BACK GROUND OF THE INVENTION
FIELD OF THE INVENTION
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This invention relates to a material of platen belonging to a lapping machine
which laps both sides of a flat shape work-piece which is required to have a very
precise flatness and parallelism, more in detail, relates to a lapping machine which
has a platen whose thermal expansion coefficient is low and thermal deformation
is very small.
DESCRIPTION OF THE PRIOR ART
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In general, the surface processing of a plate shape work-piece made of glass,
metal, semi-conductor, ceramics or carbon which is required to have precise
flatness and parallelism is carried out by a lapping machine having a platen at
both upper and lower position, or a platen at either of the positions. The work-piece
is held and pressed between said upper and lower platens or pressed onto said one
platen, the platens and the work-piece are rotated under the constant supply of
aqueous slurry containing fine abrasive grains, and the work-piece is lapped as to
have an even thickness, further to improve flatness and parallelism.
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Recently, according to the necessity for the improvement of memory capacity
and productivity of integrated circuit (IC) or large-scale integration (LSI), a silicon
water or a compound semiconductor wafer (hereinafter shortened to wafer) which
is the starting material of IC or LSI is becoming to be required to have a
homogeneous thickness and to improve the machining accuracy and the
dimensional stability, further the size of water is becoming bigger and bigger.
Therefore, for the lapping of wafer, not only a main condition of machine but also
the machine itself is becoming to be required to have excellent accuracy and
stability, further, regarding other incidental conditions, more severe control is
becoming necessary. Especially, in a case of silicon wafer, a production technique of
silicon single crystal ingot is improved, and recently it become possible to produce
a large size wafer for example 12 inch or 16 inch diameter. To carry out the
effective on these large size wafer by higher productivity, a lapping machine
having bigger size platens which can use 24 inch size or 32 inch size carrier is
becoming popular.
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The lapping platen of this invention indicates a disk plate having considerable
thickness made of metal such as cast iron, copper, tin or steel or nonmetallic
materials such as ceramics or glass, on the front surface of which, for example,
narrow grooves are engraved to form a grit pattern. The lapping platen as
mentioned above is arranged at the upper and lower position or at the lower
position of the lapping machine. At the lapping process, in a case of double sided
lapping machine, a work-piece is held and pressed between two platens, and in a
case of single sided lapping machine, a work-piece is pressed to the platen by
means of a holder. The work-piece and/or platens are rotated, under the constant
supply of polishing compound slurry containing abrasives. By the effect of the
abrasive, the surface of work-piece is removed gradually and a flat and precise
surface can be generated. In a case of above mentioned lapping, the surface
flatness of the lapped work-piece is an imitation of that of platen. Therefore, the
dimensional accuracy and the flatness of the platen must be kept in precise level.
And when the brittle material such as silicon wafer is used as a work-piece, a cast
iron platen can be ordinary used.
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As the material for the cast iron platen, so called cast iron containing 1∼5 % of
carbon by weight is generally used, whose thermal expanding co-efficiency α is in
the level of about 10∼11 × 10-6/°C. When said cast iron platen is used as the 24B
size (outer diameter Φ is 1592mm and 70mm thickness), a warp of about 70 µm
generates to the thickness direction by 1°C temperature difference. As above
mentioned, the surface flatness of the lapped work-piece is an imitation of that of
platen, meanwhile, since the upper limit of TTV for the lapped 8 inches size silicon
wafer is about 0.7 µm, a warp of 70 µm of the platen can not be permitted.
Therefore, at the practical use, it is necessary to correct the surface of platens by a
mutual lapping after adjusted to the lapping temperature. Consequently, the wear
of platen is remarkable and it is necessary to change the expensive platen every
several months. That is, the conventional type lapping machine is not only hard for
handling at the practical use, but also is not sufficient from the economical view
point.
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Namely, considering the silicon wafer lapping, by the effect of the lapping
temperature generated with lapping or the temperature generated from a driving
motor or a speed reducer, the temperature of platens raises about 5∼10°C from
the initial state and are warped. And the warped platens gives severe affect to the
dimensional accuracy of the lapped wafer. To prevent above mentioned bad
influence, the methods to control the temperature raising of platen by making a
coolant flow inside of platen and lap the work-piece at a stable cooled condition, or
to heat the platen to the stable temperature and lap the work-piece at the stable
temperature are already proposed (for example, Japanese Patent Laid open
publication 63-245368, Japanese Patent Laid open publication 4-53671 and
Japanese Patent Application 9-317735). These references are effective from the
view point of the solution of the problem, however, since these references are
characterized to attach the additional parts to the original lapping machine, they
can not be said as the fundamental solution.
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As the fundamental countermeasure to solve the above mentioned problem, the
method to use a specific cast iron alloy whose thermal expanding co-efficiency α
is low as a material of platen is also proposed. For example, as disclosed in
Japanese Patent Publication 60-51547 or Japanese Patent Publication 3-90541, an
alloy composed by carbon steel as the main component, nickel and cobalt as the sub
component is proposed as a material whose thermal expanding co-efficiency α is
lower than 5 × 10-6/°C. These alloy of low thermal expanding co-efficiency contains
about 1∼5% of carbon and large amount of nickel and cobalt.
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In a case of ordinary type cast iron, the added carbon is dispersed into fused
iron during the fusion procedure and deposits as the spherical graphite
homogeneously at the quenching process. When this cast iron is used as the
lapping platen, said spherical graphite particles exists at the surface of the platen
are removed by the friction caused by lapping action and forms fine hole. And the
hole contributes to form an adequate surface roughness and catches fine particles
of abrasive grain, and can be used as the lapping platen. Meanwhile, in a case of
above mentioned low thermal expanding co-efficiency cast iron alloy, since the
content of nonferrous metals is large, the sphering of graphite is not sufficient and
forms amorphous carbon or needle shape crystalline, and affect the property as a
platen. Such a platen has a problems to cause abnormal lines (scratches) and
deteriorate TTV, and is difficult to be used as the platen for precise lapping use.
That is, the cast iron alloy only the thermal expanding co-efficiency is low can not
be used as a lapping platen used for the purpose of ultra precision processing.
BRIEF SUMMARY OF THE INVENTION
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The inventors of this invention have carried out an intensive study to solve
above mentioned problem which conventional lapping machine has, and have
found that the use of specific Fe-Ni-Co type cast iron alloy as the material of
lapping platen gives good results and accomplished the present invention. Namely,
the inventors have took a metal component ratio of alloy into consideration as to
control the state of carbon in the matrix of cast iron alloy, and have found a specific
Fe-Ni-Co type cast iron alloy has not only low thermal expanding co-efficiency but
also has proper features necessary as a platen such as stiffness or machinability.
That is, the object of this invention is to provide a lapping machine which
possesses a platen whose distortion by heat is small and not necessary to correct
the surface so frequently.
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The above mentioned object can be accomplished by a lapping machine which
possesses a platen made of cast iron alloy containing 1.0∼4.0 wt% of carbon,
smaller than 38 wt% of summed weight of nickel and cobalt, smaller than 2.0 wt%
of silicon, smaller than 2.0 wt% of manganese, smaller than 0.1 wt% of sulfur,
smaller than 0.15 wt% of phosphorus, smaller than 0.1 wt% of magnesium and
balancing weight of iron which contains small amount of impurity, wherein
spherical graphite grains smaller than 50 µm diameter are dispersed in said cast
iron alloy by smaller than 150pieces/mm2, and the thermal expanding co-efficiency
of said cast iron alloy is smaller than 5 × 10-6/°C.
BRIEF DESCRIPTION OF THE DRAWING
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- FIG.1 is a longitudinal sectional view of the double sided lapping machine. In
the drawing, 1 indicates an upper platen, 2 indicates a lower platen and 3 indicates
a work-piece.
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DETAIL DESCRIPTION OF THE INVENTION
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A lapping machine used in this invention is indicated by FIG.1, possesses a
rotatable upper platen 1 at upper position and a rotatable lower position 2 at lower
position as to faced, a work-piece 3 such as silicon wafer is held between said
platens and rotated under the constant supply of lapping compound, and as the
platen a cast iron platen having about 50∼70 mm thickness is used. At the lapping
process, since the surface accuracy of the lapped work-piece is an imitation of that
of platen, the platen must be stable against the external condition change such as
temperature change. The specific cast iron alloy used as the lapping platen in this
invention has characteristics to satisfy above mentioned matter. That is, in this
invention, by use of Fe-Ni-Co austenitic type alloy composed mainly by carbon steel
which has good stiffness and containing smaller than 38 wt% of summed weight of
nickel and cobalt, the thermal expanding co-efficiency (linear) of platen can be
dropped to the level lower than 5 × 10-6/°C.
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Further, to disperse the carbon component as the spherical graphite
homogeneously into the matrix of the cast iron alloy structure so as to provide a
casting property, machinability and vibration absorbing ability which ordinary cast
iron has, it is necessary to add smaller than 2.0 wt% of silicon, smaller than 2.0
wt% of manganese, smaller than 0.1 wt% of sulfur, smaller than 0.15 wt% of
phosphorus, smaller than 0.1 wt% of magnesium. Thus, the above mentioned
properties are provided to the alloy and the desired low thermal expanding co-efficiency
alloy can be obtained.
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In a case of ordinary use of the cast iron, the diameter of said spherical graphite
fine particle and the dispersing density of fine particles does not effect significantly
to the ability of the cast iron, however, in a case of use as a platen, these factors are
very important. That is, in the present invention, it is necessary that the diameter
of spherical graphite dispersed in the matrix of alloy is smaller than 50 µm. When
the spherical graphite is present at the surface of lapping platen, it is removed
from the surface by the friction and the mark forms a fine pore. When the size of
fine particles of abrasive is smaller than said pore, the particles of abrasive are
caught and accumulated in the pore during the lapping and affect the surface of
work-piece. Meanwhile, when the pore size is smaller than the size of abrasives,
above mentioned problem can be effectively prevented. When the diameter of
spherical graphite is bigger than 50 µm, since abrasives and other chemicals are
accumulated in the removed mark and forms very hard particles, it can not be used
as the platen for a precision lapping.
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Further, in the present invention, it is necessary to disperse said fine particles
of spherical graphite homogeneously in the matrix of alloy by smaller than
150/mm2 density. If the dispersion in the alloy is not homogeneous, since the
abnormal scratches are generated during the lapping process, the alloy can not be
used as a platen.
THE BEST EMBODYMENT TO CARRY OUT THE INVENTION
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The present invention will be understood more readily with reference to the
following Examples, however not intended to limit the scope of the invention.
EXAMPLE
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The cast iron alloy composed by following component is prepared. Content of
carbon is 1∼2 wt%, summed weight of nickel and cobalt is 36 wt%, smaller than 2
wt% of silicon, smaller than 2.0 wt% of manganese, smaller than 0.1 wt% of sulfur,
smaller than 0.15 wt% of phosphorus, smaller than 0.1 wt% of magnesium and
balancing weight of iron. The thermal expanding co-efficiency of the obtained cast
iron alloy is 5 × 10-6/°C, in which spherical graphite particles bigger than 50 µm
diameter are not contains and the dispersing density of spherical graphite
particles is not bigger than 150/mm2. By this alloy, a platen of 24B size (outer
diameter is 1592 mm, thickness is 70 mm) is prepared and a double sided lapping
machine having said platen at upper and lower position is assembled. The work-piece
of 8 inches (200 mm) size silicon wafer is processed by said double sided
lapping machine. At the starting of the lapping, the platens are not previously
heated and any dummy runs are not carried out. From the first run, the products
whose dimensional accuracy such as TTV or surface roughness are within the limit
of quality standard can be obtained by 100% around productivity.
COMPARATIVE EXAMPLE
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The cast iron alloy composed by following component is prepared. Content of
carbon is 3.6 wt%, silicon content is 2.5 wt%, manganese content is 0.2 wt%,
smaller than 0.04 wt% of sulfur, smaller than 0.1 wt% of phosphorus, smaller than
0.06 wt% of magnesium and balancing weight of iron. The thermal expanding co-efficiency
of the obtained cast iron alloy is 11 × 10-6/°C. By this cast iron alloy, a
platen of 24B size (outer diameter is 1592 mm, thickness is 70 mm) is prepared
and a double sided lapping machine having said platen at upper and lower position
is assembled. The work-piece of 8 inches (200 mm) size silicon wafer is processed
by said double sided lapping machine. Before the starting of the lapping, the
platens are not previously heated and any dummy runs are not carried out. Until
3rd runs, the lapped work-pieces are not stable from the view point of the
dimensional accuracy and the productivity during first 3 runs is very low. From the
5th run, products having adequate quality can be obtained by high productivity.
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It is clearly understood from the above mentioned Example and Comparative
Example that by use of the lapping machine of this invention serviceability ratio of
machine and productivity are remarkably improved without previous heating of a
platen or any dummy runs. Namely, by the lapping machine of this invention it is
possible to start the actual lapping immediately.
