Conductive polymer pad for supporting a workpiece upon a workpiece support surface of an electrostatic chuck

This invention relates to an apparatus comprising a Johnsen-Rahbek electrostatic chuck having a conductive stand-off pad and a method of fabricating the chuck. More specifically, the stand-off pad is made of a conductive polymeric material, such as a polyimide, which is disposed upon a semiconducting or partially conducting layer of the chuck. The polymeric material has a controlled resistivity within a range of about 10.sup.7 -10.sup.12 ohm-cm, which allows a wafer, or other workpiece, to be supported and retained upon the electrostatic chuck via the Johnsen-Rahbek effect.

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 the use of 
conductive polymeric pads as part of an electrostatic chuck for retaining 
a wafer by the Johnsen-Rahbek effect. 
2. Description of the Background Art 
Substrate support chucks are widely used to support substrates within a 
semiconductor wafer processing system. A particular type of chuck used in 
high-temperature semiconductor wafer 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 semiconductive 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 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. 
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. 
A commonly assigned U.S. patent application Ser. No. 08/791,941, "Stand-Off 
Pad for Supporting a Wafer on a Substrate Support Chuck and Method of 
Fabricating Same", filed Jan. 31, 1997, overcomes the disadvantages of 
these prior art by the use of a polymeric stand-off pad, which supports a 
wafer in a spaced-apart relation to the chuck surface. Being less abrasive 
and more compliant than the chuck surface material, the polymeric pad 
significantly reduces particulate contamination. This commonly assigned 
patent application is hereby incorporated by reference. 
A Japanese laid-open patent application (Kokai) no. 63-194345 discloses an 
electrostatic chuck with sheets of a conductive resin material locally 
arranged on the surface of an insulating film or ceramic material. The 
capacitance between the wafer and the insulating film is reduced by the 
increased distance interposed by the thickness of the conductive resin 
layer. This leads to a chuck with improved charging and discharging time 
responses, which operates through electrostatic attraction, or coulombic 
forces, from the charge build-up on an electrode beneath the insulating 
material. 
However, the increased distance between the electrode and the wafer also 
leads to a correspondingly weaker electrostatic chucking force. Therefore, 
a need exists in the art for an electrostatic chuck that can avoid an 
abrasive contact with a wafer, reduce the amount of contaminant particles 
that may adhere to the backside of a wafer, and allow for a strong 
chucking force via the Johnsen-Rahbek effect. 
SUMMARY OF THE INVENTION 
The present invention relates to an electrostatic chuck having a stand-off 
pad made of a conductive polymeric material for retaining a substrate upon 
the chuck. With the conductive stand-off pad supporting a substrate, this 
invention offers the advantages of a non-abrasive wafer contact, reduced 
particulate contamination on the wafer backside, and a strong chucking 
force arising from the Johnsen-Rahbek effect. 
The polymeric material of the stand-off pad has superior contact properties 
as compared to the chuck body material, including being less abrasive and 
more compliant. Particle generation due to abrasive contact is therefore 
avoided. The conductive stand-off pad may be fabricated from polymeric 
materials such as polyimide, fluoropolymers, and the like, by adding 
semiconducting or other conducting species to a polymeric chain structure. 
By properly adjusting the amount of conducting species in the polymer, the 
resistivity can be controlled within a range of about 10 .sup.7 -10.sup.12 
ohm-cm which allows the chuck to operate under the Johnsen-Rahbek effect. 
Furthermore, the stand-off pad maintains a wafer, or other workpiece, in a 
spaced-apart relation to the underlying semiconducting chuck body, the 
backside of the wafer being separated from the chuck body by the thickness 
of the stand-off pad. Although the thickness of the stand-off pad is not 
critical to the present invention as it relates to the Johnsen-Rahbek 
force, it does affect a hybrid component of the total chucking force --a 
thicker stand-off pad results in a smaller hybrid component. The thickness 
of the stand-off pad should preferably be larger than the expected 
diameter of contaminant particles to avoid contaminant particles from 
adhering to the backside of the wafer during processing. 
In one embodiment of the invention, the plurality of islands forming the 
wafer stand-off pad are formed by drop dispensing a polymeric solution 
onto the chuck body, and allowing the polymer to dry and cure. In another 
embodiment, the stand-off pad is formed by spin coating a polymeric 
material onto the chuck body and then selectively etching unwanted polymer 
material using an etch mask, and the like. Alternatively, photopolymers 
may also be used in conjunction with an appropriate lithographic technique 
to form the stand-off pad. Furthermore, 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. Other pre-defined patterns such as a 
plurality of spaced-apart pads, radial strips, concentric rings, or a 
combination of radial strips and concentric rings may also be used. 
