Method for cutting a workpiece

Abstract

Method for cutting a workpiece (W), comprising: - measuring a crystal axis alignment while holding a cylindrical workpiece (W) with a workpiece holder (1); - Adjusting the workpiece holder (1) holding the workpiece (W) onto a wire saw (9) so that the measured crystal axis alignment is maintained; - then adjusting an orientation of a cut plane; and - pressing the workpiece (W) held in the workpiece holder (1) against a wire row (10) for cutting the workpiece (W), the wire row (10) being formed by winding a wire axially reciprocating around a plurality of grooved rollers, wherein the workpiece holder (1) includes a sliding portion (2) which is displaceable while holding the workpiece (W) and a fixing portion (3) for fixing the sliding portion (2), and wherein the method includes, after measuring the crystal axis alignment, shifting the sliding portion (2) so that the workpiece (W) is moved to a center of the workpiece holder (1) in such a manner that the measured crystal axis alignment is maintained, fixing the sliding portion (2) by the fixing portion (3), setting the workpiece holder (1) holding the workpiece (W) onto the wire saw (9), then adjusting the orientation of the cut plane, and pressing the workpiece (W) against the wire row (10) to cut the workpiece (W).

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for cutting a workpiece with a wire saw. STATE OF THE ART

[0002] When a workpiece is cut into wafers using a wire saw, the cutting is performed such that the cut plane has a predetermined orientation. The cutting process begins with measuring the axial alignment of the workpiece while the workpiece is held and clamped by a workpiece holder. Based on the measured axial alignment data, the workpiece holder is attached to a wire saw, the workpiece position is adjusted, and the workpiece is then cut such that the crystal axis alignment of the workpiece matches a desired wafer plane orientation.The alignment of the cut plane of the workpiece is achieved by a combination of a rotary motion about an axis perpendicular to a central axis between the bottom surfaces of the cylindrical workpiece in a plane parallel to the wire row, and a pivoting motion of the central axis between the bottom surfaces relative to the wire row plane. This method for adjusting the crystal plane alignment of a workpiece is referred to as an internal adjustment method.

[0003] When cutting a workpiece, it has been known that causing a wire to run at an angle farthest from a crystal habit line, such as a notch and an OF (orientation surface), to cut the workpiece helps prevent cracks and other damage to wafers (see Patent Document 1). Other relevant prior art is disclosed in Patent Document 2.

[0004] In addition to the internal adjustment method, there is an external adjustment method in which, when a workpiece is mounted on a workpiece holder, the alignment is adjusted by rotating the workpiece around a central axis passing through the centers of the workpiece's undersides and pivoting it in a plane parallel to the wire row plane. In the external adjustment method, the alignment of the workpiece plane is not adjusted in the wire saw.

[0005] In contrast, the internal adjustment method ensures that a fixed position of a workpiece is always at the same position with respect to the workpiece's crystallography, and enables a product cut from the workpiece to be processed in an arrangement in which fracture is least likely to occur. In particular, a silicon single crystal has a cleavage plane arrangement for each crystal axis orientation, which can be known from the relative position between an AF or notch and a central axis passing through the centers of the bottom surfaces. LIST OF CITATIONSPATENT LITERATURE Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2007-90466 Patent document 2: US 5,904,136 A SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0006] However, some alignment gauges have a limitation on the distance between a measuring section of the alignment gauge and a workpiece bottom surface. In particular, a workpiece with a length of three-quarters or less of a processable maximum length (one length of a workpiece holder) cannot be subjected to alignment measurement unless the workpiece is brought to one side of the workpiece holder and fixed thereto. Therefore, it is necessary to cut the workpiece while holding the workpiece on one side of a wire row of a wire saw. In this method, the workpiece is not always pressed symmetrically against the wire row from the start of cutting, causing fluctuation in processing pressure and varying displacement of the workpiece. Accordingly, there arises a problem that the workpiece is damaged due to warpage.

