Wafer carrier ring method and apparatus for chemical-mechanical planarization

The present invention discloses an improvement on a wafer carrier ring for use in a chemical-mechanical polishing apparatus for uniformly polishing semiconductor wafers. The apparatus comprises of a ring assembly, a stainless steel backing plate and a rubber bladder for holding the ring assembly and the backing plate. The ring assembly comprises of two rings. The first ring is made of a soft material such as Delrin or PBT for holding the stainless steel backing plate which is attached to the wafer. A top portion of the first ring is cutoff to leave an annular notch. The second ring is made of a hard material such as stainless steel and is fitted into the annular notch of the first ring. Both rings are attached to the rubber bladder through two sets of screws which are evenly spaced through a circular path concentric to the circumference of the first ring. The stainless steel material of the second ring improves the sealing to the rubber bladder and reduces the bending of the first ring during the polishing process. The results include reduced leakage from the sealing and reducing particle counts.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of semiconductor manufacturing and, 
more specifically, to an improved method and apparatus for a wafer carrier 
for chemical-mechanical planarization (CMP) usage. 
2. Description of Related Art 
Semiconductor devices are typically made up of several layers to improve 
density and reduce interconnection complexity. On each layer, devices such 
as Metal Oxide Semiconductor (MOS) transistors are formed on a silicon 
substrate. Metalized contacts and vias are formed by depositing metal 
materials such as aluminum at contact points. Layers stacked one upon 
another are interconnected through an elaborate interconnection scheme. 
Semiconductor manufacturing processes typically consist of several steps. 
One of the most important step is the planarization of the layers of the 
interconnection structure. Nonplanar surfaces create poor optical 
resolution of subsequent photolithographic processing steps, which in 
turn, prohibits the printing of high density lines. 
To ensure planar topography, various planarization techniques have been 
developed. One approach, known as chemical-mechanical planarization (CMP), 
employs polishing to remove excess materials formed on the surface of the 
semiconductor wafers. In a typical chemical-mechanical polishing method, 
as shown in FIG. 1a and FIG. 1b, a silicon substrate or wafer 100 is 
placed facing downward on an orbital motion table 110 covered with a flat 
polishing pad 120 which is coated with an active chemical mixtures 
commonly referred to as "slurry" 125. The wafer 100 is mounted upward to 
wafer pad 135 which is coupled to a backing plate 130. The backing plate 
130 and the wafer 100 are held by a wafer carrier ring 140 to prevent them 
from slipping laterally. Wafer carrier ring 140 is annular and is usually 
made of polymer such as PBT or Delrin. A rubber bladder 150 holds wafer 
carrier ring 140 and stainless steel backing plate 130. Wafer carrier ring 
140 is fastened to wafer carrier plate 260 by a set of screws 160.sub.I 
through 160.sub.P ("P" being a positive whole number). The rubber bladder 
150 is sandwiched in between wafer carrier ring 140 and wafer carrier 
plate 260 providing the sealing. The downward force F acting upon the 
backing plate 130 and the rotational movement of the polishing pad 120 
together with the slurry 125 facilitate the abrasive polishing and planar 
removal of the surface of the wafer 100. 
One major problem with the conventional CMP technique is the presence of 
defects that impact die yield and product reliability. If, for example, 
the pressure needed to ensure a quality polish process is too high, the 
wafer carrier ring could undergo extensive bending. With the extensive 
bending, the potential for defect generation is exacerbated since the 
over-stressed portion of the wafer carrier ring will be out of plane with 
the wafer, affecting slurry flow and causing wafer CMP polish 
non-uniformity. Another problem associated with the conventional CMP 
technique is that the sealing of the rubber bladder may be adversely 
affected during operation, resulting in leakage. To check for leakage, 
regular preventive and maintenance (PM) checks are regularly performed. PM 
times reduce equipment availability. 
Therefore, it is desirable to have an improvement on the 
chemical-mechanical polish head structure to reduce the bending and the 
leakage of the sealing. 
SUMMARY OF THE INVENTION 
A present invention relates to a ring assembly attached to a rubber bladder 
and configured to secure a wafer during a polishing process. The ring 
assembly comprises a plurality of rings. The first ring is made of a soft 
material, and holds a backing plate which is attached to the wafer during 
the polishing process. The second ring is made of a hard material, and is 
attached to the first ring and a wafer carrier plate for reducing leakage 
from the rubber bladder and for reducing bending of the first ring during 
the polishing process.

DESCRIPTION OF THE PRESENT INVENTION 
In the following description, for purposes of explanation, numerous details 
are set forth in order to provide a thorough understanding of the present 
invention. However, it will be apparent to one skilled in the art that 
these specific details are not required in order to practice the present 
invention. In other instances, well-known machines and chemical-mechanical 
process steps are not described in particular detail in order not to 
obscure the present invention unnecessarily. 
