Method and apparatus for simultaneously slicing multiple slices from a workpiece - Patent Application 20070122997

Abstract

A method and wire saw for simultaneously slicing multiple slices from a workpiece during a slicing operation is provided, the method comprising selecting a nonlinear pitch function dTAR(WP) in dependence on a target thickness characteristic value function TTAR(WP), a pitch function dINI(WP), and a thickness characteristic value function TINI(WP), wherein dTAR(WP) assigns pitches during the slicing operation to adjacent grooves in a casing of a wire guide roller at positions WP, TINI(WP) assigns thickness characteristic values ​​measured on the slices to slices obtained during multiple previous slicing operations by the wire saw at positions WP, dINI(WP) assigns pitches during previous slicing operations to adjacent grooves in a casing of a wire guide roller at positions WP, TTAR(WP) assigns target thickness characteristic values ​​to slices sliced ​​during the slicing operation at positions WP, and WP represents an axial position of the adjacent grooves relative to an axis of the wire guide roller.

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for simultaneously slicing multiple slices from a workpiece with a wire saw, and to a wire saw suitable for carrying out the method. [Background technology]

[0002] Prior art / issue Thin, particularly uniform slices of a particular material are required for many applications. One example of a slice that is subject to particularly stringent requirements regarding uniformity and plane parallelism of the front and back surfaces is a slice made of semiconductor material, called a "wafer," which is used as a substrate for the fabrication of microelectronic components. So-called wire sawing, in which multiple slices are simultaneously sliced ​​from a workpiece, is particularly important for producing such slices, as it is particularly economical.

[0003] The required method and apparatus are known, for example, from DE 10 2016 211 883 A1 or DE 10 2013 219 468 A1. In such a method, a wire is guided spirally around at least two wire guide rollers, which stretch a web composed of parallel wire sections across the web, facing the workpiece. The wire guide rollers have a straight cylindrical shape, are rotatable about their axes, and have a plurality of circularly closed grooves on their sides that guide the wire in a plane perpendicular to the axis. The rotation of the wire guide rollers generates relative movement between the wire sections and the workpiece. The wire saw also has a feeder, to which the workpiece is fixed via a saw beam (sacrificial beam), and the feeder feeds the workpiece onto the wire web. The relative movement in the presence of an abrasive causes material removal from the workpiece when it comes into contact with the wire web. With relative motion and continuous feeding in the presence of an abrasive, the wire sections form slice gaps in a material-removing manner in the workpiece, and the wire web slowly works its way through the entire workpiece until the wire web is completely within the saw beam, where the workpiece is connected by an adhesive joint. The workpiece is then completely separated into slices that hang from the saw beam like the teeth of a comb, held only by the adhesive joint.

[0004] Wire sawing can be differentiated according to wire-wrapped slicing and wire-ground slicing. Wire-wrapped slicing is characterized by a three-way interaction between the initially abrasive-free wire, the abrasive loosely distributed in the slurry, and the workpiece. Wire-ground slicing is characterized by a two-way interaction between the abrasive tightly entrapped on the wire surface and the workpiece.

[0005] The wire is typically composed of hypereutectoid pearlitic steel (piano wire). The steel wire is typically coated with a very thin layer of brass or zinc, the ductility of which acts as a drawing aid (solid lubricant) during wire production and as corrosion protection for the finished wire. In wire-wrap slicing, the abrasive typically consists of silicon carbide (SiC), suspended in a viscous carrier fluid composed of oil or glycol. In wire-grinding slicing, the abrasive typically consists of diamond, which is fixed to the wire surface by electroplating, synthetic resin bonding, or form-fitting (pressing, rolling). The wire is typically fed from stock in the form of a spool to the wire web. This spool is called the new wire spool. After passing through the wire web, the wire is also typically fed to stock in the form of a spool. This spool is called the worn wire spool.

[0006] In wire sawing, plain wire is used, and in wire-wrap slicing, so-called structured wire is additionally used. Plain wire has a cylindrical shape with a very large height, i.e., length, of the wire, and the wire diameter corresponds to the diameter of the cylinder. Structured wire includes plain wire with a plurality of protrusions and depressions along its entire length, arranged perpendicular to the longitudinal direction of the wire. Thus, the surface of the structured wire has protrusions and depressions, from which slurry can accumulate on the wire without being wiped off when the wire enters the slicing gap or during its further passage through the slicing gap. Structured wire has an effective diameter corresponding to the diameter of the basic plain wire and the diameter of the cylinder with the smallest diameter that completely contains all of the depressions and protrusions (the so-called outer skin). WO 2006 / 067062 A1 describes an example of a structured wire.

[0007] Slices generally have a cylindrical shape of small height and therefore have a bottom surface (back surface), a top surface (front surface), and side surfaces (edges of the slice). The front and back surfaces of the slices form their major surfaces, and the slice edges form their secondary surfaces.

[0008] The groove of the wire guide roller that first contacts the new wire being fed into the wire web may be referred to as the first groove. The half of the wire web containing the first groove may be referred to as the new wire (feed) side of the wire web. The slice whose major surface is generated by the wire section of the first groove may be referred to as the first slice, and the half of the workpiece containing the first slice may be referred to as the beginning of the workpiece. The groove that last contacts the worn wire being removed from the wire web may be referred to as the last groove. The side of the wire web containing the last groove may be referred to as the worn wire (removal) side of the wire web. The slice whose major surface is generated by the wire section of the last groove may be referred to as the last slice, and the half of the workpiece containing the last slice may be referred to as the end of the workpiece.

