EP1819473A1

EP1819473A1 - Workpiece holder and method for wire sawing

Info

Publication number: EP1819473A1
Application number: EP04822533A
Authority: EP
Inventors: Ralf Hammer; Ralf Gruzsynsky
Original assignee: Freiberger Compound Materials GmbH
Current assignee: Freiberger Compound Materials GmbH
Priority date: 2004-12-10
Filing date: 2004-12-10
Publication date: 2007-08-22
Also published as: US8061345B2; US20090064982A1; JP2008522838A; US20060243265A1; WO2006061043A1; JP4874262B2

Abstract

The invention relates to a method and to a device for separating a workpiece (1, 21) by means of a wire saw, wherein a workpiece (1, 21) is fixed to the wire saw by means of a sawing strip (2, 22). According to the inventive method, the formation of a groove or an indentation on the separating surface, along the separating gap on the crossing of the workpiece (l, 21) on the sawing strip (2, 22) is displaced towards the edge of the separating surface or completely prevented. The workpiece (1, 21) is maintained in the wire saw during the separating process by means of a sawing strip (2, 22) in such a manner that one of the two piercing points (9; 29) is arranged on the surface of the workpiece (1; 21) and also the other (10; 30) of the two piercing points (9, 10; 29, 30) is arranged on the surface of the sawing bar (2; 22), and the piercing point which is arranged on the surface of the workpiece is the inlet side piercing point.

Description

Workpiece holder and method for wire sawing

The invention relates to a method for separating a workpiece, in particular a single crystal, in a wire saw, as well as a wire saw, a saw bar for carrying out this method, and a semiconductor wafer.

It is known from EP 0 903 210 A1 that, in a generally known method of separating slices from a crystal with a wire saw, in which the crystal is fixed with a saw bar, undesired grooves (saw marks) can occur on the slices.

It is described in EP 0 903 210 A1 that the grooves are formed when the sawing wire penetrates into the saw bar. It is also known from this document that the material of the saw bar has an influence on the depth of the groove. In order to avoid the scoring is proposed in this document to use for the saw bar material with the same hardness as that of the crystal. However, this has the disadvantage that different saw bars must be kept ready for workpieces of different materials. In addition, the restriction to materials of certain hardness eliminates the possibility of optimization according to other criteria such as modulus of elasticity, thermal expansion coefficient, strength, adhesiveness, machinability and others. Furthermore, when cutting a very hard workpiece by using a saw bar of very hard material, the wear of the wire and the consumption of abrasive increases.

In FIGS. 5a and 5b, the surface profile of a GaAs wafer after separation of a GaAs single crystal by the known wire saw method is shown, as recorded with a stylus instrument (Perthometer from Mahr) along the scan line 44 in FIG. The depth of the groove or the height of the heel (saw mark) clearly stands out with 7 μm in FIG. 5a or 10 μm in FIG. 5b from the remaining surface roughness! The height of the heel may also assume significantly higher values of more than 20 μm given unfavorable choice of the properties of the abrasive.

The applicant is aware that the properties of the abrasive, such as the hard material content or the viscosity of the carrier influence the scoring. By suitable choice of the abrasive, the scoring can be minimized. On the other hand, the properties of the abrasive also affect other surface parameters of the wheels, such as the warp, the bow or the surface roughness. It is not always possible in the known wire sawing method to select the properties of the abrasive so that the formation of grooves or shoulders and the aforementioned further surface parameters are optimized at the same time, since in some cases different properties are required for both optimization criteria. Optimizing the abrasive to prevent scoring and scaling while optimizing surface parameters greatly limits the abrasive properties of the abrasive and limits the life of the abrasive. The abrasive is usually held in wire sawing in a closed circuit. When wire sawing GaAs disks, valuable gallium can be recovered from the abrasive slurry. For reasons of economy, therefore, the abrasive is preferably kept in the closed loop until it has a high content of GaAs (about 10%). However, a high GaAs content in the abrasive in the prior art process negatively affects the score and step formation as the wire passes from the workpiece to the saw bar.

After the workpiece has been cut, further surface finishing operations (e.g., lapping, grinding, polishing) may follow, depending on surface quality requirements. These methods are i.a. necessary to correct errors in wire sawing such as the aforementioned grooves or heels at the transition from the workpiece to the saw bar. However, heels with a height of more than about 20 microns can not be completely removed even with these methods. In addition, these methods are time consuming and expensive.

