US20070015444A1

US20070015444A1 - Smoothing pad for bare semiconductor wafers

Info

Publication number: US20070015444A1
Application number: US11/533,435
Authority: US
Inventors: Daniel Marks; Anthony Clark; Douglas Targgart
Original assignee: PsiloQuest
Current assignee: PsiloQuest
Priority date: 2005-01-12
Filing date: 2006-09-20
Publication date: 2007-01-18
Also published as: US20060154579A1; WO2006076060A1

Abstract

One embodiment of the present invention is a smoothing pad for bare semiconductor wafers the smoothing pad for bare semiconductor wafers. The smoothing pad comprises a smoothing body having a closed-cell thermoplastic foam comprising an ethylene vinyl acetate block copolymer comprising a vinyl acetate content ranging from about 1 to about 18 wt %. The smoothing body is substantially free of particles having an average size of greater than about 1 micron. Other aspects of the invention comprise a method of preparing a bare semiconductor wafer and a method of manufacturing a pad for smoothing bare semiconductor wafers.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent application Ser. No. 11/033,690, entitled, A THERMOPLASTIC CHEMICAL MECHANICAL POLISHING PAD AND METHOD OF MANUFACTURE, filed on Jan. 12, 2005 by Marks, et al. (“690' application”), which is currently pending. The above-listed application is commonly assigned with the present invention and is incorporated herein by reference as if reproduced herein in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed to semiconductor wafer smoothing pads, and methods of use and manufacture thereof.

BACKGROUND OF THE INVENTION

In semiconductor manufacturing, after growing a semiconductor crystal and slicing the crystal to form wafers, the wafer is subject to a preparation process. Typically, the preparation process includes one or more polishing steps to planarize the bare semiconductor wafer, followed by a smoothing step to remove scratches on the wafer's surface, and then a cleaning step to remove debris on the wafer's surface. The goals of wafer smoothing are different than the goals wafer polishing or cleaning, thereby making these separate fields of endeavor. The goal of polishing is to remove portions of the wafer itself to produce a flatter surface, while the goal of smoothing is to reduce local discontinuities on the surface of the bare wafer without removing substantial amounts of wafer material other than that needed to improve the wafer's smoothness. Unfortunately, however, the smoothing step can also inadvertently reintroduce nonplanarities or scratches into the wafer.
Accordingly, what is needed is a smoothing pad and process that can provide a highly smooth wafer surface without introducing nonplanarities and minimize the introduction of scratches into the wafer.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, the present invention provides in one embodiment, a smoothing pad for bare semiconductor wafers that comprises a smoothing body. The smoothing body includes a closed-cell thermoplastic foam having an ethylene vinyl acetate block copolymer comprising a vinyl acetate content ranging from about 1 to about 18 wt %. The smoothing body is substantially free of particles having an average size of greater than about 1 micron
Another embodiment is a method of preparing a bare semiconductor wafer that comprises smoothing the wafer. Smoothing the bare semiconductor wafer includes situating the above-described smoothing pad adjacent to a surface of the bare semiconductor wafer. Smoothing also includes moving the smoothing pad relative to the bare semiconductor wafer until the surface has a roughness of less than or equal to about 20 Angstroms.
Still another embodiment of the present invention is a method for manufacturing a pad for smoothing bare semiconductor wafers. The method comprises placing an ethylene vinyl acetate block copolymer and a foaming agent into a container. The ethylene vinyl acetate block copolymer has a vinyl acetate content ranging from about 1 to about 18 wt %. The ethylene vinyl acetate block copolymer is substantially free of particles having an average size of about 1 micron or greater. The method also includes mixing the ethylene vinyl acetate block copolymer and the foaming agent together, and then forming closed cells throughout the ethylene vinyl acetate block copolymer to thereby produce a closed-cell thermoplastic smoothing body.
The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 presents an cross-sectional view of a portion of an example smoothing pad of the present invention;
FIG. 2 illustrates, by flow diagram, an example method of preparing a bare semiconductor wafer according to the principles of the present invention; and
FIG. 3 illustrates, by flow diagram, an example method of manufacturing a pad for smoothing bare semiconductor wafers according to the principles of the present invention.

