DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] An invention for methods and systems for cleaning a surface of a semiconductor wafer are 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, by one of ordinary skill 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.

[0035] FIGS. 2A and 2B show a side view and a top view, respectively, of a cleaning system 120. The cleaning system 120 typically includes an input station 100 where a plurality of wafers may be inserted for cleaning through the system. Once the wafers are inserted into the input station 100, a wafer may be taken from the input station 100 and moved into a brush box one 102a, where the wafer is scrubbed with selected chemicals and water (e.g., de-ionized water) before being moved to a brush box two 102b.

[0036] After the wafer has been scrubbed in the brush boxes 102, the wafer is moved into a spin, rinse, and dry (SRD) station 104, where de-ionized (DI) water is sprayed onto the surface of the wafer and spun to dry. After the wafer has been placed through the SRD station 104, an unload handler 110 takes the wafer and moves it into an output station 106. The cleaning system 120 is configured to be programmed and controlled from system electronics 108.

[0037] FIGS. 3A-1 and 3A-2 show a detailed view of a cleaning apparatus inside one of the brush boxes 102. A load handler may take the wafer 200 from the input station 100 and position the wafer inside the brush box 102a. A top cleaning brush 204a and a bottom cleaning brush 204b, may be positioned on the respective top wafer surface 210a and bottom wafer surface 210b, as shown in FIG. 3A-2. A cleaning brush 204 typically has a plurality of small surface mounds 206 distributed in evenly spaced rows along the surface of the cleaning brush 204. The brushes 204 are also referred to as polyvinyl alcohol (PVA) brushes, and are very soft and porous. Therefore, the brushes 204 are capable of scrubbing a wafer clean without damaging the delicate surface. Because the brushes 204 are porous, they are able to function as a conduit for fluids that are to be applied to the wafer surface 210.

[0038] For more information on wafer cleaning systems and techniques, reference may be made to commonly owned U.S. patent application Ser. Nos.: (1) 08/792,093, filed Jan. 31, 1997, entitled “Method And Apparatus For Cleaning Of Semiconductor Substrates Using Standard Clean 1 (SC1),” and (2) Ser. No. 08/542,531, filed Oct. 13, 1995, entitled “Method and Apparatus for Chemical Delivery Through the Brush.” Both U.S. patent applications are hereby incorporated by reference.

[0039] During the cleaning process, the wafer 200 may be rotated between the cleaning brushes 204 and a set of rollers 202. The wafer preferably rotates at about 20 rotations per minute or less. It should be appreciated that this is about ⅕ the rotational speed using in conventional SRD stations. Although FIG. 3A-1 illustrates two rollers 202, it should be appreciated by one of ordinary skill in the art that additional or fewer rollers 202 can be used, preferably as long as the wafer is properly balanced between the cleaning brushes 204. As shown in FIG. 3A-2, in a typical cleaning process, the cleaning brushes 204 rotate about their radial axes and scrub the wafer surfaces 210 as the wafer rotates between the cleaning brushes 204. The surface mounds 206 assist in achieving improved scrubbing of the wafer surfaces 210.

[0040] A chemical cleaning fluid is generally applied to the wafer surfaces 210 through the brush (TTB) as the cleaning brushes 204 scrub the wafer surfaces 210. It should be appreciated by one of ordinary skill in the art that a chemical cleaning fluid may alternatively be applied by other means, such as an external drip applicator (not shown), as opposed to TTB.

[0041] Where a chemical cleaning fluid has been applied to the wafer surfaces 210 via a TTB application technique, it is generally desired to clean the wafer surfaces 210 with water (preferably de-ionized water) in order to remove the chemicals from the wafer surface 210. If left on the wafer surface 210, the chemicals may cause unwanted reactions in subsequent cleaning and post-cleaning operations.

[0042] As mentioned above, using a TTB technique to apply water to the wafer surfaces 210 is extremely inefficient. Time is wasted on loading the brushes with chemicals to the desired chemical concentration, flushing the chemicals from the brushes to perform the water cleaning, and then re-loading the brushes with chemicals again. Moreover, not only is the process inefficient, this process can be unsafe. Unwanted chemical reactions can occur, particulate generation can be promoted, and the mechanical components of the cleaning system (particularly components of the SRD station) can be placed at an increased risk of degradation. The following discussion discloses techniques for substantially eliminating the aforementioned need to flush the brushes 204 with water and thereby change the chemical concentration in the brushes 204.