From US 5,052,366 a method and a device for wire sawing is known in which the wire plane during the separation process rocking relative to the saw bar by the wire direction (direction of the wire longitudinal axis) is changed periodically relative to the feed direction. At the same time, the wire is moved along its longitudinal axis with changing sense of direction back and forth. By changing the direction of the wire during a rocking motion, the wire enters the saw bar already at an early stage of the separation process.

It is an object of the present invention to provide a method for separating a workpiece in a wire saw, in which the cut surfaces are as smooth as possible and without grooves. It is another object of the present invention To provide wire saw and a saw bar, with which the inventive method for separating a workpiece in a wire saw can be performed.

The object is achieved by a saw bar for fastening a workpiece in a wire saw according to claim 1, by a method for separating a workpiece in a wire saw according to claim 6, by a wire saw according to claim 15 or 17 and by a semiconductor wafer according to claim 18.

Further developments of the invention are specified in the subclaims.

The method according to the invention and the devices according to the invention have the particular advantage that on the cut surfaces of a workpiece separated in a wire saw, regardless of the composition of the abrasive used, the formation of scoring or shouldering during the transition of the wire from the workpiece to the sawing bar is either reduced or completely avoided can.

The property parameters of the abrasive can therefore be optimized with a purpose other than the avoidance of scoring or shouldering. The abrasive may e.g. be optimized so that surface parameters of the cut surface such as warp, bow and surface roughness are minimized. Further, in GaAs disc production, the GaAs content of the closed loop abrasive can be increased.

The invention further has the advantage that a complex post-processing of the cut surface such as lapping or grinding can be omitted. Further, in the separation of a workpiece in a wire saw, in which high demands are placed on the surface quality of the cut surface, the yield can be increased.

In addition, in the production of semiconductor wafers 43, the edge region 42 (see Fig. 3), which can not be used for the production of semiconductor devices - the so-called edge exclusion of the semiconductor wafer - can be minimized. If, in the production of wheels, the edge area in which the saw mark is located is removed after sawing by edge rounding, then the invention can minimize the removal by edge rounding, thereby saving costly material.

Other features and advantages of the invention will become apparent from the description of embodiments with reference to FIGS.

From the figures show:

Fig. 1 is a schematic sectional view of a

Workpiece with a saw bar attached thereto during the separation process in a wire saw according to a first embodiment;

Fig. Ia is a perspective view of the saw bar according to the first embodiment;

Fig. 2 is a schematic sectional view of a

Workpiece with an attached saw bar during the separation process in a wire saw according to a second embodiment; 3 shows the position of a groove on the surface of a semiconductor wafer;

4 shows a section through a surface profile of a separating surface produced by the method according to the invention in a wire saw; and

5a and 5b each show the section through a surface profile of a separation surface produced by the known method in a wire saw.

First embodiment

In Fig. 1, a workpiece 1 with a saw bar 2 when disconnected in a wire saw according to the first embodiment is shown in a schematic sectional view.

The workpiece 1 has the shape of a cylinder. The material of the workpiece 1 may be a single crystal, in particular a semiconductor single crystal such as a Si single crystal or a GaAs single crystal, but also any other material that can be cut with a wire saw. On the workpiece 1, a saw bar 2 is fixed, with which the workpiece 1 is fixed in a wire saw. The saw bar 2 may for example be made of graphite, but also of any other material that can be cut with a wire saw. In the wire saw, a wire 3 is stretched over rollers (not shown) such that a plurality of wire sections span a wire plane perpendicular to the image plane in FIG. The wire saw has a feed device 12, to which the workpiece 1 is attached via the saw bar 2 for separation. With the feed device 12, the workpiece 1 can be moved relative to the wire plane in a feed direction 4, which is perpendicular to the wire plane. In the wire saw, a device 14 for applying abrasive to the wire 3 is provided. It is Further, a drive unit (not shown) provided for moving the wire 3 along its longitudinal direction.

