Method for separating plurality of slices from workpiece during separation process by means of wire saw

Abstract

The invention relates to a method for separating a plurality of pieces from a workpiece (2) during a separating process by means of a wire saw, the wire saw comprising a control mechanism and a wire web formed by moving sections of the wire saw, and the wire web being clamped in a plane between two wire guide rollers, wherein each of the two wire guide rollers is mounted between a fixed bearing and a floating bearing, wherein the method comprises: feeding a workpiece (2) through the wire mesh by means of a control mechanism in a feeding direction perpendicular to the workpiece axis and perpendicular to the plane of the wire mesh; supplying a cutting agent to the wire mesh; supplying a temperature control medium to the workpiece; guiding a temperature control liquid through the fixed bearing according to a temperature curve and / or displacing the workpiece along the workpiece axis according to a displacement curve, wherein the temperature curve and the displacement curve counteract the shape deviation; the method is characterized in that the warpage curve of at least one wafer from the starting end (3) of the workpiece and at least one wafer from the tail end (4) of the workpiece in at least one previous separation process using the wire saw is determined; generating a difference value between the warping curves; setting the temperature and / or the volume flow rate of the cutting agent during the separation process according to the difference between the warpage curves; and setting the temperature and / or volume flow rate of a temperature control medium according to the difference between the warping curves.

Description

Technical Field

[0001] The present invention provides a method for cutting a plurality of wafers from a workpiece by means of a wire saw during a slicing operation. Background Art

[0002] This type of method is used, for example, in the production of semiconductor wafers, especially wafers made of single crystal silicon, and is referred to as the MWS (Multi-Wire Slicing) method. One configuration of this method involves feeding a workpiece, such as a cylindrical ingot of single crystal silicon, through a wire section of a saw wire that moves in a longitudinal direction, with the wire sections arranged parallel to each other to form a wire mesh, in the presence of a cutting liquid and a suspension of hard material (also called slurry), and in the process, the workpiece is cut into wafers.

[0003] During the process of this method, heat is generated at different positions on the wire saw and the workpiece, causing the workpiece and the wire mesh to move relative to each other due to thermal expansion. This relative movement results in non-uniformity of the main surfaces of the cut wafers. Efforts are being made to mitigate this adverse effect.

[0004] US2012 0 178 346A1 describes supplying a cooling liquid to the workpiece and wiping off the cooling liquid from the workpiece. Among other things, it discloses that this measure reduces the local warping of the cut wafers.

[0005] US2007 0 178 807A1 proposes adjusting the temperature and / or volumetric flow rate of the slurry supplied to the workpiece holder and the temperature and / or volumetric flow rate of the slurry supplied to the wire mesh in a manner depending on the cutting depth.

[0006] The method described in EP 3 922 386 A1 includes supplying slurry to the wire mesh, wetting the workpiece with a cooling medium, and controlling the temperature of the fixed bearings of the wire guiding rollers with a cooling liquid. The temperature of the cooling medium and the temperature of the cooling liquid follow the specifications of corresponding temperature curves that respectively specify the temperature of the cooling medium and the temperature of the cooling liquid depending on the cutting depth, and offset the shape deviation from a planar target shape determined on previously cut wafers. As an alternative or supplement to controlling the temperature of the fixed bearings, it is proposed to displace the workpiece along the workpiece axis with an actuating element, and the curve of the displacement stroke offsets the shape deviation. The shape deviation of the wafers produced in this way (shown in the form of an average warping curve) is relatively low. This method, its alternatives, and the combination of both are hereinafter referred to as the method for correcting the average warping.

[0007] During the slicing operation, the workpiece is heated and thus expands temporarily. At the start of the slicing operation, the shorter the distance between the wire section and the workpiece end side, the greater the change in the distance between the wire section and the nearest workpiece end side. If the workpiece start end is defined as the end side of the workpiece having the minimum distance from the wire mesh at the point (or position) where the saw wire enters the wire mesh, and the workpiece end is defined as the end side of the workpiece having the minimum distance from the wire mesh at the point where the saw wire exits (or leaves) the wire mesh, then it is obvious that due to the thermal expansion of the workpiece, the wafers having the shortest distances from the workpiece start end and the workpiece end exhibit the greatest warpage. In addition, such wafers have a reverse shape. The shape of the wafers cut near the workpiece start end is concave, while the shape of the wafers cut near the workpiece end is convex.

[0008] If the shape deviation is reduced by controlling the temperature of the fixed bearings and / or by displacing the workpiece, and if the graphs of the warpage curves of the wafers cut at the ends are superimposed, a set of curves (or a cluster of curves) similar to the contour of a wrapped candy is obtained. Due to the reverse nature mentioned, the change in the warpage curve is greatest at the inlet cut, the cut center, and the outlet cut. Summary of the Invention

[0009] The object of the present invention is to reduce such a change.