[0007] The present invention was achieved in view of the aforementioned problems. An object of the present invention is to provide a method for cutting a workpiece that enables alignment measurement regardless of the limitation of the distance between a measuring section of an alignment measuring device and a plane of a workpiece to be measured, and that can prevent workpiece breakage and damage due to distortion caused when the workpiece is cut. SOLUTION TO THE PROBLEM

[0008] To achieve this goal, the present invention provides a method for cutting a workpiece having the features of claim 1. The method comprises: measuring a crystal axis orientation while holding a cylindrical workpiece with a workpiece holder; setting the workpiece holder holding the workpiece to a wire saw so that the measured crystal axis orientation is maintained; then adjusting an orientation of a cut plane; and pressing the workpiece held by the workpiece holder against a wire row for cutting the workpiece, wherein the wire row is formed by winding a wire in an axially reciprocating manner around a plurality of grooved rollers, wherein the workpiece holder includes a sliding portion that is slidable while holding the workpiece and a fixing portion for fixing the sliding portion.and wherein the method includes, after measuring the crystal axis alignment, shifting the sliding portion so that the workpiece is moved to a center of the workpiece holder in such a way that the measured crystal axis alignment is maintained, fixing the sliding portion by the fixing portion, adjusting the workpiece holder holding the workpiece to the wire saw, then adjusting the orientation of the cut plane, and pressing the workpiece against the wire row to cut the workpiece.

[0009] Such a method allows the crystal axis alignment of the workpiece to be measured without limiting the length of the workpiece when measuring the crystal axis alignment of the workpiece. The workpiece can then be cut at the center of the wire row of the wire saw by moving the workpiece to the center of the workpiece holder in such a way that the crystal axis alignment of the workpiece is maintained. As a result, a processing pressure is uniformly applied to the workpiece. Thus, a wafer with a cut plane with a desired plane alignment can be cut out, while simultaneously preventing damage due to warpage and wafer breakage due to fluctuating processing pressure.

[0010] In this method, the workpiece may have a length of three-quarters or less of a length of the workpiece holder, and the crystal axis alignment may be measured after the sliding portion holding the workpiece is moved so that one end face of the workpiece is brought to one side of the workpiece holder.

[0011] Conventionally, when a short workpiece with a length of three-quarters or less of a workpiece holder length is subjected to axis alignment measurement, the alignment cannot be measured unless the short workpiece is brought to one side of the workpiece holder and fixed thereto due to the structure of an alignment measuring device. Therefore, it is necessary to cut the short workpiece while it is fixed to one side of the workpiece holder. However, according to the inventive method for cutting a workpiece, the sliding portion is shifted to move the short workpiece, whereby not only can the alignment be measured without limiting the wafer length, but also the workpiece can be cut without fluctuation of the processing pressure, even in the internal adjustment process.

[0012] The workpiece can be a silicon single crystal ingot.

[0013] In the inventive method for cutting a workpiece, a short, large-diameter blank of a silicon single crystal can be cut into silicon wafers having a large diameter with reduced distortion and breakage.

[0014] Furthermore, a workpiece holder for use in holding a cylindrical workpiece during measurement of a crystal axis alignment of the workpiece and subsequent cutting of the workpiece with a wire saw is described, which is not the subject of the invention. The workpiece holder includes a sliding portion that is slidable while holding the workpiece, and a fixing portion for fixing the sliding portion. After measuring the crystal axis alignment of the workpiece, the sliding portion is slidable so that the workpiece is moved toward a center of the workpiece holder in such a manner that the measured crystal axis alignment is maintained, and the fixing portion can fix the sliding portion, whereby the workpiece is held by the workpiece holder.

[0015] Such a workpiece holder allows the crystal axis alignment of the workpiece to be measured without limiting the length of the workpiece when measuring the crystal axis alignment of the workpiece. Furthermore, the workpiece can be cut at the center of the wire row of the wire saw by moving the workpiece to the center of the workpiece holder in such a way that the crystal axis alignment of the workpiece is maintained. As a result, processing pressure is uniformly applied to the workpiece. Thus, a wafer with a cut plane having a desired orientation can be cut out, while simultaneously preventing damage due to warpage and wafer breakage due to fluctuating processing pressure.

