Wire with fixed abrasive grain, wire saw and method for cutting a workpiece

Abstract

Wire with fixed abrasive grain, comprising a core wire and abrasive grains fixed to a surface of the core wire, wherein an abrasive grain density of 1200 grains / mm² 2 or more, where the abrasive grain density is the number of abrasive grains per unit area on the surface of the core wire and 2% or less of all distances between centroids of the abrasive grains are equal to or shorter than an average circle equivalent diameter of the total abrasive grains.

Description

TECHNICAL AREA

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

[0002] A workpiece, such as a 300 mm diameter silicon ingot, is primarily manufactured using a conventional loose-abrasive grain process. In this process, the workpiece is cut while a slurry containing suspended abrasive grains, such as silicon carbide, is applied to a wire. This process requires 20 to 25 hours for cutting when using silicon carbide abrasive grains with a grit size of #1000 to #1500 (average grain diameter: approximately 8 to 10 µm). It has recently been observed that the high integration of semiconductor devices, such as DRAM, NAND flash memory, and MPUs, is reaching technical limitations, and the cost reduction per silicon semiconductor device through miniaturization is consequently reaching its limits.With this in mind, an increase in diameter to 450 mm is being considered in order to achieve more semiconductor devices from one wafer and continuous cost reduction.

[0003] When a 450 mm diameter silicon ingot is cut using the loose-grain cutting method with abrasive grains such as silicon carbide grains, the cutting process takes approximately 40 to 50 hours, while simultaneously increasing the surface area of ​​the silicon ingot. This significantly reduces productivity per wafer.

[0004] Furthermore, the cutting of the 450 mm diameter silicon ingot utilizes a wire with fixed abrasive grains, featuring diamond abrasive grains fixed to a core wire. This wire is used to cut ingots for solar cells, such as silicon, glass, magnet, crystal, sapphire, and silicon carbide (SiC) ingots (see patent document 1). A typical wire with fixed abrasive grains is produced by fixing diamond abrasive grains with a minimum diameter of 4 µm and a maximum diameter of 16 µm to the surface of a metal core wire with a diameter of approximately 0.1 mm by electrolytic plating.

[0005] Cutting an ingot with a wire containing fixed abrasive grains uses a standard multi-wire chainsaw to cut the ingot by pressing it against the wire while a coolant, which contains no abrasive grains, is supplied to the moving wire. Compared to the conventional loose-grain method, where the ingot is cut while a slurry containing suspended abrasive grains, such as silicon carbide with an average grain diameter of approximately 10 µm, is supplied to the wire, the above method can reduce cutting time by half to one-third. The elimination of loose abrasive grains also offers significant advantages in terms of reducing the costs associated with separating abrasive grains and metal powder or disposing of waste fluid used in the cutting process. QUOTE LIST PATENT LITERATURE

[0006] Patent document 1: Japanese unexamined patent application publication number (Kokai) No. JP 2010-74056A

[0007] Patent document 2: JP 2013-43268A describes a wire made of solid abrasive wire. The wire with solid abrasive grains has solid abrasive grains on the surface of the wire core, and the wire core diameter is fixed to the wire core diameter. The wire core diameter is 80 µm or less, and the wire is bonded to the surface of the wire core with solid abrasive grains. The number of abrasive grains per unit area of ​​the core wire is 400 / m. 2 or more. The number of abrasive grains per surface unit of the wire core is 400 pieces / m. 2 or more. SUMMARY OF THE INVENTIONAL PROBLEM

[0008] In general, more precise flatness and warping of semiconductor silicon wafers are required with the miniaturization of highly integrated circuits (LSI), especially memory LSI and logic LSI. Warping is a parameter that indicates the shape when a silicon wafer is held in its natural state without any vacuum suction. As in Fig. As shown in Figure 5, a thickness center plane is used as a measurement plane, a best-fit plane of the thickness center plane is used as a reference plane, and warping is defined as the difference between the minimum and maximum values ​​obtained by subtracting the reference plane from the measurement plane. A wafer with deteriorated warping can cause focus deviation and superposition errors in a device exposure process. In particular, a large-diameter wafer, with a diameter of 450 mm, causes these problems considerably.

[0009] When grinding a silicon ingot with wire containing fixed abrasive grains, the accuracy of the wafer shape after grinding is important. TTV (the difference between the maximum and minimum thickness values ​​in the plane of the wafer) and warpage of cut wafers are compared between the following two cases: a silicon ingot with a diameter of 450 mm and a length of approximately 120 mm is ground with wire containing fixed abrasive grains for 23 hours, with diamond abrasive grains with a minimum diameter of 4 µm and a maximum diameter of 16 µm fixed to a 0.1 mm diameter core wire; the silicon ingot is ground with conventional loose abrasive grains, while a slurry containing suspended silicon carbide abrasive grains with an average abrasive grain diameter of approximately 10 µm is fed to the wire for 48 hours.

