Method of producing polishing sheet material

A method of producing an abrasive backing or a polishing sheet material which has a nonwoven fabric of a synthetic fiber having a network structure and impregnated with a polyurethane solution. The method has the steps of completely coagulating and drying the polyurethane solution to prepare a base material, removing a skin layer from each surface thereof, dipping the resultant base material in an aqueous solution of a solvent for a short period of time, and treating it in a high-temperature atmosphere for a short period of time.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of producing a polishing sheet 
material suitable for use in polishing and finishing the surface of a 
semiconductor IC wafer, glass, metal and the like. 
A conventional method of producing the polishing sheet material or cloth 
comprises impregnating a nonwoven fabric with a polyurethane resin 
solution, dipping the impregnated fabric in a water bath or an aqueous 
solution of dimethylformamide so as to effect complete coagulation, 
washing the product with water, and drying the washed product. In the 
sheet material produced by the conventional method, the polyurethane resin 
has a porous structure, so that it is too soft, thereby causing the edges 
thereof to be "sagged" and suffer from flagging or sugging at the time of 
use, and so it has been impossible to obtain a wafer having sharp edges. 
Accordingly, an attempt has been made in order to strengthen the fiber 
network structure, which includes the steps of slicing the sheet material 
obtained by the above-described conventional method, removing the skin 
layer from each surface thereof, and performing high-temperature heat 
treatment so as to melt the polyurethane resin and to effect fusion around 
the fibers. Another attempt was made to use polyurethane resins having 
different melting temperatures to facilitate the above-described melting 
and fusion (see Japanese Patent Laid-Open No. 62-297061/1987). 
The present inventor conducted a study to obtain an improved polishing 
sheet material, and proposed a method of obtaining the polishing sheet 
material which comprises impregnating a nonwoven fabric of a synthetic 
fiber having a network structure with a polyurethane solution, performing 
incomplete coagulation of the same, removing any remaining solvent in an 
atmosphere heated at 75.degree..+-.5.degree. C., and performing complete 
coagulation, as disclosed in Japanese Patent Application No. 2-7921, filed 
Jan. 17, 1990. 
Experiments have been repeated in accordance with the method described in 
the above-described Japanese Patent Laid-Open No. 62-297061/1987, and 
observations have been made by means of scanning electron 
photo-micrographs. As a result, it has been found that there are a large 
number of residual microvoid structures in the resin portion present 
between fibers forming the skin layer portion of the material. Regarding 
the attempt to eliminate the microvoids by subjecting the same to 
high-temperature heat treatment so as to effect the melting and fusion of 
the resin, it had a limitation of its own attributed to the narrowness of 
the temperature control range for the high-temperature heat treatment. On 
the other hand, a considerable improvement has been achieved by the 
invention disclosed in the above-described Japanese Application No. 
2-7921. However, in respect of the prolongation of the life of the 
polishing sheet material which is an especially strong desire at present, 
it is unsatisfactory as yet. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improvement in 
prolongation of the life of the polishing sheet material. 
Another object of the present invention is to provide an improved polishing 
sheet material structure wherein the entire layers are free of microvoid 
structures as completely as possible and the polyurethane resin fixes 
together the synthetic fibers forming a network structure, with a resin 
film formed only around the synthetic fibers, to thereby readily obtain 
the sheet material exhibiting low Taber abrasion values and being suitable 
for polishing. 
A further object of the present invention is to provide a new method of 
producing the polishing sheet material, which can remove the drawbacks 
inherent to the conventional methods. 
Another object of the present invention is to provide an improved method of 
producing the polishing sheet material, which can eliminate completely the 
microvoids. 
According to the present invention, there is provided a method of producing 
the polishing sheet material which has a nonwoven fabric of a synthetic 
fiber having a network structure and impregnated with a polyurethane 
solution, comprising the steps of completely coagulating and drying the 
polyurethane solution to prepare a base material, removing a skin layer 
from each surface thereof, dipping the resultant base material in an 
aqueous solution of a solvent for a short period of time, and treating it 
in a high-temperature atmosphere for a short period of time. 
