Patent Grant
7270597
The present invention relates to chemical mechanical polishing (CMP) techniques and related wafer cleaning and, more particularly, to improved CMP operations.
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical polishing (CMP) operations and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess metallization. After any such CMP operation, it is necessary that the planarized wafer be cleaned to remove particulates and contaminants.
A CMP system 14 typically includes system components for handling and polishing the surface of the wafer 12. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad itself is typically made of a polyurethane material. In operation, the belt pad is put in motion and then a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer is then sent to be cleaned in the wafer cleaning system 16.
It is important to clean a semiconductor chip after a semiconductor wafer 12 has undergone a CMP operation in a chemical mechanical polishing (CMP) system 14 because particles, particulates, and other residues remain on the surface of the semiconductor wafer 12 after the CMP operation. These residues may cause damage to the semiconductor wafer 12 in further post-CMP operations. The residues may, for example, scratch the surface of the wafer or cause inappropriate interactions between conductive features. Moreover, several identical semiconductor chip dies are produced from one semiconductor wafer 12. One unwanted residual particle on the surface of the wafer during post-CMP processing can scratch substantially all of the wafer surface, thereby ruining the dies that could have been produced from that semiconductor wafer 12. Such a mishaps in the cleaning operation may be very costly.
Better cleaning of the wafer can be achieved in the wafer cleaning system 16 by improving the processes used in the CMP system 14 before the wafer even gets to the wafer cleaning system 16. The CMP system 14 can be improved for the next wafer by conditioning the surface of the belt pad. Pad conditioning is generally performed to remove excess slurry and residue build-up from the clogged belt pad. As more wafers are polished, the belt pad will collect more residue build-up, which can make efficient CMP operations difficult. One well-known method of conditioning the belt pad is to rub the belt pad with a conditioning disk. The conditioning disk typically has a nickel-plated diamond grid or a nylon brush over its surface. The diamond grid is typically used to condition belt pads having a hard surface. In contrast, the nylon brush is typically used to condition belt pads having a softer surface. The conditioning of the belt pad may be done in-situ, where the belt pad is conditioned while the belt pad is polishing the wafer, or ex-situ, where the belt pad is conditioned when the belt pad is not polishing a wafer.
While conditioning disks remove slurry and residue, they inevitably remove some of the belt pad surface. Of course, removal of the belt pad surface exposes a fresh layer of the belt pad, thus increasing the polishing rate during CMP. Unfortunately, removal of the belt pad surface using conditioning methods causes the belt pad to wear out quickly, thereby driving up the cost of running the CMP system 14. On the other hand, if the belt pad is under-conditioned, the life of the belt pad may increase because less of the belt pad is removed. However, residual clogging materials will be left on the belt pad surface. Thus, the belt pad will generally not polish at an efficient rate and the CMP itself will not be of a very high quality.
For the aforementioned reasons, techniques for conditioning the belt pad are an important part of the semiconductor chip fabrication process. There is therefore a need for improved methods of conditioning the belt pad.
Broadly speaking, the present invention fills these needs by providing an improved method for conditioning a chemical mechanical polishing (CMP) pad and a system for implementing the same. The method involves a chemically treating and mechanically scraping the CMP pad. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a method for cleaning a chemical mechanical polishing (CMP) pad is provided. The CMP pad has a residue on the surface of the CMP pad. Chemicals are applied onto the surface of the CMP pad and the pad surface is rinsed so as to substantially remove by-product produced by the chemicals. A mechanical conditioning operation is performed on the surface of the pad. The wafer surface includes copper and oxide during the CMP operation.