[0010] As in Fig.As shown in Figure 6, both TTV and wafer warping are significantly worse when cut with conventional wire with fixed abrasive grains compared to TTV and wafer warping when cut with the loose abrasive grain method. Furthermore, it is known that cutting with wire with fixed abrasive grains does not significantly improve TTV and warping, even when the cutting time is extended, and the wire with fixed abrasive grains tends to wander through the workpiece during cutting. The present invention has been further developed to address the problems described above.It is an object of the present invention to provide a wire with fixed abrasive grain, a wire saw and a method for cutting a workpiece which can control the drift of the wire with fixed abrasive grain during the cutting of a workpiece and can improve TTV and warping of the wafer cut from the workpiece. PROBLEM SOLVING

[0011] To achieve this goal, the present invention provides a wire with fixed abrasive grains, comprising a core wire and abrasive grains fixed on a surface of the core wire, wherein an abrasive grain density of 1200 grains / mm² 2or more, where the abrasive grain density is the number of abrasive grains per unit area of ​​the surface of the core wire, and 2% or less of all distances between the centroids of the abrasive grains are equal to or shorter than an average circle equivalent diameter of the total abrasive grains.

[0012] The aforementioned abrasive grain density allows for the control of wire drift with fixed abrasive grains during workpiece cutting, which is caused by a low abrasive grain density. Conversely, a high abrasive grain density can easily cause agglomeration of the abrasive grains and thus lead to wire drift with fixed abrasive grains during workpiece grinding. In the present invention, however, the percentage of the intercentroidal spacing that is equal to or less than the average circle equivalent diameter is 2% or less. This percentage allows the fixed abrasive grains to be uniformly distributed, which reduces agglomeration. Wire drift with fixed abrasive grains due to agglomeration can therefore be controlled.Consequently, the wire according to the invention with fixed abrasive grain can improve the properties, such as TTV and warping of wafers cut from the workpiece, which makes it possible to obtain wafers with good dimensional accuracy at high productivity.

[0013] The abrasive grains can be diamond abrasive grains. Diamond abrasive grains are preferably used as the abrasive grains of the wire with fixed abrasive grain according to the present invention.

[0014] In the wire with fixed abrasive grain, the abrasive grains that are fixed to the surface of the core wire can have a particle size distribution in which a minimum grain diameter is 4 µm and a maximum grain diameter is 16 µm.

[0015] The abrasive grains of the wire according to the invention with fixed abrasive grain preferably exhibit the particle size distribution described above. Such abrasive grains enable the workpiece to be cut efficiently.

[0016] To achieve this objective, the present invention further provides a wire saw comprising a wire array formed by spirally winding the wire according to the invention with fixed abrasive grain around a plurality of wire guides, a workpiece feed unit for holding a workpiece and pressing the workpiece against the wire array, and a nozzle for supplying a coolant to a contact section between the workpiece and the wire, wherein the wire saw is configured to cut the workpiece into wafers by pressing the workpiece against the wire array with the workpiece feed unit, while the coolant is supplied to the wire via the nozzle.

[0017] Such a wire saw can control the deviation of the wire with fixed abrasive grain during cutting of the workpiece and improve TTV and warping of the wafers cut from the workpiece, thus making it possible to obtain wafers with good dimensional accuracy at higher productivity than in the loose abrasive grain approach.

[0018] To achieve this objective, the present invention further provides a method for cutting a workpiece, comprising: producing a wire with fixed abrasive grains having a core wire and abrasive grains fixed to a surface of the core wire; forming a wire array by helically winding the produced wire with fixed abrasive grains around a plurality of wire guides; and pressing the workpiece against the wire array while imparting an axial reciprocating motion to the wire with fixed abrasive grains to cut the workpiece into wafers, wherein the production of the wire with fixed abrasive grains comprises the steps of acquiring an image of a surface of the wire with fixed abrasive grains and extracting the abrasive grains from the image by image binarization analysis, measuring the number of extracted abrasive grains, and determining an abrasive grain density.which is the number of abrasive grains per unit area, measuring the circle equivalent diameter of the extracted abrasive grains and calculating an average circle equivalent diameter of all abrasive grains, measuring the centroids of the extracted abrasive grains and determining all distances between the centroids of the abrasive grains and selecting the wire with fixed abrasive grain, if the wire with fixed abrasive grain is sufficient so that the determined abrasive grain density is 1200 grains / mm², 2 or more and includes 2% or less of the determined distances between the centroids of the abrasive grains being equal to or shorter than the calculated average circle equivalent diameter of the total abrasive grains, where the workpiece is cut with the selected wire with fixed abrasive grain.

[0019] Such a method allows for the simple and precise selection of the wire with fixed abrasive grain, which meets the requirement that the abrasive grain density is 1200 grains / mm². 2 or more, and 2% or less of all distances between the centroids of the abrasive grains are equal to or shorter than the average circle equivalent diameter of all abrasive grains, so that the selected wire can be used to cut the workpiece. In this way, wafers can be cut from the workpiece with high dimensional accuracy and higher productivity than in the loose-grain method. Furthermore, the costs of disposing of waste slurry fluid can be reduced more significantly than in the loose-grain method.