The saw bar 2 according to the first embodiment is, as shown in Fig. Ia, substantially formed as an elongated cuboid having a recess which is adapted to the outer shape of the cylindrical workpiece. Through the recess, a bearing surface 5 of the saw bar is formed, which serves as a contact surface for the workpiece 1 in the adhered to the workpiece 1 state. In the first embodiment of the invention, the recess is formed such that the cross-sectional area A (hatched area in Fig. Ia) of the saw bar 2 perpendicular to its longitudinal axis L is asymmetrical. A flat side surface 6 of the saw bar 2 serves as a stop for attachment of the saw bar 2 to the feed device 12. This flat side surface 6 is aligned parallel to the wire plane (perpendicular to the image plane of FIG. 1) in the installed state. Seen in cross section, the two sides 13, 13 ', which connect the flat side surface 6 with the contact surface 5 of different lengths. Due to their asymmetry, the saw bar 2 is glued laterally displaced onto the workpiece 1 in the state installed in the wire saw relative to a plane 11 extending through the central longitudinal axis M of the workpiece 1 perpendicular to the wire 3. Here, the center longitudinal axis M in Fig. 1 is perpendicular to the image plane.

In operation, the saw bar 2 is secured to the workpiece 1 prior to cutting. To attach the saw bar to the workpiece, it can be glued to the workpiece, for example. As an adhesive, for example, an epoxy resin can be used. By attaching the saw bar 2 to the feed device 12, the workpiece 1 is fixed in the wire saw. For this purpose, the feed device 12 has a stop surface 6 'parallel to the wire plane. The wire sections 3 are always in one during the separation process via the rollers (not shown) Direction of sense 8 moves along its longitudinal axis, while the workpiece 1 is pressed in a feed direction 4 perpendicular to the wire plane on the wire sections 3 and moved through the wire plane. Thereby, the workpiece 1 is separated into a plurality of disks.

The workpiece 1 and the saw bar 2 constitute a composite body whose surface is pierced by each wire section 3 forming the wire plane during the separating operation at two piercing points 9 and 10. At one piercing point 9, the wire enters the separating gap along its longitudinal direction with a constant sense of direction 8, while it emerges from the separating gap at the other piercing point 10. It has been found that the cut along the cut during separation is formed at the point at which the entry-side puncture point 9 merges from the surface of the workpiece 1 to the surface of the saw bar 2. By the lateral displacement of the saw bar 2 relative to a through the center longitudinal axis M of the workpiece 1 and perpendicular to the wire 3 extending plane 11 ensures that the wire entry side only shortly before or after the complete cutting of the workpiece enters the saw bar. As a result, the score can be moved to the edge of the parting line or even into the saw bar.

In the first embodiment, it is ensured that during the separation process, during one of the two puncture points (9, 29) on the surface of the workpiece (1, 21) and at the same time the other (10, 30) of the two puncture points (9, 10, 29, 30 ) on the surface of the saw bar (2; 22), the piercing point lying on the surface of the workpiece is the entry-side piercing point.

The effect of the method according to the invention can be attributed to the fact that the transport of the inlet side of the Wire 3 applied abrasive can be transported largely without interference by the saw bar 2 in the separation gap in the workpiece 1, as long as the entry-side puncture point 9 is located on the surface of the workpiece.

Second embodiment

In the wire saw according to the second embodiment of the invention, a saw bar 22 known per se can be used whose cross-sectional area is symmetrical perpendicular to its longitudinal axis L in order to carry out the method according to the invention. A wire saw according to the second embodiment is different from the wire saw according to the first embodiment in that the saw bar 22 is held obliquely in the wire saw relative to a plane 31 perpendicular to the wire plane. For this purpose, the feed device 32 is provided with a stop surface 26 'for the saw bar 22, which is tilted against the wire plane so that the plane defined by the stop surface 26' E with the wire plane forms an angle α.

The saw bar 22 rests with a bearing surface 25 on the workpiece. The support surface 25 opposite a flat side surface 26 of the saw bar 22 serves as a stop for attachment of the saw bar 22 on the abutment surface 26 'of the feed device 32. The flat side surface 26 is in the installed state parallel to the abutment surface 26' of the feed device 32, while the support surface 25 represents a contact surface between the workpiece 21 and saw bar 22. Since the abutment surface 26 'of the advancing device 32 is tilted about the central longitudinal axis M against the wire plane by an angle α, the saw bar 22 in the installed state in the wire saw relative to a through the central longitudinal axis M of the workpiece 21 perpendicular to the wire 23 extending plane 31 laterally staggered on the workpiece 21 glued. With the wire saw according to a second embodiment and a per se known saw bar 22 thus the method according to the invention can be carried out, in which the composite of the material 21 and the saw bar 22 body is moved through the wire plane that at least temporarily a puncture point 29, by which the wire portion 23 pierces the assembled body on the surface of the workpiece 21 and at the same time the other piercing points 30, through which the wire portion 23 pierces the assembled body, on the surface of the saw bar 22 is located.