DETAILED DESCRIPTION

As part of the present invention it was discovered that an exceptionally smooth bare semiconductor wafer surface can be prepared by using a pad having a smoothing body that comprises a certain type of thermoplastic foam that is substantially free of particles having an average size of about 1 micron or greater. The combination of the thermoplastic foam and the absence of particles are important to producing a smooth surface while not introducing nonplanarities or scratches into the wafer during smoothing.
FIG. 1 presents a cross-sectional view of a portion of an exemplary smoothing pad 100 for smoothing bare semiconductor wafers 110, and in particular smoothing a surface 112 of the wafer 110. The term bare semiconductor wafer as used herein refer to a semiconductor wafer with no integrated circuit components, including sacrificial structures, formed thereon. Example bare semiconductor wafers 110 include silicon, silicon carbide or gallium arsenide wafers.
The smoothing pad 100 comprises a smoothing body 115 that has a closed-cell thermoplastic foam 120. The closed cell foam 120 comprises an ethylene vinyl acetate (EVA) block copolymer 125 comprising a vinyl acetate content ranging from about 1 to about 18 weight percent (wt %) and that is substantially free of particles having an average size of about 1 micron or greater. In some preferred embodiments, the smoothing body 115 consists essentially of the closed-cell thermoplastic foam 120, which in turn, consists essentially of the above-described ethylene vinyl acetate (EVA) block copolymer 125. The thermoplastic foam 120 can also comprise trace amounts (e.g., about 1 to 2 wt %) of foaming agents, cross-linking agents, reaction products of the cross-linking or foaming agent, or catalysts to activate the cross-linking agent or foaming agent.
The term substantially free of particles, as used herein, is defined as a particle content of about 1 wt % or less, where the particles have an average size of 1 micron or greater. While not limiting the scope of the invention by theory, it is believed that hard particles projecting from the surface of the smoothing body 115 could cause substantial material losses from the bare semiconductor wafer 110, thereby increasing the potential for scratches to be introduced into the wafer's surface 112 during smoothing. In particular, it is desirable for the smoothing body 115 to be free of particles having a hardness of about 5 mohs or greater. Examples of such particles include inorganic oxides such as silica, alumina, ceria, titania, or zirconia.
In some embodiments, however, the smoothing body 115 can include sub-micron sized particles 130. E.g., some embodiments of the smoothing body 115 include about 4.5 wt % of titania particles 130 added as a colorant, the titania particles 130 having an average size of about 0.3 microns. In some embodiments, to further facilitate the production of a smooth surface 112 and to reduce pad manufacturing costs, it is desirable for the smoothing body 115 to also be substantially free (e.g., less than about 1 wt %) of the submicron particles 130, including colorant particles having an average size of about 0.3 microns or greater.
The term smoothing pad 100, as used herein, refers to a material, or composition of materials, that is configured to smooth the wafer's surface 112 while not substantially removing material from the wafer 110. E.g., while smoothing the bare semiconductor wafer 110 with the smoothing pad 115, no more than about 1 to 2 microns per minute of a thickness 135 of the bare semiconductor wafer 110 is removed from the surface 112. This is in contrast to a conventional wafer polishing pad, which is configured to remove substantially greater amounts of material from the wafer's surface 112 (e.g., about 30 microns or more per minute of the wafer polishing). This is also in contrast to a conventional cleaning pad, which is configured to remove debris on the wafer's surface 112.
The planarity of the bare semiconductor wafer 110 can be measured by conventional stylus profiling or other techniques well known to those skilled in the art. For the purposes of the present invention, planarity was characterized by measuring the total indicated run-out (TIR). TIR is defined as the difference in height from the highest 140 and lowest point 145 on the wafer's surface 112 (FIG. 1).
The smoothness of the bare semiconductor wafer 110 can be characterized by a surface roughness measured using atomic force microscopy. The term surface roughness, as used herein, is defined as the deviation of the actual surface topography from an ideal atomically smooth and planar surface. A measure of the surface roughness is the root-mean-square deviation from a center-line-average 150 over the wafer's surface 112 (FIG. 1).
Scratches 155 in the wafer's surface 112 can be measured by conventional Light Point Defect (LPD) detection tools such as a Constellation AWIS (ADE Optical Systems Corp. Westwood, Mass.). Typically the LPD tool is configured to bin defects according to their size, e.g., scratches 155 may be classed into seven bins of linear defects ranging from 0.065 to 0.7 microns in length 157.
In some preferred embodiments, the closed-cell thermoplastic foam 120 of the smoothing body 115 is free of urethane, because the presence of urethane can make the smoothing body 115 harder than desired. Additionally, certain urethane-free smoothing bodies 115 have the advantage of not requiring conditioning before their use for smoothing.
As illustrated in FIG. 1 the smoothing body 115 can further include grooves 160 in its surface 165 to facilitate the flow and distribution of a slurry used in combination with the smoothing body 115 to smooth the wafer 110. One of ordinary skill in the art would be familiar with groove dimensions and patterns of grooves that can be used for such purposes. In some preferred embodiments, however, the surface 165 of the smoothing body 115 is free of grooves.
