Crystal waferization system and method

Abstract

A method for producing wafers or discs from industrially produced crystals, the method comprising the steps of: - scanning the volume of a crystal (1) to form a 3D volume digital model of the crystal; - recording the 3D spatial coordinates of defects (4, 4') detected during the scanning; - measuring one or more crystal axes (C1, C2, C3) provided by the crystal structure of the crystal and recording them in the 3D model of the crystal; - extracting one or more crystal cores (2) from the crystal (1) in a selected crystal direction, which is parallel or at an angle to one of the crystal axes; - slicing the crystal core orthogonally to the selected crystal direction with a wafer slicer comprising a slicing tool comprising a plurality of cutting lines or blades spaced at a regular slicing pitch (G) for cutting wafers of the same thickness (S) from the crystal core; - wherein the method comprises calculating an offset position (O) of the slicing tool along the selected crystal axis, the offset position being configured to have a minimum number of defective wafers, and - adjusting the position of the slicing tool relative to the crystal core along the selected crystal direction according to the calculated offset.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for wafering a crystal, in particular a large crystal such as a sapphire, silicon or silicon carbide crystal for optical or electronic applications. BACKGROUND

[0003] Crystals produced industrially, such as sapphire crystals, usually have a weight comprised between a few kilograms and a few hundred kilograms and have an irregular shape approximating a cone / cylinder. In this irregular 3D shape, as shown in the photograph of Figure 1 , a typical process consists in extracting cylindrical cores and then cutting these cylindrical cores into discs or circular wafers (as shown in Figure 2 ). The orientation of the axis of the cylindrical cores is in a defined direction with respect to the crystal structure of the crystal, for example, it can correspond to one of the optical axes of the crystal. Generally, industrial applications require discs or wafers of a certain diameter and thickness, and the position and number of cylindrical cores will vary from one raw crystal to another depending on the size and shape of the raw crystal.

[0004] It should be understood that, in general, the extracted cores can have a circular, rectangular or any other cross-section. It should also be understood that "discs" or "wafers" are generic terms for parallel slices of the cores, regardless of their shape or thickness.

[0005] In the conventional process, the crystal structure axes are determined by measurements using X-ray or optical measurement tools, and the optimal distribution of cores to extract from a given crystal is decided by a human operator.

[0006] However, these processes are time-consuming and costly. In addition, since the crystals contain internal defects, a certain amount of waste is generated, which ends up in the discs or wafers cut from the selected cores. Since the production of such crystals is both time-consuming and expensive, it is advantageous to reduce the amount of waste and of defective discs or wafers. SUMMARY

[0008] In view of the above, it is an object of the present invention to provide a system and a method for wafering crystals produced industrially, in order to reduce waste and increase the yield of the wafering process.

[0009] Advantageously, a wafering process for crystals produced industrially is provided, which can produce very high quality crystal discs or wafers with a low defect rate.

[0010] Advantageously, a fast and economical process for wafering crystals produced industrially is provided, in particular with an increased degree of automation and reduced human intervention.

[0011] The object of the present invention is achieved by a method for wafering a crystal according to claim 1.

[0012] Disclosed herein is a method for producing wafers from a crystal produced by industrial means, said method comprising the steps of:

[0013] - scanning the volume of the crystal, forming a 3D volume digital model of the crystal;

[0014] - recording the 3D spatial coordinates of the defects detected during the scanning process;

[0015] - measuring one or more crystallographic axes (C1, C2, C3) provided by the crystallographic structure of the crystal and recording said crystallographic axes in the 3D model of the crystal;

[0016] - extracting one or more crystal cores from the crystal in a selected crystallographic axis direction, said direction being parallel to one of said crystallographic axes or at an angle to said crystallographic axes;

[0017] - slicing the crystal core orthogonally to the selected crystallographic direction using a wafering machine comprising a slicing tool comprising a plurality of cutting lines or blades spaced apart at a regular slicing pitch (G) for cutting wafers of the same thickness (S) from the crystal core;

[0018] wherein said method comprises

[0019] - calculating an offset position (O) of the slicing tool along the selected crystallographic direction, said offset position being configured to have a minimum number of defective discs or wafers, and

[0020] - adjusting the position of the slicing tool relative to the crystal core along the selected crystallographic direction according to the calculated offset.

[0021] In one embodiment, the magnitude of the offset position varies in the range from 0 to the slicing pitch (G) thickness of the wafer.

[0022] In an advantageous embodiment, the scanning of the crystal comprises an optical scanning of the crystal.

