Method for cutting a plurality of slices from a workpiece by means of a wire saw during a series of cutting operations

Abstract

The invention relates to a method for separating a plurality of slices from a workpiece (4) by means of a wire saw, wherein the wire saw comprises a wire grid (2) formed by moving line sections of a sawing wire (3) and a control device (12), and the wire grid (2) is tensioned in a plane between two wire guide rollers (1), wherein each of the two wire guide rollers (1) is mounted between a fixed bearing (5) and a floating bearing (6). The method comprises conveying the workpiece (4) through the wire grid (2) by controlling the temperature of the fixed bearing (5) with a cooling fluid according to a specification of a temperature profile while moving the floating bearing (6) axially, and moving the workpiece (4) along the workpiece axis simultaneously according to a specification of a second correction profile.

Description

Technical Field

[0001] This invention relates to a method for cutting multiple slices from a workpiece using a wire saw during a series of cutting operations. The wire saw comprises a wire array formed by moving segments of a sawing wire and an actuating device. The wire array is tensioned in a plane between two wire guide rollers, each of which is supported between a fixed bearing and a floating bearing. During each cutting operation, in the presence of a working fluid and a hard material, the corresponding workpiece is fed through the wire array via the actuating device in a feed direction perpendicular to the workpiece axis and perpendicular to the plane of the wire array, the hard material acting abrasively on the workpiece.

[0002] Existing technology / problems

[0003] This method of cutting multiple slices can be achieved by overlapping or grinding the slices.

[0004] In the case of overlapping cuts, the working fluid is supplied in the form of a slurry composed of hard materials in a liquid carrier medium to the working space formed between the cutter surface and the workpiece. In the case of overlapping cuts, material is removed by means of the three-body interaction involving the tool carrier (cutting line), the tool (abrasive), and the workpiece.

[0005] In the case of abrasive cutting, a saw filament with a hard surface material is used, and a working fluid that does not contain abrasive material itself and serves as a cooling lubricant is provided. In the case of abrasive cutting, the material is removed by means of a two-body interaction involving the diamond-coated saw filament as a tool and the workpiece.

[0006] In the case of a conventional wire saw, each wire guide roller has a bearing near each of its end faces, which is fixedly connected to the machine frame and is called a fixed bearing, and a bearing near the opposite end face, which can move relative to the machine frame in the axial direction of the wire guide roller and is called a floating bearing.

[0007] There are known measures designed to counteract changes in the arrangement of the linear array and the workpiece relative to each other during the cutting operation in order to improve the planar parallelism of the main surface of the slice to be cut.

[0008] US 5,377,568 discloses a method in which the position of a reference surface located on the outside of a line guide roller relative to a machine frame is measured, and an increase or decrease in thermal length in the line guide roller is caused by adjusting the temperature inside the line guide roller until the change in the measured position of the reference surface is compensated again.

[0009] WO 2013 / 079683 A1 discloses a method in which the slice shapes obtained at different temperatures of an online guide roller bearing are first measured and each of these shapes is stored together with its respective associated bearing temperature, and then the bearing temperature that best matches the desired target shape is selected in subsequent cutting.

[0010] US 5,875,770 discloses a method in which the shape of a cut slice is measured, a correction curve dependent on the cutting depth is calculated by forming a difference relative to the desired shape of the slice, and in a subsequent cut, during the cutting operation, the workpiece is moved in an axial direction relative to the linear array according to the correction curve.

[0011] Despite these measures, continuous improvement is still needed, partly because these measures have only limited effectiveness, and partly because the requirements for the flatness and parallelism of the slices are becoming increasingly stringent, especially in the semiconductor industry. Summary of the Invention

[0012] The purpose of this invention is to provide slices whose shape matches the target shape as closely as possible.