A groove-free smoothing surface 165 can be advantageous when the presence of grooves changes the physical properties of the smoothing pad 100. E.g., consider instances when the grooves 160 are formed by scoring the surface 165 of the smoothing body 115 via a laser beam. In some cases, the hardness of the smoothing body 115 is altered because portions of the thermoplastic foam 120 are condensed by the heat of the laser beam. In other cases, however, the grooves 160 are made in a fashion (e.g., mechanical scoring) that does not alter the physical properties of the pad 100.
Any of the embodiments of the thermoplastic foam discussed in the 690' application can be used in the closed-cell thermoplastic foam 120 of the present invention. As pointed out in the 690' application, thermoplastic foams that comprise EVA block copolymers 125 are much more resistant to thermal and resilience breakdown as compared to e.g., blends of EVA and polyethylene. Previous experiments have shown that it is important adjust the vinyl acetate content of the EVA block copolymers 125 within a range that is low enough to impart thermal stability, but high enough to provide a resilient EVA block copolymer 125. As the vinyl acetate content is decreased, the melting point increases. But as the content of vinyl acetate in the EVA block copolymer 125 decreases, so too does the resilience. Specifically, in embodiments of the closed-cell thermoplastic foam smoothing body 115, the ethylene vinyl acetate block copolymer 120 comprises a vinyl acetate content ranging from about 1 wt % to about 18 wt %, and more preferably, from about 6 wt % to about 12 wt %.
Advantageous embodiments of the EVA block copolymer 125 have a particular hardness, resilience, melt index, and density that facilitate the preparation of the desired smoothing body 115. Some embodiments of the EVA block copolymer 125 have a hardness of about 80 to about 100 Shore A (as measured by ASTM D2240). Other embodiments of the EVA block copolymer 125 have a resilience, as characterized by a compression-set, ranging from about 30% to about 50% (ASTM D 395, 10 days at 25° C.). Still other embodiments of the EVA block copolymer 125 have a melt index ranging from about 0.2 to about 25 dg/minute. Yet other preferred embodiments of the EVA block copolymer 125 have a density ranging from about 0.93 to about 0.95 g/cm³.
As well understood by one skilled in the art, the above-described characteristics can be adjusted by changing the vinyl acetate content of the EVA block copolymer 125 within the ranges set forth above, by changing the size of the ethylene and vinyl acetate blocks and by changing the copolymer's 125 molecular weight. One of ordinary skill in the art would be familiar with how these properties of the EVA block copolymer 125 can be adjusted
In some cases, the EVA block copolymer 125 is foamed to different extents to provide the closed-cell thermoplastic foam smoothing body 115 with a range of hardness conducive to polishing certain types of materials. The extent of foaming can be characterized in terms of the density of the closed-cell thermoplastic foam smoothing body 115 produced. A smoothing body 115 having a density ranging from about 0.1 to about 0.4 gm/cm³results in a hardness ranging from about 20 to about 75 Shore A. For example, some preferred embodiments of the smoothing body 115 have a density of about 0.13 gm/cm³(about 8 lb/ft³), corresponding to a hardness ranging from about 20 to about 40 Shore A. As another example, embodiments of the smoothing body 115 having a density of about 0.29 gm/cm³(about 18 lb/ft³) corresponding to a hardness ranging from about 35 to about 75 Shore A. Still another example is a smoothing body 115 having a density of about 0.38 gm/cm³(about 24 lb/ft³) corresponding to a hardness ranging from about 35 to about 75 Shore A. Of course, smoothing bodies 115 of different density values than recited above can be used.
As noted above, the vinyl acetate content of the EVA block copolymer 125 represents a balance between thermostability and resilience. In some cases, for instance, it is desirable for the EVA block copolymer 125 to have a certain resilience that is conducive to uniform and repeatable smoothing. Resilience here is characterized by the EVA block copolymer's 125 compression-set (ASTM D 395, 10 days at 25° C.). A compression-set equals 100 percent minus the percentage return of the copolymer to its original size along the original dimension that the force was applied. Preferred embodiments of the EVA block copolymer 125 have a compression set ranging from about 30% to about 50%. Preferred embodiments of the closed-cell thermoplastic foam smoothing body 115 produced by foaming such EVA block copolymers 125 have a compression set ranging from about 2% to about 15% (ASTM D 395, 10 days at 25° C.).
To avoid distortion or melting during the elevated temperatures and pressures attained during smoothing, it is advantageous for the EVA block copolymer 125 to have a particular range of thermostability. Thermostability can be characterized in terms of the EVA block copolymer's 125 melting point or melt index as measured using ASTM D1238. Some preferred embodiments of the EVA block copolymer 125 have a melt point ranging from about 85 to about 105° C., and more preferably, about 88 to 100° C. Other preferred embodiments of the EVA block copolymer 125 have a melt index ranging from about 0.2 to about 25 dg/minute, and more preferably from about 0.2 to about 6 dg/minute. The closed-cell thermoplastic foam polishing body 110 produced by foaming these embodiments of the EVA block copolymer 120 has substantially the same melting point or melt index as the EVA block copolymer 125. In some instances by cross-linking the EVA block copolymer as discussed below, the melting point of the smoothing body 115 is higher than that of a commercially provided EVA block copolymer.
Non-limiting examples of commercially available EVA block copolymers 125 suitable for use in the present invention include copolymers available under the trademark ELVAX™ (trademark of E.I. du Pont de Nemours & Company, Wilmington, Del.), or under the trademark AIRFLEX™ (Air Products & Chemicals, Inc., Allentown, Pa.) or under the trademark Ultrathene® (Equistar Chemicals, LP, Houston, Tex.) or under the trademark Escorene (ExxonMobil Chemical, Houston, Tex.) or other suitable supplier.