[0016] The fixing portion may have a dovetail groove provided parallel to a line connecting the centers of both end faces of the workpiece, and the sliding portion may be engaged with the dovetail groove, whereby the sliding portion is slidable while maintaining the measured crystal axis orientation of the workpiece.

[0017] According to this design, the sliding portion can be moved effortlessly while maintaining the measured crystal axis orientation by a simple structure.

[0018] The workpiece to be held may have a length of three-quarters or less of the length of the workpiece holder.

[0019] The workpiece holder not only allows alignment measurement without limiting the wafer length, but also allows the workpiece to be cut without fluctuations in processing pressure, even when the object to be held is a short workpiece with a length of three-quarters or less than the length of the workpiece holder. Thus, a wafer with a cut plane and a desired alignment can be cut out, while simultaneously preventing damage due to warpage and wafer breakage due to fluctuating processing pressure.

[0020] The workpiece to be held can be a silicon single crystal ingot.

[0021] The use of the workpiece holder enables the workpiece to be cut from a silicon single crystal ingot, whose diameter has tended to increase in recent years, into large diameter silicon wafers with reduced warpage and breakage, even if the workpiece is short. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0022] The method for cutting a workpiece of the present invention and the workpiece holder described but not belonging to the invention enable a crystal axis orientation of a workpiece to be measured without limiting the length of the workpiece when measuring the crystal axis orientation of the workpiece. Furthermore, the workpiece can be cut at the center of the wire row of the wire saw by moving the workpiece to the center of the workpiece holder in such a way that the crystal axis orientation is maintained. Thus, a wafer with a cut plane having a desired orientation can be cut out, while simultaneously preventing damage due to warpage and breakage of the wafer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing an example of the workpiece holder; Fig.2 is a schematic diagram showing an example of the fixing portion of the workpiece holder; Fig. 3 is a schematic diagram showing an exemplary case where a workpiece is brought to one side of the workpiece holder; Fig. 4 is a schematic diagram showing an exemplary case where a workpiece is moved to a center of the workpiece holder; Fig. 5 is a schematic diagram showing an exemplary case where the workpiece holder is set on a crystal alignment measuring device; Fig. 6 is a schematic diagram showing an exemplary case in which a crystal alignment axis is measured while a workpiece is held by the workpiece holder; Fig.7 is a schematic diagram showing an exemplary case in which a workpiece held by the workpiece holder is cut; Fig. 8 is a graph showing a relative camber value in examples and comparative examples; and Fig. 9 is a graph showing a relationship between the relative camber value and an aspect ratio in examples and comparative examples. DESCRIPTION OF EMBODIMENTS

[0023] Embodiments of the present invention will be described below, but the present invention is not limited thereto.

[0024] As mentioned previously, there is a limitation on the distance between a measuring section of an alignment measuring device and a workpiece bottom surface. Specifically, a workpiece with a length of three-quarters or less of a workpiece holder length cannot be measured unless the workpiece is brought to one side of the workpiece holder and secured thereto. Therefore, when adjusting the cut plane in the internal adjustment process, it is necessary to cut the workpiece while holding the workpiece on one side of the wire row, causing differential displacement of the workpiece. Accordingly, a problem arises that the workpiece is damaged due to warpage.

[0025] In view of this, the inventors of the present invention carefully considered to solve this problem and accordingly describe the following: a workpiece holder not belonging to the invention, which includes a sliding portion configured to hold a workpiece and is slidable while maintaining the crystal axis alignment, can move the workpiece while holding the workpiece. Even if the workpiece is brought to one side of the workpiece holder during crystal axis alignment measurement, such a workpiece holder allows the workpiece to be moved to the center of the workpiece holder during cutting. Therefore, the workpiece can be cut at a center of a wire row into wafers having a cut plane with a desired orientation, while preventing damage due to warpage and breakage of the wafer.

[0026] Below, the workpiece holder and the inventive method for cutting a workpiece using this workpiece holder are described with respect to the Fig. 1 to 7 described.

[0027] First, the workpiece holder used in the inventive method for cutting a workpiece is described.

[0028] The workpiece holder is used to hold a workpiece while measuring the crystal axis alignment of the workpiece. Then, the workpiece holder is mounted on a wire saw while holding the workpiece and used to hold the workpiece, even when cutting the workpiece.