[0013] The object of the present invention is achieved by a method for cutting multiple slices from a workpiece using a wire saw during a series of cutting operations divided into an initial cut and subsequent cuts, wherein the wire saw includes a wire array formed by moving wire segments of a sawing wire and an actuation device, and the wire array is tensioned in a plane between two wire guide rollers, wherein each of the two wire guide rollers is supported between a fixed bearing and a floating bearing, the method comprising:

[0014] During each cutting operation, in the presence of working fluid and hard material, the corresponding workpiece is fed through the linear array via an actuating device along a feed direction perpendicular to the workpiece axis and perpendicular to the plane of the linear array. The hard material acts abrasively on the workpiece, including:

[0015] During each cutting operation, the workpiece is fed through the linear array by adjusting the temperature of the fixed bearing with cooling fluid according to a first temperature profile, which is dependent on the cutting depth and specifies the temperature of the cooling fluid, and is related to a first correction profile, which is dependent on the cutting depth and specifies the stroke of the floating bearing.

[0016] The workpiece is fed through a linear array, and simultaneously moved along the workpiece axis by means of an actuating element according to a second correction curve, which specifies the workpiece's stroke. The first and second correction curves are opposite to the shape deviation.

[0017] Determine the shape deviation during and / or before each cutting operation.

[0018] This method can be configured for overlapping or grinding slices. The cutting depth (doc) refers to the length in the region from the entry cut into the workpiece to the exit cut out of the workpiece, opposite to the feed direction.

[0019] Adjusting the temperature of the fixed bearing (hereinafter referred to as line guide temperature control, WGTC) will cause the fixed bearing and therefore its components to contract or expand in the axial direction of the line guide roller, thereby causing axial movement of the floating bearing and thus movement of the line guide roller relative to the workpiece.

[0020] Regulating the temperature of the fixed bearing thus has the same qualitative effect as moving the workpiece along its axis by means of an actuating element (hereinafter referred to as ingot positioning control, IPC). The actuating element is preferably a piezoelectric actuator. Therefore, the variable causing the corresponding stroke is the temperature of the cooling fluid in the case of WGTC, and the signal used to drive the actuating element in the case of IPC.

[0021] According to the invention, two measures are provided to minimize any possible shape deviations. This method has particular advantages. If the measures are used in combination, the magnitude (stroke) of the movement of the line guide roller relative to the workpiece can be achieved, which is greater than that possible when only one of the measures is used. The range of movement of the floating bearing and the workpiece, which linearly depends on the variable causing the movement, is wider than the corresponding range available when only one of the measures is used. From the change of the variable causing the movement to the actual occurrence of the movement, a significantly longer response time is required in the case of WGTC than in the case of IPC, especially if the actuating element is a piezoelectric actuator. Therefore, the two measures have different control bandwidths. Thus, it is advantageous to combat relatively high-frequency shape deviations, i.e., shape deviations that vary with a relatively large gradient according to the cutting depth, by means of IPC and to combat relatively low-frequency shape deviations by means of WGTC. Due to the different response times, the control by IPC can be used to suppress the overshoot of the control by WGTC. Furthermore, the control bandwidth of the combination of IPC and WGTC is greater than the control bandwidth of either IPC or WGTC. The combination of the two measures makes it possible to achieve a larger stroke per unit time because the movement of the floating bearing and the workpiece is performed by two independent actuators.

[0022] If shape deviation is determined before the cutting operation, the term shape deviation refers to the deviation of the slice's shape curve or the average shape curve of the slice from a reference shape curve.

[0023] If shape deviation is determined during the cutting operation, the term shape deviation refers to the deviation of the position of a line passing through the center of the observed cut, or the deviation of the position of the average of multiple such lines from the position of a reference trajectory. If the cutting operation is completely fault-free, i.e., there is no unexpected axial movement of the line guide rollers and / or axial movement of the workpiece, then the line and the reference trajectory have the same position with respect to a common reference. The common reference is a position that remains stationary in space, such as a position on the frame. If multiple cuts are observed, the positions of the lines passing through the center of the cuts are averaged to give the position of the average line. A point at the same height as the line or average line and the reference trajectory represents a specific cutting depth. Therefore, the distance of such a point on the line or average line to the corresponding point on the reference trajectory indicates the shape deviation at the cutting depth represented by that point.