Thus, as in the first embodiment, it is achieved that the contact surface 25 between the saw bar 22 and the workpiece 21 in the installed state exclusively or at least for the most part on one side of the through the center longitudinal axis M of the workpiece 21 and a solder on the portion of the wire 23rd , by which the workpiece 21 is moved, plane clamped 31 lies.

Preferably, the saw bar 22 in the wire saw, as shown in FIG. 2, is aligned such that it is moved exclusively on one side through the center longitudinal axis M of the workpiece 21 and a perpendicular onto the portion of the wire 23 through which the workpiece 21 is moved. spanned level 31 is located. As a result, at least one of the two puncture points 29 and 30 always lies on the surface of the workpiece 21 during the entire time of cutting the workpiece 21. The abrasive applied to the wire 23 on the inlet side can therefore be used without interruption by the saw bar during the entire separation process be transported with the wire in the separating gap.

In the second embodiment as well, it is ensured that during the separation process, during one of the two puncture points (9, 29) on the surface of the workpiece (1, 21) and simultaneously the other (10, 30) of the two puncture points (9, 10, 29, 30) lies on the surface of the saw bar (2, 22), the piercing point lying on the surface of the workpiece being the entry-side puncture point.

In Fig. 3, the position of the groove 40 is shown on a semiconductor wafer, which was prepared by the inventive method according to one of the two embodiments of a semiconductor single crystal as a workpiece. For comparison, the position of the groove 41 is shown, which is formed in the known wire sawing of the semiconductor single crystal on the semiconductor wafer. The inner region 43 is the region of the semiconductor wafer which remains after a subsequent rounding of the semiconductor wafers. With the method according to the invention, the position of the groove can be shifted into this region 42, which is removed during the rounding of the disk after the workpiece has been cut off, or the scoring can be completely avoided.

4 shows a section through the surface profile of a GaAs wafer produced by the method according to the invention. The surface profile was taken along the scan line 7 in FIG. 1 with a stylus instrument (Perthometer from Mahr). The improvement of the surface profile compared to the measurements on conventionally produced GaAs disks in FIGS. 5a and 5b can be clearly seen. Thus, no ridge or no paragraph at the transition of the cut from the workpiece to the saw bar can be seen here. It is therefore produced by the method according to the invention a disc which has no Sägemarke in the unrounded state, ie without performing an edge rounding after separation in the wire saw. This disk may consist of a polycrystalline or a monocrystalline material such as GaAs or InP. The invention has been described so that either the saw bar or a stop surface for holding the workpiece with a known saw bar in the wire saw is provided so that the contact surface between the saw bar and the workpiece in the installed state exclusively or at least for the most part on one side which lies through the center longitudinal axis M of the workpiece and a solder on the portion of the wire through which the workpiece is moved, spanned plane. However, it is also possible to combine the saw bar according to the first embodiment with a wire saw according to the second embodiment to achieve this.

The invention has been described so that the shape of the workpiece is a cylinder. The shape of the workpiece is not limited to this form. Thus, the workpiece can have any other cross-sectional area.

The feed mechanism has been described in the embodiments of the invention as being perpendicular to the wire plane. However, the feed direction may be a different direction as long as it has a component perpendicular to the wire plane. The direction of the longitudinal axis of the wire may change during the separation process relative to the saw bar, as long as the entry-side piercing point of the wire is on the surface of the workpiece and thereby does not migrate to the O- surface of the saw bar. For example, e.g. conceivable, because separation operation first in rocking mode and in the final phase without rocking with fixed alignment of the wire relative to the saw bar perform. Another possibility is to coordinate the periodic rocking movement and the change in the sense of direction of the movement of the wire along its longitudinal axis so that during the time during which one of the two puncture points lies on the workpiece and at the same time the other puncture point on the saw bar, the puncture point on the workpiece is always on the inlet side.

Claims

1. Saw bar for fastening a workpiece in a wire saw, wherein the saw bar (2) serves for holding a workpiece (1) in the wire saw, the saw bar (2) is formed with a longitudinal axis (L), and the longitudinal axis (L) vertical cross-sectional area (A) of the saw bar (2) is formed asymmetrically.

2. Saw bar according to claim 1, wherein the saw bar

(2) has substantially the shape of a cuboid with a recess through which along the longitudinal axis (L) of the cuboid adapted to the shape of the surface of the workpiece bearing surface (5) is formed.