In another aspect of the present invention, after removal of the skin layer 
from each surface of the base material, there are provided steps of 
slicing the resultant base material into a plurality of sheets, dipping 
the sheets in an aqueous solution of a solvent for a short period of time, 
and treating the sheets in a high-temperature atmosphere for a short 
period of time. 
A preferable nonwoven fabric of a synthetic fiber having a network 
structure to be used in the present invention is one which is obtained by 
forming staple fibers of a synthetic fiber, such as those of polyamide, 
polyester, polyacrylonitrile or polypropylene, into a nonwoven fabric by 
means of the conventional dry nonwoven fabric manufacturing method and 
performing needle punching to maintain the shape of the nonwoven fabric. 
Of the fibers, polyester fibers which are insoluble in solvents and have 
excellent physical properties are most desirable. 
The nonwoven fabric is impregnated with a polyurethane solution. Examples 
of the polyurethane resin to be used for this purpose include polyester 
and polyether polyurethane resins which may be used individually or in 
combination. From the viewpoint of physical properties, a preferred 
polyurethane solution is one which is obtaied by dissolving a polyether 
polyurethane resin in a solvent, such as N,N-dimethylformamide, 
N,N-dimethylacetamide or methyl ethyl ketone. Although the resin 
concentration is arbitrary, it is desirably about 20% for the convenience 
of handling. 
The nonwoven fabric is completely impregnated with the above-described 
polyurethane solution until the polyurethane resin uniformly penetrates 
into the interior of the nonwoven fabric layer, and wrung by means of a 
wringer so as to attain a predetermined resin pickup. Subsequently, the 
nonwoven fabric impregnated with the polyurethane resin is dipped in hot 
water or water so as to effect complete solvent removal and complete 
coagulation. Thereafter, water is completely removed from the treated 
nonwoven fabric by using a conventional dryer, thereby preparing a base 
material. 
The skin layers are sliced off from both of the surfaces of the base 
material thus obtained in order to remove the nonuniform resin-adherent 
portion which is attributed to the transfer of the resin by the 
treatments, such as drying, to the skin layers of both surfaces of the 
base material. 
The base material having the skin layers of both of the surfaces thereof 
sliced off may optionally be sliced into a plurality of sheets having a 
thickness suitable for the intended specific application to obtain another 
form of the base material. The base material having the skin layers of 
both surfaces thereof removed or the base material split by slicing is 
then dipped in an aqueous solution of a solvent obtained by adding water 
to a solvent capable of dissolving the polyurethane resin, for example, 
one used in the above-described polyurethane solution. The solvent 
concentration of the aqueous solution is freely selected in accordance 
with the amount of the adherent polyurethane resin and the excellent 
properties to be realized for the polishing sheet material. The dipping 
time may also be freely selected. However, a long dipping time possibly 
causes the resin which is penetrated into the nonwoven fabric to be 
entirely dissolved out. Hence, too long dipping time is unfavorable. In 
the present invention, it is generally preferred that the dipping time be 
not greater than 10 seconds. After the dipping in the aqueous solution, 
the sheet material is promptly wrung by means of a wringer and then dried 
in a high-temperature atmosphere for a short period of time, thereby 
obtaining the polishing sheet material. It is not favorable that the 
temperature of the high-temperature atmosphere is elevated to such an 
extent that the resin is melted or the synthetic fiber forming the 
nonwoven fabric is melted. In the present invention, it is preferred that 
the temperature be about 150.degree. C., and that the time be about 30 
minutes. The above-described method provides an excellent polishing sheet 
material wherein the polyurethane resin firmly bonds the synthetic fibers 
together throughout the whole layer portions of the polishing sheet 
material and coats the surface of the synthetic fibers to thereby avoid 
the presence of residual microvoids or the like.

PREFERRED EMBODIMENTS OF THE INVENTION 
The present invention will now be described in more detail with reference 
to the following Examples, which however should not be construed to be 
limiting the present invention. 
The measurements of physical properties mentioned in the Examples were 
carried out by the following methods. 
