Abstract:
The invention is directed toward a chemical mechanical polishing/planarizing (CMP) brush with an embedded mandrel. PVA foam injected into the substantially hollow mandrel expands through apertures in the mandrel to fill a mold in the desired shape of the brush with the foam in the mandrel being integral with the outer foam covering the brush.

Description:
RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 12/071,017 filed Feb. 14, 2008, now U.S. Pat. No. 7,955,693 a continuation-in-part of U.S. patent application Ser. No. 10/748,302 to Drury (now published as pre-grant publication no. 2004/0151890) filed on Dec. 31, 2003, now abandoned which is a continuation of U.S. patent application Ser. No. 09/838,138 filed on Apr. 20, 2001 now abandoned. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO SEQUENCE LISTING, A TABLE OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to cleaning devices for semiconductor wafers. More specifically, this invention relates to a novel roller constructed of a polyvinyl acetal composition having an embedded mandrel. The roller has no outer skin and a small average pore size allowing chemical flow through the roller body without clogging of the sponge pores thereby prolonging its useful life. 
     2. Background of the Invention 
     Many industries today require the ability to efficiently clean highly finished surfaces, removing particles and other surface contaminants. Specific industries having articles with highly finished surfaces include, but are not limited to, wafers of semiconductor material, and memory disks. 
     The presence of foreign material on semiconductor wafers represents a serious problem in the manufacture of integrated circuits on semiconductor wafers. In the manufacturing process, the surfaces of the wafers are initially polished prior to subsequent processing resulting in the deposition of desired circuits. A combination of chemical and mechanical polishing is generally used requiring the introduction of a polishing slurry onto the surface of the semiconductor wafer as it is being polished or cleaned. The slurry may contain fine abrasive particles and incorporate chemical cleaning compositions such as silica (SiO 2 ) and alumina (Al 2 O 3 ). When the polishing is completed the semiconductor wafers must be cleaned to completely remove residual materials to ensure that the surfaces are ready for subsequent photolithographic processing and other manufacturing steps. 
     Semiconductor wafers must be cleaned prior to any processing steps used to produce semiconductor devices. The pores of currently used chemical mechanical polishing (CMP) roller brushes quickly become clogged with chemicals and waste, reducing flow and prematurely breaking down during the cleaning process. 
     CMP roller brushes are used to clean the residual cleaning compositions from the surfaces of the semiconductor wafers. These devices for cleaning wafers generally consist of a cylindrical roller passing over a wafer. Alternatively, the rollers are used in tandem, mounted with essentially zero clearance between their cleaning/polishing surfaces. They are then counter-rotated and the wafer is passed between them to effect cleaning, polishing and/or planarization. Conventional CMP roller arrangements can include tufted nylon or other types of bristles extending from a central core and typically include sponge resins of polyvinyl acetal (PVA). The brushes are fixed horizontally and rotate as the wafer or memory disk is passed between the surfaces of the brushes while the surfaces receive large quantities of cleaning solution or de-ionized water. 
     Wafers cleaned in this manner have a high defect rate due to particles and other surface contaminants missed or passed over in the cleaning process. This results in breakage of the wafers as well as damage to wafer surfaces. 
     As noted above, more effective cleaning devices using sponge material have become available whereby a foam brush is mechanically affixed to a core or mandrel. This method is used in lieu of the more cumbersome system of mounting the brush or roller onto a core and then mounting the completed unit onto the CMP equipment. Previously known methods of manufacturing these foam brushes include molding the foam directly onto the external diameter of the core or mandrel or pre-mounting the brush onto the core or mandrel. However, the pores of the sponge material quickly become clogged with waste material and the pliability of the sponge material that is necessary for efficacy also makes them particularly prone to surface ripping. It has not been effective to clean such brushes and they are generally discarded after an undesirable amount of residual cleaning composition has been built up in the pores of the brush. Thus, the sponge brushes are generally more effective but require frequent replacement due to the accelerated wear characteristics inherent in their design. However, since such brushes are relatively costly, the dirty brushes are sometimes utilized beyond the point where they should be replaced resulting in inconsistent levels of cleanliness on the wafers lowering the yield of semiconductor product wafers. 
     3. Description Of The Related Art 
     U.S. Pat. No. 4,098,728 discloses a polyvinyl acetal (PVA) sponge and a method for making the same. In this method, pore spaces in the sponge are formed by gas bubbles; rather than pore forming chemical additives such as starch/sulfate combinations. Because the sponge disclosed in this patent does not have any starch residue, it has been particularly useful in medical applications in which starch residues can cause a foreign body reaction when in contact with human tissue. 