In another embodiment, another method for cleaning a chemical mechanical polishing (CMP) pad is provided. The CMP pad has a residue on a surface of the CMP pad as a result of performing a CMP operation on the surface of a substrate. The surface of the substrate includes substantially all copper at a beginning of the CMP operation and a combination of oxide and copper near a completion of the CMP operation. Chemicals are applied onto the surface of the CMP pad and the pad surface is rinsed so as to substantially remove the applied chemicals and the residue. When the substrate surface includes copper, the chemicals are selected from one or a combination of: NH4Cl+CuCl2+HCl, (NH4)2S2O8+H2SO4, CuCl2+NH4Cl+NH4OH, C6H8O7, NH4OH, (NH4)2HC6H5O7, HCl, HF, TMAH, SC1, chelating agents, and surfactants.
In yet another embodiment, another method for cleaning a chemical mechanical polishing (CMP) pad is provided. The CMP pad has already been used for performing a CMP operation on a wafer surface and has a residue on a surface of the CMP pad. Chemicals are applied onto the surface of the CMP pad. When the wafer surface is oxide, the chemicals are selected from one or a combination of: NH4OH+ hydrogen peroxide (H2O2)+deionized water (DIW), NH4OH, C6H8O7, (NH4)2HC6H5O7, HCl, HF, TMAH, chelating agents, and surfactants. The chemicals are allowed to react with the residue to produce a by-product. The pad surface is rinsed to substantially remove the by-product and a mechanical conditioning operation is performed on the surface of the pad.
In another embodiment, a chemical mechanical polishing (CMP) system is provided. The CMP system has CMP pad surface that has a residue. The CMP system includes a holding surface, a polishing head, and a chemical dispenser. The holding surface receives the CMP pad. The polishing head holds and applies a wafer to the CMP pad surface. The chemical dispenser uniformly applies a first pad cleaning chemical or a second pad cleaning chemical across the CMP pad surface. The first and second pad cleaning chemicals are configured to react with the residue so as to produce a by-product, substantially removing the residue from the CMP pad surface. When the wafer primarily includes copper, the chemical dispenser will apply the first pad cleaning chemicals. When the wafer primarily includes oxide, the chemical dispenser will apply the second pad cleaning chemicals.
Advantageously, by conditioning a CMP pad in accordance with any one of the embodiments of the present invention, the CMP pad will be able to provide more efficient and cleaner polishing operations over wafer surfaces (e.g., metal and oxide surfaces). Furthermore, because the wafers placed through a CMP operation using a well conditioned pad are cleaner, subsequent wafer cleaning operations will also yield improved cleaning parameters. As a result of the improved CMP and cleaning operations, the wafers and resulting integrated circuit devices may also be of higher quality and, therefore, more reliable. Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.
The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements.
An invention for methods and systems for conditioning CMP pads is disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.
Wafer cassettes 102 preferably containing at least one semiconductor wafer 101 may be provided to the CMP and cleaning unit 100. A dry robot 104 may then transfer the wafer 101 to a pre-aligner 106 where the wafer 101 is properly aligned for subsequent handling. The wet robot 108 may then transfer the wafer 101 from the pre-aligner 106 to a load/unload to a dial plate 116. A polishing head (not shown) may be used to hold the wafer 101 when the wafer is placed over the polishing pads of the CMP systems. The dial plate 116 is used to rotate the wafer 101 to subsequent CMP and cleaning locations. For instance, the dial plate 116 may be used to rotate the wafer to a first CMP system 114a, where the wafer 101 is loaded onto the polishing head. The polishing head secures the wafer 101 in place as the wafer 101 is lowered onto a linear belt polishing pad that is part of the first CMP system 114a.
After the wafer undergoes a CMP operation in the first CMP system 114a, the wafer 101 may be transferred by the dial plate 116 to an advanced polishing head 118 in a second CMP system 114b, where the wafer undergoes additional CMP operations. The wafer 101 may then be transferred to the advanced rotary module 120, where the wafer 101 may undergo pre-cleaning operations. In this example, the advanced rotary module 120 implements a soft orbital pad surface. The wafer 101 may then be loaded into a load station 124 in a wafer cleaning system 122. The wafer cleaning system 122 is generally used to remove unwanted slurry residue left over from CMP operations in the CMP systems 114. The unwanted residue may be brushed away by operations in the brush boxes 126.