[0029] As in Fig. 1, the workpiece holder 1 includes a sliding portion 2 and a fixing portion 3.

[0030] As in Fig. 3 and Fig.4, the sliding portion 2 can hold a workpiece W by a carrier 4 connected thereto, and can be slid on the fixing portion 3 while holding the workpiece W.

[0031] The fixing portion 3 preferably has a dovetail groove 5 provided parallel to a line connecting the centers of both end faces of the held workpiece W, and the sliding portion 2 can be engaged with the dovetail groove 5, whereby the sliding portion 2 is slidable while maintaining the measured crystal axis orientation of the workpiece W.

[0032] According to this design, the sliding portion can be moved effortlessly while maintaining the measured crystal axis orientation by a simple structure.

[0033] As in Fig.2, fixing screws 6 are provided which penetrate from a side surface of the fixing portion 3 to the dovetail groove 5, and the sliding portion 2 which engages with the dovetail groove 5 is fixed to the fixing portion 3 with the fixing screws 6.

[0034] The workpiece holder 1 is particularly suitable for holding a short workpiece W having a length of three-quarters or less of a length of the workpiece holder 1.

[0035] When measuring a crystal axis alignment, even if the object is a short workpiece, the workpiece holder 1 enables the crystal axis alignment to be measured after the sliding portion 2 holding the workpiece W is moved so that an end face of the workpiece W is brought to one side of the workpiece holder 1, as shown in Fig. 3. As shown in Fig.4, the short workpiece W may then be moved to a center of the workpiece holder 1 and then fixed while being held to cut the workpiece W with a wire saw.

[0036] As in Fig. 3 and Fig. 4, a center mark M1 may be applied to a midpoint of the length of the fixing portion 3, and a center mark M2 indicating a center point of the workpiece may be previously applied to the sliding portion 2. When the sliding portion 2 is displaced so that the center marks M1, M2 coincide with each other, as shown in Fig. 4, the short workpiece W can be moved precisely to the center of the workpiece holder 1.

[0037] Furthermore, the workpiece to be held by the workpiece holder 1 can be a silicon single crystal ingot.

[0038] As the diameter of a silicon single-crystal ingot increases, it is often necessary to cut a short ingot with a large diameter. In this case, using the workpiece holder 1, a large-diameter silicon wafer with reduced warpage and breakage can be obtained. Of course, the workpiece to be cut is not limited to a silicon single crystal, and can be a compound semiconductor, an oxide single crystal, quartz, and the like.

[0039] Next, the inventive method for cutting a workpiece using the workpiece holder 1 will be described.

[0040] In order to cut the workpiece precisely, first, a workpiece W is connected to a carrier 4, with the workpiece W held by a sliding portion 2 of a workpiece holder 1. At this time, the workpiece W is connected at an angle farthest away from a crystal habit line of the workpiece with respect to the bottom surface of the sliding portion 2. In other words, the workpiece W is preferably connected to the carrier 4 at such an angle that the cleavage direction is sufficiently away from the wire travel direction. In this way, almost no cracking of the wafers occurs because the cleavage direction can be sufficiently removed from the wire travel direction during cutting. In this case, alignment based on a notch d or an AF is possible because the crystal habit line has already been removed from the workpiece by cylindrical grinding or another method.

[0041] Then the workpiece holder 1, which holds the workpiece W, is adjusted to an alignment measuring device 7, as shown in Fig. 5 shown.

[0042] In this process, as in Fig. 6, an end face of the workpiece W is brought close to an alignment measuring section 8 capable of measuring a crystal axis alignment of a workpiece W closely within a certain distance to measure the crystal axis alignment.

[0043] After measuring the crystal axis alignment, the sliding section 2 of the workpiece holder 1 is moved so that the workpiece W is moved to the center of the workpiece holder in such a way that the measured crystal axis alignment is maintained. In this process, as shown in Fig.4, when the aforementioned center marks M1, M2 are applied as marks and the sliding section is moved to a position in which both marks are aligned, the workpiece W can be moved effortlessly to the center of the workpiece holder while maintaining the measured crystal axis alignment. Then, after the sliding section 2 has been fixed with the fixing screws 6 of a fixing section 3, the workpiece holder 1, as shown in Fig. 7, is mounted on a wire saw 9. The wire saw 9 is provided with a wire row 10 formed by winding a wire in an axially reciprocating motion around a plurality of grooved rollers (not shown). The workpiece holder 1 is positioned above the wire row 10.

