Stand-off pad for supporting a wafer on a substrate support chuck

A stand-off pad, and method of fabricating the same, for supporting a workpiece in a spaced apart relation to a workpiece support chuck. More specifically, the wafer stand-off pad is fabricated of a polymeric material, such as polyimide, which is disposed upon the support surface of the chuck. The stand-off pad maintains a wafer, or other workpiece, in a spaced apart relation to the support surface of the chuck. The distance between the underside surface of the wafer and the chuck is defined by the thickness of the stand-off pad. This distance should be larger than the expected diameter of contaminant particles that may lie on the surface of the chuck. In this manner, the contaminant particles do not adhere to the underside of the wafer during processing and the magnitude of the chucking voltage is maintained between the workpiece and the chuck.

BACKGROUND OF THE DISCLOSURE
 1. Field of the Invention
 The invention relates to a substrate support chuck within a semiconductor
 processing system. More particularly, the invention relates to a stand-off
 pad disposed upon the surface of a substrate support chuck for supporting
 a semiconductor wafer such that the surface of the wafer that faces the
 chuck is spaced-apart and substantially parallel to the surface of the
 chuck.
 2. Description of the Background Art
 Substrate support chucks are widely used to support substrates within a
 semiconductor processing system. A particular type of chuck used in
 high-temperature semiconductor processing systems, such as
 high-temperature physical vapor deposition (PVD), is a ceramic
 electrostatic chuck. These chucks are used to retain semiconductor wafers,
 or other workpieces, in a stationary position during processing. Such
 electrostatic chucks contain one or more electrodes imbedded within a
 ceramic chuck body. The ceramic material is typically aluminum-nitride or
 alumina doped with a metal oxide such as titanium oxide (TiO.sub.2) or
 some other ceramic material with similar resistive properties. This form
 of ceramic is partially conductive at high temperatures.
 In use, a wafer rests flush against the surface of the chuck body as a
 chucking voltage is applied to the electrodes. Because of the conductive
 nature of the ceramic material at high temperatures, the wafer is
 primarily retained against the ceramic support by the Johnsen-Rahbek
 effect. Such a chuck is disclosed in U.S. Pat. No. 5,117,121 issued May
 26, 1992.
 One disadvantage of using a chuck body fabricated from ceramic is that,
 during manufacture of the support, the ceramic material is "lapped" to
 produce a relatively smooth surface. Such lapping produces particles that
 adhere to the surface of the support. These particles are very difficult
 to completely remove from the surface. Additionally, the lapping process
 may fracture the surface of the chuck body. Consequently, as the chuck is
 used, particles are continuously produced by these fractures. Also, during
 wafer processing, the ceramic material can abrade the wafer oxide from the
 underside of the wafer resulting in further introduction of particulate
 contaminants to the process environment. During use of the chuck, the
 particles can adhere themselves to the underside of the wafer and be
 carried to other process chambers or cause defects in the circuitry
 fabricated upon the wafer. It has been found that tens of thousands of
 contaminant particles may be found on the backside of a given wafer after
 retention upon a ceramic electrostatic chuck.
 Japanese Patent Application No. 60-261377, published Dec. 24, 1985,
 discloses a ceramic electrostatic chuck having an embossed support
 surface. The embossing reduces the surface area of the ceramic support
 that contacts the wafer. Consequently, the number of contaminant particles
 transferred to the wafer is reduced. However, such an embossed surface
 maintains some degree of contact between the ceramic material and the
 underside of the wafer. Thus, contamination, though reduced, is still
 substantial.
 Similarly, substrate support chucks that are used in low-temperature
 processing (e.g., less than 300 degrees Celsius) may also produce
 contaminant particles that interfere with wafer processing. Such
 low-temperature chucks include electrostatic chucks and mechanical
 clamping chucks which contain wafer support surfaces that are typically
 fabricated from dielectric materials such as alumina. These types of
 chucks have also been found to produce particulate contaminants that can
 adhere to the underside of the wafer during processing.