Each of the brush boxes 126 includes a set of PVA brushes that are very soft and porous. Therefore, the brushes are capable of scrubbing the wafer clean without damaging the delicate surface. Because the brushes are porous, they are also able to function as a conduit for fluids that are to be applied to the wafer surface during cleaning. These cleaning operations typically implement chemicals as well as deionized (DI) water. By way of example, SC1, water, citric acid (C6H8O7), ammonium hydroxide (NH4OH), ammonium citrate ((NH4)2HC6H5O7), hydrochloric acid (HCl), hydrofluoric acid (HF), or Tetramethylammonium Hydroxide (TMAH), alone or in combination, can be applied to the wafer surface during cleaning. According to one embodiment, the SC1 solution implemented is approximately one NH4OH, four parts peroxide (H2O2), and twenty part H2O, by volume. Of course, the concentration of the components in the SC1 solution may be varied depending upon the specific application. Furthermore, the SC1 solution is applied for a predetermined amount of time. The amount of time the SC1 scrubbing process is applied can be adjusted. For instance, the SC1 solution may be dispensed through the brush for a variable length of time. In another embodiment, chelating agents, surfactants, or chemical mixtures can be implemented to clean wafer surface.
For more information on wafer cleaning systems and techniques, reference may be made to commonly owned U.S. Pat. No. 5,858,109 issued on Jan. 12, 1999, entitled “Method And Apparatus For Cleaning Of Semiconductor Substrates Using Standard Clean 1 (SC1),” and U.S. Pat. No. 5,806,106 issued on Sep. 15, 1999, entitled “Method and Apparatus for Chemical Delivery Through the Brush.” Both United States Patents are hereby incorporated by reference.
A spin station 128 may be used to finalize the cleaning operations of the wafer 101. The wafer 101 may then be transferred to the wet queue 110, where the wafer 101 awaits to be transferred to post-CMP processing.
In some cases, the CMP operation is used to planarize materials such as oxide, and in other cases, it may be used to remove layers of metallization. The rate of planarization may be changed by adjusting the polishing pressure 152. The polishing rate is generally proportional to the amount of polishing pressure 152 applied to the linear belt polishing pad 156 against the polishing pad stabilizer 158. After the desired amount of material is removed from the surface of the wafer 101, the polishing head 150 may be used to raise the wafer 101 off of the linear belt polishing pad 156. The wafer is then ready to proceed to the advanced polishing head 118 or to the wafer cleaning system 122.
Better cleaning of the wafer can be achieved in the wafer cleaning system 122 by improving the processes used in the CMP system 114 before the wafer even gets to the wafer cleaning system 122. The CMP system 114 can be improved for the next wafer by conditioning the surface of the linear belt polishing pad 156. Conditioning of the pad may be performed by removing excess slurry and residue build-up from the clogged belt pad. As more wafers are planarized, the belt pad will collect more residue build-up, which can make efficient CMP operations difficult. One method of conditioning the belt pad is to use a polishing pad conditioning system 166. A conditioning head 170 is preferably used to hold (and in some embodiments rotate) a conditioning disk 172 as a conditioning track 168 holds the conditioning head 170. The conditioning track 168 moves the conditioning head 170 back and forth as the conditioning disk 172 scrapes the linear belt polishing pad 156, preferably with a nickel-plated conditioning disk.
The conditioning disk 172 preferably has a nickel-plated diamond grid or a nylon brush over its surface. The diamond grid is preferably used to condition belt pads having a hard surface. The nylon brush is preferably used to condition belt pads having a softer surface. The conditioning of the belt pad may be done in-situ, where the belt pad is conditioned while the belt pad is polishing the wafer, or ex-situ, where the belt pad is conditioned when the belt pad is not polishing a wafer. Unfortunately, although scraping the belt removes slurry and residues, it inevitably wears away the belt pad itself such that about 200 angstroms of belt pad material is removed from the belt during each conditioning operation.