(1) Density (g/cm.sup.3) 
From a sample, 50 mm wide edges were cut off to prepare three test pieces 
of 100.times.100 mm in size. The weight of each of the pieces was measured 
to calculate an average g/cm.sup.3 value from the measured weights. 
(2) Compressibility, elastic modulus in compression (%) 
Three test pieces each having a size of 2.times.2 cm were cut off. Using a 
YSS-system, Schopper-type of thickness meter manufactured by Yasuda Seiki 
Co., Ltd., Japan, the thickness t.sub.0 was measured after compression 
under a load of 100 g for 30 seconds. Subsequently, the thickness t.sub.1 
was measured after compression under a load of 100 g for 30 seconds. 
Subsequently, the thickness t.sub.1 was measured after continued 
compression under a load of 900 g for 5 minutes. 
Thereafter, the load of 900 g was removed, and each of the test pieces was 
allowed to stand still for 5 minutes without any load application. Again, 
100 g compression was applied for 30 seconds, and then the thickness 
t.sub.2 was measured. Calculation was performed by the following formulae. 
EQU compressibility=(t.sub.0 -t.sub.1 /t.sub.0).times.100 
EQU elastic modulus in compression=(t.sub.2 -t.sub.1 
/t.sub.0-t.sub.1).times.100 
(3) Hardness 
A durometer of Type A (manufactured by The shore Instrument & Mfg. Co.) was 
used, and four test pieces of 2.times.2 cm in size which were piled one 
upon another so as to have a thickness of 4.5 mm or more were disposed on 
a test mount, and a pressurized needle was caused to fall thereon from a 
25 mm height. Measurement was made at three points of the test pieces 
piled to calculate an average value. 
(4) Water vapor permeability (mg/cm.sup.2 /hr) 
Measured according to the Japanese Industrial JIS K-6549. The exployed 
sealing wax was composed of beeswax and paraffin in proportions of 60:40. 
(5) Taber abrasion (mg/1000 cycles) 
Tests were made by using a Taber abrasion tester munufactured by Toyo 
Measuring Instument Co., Ltd., Japan. The test piece was allowed to stand 
still until it assumed normal condition (room temperature: 
20.degree..+-.2.degree. C., humidity: 65.+-.2%). Prior to measurement, 
adherent fibers or the like were completely brushed off. The weight of the 
test piece A (A mg) was then measured by a chemical balance, and the test 
piece was mounted on the tester wherein the load was set at 500 g, 
abrasion wheel H-22 was applied and the revolution was set at 1000 cycles 
to conduct abrasion test. After the completion of the operation, the 
weight of the resultant test piece B (B mg) freed of the abrasion layer 
was measured. The testing was repeated twice and the results were 
calculated by the following formula to obtain an average value: 
EQU Taber abrasion=A-B 
EXAMPLE 1 
Use was made of nonwoven fabric having a thickness of 4.2 mm and an areal 
density of 880 g/m.sup.2 as prepared by laminating webs of 3-denier 
polyester fibers having a fiber length of 51 mm and performing needle 
punching at a rate of 700 needles per cm.sup.2. The nonwoven fabric was 
impregnated with a solution of an ether polyurethane resin (trade name: 
Sanprene, a product of Sanyo Chemical Industry Co., Ltd., Japan, average 
molecular weight: 2000, a dimethylformamide solution having a resin 
concentration of 20%), and wrung by means of a roll press at a clearance 
of 3.5 mm under a pressure of 3 Kg/cm.sup.2 so as to effect uniform 
impregnation. 
The resultant resin-impregnated nonwoven fabric was dipped in a water 
stream at ordinary temperature for 10 hours to effect complete solvent 
removal and complete coagulation. Subsequently, the resultant nonwoven 
fabric was taken out of the water, subjected to water washing and pressing 
three times, and dried in a dryer kept at 100.degree. C. for 2 hours. 