Other non-limiting examples of preferred EVA block copolymers 125 include ELVAX 460, which has a vinyl acetate content of 18 wt %, a melting point of 88° C., a compression set of 50%, a density of 0.941 g/cm³, and Shore A hardness of 90; ELVAX 550, which has a vinyl acetate content of 15 wt %, a melting point of 93° C., a compression set of 54%, a density of 0.935 g/cm³, and Shore A hardness of 92; ELVAX 660, which has a vinyl acetate content of 12 wt %, a melting point of 96° C., a compression set of 39%, density of 0.933 g/cm³, and Shore A hardness of 95; ELVAX 670, which has a vinyl acetate content of 12 wt %, a melting point of 96° C., a compression set of 53%, a density of 0.933 g/cm³, and Shore A hardness of 96; ELVAX 750, which has a vinyl acetate content of 9 wt %, a melting point of 100° C., a compression set of 46%, a density of 0.930 g/cm³and Shore A hardness of 95; ELVAX 760, which has a vinyl acetate content of 9 wt %, a melting point of 100° C., a compression set of 33%, a density of 0.930 g/cm³, and Shore A hardness of 97.
In some preferred embodiments of the closed-cell thermoplastic foam smoothing body 115 of the EVA block copolymer 125 is cross-linked. While not limiting the scope of the invention by theory, it is believed that a smoothing body 115 comprising a cross-linked EVA block copolymer 125 advantageously makes the smoothing body 110 less prone to inelastic or plastic deformation during smoothing as compared to a smoothing body 110 comprised of non-cross-linked EVA block copolymers 125. In addition, cross-linking is thought to beneficially increase the chemical resistance of the smoothing body 110 to degradation due to slurry chemistries. Furthermore, because cross-linking increases the effective molecular weight, it is thought that the cross-linked EVA block copolymer 125 advantageously has an increased tear resistance, increased tensile strength, decreased tensile elongation and decreased dimensional thermal stability. Additional, it is thought that the cross-linked EVA block copolymer 125 has a distribution of melting points that is on average higher than a non-cross-linked EVA block copolymer 125
As well known to those skilled in the art the extent of cross-linking can be characterized in terms of a gel fraction, the fraction of the EVA block copolymer 125 that cannot be extracted by a solvent, in accordance with ASTM D 2765. Preferred embodiments of the smoothing body 115 have a gel fraction ranging from about 60′ to about 95%.
In certain preferred embodiments of the present invention, an interior of the closed-cell thermoplastic foam smoothing body 115 comprise cells 170 having an average size 172 ranging from about 50 to about 300 microns, and more preferably, from about 100 to about 200 microns. The term cell 170 as used herein, refers to any volume defined by a membrane within the EVA block copolymer 125 occupied by air, or other gases from a foaming agent. Of course, as well known to those skilled in the art, the foaming gases inside the cells can out-gas after the manufacture of the closed-cell thermoplastic polishing body, and be substantially replaced by air. Cell size 172 can be determined using standardized protocols, such as developed and published by the American Society for Testing and Materials (West Conshohocken, Pa.), such as ASTM D3576, incorporated herein by reference.
Some preferred embodiments of the pad 100 comprise concave cells 175 at the pad's surface 165 having an average diameter 178 that is substantially the same as the average size 172 as the cells 170. The concave cells 175 are formed from the cells 170 upon exposing the closed cell thermoplastic foam 120, as further discussed below. In certain preferred embodiments, the cells 170 are substantially spheroidal. However, the cells 170 and concave cells 175 can also comprise irregular shapes.
In some embodiments, the pad 100 further comprises an optional backing film 180 coupled to the smoothing body 115. The backing film 180 can comprise any of the embodiments of the backing film, and be coupled to the smoothing body 115, as described in U.S. Pat. No. 6,838,169, which is incorporated by reference herein in its entirety.
The backing film 180 can comprise a thermoplastic that is preferably non-foamed. Example thermoplastics include high density polyethylene (HDP; density greater than about 0.96 gm/cc), and in some cases, a condensed high density polyethylene. Examples of a HDP suitable for use as a backing film 180 are product numbers DGDA-2490 and DGDA-2480 (Dow Chemical Corp), Product numbers Paxon BA7718 and Escorene HD7845 (Exxon Corp.). Other suitable materials for the backing film 180 include condensed low density polyethylene (LPDE), linear low density polyethylene (LLDPE) (density of 0.94 g/cc or less), polypropylene (PP), thermoplastic elastomers (TPE), thermoplastic rubber (TPR), polycarbonate (PC), poly vinyl chloride (PVC), polyamide 6,6, adipic-acid-1,6-hexanediamine polymer (PA6), thermoplastic polyurethane (TPU), or a non-foamed (e.g., condensed) ethyl vinyl acetate polyolefin co-polymers (EVA-PO). A preferred example non-foamed EVA-PO suitable for the backing film 180 is ELVAX™ 265, which has a vinyl acetate content of about 28 wt %, a melting point of about 73° C., a compression set of about 49%, a density of about 0.951 g/cm³, and Shore A hardness of about 86.
The coupling between the backing film 180 and the smoothing body 115 can be direct, e.g., by forming a thermal weld 185 between the backing 180 and the smoothing body 115, or indirect, using e.g., an adhesive such as a pressure sensitive adhesive (PSA), or a combination of direct or indirect coupling. The term thermal weld 185 as used herein refers to portions of the thermoplastic foam 120 and backing film 180 that have condensed and coalesce with each other to form a strong bond between these two materials. In some embodiments, direct coupling through the formation of a thermal weld 185 is preferable because the smoothing pad 100 is less prone to delamination or distortion during the smoothing process. Additionally, a second adhesive 187, e.g., the same or different PSA can be attached to the side 190 of the backing film 180 that does not contact the body 115. Alternatively, in embodiments where there is no backing film 180 the adhesive 185 can be attached to the non-smoothing side 192 of the body 115. The first or second adhesive 185, 187 can facilitate coupling of the pad 100 to the platen 194 of a smoothing tool.
