Wire saw and method of slicing a cylindrical workpiece, e.g. an ingot

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. Japanese Patent Publication No. 07001442A discloses a method of improving the uniformity of thickness of wafers produced, wherein the temperature of an abrasive grain slurry is varied to stabilise deflection of the roller and improve cutting accuracy. 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. To achieve the above object, according to a first aspect of 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 comprising a wire and a slurry feeder, wherein the feed rate of slurry fed to the workpiece is varied according to the cutting length in the workpiece or the angle between a wire and the circumference of the workpiece, such that the work amount on the workpiece is adjusted to a constant value. Preferably, the feed rate of slurry fed to a cylindrical workpiece is increased as the cutting length in the workpiece increases, and subsequently the feed rate of slurry is decreased as the cutting length decreases. Also, preferably, the feed rate of slurry is varied such that as compared with the rate of increase in the feed rate of slurry during slicing from a slicing start portion to the central portion of a cylindrical workpiece, the rate of decrease in the feed rate of slurry during slicing from the central portion of the workpiece to a slicing end portion is made lower. The term "cutting length" is used herein to refer to the length of cut portion of the workpiece in contact with the wire. The feed rate of slurry fed to the cylindrical workpiece can be varied through a change of the flow rate of slurry fed to a wire array or a change of the feed position of slurry. Preferably, the feed position of slurry is varied along a wire between a position in the proximity of a cylindrical workpiece and a position corresponding to a far end of a roller as viewed from the workpiece. In this case, the feed position of slurry may be located most distant from the workpiece when slicing starts, and may be moved toward the workpiece as the cutting length increases. When the cutting length reaches a maximum, the feed position of slurry may be located closest to the workpiece. Subsequently, as the cutting length decreases, the feed position of slurry may be moved away from the workpiece. A wire saw of the present invention for slicing a cylindrical workpiece of a fragile material, such as a semiconductor ingot, comprises a wire, a slurry feeder and means for varying the feed rate of slurry fed to the workpiece according to the cutting length in the workpiece or the angle between the wire and the circumference of the workpiece, such that the work amount on the workpiece is adjusted to a constant value. The means for varying the feed rate of slurry fed to a workpiece may be means for varying the flow rate of slurry or means for varying the feed position of slurry. The means for varying the feed position of slurry can move a slurry feed nozzle toward or away from a workpiece along the a wire. In a method and a wire saw for slicing a cylindrical workpiece according to the present invention, the amount of slurry fed to the workpiece is varied according to the cutting length in the workpiece or the angle between the 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 schematic cross-section of a wire saw according to a first embodiment of the present invention; FIG. 4 is a schematic cross-section of a wire saw according to a second embodiment of the present invention; FIG. 5 is a graph showing variation of the flow rate of slurry when a cylindrical workpiece is sliced according to the first embodiment; FIG. 6 is a view showing positions on a sliced wafer where thickness is measured; and FIG. 7 is a graph showing variation of the feed position of slurry when a cylindrical workpiece is sliced according to the second embodiment. 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 decreases 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 starts, 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 becomes 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 slurry according to the cutting length in the workpiece or the angle between a wire and the circumference of the workpiece. Embodiments 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. FIG. 3 schematically shows the structure of a first embodiment of the present invention. As shown in FIG. 3, abrasive slurry is fed from slurry feed nozzles 50A and 50B located above the centrelines of the rollers 10A and 10B, respectively. Slurry is fed from the slurry feed nozzle 50A when the wire 12 is being fed in a direction from the roller 10A to the roller 10B, and from the slurry feed nozzle 50B when the wire 12 is being fed in the opposite direction. A number of the slurry feed nozzles 50A and 50B are disposed along longitudinal directions of the rollers 10A and 10B, respectively, so that slurry is fed in a curtain-like form. In the first embodiment, the flow rate of slurry fed from the slurry feed nozzle 50A or 50B 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, thereby varying the feed rate of the slurry fed to the workpiece 40. The flow rate of the slurry is