[0020] The image of the surface of the wire with the fixed abrasive grain can be captured using a scanning electron microscope or a confocal laser microscope.

[0021] In the present invention, such devices are preferably used to observe the surface of the wire with fixed abrasive grain.

[0022] The abrasive grains can be diamond abrasive grains. Diamond abrasive grains are preferred over the abrasive grains of wire with fixed abrasive particles.

[0023] The abrasive grains, which are fixed to the surface of the core wire in the manufactured wire with fixed abrasive grain, can have a particle size distribution in which a minimum grain diameter is 4 µm and a maximum grain diameter is 16 µm.

[0024] In the present invention, abrasive grains exhibiting such a fine particle size distribution are preferably used. Such abrasive grains enable the workpiece to be cut efficiently. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0025] The wire with fixed abrasive grains, the wire saw, and the workpiece cutting method according to the invention can significantly reduce cutting time compared to cutting with loose abrasive grains and significantly improve dimensional accuracy, such as TTV and wafer warpage, compared to cutting in the conventional fixed abrasive grain method. Thus, the invention, which enables the high-productivity cutting of a silicon ingot with the wire with fixed abrasive grains, can be applied to large-diameter silicon wafers for memory LSI or logic LSI, which require strict wafer dimensional accuracy. Furthermore, the elimination of loose abrasive grains allows for a significant cost reduction in wafer manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS Fig.Figure 1 is a schematic diagram showing an example of the wire saw according to the invention; Fig. Figure 2 is a flowchart showing an example of the inventive method for cutting a workpiece; Fig. Figure 3 is a graph showing the results of the TTV and warping of examples 1 to 3 and comparison examples 1 and 2; Fig. Figure 4 is a graph showing the results of the TTV and warping of comparison examples 1 to 4; Fig. 5 is an explanatory diagram showing the definition of warping; Fig. Figure 6 is a graph showing the values ​​of TTV and warping of the wafers cut using the loose abrasive grain method and the conventional fixed abrasive grain method; Fig. Figure 7 is an image showing an example of image binarization analysis using image processing software; and Fig.Figure 8 is a schematic diagram showing an example of a manufacturing device usable for the production of the wire according to the invention with fixed abrasive grain. DESCRIPTION OF THE EXECUTION FORMS

[0026] The following will describe embodiments of the present invention, however the present invention is not limited thereto.

[0027] As mentioned above, the method of cutting a workpiece with the wire with fixed abrasive grain can cut the workpiece in a short time, however, the shape accuracy of the cut-out wafer deteriorates compared to the method of cutting a workpiece with the loose abrasive grain approach.

[0028] In order to solve the problems, the present inventor has repeatedly and ambitiously carried out the following investigations.

[0029] It is assumed that the deterioration in wafer shape accuracy resulting from the use of wire with fixed abrasive grains is caused by the wire's drift during cutting. The present inventor has assumed that this wire drift is caused by variations in the abrasive grain density on the surface of the wire with fixed abrasive grains and has conducted repeated experiments.

[0030] The following results were observed: Form accuracy, such as TTV and warping, tends to improve with an increase in abrasive grain density, which is the number of abrasive grains per unit area. However, TTV and warping are worse than with the loose abrasive grain cutting method. Conversely, a wire with fixed abrasive grains and a higher than specified grain density significantly degrades form accuracy, such as TTV and warping. This means that wafers with good form accuracy cannot be achieved solely by increasing the number of abrasive grains.

[0031] The inventor then investigated the cause of the deterioration of the wafer shape due to the wire with fixed abrasive grain having more than a prescribed density of abrasive grains and confirmed that the wafer shape accuracy is altered by a dispersion state (an agglomerated state) of the abrasive grains on the surface of the wire with fixed abrasive grain and that good shape accuracy can be achieved when the abrasive grains are uniformly distributed in a previously described pattern.

[0032] Based on these findings, the inventor discovered that the deterioration of wafer shape accuracy can be controlled by increasing the density of the abrasive grains on the surface of the core wire, i.e., by increasing the number of abrasive grains and uniformly dispersing them. The inventor then found that the aforementioned problems can be solved by using a wire with fixed abrasive grains, exhibiting an abrasive grain density of 1200 grains / mm². 2 or more is sufficient and the percentage of the distance between the centroids of the abrasive grains that are equal to or shorter than the average circular equivalent diameter of the entire abrasive grains is 2% or less of all distances between the centroids of the abrasive grains, thereby completing the present invention.

[0033] The present invention will be described in more detail below.

[0034] First, the wire according to the invention with fixed abrasive grain will be described in detail.

[0035] The wire with fixed abrasive grain has a core wire and abrasive grains that are fixed to the surface of the core wire.