In some preferred embodiments, the backing film 180 comprises, and in some cases, consists essentially of, a soft and pliable material such as EVA-PO or LLDPE. Such materials are desirable because they are conducive to forming a strong thermal weld 185 with the foamed EVA block copolymer 125. Such materials are also desirable because they do not substantially increase the hardness of the smoothing pad 100 as compared to the hardness of the smoothing body 115 alone.
In some preferred embodiments, the backing film 180 has a thickness 188 of about 2 mil or less. Such low thicknesses 188 as compared to the smoothing body 115 are desirable because it renders the hardness of the backing film 180 unimportant to the overall hardness of the smoothing pad 100. More generally, when the ratio of a thickness 190 of the smoothing body 115 to the thickness 188 of the backing film 180 is about 35:1 or greater. E.g., in some embodiments the smoothing body 115 has a thickness 190 of about 70 mil and the backing film 180 has a thickness of about 2 mil. In some cases, however, it is desirable for the smoothing pad's total thickness 193 to not exceed about 100 mil because, e.g., of constrains on the thicknesses that can be used in a smoothing apparatus or that can be accommodated in pad manufacturing equipment. Thus, in some embodiments, the ratio of the thickness 190 of the smoothing body 115 to the thickness 188 of the backing film 180 ranges from about 35:1 to 50:1. When such thickness ratios are present, then the hardness of the material that the backing film 180 is composed of has no substantial effect on the hardness of the pad 100. That is, the hardness of the smoothing pad 100 is dominated by the hardness of the smoothing body 115.
Consider embodiments of the smoothing pad 100 that comprise a smoothing body 115 having a thickness 190 of about 70 mil that is thermally welded to a 2 mil thick EVA-PO backing film 180 and a pressure sensitive adhesive 187 having a thickness 196 of about 10 mil. The hardness of the pad 100 (tested on the smoothing body's surface 165) has a Shore A hardness that ranges from about 30 to about 50, and in some cases, a Shore A hardness of about 35 to about 41.
Yet another embodiment of the present invention is a method of preparing a bare semiconductor wafer. FIG. 2 presents an example flow diagram of a method 200 of preparing the bare semiconductor wafer. As illustrated, in FIG. 2 the preparation step 200, occurs as part of an integrated circuit fabrication process 202, between a step 204 to slice a semiconductor crystal to form a wafer and a step 206 of forming integrated circuit components in or on the wafer.
The method of preparing the wafer 200 comprises a step 210 of smoothing a bare semiconductor wafer. Any embodiments of the smoothing pad such as described above in the context of FIG. 1 can be used in the smoothing step 210. The smoothing step 210, in turn, includes a step 220 of situating a smoothing pad adjacent to a surface of the bare semiconductor wafer, and a step 230 of moving the smoothing pad relative to the bare semiconductor wafer. Situating the smoothing pad according to step 210 can include coupling the smoothing pad to the platen and the wafer to a carrier head of a smoothing apparatus, which can be a conventional chemical mechanical polishing apparatus, such as discussed in the 690' application or the U.S. Pat. No. 6,838,169 patent. Movement in accordance with step 230 can include repetitive (circular) or random lateral movements of either the pad or the wafer, or both, as well as applying a force between the pad and the wafer (e.g., a down-force of about 5 psi in some cases). The moving step 230 is continued until the surface of the wafer has a roughness of less than or equal to about 20 Angstroms. In some cases the roughness ranges from about 2 to about 20 Angstroms.
It is preferable that the smoothing step 210 does not inadvertently remove substantial amounts of material from the wafer because this increases the possibility of nonplanarities being introduced into the wafer. E.g., in some preferred embodiments, less than about 1.5 microns/min of the bare semiconductor wafer is removed during the moving step 210, and a planarity of the bare semiconductor wafer after the smoothing step 210 has a TIR of about 0.15 microns or less. More preferably, less than about 0.3 microns/min of the bare semiconductor wafer is removed during the moving step 210, and a planarity of the bare semiconductor wafer after the smoothing step 210 has a TIR of about 0.11 microns or less.
Several experimental smoothing pads were fabricated and tested for their ability to perform the smoothing step 210. One pad (designated Pad-A) comprised a smoothing body (e.g., body 115 in FIG. 1) having a thickness (e.g., thickness 190 in FIG. 1) of about 70 mil and no backing film. Thus, the total pad thickness was about 70 mil (e.g., thickness 193 in FIG. 1). The smoothing body was made of an EVA block copolymer (e.g., EVA block copolymer 125 in FIG. 1) having a density of about 0.29 μm/cm³(about 18 lb/ft³), corresponding to a hardness ranging from about 40 to about 75 Shore A. The smoothing body also contained filler particles (about 15 wt %; e.g., particles 130 in FIG. 1) made of Silicon Dioxide (silica) and having an average size of about 11 micron. The smooth step 210 for Pad-A, and subsequently described pads, comprised a down force of about 5 psi. It was found that Pad-A smoothed a bare silicon wafer to a surface roughness of about 20 Angstroms or less (root-mean-square deviation). However, the presence of nonplanarities and scratches on the bare wafer's surface were noted.
Another pad (designated Pad-B) comprised the same thickness smoothing body plus a 32 mil backing film thermally welded to the smoothing body (e.g., total pad thickness was about 72 mil). An adhesive (e.g., adhesive 187 in FIG. 1) was coupled to the backing film (e.g., backing film 180 in FIG. 1). The smoothing body was made of an EVA block copolymer having a density of about 0.13 gm/cm³(about 8 lb/ft³), corresponding to a hardness ranging from about 20 to about 40 Shore A. The smoothing body contained the same amount and type of filler particle as described for Pad-A. The backing film was made of HDP and had a density of about 0.96 gm/cc. It was found that Pad-B smoothed a bare silicon wafer to a surface roughness of about 20 Angstroms or less. However, the presence of nonplanarities and scratches on the bare wafer's surface were noted.