[0009] One problem with wire sawing is that the thickness of the slices obtained by the slicing operation has a finite distribution. Typically, further machining of the wire sawed slices, for example by lapping, grinding, etching, or polishing, or a series of these steps, imposes strict requirements on the thickness distribution of the wire sawed slices. For example, to completely remove the surface layer damaged by wire sawing, in each case, a minimum material thickness above the target slice thickness required for subsequent material removal must be provided. None of the slices, or only a maximum percentage of the slices determined by economic considerations, must be below the minimum thickness. At the same time, wire sawed slices often must not exceed a defined maximum thickness for subsequent machining, for example, because subsequent excessive material removal is uneconomical. For example, slices in a lapping operation following wire sawing, in which multiple slices are simultaneously machined as a so-called batch process, must have a very narrow distribution of starting thicknesses. Otherwise, the load of the working disks will be unevenly distributed to the individual slices, which could overload the slices and break them, or the working disks may become misaligned, resulting in slices with a wedge-shaped thickness profile. For example, in the subsequent grinding machining process, the grinding wheels may also become clogged and dull, or in the case of multiple grinding wheels (double-sided machining), there may be uneven material removal from the front and back sides of the slices if too much material is removed in the grinding operation. In addition to the minimum and maximum thicknesses mentioned by way of example, further thickness parameters are possible. Wire-sawed slices must have a narrow distribution for this thickness parameter so that subsequent machining operations after wire-sawing can be performed economically and slices with the required high degree of uniformity and plane parallelism on the front and back sides are obtained. This thickness parameter may, for example, be the average slice thickness.

[0010] The grooves of wire guide rollers generally have a V- or U-shaped cross-sectional profile, which includes groove sides, a groove base, and a recess depth. The groove sides connect to the ungrooved cylindrical sides of the cylinder, giving the wire guide roller the shape of the cylinder. The groove sides also have an opening angle that "catches" the wire and prevents it from accidentally "jumping out" during rotation of the wire guide roller. The groove base is the part of the profile closest to the wire guide roller axis where a wire fed into the wire guide roller with a defined tension will be positioned due to said tension. The recess depth (groove depth) represents the distance from the side of the wire guide roller to the point of the groove closest to the wire guide roller axis. Straight shapes (pot-shaped), pointed shapes (V-shaped), and curved shapes (U-shaped) are known for the groove base.

[0011] It is known that in wire-wrap slicing, the rolling of loose abrasive between the wire and workpiece, which causes the friable and erosive removal of material from the workpiece, also results in the unwanted removal of material from the wire, and the diameter of a plain wire or the effective diameter of a structured wire decreases as a result of wear due to abrasion. In contrast, in wire-grinding slicing, no movement occurs between the abrasive and the wire. In wire-wrap slicing, the wire diameter changes only slightly, specifically as a result of very little diamond abrasion or as a result of individual diamonds occasionally breaking free from their bond with the wire surface.

[0012] It is further known that, particularly in the case of wire-wrapped slices, a constant pitch of the grooves of the wire guide rollers results in an increase in slice thickness from the new wire side to the worn wire side, due to the wire becoming thinner as a result of wear.

[0013] Measures are known in the prior art to prevent non-uniform slice thicknesses from occurring in this manner.

[0014] DE 10 2010 005 718 A1 describes a wire guide roller in which the radius of curvature of the base of the U-shaped grooves, the opening angle of the groove sides, or the groove depth of each groove is varied along the wire guide roller axis. Variation of the groove pitch is also provided.

[0015] Many patents describe cylindrical wire guide rollers with a continuously decreasing inter-groove pitch from the first groove to the last groove, including CN 2015 166 49 U, KR 100 445 192 B, CN 2012 255 84 Y, and CN 101 879 759 A. Summary of the Invention [Problem to be solved by the invention]

[0016] However, none of the wire guide rollers known in the prior art solve the problem of widely distributed slice thickness after wire sawing.

[0017] The object of the present invention is therefore to identify a method for providing slices with a distribution narrow enough for the requirements regarding uniformity and plane parallelism of the front and back surfaces with respect to thickness parameters that are critical for subsequent machining. [Means for solving the problem]

[0018] The object of the invention is to provide a method for simultaneously slicing a plurality of slices from a workpiece having a workpiece axis by means of a wire saw during a slicing operation, the method comprising the steps of moving the wire of the wire saw relative to the workpiece by rotation of a wire guide roller of the wire saw about its axis to lay a wire web made up of wire sections of the wire, the wire guide roller having a casing provided with wire guide grooves, the method further comprising: feeding the workpiece into the wire web perpendicular to the wire sections in the presence of an abrasive, the wire sections machining the workpiece completely through the workpiece; A nonlinear pitch function dTAR(WP) is selected depending on the target thickness characteristic value function TTAR(WP), the pitch function dINI(WP), and the thickness characteristic value function TINI(WP); dTAR(WP) assigns a pitch to adjacent grooves in the casing of the wire guide roller at position WP during the slicing operation; TINI(WP) assigns thickness characteristic values ​​measured on slices to slices obtained during a plurality of preceding slicing operations by the wire saw at position WP; dINI(WP) assigns a pitch to adjacent grooves in the casing of the wire guide roller at position WP during the preceding slicing operation; TTAR(WP) assigns a target thickness characteristic value to a slice sliced ​​during a slicing operation at location WP; WP is achieved through a method that represents the axial position of adjacent grooves relative to the axis of the wire guide roller.