 Therefore, a need exists in the art for an apparatus that reduces the
 amount of contaminant particles that adhere to the underside of the wafer
 while supported upon a chuck.
 SUMMARY OF THE INVENTION
 The disadvantages of the prior art are overcome by the present invention of
 a stand-off pad for supporting a wafer, or other workpiece, in a
 spaced-apart relation to a chuck, or other workpiece support. More
 specifically, the invention is a stand-off pad disposed upon the support
 surface of the chuck. The material of the stand-off pad has superior
 contact properties as compared to the chuck surface material including
 being less abrasive and more compliant. The stand-off pad may be
 fabricated from polymeric materials such as polyimide, fluoropolymers, and
 the like.
 The stand-off pad maintains a wafer, or other workpiece, in a spaced apart
 relation to the support surface of the chuck. The distance between the
 underside surface of the wafer and the chuck is defined by the thickness
 of the stand-off pad. This distance should be larger than the expected
 diameter of contaminant particles that may lie on the surface of the
 chuck. In this manner, the contaminant particles do not adhere to the
 underside of the wafer during processing.
 In a specific embodiment of the invention, the wafer stand-off pad is
 comprised of a plurality of individual support pads (islands). The islands
 are fabricated by dispensing a solution of polymeric material using a drop
 dispenser on the surface of the chuck, then drying and curing the
 material.
 In another embodiment of the invention, the wafer stand-off pad is formed
 by spin coating a polymer material onto the chuck and then selectively
 etching unwanted polymer material using an etch mask, and the like. The
 stand-off pad may also be fabricated by forming a pattern that is die cut
 from a sheet of polymeric material to yield a web pattern, i.e., a
 plurality of islands interconnected by connector strips. The stand-off pad
 may also be a predefined pattern such as a plurality of spaced-apart pads,
 radial strips, concentric rings, or a combination of radial strips and
 concentric rings.
 In use, the web is placed on the ceramic surface or in a corresponding
 recess pattern formed in the surface of the chuck, and held thereon with
 an adhesive or by physical means (e.g., friction). With this
 configuration, the web can be removed for cleaning and replaced when worn.
 As a result of using the invention during processing of semiconductor
 wafers, the number of particulate contaminants adhered to the underside of
 a wafer after processing has been reduced from tens of thousands of
 particles to hundreds of particles. This substantial improvement in
 particle count has significantly decreased the number of wafers that are
 found defective during processing. Additionally, while using the invention
 with electrostatic chucks current leakage through the wafer and chuck has
 been reduced due to the insulating characteristics of the polymer material
 of the wafer stand-off pad.

DETAILED DESCRIPTION
 FIG. 1 depicts a cross-sectional view of a wafer stand-off pad 102 of the
 present invention supporting a wafer 106 above a the surface 114 of an
 electrostatic chuck 100. To illustrate the use of the invention, FIG. 1
 depicts the stand-off pad 102 supporting a semiconductor wafer 106. FIG. 2
 depicts a top plan view of an illustrative pattern for the stand-off pad
 102 of FIG. 1 (without the wafer 106). For best understanding of the
 invention, the reader should simultaneously refer to both FIGS. 1 and 2
 while reading the following disclosure.
 Although the preferred embodiment of the present invention is discussed as
 used in conjunction with a ceramic electrostatic chuck, the invention is
 also useful in supporting substrates above any form of chuck including
 non-ceramic electrostatic chucks, mechanical clamping chucks, and the
 like. One important feature of the invention is that the stand-off be
 fabricated from a polymeric material such as polyimide or a fluoropolymer
 that has contact properties that are different from the chuck material.