The chemicals 180 are preferably supplied from a chemical source 176, which may be located inside the CMP and cleaning unit 100 or may be located externally. A conduit 178 leading from the chemical source 176 to the chemical dispenser 174 is preferably used to provide the pathway for the chemicals 180 to reach the chemical dispenser 174. In one embodiment, depending on the desired interaction of the chemicals with the materials left on the surface 156a after the CMP operation, the chemicals assist in achieving certain advantageous results. For example, the chemicals can react with and substantially dissolve the residue of the materials removed from wafer 101 and the slurry used in the CMP operation. As mentioned above, the CMP operation polishes material from the wafer 101, thereby leaving wafer material residue on the surface 156a of the linear belt polishing pad 156. After the chemicals react with the residue, substantially all of the resulting film on the surface 156a may be rinsed away with a rinsing liquid, which is preferably DI water. The result is a linear belt polishing pad 156 that has been chemically treated before being conditioned and made ready for another CMP operation on a next wafer.
The additional operation of chemically treating the linear belt polishing pad 156 may provide several advantages over traditional cleaning methods. An additional operation of chemical treatment substantially reduces the amount of pressure and the amount of time needed for applying the wafer to the polishing pad during a subsequent CMP operation because the polishing pad is cleaner and thereby more efficient. With a cleaner polishing pad, the necessary pressure is typically between about 3 and 4 pounds per square inch (psi), and the necessary time for polishing a wafer is typically about 60 seconds. For comparison purposes, if no chemical treating is performed on the pad surface, the time for polishing a subsequently applied wafer is likely to be substantially more at about 2 minutes.
Further, an additional operation of chemical treatment saves a substantial amount of the pad material from being unnecessarily scraped away. As mentioned above, typical conditioning techniques primarily rely on the scraping away of about 200 angstroms of polishing pad material each time conditioning is performed. In a traditional conditioning technique, for example, where chemical treatment is not performed, a hard polishing pad may be usable for about 300 to 500 CMP operations. However, by implementing chemical treatments, as described above, a typical hard polishing pad may be usable for up to about 800-1000 CMP operations. This increase in pad lifetime is primarily due to the fact that the subsequent scraping operation does not have to be so intensive. An extended pad life leads to less downtime for maintenance and repair. Less downtime in turn leads to a significantly lower cost of ownership.
Still further, the chemical treatment of the present invention may safeguard the fabrication system from some of the consequences of over or under-conditioning. If a polishing pad is over-conditioned, the pad will likely not perform as expected, and the material on the surface of the conditioning disk may degrade prematurely. The material over the surface of the conditioning disk may include a diamond grid, which is likely to be very costly to replace. Additionally, through its wearing-out stages, fragments of the diamond grid are likely to shed onto the pad surface and the surface of the wafer. Such unwanted shedding will likely require the entire wafer to be discarded.
On the other hand, if a polishing pad is under-conditioned, unwanted residual material may be left on the polishing pad. It is well-known in the art that it is important that a wafer be adequately cleaned after a CMP operation because of these slurry residues, which may cause damage to the wafer in post-CMP operations or in the operation of a device. The residues may, for example, cause scratching of the wafer surface or cause inappropriate interactions between conductive features. Moreover, a multitude of identical semiconductor chip dies are produced from one semiconductor wafer. One unwanted residual particle on the surface of the wafer during post-CMP processing can scratch substantially all of the wafer surface, thereby ruining the dies that could have been produced from that semiconductor wafer. Such a mishaps in the cleaning operation may be very costly. Accordingly, the chemical treatment operation provides a polishing pad that is in better condition for CMP operations, thereby providing stable removal rate and also reducing the risk of having unwanted particulates and residues left on the wafer in subsequent fabrication processes. Fewer unwanted residues and particulates lead to fewer defective wafers and, thus, an increase in yield.