[0023] In an advantageous embodiment, the crystal core is placed on a holder, said holder and crystal core being placed in the slicing machine.

[0024] In an advantageous embodiment, after the extraction of the crystal core from the crystal, the crystal core axes are measured again and the crystal core axes position is adjusted by the holder so that the slicing tool cuts the wafers orthogonally to the adjusted crystallographic axes.

[0025] In an advantageous embodiment, the maximum diameter (Dmax) of the core to be cut from the crystal is such that, by calculation using the 3D volumetric digital model, the minimum number of defects is found in the wafer to be cut from the crystal.

[0026] In an advantageous embodiment, the absolute value of the offset is calculated from a reference position on the surface of the crystal before the core taking operation.

[0027] In an advantageous embodiment, the step of scanning the crystal for defects and geometry is performed after the core taking operation on the top and / or bottom end of the original crystal.

[0028] In an advantageous embodiment, the crystal comprises a plurality of crystal axes, the method comprising generating planes intersecting the core or the wafer, the planes being parallel to an axis of the core and orthogonal to the crystal axes, calculating the number of defects between the planes and the outer contour of the core or the wafer, selecting one of the planes for slicing, the plane having the maximum number of defects in the removal area between the plane and the outer contour.

[0029] In an advantageous embodiment, the non-circular pattern, for example a pattern of semiconductor dies to be cut from the wafer, is oriented according to one of the calculated plurality of crystal axes (C2, C3) so that the number of defects found in the area of the dies or in the minimum number of dies is minimum and the number of defects found in the removal area between the outer contour of the die pattern and the outer circumferential edge of the wafer is maximum.

[0030] In an advantageous embodiment, the die pattern is contained in the 3D volumetric digital model of the crystal for calculating the offset to take into account the defects in the waste area between the outer diameter of the wafer and the dies to be cut from the wafer.

[0031] It is also disclosed a system for producing wafers from industrially produced crystals, the system comprising:

[0032] a scanner for scanning the volume of the crystal, and a program module for forming a 3D volumetric digital model of the crystal from the output of the scanner;

[0033] the program module is further configured to record the 3D spatial coordinates of the defects detected by the scanner;

[0034] wherein the program module is configured to calculate the offset position (O) of the slicing tool along the selected crystal axis configured to have the minimum number of defective wafers.

[0035] In an advantageous embodiment, the scanner for scanning the crystal comprises an optical scanner.

[0036] In an advantageous embodiment, the system further comprises:

[0037] a measuring device for measuring one or more crystal axes (C1, C2, C3) provided by the crystal structure of the crystal and recording the one or more crystal axes in the 3D model of the crystal.

[0038] In an advantageous embodiment, the system further comprises:

[0039] a core taking device for cutting one or more cylindrical crystal cores (2) from the crystal (1) in the direction of the selected crystal axis; and

[0040] a wafer slicer comprising a slicing tool comprising a plurality of cutting lines or blades spaced apart at a regular slicing pitch (G) for cutting wafers of the same thickness (S) from the crystal core.

[0041] The system can be configured to perform the method of producing wafers as described in any of the above embodiments.

[0042] Other objects and advantageous aspects of the present invention will become apparent from the claims, the following detailed description, and the accompanying drawings.

[0043] BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 Photo of a sapphire crystal produced industrially.

[0045] Figure 2 Schematic of a wafering process for a crystal known to be produced industrially, depicting the original crystal and a selected location of a crystal core, and the process of cutting one of the crystal cores into a circular disc.

[0046] Figure 2a Simplified schematic of a crystal core cutting tool and a crystal.

[0047] Figure 2b Simplified perspective schematic of a cylindrical crystal core placed on a crystal core holder being sliced into a disc by a wire slicer.

[0048] Figure 3a Schematic of a cylindrical crystal core cut from an original crystal prior to being sliced into a disc, with some defects present within the crystal core.

[0049] Figure 3b Simplified schematic of a cutting grid in a crystal core of a crystal and a wafer.

[0050] Figure 3c Schematic showing the optimal cutting arrangement for the defects shown in Figure 3a Figure 3a Schematic showing the optimal cutting arrangement for the defects shown in​

[0051] Figure 3d A schematic view of an unoptimized cutting arrangement for embodiments of the present application. Figure 3a

[0052] Figure 4 A plan view of a crystal core, where a plane parallel to the cylinder axis is required.