minimised when slicing starts, then increased as the cutting length increases, and subsequently decreased as the cutting length decreases. The cutting length can be detected from the relative position between the wire 12 and the workpiece 40. For example, the cutting length can be calculated from the feed amount of the workpiece holder 42 relative to the wire 12 and the diameter of the workpiece 40. The flow rate of the slurry may be varied by changing the amount of the slurry discharged from a slurry feed pump (unillustrated), which sends the slurry under pressure from a slurry tank (unillustrated) to the slurry feed nozzle 50A or 50B, or by installing a flow controller between the slurry tank and the slurry feed nozzle 50A or 50B. FIG. 4 schematically shows the structure of a second embodiment of the present invention. As shown in FIG. 4, a slurry feed nozzle 50A or 50B can be moved along a wire 12 toward or away from a workpiece 40 to thereby vary the feed rate of the slurry fed to the workpiece 40. The slurry feed nozzle 50A or 50B is located most distant from the workpiece 40 (for example, a position indicated by numeral 52A or 52B) when slicing starts, and then moved toward the workpiece 40 as the cutting length increases. When the workpiece 40 is being sliced at the central portion thereof, the slurry feed nozzle 50A or 50B is located closest to the workpiece 40 (for example, a position indicated by numeral 54A or 54B). Subsequently, as the cutting length decreases, the slurry feed nozzle 50A or 50B is moved away from the workpiece 40. The cutting length is detected from the relative position between the wire 12 and the workpiece 40 as previously described. A moving mechanism 56A or 56B for moving the slurry feed nozzle 50A or 50B is preferably a combination of a drive motor attached to the slurry feed nozzle 50A or 50B and an encoder for detecting the position of the slurry feed nozzle 50A or 50B. When the slurry feed nozzle 50A or 50B is moved in a direction opposite to the workpiece 40 from a position located just above the centreline of the roller 10A or 10B, slurry drops on the roller 10A or 10B at its outer side. As a result, part of dropping slurry is scattered due to the centrifugal force of the rotating roller 10A or 10B, resulting in a reduction in the feed rate of the slurry fed to the workpiece 40 as compared with the case where slurry drops from just above the centreline of the roller 10A or 10B. The feed rate of the slurry fed to the workpiece slicing section is minimized when the slurry dropping position is located above the far end of the roller 10A or 10B as viewed from the workpiece 40 (a position indicated by numeral 52A or 52B). However, in order to secure the feed rate of the slurry required at the time of starting slicing, the slurry dropping position is preferably located approximately not more than 0.9r distant as measured opposite to the workpiece 40 from a position located just above the centreline of the roller 10A or 10B (r is the radius of the roller 10A or 10B). The amount of scattering slurry is, for example, approximately 50% of discharge from the slurry feed nozzle 50A or 50B when the slurry dropping position is located 0.5r distant as measured opposite to the workpiece 40 from a position located just above the roller centerline. As the slurry feed nozzle 50A or 50B is moved toward the workpiece 40, the feed rate of the slurry fed to the workpiece slicing section increases. When the slurry feed nozzle 50A or 50B is located in the proximity of the workpiece 40 as indicated by numeral 54A or 54B, the feed rate of the slurry to the workpiece slicing section is maximized. As described above, by moving the slurry feed position along the wire 12 within the range from a position located in the proximity of the workpiece 40 to a position corresponding to the far end of the roller 10A or 10B as viewed from the workpiece 40, the feed rate of the slurry fed to the workpiece slicing section can be varied as desired. Examples of the first and second embodiments according to the present invention and a comparative example will now be described. Example 1: An ingot of silicon single-crystal semiconductor having a diameter of approximately 200mm was sliced using a wire saw of the above-described first embodiment. The flow rate of the slurry was varied according to a pattern shown in FIG. 5, which defines a variation in the flow rate with respect to the cutting depth in the workpiece. Specifically, a discharge from a slurry feed pump was 50 1/min when slicing was started, 120 1/min when the central portion of the ingot was subjected to slicing, and 70 1/min when slicing was ended. 10 ingots were sliced to obtain approximately 2500 wafers, while the flow rate of the slurry was varied according to the pattern of FIG. 5. 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. 