[0019] The pitch dTAR(WP) is preferably selected such that the formula dTAR(WP)=dINI(WP)+TTAR(WP)−TINI(WP) is satisfied.

[0020] The method considers slice thickness characteristics from i preceding slicing operations with a wire saw, and uses a wire guide roller with an initial distribution of groove pitches corresponding to a pitch function dINI(WP). A second pitch function dINI(WP) is freely defined in advance and assigns, for the preceding slicing operation, adjacent grooves in the casing of the wire guide roller a groove pitch that depends on the position WP of the adjacent grooves. This groove pitch also corresponds to the distance between two wire sections in the wire web that extend through the adjacent grooves. The position WP is preferably a discrete length specification that indicates the axial position of adjacent grooves relative to the axis of the wire guide roller and thus also the position of the slices in the wire web.

[0021] The initial distribution according to the pitch function dINI(WP) can be selected, for example, so that the pitch between adjacent grooves continuously decreases by the same amount with the position WP. The method according to the invention further includes specification of a target thickness characteristic function TTAR(WP) that assigns a target thickness characteristic value to the slices sliced ​​during the slicing operation at the position WP. Particularly suitable target thickness characteristic values ​​are minimum thickness, maximum thickness, and standard thickness. The target thickness characteristic value in particular complies with the requirements regarding material removal of subsequent machining steps. The target thickness characteristic value can, for example, be the minimum thickness that each slice needs to have so that there is sufficient material removal available for a subsequent grinding machining, e.g., as a subsequent machining step, to completely remove the near-surface layer damaged by wire sawing.

[0022] The thickness characteristic value may be based on a full area measurement. However, a measurement of the surface of the slice along its diameter, especially along its diameter in the feed direction, is sufficient and preferred for determining the thickness characteristic value. The measurement along the diameter in the feed direction is also considered as the basis for the thickness characteristic values ​​of the examples and comparative examples presented later.

[0023] From the i preceding slicing operations, for each position WP, corresponding i slices with a thickness characteristic value TINI(WP) result. The thickness characteristic value function TINI(WP) assigns a common thickness characteristic value to the slices obtained during the preceding slicing operations with the wire saw at the position WP, measured and calculated by averaging (arithmetic mean) or as quantiles. The minimum thickness of a slice is, for example, such a thickness characteristic value and corresponds to the value of the smallest distance between corresponding measurement points on the front and back surfaces of the slice. The number of measurement points on the slice surface is preferably at least 25. The measurement points are preferably located along a diameter in the feed direction. Instead of the average value of the minimum thickness, it is also possible to use, for example, the average value of the maximum thickness or the average value of the standard thickness of the slices at the position WP as the thickness characteristic value, or to use the average value of the thicknesses of the minimum, maximum or standard thickness quantiles.

[0024] All of the slices are evaluated from the same location WP on the wire web for a thickness characteristic value. The thickness characteristic value can be, for example, the (-3σ) quantile (=0.135 percentile). The (-3σ) quantile of the minimum thickness of the slices from the preceding slicing operation means that 0.135% of all slices at location WP are below this minimum thickness.

[0025] As an example, the target thickness characteristic value function TTAR(WP) may be defined such that it assigns the same constant, TTAR(WP) = constant, to each location WP, in that, for example, the (-3σ) quantile of the minimum thickness of the sliced ​​slices is intended to have the same value for all locations WP.

[0026] Based on the defined target thickness characteristic value function TTAR(WP) and the thickness characteristic value function TINI(WP) resulting from the measurement, a first pitch function dTAR(WP) is determined, which assigns to each position WP the groove pitch between two adjacent grooves having this position. The first pitch function dTAR(WP) is preferably obtained by the formula dTAR(WP)=dINI(WP)+TTAR(WP)-TINI(WP).

[0027] The invention can be practiced with plain wire, a plain wire having a straight cylindrical shape of a very large height, i.e., length, of wire, the diameter of the cylinder representing the diameter of the plain wire and the axis of the cylinder representing the longitudinal wire axis of the plain wire.

[0028] This invention is particularly preferably implemented by the wire wrap slicing method using a structured wire, which is a plain wire having a plurality of depressions and protrusions along its length in a direction perpendicular to the wire axis. For a structured wire, the longitudinal axis of the wire is the axis of a straight cylinder of the smallest diameter that would completely contain the structured wire (sheath), and the diameter of this cylinder is called the effective diameter of the structured wire.

[0029] The invention can be practiced using a unidirectional wire drawing configuration. In unidirectional wire sawing, the wire is moved in exactly one direction of its longitudinal axis throughout the entire duration of the slicing motion. In this case, the speed of wire feed can be variable.

[0030] The invention is particularly preferably implemented using the Pilgrim Step method, which describes wire movement in a continuous sequence of pairs of direction reversals throughout the slicing motion, where one pair of direction reversals involves moving the wire a first length in a first direction along the wire's length and then moving the wire a second length in a second direction opposite the first direction, the first length being selected to be greater than the second length. The pairs of direction reversals may differ in terms of length and speed of wire movement.