[0010] The object of the present invention is achieved by a method for cutting a plurality of wafers from a workpiece by means of a wire saw during a slicing operation, the wire saw comprising actuating means and a wire mesh formed by a moving wire section of a saw wire, and the wire mesh being tensioned in a plane between two wire guide rollers, each of the two wire guide rollers being supported between a fixed bearing and a floating bearing, the method comprising:

[0011] feeding the workpiece through the wire mesh by means of the actuating means in a feed direction perpendicular to the workpiece axis and perpendicular to the plane of the wire mesh;

[0012] supplying a cutting agent to the wire mesh;

[0013] supplying a temperature control medium to the workpiece;

[0014] guiding a temperature control liquid through the fixed bearing according to a temperature curve and / or displacing the workpiece along the workpiece axis according to a displacement curve, the temperature curve and the displacement curve canceling the shape deviation;

[0015] characterized in that

[0016] determining the warpage curves of at least one wafer from the workpiece start end and at least one wafer from the workpiece end in at least one previous slicing operation using the wire saw;

[0017] Generate a difference between the warpage curves;

[0018] Adjust the temperature and / or volumetric flow rate of the cutting agent during the slicing operation based on the difference between the warpage curves; and

[0019] Adjust the temperature and / or volumetric flow rate of the temperature control medium based on the difference between the warpage curves.

[0020] To control the temperature of a workpiece during a slicing operation, the method uses data specific to a wire saw, as they are provided by wafers that have been produced using the same wire saw. The wire saw includes two or more wire guiding rollers. Determine the respective warpage curves in the cutting direction along the diameter of the wafer in one or more slicing operations prior to the slicing operation using the wire saw. Select at least two wafers, which are from the workpiece start end and the workpiece end. Average (or average out) the warpage curves of one or more wafers from the workpiece start end and the warpage curves of one or more wafers from the workpiece end to generate respective common warpage curves representative of the wafers from the workpiece start end and the wafers from the workpiece end. Subsequently generate a difference between the warpage curves of one or more wafers from the workpiece end and one or more wafers from the workpiece start end in at least one previous slicing operation. If the difference between the warpage curves is based on the warpage curves of wafers from the workpiece start end and wafers from the workpiece end in a plurality of previous slicing operations, average the warpage curves of the wafers from the workpiece start end and the warpage curves of the wafers from the workpiece end respectively before generating the difference to generate the respective warpage curves. The temperature adjustment and / or volumetric flow rate adjustment of the cutting agent and the temperature control medium during the slicing operation is based on the difference between the warpage curves. In other words, the temperature and / or volumetric flow rate of the cutting agent and the temperature control medium is specified based on the difference between the warpage curves, and the temperature control performance can be divided differently between the cutting agent and the temperature control medium. Preferably, at the start of the slicing operation, the temperature control performance on the temperature control medium side is emphasized, while at the end of the slicing operation, the temperature control performance on the cutting agent side is emphasized, because the side surface of the workpiece that can be wetted by the temperature control medium decreases as the cutting depth increases.

[0021] If the difference between the warpage curves at a determined cutting depth is positive, more heat is supplied to the workpiece as compared to the heat supply in the case where the difference is zero. If the difference between the warpage curves at a determined cutting depth is negative, more heat is withdrawn from the workpiece as compared to the heat supply in the case where the difference is zero. It is possible to lower the temperature of the cutting agent, increase the volume flow rate of the cutting agent, lower the temperature of the temperature control medium or increase the volume flow rate of the temperature control medium in response to a negative difference between the warpage curves. Correspondingly, it is possible to increase the temperature of the cutting agent, lower the volume flow rate of the cutting agent, increase the temperature of the temperature control medium or lower the volume flow rate of the temperature control medium in response to a positive difference between the warpage curves. It is possible to select one or a combination of more of these response possibilities. In particular, after a cutting depth of not less than 85% has been reached, it is preferable to omit the response by means of the temperature control medium and to regulate the supply of the temperature control medium.

[0022] Of course, it is also possible to generate a difference between the warpage curves from one or more wafers from the starting end of the workpiece and one or more wafers from the end of the workpiece. Then, the difference has the opposite sign and is correspondingly responded to in the opposite way. For example, in the case of a positive difference, less heat is supplied to the workpiece as compared to the heat supply in the case of a zero difference. Correspondingly, one or more of these measures are used to respond to a positive difference between the warpage curves: lowering the temperature of the cutting agent, increasing the volume flow rate of the cutting agent, lowering the temperature of the temperature control medium, and increasing the volume flow rate of the temperature control medium.

