Wire saw and method of slicing a cylindrical workpiece

Abstract: In a wire saw for slicing a cylindrical workpiece such as a silicon ingot so as to obtain wafers or the like, the feed rate of slurry, fed to the workpiece, the viscosity of the slurry, or the feed rate of a wire is varied according to the cutting length in the workpiece or the angle between the wire and the circumference of the workpiece. It becomes possible to obtain wafers or the like, each having a uniform thickness.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a wire saw and a method of slicing a cylindrical workpiece of a fragile material such as a semiconductor ingot of silicon, gallium arsenide, or the like, so as to obtain wafers or wafer-shaped plates. Description of the Related Art: In a wire saw, a wire is spirally wound around a plurality of rollers such that turns of the wire extend between rollers in parallel at a predetermined pitch. A cylindrical workpiece of a semiconductor ingot or the like or a quadrangularly prismatic workpiece of a synthetic quartz ingot or the like is pressed against the thus-arranged wire. Simultaneously, the wire is moved in its lengthwise direction, while a machining solution containing abrasives (hereinafter referred to as slurry) is fed between the workpiece and the wire, thereby concurrently slicing the workpiece into many (for example, several hundred) wafers having a predetermined thickness. However, in contrast with a quadrangularly prismatic workpiece of a synthetic quartz ingot or the like, when a cylindrical workpiece of a semiconductor ingot is sliced using the above-described wire saw, an obtained wafer has a problem that thickness is different among a portion where slicing started, a central portion, and a portion where slicing ended. That is, as shown in FIG. 1A, the thickness of the portion of wafer where slicing started or ended becomes relatively thin, while the thickness of the central portion of a wafer becomes relatively thick. That is, the thickness of each sliced wafer varies such that the wafer is convex on both sides thereof. This variation of thickness is observed in almost all wafers which are obtained from the same workpiece in a single slicing process. Also, this variation of thickness is peculiar to the slicing of a cylindrical workpiece through the use of a wire saw and tends to intensify as the diameter of a cylindrical workpiece increases. Therefore, this variation of thickness raises a significant problem in slicing of wafers from a silicon semiconductor single crystal, because these wafers are required to have a uniform thickness, and the degree of integration of devices has been increased, so that the diameter of each wafer has been increased to 200mm, 300mm, or even more. Japanese Patent Publication No. 57041118A (Sumitomo Electric Ind. Ltd.), also published in the Patent Abstracts of Japan volume 006, No. 109 (M-137), discloses a method for cutting a work piece at a fixed cutting speed. This is achieved by varying the torque applied to a revolution shaft that is used for shifting the workpiece upwards so that the pushing force of the workpiece as it is cut is varied in proportion to the length of wire in contact with the work piece. Japanese Patent Publication No. 07052149A discloses a method for improving the uniformity of thickness of wafers produced, wherein the speed of insertion of a round ingot through the cutting wires is varied, the speed being slowest in the middle portion of the ingot. SUMMARY OF THE INVENTION The present invention has been accomplished to solve the above-mentioned problems, and it is an object of the invention to provide a method and apparatus for slicing a cylindrical workpiece into wafers, each having a uniform thickness, with a wire saw. According to the present invention, there is provided a method of slicing a cylindrical workpiece of a fragile material such as a semiconductor ingot with a wire saw, wherein the feed rate of a wire is varied according to the cutting length in the workpiece or the angle between a wire and the circumference of the workpiece, such that a work amount on the workpiece is adjusted to a constant value. Through the change of the feed rate of the wire, a work amount on the workpiece is adjusted to a constant value, thereby uniformizing the thickness of a sliced wafer. The term "cutting length" is used herein to refer to the length of cut portion of the workpiece in contact with the wire. Preferably, the feed rate of the wire is increased as the cutting length in a cylindrical workpiece increases, and subsequently is decreased as the cutting length in the workpiece decreases. In this case, through the change of the feed rate of the wire according to the cutting length in a cylindrical workpiece, a wafer having a uniform thickness can be obtained. Preferably, the feed rate of the wire is varied such that as compared with the rate of increase in the wire feed rate during slicing from a slicing start portion to the central portion of a cylindrical workpiece, the rate of decrease in the wire feed rate during slicing from the central portion of the workpiece to a slicing end portion is made lower. In this case, through the employment of a different rate of change in the feed rate of the wire between slicing from a slicing start portion to the central portion of a cylindrical workpiece and slicing from the central portion to a slicing end portion, the thickness of a wafer can be made uniform. A wire saw of the present invention for slicing a cylindrical workpiece of a fragile material such as a semiconductor ingot comprises means for varying the feed rate of the wire according to the cutting length in the workpiece or the angle between a wire and the circumference of the workpiece, such that a work amount on the workpiece is adjusted to a constant value. Through the use of the above-described wire saw, a method according to the present invention can be carried out. In a method and a wire saw for slicing a cylindrical workpiece according to the present invention, the feed rate of the wire is varied according to the cutting length in the workpiece or the angle between a wire and the circumference of the workpiece, thereby significantly decreasing variation in the thickness of a wafer. As an additional effect produced, the rate of generation of a cracked wafer during slicing is decreased, thereby improving the yield of slicing and productivity. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a schematic view showing the cross-section of a wafer obtained by slicing through the use of a conventional apparatus and method; FIG. 1B is a schematic view showing the cross-section of a wafer obtained by slicing through the use of an apparatus and a method according to the present invention; FIG. 2 is a schematic view of a wire saw; FIG. 3 is a view showing positions on a sliced wafer where thickness is measured; and FIG. 4 is a graph showing variation of a wire feed rate when a cylindrical workpiece is sliced according to a fourth embodiment of the present invention. DESCRIPTION OF THE INVENTION AND EMBODIMENTS The present invention will now be described in detail. The inventors of the present invention investigated the cause of variations in the thickness of wafers which were obtained by slicing a cylindrical workpiece of a semiconductor ingot or the like. The results of the investigation implied that since the cutting length is different, for example, among the portion of the workpiece where slicing starts, the central portion of the workpiece, and the portion of the workpiece where slicing ends, the work amount of a wire and slurry on the workpiece is different among these portions. That is, when slicing starts or ends, the cutting length is short, and thus the work amount of the wire and slurry on the workpiece becomes relatively large. As a result, the removal amount or stock removal during slicing becomes relatively large, so that the thickness of each wafer deceases at these portions. By contrast, at the central portion of the workpiece, the cutting length becomes relatively long, and thus the work amount of the wire and slurry on the workpiece becomes relatively small. As a result, the removal amount or stock removal during slicing becomes relatively small, so that the thickness of each wafer increases at the central portion. The angle between a wire and the circumference of a workpiece is acute during slicing from a slicing start portion to the central portion of a workpiece, and subsequently becomes obtuse during slicing from the central portion to a slicing end portion. Particularly, when slicing start, the angle between the wire and the circumference of the workpiece is acute to form a wedge shape, and consequently slurry enters a cutting area efficiently. By contrast, during slicing from the central portion to a slicing end portion, the angle between a wire and the circumference of a workpiece becomes obtuse, and consequently slurry enters the cutting area less efficiently. Accordingly, when slicing starts, the work amount of a wire and slurry on a workpiece become particularly large. As a result, the removal amount or stock removal during slicing becomes particularly large when slicing starts. Consequently, the thickness of a wafer at the portion where slicing started is smaller than that at the portion where slicing ended even though the cutting length is substantially identical between the portions, resulting in the shape of a cross-section of a wafer as shown in FIG. 1A. The present inventors came up with the idea of decreasing variation in the thickness of each wafer by making constant a work amount on a cylindrical workpiece irrespective of the cutting length or the angle between a wire and the circumference of the workpiece, and confirmed that the work amount on the workpiece can be made constant by changing the feed rate of a wire according to the cutting length in the workpiece or the angle between a wire and the circumference of the workpiece. An embodiment of the present invention will next be described with reference to the drawings. However, the present invention is not to be limited thereto. FIG. 2 schematically shows the structure of a wire saw. In this wire saw, a wire 12 supplied from one of two wire take-up reel bobbins 22 and 32, which are driven by torque motors 24 and 34, respectively, is spirally wound around a set of three rollers 10A, 10B, and 10C, and is then taken-up by the other of the take-up reel bobbins 22 and 32. Portions of the wire 12 extends between two rollers 10A and 10B in parallel at a predetermined pitch so as to slice a workpiece. A drive motor 7 connected to the roller 10C, located underneath the rollers 10A and 10B, rotates the roller 10C to thereby feed the wire 12 at a predetermined rate. While the wire 12 is being unwound from the wire take-up reel bobbin 22 and taken up onto the other wire take-up reel bobbin 32 at a predetermined feed rate of the wire 12, and vice versa, via tension-adjusting mechanisms 20 and 30, a cylindrical workpiece 40 held by the workpiece holder 42 is pressed against a workpiece slicing section located between the two rollers 10A and 10B to thereby slice the workpiece 40. During this slicing operation, slurry from an unillustrated slurry feeder is being fed into the workpiece slicing section via the wire 12. Next, an embodiment of the present invention will be described. In a wire saw according to the embodiment, the feed rate of a wire 12 for slicing a workpiece 40 is varied according to the cutting length in the workpiece 40 or the angle between the wire 12 and the circumference of the workpiece 40. FIG. 4 shows a pattern of varying the feed rate of the wire 12. The feed rate of the wire 12 is lowest when slicing starts, then is increased with the cutting length in the workpiece 40 until it reaches a maximum for slicing the workpiece 40 at the central portion thereof, and subsequently decreased as the cutting length decreases. The wire feed rate is varied such that as compared with the rate of increase in the wire feed rate during slicing from a slicing start portion to the central portion of a cylindrical workpiece, the rate of decrease in the wire feed rate during slicing from the central portion of the workpiece to a slicing end portion is made lower. This variation of the wire feed rate copes with the fact that as shown in FIG. 1A, the work amount of a wire and slurry on a cylindrical workpiece is relatively small during slicing from the central portion of the workpiece to a