3. Saw bar according to claim 2, wherein the contact surface (5) is adapted to the shape of a cylinder.

4. Saw bar according to claim 2 or 3, wherein one of the contact surface (5) opposite surface is formed as a stop surface (6) for orientation in the wire saw.

5. Saw bar according to claim 4, wherein the cross-sectional area (A) perpendicular to the longitudinal axis (L) of the saw bar two the contact surface (5) and the abutment surface (6) connecting sides (13, 13 ') which have different lengths.

A method of separating a workpiece (1; 21) in a wire saw, the workpiece (1; 21) being fixed to a saw bar (2; 22) such that the workpiece (1; 21) and the saw bar (2; 22 ) form a coherent body and the method includes the following step: Moving a wire (3; 23) through the workpiece (1; 21) for separating the workpiece (1; 21), the wire (3; 23) being separated at two opposing puncture points (9, 10; 29, 30) during the separating operation. the surface of the body is pierced, the wire (3; 23) is moved along its longitudinal axis with a constant or changing sense of direction, so that one of the two puncture points is defined as entry-side and the other as exit-side puncture point, and during one of the two puncture points (9; 29) on the surface of the workpiece (1, 21) and at the same time the other (10; 30) of the two puncture points (9, 10, 29, 30) on the O- surface of the saw bar (2, 22) located on the Surface of the workpiece lying piercing point is the entry-side puncture point.

7. The method of claim 6, wherein the wire is moved along its longitudinal direction during the separation process along its longitudinal axis with a constant sense of direction.

A method according to claim 6 or 7, wherein the orientation of the longitudinal axis of the portion of the wire (3; 23) through which the workpiece (1; 21) is moved relative to the saw bar (2; 22) is constant during the separating operation ,

A method according to claim 6 or 7, wherein the orientation of the longitudinal axis of the portion of the wire (3; 23) through which the workpiece (1; 21) is moved is changed periodically relative to the saw bar, and at the same time the direction of movement of the Wire is changed periodically along its longitudinal axis.

10. The method according to any one of claims 6 to 9, wherein the entry-side puncture point (9, 10, 29, 30) during the the entire cutting of the workpiece (1, 21) on the surface of the workpiece (1, 21).

11. The method according to any one of claims 6 to 10, wherein the workpiece (1; 21) has the shape of a cylinder at least in a portion thereof.

12. The method according to any one of claims 6 to 11, wherein the workpiece (1; 21) is a semiconductor single crystal.

13. The method according to any one of claims 6 to 12, wherein the workpiece (1, 21) is fixed by gluing to the saw bar (2; 22).

14. The method according to any one of claims 6 to 13, wherein the workpiece (1, 21) with the saw bar (2; 22) is held in the wire saw that the contact surface between the saw bar and the workpiece in the installed state exclusively on one side which lies through the center longitudinal axis (M) of the workpiece and a solder on the portion of the wire (3; 23) through which the workpiece (1; 21) is moved, plane (11; 31).

15. Wire saw comprising: a wire (23); a feed device (24), with which a relative movement between the workpiece (21) and wire (23) can be performed such that the workpiece (21) along a feed direction (24) through the wire (23) is moved; and a drive device that moves the wire (23) along its longitudinal axis during the separation process; wherein the workpiece (21) is secured with a saw bar (22) to a stop surface in the wire saw; and the plane (E) defined by the abutment surface (26) is tilted by an angle α relative to the portion of the wire through which the workpiece is moved.

16. Wire saw according to claim 15, wherein the stop surface is provided on the feed device.

17. Wire saw comprising: a wire (23); a feed device (24), with which a relative movement between the workpiece (21) and wire (23) can be performed such that the workpiece (21) along a feed direction (24) through the wire (23) is moved; a drive device that moves the wire (23) along its longitudinal axis during the separation process; and a saw bar (2; 22) for fixing the workpiece (1; 21) in the wire saw; wherein the workpiece with the saw bar in the installed state is held in the wire saw so that the contact surface between the saw bar (2; 22) and the workpiece (1; 21) predominantly on one side of the through the central longitudinal axis (M) of the workpiece (1; 21) and a solder on the portion of the wire (3; 23) through which the workpiece (1; 21) is moved, plane spanned (11; 31) is located.

18. Disc made of a polycrystalline or monocrystalline material, which has no Sägemarke in the unrounded state.