[0043] FIGS. 3B-1 and 3B-2 show a process of applying water to the wafer surfaces 210 by way of nozzles 220, in accordance with one embodiment of the present invention. The top cleaning brush 204a may be raised away from the top wafer surface 210a, as depicted in FIG. 3B-2. As mentioned above, the wafer 200 may be held by the bottom cleaning brush 204b and a set of rollers 202. Although FIG. 3B-2 illustrates two rollers 202, it should be appreciated that additional or fewer rollers 202 can be used, preferably as long as the wafer is properly held on the bottom cleaning brush 204b. In this embodiment, a liquid source 222 is used to supply liquid through a conduit that leads to the nozzles 220. The nozzles 220 may be configured to evenly and quickly distribute water over the wafer surfaces 210 as the wafer 200 rotates and is balanced on the bottom cleaning brush 204b and the two rollers 202.

[0044] In a preferred embodiment, two top nozzles 220a may be used for applying water to the top surface 210a, while two bottom nozzles 220b may be used for applying water to the bottom surface 210b. Such an embodiment includes a total of four nozzles 220, as shown in FIG. 3B-2. In an alternative embodiment (not shown), two nozzles may be used for applying water to the top surface, while one nozzle may be used for applying water to the bottom surface. In other embodiments of the present invention, additional or fewer nozzles 220 may be used, preferably as long as the nozzles 220 are positioned in such a way as to distribute water evenly over at least the top surface 210a of wafer 200.

[0045] The flow rate of the water out of any one of the nozzles 220 is preferably between about 150 ml/minute and about 750 ml/minute, more preferably between about 300 ml/minute and about 600 ml/minute, and most preferably about 500 ml/minute. The water pressure of any one of the nozzles 220 is preferably between about 20 psi and about 50 psi, more preferably between about 25 psi and about 45 psi, and most preferably about 35 psi. The duration for rinsing the wafer surfaces 210 is preferably selected to be between about 5 seconds and about 60 seconds, more preferably between about 10 seconds and about 45 seconds, and most preferably about 15 seconds.

[0046] In cases where hydrofluoric (HF) acid is used in the cleaning, it is generally desired that the pH of the fluids that remain over the wafer surfaces 210 be at least about 4 or greater after the rinsing with the water (pH value of 4 corresponds to HF concentration of about 2.3 ppm (parts per million)). Note that water has a pH of about 7. In an alternative embodiment, where base cleaning is performed, it is generally desired that the pH of the fluids that remain over the wafer surfaces 210 be at most about 8.5 or less after the rinsing with the water that has a pH of about 7.

[0047] In typical cases where acid cleaning is performed, the cleaning pH level is typically set to about 2 (pH value of 2 corresponds to HF concentration of about 3500 ppm). Thus, the desire to bring up the pH level in acid chemicals is primarily due to the fact that acids having pH levels less than about 4 can cause unwanted reactions in subsequent cleaning operations or cause the cleaning station equipment to degrade. The rapid rinsing with the nozzles 220 therefore enables quick removal of most of the cleaning chemicals and also assists in efficiently increasing the pH of any remaining surface chemicals. Bringing up the pH level also adds additional safety to the SRD station, which is easily accessible to operators of an SRD station. For operators handling HF, the permissible exposure limit (PEL) for HF is about 3 ppm.

[0048] FIGS. 3B-3 and 3B-4 show a process of applying water to the wafer surfaces 210 by way of nozzles 220, the wafer being rotated by four rollers 202, in accordance with one embodiment of the present invention. The top cleaning brush 204a may be raised away from the top wafer surface 210a, as depicted in FIG. 3B-4. Likewise, the bottom cleaning brush 204b may be removed from the bottom wafer surface 210b. The wafer 200 may be held by a set of rollers 202. Although FIG. 3B-3 illustrates four rollers 202, it should be appreciated that additional or fewer rollers 202 can be used, preferably as long as the wafer is properly held between the rollers 202. In this embodiment, a liquid source 222 is used to supply liquid through a conduit that leads to the nozzles 220. The nozzles may be configured to evenly and quickly distribute water over the wafer surfaces 210 as the wafer rotates and is held on the four rollers 202.

[0049] In a preferred embodiment, two top nozzles 220a may be used for applying water to the top surface 210a, while two bottom nozzles 220b may be used for applying water to the bottom surface 210b. Such an embodiment includes a total of four nozzles 220, as shown in FIG. 3B-2. In an alternative embodiment (not shown), two nozzles may be used for applying water to the top surface, while one nozzle may be used for applying water to the bottom surface. In other embodiments of the present invention, additional or fewer nozzles 220 may be used, preferably as long as the nozzles 220 are positioned in such a way as to distribute water evenly over at least the top surface 210a of wafer 200.

[0050] FIG. 4 shows an enlarged view of one of the nozzles 220 fixedly positioned over the wafer 200, in accordance with one embodiment of the present invention. Although FIG. 4 shows a view of one of the top nozzles 220a, it should be apparent that the following discussion is applicable to any one of the bottom nozzles 220b as well.