Still another pad (Pad-C) comprised a smoothing body of the same thickness, EVA block copolymer and filler particles as described above for Pad-B, but with no backing film. Pad-C also smoothed a bare silicon wafer to a surface roughness of about 20 Angstroms or less. There were less nonplanarities associated with Pad-C as compared to Pad-B, although surface scratches were still present. Additionally, it was found that the adhesion between the pad and an adhesive (e.g., adhesive 185 in FIG. 1) failed sooner than desired (e.g., after smoothing less than about 100 wafers). The adhesion between the adhesive and the platen of the smoothing tool did not fail.
Yet another pad (Pad-D) comprised a smoothing body of the same thickness, EVA block copolymer as described above for Pad-B, except that the smoothing body was substantially free of 1 micron or greater filler particles, and there was no backing film. Pad-D also smoothed a bare silicon wafer to a surface roughness of about 20 Angstroms or less. Again, there were less nonplanarities associated with Pad-D as compared to Pad-B. Additionally, there were less surface scratches as compared to Pad A through Pad C. Similar to Pad-C, however, it was found that Pad-D's adhesion to adhesive failed sooner than desired (e.g., after smoothing less than about 100 wafers).
Another pad (Pad-E) comprised a smoothing body as described above for Pad-D plus a 2 mil thick backing film made of non-foamed EVA-PO. Pad-E was free of groves. The non-foamed EVA-PO had a density of about 0.951 g/cm³, and Shore A hardness of about 86. Pad-E also smoothed a bare silicon wafer to a surface roughness of about 20 Angstroms or less. Like Pad-D, there were less nonplanarities and surface scratches as compared to Pad-B. Additionally, it was found that Pad-E had improved adhesion to the adhesive, as indicated by longer times until failure (e.g., after smoothing greater than about 1500 wafers).
As further illustrated in FIG. 2, the method 200 can further comprise a first polishing step 240 and a second polishing step 250. As illustrated in FIG. 2 the first polishing step 240 is performed before the smoothing step 210. Those of ordinary skill in the art would be familiar with the conventional types of polishing pads, and polishing methods that can be used to planarize the wafer in step 240. One skilled in the art would appreciate that the method of preparation 200 could include several additional polishing steps before the smoothing step 210 as well as one or more cleaning steps after the smoothing step 210.
Analogous to that described for the smoothing step 210, the first polishing step 240 can comprise situating a polishing pad adjacent to the surface of the bare semiconductor wafer (step 242) and moving the polishing pad relative to the bare semiconductor (step 245).
As also illustrated in FIG. 2, in some embodiments there can be a second polishing step 250 performed after the smoothing step 210. Typically, the second polishing step 250 is less aggressive than the first polishing step 240, and is configured to make relatively small changes to the wafer's finish. The second polishing step 250 can comprise situating a polishing pad adjacent to the surface of the bare semiconductor wafer (step 252) and moving the polishing pad relative to the bare semiconductor (step 255).
In some preferred embodiments, the bare wafer polished in step 240, in step 250, or both, use a pad that is different than the smoothing pad used in step 210, because the goals of wafer polishing and wafer smoothing are different. E.g., it can be disadvantageous to use a pad containing hard particles and higher hardness materials (e.g., material having a Shore A hardness of greater than about 40 in some cases of silicon wafer smoothing) for both the polishing and smoothing steps 210, 240, 250 because the hard particles in the pad, or the hard pad material itself, can promote the production of scratches or introduce roughness and nonplanarities during the smoothing step 210.
Yet another embodiment of the present invention is a method for manufacturing a smoothing pad. Turning to the example flow diagram depicted in FIG. 3, the method 300 comprises placing into a container: an EVA block copolymer in step 310, an optional colorant in step 320, and a foaming agent in step 330.
The EVA block copolymer can comprise any of the embodiments described above. Copolymerisation products of ethylene with vinyl acetate and can be produced by any conventional process well know to those skilled in the art, including bulk continuous polymerization or solution polymerisation. As noted above, the EVA block copolymer is substantially free of particles having an average size of about 1 micron or greater. The optional colorant can include submicron particles such as about 0.3 micron titania particles.
In some cases the foaming agent can comprise a heat-activated foaming agent, such as azodicarbonamide, preferably combined with a catalyst such as zinc oxide. In some preferred embodiments, for example, azodicarbonamide and zinc oxide are heat activated at a temperature ranging from about 149 to about 165° C. In other cases the foaming agent comprises a blowing agent, such as nitrogen or fluorocarbon gas. In still other embodiments, the foaming agent comprises a combination of heat-activated foaming agent and blowing agent.
The method for preparing the smoothing pad comprises a step 340 of mixing together the EVA block copolymer, foaming agent and optional colorant. Any pair or all three of the EVA block co-polymer, colorant and foaming agent can be mixed together before or after being placed in the container.