     The use of synthetic sponges for cleaning devices is well known. For example, U.S. Pat. No. 4,566,911 discloses a roller scrubbing device using a PVA material for cleaning semiconductor chips having a surface layer of elastic PVA material with an average pore diameter ranging from 10 to 200 microns. U.S. Pat. No. 6,080,092 discloses an industrial sponge roller device with a cylindrical body of PVA material and a plurality of projections of a truncated conical shape extending from an outer surface of the cylindrical body. 
     Conventional synthetic sponges have a polymer structure with “dead end pockets” formed therein that trap residue and trace amounts of metals and have non-uniform pore sizes causing fluid backup and residue deposit. As the sponge wears, these metals can come out of the sponge in the form of particulate matter. Such particulate matter can damage the surfaces that are to be cleaned. Further, this type of sponge has tiny fibrils in the pores thereof that are a result of spaces between the pore forming chemical additive during a cross-linking reaction. “Cross-linking” is the formation of ester bonding between chains of the two adjacent hydroxyl groups that occur with the reaction of polyvinyl alcohol and aldehyde. This reaction hardens and strengthens the resulting material. 
     These conventional synthetic sponge rollers are generally manufactured by affixing the foam roller material about a more rigid core or mandrel. However, previously known methods of manufacturing CMP brushes results in insufficient adhesion of the foam brush material to the core or mandrel, resulting in slippage and distortion during use. In operation, the foam brush roller is generally rotated at approximately 500 revolutions per minute with the result that irregular deformation of the sponge material occurs due to impingement against the wafer material and centrifugal forces exerted by rotation. Moreover, axial distortion or twisting of the foam brush occurs due to irregular slippage of the roller about the mandrel. The result is an unacceptable degree of irregularity in the planarization, cleaning, and/or polishing of the wafer surface. 
     SUMMARY OF THE INVENTION 
     These disadvantages are solved by the present invention in which a substantially hollow mandrel is injected with foam that subsequently flows through fenestrations in the mandrel, thereby forming a foam CMP brush. The foam material inside the mandrel of the brush forms a unitary whole with the foam forming the external portion of the brush. Hence, structural integrity of the brush is increased without any appreciable decrease in the flow properties therein. Inspection of a cross section of the mandrel will indicate that its fenestrations are positioned such that opposing cleaning, polishing, and/or planarizing surfaces of the brush are connected through the central axis of the brush, further increasing structural integrity. The resulting substantially skinless CMP roller of polyvinyl acetal is firmly linked to the mandrel to prevent slippage and distortion, and the usable lifetime of the brush is substantially increased compared to previously known devices. Additionally, the CMP roller has a uniform small pore size throughout the material in which over 90% of the pores of the material range from 7-40 microns in diameter with a mean flow pore diameter of about 20 microns and a mean flow pore pressure of 0.334 PSI, with a dry flow rate through the material ranging from 95.0 L/min to 25.0 L/min and a wet flow of about 80.0 L/min to 6.5 L/min. Due to the skinless open cell structure of the foam, the core is able to take up and pass liquids through the brush closely approximating the flow performance of a hollow core brush. 
     Accordingly, it is an object of the present invention to provide a new and improved polyvinyl acetal sponge roller for use in cleaning semiconductor wafers which has greater durability than prior sponge rollers. 
     Another object of the present invention is to provide an improved even flow rate of cleaning solution from the center of the core of the roller to the outside of the diameter of the roller through the sponge material for even application of cleaning solution to the surfaces of the articles being cleaned without clogging. 
     It is another object of the invention to provide a roller constructed with a material which allows better cleaning of surface materials using less cleaning solution and uniform solvent delivery at low rates. 
     It is still another object of the present invention is to provide an improved sponge roller for cleaning semiconductors with better strength characteristics. 
     It is yet another object of the present invention to provide a sponge roller capable of removing ultra-fine particles from hydrophobic surfaces and adding to the life usage of the sponge material used in cleaning the semiconductor wafers. 
     These and other objects, advantages, and novel features of the present invention will become apparent when considered with the teachings contained in the detailed disclosure along with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the chemical mechanical polishing (CMP) brush. 