In another embodiment, the web is placed on the chuck body or in a 
corresponding recess pattern formed in the surface of the chuck body, and 
held in place with an adhesive or by other physical means (e.g., 
friction). This configuration facilitates the removal of the web for 
cleaning or replacement.

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 the surface 114 of an 
electrostatic chuck (ESC) 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 chuck body 112, the invention applies 
equally to a non-ceramic chuck body as well. A key feature of the present 
invention is that the polymeric pad 102 be made of a conductive material, 
with resistivity intermediate between that of an insulator and a 
conductor. Such intermediate resistivity allows the ESC 100 to operate via 
the Johnsen-Rahbek (J-R) effect, which provides a chucking force which is 
considerably stronger than that from electrostatic, or coulombic force 
alone. For example, a polymeric material with resistivity in the range of 
approximately 10.sup.7 -10.sup.12 ohm-cm will enable a workpiece or wafer 
substrate to be retained upon the chuck 100 by the Johnsen-Rahbek effect. 
This is several orders of magnitude lower than the resistivity typically 
used for an electrostatic chuck, which is about 10.sup.15 ohm-cm. 
Conductive polymers can be formed by adding conducting or semiconducting 
species to an otherwise non-conducting polymeric chain structure, such as 
polyimide, fluoropolymer and the like. For example, these conducting or 
semiconducting species may include carbon or silicon. By properly 
adjusting the amount of "dopants" in the polymeric chain, one can form 
conductive polymers with resistivities that can be controlled within 
certain desirable range, depending on the specific application needs. Note 
that although there is a certain temperature dependence in the resistivity 
of conducting polymers, the resulting resistivity change is typically less 
than an order of magnitude for the specific applications of interest, 
especially for applications below 300.degree. C., where the temperature is 
controlled to within a relatively narrow range. Therefore, the temperature 
effect on the resistivity will generally not affect the conductive 
polymer's functionality as applied to a Johnsen-Rahbek chuck. 
In a preferred embodiment, the electrostatic chuck 100 contains one or more 
electrodes 116 embedded within a ceramic chuck body 112. The ceramic chuck 
body 112 is, for example, fabricated of aluminum-nitride or boron-nitride. 
Such a partially conductive (semiconductive) ceramic material promotes the 
J-R effect which retains the wafer 106 during high temperature processing. 
Other semiconductive ceramics such as alumina doped with a titanium oxide 
or a chromium oxide also form useful high temperature chuck materials. If 
the chuck 100 is to be used at low temperatures only, then other ceramic 
and/or dielectric materials may be used, as long as the resistivity falls 
within a range appropriate for the J-R effect. 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. 
FIG. 2 depicts a top plan view of a pattern for an illustrative stand-off 
pad 102 made of a conducting polymeric material. As depicted using solid 
lines, a plurality of individual islands 206 collectively form the pad 
102. Typically, each island 206 has a diameter of approximately 1-10 mm, 
preferably 2-3 mm. They are spaced from one another and, depending upon 
the size and spacing of the islands, contact between 2% to 75% of the 
underside surface 108 of the wafer 106. Preferably, the islands 206 
contact approximately 5% to 60% of the surface area of the wafer 106. The 
number, spacing and size of the islands 206 are related to the amount of 
clamping force required. Since the Johnsen-Rahbek chucking force is 
directly proportional to the surface contact area between the stand-off 
pad 102 and the wafer 106, for large clamping forces, the islands 206 
should either be relatively large or positioned relatively densely near 
one another. Note that the thickness of the conductive polymeric pad 102 
does not affect the Johnsen-Rahbek force, although it may play a role in 
the total chucking force by contributing to a hybrid component, i.e., a 
combination of the Johnsen-Rahbek effect and the Coulombic effect. This 
hybrid force varies inversely with the thickness of the polymeric pad 102. 
Therefore, depending on the specific combination of thickness and 
resistivity of the polymer pad 102, the hybrid component may become 
comparable to the J-R force under certain circumstances. 
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 202, for example, are spaced 
from one another by approximately 0.64 cm. Furthermore, the rings 202 
and/or the radial strips 204 could each be used separately as the wafer 
stand-off pad 102 with or without islands 206. 
The key feature of the invention is that the wafer 106 is retained over the 
conductive polymeric pad 102 of an ESC 100 by the Johnsen-Rahbek effect. 