The skin layer of each of the two surfaces of the obtained 
resin-impregnated nonwoven fabric was removed in a thickness of 0.8 mm, 
totally 1.6 mm, and the resultant nonwoven fabric was sliced at the center 
thereof into two split resin-impregnated nonwoven fabrics having one half 
of the original thickness. From the resultant nonwoven fabrics, seven 
samples were taken, respectively dipped for 5 seconds in aqueous 
dimethylformamide solutions having dimethylformamide to water weight 
ratios of 1:1, 2:1, 3: 1, 4: 1, 5:1, 6:1 and 7 : 1. After the completion 
of the 5-second dipping, the samples were taken out, wrung by a roll press 
at a clearance of 3.5 mm under a pressure of 3 Kg/cm.sup.2, and treated 
in a dryer wherein a temperature atmosphere of 140.degree. C. was formed 
for 20 minutes. Thus, the polishing sheet material sample Nos. 1 to 7 were 
obtained. The amount of an adherent resin was 35%. With respect to the 
sample Nos. 1 to 7 and sample No. 8 which was obtained by the 
above-described splitting and not subjected to the dipping treatment in 
the aqueous dimethylformamide solutions, the physical properties were 
measured. The results are given in Table 1. The scanning electron 
photomicrographs depicting the surface conditions of sample Nos. 5 and 8 
are respectively shown in FIGS. 1 and 2. No microvoid is observed in the 
polishing sheet material produced by the method of the present invention. 
TABLE 1 
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Sample No. 
1 2 3 4 5 6 7 8 
______________________________________ 
Thickness 
1.38 1.33 1.37 1.40 1.38 1.35 1.33 1.40 
Density 0.38 0.39 0.38 0.38 0.38 0.34 0.32 0.38 
Compress- 
4.2 4.5 4.0 3.5 3.2 3.3 4.2 4.0 
ibility 
Elastic 76.5 79.6 80.1 82.0 84.6 84.4 80.8 74.6 
modulus in 
compres- 
sion 
Hardness 
74.3 75.7 76.7 78.0 79.8 79.3 74.2 73.8 
Water 12.4 12.5 13.3 13.3 13.4 13.2 14.2 9.7 
vapor 
perme- 
ability 
Taber 94.0 70.1 67.7 39.5 28.2 40.1 69.3 103.0 
abrasion 
______________________________________ 
EXAMPLE 2 
The polishing sheet material sample Nos. 9 to 15 were prepared in 
substantially the same manner as that of Example 1, except that the 
resin-impregnated nonwoven fabric was wrung by a roll press at a clearance 
of 2.7 mm under a pressure of 3 kg/cm.sup.2 while it was performed at a 
clearance of 3.5 mm under a pressure of 3 Kg/cm.sup.2 in Example 1. 
Regarding these polishing sheet materials, the amount of an adherent resin 
was 25%. The physical property measurement results of sample Nos. 9 to 15 
are as shown in Table 2. 
TABLE 2 
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Sample No. 
9 10 11 12 13 14 15 
______________________________________ 
Thickness 1.36 1.34 1.33 1.33 1.36 1.33 1.39 
Density 0.31 0.30 0.33 0.30 0.32 0.31 0.32 
Compressiblity 
6.0 6.5 6.1 6.3 6.3 6.2 6.5 
Elastic modulus 
71.9 75.7 74.4 76.1 78.9 73.9 72.4 
in compression 
Hardness 74.5 75.0 73.2 75.7 74.3 73.3 71.7 
Water vapor 
13.9 14.1 14.4 14.5 14.9 15.0 15.5 
permeability 
Taber abrasion 
102.6 94.6 87.8 73.8 71.6 85.4 96.0 
______________________________________ 
EXAMPLE 3 
The polishing sheet material sample Nos. 16 to 22 were prepared in 
substantially the same manner as that of Example 1, except that the 
resin-impregnated nonwoven fabric was wrung by a roll press at a clearance 
of 4.1 mm under a pressure of 3 Kg/cm.sup.2. Regarding these poliching 
sheet materials, the amount of an adherent resin was 45%. The physical 
property measurement results of sample Nos. 16 to 22 are as shown in Table 
3. 
TABLE 3 
______________________________________ 
Sample No. 