[0036] The core wire can be made of metal. Its diameter can be, for example, approximately 0.1 mm.

[0037] The abrasive grains, which are fixed to the surface of the core wire of the wire according to the invention with fixed abrasive grain, can have a particle size distribution in which a minimum grain diameter is 4 µm and a maximum grain diameter is 16 µm.

[0038] If the wire with fixed abrasive grain has abrasive grains with such a fine particle size distribution, the form quality, such as TTV and wafer warping, is improved more significantly.

[0039] The abrasive grains fixed to the core wire can be diamond abrasive grains.

[0040] Diamond abrasive grains are particularly preferred over wire abrasive grains with fixed abrasive grains.

[0041] The wire according to the invention with fixed abrasive grain is characterized by the fact that the abrasive grain density, which is the number of abrasive grains per unit area on the surface of the core wire, is 1200 grains / mm². 2 or more, and the percentage of intercentroidal distances that are equal to or shorter than the average circle equivalent diameter of all abrasive grains is 2% or less in the distribution of all intercentroidal distances of the abrasive grains.

[0042] Essentially, the more abrasive grains are fixed to the surface, the more effectively the deterioration of wafer shape accuracy can be controlled. If the number of abrasive grains per unit area is 1200 grains / mm² 2 or more, as in the present invention, the deterioration of dimensional accuracy due to the absence of abrasive grains can be controlled, and the workpiece can be cut into wafers with improved shape. The density can be 1500 grains / mm². 2 or more.

[0043] If the number of abrasive grains is increased and the grains accumulate on a specific section of the core wire's surface (i.e., if the grains agglomerate), the wire with the fixed abrasive grain will drift during workpiece cutting, and the wafer shape accuracy will deteriorate. This agglomeration of the abrasive grains can be detected as grain overlap. It can be inferred that, of all the distances between the centroids of the abrasive grains, the higher the percentage of the distance that is equal to or shorter than the average circle equivalent diameter of the abrasive grains, the more abrasive grains will agglomerate.

[0044] Therefore, if the percentage of intercentroidal spacing equal to or less than the average circle equivalent diameter is 2% or less of all intercentroidal spacings of the abrasive grains, as in the present invention, non-uniformity in the distribution of the abrasive grains is effectively prevented, and the abrasive grains are dispersed uniformly without agglomeration. The deterioration of wafer shape accuracy caused by agglomeration is thus reduced.

[0045] An exemplary method for producing the wire according to the invention with fixed abrasive grain will now be described.

[0046] The wire according to the invention with fixed abrasive grains can be produced by fixing abrasive grains, such as diamond abrasive grains, to the surface of the metallic core wire by electrolytic nickel plating. The metallic core wire can, for example, be piano wire.

[0047] In particular, the wire with fixed abrasive grain can be produced using a manufacturing apparatus, as in Fig. 8 shown, to be produced.

[0048] In apparatus 20 for producing a wire with fixed abrasive grain, shown in Fig.8. First, stains and rust are removed from the surface of the core wire 24 in a pretreatment tank 21 containing an acetone solution 22 and a hydrochloric acid solution 23. The core wire 24 is then passed through an electroplating solution 27 containing dispersed abrasive grains 26, such as diamond abrasive grains, in an electroplating tank 25, so that the diamond abrasive grains are bonded to the surface of the core wire 24 by electroplating together with nickel ions eluted from a positively charged nickel electrode. The wire with the abrasive grain 1 fixed can thus be produced.The embodiment described above is a typical manufacturing process and apparatus used in the method, but the process for producing the wire with fixed abrasive grain according to the invention is not limited to it, and the wire with fixed abrasive grain according to the invention can be produced by any other manufacturing process.

[0049] The wire saw according to the invention is now referred to as Fig. 1. will be described.

[0050] As in Fig. Figure 1 shows a wire saw 11 of the fixed abrasive grain type, which mainly comprises a wire with fixed abrasive grain 1 for cutting a workpiece W, grooved rollers 2, mechanisms 3, 3' for applying a tensile force, a workpiece feed unit 4, and a coolant feed unit 5. The wire with fixed abrasive grain 1 is the wire with fixed abrasive grain according to the invention described above.

[0051] The wire with fixed abrasive grain 1 is unwound from a wire reel 6 and enters the grooved roller 2 via a traverser after passing through the traction mechanism 3, which includes a powder clutch (a constant-torque motor) and a dancer roller (a dead weight). The grooved rollers 2 can be formed by press-fitting a polyurethane resin around a steel cylinder and then cutting grooves at regular intervals into their surface.

[0052] The wire with fixed abrasive grain 1 is wound approximately 300 to 400 times around the grooved rollers 2 to form a wire strand. After passing through the other mechanism 3' for applying tensile force, the wire with fixed abrasive grain 1 is wound around the other wire spool 6'. A drive motor 10 can impart a reciprocating motion to the wound wire with fixed abrasive grain 1. During the reciprocating motion of the wire with fixed abrasive grain 1, it can move in each direction over varying distances; therefore, the distance of movement in one direction can be greater than in the other, allowing a new strand of wire to be fed in. This enables the new strand to be fed in the direction of the greater distance while the reciprocating motion of the wire with fixed abrasive grain 1 continues.