[0023] Due to the proposed supply manner of the cutting agent and the temperature control medium, the degree of change in the shape (represented as the warpage curve) of the wafer in the slicing operation is significantly smaller as compared to the case without such measures.

[0024] The cutting agent is a slurry, that is, a suspension of hard material particles in a liquid. The temperature control medium can be such a slurry, a slurry of different components or a liquid without hard material particles therein.

[0025] Guiding a temperature control liquid through a fixed bearing and / or displacing the workpiece along the workpiece axis affects the appearance of the wafer in a slicing operation (which is represented as the average warpage curve after averaging the warpage curves of the wafer). The temperature of the temperature control liquid depending on the cutting depth follows a temperature curve that cancels (or suppresses) the shape deviation, and causes an axial displacement of the floating bearing by means of thermal expansion or thermal contraction of the fixed bearing. Accordingly, the displacement of the workpiece depending on the cutting depth follows a displacement curve that indicates the direction and magnitude of the specified displacement and cancels the shape deviation. Preferably, the temperature curve and / or the displacement curve cancels the shape deviation indicated by the average warpage curve of the wafer that has been cut during at least one previous slicing operation, or the shape deviation as defined in EP 3 922 386 A1. Guiding the temperature control liquid through the fixed bearing according to the specification of the temperature curve and displacing the workpiece along the workpiece axis according to the specification of the displacement curve can be performed simultaneously. However, one or the other of them can also be omitted.

[0026] Furthermore, it is preferable to ensure that the cutting agent and the temperature control medium influence each other as little as possible by wiping the temperature control medium off the workpiece and discharging it before it falls onto the wire mesh. This can be performed, for example, as described in US2012 0 178 346A1.

[0027] The expression "warpage of the wafer" refers to the sum of the maximum upward and downward distance values between a reference plane and the median plane of the relaxed (or relaxed) wafer, where the reference plane is the plane that best fits the median plane. The measurement of warpage is described in standard SEMIMF1390. The warpage curve (linear shape scan) corresponds to a one-dimensional profile obtained when observing the difference between the median curve of the median plane and the reference line of the reference plane along the diameter, preferably in the cutting direction, or deviating from the cutting direction by at most a few degrees if the influence of the directional notch is to be avoided. The influence of gravity on the warpage curve can be taken into account by the vertical arrangement of the wafer when measuring the warpage or by correction when calculating the difference. The warpage curve can be filtered or other mathematical operations can be performed, as long as the resulting changes cancel each other out in the generation of the warpage curve difference according to the present invention.

[0028] The present invention will be described below with reference to the accompanying drawings. Description of the Drawings

[0029] Figure 1 Schematically shows the cutting profile (or trace) of the saw wire of the wire mesh passing through the workpiece, which profile typically results from the temporary thermal expansion of the workpiece, despite the application of a method for correcting the average warpage.

[0030] Figure 2 (Top) shows an overview of the typical warpage curves of the wafers from the starting end and from the ending end of the workpiece.

[0031] Figure 2 (Middle) shows the average warp curves of wafers from the starting end of the workpiece and from the ending end of the workpiece.

[0032] Figure 2 (Bottom) shows the difference between the average warp curves from the starting end of the workpiece and from the ending end of the workpiece.

[0033] Figure 3 (Left half) shows the curves of the volumetric flow rate (top) and temperature (bottom) of the cutting agent depending on the cutting depth.

[0034] Figure 3 (Right half) shows the curves of the volumetric flow rate (top) and temperature (bottom) of the temperature control medium depending on the cutting depth.

[0035] Figure 4 (Top) shows the variation of the superimposed warp curves of wafers from the starting end of the workpiece and from the ending end of the workpiece, where the wafers have been cut from the workpiece without using the present invention.

[0036] Figure 4 (Bottom) shows the variation of the superimposed warp curves of wafers from the starting end of the workpiece and from the ending end of the workpiece, where the wafers have been cut from the workpiece that has been cut into wafers in accordance with the present invention.

[0037] List of reference numerals used

[0038] 1 Line section

[0039] 2 Workpiece

[0040] 3 Starting end of the workpiece

[0041] 4 Ending end of the workpiece

[0042] D Cutting depth

[0043] WP Warp curve

[0044] ΔWP Difference in warp curves

[0045] V Volumetric flow rate

[0046] T Temperature

[0047] SML Cutting agent

[0048] TPM Temperature control medium Detailed description

[0049] Detailed Description of Exemplary Embodiments According to the Present Invention

[0050] Figure 1Schematically shows the cutting profile of the wire section 1 of the wire mesh, which typically results from the temporary thermal expansion of the workpiece 2. If the obtained wafer is rotated clockwise by 90°, the wafer from the starting end 3 of the workpiece has a concave shape, while the wafer from the end 4 of the workpiece has a convex wafer shape.