[0031] As an example, the first length may be 320 m and the second length 240 m. Thus, during one pair of direction reversals, 320 m + 240 m = 560 m of wire is moved through the slice gap. However, within one such complete Pilgrim cycle, the wire is further moved by a total of 320 m - 240 m = 80 m from the stock of new wire to the stock of worn wire. In the example described, wire sawing in the Pilgrim step method results in wire utilization with a factor of (320 + 240) / (320 - 240) = 560 / 80 = 5. This is advantageous because in the case of only one-time wire use (unidirectional slice method), very high wire consumption occurs and only a small amount of worn wire is worn, which is very uneconomical. The selection of the first length L1 and the second length L2 preferably results in a utilization factor r=(L1+L2) / (L1-L2) being selected such that, when the tensile stress used when the wire is fed into or removed from the wire web is selected, the resulting slice gaps have width variations that would result in unevenness and lack of planar parallelism in the planarity of the front and back surfaces of adjacent slices that bound the slice gap, making the slices unsuitable for the claimed application, such that the wire still does not break and still does not become significantly non-circular (elliptical) as a result of uneven wear.

[0032] The axis of the workpiece is preferably oriented parallel to the axis of the wire guide roller. However, it may be advantageous to rotate the axis of the workpiece relative to the direction of the axis of the wire guide roller, for example, if the workpiece is an ingot made of a monocrystalline semiconductor material and slices (wafers) with a determined crystallographic misorientation are desired. Misorientation is advantageous, for example, if, after wire sawing and subsequent machining (lapping, grinding, etching, polishing), the wafer is additionally provided with a layer made of a further epitaxially applied monocrystalline semiconductor material, since misorientation is accompanied by an increased density of atomic steps at the growth interface, which allows for particularly defect-free growth of the epitaxial layer.

[0033] The method can also be realized using a wire guide roller with grooves of a depth corresponding to a depth function t(WP) that depends on the position WP. Said depth is preferably selected to be proportional to the reduction in wire diameter (plain wire) or reduction in effective diameter (structured wire) as a result of wire wear. To this end, groove depth is understood to mean the extent of the groove in the direction towards the wire guide roller axis, i.e. the difference between the radius of a straight cylinder (where the wire guide roller has said cylindrical shape) and the distance from the wire guide roller axis of the point of the groove profile (where said point has the smallest distance from the wire guide roller axis).

[0034] An additional object is to provide a wire saw for simultaneously slicing a plurality of slices from a workpiece, the wire saw including a wire guide roller having a wire web stretched therethrough and an axis and a casing, the casing being provided with a plurality of grooves for guiding the wire, the pitch between adjacent grooves at a position WP following a nonlinear pitch function dTAR(WP), where WP represents the axial position of adjacent grooves relative to the axis of the wire guide roller.

[0035] The nonlinear pitch function dTAR(WP) is preferably selected so that the equation dTAR(WP) = dINI(WP) + TTAR(WP) - TINI(WP) is satisfied, where TTAR(WP) is a target thickness characteristic value function that assigns a target thickness characteristic value to a slice sliced ​​at position WP, TINI(WP) is a thickness characteristic value function that assigns thickness characteristic values ​​measured on the slice to slices obtained during multiple previous slicing operations by the wire saw at position WP, and dINI(WP) is a pitch function that assigns the pitch that was present during the previous slicing operations to adjacent grooves in the casing of the wire guide roller at position WP.

[0036] Exemplary embodiments of the invention are described below with reference to the drawings, in which: FIG. [Brief explanation of the drawings]

[0037] [Figure 1] FIG. 1 shows elements of a wire saw. [Figure 2] FIG. 2 is a cross-sectional view of a wire guide roller. [Figure 3] FIG. 10 is a diagram showing the (−3σ) quantile of the minimum thickness TMIN of the comparative example and the first example depending on the position WP. [Figure 4] FIG. 10 is a diagram showing the groove pitch WGRP depending on the position WP for the comparative example, the first example, and the second example. [Figure 5] FIG. 10 is a diagram showing the cumulative frequency CF of the minimum thickness TMIN and maximum thickness TMAX of slices from a comparative example, a first example, and a second example. [Figure 6] FIG. 10 shows the maximum thickness TMAX of the slice associated with the comparative example and the first example, depending on the position WP. [Figure 7] FIG. 10 shows the maximum thickness TMAX of the slices from the comparative example and the second example depending on the position WP. [Figure 8] FIG. 10 shows the minimum thickness TMIN of the slices associated with the comparative example and the second example, depending on the position WP. [Figure 9] FIG. 10 shows the (+3σ) quantile of the maximum slice gap width KMAX from lap slice operation using plain wire and structured wire. [Figure 10] FIG. 1 illustrates the characteristic thickness profile of a lap-sliced ​​semiconductor slice. DETAILED DESCRIPTION OF THE INVENTION