[0053] The coolant supply unit 5 includes a tank 7, a cooler 8, and a nozzle 9. The nozzle 9 is located above the wire array, which is formed by winding the wire with fixed abrasive grain 1 around the grooved rollers 2. The nozzle 9 is connected to the tank 7, and the coolant, the temperature of which is controlled by the cooler 8, is supplied to the wire 1 with fixed abrasive grain through the nozzle 9.

[0054] The workpiece W is held by the workpiece feeder 4. The workpiece feeder 4 is configured to guide the workpiece W downwards from above the wire to below the wire, pressing the workpiece W against the reciprocating wire with fixed abrasive grain 1 and cutting the workpiece. At this point, the held workpiece W can be fed at a pre-programmed feed rate with a predetermined feed quantity by computer control. After the workpiece W has been cut, it can be moved in a direction opposite to the feed direction to pull the cut workpiece W out of the wire array.

[0055] Such a wire saw, including the wire according to the invention with fixed abrasive grain, can significantly reduce the cutting time compared to the wire saw of the loose abrasive grain type and can cut a workpiece with better dimensional accuracy than a conventional wire saw of the type with fixed abrasive grain.

[0056] The inventive method for cutting a workpiece will now be described.

[0057] The inventive method for cutting a workpiece includes the steps described below, as in Fig. 2 shown, one.

[0058] First, a wire with fixed abrasive grains is produced. At this point, an image of the surface of the wire with fixed abrasive grains is taken, and the abrasive grains are extracted from the image using image binarization analysis (p. 101 in Fig. 2).

[0059] The image of the surface of the wire with fixed abrasive grain can be captured using a scanning electron microscope or a confocal laser microscope.

[0060] In particular, a scanning electron microscope or a confocal laser microscope with an imaging depth that is almost equal to the radius of the wire with fixed abrasive grain is preferably used to capture the image of the surface of the wire with fixed abrasive grain.

[0061] The captured image can be processed by image binarization using, for example, the image processing software WinRoof (available from MITANI Corp.), as shown in Fig. 7 shown, to be analyzed.

[0062] Then the number of abrasive grains extracted by image binarization analysis is measured, and the abrasive grain density, which is the number of abrasive grains per unit area, is determined (S102 in Fig. 2).

[0063] Then the circle equivalent diameters of the extracted abrasive grains are measured and an average circle equivalent diameter of all abrasive grains is calculated (S103 in Fig. 2).

[0064] Then the centroids of the extracted abrasive grains are measured and all distances between the centroids of the abrasive grains are determined (S104 in Fig. 2).

[0065] In this way, the abrasive grain density, which serves as an index of the number of abrasive grains on the surface of the wire with fixed abrasive grain and the dispersion status of the abrasive grains, the average circle equivalent diameter and the distances between the centroids of the abrasive grains are determined.

[0066] The sequence of steps for determining the abrasive grain density, based on the extracted abrasive grains (S102 in Fig. 2), the step of calculating the average circle equivalent diameter (S103 in Fig. 2), and the step of measuring the intercentroidal distances (S104 in Fig. 2) is not in the order shown in the flowchart of Fig. 2 limited; these three steps can be performed in any order.

[0067] The captured image of the wire's surface with the abrasive grain fixed is a planar image, while the surface of the actual wire has a cylindrical shape. Therefore, if someone wishes to accurately measure the area that serves as the denominator of the abrasive grain density, the planar image could be converted to the cylindrical shape before the measurement. Similarly, the planar image could be converted to the cylindrical shape beforehand to measure all the distances between the centroids of the abrasive grains. Conversely, if someone wants to calculate these indices as simply as possible, the area used as the denominator of the abrasive grain density and all the distances between the centroids of the abrasive grains can be measured using the planar image as is. Thus, the measurement, including the conversion to the cylindrical shape, can be appropriately determined according to the desired measurement accuracy, cost, etc.

[0068] Then the wire with fixed abrasive grain is selected (S105 in Fig. 2).

[0069] In this step, the wire with fixed abrasive grain is selected if the wire with fixed abrasive grain meets the requirement that the abrasive grain density is determined in the step of determining the abrasive grain density (S102 in Fig. 2), 1200 grains / mm 2 or more and 2% or less of all intercentroidal distances of the abrasive grains, determined in the step of measuring the intercentroidal distances (S104 in Fig. 2), are equal to or shorter than the average circle equivalent diameter of the total abrasive grains, calculated in the step of calculating the average circle equivalent diameter (S103 in Fig. 2).

