. j0052| Thus, to effectuate a substantially hydrophobic surface, the HF solution has a concentration of about 0.1 % to 5 % by weight of HF in water, preferably a concentration of about 0, 1 % to 0.5 % by weight of HF in water. The HF solution is supplied to the surface for a period greater Shan about 60 seconds, and preferably within the range of about 200 AQQ seconds. To effectuate a substantially hydrophilic surface, the HF solution likewise has a concentration of about 0.1 % to 5 % by weight of HF in water, preferably a concentration of about 0, J % to 0.5 % by weight of HF in water. The HV solution is supplied to the surface for a short period of time, roughly between about 1-45 seconds, preferably about 5-20 seconds.
|(M)53j Referring to FIG. S, experiments show the effect of different dissolved gases in rinse water on substrate particle addition greater than 100 ran. Such experiments were conducted with application of HF (at a concentration of 100:1) for 1 minute at 25 degrees Celsius, followed by a 1>1 water rinse a«d IPA dry. Gases including N:. CDA, O 2 and CO> were injected to the rinse water to at or over the saturation limit. (Dissolved gasses can also include but are not limited to carbon monoxide, nitric oxide. hydrogen, methane, and the like.) it was previously demonstrated that dissolved N ∑ gas at about 20 ppb in water and having an aqueous solution thereof with a low pH (~pH2-3) eouJd suppress the formation of watermarks (typically known for ihis process). Many semiconductor fabricators use Nj- purgcd and -blanketed water to k(x>p OJ content low to suppress the formation of watermarks. Contrary to industry practice, experiments show that excessive dissolved gas, irrespective of type, in the rinse DIW generates particles on the hydrophobic waters. In contrast, degasified rinse water produces the lowest level of particle addition. A DIVV pressure of 20 psrg on one side of the aerator's membrane and 2 7 psig of gas input on the other side of the membrane is sufficient to provide Ae dissolved gas above its saturation Iiπrit in the rinse water. Through experimentation, when the DIW was dispensed into a glass beaker, micro-hubbles from the pressure drop were visually observed. Once solids collected at the gas-liquid interface contact the hydrophobic wafer surface, they yield high particle counts. 10054} Specifically, experiments have shown that using DIW aerated with N 2 gas has the effect of adding 205, 137. 184 and \69 particles to the substrate surface. Likewise. DlW
aerated with CO ? gas has the effect of adding 3604, ISO, 160 and Sl particles to the substrate surface. Generally, the aeration of gas in DIW correlates to particle addition in excess of at least 550 on average.
{θ055J It has been found through experimentation, however, that removing dissolved gasse* in the DlW correlates to the lowest particle count on the substrate surface. Degasiδed DIW provided {he lowest particles added, wherein the particles added numbered 1, 7, 5 and -4 particles. The reason for the discrepancy in particle addition with respect to degassed DIW compared to DIW injected or aerated with gas, is that at these levels of dissolved gasses. solids υoiiϋci at the air-liquid interface. Once in contact with ihe wafers' surface they yield high particles.
[0056) Thus, while some gases such as nitrogen are helpful to prevent the formation of watermarks, in utilizing the process of one embodiment of the present invention, the preferred approach is to provide DlW or rinse water that is gas-free and solids-free. In such an embodiment, prior to rinsing ihe substrate with DlW. substantially all of {he gas eutrøuied m the DIW is removed from the DlW. Preferably, and as described previously, the DIW should contain less than 1 ppb of dissolved oxygen and less than 10 ppb of total dissolved gasses. Prior to rinsing the substrate with DlW. the DlW can be filtered through a filtration system, including hut not limited to a point-of-use (POU) filtration system, Preferably, the POLf filtration has a pore rating of about 0.01 Io about 0.03 μin.