 In the preferred embodiment, the electrostatic chuck 100 contains one or
 more electrodes 116 imbedded within a ceramic chuck body 112. The ceramic
 chuck body is, for example, fabricated of aluminum-nitride or
 boron-nitride. Such a partially conductive ceramic material promotes the
 Johnsen-Rahbek effect which retains the wafer during high temperature
 processing. Other partially conductive ceramics also form useful high
 temperature chuck materials such as alumina doped with a titanium oxide or
 a chromium oxide. If the chuck is to be used at low temperatures only,
 then other ceramic and/or dielectric materials such as alumina are used to
 form the chuck body. An illustrative ceramic electrostatic chuck is
 disclosed in commonly assigned U.S. Pat. No. 5,511,799 issued Apr. 30,
 1996, herein incorporated by reference. Examples of non-ceramic
 electrostatic chucks are disclosed in U.S. Pat. No. 4,184,188 issued Jan.
 15, 1980 and U.S. Pat. No. 4,384,918 issued May 24, 1983, both of which
 are incorporated herein by reference.
 The stand-off pad 102 is comprised of a plurality of islands 206 positioned
 on the support surface 114 of the chuck 100. Typically, each island has a
 diameter of approximately 10-200 .mu.m, spaced equidistantly from one
 another and, depending upon the size and spacing of the islands, contact
 between 5% to 75% of the underside surface of the wafer. Preferably, the
 islands contact approximately 10% to 25% of the surface area of the wafer.
 Generally, the number, spacing and size of the islands is determined by
 the amount of clamping force applied by the electrostatic chuck. For
 example, if the amount of force is large and the islands are spaced
 relatively distant from one another, the wafer may bow between the
 islands. Consequently, for large clamping forces, the islands should
 either be relatively large or positioned near one another.
 FIG. 2 depicts a top plan view of a pattern for an illustrative stand-off
 pad. As depicted using solid lines, a plurality of individual islands 206
 collectively form the pad 102. Alternatively, the islands 206 are
 interconnected by connecting strips 202 and 204 (shown in phantom) to form
 a web 208. More specifically, the connecting strips are a plurality of
 concentric rings 202 and radially extending connector strips 204. The
 rings, for example, are spaced from one another by approximately 0.64 cm.
 Also, the rings and/or the radial strips could each be used separately as
 the wafer stand-off pad with or without islands 206.
 The key feature of the invention is that the wafer is supported in a
 spaced-apart relation to the surface of the chuck by a stand-off pad. The
 particular stand-off pad pattern and pad material is defined by the
 particular application for the chuck. chucking voltage, chucking force,
 wafer thickness, the chuck electrode pattern, the particular process that
 the wafer is to be subjected and the like, are such factors.
 Typically, the stand-off pad 102 is disposed upon the support surface 114
 of the chuck body 112 by dispensing a polymer solution using a drop
 dispenser. After dispensing the polymer solution, the polymer is dried and
 cured. This method produces the plurality of individual support pads
 (islands 206) that are permanently adhered to the support surface of the
 chuck.
 The stand-off pad may also be formed by spin coating the polymeric material
 onto the chuck surface. The coating of polymer may then be selectively
 etched to remove unwanted polymeric material and leave the stand-off pad
 on the support surface. The stand-off pad may be etched to form individual
 islands 206 or a web 208 of interconnected islands. Other methods such as
 decal transfer or stencil intaglio printing methods may also be used to
 form the stand-off pad.
 To produce the web 205, a pattern is die cut from a sheet of polymeric
 material. A stand-off pad having a web pattern does not require attachment
 to the chuck surface by an adhesive. As such, the web is easily removed
 from the surface of the chuck for cleaning or replaced by another
 stand-off pad when worn or otherwise damaged. Alternatively, the stand-off
 pad can be formed by dip coating a die-cut core of a thin metal sheet,
 such as aluminum, in a solution of a polymer, such as polyimide, dissolved
 in a solvent, such as N-methyl pyrrolidine (NMP). The metal core adds
 support to the web, aiding in its placement on and removal from the
 ceramic surface.