[0038] DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS IN ACCORDANCE WITH THE INVENTION FIG. 1 illustrates elements of a wire saw that contribute to an understanding of the present invention. Wire 1 is fed from a stock (a new wire spool, not shown) in direction 9 and spirally guided through grooves 18 around at least two wire guide rollers 27 to produce a wire web 11 composed of wire sections 2 extending parallel to one another. The example wire saw in FIG. 1 shows two wire guide rollers 27: left wire guide roller 3 and right wire guide roller 4. Rotation of wire guide rollers 3 and 4 about axes 5 and 6 in directions 7 and 8 results in the wire sections 2 and wire web 11 moving relative to a workpiece 12 in direction 13. Axes 5 and 6 of wire guide rollers 3 and 4 are oriented parallel to one another. Workpiece 12 has an axis 14 and is connected to saw beam 15 via adhesive joint 16. The saw beam is fixed to a feeder (not shown) that feeds workpiece 12 perpendicular to wire web 11 in direction 17. Preferably, the workpiece 12 is an ingot made of a single-crystal semiconductor material, such as single-crystal silicon. In this case, the workpiece has a straight cylindrical shape and is provided with a crystal orientation identification notch 26 parallel to its axis 14. On both sides of the workpiece 12, the wire web 11 is supplied by nozzles 19 and 20 with jets 22 and 23 of liquid cutting aid through nozzle openings 21. When the wire saw is operated in an abrasive slicing process, the cutting aid is a cooling lubricant that does not contain abrasives, and the wire 1 is a piano wire with a surface in which abrasive diamond particles are firmly embedded. When the wire saw is operated in a lap slicing process, the cutting aid is a slurry of silicon carbide (SiC) in a liquid carrier made of oil or glycol.

[0039] As a result of feeding in direction 17, wire web 11 comes into contact with workpiece 12. As a result of relative motion 13 between wire web 11 and workpiece 12, wire sections 2, in the presence of an abrasive agent, cause material removal from workpiece 12. Upon further feeding 9 with relative motion 13, material removal caused by wire sections 2 forms slice gaps 25 in workpiece 12. The depth in workpiece 12 to which the wire sections penetrate through the workpiece is called cutting depth 24. When cutting depth 24 exceeds the diameter of the workpiece, all of wire sections 2 of wire web 11 have sliced ​​through the workpiece to reach holding beam 15, and the workpiece is completely separated into slices. After passing through the wire web, wire 1 exits wire web 11 in direction 10 toward a used wire stock (wear wire spool, not shown).

[0040] FIG. 2 shows a wire guide roller 27 having a diameter D and a length L in a cross section including the axis 6, and including a core 28 and a casing 29, which may include, for example, a chamber for cooling the wire guide roller. The casing 29 has a coating made of a wear-resistant material, for example, a thermosetting polyurethane. The coating is provided with annular closed grooves 18, which are arranged in a plane perpendicular to the axis 6 and extend around the circumference of the wire guide roller. According to the invention, the pitch of adjacent grooves, which depends on the position WP measured along the axis 6 from the first groove having a depth t1 to the last groove, is selected so that the pitch follows a nonlinear pitch function dTAR(WP). The nonlinear pitch function dTAR(WP) depends on the slice thickness characteristic value of the preceding slicing operation, the pitch function of the groove pitch during the preceding slicing operation, and a predetermined target thickness characteristic value function TTAR(WP). dTAR(WP) varies nonlinearly, as indicated by different pitches, for example, the first groove pitch d1 and the arbitrary groove pitch d. The grooves have an opening angle α of their groove sides and a radius of curvature r of their groove bases. Furthermore, according to one embodiment of the present invention, the groove depth t can vary with position WP according to a depth function t(WP), as shown by the exemplary depth t1 of the first groove and any groove depth t. The remaining cylindrical regions of the sidewalls between the grooves have different widths according to a width function s(WP), depending on the groove pitch dTAR(WP), which is variable with position WP, and optionally depending on the variable groove depth t(WP). Width s1 exemplarily indicates the cylindrical portion between the first and second grooves.

[0041] FIG. 10 shows a schematic thickness diagram of a semiconductor slice 60 obtained via wire-wrap slicing, i.e., a perspective view of a flat central area 59 between the front and back surfaces of the semiconductor slice. Any undulations of the slice that would appear due to the curvature of the central area 59 are absent from this thickness diagram. The central area includes the exact center point of the path connecting corresponding points on the front and back surfaces, respectively. In each case, the corresponding points are connected via paths perpendicular to the central area. A semiconductor slice 60 obtained by wire-wrap slicing typically has a zone of reduced slice thickness, called a cut wedge 56, in the thickness diagram, at the region where the workpiece first contacts the wire web. The cutting, or feeding, of the workpiece during the slicing operation proceeds from left to right along a scan line 58 in FIG. 10. The wire moves perpendicular to the feed direction, i.e., perpendicular to the scan line 58. Thickness characteristics are preferably measured parallel to the scan line 58.

[0042] The cutting wedge 56 is created because new wire is present in the wire web at the start of the slicing operation, causing a wider slice gap and therefore a reduced slice thickness. The wire stock present in the wire web at the start of the slicing operation is the wire stock last cut by the saw beam in the preceding slicing operation. The saw beam is generally made of a material that is softer and easier to machine than semiconductor materials. For example, the saw beam may be made of sintered carbon, synthetic plastic, or glass. Therefore, the saw beam cutting only causes a small amount of wear on the wire present in the wire web at the start of the slicing operation. As the wire is further fed and the initial wear of the wire due to contact with the workpiece occurs, as soon as a cut depth of a few centimeters into the workpiece is achieved, the wire stock in the wire web is replaced with a wire section whose diameter is determined by the ratio of chip volume to wire feed rate, resulting in a thinner slice gap and a larger slice thickness.