slicing end point as compared with slicing from a slicing start point to the central portion of the workpiece. In order to vary the feed rate of the wire 12, a servomotor, for example, may be used as a drive motor 7 connected to a roller 10C to notate the roller 10C. Through the control of the rotational speed of this servomotor, the feed rate of the wire 12 can be controlled according to a predetermined pattern. In step with this variation of the wire feed rate, the wire take-up/release rate of wire take-up reed bobbins 22 and 32 may be controlled by means of torque motors 24 and 34. The cutting length can be determined from the relative position between the wire 12 and the workpiece 40, for example, from the diameter of the workpiece 40 and the feed amount of the workpiece holder 42 relative to the wire 12. An example of the embodiment according to the present invention and a comparattve example will now be described. Example 1: An ingot of silicon single-crystal semiconductor having a diameter of approximately 200 mm was sliced using a wire saw and method of the present invention. A wire feed rate was varied according to a pattern shown in FIG. 4, which defines a variation in the wire feed rate with respect to the cutting depth in the workpiece. Specifically, the wire feed rate was 100 m/min when slicing was started, then was increased with the cutting length in the ingot until it reaches 650 m/min for slicing the ingot at the central portion thereof, and subsequently decreased with the cutting length until it reaches 300 m/min for slicing the ingot at the slicing end portion thereof. 10 ingots were sliced to obtain approximately 2500 wafers, while the wire feed rate was varied according to the pattern of FIG. 4. The thickness of each of the thus-obtained wafers was measured at 5 points, namely a wafer center and 4 peripheral points (located 3mm inside as measured from the wafer edge), as shown in FIG. 3, thereby obtaining a difference TV5 between the maximum and minimum thickness measurements. Comparative Example 1: An ingot of silicon single-crystal semiconductor having a diameter of approximately 200 mm was sliced in a manner similar to that of Example 1, except that the wire feed rate was maintained constant at 500 m/min during slicing. 5 ingots were sliced to obtain approximately 1000 wafers. The thickness of each of the thus-obtained wafers was measured at 5 points, namely a wafer center and 4 peripheral points (located 3mm inside as measured from the wafer edge), as shown in FIG. 3, thereby obtaining a difference TV5 between the maximum and minimum thickness measurements. Table 1 shows the test results of Example 1 and Comparative Example 1. Ingots (pcs) Wafers (pcs) Average TV5 (µm) Crack generation (%) Example 1 10 2512 10 0.2 Comparative Example 1 5 1032 25 3.4 The cross-section of those wafers which are obtained through the use of a method and apparatus of the present invention is as shown in FIG. 1B, whereas the cross-section of those wafers which are obtained through the use of a conventional method and apparatus or in Comparative Example 1 is convex on both sides thereof as shown in FIG. 1A, indicating that the present invention provides a significant improvement. As seen from Table 1, compared with Comparative Example 1, Example 1 of the present invention shows a smaller value of TV5 that indicates the degree of variation in the thickness of each wafer. Further, Example 1 shows a significant improvement in a crack generation percentage with wafers, probably owing to a smooth flat cut surface, as shown in FIG. 1B. Accordingly, the present invention can decrease variation in the thickness of each wafer while improving the yield of slicing and productivity. The present invention is not limited to the above-described embodiment. The above-described embodiment is a mere example, and those having the substantially same structure as that described in the appended claims and providing the similar action and effects are included in the scope of the present invention. For example, the term "cylindrical workpiece" appearing in the above description of the present invention does not necessarily denote the only prismatic workpiece having a completely circular cross-section. Cylindrical workpieces include workpieces having an elliptic cross-section, workpieces having a circular cross-section and a partial cut (so-called orientation flat), and the like. So long as a workpiece has a cross-section such that the cutting length or the angle between a wire and the circumference of the workpiece varies as slicing progresses, a pattern of varying a wire feed rate may be determined accordingly.

Claims: A method of slicing a cylindrical workpiece of a fragile material such as a semiconductor ingot with a wire saw, wherein the feed rate of a wire is varied according to the cutting length in the workpiece or the angle between the wire and the circumference of the workpiece, such that a work amount on the workpiece is adjusted to a constant value. A method of slicing a cylindrical workpiece according to Claim 1, characterized in that the feed rate of the wire is increased as the cutting length in the cylindrical workpiece increases, and subsequently is decreased as the cutting length decreases. A method of slicing a cylindrical workpiece according to Claim 1 or 2, characterized in that the feed rate of the wire is varied such that as compared with the rate of increase in the feed rate of the wire during slicing from a slicing start portion to the central portion of the cylindrical workpiece, the rate of decrease in the feed rate of the wire during slicing from the central portion of the workpiece to a slicing end portion is made lower. A wire saw for slicing a cylindrical workpiece of a fragile material such as a semiconductor ingot, characterized by comprising means for varying the feed rate of a wire according to the cutting length in the workpiece or the angle between a wire and the circumference of the workpiece, such that a work amount on the workpiece is adjusted to a constant value.

Inventors: HAYAKAWA KAZUO, KIUCHI ETSUO, TOYAMA KOHEI, HAYAKAWA, KAZUO, KIUCHI, ETSUO, TOYAMA, KOHEI
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1997
Main classification: B28D5
IPC codes: B28D5