Chemicals to be applied to the surface 156a depend on the type of slurry used during the CMP operation and the type of material polished away from the wafer 101 during the CMP operation. The following discussion discloses various types of fabrication processes and respective preferred chemicals for conditioning the polishing pad.
The method then moves to operation 412 where an even coat of chemicals is distributed onto the pad surface. In general, it is preferred that the linear belt polishing pad 156 be moving. In one example, the linear belt polishing pad 156 can be traveling at a rate of about 100 feet per minute. After the chemicals are distributed, the chemicals are allowed to react with the residue film 214 on the pad surface to produce a water soluble by-product. The chemicals may be in the form of a solution that contains DI water and hydrochloric acid (HCl). The concentration of HCl in the solution is preferably between about 0.05% and about 1.0% by weight, more preferably between about 0.2% and about 0.8% by weight, and most preferably about 0.5% by weight. The remainder of the solution is preferably DI water. The waiting time for allowing this solution to react with the residue is preferably between about 30 seconds and about 3 minutes, more preferably between about 60 seconds and about 2 minutes, and most preferably about 90 seconds. The chemical reaction that occurs here is likely to be CuOx+HCl→CUCl2+H2O, where the by-product CuCl2+H2O is a water soluble material.
Another solution of chemicals contains DI water, NH4Cl, CuCl2, and HCl. The concentration of NH4Cl is preferably between about 0.5 and about 2.4 moles per liter. The concentration of CuCl2 is preferably between about 0.5 and about 2.5 moles per liter. The concentration of HCl is preferably between about 0.02 and about 0.06 moles per liter. The remainder of the solution is preferably DI water.
Still another solution of chemicals contains DI water, ammonium persulfate ((NH4)2S2O8), and sulfuric acid (H2SO4). The concentration of (NH4)2S2O8 is preferably between about 0.5 and about 1.0 molar. The concentration of H2SO4 is preferably between about 0.25 and about 0.5 molar. The remainder of the solution is preferably DI water. The waiting time for allowing this solution to react with the residue is preferably between about 30 and 180 seconds, and most preferably about 60 seconds.
Yet another solution of chemicals contains DI water, copper chloride (CuCl2), ammonium chloride (NH4Cl), and ammonium hydroxide (NH4OH). The concentration of CuCl2 is preferably between about 2 and about 5 grams per liter. The concentration of NH4Cl is preferably between about 5 and about 10 grams per liter. The concentration of NH4OH, is preferably between about 0.2% and about 0.5% by weight. The remainder of the solution is preferably DI water. The waiting time for allowing this solution to react with the residue is preferably between about 30 and about 180 seconds, and most preferably about 60 seconds.
Of course, one of ordinary skill in the art must appreciate that additional chemicals in the form of solutions may also be applied. For instance, the solution of chemicals can include one or a combination of chemicals such as citric acid, ammonium hydroxide, ammonium citrate, hydrochloric acid, and hydrofluoric acid, chelating agents, SC1, and surfactants.
Next, in operation 414 the pad surface is rinsed with DI water to substantially remove the soluble by-product. A mechanical conditioning operation 416 is then performed on the pad. The conditioning disk 172 may be applied to the surface of the polishing pad at a pressure preferably set between about 1 and about 2 pounds per square inch. At this point, where the pad has been conditioned and prepared to polish a next wafer, the operation moves to operation 418 where a wafer is polished. The polished wafer is subsequently moved to a post-CMP cleaning operation 420. The method now moves to a decision operation 422 where it is determined whether a next wafer is to undergo a CMP operation. If there is not a next wafer, the method is done. However, if there is a next wafer, the method goes back to and continues from operation 412. The foregoing cycle continues until there is no next wafer at decision operation 422.
The foregoing discussion disclosed techniques for removing unwanted materials from a polishing pad where a CMP operation has been performed on metallization material. The following discussion includes disclosure of techniques for cleaning and conditioning a polishing pad where a CMP operation has been performed on dielectric materials or materials that are substantially oxide-based.