[0043] In the region of maximum wire engagement length into the workpiece, i.e., at the center of the scan line 58, the thickness of the wire-wrapped sliced ​​semiconductor slice 60 has a saddle shape 57. This is due to the fact that the wire sections lose most of the cutting aid (slurry) surrounding them during penetration into the workpiece and further penetration toward the workpiece center, which also determines the width of the slice gap created as the cutting aid is wiped off and used up at the workpiece edge and drips off. FIG. 10 shows two such zones of reduced thickness 57 at the edge of the semiconductor slice, facing each other in the wire extension direction. The wire sections are therefore penetrating the workpiece in both directions during Pilgrim Step sawing.

[0044] This behavior and the resulting slice shapes illustrated explain the observed profile of the maximum width of the slice gap as a function of the position WP shown by FIG. 9. The set of points 51 and the associated regression curve 52 show the profile of the (+3σ) quantile of the maximum width of the slice gap from approximately 2000 slicing operations in which a silicon slice (wafer) having a diameter of 300 mm was sliced ​​using a plain wire. For comparison, the set of points 49 and the associated regression curve show the profile of the (+3σ) quantile of the maximum width of the slice gap as a function of the position WP when approximately 1000 slicing operations were performed using a structured wire to obtain a semiconductor slice having a diameter of 300 mm.

[0045] The profile of regression curve 52 indicates that when a plain wire is used, the maximum slice gap width has a nonlinear dependency on position WP, and therefore cannot be compensated for by, for example, a linear reduction in groove pitch so that slices having as similar a thickness as possible can be obtained from each position WP. The set of points 49 and the associated profile of regression curve 50 indicate that when a structured wire is used, the maximum slice gap width depending on position WP may even increase in some places. Regression curve 50 has, for example, a section 53 with a nonlinearly decreasing maximum slice gap width, as well as a section 54 where the maximum slice gap width increases with position WP, despite the fact that the effective diameter of the structured wire decreases with the cumulative engagement length into the workpiece, and a section 55 with a nonlinearly decreasing maximum slice gap width.

[0046] Figure 3 shows, as a comparative example, a set of points 30 representing the (-3σ) quantile of the minimum thickness T of a semiconductor slice obtained from approximately 1,000 slicing runs by lap slicing using a structured wire with a core wire diameter of 175 μm, plotted against the position WP in the wire web. A wire guide roller with a linearly decreasing pitch between adjacent grooves from the first to the last groove was used to cut a straight cylindrical silicon (100) single crystal. Here, up to three ingots with total lengths of up to 400 mm (variable) were sequentially loaded onto the saw beam in one slicing run. Despite the large number of slicing runs, and due to the generally noisy (-3σ) quantile for statistical reasons, only every 25th semiconductor slice was measured, so that the set of points 30 had a significant spread with a third-order polynomial regression 31. A 60 km length of structured wire was used per slicing operation, resulting in a 12 μm difference between the effective diameter of the new wire fed into the wire web and the effective diameter of the used wire removed from the wire web. To ensure that the minimum thicknesses of the first and last semiconductor slices were preferably identical (TMIN), the groove pitch between the first and second grooves was 1134 μm, and the groove pitch between the penultimate and last grooves was 1122 μm. The sum of the linear scaling was therefore 1134 μm - 1122 = 12 μm. However, despite the linear scaling of the groove pitch, it was shown that the semiconductor slices from the intermediate positions deviated from each other by a maximum of 5.7 μm for the regression curve 31 and a maximum of 12.3 μm for the quantiles, according to the set of points 30. Thus, despite the linear scaling of the groove pitch, valuable semiconductor material was wasted due to the production of unnecessarily thick semiconductor slices.

[0047] FIG. 4 shows the initial distribution of groove pitch 34 as a function of pitch for the comparative example, the first example, and the second example. In each case, a linear decrease in groove pitch was selected. For the first and second examples, this profile of groove pitch 34 corresponds to the function dINI(WP), and the slicing operation of the comparative example forms the preceding slicing operation. FIG. 4 also shows the profile of groove pitch 35, which corresponds to the nonlinear pitch function dTAR(WP) calculated by the formula dTAR(WP) = dINI(WP) + TTAR(WP) - TINI(WP) for the first example. The function TINI(WP) represents the thickness of the (-3σ) quantile of the minimum thickness of the slice of the preceding slicing operation, and the target thickness characteristic value function represents a constant (-3σ) quantile of the minimum thickness (TTAR(WP) = 878.6 μm). The profile of groove pitch 35 not only monotonically decreases, but also temporarily has an increasing region. The region of this profile has a structure similar to that of regions 53, 54, and 55 of the regression curve 50 of the (+3σ) quantile of the maximum slice gap width as shown in FIG.

[0048] FIG. 3 shows a set of points 32 representing the (-3σ) quantiles of minimum thickness T from approximately 1,000 slicing operations using lap slicing as a first example, where the thickness characteristic was measured only for every 25th semiconductor slice. Except for the groove pitch, the conditions in the lap slicing were the same as those in the comparative example. The groove pitch 35 followed a nonlinear pitch function dTAR(WP) calculated according to the formula: The fitted polynomial 33 to the set of points 32 is substantially constant for all positions WP in the wire web. The method performed in accordance with the present invention thus provided semiconductor slices with minimum thickness (-3σ) quantiles narrowly distributed around the target value of 878.6 μm.

[0049] 5 shows the cumulative frequencies of the minimum and maximum slice thicknesses TMIN and TMAX of all measured slices, i.e., cumulative frequency 37 of the minimum slice thickness of the comparative example and cumulative frequency 39 of the minimum slice thickness of the first example. Also plotted are cumulative frequencies 38 of the maximum slice thickness of the comparative example, cumulative frequency 40 of the maximum slice thickness of the first example, and cumulative frequencies 41 and 42 of the minimum and maximum slice thicknesses TMIN and TMAX of the second example.