|0057} Referring now to FIG. 6, the effect of the rotational speed during the DiW rinse cycle ((JO seconds) on the particle performance of hydrophobic wafers for two different drying methods is illustrated. λs can be seen ftom the data, the spin drying method results in higher particle addition ihe IPA drying method. The effect of spin drying is depicted by ihe solid diamond shaped data points, and ϊPA drying is shown by ihe s<juare data points. With its low surface tension, IPA displaces ihe DIW from the wafer surface and thereby captures more water droplets than does spin drying. When the water droplets are left to evaporate on the wafer surface, noti-volatile silicic acid (IioSiOs), which results from the reaction between silicon and dissolved Oi, precipitates to form particles. The particle mapping of FlG. 6 also shows that the particle addition increases with increased rotational speed during the rinse step, forming star-bursting like streak patterns, ϊt was discovered that at high rotational speeds, highly insulating DIW sheers across the Si surface, creating high levels of static- charge that increases Ae particle deposition, ϊn addition, high speed spinning decreases the water boundary by creating smaller droplets and thus enhances O^ absorption by diffusion, thereby leading to higher silicate concentrations in individual droplets. Small droplets easily
evaporate prematurely, thus leaving particles on die wafer. Rinsing with a conductive solution such as 100:1 DIW: HF, however, with the same high rotational speed has led to she disappearance of the streak patterns and lesser particle adders. The difference in particle results between using a pure DIW rinse and 100: 1 DlW: HF rinse is shown in FIG..7. [θO5$j Is has also been observed that a lower rotational speed during the cleaning and processing steps of the present invention correlates to lower particle counts on the wafer surface. Experiments were conducted with application of HF (at a concentration of 100: 1) for 1 minute at 25 degrees Celsius, followed by a Dϊ water rinse and IFA dry. As seen in FIO. 8, a rotational speed of 100 rpm (RPM) produced particle additions of 12, 8, 4 and -1 particles, while a rotational speed of 300 rpm (3X RPM) particle additions of i, 7, 5 and -4 particles. At rotational speeds generally higher than 600 RPM, however, a trend of higher addition of particles is observed with increasing rotational speed. λ rotational speed of 1000 rpm (K)X RPM) produced particle additions of 1023, 190, 57 and 147 particles. At higher rotational speeds, it is believed that the water boundary layer is decreased and the potential for gas absorption by diffusion is higher. (Referring back to FlO. 5, experimentation shows that the higher the gas content in the DlW, the higher the particle counts on the substrate surface.) Also, at higher ψesόs the concentration of solids per unit volume is bjgher (due to ihe thinning of liquid layer at higher speeds) which results in higher residual particles on the wafer surface at the end of the cycle. Thus, ir» a preferred embodiment, the rotational speed during the rinsing and drying step is kept constant at a speed in the range below 400 rotations per minute (rpm), preferably between about 1-200 rpm.
[θ059J Referring to FiG. 9, experiments show the effect of DIW rinse lime at 60 seconds and 5 seconds on particle addition, which were conducted on hydrophobic substrates prior to a spin dry. It has been discovered that a shorter DlW rinse time correlated to a lower particle count on the substrate surface. λs seen ia FϊG. 9. a rinse time (at a constant rotational speed of 800 rpm} of 5 seconds produced particle additions of 1 125 and 982 partieies. On lhe contrary, a longer rinse time (at a constant rotational speed uf 800 rpm) of 60 seconds produced particle additions of 2990 and 3320 particles. Thus, at lower rinse times, a lower total volume of DIW is applied to the substrate surface, which translates into a lower total number of panicles that are added to the substrate surface. The rinse time, however, needs to be sufficient for effective rinsing of the Hf solution from the substrate surface. In one embodiment, the DIW rinse time is between about 1-60 seconds. In Ά preferred embodiment, the DlW rinse time is between about 5-20 seconds.
{0060] After the DIW rinse step, a spin step may optionally be performed at a high, rpm in one embodiment It i*s understood, however, that the spin slep may be conducted at a low rpm less than 500 rpm. which in some embodiments of the present invention, is preferred. As shown in FlG. 10, experiments show the effects of a high rpm final spirt step in the process chamber 10 wilh an axial or dowa flow exit standard exhaust 50, which will be discussed is. greater detail below and in FIGS. 12a and 12b. Generally, the rpm spin step Is performed after the application of an aqueous solution of HF followed by a DlW rinse step. The high rprn can be any rpin greater than 500 rpm; however, it is preferred thai ihat the high rpm be greater than 1000 rpm. Such a high RPM final spin produced panicle additions on a wafer surface ranging from approximately 20 to 150 panicles additions per run. As shown in FlG. ] 1, additional experiments show the effects of particle additions on a wafer surface without a high RPM final spin, likewise conducted within the process chamber 16 with a standard axial or down flow exit exhaust 50. From FlG. 1 1, the particle additions ranged from approximately -10 to 40, where a high RPM final spin was applied to the wafer (which is less than the approximate range as shown in FIG. 10). Thus, in one embodiment, it is preferred to not apply a high RPM final spin to the wafer.
