Polishing pad

A polishing pad for semiconductor wafers, having a face shaped to provide a constant, or nearly constant, surface contact rate to a workpiece such as a semiconductor wafer, in order to effect improved planarity of the workpiece. The favored face shape is a sunburst pattern having nontapered rays, coaxial with the pad's rotation.

BACKGROUND OF THE INVENTION 
1. 1 Field of the Invention 
This invention relates to the grinding or polishing of a workpiece, in 
particular the polishing of a semiconductor wafer surface to a high degree 
of planarity. 
2. Description of the Related Art 
In the manufacture if integrated circuits, for example, planarity of the 
underlying semiconductor substrate or wafer is very important. Critical 
geometries of integrated circuitry are presently in the neighborhood of 
less than 1 micron. These geometries are by necessity produced by 
photolithographic means: an image is optically or electromagnetically 
focused and chemically processed on the wafer. If the wafer surface is not 
sufficiently planar, some regions will be in focus and clearly defined, 
and other regions will not be defined well enough, resulting in a 
nonfunctional or less than optical circuit. Planarity of semiconductor 
wafers is therefore necessary. 
Chemical and mechanical means, and their combination (the combination being 
known as "mechanically enhanced chemical polishing"), have been employed, 
to effect planarity of a wafer. In mechanically enhanced chemical 
polishing, a chemical etch rate on high topographies of the wafer is 
assisted by mechanical energy. 
FIGS. 1a and 1b illustrate the basic principles used in prior art 
mechanical wafer polishing. A ring-shaped section of a polishing pad 
rotates at W.sub.P radians per second (R/s) about axis O. A wafer to be 
polished is rotated at W.sub.W R/s in the opposite sense. The wafer may 
also be moved in 
directions +X and -X relative to O, the wafer face being pressed against 
the pad face to accomplish polishing. The pad face may not itself be 
abrasive. Actual removal of surface material from the wafer is often 
accomplished by a mechanically abrasive slurry, which may be chemically 
assisted by an etchant mixed in with the slurry. 
FIG. 2 helps to clarify rotation W.sub.W and the ring shape of the pad in 
FIG. 1. For a generic circular pad rotating at W R/s, the linear speed of 
the polishing face at any given radius will vary according to the 
relationship L=W.times.R, where L is in cm/s for W in R/s and R in cm. It 
can be seen, for example, that linear speed L.sub.2 at large radius 
R.sub.2 is greater than linear speed L.sub.1 at small radius R.sub.1. 
Consider now that the pad has a surface contact rate with a workpiece that 
varies according to radius. Portions of a workpiece, such as a wafer, 
contacting the pad face at radius R.sub.1 experience a surface contact 
rate proportional to L.sub.1. Similarly, portions of the wafer contacting 
the pad face at radius R.sub.2 will experience a surface contact rate 
proportional to L.sub.2. Since L.sub.2 &gt;L.sub.1, it is apparent that a 
workpiece at radius R.sub.2 will receive more surface contact than a 
workpiece at radius R.sub.1. If a wafer is large enough in comparison to 
the pad to be polished at both R.sub.1 and R.sub.2, the wafer will be 
polished unevenly: the portions of the wafer at R.sub.2 will be polished 
faster than the portions of wafer at R.sub.1. The resulting non-planarity 
is not acceptable for high precision polishing required for semiconductor 
wafers. 
Referring again to the prior art of FIG. 1, a common approach by which 
prior art attempts to overcome non-uniform surface contact rate is by 
using a ring-shaped pad or the outer circumference of a circular pad, to 
limit the difference between the largest usable radius and smallest usable 
radius, thus limiting surface contact rate variation across the pad face, 
and by moving the wafer and negatively rotating it, relative to the pad 
and its rotation. The combination is intended to limit the inherent 
variableness of the surface contact rate across the wafer, thereby 
minimizing non-planarity. Such movement of the wafer with respect to the 
polishing pad's axis of rotation requires special gearing and design 
tolerances to perform optimally. 
It is an object of the present invention to provide a polishing pad capable 
of providing a substantially constant, radially independent surface 
contact rate, improving planarity of a workpiece polished thereby. 
SUMMARY OF THE INVENTION 
According to the invention, a polishing pad is provided, having its face 
shaped to provide a constant, or nearly constant, surface contact rate. 
The preferred embodiment is a rotatable circular pad having a face formed 
into sunburst pattern with nontapered rays. The sunburst pattern is 
coaxial with the pad's rotation. 
Alternate face patterns are also disclosed, each providing a constant 
surface contact rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 3 shows different embodiments of the invention. Quadrant II 
illustrates the preferred embodiment. With reference to FIGS. 3 and 4, a 
polishing pad face 25 is interrupted with voids 27. The voids 27 form the 
polishing pad face 25, which form it into rays 31, each having parallel 
edges 32 (nontapered). Rays 31 meet each other at radius R.sub.1, and 
continue outward to R.sub.O, as shown in quadrant I. 
Because rays 31 have parallel edges 32, a workpiece P that is stationary 
with reference to the polishing pad's axis of rotation 0 will experience 
the same surface contact rate at any radius R between R.sub.I and R.sub.O. 
Planarity across the finished surface of P is therefore obtainable without 
movement of workpiece P with respect to O, simply by pressing P against 
the pad face within the bounds of R.sub.1 and R.sub.O. 
Other embodiments are conceivable. Quadrant III of FIG. 3 shows grooves 33 
formed in the pad face such that a distance between any two grooves is 
oppositely related to the radius from O of the inner of the two 
grooves--that is, the distance between any two grooves decreases with 
increasing radius. The grooves so arranged are able to provide a constant 
surface contact rate between R.sub.I and R.sub.O. Two orthogonal series of 
parallel grooves are shown in quadrant III. 
As shown in quadrant IV, circular voids 37 govern the pad face to achieve 
the same inventive effect. The voids are formed in the pad face such that 
the size of any void is cooperatively related to its radius from O--that 
is, void size increases with increasing radius. 
I wish it to be understood that the term "polish" as used herein 
circumscribes abrasive activity such as grinding or polishing, by use of: 
slurry; abrasive grains embedded in the polishing pad face; chemical 
means; mechanically enhanced chemical polishing; any combination thereof. 
It should also be understood that I consider my invention to have utility 
with workpieces of varying constituency, including semiconductors (such as 
silicon, germanium, and Group III-V semiconductors such as gallium 
arsenide), and optical materials (such as glass), among others. Further, 
although only three face patterns are disclosed herein, I wish it to be 
understood that I consider my invention to include any polishing pad face 
pattern capable of providing a constant or nearly constant surface contact 
rate to a workpiece.