[0050] FIG. 6 shows a set of points 43 at the (+3σ) quantile of the maximum slice thickness TMAX for the comparative example and an associated regression curve 44, and a set of points 45 at the (+3σ) quantile of the maximum slice thickness TMAX for the first example and an associated regression curve 46.

[0051] The results of the second example show that groove pitch can also be optimized, for example, toward a maximum slice thickness that is as uniform as possible. FIG. 7 shows a set of (+3σ) quantile points 61 of maximum slice thickness and an associated regression curve 62 when groove pitch varies with position in the wire web WP during the slicing operation, such as groove pitch 36 shown in FIG. 4 . The profile corresponds to the profile of the nonlinear pitch function dTAR(WP), and in solving the equation, the (+3σ) quantile of maximum thickness was estimated as the thickness characteristic value for the thickness characteristic value function TINI(WP), and a constant (+3σ) quantile of maximum thickness was estimated for the target thickness characteristic value function. For comparison, FIG. 7 again shows a set of (+3σ) quantile points 43 of maximum slice thickness and an associated regression curve 44 for the comparative example.

[0052] FIG. 8 shows a set of points 47 of the (-3σ) quantile of minimum slice thickness 47 for the second example, and an associated regression curve 48, and also, for comparison, a set of points 30 of the (-3σ) quantile of minimum slice thickness 31 for the comparative example.

[0053] Table 1 summarizes the results.

[0054] [Table 1]

[0055] Table 1 shows the variance VAR of the fitted polynomial on which the regression curve is based, and the difference DIFF for the position WP on the wire web where the thickness distribution for the two examples changed relative to the comparative example. In the first example (where the target thickness characteristic function defines a constant (-3σ) quantile of the minimum slice thickness TMIN), the variance in the comparative example (linear reduction in groove pitch) decreased from 5.2 μm to 0 μm. In contrast, the variance for the (+3σ) quantile of the associated maximum slice thickness TMAX increased from 38.3 μm to 58.6 μm. In the second example (where the target thickness characteristic function specifies a constant (+3σ) quantile of the maximum slice thickness TMAX), the variance of the (+3σ) quantile of the maximum slice thickness TMAX decreased from 38.3 μm to 0 μm; in contrast, the variance of the associated (-3σ) quantile of the minimum slice thickness TMIN increased from 5.2 μm to 63.5 μm.

[0056] The above description of exemplary embodiments should be understood as illustrative. The disclosure made thereby, on the one hand, enables a person skilled in the art to understand the present invention and its associated advantages, and, on the other hand, also includes changes and modifications to the described structures and methods that are obvious to those skilled in the art as well. Therefore, all such changes and modifications, as well as equivalents, are intended to be covered by the scope of protection of the claims. [Explanation of symbols]

[0057] 1 wire 2 wire sections 3 Left wire guide roller 4 Right wire guide roller 5 Left wire guide roller shaft 6 Right wire guide roller shaft 7 Rotation of the left wire guide roller 8 Rotation of the right wire guide roller 9 Wire feeding (new wire) 10 Wire removal (worn wire) 11 Wireweb 12 workpieces 13 Wire section movement direction 14 Workpiece axis 15 Saw Beam 16 Adhesive joints 17 Feeding direction 18 Groove 19 Left nozzle 20 Right nozzle 21 Exit opening 22 Left side injection 23 Right side injection 24 cutting depth 25 slice gap 26 Identification notch 27 Wire guide roller 28 Wire guide roller core 29 Wire guide roller covering 30. (-3σ) quantile of minimum thickness with linearly decreasing groove pitch for position WP Regression curve for 31 30 32. (-3σ) quantile of minimum thickness with nonlinearly varying groove pitch for position WP Regression curve for 33 32 34 Linearly decreasing groove pitch 35 Groove pitch for a constant (-3σ) quantile of minimum thickness 36 Groove pitch for a constant (+3σ) quantile of maximum thickness 37 Cumulative frequency CF of minimum thickness with linearly decreasing groove pitch 38 Cumulative frequency of maximum thickness with linearly decreasing groove pitch CF 39 Cumulative frequency of minimum thickness with nonlinearly varying groove pitch optimized for a constant (-3σ) quantile of minimum thickness CF 40 Cumulative frequency of maximum thickness with nonlinearly varying groove pitch optimized for a constant (-3σ) quantile of minimum thickness CF 41 Cumulative frequency of minimum thickness with nonlinearly varying groove pitch optimized for a constant (+3σ) quantile of maximum thickness CF 42 Cumulative frequency of maximum thickness with nonlinearly varying groove pitch optimized for a constant (+3σ) quantile of maximum thickness CF 43 (+3σ) quantile of maximum thickness TMAX versus position WP with linearly decreasing groove pitch Regression curve for 44 43 45 (+3σ) quantile of maximum thickness TMAX versus position WP with nonlinear groove pitch optimized for a constant (-3σ) quantile of minimum thickness Regression curve for 46 45 47 (+3σ) quantile of minimum thickness TMIN versus position WP with nonlinear groove pitch optimized for a constant (+3σ) quantile of maximum thickness Regression curve for 48 47 49 (+3σ) quantile of maximum slice gap width KMAX with linearly decreasing groove pitch for structured wire Regression curve for 50 49 51 (+3σ) quantile of maximum slice gap width KMAX with linearly decreasing groove pitch for plain wire Regression curve for 52 51 53 Sections with monotonically decreasing 54 Sections with Monotonically Increasing 55 Sections with monotonically decreasing 56 Cutting wedge 57 Saddle 58 Scanning direction in feed direction 59 Slice Center Plane 60 slices 61 (+3σ) quantile of maximum thickness TMAX versus position WP with nonlinear groove pitch optimized for a constant (+3σ) quantile of maximum thickness Regression curve for 62 61 α Opening angle CF cumulative frequency d Groove pitch d1 Pitch between the second groove and the first groove dINI(WP) pitch function dTAR(WP) nonlinear pitch function D diameter of wire guide roller DIFF i Disconnection counter KMAX Maximum width of slice gap L Length of wire guide roller r Radius of curvature of the base of the groove s Width of the cylindrical part between adjacent grooves s1 Width of the cylindrical portion between the first and second grooves s(WP) width function t Groove depth t(WP) depth function t1 First groove depth TMIN Minimum slice thickness TMAX Maximum slice thickness TINI(WP) Thickness characteristic function TTAR(WP) target thickness characteristic function VAR variance WGRP Wire guide roller groove pitch WP: The groove pitch position or slice position of two adjacent grooves on the wire guide roller