[0044] Subsequently, the position of the workpiece W is adjusted so that the crystal axis orientation of the workpiece W matches a desired plane orientation of the wafer after cutting, by a combination of a rotational movement about an axis perpendicular to a central axis between the bottom surfaces (between both end surfaces) of the workpiece W in a plane parallel to the wire row 10 and a swinging movement for rotating the central axis between the bottom surfaces in a plane perpendicular to the wire row 10. Even after the workpiece holder 1 is attached to the wire saw 9, the adjustment of the cut plane orientation of the workpiece can be performed using, for example, a wire saw equipped with a tilting mechanism.

[0045] The workpiece W is then moved downwards and pressed against the wire row 10 to cut the workpiece W.

[0046] As mentioned above, the inventive method for cutting a workpiece enables measurement of the crystal axis orientation without limiting the length of the workpiece W in measuring the crystal axis orientation. Then, the workpiece W is moved to the center of the workpiece holder 1 in such a manner that the crystal axis orientation of the workpiece W is maintained, whereby the workpiece can be cut at the center of the wire row 10 of the wire saw 9. As a result, a processing pressure is uniformly applied to the workpiece. Thus, a wafer with a cut plane having a desired plane orientation can be cut out, while simultaneously preventing wear due to warpage and breakage of the wafer due to fluctuating processing pressure.

[0047] Further, in measuring the crystal alignment, the workpiece may have a length of three-quarters or less of a length of the workpiece holder, and the crystal axis alignment may be measured after the sliding portion holding the workpiece is moved so that an end face of the workpiece is brought to one side of the workpiece holder.

[0048] Conventionally, when a short workpiece with a length of three-quarters or less of a workpiece holder length is subjected to axis alignment measurement in the internal adjustment method, the alignment cannot be measured unless the short workpiece is brought to one side of the workpiece holder and fixed thereto due to the structure of an alignment measuring device. Therefore, it has become necessary to cut the short workpiece while fixing it to one side of the workpiece holder. However, according to the inventive method for cutting a workpiece, the sliding portion is shifted to move the short workpiece, whereby not only can the alignment be measured regardless of the wafer length, but also the workpiece can be cut without fluctuation of the processing pressure.

[0049] Furthermore, the workpiece W can be a silicon single crystal ingot.

[0050] In recent years, the diameter of a silicon single-crystal ingot has become increasingly larger, and therefore, it is often necessary to cut a short ingot with a large diameter. In this case, too, the inventive method for cutting a workpiece in the internal adjustment process can provide silicon wafers with reduced warpage and breakage, since the arrangement of the cleavage plane can be easily determined from a crystal habit line. EXAMPLE

[0051] Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited thereto. (Examples 1 to 5)

[0052] Using a workpiece holder 1 as shown in Fig. 1, a workpiece was placed on one side of the workpiece holder as shown in Fig.6, to measure a crystal axis alignment. Subsequently, the sliding section was shifted to the center of the workpiece holder in such a way that the measured crystal axis alignment was maintained, and the sliding section was secured to the mounting section. The workpiece holder was then set on a wire saw, as shown in Fig. As shown in Figure 7, an orientation of a cut plane was adjusted, and the workpiece was pressed against a wire row and cut. The workpiece to be cut was a silicon single crystal ingot with a diameter of 200mm and an aspect ratio (%) with respect to the length of the workpiece holder (a length that is Fig.1 of Lx) of 3 / 4 (=0.75) or less. The silicon single-crystal ingot with an aspect ratio (length of the workpiece / length of the workpiece holder) of 0.60 (Example 1), 0.51 (Example 2), 0.45 (Example 3), 0.30 (Example 4), or 0.24 (Example 5) was repeatedly sliced into wafers.