[0024] Observation of one or more incisions is preferably performed by irradiation with light, IR, X-rays, or gamma rays. Additionally, mechanical scanning of the incisions or measurement of their inductance or capacitance may also be considered.

[0025] Determine shape deviations during and / or before each cutting operation.

[0026] According to one embodiment, the determination of shape deviation is based on observing one or more cuts and establishing two closed control loops. The first control loop responds to control errors, i.e., to the determined shape deviation, by means of a WGTC (WGed Controlled Transformer), and the second control loop responds by means of an IPC (Integrated Circuit Control), wherein the movement of the floating bearing and the workpiece to correct the shape deviation is performed using work distributed between the loops. The floating bearing is moved by means of the WGTC according to a first correction curve, which determines the stroke of the floating bearing based on the cut depth. A temperature curve, which specifies the coolant temperature based on the cut depth, is correlated with the first correction curve. The required temperature change of the cooling fluid to cause the specified stroke of the floating bearing of the corresponding line guide roller is determined beforehand through experiments. The first correction curve determines the proportion of stroke performed by the floating bearing based on the cut depth to reduce the shape deviation. The workpiece is moved by means of the IPC according to a second correction curve. The second correction curve determines the proportion of stroke performed by the workpiece based on the cut depth to reduce the shape deviation. The sum of the two proportions corresponds to the stroke required to reduce the determined shape deviation. The stroke proportions can be distributed equally or differently.

[0027] According to another embodiment, the determination of shape deviation is based on a comparison of the average shape curve of the cut slices with a reference shape curve. This comparison provides a basis for determining the required stroke, dependent on the cutting depth, even before the cutting operation, to avoid shape deviation, which is expected in the absence of countermeasures, given this comparison. The entire correction curve is divided into first and second correction curves, and this determines the stroke ratio to be performed by means of WGTC and IPC. In this case, the stroke ratio can also be divided equally or in other proportions.

[0028] According to another embodiment, shape deviations are determined based on observation of one or more incisions and a comparison of the average shape curve of the cut slices with a reference shape curve. Shape deviations determined by observing one or more incisions are designated as a first or alternatively a second correction curve, and shape deviations determined by comparing the average shape curve with the reference shape curve are designated as a second or alternatively a first correction profile. Therefore, if WGTC is used to determine the shape deviations occurring during each cutting operation, then IPC is used to determine the shape deviations determined prior to each cutting operation, or vice versa.

[0029] The surface of the slice consists of a main surface and edge surfaces. The main surface includes the front and back sides of the slice. The slice can be measured by being arranged between a pair of sensors, which is common in the case of warped measurements. Each sensor measures the distance to the main surface facing the slice at a measurement point. The measurement points can be distributed on the main surface or located on the slice diameter at a deviation of no more than ±20° from the feed direction. The measurement points are preferably located at position i along the diameter of the slice, specifically opposite to the feed direction, and thus each measurement point is associated with a specific cutting depth. The density of measurement points is preferably not less than one per centimeter, and the distance between a measurement point and its nearest neighbor is preferably the same for all measurement points.

[0030] The shape curve of the slice is the line connecting measurement points si, which are calculated at position i according to the rule si = D - (FDi - BDi), where D is the distance between the sensors, FDi is the distance between the upper sensor and the corresponding measurement point on the front side of the slice, and BDi is the distance between the lower sensor and the corresponding measurement point on the rear side of the slice. It should be noted that alternative definitions of the shape curve can also be used to implement this invention, provided that such alternative definitions encode the shape of the slice based on the cutting depth.