[0070] Then, a wire array is formed by spirally winding the selected wire with fixed abrasive grain around the wire guides, and the workpiece is pressed against the wire array while an axial reciprocating motion is imparted to the wire with fixed abrasive grain and a coolant is supplied for cutting the workpiece in wafer (S106 in Fig. 2) The workpiece is thus completely cut.

[0071] The method, as in the present invention, which includes taking an image of the surface of the wire with fixed abrasive grain and measuring the abrasive grain density, the intercentroid distances and the average circle equivalent diameters, based on the taken image, in order to evaluate the number and dispersion state (agglomeration state) of the abrasive grains, can easily and precisely evaluate the condition of the abrasive grains on the surface of the wire with fixed abrasive grain.

[0072] As an example of a conventional method for evaluating and inspecting a wire with fixed abrasive grains, a method is proposed that involves irradiating the wire with light, recording a projection image of the wire, and determining the number of abrasive grains, the intervals between the grains, etc. (see, for example, Japanese unexamined patent application publication number (Kokai) No. JP 2006-95644A). However, the area where the abrasive grains are observed is limited to a small section when the grains are detected by projection. Furthermore, the conventional method, which measures the distances between the abrasive grains by determining a standard deviation from a contour of the projection image, can hardly distinguish agglomeration of the abrasive grains.

[0073] In contrast, the present invention can easily and precisely evaluate the condition of the abrasive grains on the surface of the wire with fixed abrasive grain, as described above.

[0074] In this way, the method according to the invention enables the simple and precise selection of the wire with fixed abrasive grain, which satisfies the requirement that the number of fixed abrasive grains is large, i.e., the abrasive grain density is 1200 grains / mm². 2or more, and the abrasive grains are uniformly dispersed without an unbalanced distribution, meaning that 2% or less of the distances between the centroids of the abrasive grains are equal to or shorter than the average circle equivalent diameter. This allows for control of wafer shape accuracy degradation. Furthermore, cutting with the fixed abrasive grain approach enables a significant reduction in cutting time, an improvement in productivity, and a reduction in the costs required to manufacture wafers.

[0075] The abrasive grains are preferably diamond abrasive grains.

[0076] Diamond abrasive grains are often used as the abrasive grains in the fixed abrasive grain procedure.

[0077] The abrasive grains, which are fixed to the surface of the core wire in the manufactured wire with fixed abrasive grain, can have a particle size distribution in which a minimum grain diameter is 4 µm and a maximum grain diameter is 16 µm.

[0078] If the abrasive grains exhibit such a particle size distribution while satisfying the above abrasive grain density and the above percentage of the distance between the centroids of the abrasive grains, wafers with improved dimensional accuracy can be obtained. The particle size distribution can be adjusted to the above range by first manufacturing abrasive grains that satisfy the above particle size distribution and fixing the abrasive grains to the core wire.

[0079] The inventive method for cutting a workpiece is suitable for cutting a silicon single-crystal ingot, in particular with a diameter of 450 mm or more.

[0080] When such a large-diameter silicon single-crystal ingot is cut with a wire containing fixed abrasive grains, the cutting process using the conventional wire with fixed abrasive grains significantly degrades the dimensional accuracy. However, the inventive method for cutting a workpiece can significantly control the deterioration in dimensional accuracy. EXAMPLE

[0081] The present invention will be described in more detail below with reference to the examples and comparative examples, but the present invention is not limited thereto. (Example 1)

[0082] A wire with a fixed abrasive grain, selected according to the inventive method for cutting a workpiece, shown in Fig. 2, was attached to a wire saw, as in Fig.Figure 1 shows that a workpiece was then cut using a wire saw in accordance with the inventive method for cutting a workpiece. Then, the TTV and warpage of all wafers cut from the workpiece were measured and their average values ​​were calculated.

[0083] The wire with fixed abrasive grain used in this example had a core wire with a diameter of 0.1 mm and diamond abrasive grains fixed to the core wire with a particle size distribution in which a minimum diameter was 6 µm and a maximum diameter was 12 µm.

[0084] The wire with fixed abrasive grain had an abrasive grain density of 1200 grains / mm². 20.9% of all distances between the centroids of the abrasive grains were equal to or shorter than the average circle equivalent diameter of all the abrasive grains. An image of the surface of the wire with the abrasive grain fixed was acquired using a scanning electron microscope. The acquired image was analyzed by image binarization using WinRoof image processing software (available from MITANI Corp.).

[0085] The workpiece to be cut was a silicon single-crystal ingot with a diameter of 450 mm and a length of about 120 mm.

[0086] Table 1 summarizes the abrasive grain density (grains / mm²). 2), average circle equivalent diameter (µm), average intercentroid distance (µm) and percentage (%) of the distance that is equal to or shorter than the average circle equivalent diameter of all the abrasive grains, of all the distances between the centroids of the abrasive grains in Example 1, Examples 2, 3 and Comparative Examples 1 to 4, described later, together.