[0051] In a preferred embodiment, the position of the nozzle 220 relative to the wafer 200 may be defined by three parameters. First, the nozzle 220 may be positioned relative to the wafer surface 210 such that the plane of the wafer surface 210 and the radial axis of the nozzle 220 form an angle θ. Second, the nozzle 226 may be positioned such that outer side 306 of the nozzle opening 308 is inward from the wafer edge 310 a predetermined edge distance 302. Third, the nozzle 220 may be positioned such that the outer side 306 of the nozzle opening 308 is above the wafer surface 210 a predetermined raised distance 304.

[0052] The angle θ is preferably between about 10 degrees and about 35 degrees, more preferably between about 15 degrees and about 25 degrees, and most preferably about 20 degrees. The edge distance 302 is preferably between about 3 mm and about 20 mm, and most preferably about 5 mm. The raised distance 304 is preferably about 5 mm. The raised distance 304 can vary between about 2 mm and about 15 mm.

[0053] FIG. 5 shows a flow chart for a wafer cleaning process 400, according to one embodiment of the present invention. The process 400 starts in operation 402, where a semiconductor wafer 200 may be loaded into a brush box 102. The process 400 then moves to operation 404, where a chemical cleaning may be performed on the wafer surface 210 in the brush box 102. This chemical cleaning is preferably performed with two brushes 204, as discussed above with reference to FIGS. 3A-1 and 3A-2. Next, the process 400 will move to operation 406, where a brush (preferably the top brush 204a) is removed from contact with the wafer surface 210. The wafer 200 is preferably held between the bottom cleaning brush 204b and a set of rollers 202 as the wafer 200 rotates, as discussed above with reference to FIGS. 3B-1 and 3B-2. After operation 406, the process 400 moves to operation 408, where a flow of cleaning fluid is delivered to the wafer surface 210 by way of a nozzle system. This cleaning fluid is preferably de-ionized water which is configured to rinse-off unwanted chemicals and particulates from the wafer surface 210 after the chemical cleaning. As mentioned above, the application of this cleaning fluid after the chemical cleaning avoids the need to clean the wafer 200 using a time consuming and less thorough TTB process.

[0054] After operation 408, the process 400 moves to operation 410, where the wafer 200 is moved to a spin, rinse and dry (SRD) station 104. Instead of moving to operation 410, the process 400 may alternatively transport the wafer 200 to a second brush box 102b, and any combination of operations 404, 406 and 408 may be performed in the second brush box 102b as well. After operation 410 or after the second brush box 102b, the process 400 will proceed to operation 412, where fabrication operations following the foregoing cleaning operations may be performed on the wafer 200.

[0055] The process 400 then proceeds to a decision operation 414, where it is determined whether a next wafer is to be cleaned. If there is no next wafer to be cleaned, the process 400 is done. On the other hand, if a next wafer is desired to be cleaned, the process 400 will go back to operation 402 where another semiconductor wafer is loaded into the brush box 102. The foregoing cycle continues preferably until there is no next wafer at decision operation 414.

[0056] FIG. 6 shows a flow chart for an alternative wafer cleaning process 500, according to one embodiment of the present invention. The process 500 starts in operation 502, where a semiconductor wafer 200 is loaded into a brush box. The process 500 then moves to operation 504, where a chemical cleaning is performed on the wafer surface 210 in the brush box 102. This chemical cleaning is preferably performed with two brushes 204, as discussed above with reference to FIGS. 3A-1 and 3A-2. Next, the process 500 moves to operation 506, where the brushes 204 are removed from contact with the wafer surface 210. The wafer 200 is preferably held between a set of rollers 202 as the wafer 200 rotates, as discussed above with reference to FIGS. 3B-3 and 3B-4. After operation 506, the process 500 proceeds to operation 508, where a flow of cleaning fluid is delivered to the wafer surface 210 by way of a nozzle system. The benefits of this cleaning fluid delivery were discussed above.

[0057] Next, the process 500 moves to operation 510, where the wafer 200 is transported to a spin, rinse and dry (SRD) station 104. Alternatively, instead of moving to operation 510, the process 500 may move to a second brush box, where any combination of operations 504, 506 and 508 may be performed. After operation 510 or after the second brush box, the process 500 may proceed to operation 512, where fabrication operations following the foregoing cleaning operations may be performed on the wafer 200.

[0058] The process 500 then proceeds to a decision operation 514, where it is determined whether a next wafer is to be cleaned. If there is no next wafer to be cleaned, the process 500 is done. On the other hand, if a next wafer is desired to be cleaned, the process 500 goes back to operation 502 where another semiconductor wafer is loaded into the brush box. The foregoing cycle continues preferably until there is no next wafer at decision operation 514.

[0059] Specific reference has been made to wafer cleaning systems that implement a through the brush (TTB) technique. However, the cleaning methods of the present invention can be applied to other types of cleaning systems, such as those that implement a chemical drip applicator. Thus, by implementing these wafer cleaning methods, the overall cleaning system will generate a higher quality cleaned wafers.

[0060] 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.