         FIG. 2  is a perspective view of the chemical mechanical polishing (CMP) brush shown in  FIG. 1  depicting the opposite, female end of its mandrel. 
         FIG. 3  is a perspective view of the mandrel of the chemical mechanical polishing (CMP) brush with the foam outer member removed. 
         FIG. 4  is a cross section view of the chemical mechanical polishing (CMP) brush along a lengthwise plane bisecting adjacent, lengthwise apertures in the mandrel, and 
         FIG. 5  is a cross section view of the chemical mechanical polishing (CMP) brush across its diameter., bisecting circumferential apertures in the mandrel. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The best mode and the preferred embodiment of the novel semiconductor cleaning device of the present invention is shown generally in  FIGS. 1 through 5 . These figures are merely an illustration and should not limit the scope of the claims herein. The cleaning device can range in size and shape, depending upon the application. One of ordinary skill in the art would recognize other variations, modifications, and alternatives 
       FIGS. 1 and 2  are perspective views of a preferred embodiment of the CMP roller brush  10  showing opposite ends of the device.  FIG. 1  includes a depiction of the mandrel male engaging end  52 .  FIG. 2  is a reversed complimentary mandrel female engaging element  51 . The eccentric shapes of either element  52 ,  51  facilitate rotation of the brush about its long axis via the CMP roller brush drive mechanism (not shown). Note also the filling aperture  53  of the female engaging element  51  for ingress of PVA foam material during the manufacturing of the CMP brush  10 . The filling aperture  53  is also used to provide various cleaning, solvating, and/or polishing admixtures to the brush while in operation. 
     According to the preferred embodiment, the cleaning device is shaped as a cylindrical roller  10 , including a foam outer member  12  and mandrel  50 . The foam outer member  12  is substantially skinless and includes homogeneous small pore distribution and sizing throughout. In a preferred embodiment, the surface of the foam outer member  12  includes a plurality of frusto-pyramidal protuberances  14  disposed with the bases of opposing faces generally abutting one another such that a midline traced through abutting protuberances  14  forms a succession of generally helical patterns on the surface  13  of the foam outer member  12 . A clearance channel  16  containing no protuberances is positioned longitudinally along the length of the surface of the CMP brush  10 . 
     The foam outer member  12  is constructed of polyvinyl acetal material and has an outer diameter of about 70 mm and an inner diameter of about 30 mm with a thickness of about 20 mm and has a length ranging from 150 mm to 450 mm, preferably 317 mm, and is able to hold tight specifications. The material has a consistent durometer ranging from between 5-9 depending on the wetness of the material and has a uniform small pore size which provides a flow through rate which does not put pressure on the roller or distort it during the cleaning process. This allows even flow of chemicals from the center of the mandrel to the outside of the diameter without sacrificing strength. It is thus easier to pass chemistry through the roller. In combination with the centripetal forces due to rotation of the device, contaminants are flushed out such that the CMP brush  10  is essentially self-cleaning. Average flow requirements are reduced from 500-700 ml/minute to 120-180 ml/minute. The material has a higher rate for a better draw of debris, residue and chemistry from patterned wafer surfaces resulting in a cleaner and dryer end product. The CMP brush  10  can have shapes and sizes to meet the particular cleaning application for devices such as semiconductor wafers, hard disks, and other applications. 
     The porosity of the PVA material is uniform with at least 90% and preferably about 95% of the pore sizes being below 40 microns with the average pore diameter size or opening being about 20 microns. The pore size diameter opening ranges from about 7 microns to about 40 microns. Thus, the material has good flow properties, enhancing the performance of the CMP roller  10 . 
     The foam outer member  12  has a pore ranging from the smallest detected pore diameter of 7.1912 microns with a smallest detected pore pressure of 0.923 PSI. The foam outer member  10  has a mean flow pore diameter of 20.0 microns with a mean flow pore pressure of 0.334 PSI. The material has a bubble point pressure of 0.026 PSI and maximum pore size distribution of 5.1417 and a diameter at maximum pore size distribution of about 14.34 microns. In measuring the flow rate through the material, at least 90% and preferably 95% of the pores have a diameter of 7 microns through 40 microns with a dry flow ranging from about 95.0 L/min to about 25.0 L/min and a wet flow of about 80.0 L/min to 6.5 L/min. The aforenoted measurements were determined using a standard capillary flow analysis. The flow was measured under the Darcy method of measuring flow using water as the fluid, with a fluid viscosity of 1.000 CP with an average Darcy Permeability Constant of 5.1849. 