[0087] The measurement results of the TTV and the warping of the cut-out wafers in examples 1 to 3 and the comparison examples 1 to 4 are shown in Table 1 and Fig. 3 shown.

[0088] In Example 1, the cut-out wafers had an average TTV of 12.55 (µm) and an average warpage of 15.1 (µm); thus, the obtained wafers had a much better shape accuracy than those in the comparison examples described later. (Example 2)

[0089] A silicon single-crystal ingot was cut under the same conditions as in Example 1, except that the wire used with fixed abrasive grain had an abrasive grain density of 1500 grains / mm². 2 The test showed that 2.0% of all distances between the centroids of the abrasive grains were equal to or shorter than the average circle equivalent diameter of all the abrasive grains. Then the average TTV and warpage of all cut wafers were calculated.

[0090] Consequently, the wafers had, as shown in Table 2 and Fig. Figure 3 showed an average TTV of 11.8 (µm) and an average warpage of 12.5 (µm); thus, the obtained wafers had a much better dimensional accuracy than those in the later described comparison examples. (Example 3)

[0091] A silicon single-crystal ingot was cut under the same conditions as described in Example 1, except that the wire used with fixed abrasive grain had an abrasive grain density of 1500 grains / mm². 2 The test showed that 1.5% of all distances between the centroids of the abrasive grains were equal to or shorter than the average circle equivalent diameter of all the abrasive grains. Subsequently, the average TTV and warpage of all cut wafers were calculated.

[0092] Consequently, as shown in Table 2 and Fig. Figure 3 shows that the wafers had an average TTV of 12.43 µm and an average warpage of 10.45 µm; thus, the obtained wafers had much better dimensional accuracy than those in the comparison examples described later. The TTV and warpage values ​​were equal to the dimensional quality obtained by cutting using the loose-abrasive grain method (see Figure 3). Fig.6) This confirmed that the present invention, which uses wire with fixed abrasive grains, can significantly reduce cutting time and achieve wafers with a dimensional accuracy equivalent to that obtained with loose abrasive grains. Additionally, TTV and warpage were improved in Examples 2 and 3 compared with Example 1. This is assumed to be due to the higher abrasive grain density, namely 1500 grains / mm². 2 , is caused, in comparison with Example 1, while the above percentage was maintained at 2% or less in the distribution of the distances between the centroids of the abrasive grains. (Comparative example 1)

[0093] A silicon single-crystal ingot was cut under the same conditions as in Example 1, except that the wire used with fixed abrasive grain had an abrasive grain density of 900 grains / mm². 2exhibited, and 1.6% of all distances between the centroids of the abrasive grains were equal to or shorter than the average circle equivalent diameter of all abrasive grains. Then the average TTV and warpage of all cut-out wafers were calculated.

[0094] Consequently, as in Fig. 3 and Fig. Figure 4 and Table 2 show that the wafers have an average TTV of 26.81 µm and an average warpage of 32.22 µm. It has thus been shown that when the number of abrasive grains is low, i.e., when the abrasive grain density is less than 1200 grains / mm² 2 This results in both a deterioration in TTV and wafer warping, and the wafers exhibit significantly poorer shape accuracy. (Comparative example 2)

[0095] A silicon single-crystal ingot was cut under the same conditions as in Example 1, except that the wire used with fixed abrasive grain had an abrasive grain density of 1000 grains / mm². 2 exhibited, and 1.8% of all distances between the centroids of the abrasive grains were equal to or shorter than the average circle equivalent diameter of all abrasive grains. Then the average TTV and warpage of all cut-out wafers were calculated.

[0096] Consequently, as in Fig. 3 and Fig. As shown in Figure 4 and Table 2, the wafers have an average TTV of 20 (µm) and an average warpage of 28 (µm). It has thus been shown that if the abrasive grain density is less than 1200 grains / mm² 2 This results in both a deterioration in TTV and wafer warping, and the wafers exhibit significantly poorer shape accuracy. (Comparative example 3)

[0097] A silicon single-crystal ingot was cut under the same conditions as in Example 1, except that the wire used with fixed abrasive grain had an abrasive grain density of 1200 grains / mm². 2 exhibited, and 2.8% of all distances between the centroids of the abrasive grains were equal to or shorter than the average circle equivalent diameter of all abrasive grains. Then the average TTV and warpage of all cut-out wafers were calculated.

[0098] Consequently, as in Fig. Figure 4 and Table 2 show that the wafers have an average TTV of 16.64 µm and an average warpage of 18.25 µm. It has thus been shown that even if the abrasive grain density is 1200 grains / mm² 2or more, both the TTV and the warping of the wafers deteriorate when more than 2% of the distances between the centroids of the abrasive grains are equal to or shorter than the average circle equivalent diameter and the abrasive grains were agglomerated with the increase in abrasive grain density, and the wafers exhibited significantly worse shape accuracy. (Comparative example 4)

[0099] A silicon single-crystal ingot was cut under the same conditions as in Example 1, except that the wire used with fixed abrasive grain had an abrasive grain density of 1500 grains / mm². 2 exhibited, and 4.3% of all distances between the centroids of the abrasive grains were equal to or shorter than an average circle equivalent diameter of all abrasive grains. Then the average TTV and warpage of all cut-out wafers were calculated.