[0031] The average shape curve of a slice is a shape curve obtained by averaging the shape curves of multiple slices. The reference shape curve is the desired shape curve, preferably the shape curve of a slice with perfectly flat and mutually parallel main surfaces. The average shape curve of the slices is determined, preferably derived from at least 1 to 5 cutting operations performed by the same wire saw, where these cutting operations immediately precede those performed by the wire saw. The selection of slices for generating the average shape curve can be based on slices or cuts, or may include both. In the case of slice-based selection, certain slices from the cutting operations are used to determine the corresponding average shape curve by averaging, while others are excluded. For example, only slices with specific positions in the workpiece are considered during the averaging process, such as only every 15th to 25th slices along the workpiece axis. Another possibility based on slice selection is to exclude some slices from all the slices in the cutting operations that have shape curves with the largest and smallest deviations from the average shape curve of all slices (so-called trimmed average). Alternatively, slices whose shape curves deviate from the average shape curve of all slices in the cutting operations by more than 1 to 2 sigma can be excluded from the average slices. In the cut-based selection, all slices from at least one cut operation are used to determine the average shape curve and all slices from at least one other cut operation are excluded.

[0032] During a series of cutting operations, the average shape profile of the slices changes. These changes are preferably used to evaluate the performance of the wire saw. They can indicate wear on the surface layer of the sawing wire and / or the wire guide roller, or on any other wear-affected component of the wire saw. Therefore, preferably, a threshold for shape deviation is defined, and when this threshold is reached or exceeded, maintenance activities (predictive maintenance activities) are initiated instead of further cutting operations. Even before such a threshold is reached, such changes can be used as an opportunity to take adjustment measures to counteract the deterioration of work results due to wear. Such adjustment measures can be, for example, changing the composition and / or temperature of the working fluid and / or changing the linear speed and / or other process-specific parameters.

[0033] The cutting operation that occurs after a change in the saw system represents a special case. This change occurs, for example, when there is a change in the wire guide roller, a mechanical adjustment to the wire saw, or a change in the physical or chemical properties of the working fluid. The first cutting operation after changing the saw system, the so-called initial cut, preferably comprises 1 to 5 cutting operations. For the initial cut, the shape deviation is preferably determined by comparing the average shape curve of the slice with a reference shape curve, wherein the average shape curve of the slice produced by the same wire saw during one or more initial cutting processes performed before changing the saw system is used.

[0034] The wire saw used according to the invention comprises two or more wire guide rollers, such as three or four. The temperature adjustment of the fixed bearings of the wire guide rollers can be limited to two wire guide rollers, by which the wire array is tensioned between the two wire guide rollers, and by means thereof the workpiece is fed.

[0035] The workpiece is preferably made of a semiconductor material, such as silicon, in a polycrystalline or monocrystalline state. The outer perimeter of the workpiece's cross-section is square, rectangular, or circular. In the case of a cylindrical workpiece, the workpiece axis extends through the center of the cylinder. The method according to the invention is particularly suitable for producing circular semiconductor wafers made of monocrystalline silicon with a diameter of at least 200 mm, and particularly at least 300 mm.

[0036] The details of the present invention are described below with reference to the accompanying drawings.

[0037] Brief description of the attached figures

[0038] Figure 1 The features of a wire saw that functions in the use of the present invention are schematically shown.

[0039] Figure 2 More details of the wire saw are shown.

[0040] Figure 3 The path of the cut through the workpiece is shown, as well as the envisioned path represented by the reference trajectory.

[0041] Figure 4 The arrangement of slices between two sensors is shown to determine their shape deviations before the cutting operation.

[0042] Figure 5 This demonstrates the advantages gained when adding the magnitudes of WGTC and IPC.

[0043] Figure 6 The range is shown, within which the magnitude of the shift A depends linearly on the variable causing the shift.

[0044] Figure 7 This demonstrates the favorable division of the overall correction curves between WGTC and IPC.

[0045] Figure 8 The results show that combining WGTC and IPC can result in greater travel per unit time.

[0046] Figure 9 The diagram shows how the favorable division of the overall correction curves between WGTC and IPC suppresses amplitude overshoot, which would be observed if WGTC were used as the sole measurement.