[0053] Figure 5 A schematic plan view of a wafered semiconductor disc, such as a silicon carbide crystal, showing the location of semiconductor dies to be cut from the disc.

[0054] Figure 6 A block diagram showing the method employed according to an embodiment of the present application.

[0055] Detailed description of the application

[0056] Referring to the drawings, a raw crystal 1 of industrial production has a roughly conical irregular 3D shape 9 and a crystal structure defining one or more crystal axes corresponding to specific orientations of the crystal lattice. For example, in a sapphire crystal there is an optical axis (also commonly referred to as the c-axis) Cl which eliminates the intrinsic birefringence properties of the sapphire crystal. The selected crystal axis Cl can also be located in the crystal plane that is most resistant to applied forces applied perpendicular to the plane. The plane is 20% more difficult to machine than other crystallographic orientations. In many applications, it is required to cut the wafer at a specific angle of one or more axes, for example orthogonal to the optical axis Cl.

[0057] For semiconductor circuits, the orientation of the crystal lattice is also important and the wafer should be cut along a specific direction in the crystal. The crystal lattice direction can be determined by various known measurement means, for example using optical or x-ray measurement systems.

[0058] Once the axis is determined, the orientation of the cylindrical crystal core within the raw crystal volume can be determined and a disc or wafer can be cut therefrom. In many applications, the required wafer diameter Dn is predetermined and in the raw crystal different diameters or surface areas of wafers can be extracted for possibly different applications or the same application. Using the example of a sapphire crystal, wafers of different diameters can be used to produce optical instruments lenses of different sizes. Typically, larger diameter wafers are more valuable and rarer, and therefore the number of wafers extracted from each raw crystal is obviously less (sometimes only one).

[0059] After the selected crystal core is determined, a suitable crystal core cutting tool 6 can be used to cut the cylindrical crystal core 2, 2a, 2b, 2c.

[0060] ​For the subsequent wafer slicing process, the core is usually placed in a polymer holder or support 8 which is combined with the crystal core and disappears in the wafer slicing process.

[0061] The support 8 can be manufactured by an additive process such as 3D printing or by a molding process or a subtractive process, for example machined from a block of support material and adapted to the shape and size of the core. In this respect, while the crystal axes (such as the optical axis CI) are determined to extract the core, their measurement can be slightly inaccurate and can be further corrected by the position of the core 2 within the support 8. This will allow, for example, to cut the wafer orthogonally to the measured crystal axis CI with a higher precision, the support 8 being located within the wafer slicer.

[0062] The disc or wafer slicer is usually formed by a plurality of wires, usually diamond-coated wires or blades, arranged at regular intervals and cutting the core orthogonally to a specific direction, for example the optical axis.

[0063] It is worth noting that the core direction can be defined as the crystal axis, but is not always equal to the crystal axis. For example, in some applications, the cut angle of sapphire is at an angle of 20.5 degrees to the optical axis. The term "crystal direction" used herein corresponds to the "core direction", which, as mentioned previously, can be parallel to the lattice axis or at an angle with respect to the lattice axis.

[0064] Cooling and lubricating fluids can be used during the extraction of the core and the slicing process, which usually takes several hours or days. The extraction of the core and the slicing process of the wafer are themselves well-known processes and do not need to be further described herein. The two discs at the ends of the core can be removed as waste, and the slices are then usually re-inspected to determine which slices are free of defects or have defects below an acceptable threshold, and which slices are rejected as having defects above the threshold. In many industrial wafer processing processes, usually about 5% to 30% of the slice discs are rejected due to internal crystal defects, but this depends largely on the quality of the crystal itself and the diameter of the disc or wafer.

[0065] According to one aspect of the application, a full 3D scan of the crystal is performed prior to the core extraction process, but also optionally after the gem cutting and top and bottom end grinding or slicing processes, and the 3D X, Y, Z coordinates of each defect within the volume of the crystal are mapped. These X, Y, Z coordinates can be mapped relative to the crystal or to a geometric reference (point, line, surface) fixed relative to the edge of the core. The original crystal or the holder fixed on the original crystal can be marked with a reference point or shape. A digitized 3D model of the crystal can thus be constructed and the defects located therein.

[0066] The crystal axes are measured by known measuring instruments (such as using x-rays) and these crystal axis directions are also recorded in the digitized 3D model of the crystal.

[0067] The core can then be selected to cut the crystal in the selected crystal axis direction and the position of the core optimized to ensure that the most valuable core (i.e. the largest core) has the least defects within its volume. This optimization can be performed with the aid of a computer program which seeks to minimize the number of defects in a core of a given diameter by varying the position of the core within the original crystal and calculating the number of defects at each position.