After performing steps 310 through 340, in certain preferred embodiments, the EVA block co-polymer is cross-linked in step 350. In some cases cross-linking is carried out after the mixing step 340 and at substantially the same time that the closed cells are being formed as further discussed below. It is desirable for cross-linking to be done on the EVA block co-polymer when heated to a molten state so that EVA block co-polymer has a substantially amorphous structure. For example, in some preferred embodiments, heating the EVA block co-polymer to a molten state and heat activating a foaming agent are carried out at the same time.
Preferred cross-linking agents used in step 350 are organic peroxide, and more preferably dialkyl peroxides, such as dicumyl peroxide or a,a′-bis(t-butylperoxy)diisopropylbenzene, commercially available as Di-Cup® and Vul-Cup®, respectively (Geo Specialty Chemicals, Cleveland, Ohio). In some preferred embodiments the cross-linking agent is heat activated at about 149 to about 165° C. Of course, other conventional cross-linking agents well known to those skilled in the art could be used. One skilled in the art would be familiar with the amounts of cross-linking agent and procedures to prepare the cross-linked EVA block copolymers. Preferably, the cross-linked EVA block copolymer has a gel fraction ranging from about 60% to about 95%.
Closed cells are formed, in step 360, throughout the EVA block copolymer to produce a closed-cell thermoplastic foam smoothing body. Any conventional foaming process well known to those of ordinary skill in the art can be used to form the closed cells. For instance, a heat-activated foaming agent can be decomposed in step 362 by heating with a catalyst to produce foaming gases such as carbon monoxide and nitrogen gas. Alternatively, in step 364, a blowing agent, such as carbon dioxide or nitrogen gas, can be introduced into the thermoplastic smoothing body, preferably under pressure in a closed container. In still other instances, forming the closed-cell thermoplastic foam smoothing body comprises both steps 362 and 364.
In some preferred embodiments, forming the closed-cell thermoplastic foam smoothing body is at least partially carried out while the container is open, in step 370. In some cases, for example, EVA block copolymer, foaming agent, and optional cross-linking agent, are added to the container in steps 310 to 340, the container is closed, and then the container is heated to a temperature ranging from about 149 to about 165° C. The increased pressure inside the container substantially suppresses the formation of cells. After a suitable period to allow cross linking of the EVA block copolymer and heat activation of the foaming agent as per step 362, the container is opened. The reduced pressure associated with opening the container in step 370 allows gases from the foaming agent to expand, resulting in the formation of closed cells to their final dimensions.
Surprisingly, thermoplastic foam smoothing bodies whose closed-cells that are at least partially formed in an open container, particularly when using a heat activated foaming agent, have superior smoothing parameters as compared to thermoplastic foam smoothing bodies made in a closed container. While not limiting the scope of the present invention by theory, it is thought that superior smoothing parameters of such a smoothing body are due to the formation of closed cells having uniform dimensions. The advantageous use of an open container is surprising because it is expected that the use of a closed container would provide batches of closed-cell thermoplastic foam smoothing bodies having more uniform properties.
It is still within the scope of the present invention, however, for closed-cell formation to be carried out entirely in closed containers, in step 375. As an example, in some embodiments the EVA block copolymer, filler particles, foaming agent, and optional cross-linking agent, are added to the container in steps 310 to 340 and the closed container is heated to a temperature ranging from about 149 to about 165° C. After a time sufficient to allow cross linking of the EVA block copolymer and heat activation of the blowing agent, the contents of the container are transferred to a second container having a larger volume than the first container and subject to continued heating at the above-cited temperature range. The extended period of heating and lower pressure in the second container, as a result of the larger volume of the second container as compared to the first container, allow gases from the blowing agent to expand resulting in formation of closed cells in step 375. One skilled in the art would understand that step 375 could further include heating to other temperature ranges, or the use of a plurality of containers to facilitate the formation of the closed-cells.
In certain optional embodiments, the method 300 can further comprise, in step 380, exposing cells within the closed-cell thermoplastic foam smoothing body to form a surface comprising concave cells. The surface of concave cells can be formed by skiving the closed-cell thermoplastic foam substrate. The term skiving as used herein is defined as any process to a cut away a thin layer of the surface of the substrate so as to expose concave cells within the substrate. Skiving can be achieved using any conventional technique and device well known to one of ordinary skill in the art. In some cases exposing comprises fixing the thermoplastic foam substrate on a planar surface and cutting away a thin layer of the closed-cell thermoplastic foam to provide a thermoplastic foam smoothing body ranging in thickness from about 1000 microns to about 3000 microns, and in some cases, from about 70 mil to 100 mil (about 1778 to 2540 microns).
In other optional embodiments, the method 300 also comprises, in step 390, coupling the thermoplastic foam substrate to a backing film, such as HDP, EVA-PO or LLDPE thermoplastics as described above, and having a thickness of about 2 mil or less. In certain preferred embodiments, direct coupling 390 is achieved by thermal welding the thermoplastic backing and thermoplastic foam substrate together. In other embodiments, coupling is indirect via chemical bonding using a conventional adhesive, such as epoxy or other materials well known to those skilled in the art.
Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the scope of the invention.