\QM\] IH ao alternative embodiment, a Qaal spin step after the DϊW rinse step is desirable. IB such an embodiment, the (spinning) wafer sαrface becomes exposed to a gas supplied to and/or present within the process chamber 10. It has been discovered through experimentation that the wafer surface is very sensitive to air movement and gas pressure buildup in the process chamber 10. As will be discussed below, the greater the buildup of pressure the greater the addition of particles on the wafer surface. The buildup of pressure from gas supplied to the process chamber 10 can be caused by high system impedance that does not allow gas to exit the process chamber 10 quickly enough, at the right time, or at the right direction. Referring to FKJS. 12a and 12b, the configuration of the (down flow exit) standard exhaust 50, which is operably connected to the process chamber 10 at opening 52, is shown. The standard exhaust sel-up is a -0.35 inches. An area within the standard exhaust 50 u composed of a plurality of baffles that direct or bend the flow of gas to standard exit 54. Such air flow is characterized as axial or down flow exit.
[0062 j As shown in FIGS. 13a and 13b, one embodiment of the process chamber 10 having an improved exhaust system is shown. As shown m. FlG, 13b, the air Hows through one tangential gas exit 56 in a tangential direction relative to the spinning wafer. The process chamber 10 having art improved exhaust system can also be comprised of two tangential gas exits 56, 58s as shown in FlG 13c. Such an improved exhaust system {whether haying a
single gas exit or multiple gas exits) provides for substantially tangential ami/or horizontal gas exit(s) with no bends to the air flow. This aids in preventing gas pressure buildup in the process chamber 10. which eofttributCvS to a lower system impedance as there are no bends to inhibit flow of the ga& through the gas exit.
[0063] As shown in FlG. 14, experiments show particle additions oκ a wafer surface in conjunction with a final spin after a modification of the chamber 10 to include one tangential airflow exit and one axial (down flow) exit, where the exhaust set up was at -0.3 inches. Referring back to FIG. 13b, the modification is such that the process chamber 10 k opcrably connected to a standard down flow exit exhaust 50 and a tangential gas exit 56. Experiments show that the air flow inside of the process chamber 10 was three-dimensional and the reference air velocity within the process chamber lft was approximately 208 FPM (feet per minute). The approximate particle addition was 44 particles (greater than 0.1 um) per ran, and ranged from about 12 particle additions to about 6? particle additions. {0064} As shown in FlG. 15, experiments show particle additions on a wafer surface ia conjunction with a final spin after a second chamber modification. In the second chamber modification, referring back to FIG. 13c, two gas exits 56, 58 were operably connected to the process chamber 10. Both the tangential gas exits 56, 58 allowed for gas to leave the process chamber 10. The reference air velocity within (he process chamber 10 was approximately 208 FPM (feet per minute). The approximate particle addition was 10 particles (greater than 0.1 urn) per ran, and ranged from about -15 to about 48 particle additions. When comparing the experiments shown in FIO. 14 and in FIG. 15, the approximate particle addition wilh two tangential gas exits 56, 58 is lower (about iO particle additions) than with a tangential gas exit 56 and standard down flow exit exhaust 50 (about 44 particles additions). Referring back to FlG. 12b, bailies within the standard down flow exhaust 50 impede the flow of gas to tire standard exit 54, thus causing high system impedance and pressure. Thus, use of two tangential gas exits 56, 58 relatively increases the gas movement with the chamber and lowers the system impedance, as gas flow management is critical in achieving lower particle addition.
[0065] Other experiments were conducted modifying the standard exhaust 50 to remove the baffles. As can b<; seen in FIO. 16, a final spin was conducted within a process chamber 10 with a -0.5 in standard exhaust without baflles. and with a reference air velocity of 206 PPM. It was observed that additions ranged from approximately 4# to SS particles, with an average of 75 particle additions per run. If was also observed that a sufficient air volume must be drawn out in order Io avoid circulation inside the process chamber 10. As seen in FIG. 17.
another experiment was conducted, which included a final spin within a process chamber 10 with a -1.04 in standard exhaust without baffles, and with a reference air velocity of about ] 50 FPM. it was observed that additions ranged from approximately -4 to ! 5 particles, with an average of 5 particle additions per run. If was observed lhere was insufficient air flow drawn out of process chamber 10.
(00661 Referring next to FIG. 18, the effects of chamber exhaust, or airflow velocity, on particle performance is shown. Experiments were conducted on 300 mm wafers utilizing the HF-last process, wherein there were 6 to 8 wafers tested for each data point Ai low airflow velocities, there is a potential for cross contamination due to insufficient air draw. At high airflow velocities, vortices will be created due to decreased pressure inside the process chamber, it was discovered that optimum airflow velocities Si 1 C required to yield minimum particle additions. The preferred range for the chamber used and the conditions of the experiment was discovered to be between 250 and 290 cubic feet per minute. In a single wafer too! running wet processes, wafer spinning easily generate* liquid aerosols. Careful attention should be given to ihe chamber design and airflow adjustment that provide optimal and directional airflow field to prevent turbulence near the wafer surface. Otherwise, the aerosols would deposit on sensitive wafer surface and become Ike source of contamination. }0067| Referring to FlG, 19, experiments show ihe effect of IPA concentration tising a single canister (bubbler) versus a dual canister (dual bubblers) in series for varying etch times on substrate particle addition greater than 100 nm. It has been found that the concentration of ϊPA vapor is important in achieving iow particle counts on the wafer surface. The higher the concentration of IP A vapor used in the HF-iast process produced wafer surfaces with relatively low particle counts. Characterized by creating low surface tension, IPA displaces the DiW from the wafer surface and thereby leaves fewer particles behind. If the water ts left to evaporate (or takes longer to dry)- it will leave etch by-products "silicates" behind for higher particle counts.