 The material of the stand-off pad has superior contact properties as
 compared to the surface material of the chuck. For example, the stand-off
 pad material is less abrasive and more compliant, i.e., produces less
 particles, than the surface material of the chuck. Furthermore, selecting
 a compliant material also prevents breakage of the wafer. During rapid
 wafer transport in the semiconductor processing system, wafers may break
 upon placement on the chuck. Preferably, the material chosen for the wafer
 stand-off pad is a material that absorbs the shock of the wafer placed on
 the stand-off pad. Typically, the material used to form the stand-off pad
 is a polymeric material such as polyimide or some other material that has
 similar properties, such as TEFLON.RTM. or other fluoropolymers.
 The pad has a pre-defined thickness that maintains the wafer 106, or other
 workpiece, above the support surface 114 such that particles 110 on the
 support surface do not contact the wafer surface. An illustrative
 thickness is approximately 50 microns. The stand-off pad is easily cleaned
 to ensure that any surfaces that contact the wafer 106 are substantially
 free of contaminants. Importantly, the contaminants tend to become trapped
 in the spaces 120 defined by the stand-off pad.
 To facilitate heat transfer from the wafer to the chuck body, a heat
 transfer medium, e.g., a gas such as helium, is pumped into the space 120
 between the backside surface of the wafer 108 and the support surface 114
 of the chuck body 112. This cooling technique is known as "backside
 cooling". The heat transfer medium is provided via a port 320 that is
 formed through the chuck body 112. The medium is typically supplied to the
 underside of the wafer at a rate of 2-30 sccm. The medium generally flows
 from the port 320 outward toward the edge of the wafer and escapes into
 the reaction chamber environment. Such backside cooling is well-known in
 the art and is disclosed, for example, in commonly assigned U.S. Pat. No.
 5,228,501, issued to Tepman et al. on Jul. 20, 1993. Importantly, when
 backside cooling is used, the wafer stand-off pad pattern has a dual
 purpose: (1) to support the wafer to reduce backside particle adherence
 and (2) to create heat transfer medium distribution channels upon the
 support surface of the chuck. However, additional heat transfer medium
 distribution channels (not shown) may be formed in the surface of the
 chuck body to further aid distribution of the heat transfer medium across
 the underside of the wafer 106. Such patterns of backside gas distribution
 channels vary in design and complexity, depending upon the application of
 the chuck.
 FIG. 3 depicts a cross-sectional view of a stand-off pad 102 of the present
 invention, disposed in a recess 302 formed in the surface of a chuck 114.
 Specifically, the recess 302 in the chuck surface is patterned to match
 the pattern of the pad 102. The recess 302 in the surface of the ceramic
 chuck has a depth less than the thickness of the wafer stand-off pad. The
 recess is milled, or otherwise formed, in the surface of the chuck.
 Preferably, the depth of the recess is 5-100 .mu.m less then the thickness
 of the wafer stand-off pad. As such, the stand-off pad projects above the
 surface of the chuck. Placing the stand-off pad in the recess aids in
 securing the stand-off pad to the chuck, and prevents movement of the
 stand-off pad during processing. The recessed pattern may also correspond
 to the backside gas distribution channels in the chuck surface.
 Using the stand-off pad in conjunction with a ceramic chuck has resulted in
 substantially decreased particulate contamination of wafers. Empirical
 data shows that a conventional ceramic chuck supporting a wafer directly
 upon its support surface can transfer tens of thousands of particles to
 the underside of a wafer. However, using the stand-off pad of the present
 invention reduces the particle count for particles located on the
 underside of a wafer to hundreds of particles. Importantly, the wafer
 stand-off pad does not significantly interfere with the clamping process
 or impact the clamping force that retains the wafer upon the chuck.
 Although various embodiments which incorporate the teachings of the present
 invention have been shown and described in detail herein, those skilled in
 the art can readily devise many other varied embodiments that still
 incorporate these teachings.