[0047] List of reference numerals

[0048] Line guide roller

[0049] 2-line array

[0050] 3 sawing lines

[0051] 4 workpieces

[0052] 5 Fixed supports

[0053] 6 floating bearings

[0054] 7 racks

[0055] 8-sided layer

[0056] 9 channels

[0057] 10 directional arrows

[0058] 11 directional arrows

[0059] 12 Actuating Devices

[0060] 13-cut detector

[0061] 14 Data Processing Units

[0062] 15 Actuating Elements

[0063] 16 control units

[0064] 17 incisions

[0065] 18 Reference Trajectories

[0066] 19 Upper Sensor

[0067] 20 Lower Sensors

[0068] 21 slices

[0069] Detailed description of exemplary embodiments according to the present invention

[0070] A wire saw suitable for performing the method according to the invention comprises a wire array 2 consisting of moving wire segments of a cutting wire 3, the wire array 2 being tensioned in a plane between two wire guide rollers 1. During the cutting operation, a workpiece 4 is fed through the wire array 2 in a feed direction perpendicular to the workpiece axis and perpendicular to the plane of the wire array 2 by means of an actuation device 12. During this operation, each of the wire guide rollers 1 and the workpiece 4, which are tensioned in the wire array 2, is moved in the axial direction according to direction arrows 10 and 11, depending on first and second correction curves. The first and second correction curves cancel out shape deviations determined before or during the cutting operation. To determine the shape deviations during the cutting operation, there is a cut detector 13 for observing the cut and a data processing unit 14 for generating the first and second correction curves. The data processing unit 14 transmits a control signal to an actuation element 15, which, depending on the cutting depth, causes the workpiece 4 to move in the direction of the workpiece axis indicated by direction arrow 11 according to the second correction curve.

[0071] like Figure 2 As shown, a wire guide roller 1 is mounted between a fixed bearing 5 and a floating bearing 6. The fixed bearing 5 and the floating bearing 6 are supported on a frame 7. The wire guide roller 1 has a surface layer 8 with grooves in which the cutting line 3 travels. The fixed bearing 5 includes a channel 9 through which cooling fluid for regulating the temperature of the fixed bearing 5 passes. If the temperature of the fluid increases, the thermal expansion of the fixed bearing 5 causes the wire guide roller 1 to move axially in the direction of the floating bearing 6, and the floating bearing 6 to move outward relative to the frame 7 in the direction of the axis of the wire guide roller 1 indicated by the directional arrow 10. If the temperature of the cooling fluid decreases, it causes the wire guide roller 1 and the floating bearing 6 to move in opposite directions. The temperature of the cooling fluid is defined by a temperature profile dependent on the cutting depth, which is related to a first calibration curve. A control unit 16 connected to a heat exchanger and a pump ensures that the fluid passing through the fixed bearing 5 has the temperature required by the corresponding temperature profile when a specific cutting depth is reached. A data processing unit 14 transmits this temperature profile to the control unit 16, which causes the floating bearing to move according to the first calibration curve.

[0072] Figure 3 The path through the cut 17 of workpiece 4 and the envisioned path represented by reference trajectory 18 are shown. The corresponding deviation of the line through the center of the cut from the reference trajectory corresponds to the overall correction curve, which is divided between WGTC and IPC in the form of first and second correction curves.

[0073] Figure 4The arrangement of a slice between two sensors 19 and 20 is shown for determining its shape deviation prior to the cutting operation. Sensors 19 and 20 measure the distance FDi from the front side of the upper sensor 19 to the front side of the slice 21 and the distance BDi from the rear side of the slice 21 to the lower sensor 20 at specific positions i along the diameter of the slice 21 in the feed direction, according to a specific cutting depth. The slice shape curve is a line connecting the measured values ​​si calculated according to the rule si = D - (FDi - BDi), where D represents the distance between the sensors. The slice shape deviation is obtained by comparing the slice shape curve with a reference shape curve. The deviation from the reference shape curve, dependent on the cutting depth, corresponds to an overall correction curve, which is divided between WGTC and IPC in the form of first and second correction curves.