[0053] After completion of cutting, warp, an index indicating the degree of warpage of a wafer, was measured using a wafer shape measuring device, MX204-8-37, manufactured by E+H Metrology GmbH.

[0054] The result is shown in Table 1, Fig. 8, and Fig. 9. In Examples and Comparative Examples, a relative value (%) of warp expressed by (average value of warp of wafers cut from a single workpiece) / (maximum value of warp in Comparative Example)×100 was used as an evaluation index for warp.

[0055] As shown in Table 1, Fig. 8, and Fig.As shown in Figure 9, the relative warp values in Examples 1 to 5 were almost half or less than those in Comparative Examples described later, and it was confirmed that the flatness was improved. Accordingly, it was confirmed that a short workpiece with a length of three-quarters or less of the length of the workpiece holder could be cut as well as a workpiece with an ordinary length, while inhibiting the warp damage of the wafers compared to the conventional cutting method. (Comparison examples 1 to 5)

[0056] A workpiece was cut under the same conditions as in the examples, except that the workpiece holder was not used. This means that the workpiece was moved to one side of the workpiece holder to allow measurement of the crystal axis alignment of the short workpiece, and cutting was performed while holding the workpiece on one side. The wafer warp was then evaluated in the same way as in the examples.

[0057] In Comparative Examples 1 to 5, the silicon single crystal ingot was repeatedly sliced into wafers having an aspect ratio of 0.51 (Comparative Example 1), 0.45 (Comparative Example 2), 0.51 (Comparative Example 3), 0.21 (Comparative Example 4), or 0.34 (Comparative Example 5).

[0058] The result is shown in Table 1, Fig. 8, and Fig.9. In Comparative Examples 1 to 5, the workpiece could not be cut at the center of the wire row, and the processing pressure was not uniformly applied to the workpiece. Therefore, the relative value of warp was almost twice that of the examples, and it was confirmed that the flatness was inferior compared to the examples.

[0059] The results of the examples and comparative examples are summarized in Table 1. [Table 1] Aspect ratio (%) relative warp value (%) Example 1 0,60 26,9 Example 2 0,51 25,7 Example 3 0,45 37,4 Example 4 0,30 30,6 Example 5 0,24 30,4 Comparison example 1 0,51 73,4 Comparison example 2 0,45 78,9 Comparison example 3 0,51 65,9 Comparison example 4 0,21 58,6 Comparison example 5 0,34 77,3

[0060] It should be noted that the present invention is not limited to the above embodiment.

Claims

[1] A method for cutting a workpiece (W), comprising: - measuring a crystal axis alignment while holding a cylindrical workpiece (W) with a workpiece holder (1); - Adjusting the workpiece holder (1) holding the workpiece (W) onto a wire saw (9) so that the measured crystal axis alignment is maintained; - then adjusting an orientation of a cut plane; and - pressing the workpiece (W) held in the workpiece holder (1) against a wire row (10) for cutting the workpiece (W), the wire row (10) being formed by winding a wire axially reciprocating around a plurality of grooved rollers, wherein the workpiece holder (1) includes a sliding portion (2) which is displaceable while holding the workpiece (W) and a fixing portion (3) for fixing the sliding portion (2), and wherein the method includes, after measuring the crystal axis alignment, shifting the sliding portion (2) so that the workpiece (W) is moved to a center of the workpiece holder (1) in such a manner that the measured crystal axis alignment is maintained, fixing the sliding portion (2) by the fixing portion (3), setting the workpiece holder (1) holding the workpiece (W) onto the wire saw (9), then adjusting the orientation of the cut plane, and pressing the workpiece (W) against the wire row (10) to cut the workpiece (W). [2] A method for cutting a workpiece (W) according to claim 1, wherein the workpiece (W) has a length of three-quarters or less than a length of the workpiece holder (1), and wherein the crystal axis orientation is measured after the sliding portion (2) holding the workpiece (W) is shifted so that an end surface of the workpiece (W) is brought to one side of the workpiece holder (1). [3] A method of cutting a workpiece (W) according to claim 1 or claim 2, wherein the workpiece (W) is a silicon single crystal ingot.

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