Claims

1. 1. A method for simultaneously slicing a plurality of slices from a workpiece having a workpiece axis with a wire saw during a slicing operation, comprising: The method further comprises a step of moving a wire of the wire saw relative to the workpiece and tensioning the wire by rotating a wire guide roller of the wire saw about an axis, the wire guide roller having a casing with a wire guide groove formed therein, feeding the workpiece perpendicular to the wire in the presence of an abrasive, the wire slicing the workpiece; The method comprises: a nonlinear pitch function dTAR(WP) is determined based on the target thickness characteristic value function TTAR(WP), the pitch function dINI(WP), and the thickness characteristic value function TINI(WP); dTAR(WP) assigns a pitch during the slicing operation to adjacent grooves in the casing of the wire guide roller at position WP; TINI(WP) assigns thickness characteristic values ​​measured on the slices to slices obtained during a plurality of previous slicing operations by the wire saw at the position WP; dINI(WP) assigns the pitch during the preceding slicing operation to adjacent grooves in the casing of the wire guide roller at the position WP; TTAR(WP) assigns a target thickness characteristic value to a slice sliced ​​during the slicing operation at the position WP; the position WP represents the axial position of the adjacent grooves relative to the axis of the wire guide roller; 10. A method according to claim 9, wherein i preceding slicing operations are performed and the nonlinear pitch function dTAR(WP) is selected such that the equation dTAR(WP)=dINI(WP)+TTAR(WP)−TINI(WP) is satisfied.

2. 2. The method of claim 1, wherein the measured thickness characteristic value is a mean minimum thickness, a mean maximum thickness, a mean standard thickness, or a mean thickness of a quantile of the minimum, maximum, or standard thickness of the measured slices.

3. 3. The method of claim 1 or claim 2, wherein TTAR(WP) is constant.

4. 4. The method of claim 1, wherein the wire is a smooth piano wire composed of hypereutectoid pearlite, and the abrasive is provided in the form of a slurry composed of silicon carbide (SiC) in a carrier fluid composed of glycol or oil.

5. The method of claim 3 , wherein the wire is additionally provided with a plurality of protrusions and depressions perpendicular to the longitudinal direction of the wire.

6. 6. The method according to claim 1, wherein the step of moving the wire is in a continuous sequence of pairs of direction reversals, each pair of direction reversals comprising first moving the wire a first length in a first direction in the wire longitudinal direction and then moving the wire a second length in a second direction diametrically opposite to the first direction, the first length being selected to be greater than the second length.

7. The method according to any one of claims 1 to 6, wherein the workpiece axis is oriented parallel to the axis of the wire guide roller.

8. 8. A method according to any one of claims 1 to 7, wherein the groove has a depth according to a depth function t(WP), which assigns to the groove at the position WP a depth proportional to the outer skin of the wire.

9. 1. A wire saw for simultaneously slicing multiple slices from a workpiece, comprising: a wire guide roller having a shaft and a casing, the wire guide roller having a wire stretched therethrough; The casing is provided with a plurality of grooves for guiding wires, The pitch between adjacent grooves at position WP follows a nonlinear pitch function dTAR(WP), the position WP represents the axial position of the adjacent grooves relative to the axis of the wire guide roller; said non-linear pitch function dTAR(WP) is selected such that the equation dTAR(WP)=dINI(WP)+TTAR(WP)−TINI(WP) is satisfied; TTAR(WP) is a target thickness characteristic value function that assigns a target thickness characteristic value to a slice sliced ​​at the location WP; TINI(WP) is a thickness characteristic value function that assigns thickness characteristic values ​​measured on slices obtained during a plurality of previous slicing operations by the wire saw at the position WP to the slices; dINI(WP) is a pitch function that assigns the pitch that existed during the previous slicing operation to adjacent grooves in the casing of the wire guide roller at the position WP.

10. 10. The wire saw of claim 9, wherein the measured thickness characteristic value is an average minimum thickness, an average maximum thickness, an average standard thickness, or an average thickness of a quantile of the minimum, maximum, or standard thickness of the measured slices.

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