     In the present embodiment, the roller devices  10  are made using a suitable material that is firm, porous, elastic, and has certain abrasion resistiveness. The primary raw starting material for the device is air blown polyvinyl alcohol which is used to form a polyvinyl acetal porous elastic material with a uniform cell structure. The porous material varies in characteristic depending upon cleanliness, type of pore forming agent or process, type of aldehyde employed for the conversion of a polyvinyl alcohol to a polyvinyl acetal, and other factors. These factors also include the relative proportions of reactants, reaction temperature and time, and the general condition and starting materials in the manufacturing process. 
     Turning now to  FIGS. 3 ,  4 , and  5 , in manufacture, the mandrel  50  is placed into a mold having the generally desired shape of the final product and injected through its filling aperture  53  with the polyvinyl acetal (PVA) material described above. The PVA expands through the apertures  54  in the mandrel  50  to fill the mold cavity. Prior to injection, the PVA material is heated and solubilized at about 190 degrees Fahrenheit, mixed with cross linking agent and catalyzed. After injection, the now formed CMP brush  10  is slowly cured before removal from the mold. After removing the molded polyvinyl acetal material from the cast, the shaped material is washed with a di-water carrier several times to remove the forming formaldehyde so that the formaldehyde is undetectable by high pressure liquid chromatography but believed to be less than 0.1 part per million. It is thereafter saturated in a 1%-5%, preferably 1.5%, H 2 O 2  solution to inhibit microbial formation during storage. Packaged in this manner, the CMP roller brush has a shelf life of at least three years. 
     The number, size, and positioning of the apertures  54  in the mandrel  50  promote the structural integrity of the CMP roller brush  10 . Increasing the size and/or number of apertures  54  promotes better flow characteristics through the foam outer member  12  but tends to reduce rigidity of the mandrel  50 , and therefore the rotational integrity of CMP brush  10  in toto. The result is a lack of uniformity during cleaning, polishing, and/or planarization. Conversely, decreasing the size and/or number of apertures  54  will tend to promote rotational integrity but at the expense of fluid flow through the brush  10 , thereby reducing its useful lifetime. 
     The percentage area of contact necessary to stabilize and connect the outer foam member  12  to the inner foam core  30  varies by the amount of foam comprising the outer diameter of the CMP brush  10 . Comparatively large volumes of foam comprising the foam outer member  12  require comparatively greater percentages of connection to the inner foam core  30 . 
     To ensure that the foam outer member  12  remains affixed to mandrel  50 , greater than 2% of the surface area of the mandrel must be fenestrated or apertured to allow for the foam comprising the outer foam member  12  to adequately interconnect with the inner foam core  30 . Approximately 20% to 25% fenestration of the mandrel  50  is ideal in order to adequately interconnect the inner foam core  30  and outer foam member  12  while maximizing fluid flow through the CMP brush  10 . In a preferred embodiment of a CMP roller brush  10  with an overall outside diameter of 70 mm, length of 317 mm, and a mandrel with 30 mm outside diameter, 21.6% of the mandrel surface was fenestrated (i.e., approximately 7,296 mm 2  surface area of the fenestrations versus approximately 33,755 mm 2  surface area of the mandrel). This allows the structural intergrity of the mandrel to remain intact while allowing sufficient flow through of the foam to properly affix the foam to the mandrel. Less than 2% fenestration does not provide sufficient contact, permitting the foam outer member  12  to rotate and distort on the mandrel  50 . Depending on the type of material used for the mandrel, the contact area permissible for flow through can be as great as 80%. Greater than 80% fenestration of the mandrel permits a degree of flexibility in the mandrel that renders it useless. 
     In another preferred embodiment with an overall outside diameter of 38 mm, the degree of fenestration required to adequately affix the foam outer member  12  to the mandrel is 12.7% due to the reduced volume of the foam outer member  12 . In this embodiment, the surface area of the mandrel is 20,441 mm 2  and the degree of fenestration necessary to affix the foam outer member  12  while maintaining the integrity of the mandrel  50  and ensuring adequate flow characteristics in the entire CMP brush unit  10  is 2,507 mm 2 . 
     The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. However, the invention should not be construed as limited to the particular embodiments which have been described above. Instead, the embodiments described here should be regarded as illustrative rather than restrictive. Variations and changes may be made by others without departing from the scope of the present invention as defined by the following claims: