U.S. patent application number 12/269738 was filed with the patent office on 2009-03-12 for workpiece mounting and method for wire sawing.
This patent application is currently assigned to FREIBERGER COMPOUND MATERIALS GmbH. Invention is credited to Ralf Gruzsynsky, Ralf Hammer.
Application Number | 20090064982 12/269738 |
Document ID | / |
Family ID | 34959411 |
Filed Date | 2009-03-12 |
United States Patent
Application |
20090064982 |
Kind Code |
A1 |
Hammer; Ralf ; et
al. |
March 12, 2009 |
WORKPIECE MOUNTING AND METHOD FOR WIRE SAWING
Abstract
A method and a device for cutting a workpiece in a wire saw is
described, wherein a workpiece is fixed in a wire saw by means of a
mounting beam. In the method according to the invention, the
generation of a mark or a step on the cutting area along the
cutting slit at the transition from the workpiece to the mounting
beam is moved further to the edge of the cutting area or is avoided
entirely. Therefor, the workpiece is held during the cutting
operation in the wire saw by a mounting beam such that while one of
the two piercing points lies on the surface of the workpiece and
while simultaneously the other of the two piercing points lies on
the surface of the mounting beam, the piercing point lying on the
surface of the workpiece is the entry side piercing point.
Inventors: |
Hammer; Ralf; (Freiberg,
DE) ; Gruzsynsky; Ralf; (Brand-Erbisdorf,
DE) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
FREIBERGER COMPOUND MATERIALS
GmbH
|
Family ID: |
34959411 |
Appl. No.: |
12/269738 |
Filed: |
November 12, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11344540 |
Feb 1, 2006 |
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12269738 |
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PCT/EP2004/014108 |
Dec 10, 2004 |
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11344540 |
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60649830 |
Feb 2, 2005 |
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Current U.S.
Class: |
125/16.02 ;
125/16.01; 125/35 |
Current CPC
Class: |
B23D 57/0046 20130101;
B28D 5/045 20130101; B28D 5/0082 20130101 |
Class at
Publication: |
125/16.02 ;
125/35; 125/16.01 |
International
Class: |
B28D 1/06 20060101
B28D001/06; B28D 7/04 20060101 B28D007/04 |
Claims
1. A mounting beam for use in a wire saw to hold a work piece
comprising at least one cylindrically shaped portion, the mounting
beam having a longitudinal axis (L) and a cross sectional area (A)
in a plane perpendicular to the longitudinal axis (L) that is
asymmetric with respect to a mirror plane that includes a feed
direction of the wire saw, and the mounting beam further comprising
a bearing surface for the work piece which has a configuration that
is adapted to the cylindrical shape of the work piece.
2. A mounting beam according to claim 1, wherein the mounting beam
has essentially the form of a cuboid having a recess formed along
the longitudinal axis (L) of the cuboid, wherein the recess forms
the bearing surface.
3. A mounting beam according to claim 1, wherein a surface of the
mounting beam opposite to the bearing surface comprises a limit
stop face for orientation in the wire saw.
4. A mounting beam according to claim 3, wherein the cross
sectional area (A) perpendicular to the longitudinal axis (L) of
the mounting beam has two opposing lateral sides connecting the
bearing surface and the limit stop face, wherein the two sides have
different lengths.
5. A wire saw for cutting a workpiece, comprising: a wire; a feed
device for producing a relative movement between the workpiece and
the wire to perform a cutting of the workpiece, as the workpiece is
moved through the wire along a feed direction; a drive unit which
moves the wire along its longitudinal axis during the cutting; and
a mounting beam according to claim 1, wherein the workpiece is
attached to the mounting beam at a limit stop face in the wire
saw.
6. A wire saw according to claim 5, wherein the limit stop face is
located at the feed device.
7. A wire saw for cutting a workpiece, comprising: a wire; a feed
device for producing a relative movement between the workpiece and
the wire to perform a cutting of the workpiece, as the workpiece is
moved through the wire along a feed direction; a drive unit which
moves the wire along its longitudinal axis during the cutting; and
a mounting beam for attaching the workpiece in the wire saw,
wherein in the mounted state the workpiece is held in the wire saw
by the mounting beam in such a manner that the contact surface
between the mounting beam and the workpiece lies pre-dominantly on
the side of a plane that contains the longitudinal axis (M) of the
workpiece and that is perpendicular to the longitudinal axis of a
section of the wire, relative to which the workpiece is moved to
perform cutting.
8. A wire saw for cutting a workpiece that has at least one
cylindrically shaped portion, comprising: a wire; a feed device for
producing a relative movement between the workpiece and the wire to
perform a cutting of the workpiece, as the workpiece is moved
through the wire along a feed direction; a drive unit which moves
the wire along its longitudinal axis during the cutting; and a
mounting beam comprising a bearing surface for the workpiece which
is adapted to the cylindrical shape of the workpiece, wherein the
workpiece is attached to the mounting beam at a limit stop face in
the wire saw, and a plane (E) defined by the limit stop face is
inclined at an angle .alpha. with respect to the longitudinal axis
of a section of the wire, relative to which the workpiece is moved
to perform cutting.
Description
[0001] The present application is a divisional of U.S. application
Ser. No. 11/344,540, filed Feb. 1, 2006, which is a continuation of
PCT/EP2004/014108 filed Dec. 10, 2004, the entire contents of which
are incorporated herein by reference.
[0002] The invention relates to a method for cutting a workpiece,
especially a single crystal, in a wire saw, as well as to a wire
saw, to a mounting beam for performing that method and to a
semiconductor wafer.
[0003] From EP 0 903 210 A1 it is known that undesirable marks
(sawing marks) may occur on wafers in a generally known method of
cutting wafers from a crystal by a wire saw in which the crystal is
mounted by a mounting beam.
[0004] It is described in EP 0 903 210 A1 that the marks are
generated when the sawing wire penetrates into the mounting beam.
Further, it is known from this document that the material of the
mounting beam has an influence on the depth of the mark. To avoid
generation of marks, it is proposed in this document to use a
material having the same hardness as that of the crystal for the
mounting beam. However, this has the drawback that different
mounting beams have to be kept ready for workpieces of different
materials. Moreover, the possibility of optimization according to
other criteria such as the coefficient of elasticity, the
coefficient of thermal expansion, the stability, the adhesion, the
machinability and others does not apply because of the restriction
to materials of a certain hardness. Furthermore, also the wear of
the wire and the consumption of slurry increase for cutting of a
very hard workpiece by the use of a mounting beam of a very hard
material.
[0005] FIGS. 5a and 5b each show the surface profile of a GaAs
wafer after cutting of a GaAs single crystal by a known method of
wire sawing as taken by a contact stylus instrument (Mahr
Perthometer) along the scanning line 44 in FIG. 3. With 7 .mu.m in
FIG. 5a and 10 .mu.m in FIG. 5b the depth of the mark or the height
of the step (sawing mark) stands out significantly from the
remaining surface roughness. For an adversarial choice of
properties of the slurry, the height of the step may even become a
higher value of more than 20 .mu.m.
[0006] It is known to the applicant that the properties of the
slurry such as the content of hard material or the viscosity of the
carrier material have an influence on the generation of marks. The
generation of marks may be minimized by appropriate choice of the
slurry. On the other hand, the properties of the slurry have also
an influence on other surface parameters of the wafers such as the
warp, the bow or the surface roughness. In the known wire sawing
method, it is not always possible to choose the properties of the
slurry such that the generation of marks or steps and the
aforementioned other surface parameters are optimized at the same
time, because in part different properties are required for these
two optimization criteria. With optimization of the slurry for
avoiding generation of marks and steps and with simultaneous
optimization of surface parameters, the operational range of the
slurry properties is significantly restricted and the life time of
the slurry is limited.
[0007] During wire sawing the slurry is kept in a closed loop in
the majority of cases. Valuable gallium can be recovered from the
slurry when wire sawing GaAs wafers. Therefore, because of economic
reasons, the slurry is preferably kept in a closed loop until it
has a high content of GaAs (about 10%). However, a high content of
GaAs has a negative influence on the generation of marks and steps
at the transition of the wire from the workpiece into the mounting
beam.
[0008] After cutting the workpiece, further surface treatment steps
(e.g. lapping, grinding, polishing) may follow depending on the
requirements for the surface quality. These processes are inter
alia necessary to correct defects in wire sawing such as the
aforementioned marks or steps at the transition from the workpiece
into the mounting beam. However, steps having a height of above 20
.mu.m can also not be removed entirely by this method. Further,
these methods cost time and money.
[0009] From U.S. Pat. No. 5,052,366 a method and a machine for wire
sawing is known wherein the wire plane rocks relative to the
mounting beam during the cutting process, the direction of the wire
(direction of the longitudinal axis) being periodically varied
relative to the feed direction. Simultaneously, the wire is
reciprocated with an alternating sense of direction. Due to the
variation of the direction of the wire during the rocking movement,
the wire penetrates into the mounting beam already in an early
phase of the cutting operation.
[0010] It is the object of the present invention to provide a
method for cutting a workpiece in a wire saw wherein the cutting
areas are as smooth as possible and show no mark. Furthermore, it
is the object of the invention to provide a wire saw and a mounting
beam by which the method for cutting a workpiece in a wire saw
according to the invention may be performed.
[0011] The object is solved by a mounting beam for fixing a
workpiece in a wire saw according to claim 1, by a method for
cutting a workpiece in a wire saw according to claim 6, by a wire
saw according to claim 15 or 17 and by a wafer according to claim
18.
[0012] Further developments of the invention are characterized in
the sub-claims.
[0013] The method according to the invention and the devices
according to the invention provide the advantage that, on the
cutting area of a workpiece cut in a wire saw, the generation of
marks or steps at the transition of the wire from the workpiece
into the mounting beam can be either reduced or totally
avoided.
[0014] Therefore, the property parameters of the slurry may be
optimized with another objective as avoiding the generation of
marks or steps. The slurry may e.g. be optimized to minimize
surface parameters of the cutting area like warp, bow or surface
roughness. Furthermore, the GaAs content of the slurry in the
closed loop may be increased in the manufacture of GaAs wafers.
[0015] Further, the invention provides the advantage that a costly
postprocessing of the cutting area by lapping or grinding can be
omitted.
[0016] Furthermore, the yield may be increased in cutting a
workpiece in a wire saw wherein high standards have to be met for
the quality of the cutting area.
[0017] Furthermore, the edge region 42 (see FIG. 3) which can not
be used for the manufacture of semiconductor devices--the so-called
edge exclusion--may be minimized in the manufacture of
semiconductor wafers 43. If the edge region wherein the marks are
located is removed by edge rounding after cutting in the
manufacture of wafers, the removal by edge rounding may be
minimized and valuable material may be saved by the invention.
[0018] Further features and advantages of the invention may be
derived from the description of embodiments accompanied by the
Figures.
[0019] In the Figures:
[0020] FIG. 1 is a schematic sectional view of a workpiece with a
mounting beam attached thereto during the cutting operation in a
wire saw according to a first embodiment;
[0021] FIG. 1a is a perspective view of the mounting beam according
to the first embodiment;
[0022] FIG. 2 is a schematic sectional view of a workpiece with a
mounting beam attached thereto during the cutting operation in a
wire saw according to a second embodiment;
[0023] FIG. 3 shows the position of a mark on the surface of a
semiconductor wafer;
[0024] FIG. 4 is the sectional view through a surface profile of a
cutting area produced by a method according to the invention in a
wire saw; and
[0025] FIG. 5a and 5b are each sectional views through a surface
profile of a cutting area produced by a known method in a wire
saw.
FIRST EMBODIMENT
[0026] In FIG. 1, a schematic sectional view of a workpiece with a
mounting beam 2 is shown during the cutting operation in a wire saw
according to the first embodiment.
[0027] The workpiece 1 is in the shape of a cylinder. The material
may be a single crystal, especially a semiconductor single crystal
such as a Si single crystal or a GaAs single crystal, but also any
other material which can be cut by a wire saw. A mounting beam 2 by
which the workpiece 1 is fixed in a wire saw is attached on the
workpiece 1. The mounting beam may be e.g. of graphite but may be
also of any other material which can be cut by a wire saw. In the
wire saw, a wire 3 is stretched around rollers (not shown) such
that a plurality of wire sections span a wire plane perpendicular
to the drawing plane of FIG. 1. The wire saw has a feed device 12
to which the workpiece is attached by means of the mounting beam
for cutting. The workpiece 1 can be moved relative to the wire
plane in a feed direction 4 which is perpendicular to the wire
plane by the feed device 12. A device 14 for applying slurry onto
the wire is provided in the wire saw. Furthermore, a drive unit
(not shown) for moving the wire 3 along its longitudinal direction
is provided.
[0028] The mounting beam 2 according to the first embodiment is
essentially formed as a longish cuboid which has a recess being
adapted to the outer form of the cylindrical workpiece. By the
recess, a bearing surface 5 of the mounting beam is formed, which
serves as a contact surface for the workpiece 1 when the it is
bonded onto the workpiece 1. In the first embodiment, the recess is
formed such that the cross sectional area A (shaded area in FIG.
1a) of the mounting beam 2 perpendicular to its longitudinal axis L
is asymmetric. A flat side surface 6 of the mounting beam 2 serves
as a limit stop for attachment of the mounting beam 2 to the feed
device 12. In a mounted state, this flat side surface 6 is aligned
parallel to the wire plane (perpendicular to the drawing plane of
FIG. 1). When viewed in cross section, the two sides 13, 13' which
connect the flat side surface 6 with the contact surface 5 have
different lengths. Due to its asymmetry, the mounting beam 2 is
bonded onto the workpiece laterally displaced relative to a plane
11 which includes the longitudinal center axis M of the workpiece 1
and which is perpendicular to the wire 3 when it is mounted into
the wire saw. Here, the longitudinal center axis M in FIG. 1 is
perpendicular to the drawing plane.
[0029] In operation, the mounting beam 2 is attached to the
workpiece 1 before the cutting. To fix the mounting beam onto the
workpiece, it may be e.g. bonded onto the workpiece 1. E.g. an
epoxy resin may be used as an adhesive therefor. By fixing of the
mounting beam 2 to the feed device 12 the workpiece 1 is fixed in
the wire saw. Therefor the feed device 12 is provided with a limit
stop face 6' which is parallel to the wire plane. The wire sections
3 are always moved with the same sense of direction 8 along its
longitudinal axis while the workpiece 1 is pressed in a direction
perpendicular to the wire plane onto the wire sections 3 and moved
through the wire plane. Thereby the workpiece 1 is cut into a
plurality of wafers.
[0030] The workpiece 1 and the mounting beam 2 form a continuous
body, the surface of which is pierced by each of the wire sections
3 forming the wire plane at two piercing points 9 and 10 during the
cutting operation. At the one piercing point 9 the wire enters the
cutting slit with constant sense of direction 8 along its
longitudinal direction, while it exits the cutting slit at the
other piercing point 10. It turns out that the mark along the cut
is formed at that position where the piercing point 9 on the entry
side moves from the surface of the workpiece 1 onto the surface of
the mounting beam 2. By laterally displacing the mounting beam
relative to the plane 11 which includes the longitudinal center
axis M of the workpiece 1 and which is perpendicular to the wire 3
it is achieved that the wire penetrates into the mounting beam on
the entry side only shortly before or after the workpiece is
completely cut through. Thereby the mark can be shifted towards the
edge of the cutting area or even into the mounting beam.
[0031] In the first embodiment it is ensured that in the cutting
operation, while one of the two piercing points (9; 29) is on the
surface of the workpiece and simultaneously the other (10; 30) of
the two piercing points is on the surface of the mounting beam (2;
22), the piercing point lying on the surface of the workpiece is
the piercing point on the entry side.
[0032] The effect of the method according to the invention can be
attributed to the fact that the slurry applied on the wire on the
entry side can be transported into the cutting slit in the
workpiece 1 without interference of the mounting beam 2 as long as
the piercing point 9 on the entry side is on the surface of the
workpiece.
SECOND EMBODIMENT
[0033] With the wire saw according to the second embodiment of the
invention a mounting beam 22 known as such the cross sectional area
of which perpendicular to the longitudinal axis L is symmetric may
be used to perform the method according to the invention. A wire
saw according to the second embodiment differs from the wire saw
according to the first embodiment in that the mounting beam 22 is
hold in the wire saw in an inclined manner relative to a plane
perpendicular to the wire plane. For that purpose, the feed device
32 is provided with a limit stop face 26' for the mounting beam 22
which is inclined such that the plane E defined by the limit stop
face 26' encloses with the wire plane an angle .alpha..
[0034] The mounting beam 22 bears with its bearing face 25 on the
workpiece. A side surface 26 of the mounting beam 22 facing the
bearing face 25 provides a limit stop for attachment of the
mounting beam 22 to the limit stop face 26' of the feed device 32.
In a mounted state, the flat side surface 26 is parallel to the
limit stop face 26' of the feed device 32, while the bearing face
25 provides a contact surface between workpiece 21 and mounting
beam 22. Since the limit stop face 26' of the feed device 32 is
rotated around the longitudinal center axis M by an angle .alpha.
relative to the wire plane, the mounting beam 22 is bonded onto the
workpiece laterally displaced relative to a plane which includes
the longitudinal center axis M of the workpiece 21 and which is
perpendicular to the wire 23 when it is mounted into the wire
saw.
[0035] With the wire saw according to a second embodiment and a
mounting beam 22 known as such, the method according to the
invention can be performed wherein the body composed of the
workpiece 21 and the mounting beam 22 is moved through the wire
plane such that at least temporary one piercing point through which
the wire section 23 penetrates the composed body lies on the
surface of the workpiece 21, while simultaneously the other
piercing point 30 through which the wire section 23 penetrates the
composed body lies on the surface of the mounting beam 22.
[0036] Thereby, as in the first embodiment, it is achieved that the
contact surface 25 between the mounting beam 22 and the workpiece
21 in a mounted state is only or at least predominantly on one side
of the plane 31 which is spanned by the longitudinal center axis M
of the workpiece 21 and a perpendicular on the section of the wire
23 through which the workpiece 21 is moved.
[0037] Herein, the mounting beam 22 is preferably oriented in the
wire saw such that it is only on one side of the plane 31 which is
spanned by the longitudinal center axis M of the workpiece 21 and a
perpendicular on the section of the wire 23 through which the
workpiece 21 is moved. Thereby at least one of the two piercing
points 29 and 30 lies on the surface of the workpiece 21 throughout
the entire process of cutting the workpiece 21. The slurry applied
on the wire 23 on the entry side can thus be transported into the
cutting slit by the wire without interference by the mounting beam
throughout the entire cutting process.
[0038] Also in the second embodiment, it is ensured that during the
cutting process, while one of the two piercing points lies on the
surface of the workpiece 21 and simultaneously the other of the two
piercing points 29, 30 lies on the surface of the mounting beam 22,
the piercing point lying on the surface of the workpiece is the
entry side piercing point.
[0039] In FIG. 3, the position of the mark 40 on a semiconductor
wafer is shown which has been manufactured from a semiconductor
single crystal as a workpiece by the method of the invention
according to one of the two embodiments. For comparison, the
position of the mark 41 is indicated which is formed in the known
wire sawing of a semiconductor single crystal on the semiconductor
wafer. The inner area 43 is that area of the semiconductor wafer
which remains after subsequent rounding of the semiconductor
wafers. By the method of the invention, the position of the mark
can be shifted into this area which is removed in rounding of the
wafer subsequent to cutting of the workpiece, or the generation of
marks can be avoided at all.
[0040] FIG. 4 shows a section through the surface profile of a GaAs
wafer manufactured by the method according to the invention. The
surface profile was therefor taken along the scanning line 7 in
FIG. 1 by a contact stylus instrument (Mahr Perthometer). The
improvement of the surface profile can be clearly seen when
comparing with measurements of conventionally manufactured GaAs
wafers in FIGS. 5a and 5b. No mark or step is visible here at the
transition of the cut from the workpiece to the mounting beam.
Therefore, by the method of the invention a wafer is manufactured
which shows no marks in a state in which it is not rounded, i.e.
without performing edge rounding subsequent to cutting in the wire
saw. This wafer may be made of a polycrystalline or single
crystalline material like GaAs or InP.
[0041] The invention was described such that either the mounting
beam or the limit stop face for supporting the workpiece with a
known mounting beam in the wire saw is provided such that the
contact surface between the mounting beam and the workpiece in a
mounted state is only or at least predominantly on one side of the
plane which is spanned by the longitudinal center axis M of the
workpiece and a perpendicular on that section of the wire through
which the workpiece is moved. However, it is also possible to
combine a mounting beam according to the first embodiment with a
wire saw according to the second embodiment to achieve that.
[0042] The invention was described such that the workpiece has the
shape of a cylinder. However, the shape of the workpiece is not
limited to that. Accordingly, the cross sectional area of the
workpiece may have any other shape.
[0043] The feed device was described in the embodiments of the
invention such that it is perpendicular to the wire plane. However,
the feed direction may be any other direction as long as it has a
component perpendicular to the wire plane. The direction of the
longitudinal axis of the wire may vary relative to the mounting
beam during the cutting process as long as the entry side piercing
point of the wire is herein on the surface of the workpiece and
does not move onto the surface of the mounting beam thereby.
Accordingly, it is possible to perform the cutting operation first
in a rocking mode and in a final phase without rocking with fixed
orientation of the wire relative to the mounting beam. Another
possibility is to coordinate the periodical rocking movement and
the variation of the sense of direction of the movement of the wire
along its longitudinal axis such that during that time during which
one of the two piercing points lies on the workpiece and
simultaneously the other piercing point lies on the mounting beam,
the piercing point on the workpiece is always on the entry
side.
* * * * *