Claims

1. A smoothing pad for bare semiconductor wafers, comprising:

a smoothing body including:

a closed-cell thermoplastic foam comprising an ethylene vinyl acetate block copolymer having a vinyl acetate content ranging from about 1 to about 18 wt %,

wherein said smoothing body is substantially free of particles having an average size of greater than about 1 micron.

2. The pad as recited in claim 1, wherein a surface of said smoothing body is free of grooves.

3. The pad as recited in claim 1, wherein said smoothing body is substantially free of particles having an average size of about 0.3 microns or greater.

4. The pad as recited in claim 1, wherein an interior of said closed-cell thermoplastic foam smoothing body comprise cells having an average size ranging from about 50 to about 300 microns, and a smoothing surface of said closed-cell thermoplastic comprises concave cells having said average size.

5. The pad as recited in claim 1, wherein said ethylene vinyl acetate block copolymer has a vinyl acetate content ranging from about 6 to about 12 wt %.

6. The pad as recited in claim 1, wherein said ethylene vinyl acetate block copolymer has a melt index ranging from 0.2 to about 25 dg/min (ASTM D1238).

7. The pad as recited in claim 1, wherein said closed-cell thermoplastic foam smoothing body has a hardness ranging from about 20 to about 75 Shore A.

8. The pad as recited in claim 1, wherein said closed-cell thermoplastic foam smoothing body has a gel fraction ranging from about 60 to about 90 percent.

9. The pad as recited in claim 1, wherein said thermoplastic foam smoothing body has a density ranging from about 0.1 to about 0.4 g/cm³.

10. The pad as recited in claim 1, further including a backing film coupled to said smoothing body by a thermal weld.

11. The pad as recited in claim 10, wherein said backing film has a thickness of about 2 mil or less.

12. The pad as recited in claim 10, wherein said backing film comprises a condensed ethylene vinyl acetate polymer.

13. The pad as recited in claim 1, wherein a ratio of a thickness of said smoothing body to a thickness of a backing film coupled to said smoothing body is about 35:1 or greater.

14. A method of preparing a bare semiconductor wafer, comprising:

smoothing a bare semiconductor wafer including:

situating a smoothing pad adjacent to a surface of said bare semiconductor wafer; and

moving said smoothing pad relative to said bare semiconductor wafer until said surface has a roughness of less than or equal to about 20 Angstroms, wherein said smoothing pad includes a smoothing body having:

a closed-cell thermoplastic foam composed of an ethylene vinyl acetate block copolymer having a vinyl acetate content ranging from about 1 to about 18 wt % and said smoothing body is substantially free of particles having an average size of greater than about 1 micron.

15. The method as recited in claim 14, wherein said surface roughness after said smoothing ranges from about 2 to about 20 Angstroms.

16. The method as recited in claim 14, wherein less than about 1.5 microns/min of said bare semiconductor wafer is removed during said smoothing and a planarity of said bare semiconductor wafer after said smoothing has a TIR of about 0.15 microns.

17. The method as recited in claim 14, further comprising polishing said bare semiconductor wafer, including:

situating a polishing pad adjacent to said surface of said bare semiconductor wafer; and

moving said polishing pad relative to said bare semiconductor,

wherein said polishing pad is different than said smoothing pad.

18. A method of manufacturing a pad for smoothing bare semiconductor wafers, comprising:

placing an ethylene vinyl acetate block copolymer and a foaming agent into a container, wherein said ethylene vinyl acetate block copolymer having a vinyl acetate content ranging from about 1 to about 18 wt % and said ethylene vinyl acetate block copolymer is substantially free of particles having an average size of about 1 micron or greater;

mixing said ethylene vinyl acetate block copolymer and said foaming agent together; and then forming closed cells throughout said ethylene vinyl acetate block copolymer to thereby produce a closed-cell thermoplastic smoothing body.

19. The method as recited in claim 18, further comprising thermally welding said closed-cell thermoplastic foam smoothing body to a backing film.

20. The method as recited in claim 19, wherein said backing film is composed of a non-foamed ethylene vinyl acetate polymer and has a thickness of about 2 mil or less.