[0051] Figure 2 (Top) shows the typical warpage curves WP of the wafers from the starting end of the workpiece and from the end of the workpiece. The warpage curves are determined using a capacitive operating measuring device of type MX 7012 from E+H Metrology GmbH, Germany. The variation of the warpage curves is particularly evident at the beginning, middle, and end of the slicing operation. Figure 2 (Middle) shows two warpage curves obtained by taking the average, which represent the wafers from the starting end of the workpiece and from the end of the workpiece, and Figure 2 (Bottom) shows the difference ΔWP between these warpage curves.

[0052] During the slicing operation, the temperature of the cutting agent, the temperature of the temperature control medium, and / or the volumetric flow rate at which the cutting agent is supplied to the wire mesh and the temperature control medium is supplied to the workpiece are based on such a warpage curve difference and thus depend on the data determined after at least one previous slicing operation.

[0053] In the case where the difference is zero, the temperature or volumetric flow rate provided for the process is adjusted respectively. In the case of a positive difference, a correspondingly higher temperature or a lower volumetric flow rate is adjusted, and in the case of a negative difference, a correspondingly lower temperature or a higher flow rate is adjusted.

[0054] As an example, Figure 3 shows curves of the volumetric flow rate V (top) and temperature T (bottom) of the cutting agent SML (left half) and the temperature control medium TPM (right half) depending on the cutting depth D. The silicon semiconductor wafer is sliced from a single crystal. The curves are based on the difference between the warpage curves, which is shown in Figure 2 (Bottom). This difference is greater than zero at the start and end of the cutting process and less than zero at the intermediate cutting depth. The curves of the temperature and volumetric flow rate according to Figure 3 are consistent with this. Thus, during the intermediate cutting depth, the volumetric flow rate of the cutting agent is adjusted to be relatively high, and the temperature of the cutting agent is adjusted to be relatively low. This also applies to the volumetric flow rate and temperature of the temperature control medium. The temperature control performance of the cutting agent and the temperature control medium has different weights during the cutting process. Towards the end of the cutting process, the temperature control performance lies entirely on the cutting agent side because at this point, the temperature control medium is no longer supplied to the workpiece.

[0055] Related to the warpage curves of the wafers from the starting end and the end of the workpiece shown simultaneously,Figure 4 Shows the difference between the method according to the present invention (bottom) and a method of adjusting the temperature and / or volumetric flow rate of the cutting agent and the temperature control medium without relying on the warpage curve difference. In the case of applying the method according to the present invention, the change in the warpage curve is quite insignificant, especially at the beginning, middle, and end of the cutting process.

Claims

1. A method for cutting a plurality of wafers from a workpiece by means of a wire saw during a slicing operation, the wire saw comprising actuation means and a wire mesh formed by a moving wire section of a saw wire, and the wire mesh being tensioned in a plane between two wire guide rollers, each of the two wire guide rollers being supported between a fixed bearing and a floating bearing, the method comprising: feeding the workpiece through the wire mesh by means of the actuation means in a feed direction perpendicular to the workpiece axis and perpendicular to the plane of the wire mesh; supplying a cutting agent to the wire mesh; supplying a temperature control medium to the workpiece; guiding a temperature control liquid through the fixed bearing according to a temperature curve and / or displacing the workpiece along the workpiece axis according to a displacement curve, the temperature curve and the displacement curve compensating for shape deviations; characterized in that determining a warpage curve of at least one wafer from the starting end of the workpiece and at least one wafer from the end of the workpiece in at least one previous slicing operation using the wire saw; generating a difference between the warpage curves; adjusting the temperature and / or volumetric flow rate of the cutting agent during the slicing operation based on the difference between the warpage curves; and adjusting the temperature and / or volumetric flow rate of the temperature control medium based on the difference between the warpage curves.

2. The method according to claim 1, wherein Focus on the temperature control performance of the temperature control medium at the start of the slicing operation, and focus on the temperature control performance of the cutting agent at the end of the slicing operation.

3. The method according to claim 1, characterized in that, Wiping off the temperature control medium from the workpiece.

4. The method according to any one of claims 1 to 3, characterized in that, The average warpage curve of the wafers cut during the at least one previous slicing operation indicates the shape deviation.

5. The method according to any one of claims 1 to 4, characterized in that, After a cutting depth of not less than 85% has been reached, adjust the supply of the temperature control medium to the workpiece.

6. The method according to any one of claims 1 to 5, characterized in that, The difference between the warpage curves is based on the warpage curves of the wafers from the starting end of the workpiece and the wafers from the end of the workpiece in a plurality of previous slicing operations. Before generating the difference, the warpage curves of the wafers from the starting end of the workpiece and the warpage curves of the wafers from the end of the workpiece are averaged respectively to generate corresponding warpage curves.

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