[0074] Figure 5 This demonstrates the advantages gained when adding the amplitudes of WGTC and IPC. If the entire calibration curve requires a stroke of amplitude A on the order of the dashed line, this stroke cannot be accomplished by WGTC or IPC alone, as the achievable stroke using the respective individual measurements is insufficient. Only the combination of WGTC and IPC makes this possible.

[0075] Figure 6 The results show that within a range B2 with a rest position B1, the magnitude of the movement A is linearly dependent on the variable causing the movement (input, I). Beyond this range, deviations from linear behavior occur, resulting in a Δ error. By combining WGTC and IPC, the range B2 of the overall system's linear response is increased.

[0076] Figure 7 This demonstrates a favorable division of the overall correction curves between WGTC and IPC, which includes compensating for low-frequency shape deviations using WGTC and the first correction curve, and compensating for high-frequency shape deviations using IPC and the second correction curve. For this purpose, a crossover is preferably used, which, with the aid of a low-pass filter (LP) and a high-pass filter (HP), allocates the low-frequency portion of the shape deviation to correction using WGTC and the high-frequency portion to correction using IPC.

[0077] Figure 8 The results show that by using a combination of WGTC and IPC, a larger total amplitude A2+A1 can be achieved per unit time Δt because the movement of the floating bearing and the workpiece is performed by two independent actuators. When using WGTC or IPC alone, only one amplitude of A1 and A2, respectively, is possible within that time unit.

[0078] Figure 9The diagram shows how the favorable division of the overall correction curves between WGTC and IPC suppresses the overshoot of amplitude A over time t (right-hand plot), which would be observed if WGTC were used as the sole measurement (left-hand plot).

[0079] The above description of the illustrative embodiments should be understood as exemplary. This disclosure enables those skilled in the art to understand the invention and its related advantages, while also encompassing changes and modifications to the described structures and methods that are readily apparent to those skilled in the art.

Claims

1. A method for cutting multiple slices from a workpiece using a wire saw during a series of cutting operations divided into an initial cut and subsequent cuts, wherein, The wire saw includes a wire array formed by moving wire segments of a sawing wire and an actuation device, and the wire array is tensioned in a plane between two wire guide rollers, wherein each of the two wire guide rollers is supported between a fixed bearing and a floating bearing, the method comprising: During each cutting operation, in the presence of working fluid and hard material, the corresponding workpiece is fed through the linear array by means of the actuating device along a feed direction perpendicular to the workpiece axis and perpendicular to the plane of the linear array, and the hard material abrades the workpiece, including: During each cutting operation, the workpiece is fed through the linear array by adjusting the temperature of the fixed bearing with cooling fluid according to a temperature profile specified by the cutting depth and by simultaneously moving the floating bearing axially. This temperature profile, which determines the temperature of the cooling fluid based on the cutting depth, is correlated with a first correction curve, which in turn determines the stroke of the floating bearing based on the cutting depth. The workpiece is fed through the linear array, and simultaneously moved along the workpiece axis by means of an actuating element according to a second correction curve, which specifies the workpiece's stroke, wherein the first and second correction curves are opposite to the shape deviations; and The shape deviation is measured by comparing the position of a line passing through the center of at least one cut with the position of a reference trajectory during each cutting operation, and determined by comparing the average shape profile of the cut slices with a reference shape profile before each cutting operation.

2. The method of claim 1, wherein the average shape profile is determined by averaging the shape profile of the selected slice, selected based on slice, based on cut, or based on both slice and cut.

3. The method of claim 1 or 2, wherein the cut slice originates from at least 1 to 5 cut operations immediately preceding the corresponding cut operation.

4. The method of any one of claims 1 to 3, wherein if the shape deviation determined prior to the corresponding cutting operation reaches or exceeds a defined threshold, maintenance measures are initiated instead of the corresponding cutting operation.

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