[0100] Consequently, as in Fig. Figure 4 and Table 2 show that the wafers had an average TTV of 22.57 µm and an average warpage of 29.04 µm. It has thus been shown that as agglomeration of the abrasive grains progresses with increasing abrasive grain density, both TTV and warpage of the wafers deteriorate more significantly, and the wafers exhibit significantly poorer dimensional accuracy. [Table 1] wire Abrasive grain density [grains / mm²] 2 ] Average circle equivalent diameter [µm] Intercentroidal distance (distribution) Average [µm] Percentage of ≤average circle equivalent diameter Comparative example 1 900 11,7 146 1,60 % Comparative example 2 1000 13,1 142,5 1,80 % Comparative example 3 1200 12 141,8 3,80 % Comparative example 4 1500 12,3 137,1 4,30 % Example 1 1200 14,9 140,4 0,90 % Example 2 1500 12,8 139,5 2,00 % Example 3 1500 15,8 142,8 1,50 % [Table 2] wire TTV [µm] Warping [µm] Comparative example 1 26,81 32,22 Comparative example 2 20 28 Comparative example 3 16,64 18,25 Comparative example 4 22,57 29,04 Example 1 12,55 15,1 Example 2 11,8 12,5 Example 3 12,43 10,45

[0101] It should be noted that the present invention is not limited to the preceding embodiment. The embodiment is merely exemplary, and any examples that exhibit essentially the same features and demonstrate the same functions and effects as in the technical concept described in the claims of the present invention are included within the technical scope of protection of the present invention.

Claims

[1] Wire with fixed abrasive grain, comprising a core wire and abrasive grains fixed on a surface of the core wire, wherein an abrasive grain density of 1200 grains / mm² 2 or more, where the abrasive grain density is the number of abrasive grains per unit area on the surface of the core wire and 2% or less of all distances between centroids of the abrasive grains are equal to or shorter than an average circle equivalent diameter of the total abrasive grains. [2] The wire with fixed abrasive grain according to claim 1, wherein the abrasive grains are diamond abrasive grains. [3] The wire with fixed abrasive grain according to claim 1 or 2, wherein the abrasive grains which are fixed on the surface of the core wire have a particle size distribution in which a minimum grain diameter is 4 µm and a maximum grain diameter is 16 µm. [4] Wire saw comprising a wire array formed by spiral winding of a wire with fixed abrasive grain according to any one of claims 1 to 3, comprising a plurality of wire guides, a workpiece feed unit for holding a workpiece and pressing the workpiece against the wire array, and a nozzle for supplying a coolant to a contact section between the workpiece and the wire, wherein the wire saw is configured to cut the workpiece into wafers by pressing the workpiece against the wire array with the workpiece feed unit while the coolant is supplied to the wire through the nozzle. [5] Method for cutting a workpiece, comprising: Manufacturing a wire with fixed abrasive grains using a core wire and abrasive grains fixed to a surface of the core wire; forming a wire array by spirally winding the manufactured wire with fixed abrasive grains around a plurality of wire guides; and pressing the workpiece against the wire array while an axial reciprocating motion is imparted to the wire with fixed abrasive grains to cut the workpiece into wafers, wherein the manufacturing of the wire with fixed abrasive grains includes the steps of Taking an image of a surface of the wire with fixed abrasive grain and Extracting the abrasive grains from the image using image binarization analysis, Measuring the number of extracted abrasive grains and determining an abrasive grain density, which is the number of abrasive grains per unit area, Measuring the circle equivalent diameters of the extracted abrasive grains and calculating an average circle equivalent diameter of all abrasive grains, Measuring the centroids of the extracted abrasive grains and determining all distances between the centroids of the abrasive grains and Selecting the wire with fixed abrasive grain, if the wire with fixed abrasive grain meets the requirement that the specific abrasive grain density is 1200 grains / mm². 2 or more, and 2% or less of the determined distances between the centroids of the abrasive grains are equal to or shorter than the calculated average circle equivalent diameter of the total abrasive grains, the workpiece being cut with the selected wire with fixed abrasive grain. [6] The method for cutting a workpiece according to claim 5, wherein the image of the surface of the wire with fixed abrasive grain is recorded using a scanning electron microscope or a confocal laser microscope. [7] The method for cutting a workpiece according to claim 5 or claim 6, wherein the abrasive grains are diamond abrasive grains. [8] The method for cutting a workpiece according to any one of claims 5 to 7, wherein the abrasive grains fixed to the surface of the manufactured wire with fixed abrasive grain have a particle size distribution in which a minimum grain diameter is 4 µm and a maximum grain diameter is 16 µm.

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