16 17 18 19 20 21 22 
______________________________________ 
Thickness 1.37 1.32 1.32 1.31 1.36 1.35 1.35 
Density 0.42 0.43 0.41 0.41 0.40 0.41 0.38 
Compressibility 
3.9 3.3 3.2 3.2 3.2 3.5 3.3 
Elastic modulus 
75.0 74.6 77.9 80.5 82.9 80.0 76.0 
in compression 
Hardness 78.3 77.2 78.0 80.5 79.8 79.2 79.0 
Water vapor 12.0 12.9 11.9 11.8 11.6 11.8 13.1 
permeability 
Taber abrasion 
95.8 68.4 59.2 54.6 42.4 56.4 60.2 
______________________________________ 
COMATIVE EXAMPLE 1 
Use was made of a nonwoven fabric having a thickness of 4.2 mm and an areal 
density of 860 g/m.sup.2 as produced by laminating webs of 3-denier 
polyester fibers having a fiber length of 51 mm and performing needle 
punching at a rate of 700 needles per cm.sup.2. This fabric was dipped in 
the polyurethane solution used in Example 1, and wrung by means of a roll 
press at a clearance of 3.5 mm under a pressure of 3 Kg/cm.sup.2 so as to 
effect uniform impregnation. 
The resultant resin-impregnated nonwoven fabric was dipped in water at 
ordinary temperature (20.degree. C.) for 50 minutes to effect incomplete 
coagulation, wrung by means of a roll press, and immediately dried in a 
dryer kept at 75.degree..+-.5.degree. C. for 2 hours. In the resultant 
nonwoven fabric, the amount of an adherent resin was 38%. The resultant 
nonwoven fabric was sliced into two split resin-impregnated nonwoven 
fabrics each having a thickness of 2.1 mm, and the skin layer of each of 
the resultant nonwoven fabrics was removed in a thickness of 0.8 mm. As a 
result, two polishing sheet materials each having a thickness of 1.3 mm 
wherein the amount of an adherent resin was 35%, were obtained. The 
physical properties of the polishing sheet materials were measured. The 
density was 0.41 g/cm.sup.3, the compressibility 3.3%, the elastic modulus 
in compression 82.3%, the hardness 80.5, the water vapor permeability 13.8 
mg/cm.sup.2 /hr, and the Taber abrasion 61.6 mg/1000 cycles. A scanning 
electron photomicrograph of the surface fiber condition is as shown in 
FIG. 3. As is apparent from FIG. 3, there is no microvoid observed, but 
there is a pretty large amount of residual resin films between fibers. 
Application Example 
Each of the polishing sheet materials of sample Nos. 5, 13 and 20 
respectively obtained in Examples 1, 2 and 3 and the polishing sheet 
material obtained in Comparative Example 1 was bonded by a double coated 
adhesive tape to the whole surface of the surface plate (diameter 812 mm) 
of a one side polishing machine manufactured by Speedfam Co., Ltd., Japan 
(model 32 SPAW). By using colloidal silica particles, a silicon wafer (125 
mm in diameter) was polished under such conditions that the revolution 
rate was 87 rpm, the applied pressure was 500 g/cm.sup.2, and one cycle 
was 20 minutes. 
As a result, sample Nos. 5, 13 and 20 respectively withstood the use for 
120, 130 and 110 hours, whereas the polishing sheet material of the 
Comparative Example withstood the use for only 83 hours. The wafers 
polished by the polishing sheet materials backings of the Examples were 
superior in flatness and prevention of surface flagging to the wafer 
polished by the polishing sheet material of the Comparative Example. 
As is apparent from the foregoing description, according to the method of 
the present invention, the polishing sheet material is obtained by 
subjecting a polyurethane-impregnated nonwoven fabric to comlete 
coagulation, drying, and removal of the skin layer from both surfaces 
thereof to prepare a base material, which is optionally sliced into at 
least two split base materials each having a thickness suitable for the 
intended specific application, and dipping the base material in an aqueous 
solution of a solvent for a short period of time, followed by treatment in 
a high-temperature atmosphere for a short period of time. By virtue of 
this method, the obtained polishing sheet material is thoroughly free of 
microvoids, exhibits the most desirable physical properties as the 
polishing sheet material having lower Taber abrasion, and ensures a 
prolonged service life.