[0θ68J It has been found that high IPA vapor enhances the drying of hydrophilie wafers. This effect can be seen on planar hydrophobic wafers, where ihe higher the IPA concentration (through use. of a dual bubbler canister as compared to a single bubbler canister) the lower the particle addition. For single IPA bubbler canisters, etch times of 180 seconds, 240 seconds and 3(K) seconds, produced parijck additions of 29, 13 and 1438 particles, respectively. On the contrary, for double ϊPA bubbler canisters, etch times of 180 seconds, 240 seconds and 300 seconds, produced particle additions of 2, 2 and 4 particles, respectively.
(0069) The double canister provides about a 20% higher concentration of IPA than the single Canister, It has been reported that high IPA vapor enhances the drying performance of hydrciphϋic wafers. Drying with IPA vapor generated through double canisters connected in series seems k> yield lower particle addition. Hydrophobic wafers are extremely sensitive to the environment around them, especially during wafer spinning. More IPA enhances drying by displacing the DIW from the wafer surface more efficiently, thereby leaving fewer particles behind. This effect is highly magnified when testing patterned wafers with high aspect ratio trenches. The IPA vapor is required for displacement oϊ water or liquids from ihc high aspect ratio trenches to prevent leaving residues behind.
{0θ70J While the drying fluid can be any number of existing drying fluids, Nj/iPA vapor is a preferred. Referring back to FuG. 1 , the formation of N 2 /IPA vapor achieved through the use of two bubbler canisters 42, 44 sequentially and opcrably aligned with one another is described. First, N 2 gas is introduced into a first bubbler canister 44 through an N 2 supply line 46. The Ts ; supply Ike 46 is positioned within the first bubbler canister 44 and submerged in IPA liquid. The open end of the N 2 supply line 46 is positioned close to the bottom of first bubbler canister 44. The Ni gas exits from the open end of the Nj supply line 46 where the Nj gas naturally forms bubbles in the IPA liquid. The Na bubbles rise through th$ IPA liquid, thereby forming NViPA vapor in the open space above the IPA liquid and within the first bubbler canister 44, The N2/IPA is then drawn into a second supply line 40. which is introduced into second bubbler canister 42.
[0071 j Similar to the process involving bubbler canister 44, the second supply line 40 is positioned within the. second bubbler canister 42 and submerged in IPA liquid. The open end of the second supply line 40 is positioned close to the bottom of the second bubbler canister 42, where the NVϊPA vapor exits from the open end of the second supply line 40. The N 2 ZIPA vapor forms bubbles in tbe IPA liquid > which then rise to the top to form a highly concentrated Nj/IPA vapor. Such Nj/IPA vapor has a higher concentration of IPA compared to if only one bubbler was used. The highly concentrated Nϊ.TPλ vapor is then drawn out of the second bubbler canister 42 through the main Nj/TPA supply line 48. (0072] Thus, the use of a multi-canister configuration, which in one embodiment incorporates first and second bubbler canisters 42, 44, provides a stable and high concentration of N^/IPA vapor. Jt has been discovered thai promoting a longer exposure time between the N?. gas and liquid IPA allows the IPA to saturate the Nj gas before exiting into the main drying fluid supply line. Providing two canisters 42, 44 in a sequential
configuration allows the N^/IPA io reach a stable IPA concentration. \t also allows the
N 2 ZiPA vapor io have a high IPA concentration.
|0β?3] Hydrophobic wafers are extremely sensitive to the environment around them especially when spinning. More ϊPA enhances drying of the substrate surface by displacing the DIW quicker and therefore leaving fewer particles behind. This effect is highly magnified when iesting high aspect ratio trenches, ϊt is believed that higher amounts of IPA vapor will be required to displace water or liquids from these deep trenches in order to leave no residues behind.
(0074) While a number of embodiments of the current invention have been described and illustrated in detail, various alternatives and modifications will become readily apparent to those skilled in the art without departing from the spirit and scope of the invention.
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