The particular stand-off pad pattern and pad material is defined by the 
specific application for the chuck 100. Factors to be considered include 
chucking voltage, chucking force, wafer thickness, the chuck electrode 
pattern, the process temperature, and so on. 
Typically, the stand-off pad 102 is disposed upon the top 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 top surface 114 of the 
chuck 
The stand-off pad 102 may also be formed by spin coating a polymeric 
material onto the ceramic chuck body 112 at a thickness of about 1-200 
.mu.m, and preferably about 5-10 .mu.m. The thickness, however, is not 
critical because it does not affect the Johnsen-Rahbek chucking force, 
although it may affect the hybrid component of the total chucking force. 
By using lithography and sputter etching, the spin-coated polymer layer 
may then be selectively etched to form the stand-off pad 102 upon the 
ceramic chuck body 112. For example, the stand-off pad 102 may be etched 
to form individual islands 206 or a web 208 of interconnected islands 206. 
Alternatively, photosensitive polymers may be used as the conducting 
polymeric material, and the stand-off pad 102 may be formed by an 
appropriate lithographic technique. Other methods such as decal transfer 
or stencil intaglio printing methods may also be used to form the 
stand-off pad 102. 
To produce the web 208, a pattern is die cut from a sheet of polymeric 
material. A stand-off pad 102 having a web pattern does not require 
attachment to the top surface 114 by an adhesive. As such, the web 208 is 
easily removed from the top surface 114 of the chuck body 112 for cleaning 
or replaced by another stand-off pad 102 when worn or otherwise damaged. 
Alternatively, the stand-off pad 102 can be formed by dip coating a 
die-cut core of a thin metal sheet, such as aluminum, in a solution of a 
conductive polymer, such as doped polyimide, dissolved in a solvent, such 
as N-methyl pyrrolidine (NMP). The metal core adds support to the web 208, 
aiding in its placement on and removal from the ceramic surface 114. 
Being less abrasive and more compliant, the polymeric pad 102 produces less 
particles than the ceramic surface 114 of the chuck body 112 upon contact 
with a wafer 106. A compliant material pad also minimizes breakage of the 
wafer 106 during rapid wafer transport upon placement on the chuck 100. In 
the present invention a doped polyimide is used to form the polymeric 
stand-off pad 102. Other compliant materials with similar conducting 
properties, i.e., resistivity in the range of approximately 10.sup.7 
-10.sup.12 ohm-cm may also be used to reduce particulate contamination 
which may otherwise arise from abrasive contact with the backside 108 of 
the wafer 106. 
To facilitate heat transfer from the wafer 106 to the chuck body 112, a 
heat transfer medium, e.g., a gas such as helium, is pumped into the 
space, or channel 120 between the backside surface 108 of the wafer 106 
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 220 that is formed through the chuck body 112. The medium is 
typically supplied to the underside 108 of the wafer 106 at a rate of 2-30 
sccm. The medium generally flows from the port 220 outward toward the edge 
of the wafer 106 and escapes into the reaction chamber environment. Inert 
gases such as helium and argon are suitable as a heat transfer medium. 
Such backside cooling is well-known in the art and is disclosed, for 
example, in a 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 
conductive polymer stand-off pad pattern has a three-fold purpose: (1) to 
support the wafer 106 to reduce backside particle adherence, (2) to 
provide wafer chucking from the J-R effect, and (3) to create heat 
transfer medium distribution channels upon the top surface 114 of the 
chuck body 112. Additional heat transfer medium distribution channels (not 
shown) may also be formed in the top surface 114 of the chuck body 112 to 
further aid distribution of the heat transfer medium across the underside 
108 of the wafer 106. Such patterns of backside gas distribution channels 
vary in design and complexity, depending upon the application of the chuck 
100. 
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 114 of the chuck 
body 112. Specifically, the recess 302 in the surface 114 is patterned to 
match the pattern of the pad 102. The recess 302, which is milled, or 
otherwise formed, in the surface 114 of the ceramic chuck 100, has a depth 
less than the thickness of the wafer stand-off pad 102. The depth of the 
recess 302 may be in the range of 5-200 .mu.m, and preferably 50-125 
.mu.m. As such, the conducting stand-off pad 102 projects above the 
surface 114 of the chuck body 112. Placing the stand-off pad 102 in the 
recess 302 aids in securing the stand-off pad 102 to the chuck 100, and 
prevents movement of the stand-off pad 102 during processing. The recessed 
pattern may also correspond to the backside gas distribution channels in 
the chuck surface 114. 
Using the stand-off pad 102 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. 
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.