In general, the present invention discloses an improvement on the 
chemical-mechanical planarization ("CMP") technique. The wafer carrier 
ring is partially cut-off to reduce the thickness from the top. A 
stainless steel ring which acts as a seal ring is fitted into the cut-off 
portion of the wafer carrier ring. This stainless steel seal ring provides 
an air-tight seal for the wafer carrier ring and wafer carrier plate by 
minimizing ring deflection during the polishing process. 
Referring to FIG. 2, a cross-sectional side view of one embodiment of a 
tool assembly used for CMP is shown. The tool assembly 200 includes frame 
structure 202, ring assembly 201, backing plate 130, and rubber bladder 
230. Frame structure 202 includes wafer carrier plate 260, retaining ring 
245, and backing plate screw 250. Ring assembly 201 includes seal ring 
210, wafer carrier ring 240, and seal ring screw 220. Wafer carrier plate 
260, rubber bladder 230, seal ring 210 and wafer carrier ring 240 are 
fastened by a set of eight wafer carrier screws 160.sub.1 through 
160.sub.8 as shown in FIG. 4. In addition, stainless steel seal ring 210 
is fastened to wafer carrier plate 260 and rubber bladder 230 through a 
set of eight seal ring screws 220.sub.1 through 220.sub.8 as shown in FIG. 
4. 
The wafer carrier ring 240 is the annular ring that holds the stainless 
steel backing plate 130. Backing plate 130 is attached to the retaining 
ring 245 by backing plate screw 250. Rubber bladder 230 is positioned 
between wafer carrier plate 260 and the ring assembly, consisting of seal 
ring 210 and wafer carrier ring 240, to provide the sealing. 
An upper portion of wafer carrier ring 240 is removed by cutting off from 
the outer circumference to create an annular notch. Seal plate 210 is 
positioned on wafer carrier ring 240 in place of the removed portion. Seal 
plate 210 therefore is also an annular ring fitted on wafer carrier ring 
240. Seal plate 210 is preferably made of a hard material having a Knoop 
hardness value ranging from approximately 2000-7000. The hard material 
also has suitable chemical reactions during the polishing process. One 
such material is stainless steel. Wafer carrier ring 240 is preferably 
made of a soft material having a Rockwell R or Rockwell M hardness value 
between the ranges of approximately 90-150. Some examples of these soft 
materials are Polybutylene terephthalate (PBT), Delrin (acetyl plastic), 
Polyester (PET), and Poly ether ether ketone (PEEK). The soft material 
also has suitable chemical reactions during the polishing process. It 
should be appreciated that alternative materials having desirable 
properties, such as those described above, may be substituted without 
departing from the scope of the present invention. 
An important feature of the present invention is the attachment of the 
stainless steel seal ring 210 to the wafer carrier ring 240 to reduce 
leakage and bending. FIG. 3 provides a cross-sectional side view with 
specific dimensions for one embodiment of the present invention. Referring 
to FIG. 3, the length A of the annulus of the wafer carrier ring 240 is 
approximately 2.05" with tolerances .+-.0.005". The length B of the cutoff 
portion or the annular notch from the outer circumference is 1.545" to 
1.555" leaving the inner annular distance C of about 0.5". The inner 
thickness H of the wafer carrier ring 240 is approximately 0.370" with 
tolerances .+-.0.005". The outer thickness K of the wafer carrier ring 240 
is approximately 0.185" with tolerances +0.005" and -0.000". The drill 
clearance 225 is located approximately 1.000" with tolerance.+-.0.005" 
from the outer circumference. Drill clearance 225 has a diameter of about 
0.375" with depth sufficient to accommodate the head of seal ring screw 
220. The addition of the stainless steel seal ring 210 improves the 
sealing and reduces bending of wafer carrier ring 240. It is contemplated 
that these specific dimensions are for illustrative purposes of this 
embodiment. Of course, other dimensions may be used. 
Stainless steel seal ring 210 is fitted into the cut-off portion or the 
annular notch of wafer carrier ring 240. Stainless steel seal ring 210 has 
length L, from the outer circumference of the ring assembly, of 
approximately 1.550" with a thickness of approximately 0.185" to 0.180". A 
set of seal ring screws 220.sub.1 through 220.sub.8 are used to attach 
stainless steel seal ring 210 and rubber bladder 230 to wafer carrier 
plate 260. 
FIG. 4 shows the location of the set of eight wear ring screws 160.sub.1 
through 160.sub.8 and the set of eight seal ring screws 220.sub.1 through 
220.sub.8 as viewed from the top. Wafer carrier ring screws 160.sub.1 
through 160.sub.8 and seal ring screws 220.sub.1 through 220.sub.8 are 
alternately located on a circular path 400 and located evenly over the 
entire ring. In other words, wafer carrier ring screws 160.sub.1 through 
160.sub.8 are interleaved with seal ring screws 220.sub.1 through 
220.sub.8 at equal arc distances. The angular separation T, as measured 
from the center O of the ring, between two consecutive screws of the same 
type is 45 degrees. The angular separation U as measured from the center 
between two consecutive screws of different types is 22.5 degrees. The 
arrangement of the screws on the ring is to ensure the even distribution 
of the force during rotation and tightened sealing of the rubber bladder. 