6, thereby obtaining a difference TV5 between the maximum and minimum thickness measurements. Example 2: An ingot of silicon single-crystal semiconductor having a diameter of approximately 200mm was sliced using a wire saw of the above-described second embodiment. The flow rate of slurry was maintained constant at 120 1/min during slicing. A slurry feed position was varied according to a pattern shown in FIG. 7, which defines a variation in the slurry feed position with respect to the cutting depth in the workpiece. Specifically, the slurry feed position was located 0.9r distant as measured opposite to a workpiece from a position located just above the centreline of a roller when slicing was started, located just above the roller centerline when the central portion of the ingot was subjected to slicing, and located 0.6r distant as measured toward the workpiece from a position located just above the roller centerline when slicing was ended. 10 ingots were sliced to obtain approximately 2500 wafers, while the slurry feed position was varied according to the pattern of FIG. 6. 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. 6, 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 200mm was sliced using a wire saw of the above-described first embodiment in a manner similar to that of Example 1 except that the feed rate of the slurry was maintained constant at 120 1/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. 6, thereby obtaining a difference TV5 between the maximum and minimum thickness measurements. Table 1 shows the test results of Examples 1 and 2 and Comparative Example 1. Ingots (pcs) Wafers (pcs) Average TV5 (µm) Crack Generation (%) Example 1 10 2503 10 0.2 Example 2 10 2475 12 0.3 Comparative Example 1 5 1036 25 3.5 In Example 1, only the flow rate of the slurry is varied, while in Example 2, only a slurry feed position is varied. However, both the flow rate of the slurry and the slurry feed position may be varied in combination so as to more finely control the feed amount of the slurry. 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, Examples 1 and 2 of the present invention show a smaller value of TV5 that indicates the degree of variation in the thickness of each wafer. Further, Examples 1 and 2 show 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 embodiments. The above-described embodiments are mere examples, 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 slurry feed rate, a slurry viscosity or 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 comprising a wire and a slurry feeder, characterized in that the feed rate of slurry fed to the workpiece is varied according to the cutting length in the workpiece or the angle between the wire and the circumference of the workpiece, such that the 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 slurry fed to the cylindrical workpiece is increased as the cutting length in the workpiece increases, and subsequently the feed rate of the slurry 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 slurry is varied such that as compared with the rate of increase in the feed rate of the slurry 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 slurry during slicing from the central portion of the workpiece to a slicing end portion is made lower. A method of slicing a cylindrical workpiece according to any one of Claims 1 - 3, characterized in that the feed rate of the slurry fed to the cylindrical workpiece is varied by changing the flow rate of the slurry fed to the wire. A method of slicing a cylindrical workpiece according to any one of Claims 1 - 4, characterized in that the feed rate of the slurry fed to the cylindrical workpiece is varied by changing the feed position of the slurry fed to the wire. A method of slicing a cylindrical workpiece according to Claim 5, characterized in that the feed position of the slurry is varied along the wire between a position in the proximity of the cylindrical workpiece and a position corresponding to a far end of a roller as viewed from the workpiece. A method of slicing a cylindrical workpiece according to Claim 5 or 6, characterized in that the feed position of the slurry is located most distant from the workpiece when slicing starts and is moved toward the workpiece as the cutting length increases, so that the feed-position of the slurry is located closest to the workpiece when the cutting length reaches a maximum, and the feed position of the slurry is moved away from the workpiece as the cutting length decreases. A wire saw for slicing a cylindrical workpiece of a fragile material, such as a semiconductor ingot, comprising a wire and a slurry feeder, characterized by comprising means for varying the feed rate of slurry fed to the workpiece according to the cutting length in the workpiece or the angle between the wire and the circumference of the workpiece, such that the work amount on the workpiece is adjusted to a constant value. A wire saw according to Claim 8, characterized in that the means for varying the feed rate of the slurry fed to the workpiece is means for varying the flow rate of the slurry. A wire saw according to Claim 8, characterized in that the means for varying the feed rate of the slurry fed to the workpiece is means for varying the feed position of the slurry. A wire saw according to Claim 10, characterized in that the means for varying the feed position of the slurry is means for moving a slurry feed nozzle toward or away from the workpiece along the wire.

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