The method then moves to operation 512 where an even coat of chemicals is distributed onto the pad surface. After the chemicals are distributed, the chemicals are allowed to react with the residue 310 on the pad surface to produce a soluble by-product and to modify the pad surface having embedded SiO2 particles. The chemicals may be in the form of a solution that contains DI water and ammonium hydroxide (NH4OH). The concentration of NH4OH in the solution is preferably between about 0.5% and about 2.5% by weight, more preferably between about 0.7% and about 1.5% by weight, and most preferably about 1.0% by weight. The remainder of the solution is preferably DI water. The waiting time for allowing this solution to react with the residue is preferably between about 45 seconds and about 3 minutes, more preferably between about 50 seconds and about 2 minutes, and most preferably about 60 seconds. This solution is preferably allowed to react at about an ambient room temperature of 21 degrees Celsius. By running the method at room temperature, there is advantageously no need for extra mechanical, electrical and control equipment to modify the temperature of the applied solution.
Another solution of chemicals contains DI water, ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2), and DI water. The concentration of NH4OH is preferably about 1% by weight. The volume ratio of NH4OH:H2O2:DI water is preferably about 1:4:20, and most preferably about 1:1:5. The waiting time for allowing this solution to react with the residue is preferably between about 30 and about 180 seconds, and most preferably about 60 seconds. This solution may also be applied to the polishing pad at a heated temperature that is preferably between about 40 degrees Celsius and about 80 degrees Celsius, and most preferably about 60 degrees Celsius.
It must be appreciated by one of ordinary skill in the art that additional chemicals in the form of solutions may also be applied. For instance, the solution of chemicals can include one or a combination of chemicals such as citric acid, ammonium hydroxide, ammonium citrate, hydrochloric acid, hydrofluoric acid, chelating agents, or surfactants.
Operation 512 is followed by operation 514 where the pad surface is rinsed with DI water to substantially remove particulates and the oxide by-product. In general, the residue will be substantially dissolved and substantially removed. Next, a mechanical conditioning operation 516 is performed on the pad. At this point, where the pad has been conditioned and prepared to polish a wafer, the operation moves to operation 518 where a wafer is polished. The polished wafer is subsequently moved to a post-CMP cleaning operation 520. Next, the method moves to a decision operation 522 where it is determined whether a next wafer is to undergo a CMP operation. If there is not a next wafer, the method is done. However, if there is a next wafer, the method goes back to and continues from operation 512. The foregoing cycle continues until there is no next wafer at decision operation 522.
It should be understood that although specific reference has been made to belt-type CMP machines, the conditioning methods of the present invention could be applied to other types of CMP machines, such as those that implement rotary mechanisms with round pads. Thus, by implementing these pad conditioning methods, the complete CMP and cleaning operations will generate a higher yield of quality planarized and cleaned metal and oxide surfaces.
While this invention has been described in terms of several preferred embodiments, it will be appreciated that those skilled in the art upon reading the preceding specifications and studying the drawings will realize various alterations, additions, permutations, and equivalents thereof. It is therefore intended that the present invention includes all such alterations, additions, permutations, and equivalents as fall within the true spirit and scope of the invention.
This Application is a divisional of the U.S. patent application Ser. No. 10/000,494, filed on Oct. 30, 2001, now U.S. Pat. No. 6,994,611 which in turn is a divisional of the U.S. Patent Application Ser. No. 09/322,198 filed May 28, 1999, now U.S. Pat. No. 6,352,595 issued on Mar. 5, 2002. The Patent Application and the Patent are incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5725417 | Robinson | Mar 1998 | A |
| 6234877 | Koos et al. | May 2001 | B1 |
| 6258721 | Li et al. | Jul 2001 | B1 |
| 6312319 | Donohue et al. | Nov 2001 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20060040595 A1 | Feb 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10000494 | Oct 2001 | US |
| Child | 11256293 | US | |
| Parent | 09322198 | May 1999 | US |
| Child | 10000494 | US |