The improvement as described in the present invention includes the 
placement of a stainless steel seal ring 210 on the cutoff portion of the 
wafer carrier ring 240. This placement of the stainless steel seal ring 
210 provides a number of advantages. 
The first advantage is that the seal ring 210 reduces leakage by improving 
the sealing effect. The stainless steel sealing ring 210 provides better 
sealing to the rubber bladder 230 than the wafer carrier ring 240 alone 
because of the metal-to-metal mounting as opposed to the delrin-to-metal 
mounting. The seal ring screws 220.sub.1 through 220.sub.8 attach the 
stainless steel sealing ring 210 with to the rubber bladder 230 to the 
wafer carrier plate 260 providing a tighter attachment. With better 
sealing, set-up parameters such as force and rotation speed can be 
accurately programmed providing better control of the equipment. 
The second advantage is that there are less checking and repairing steps 
thanks to reduced leakage. With the new seal ring and the rubber bladder 
being attached to the wafer carrier plate with seal ring screws 220.sub.l 
through 220.sub.8, the wafer carrier ring screws 160.sub.1 through 
160.sub.8 are not overtorqued because stainless steal seal ring 210 now 
holds the seal rather than the wafer carrier ring 240. The preventive and 
maintenance (PM) time is reduced and the system down time is reduced. 
Productivity is thus improved. In addition, once the seal is established, 
the wafer carrier ring can be removed without affecting the pressure and 
vacuum seal. 
The third advantage is that there is no bending of the wafer carrier ring 
240. Since wafer carrier ring 240 is made of soft material such as Delrin 
or PBT which has a small modulus of elasticity, it tends to bend down 
under the pressure of the force F during rotation. The bending causes the 
wafer carrier ring 240 to touch the surface of the polishing pad and the 
wafer carrier ring 240 may be ground away. This grinding of the Delrin or 
PBT material may generate extra particles and cause impurities added to 
the wafer. In the present invention, the addition of the stainless steel 
seal ring prevents bending because stainless steel has a large modulus of 
elasticity. Without bending, there are no excess particles deposited on 
the surface of the wafer resulting in a smoother surface. 
The advantages of the present invention are confirmed through extensive 
test data using the improved apparatus as described above. The main 
measurement for the improvement is the particle count. The particle count 
results are generated by scanning the wafer before and after the 
manufacturing process and computing the difference in particles as 
.DELTA.p=p.sub.2 -p.sub.1 where p.sub.1 and P.sub.2 are particle counts 
before and after the polish process. A negative value of .DELTA.p 
indicates that the polishing actually reduces the particle count. 
Table 1 shows the results of the overall average particle count within one 
standard deviation (.+-..sigma.). Referring to Table 1, the particle count 
is significantly reduced from an average of 3.84 to -589.8. In addition, 
both polish rate and polish rate range are unaffected. Consequently, the 
manufacturing process is essentially unchanged but particles are 
substantially reduced. 
TABLE 1 
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p1 (Avg .+-. .sigma.) 
p2 (Avg .+-. .sigma.) 
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Particles 3.84 .+-. 125.98 
-589.8 .+-. 372.1 
Tungsten Polish Rate 
3029.5 .+-. 138.5 
2952.2 .+-. 120.8 
Polish Rate Range 
855.7 .+-. 266.8 
1038.1 .+-. 298.1 
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Table 2 shows the results as a summary of particle data according to 
particle size. The numbers shown are differences in particle counts before 
and after the manufacturing process. There are three sizes: 0.25 .mu.m, 
0.5 .mu.m, and 1.0 .mu.m. The results are shown for three different 
systems using the improved apparatus of the present invention. In all 
three cases, the particle counts using the improved apparatus of the 
present invention are much less than the original apparatus. 
TABLE 2 
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Particle Size 
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Ring set-up 
0.25 .mu.m 0.5 .mu.m 1.0 .mu.m 
Original 13.41 .+-. 31.1 
11.93 .+-. 32.7 
-0.59 .+-. 2.98 
Ring 1 10.11 .+-. 9.1 
0.35 .+-. 2.17 
-2.24 .+-. 6.83 
Ring 2 11.93 .+-. 32.7 
-0.86 .+-. 2.84 
-1.35 .+-. 4.02 
Ring 3 10.24 .+-. 25.7 
-1.35 .+-. 3.97 
-0.58 .+-. 2.97 
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While this invention has been described with reference to illustrative 
embodiments, this description is not intended to be construed in a 
limiting sense. Various modifications of the illustrative embodiments, as 
well as other embodiments of the invention, which are apparent to persons 
skilled in the art to which the invention pertains are deemed to lie 
within the spirit and scope of the invention.