[0068] The computer aided optimization process can therefore be configured to position the largest diameter core in the optimum position, i.e. the position within the core volume which has the least defects, and then position the remaining smaller diameter cores in the remaining volume around the largest core. If there is one largest core and one second largest core, and then cores of smaller diameter than the second largest core, then the optimization process can position the second largest core in the optimum position within the core which has the least defects before positioning the remaining cores. Depending on the application, the cores can have predetermined diameters, or their diameters can be different from the single largest core in the original crystal which has the largest possible diameter and the other smaller diameter cores, to optimize the volume of useful material and reduce the volume of waste material. The latter will depend on the requirements of the wafer for the intended application. Alternatively, the optimization can be performed to maximize the value of all the extracted cores collectively.

[0069] Figure 3a A core is shown, for example a largest diameter Dmax core extracted from an original crystal, which has a useful volume 10 from which wafers can be cut normal to the crystal axis Cl. As mentioned above, wafers are often required for specific uses and need to have a certain thickness S, and such wafers can be cut with a wire saw having a regular cutting grid.

[0070] According to one aspect of the application, there can be a number of discs with defects 4 depending on the position of the cutting grid along the crystal axis, which ranges between a minimum number of discs with defects and a maximum number of discs with defects. The spacing between the cutting lines, which corresponds to the range of possible axial positions of the cutting lines, can be adjusted to optimize the number of defect free discs, i.e. discs which have no defects or only defects below an acceptable threshold defined by the requirements of the application.

[0071] According to one aspect of the invention, the position of the dicing line is adjusted by an offset O, which is within the range of 0 and the grid spacing G between the dicing line 7. The offset O is calculated by adding an absolute value to a reference plane or reference point, such as the end face 5 of the core or any other fixed reference point or shape on the original crystal 1, crystal core 2, or support 8. This process can be automated in a simple manner using a digital 3D model of the crystal, in which the X, Y, Z positions of defects and the crystal axis orientation are recorded, so that the optimal offset can be calculated at any time before wafer slicing, including before removing the crystal core 2 from the original crystal 1. The position of the offset O can be adjusted based on any other reference position, such as a reference position provided on the core support. The offset position and position correction of the crystal relative to the machine can be achieved by repositioning the core on the support, or by adjusting the geometry of the support to present a reference surface thereon, which adjusts the position of the support 8 relative to the dicing machine, or by sending instructions to the dicing machine to adjust the position of the support such that the calculated offset accurately positions the dicing grid relative to the reference position of the core or its support.

[0072] When calculating the offset, the thickness of the material removed during the slicing process must be taken into account. This process optimization can change the offset from 0 to the grid spacing G to determine the number of wafer slices with defects below a predetermined acceptable threshold. Thus, some defects may be within the material removed by the thickness B and therefore no longer exist in the wafer, or defects that may exist in two adjacent wafers may be transferred to a single wafer.

[0073] exist Figure 3c In China, with Figure 3a Taking the defects found in the embodiments as an example, the offset was calculated from a reference point on an outer point of the crystal surface 9, with a corresponding offset of 68.5 mm. As a result, defects were found in 3 out of 18 wafer slices. These 3 wafers... Figure 3c The black area is used to indicate this. In this particular embodiment, if a suboptimal offset of 69.5 mm is used, six wafer slices contain defects ( Figure 3d Therefore, a significant advantage of this embodiment is that it can calculate the optimal offset and deflect the position of the linear grid along the selected crystal axis to reduce waste. Furthermore, by scanning defects and subsequently 3D modeling their spatial coordinates, and using the crystal axis and outer contour of the crystal as reference points, the optimal cutting arrangement can be automatically and accurately adjusted to minimize waste.

[0074] Reference Figure 4In certain applications, a plane parallel to the cylinder axis and orthogonal to the crystal axis or crystal direction is required. In some crystals, such an axis or direction is replicated by the crystal symmetry, and therefore, multiple equivalent directions C1.1, C1.2, C1.3 are provided to orient the plane. In such applications, the spatial coordinates of the defects 4, 4’ can also be used to optimize the selection of the flat cut planes 13a, 13b, 13c forming the cylinder chord, to produce a flat chord surface with the most defects 4’ in the waste volume 11. The flat chord surface cut from the crystal core or wafer disc can be used for various applications, including, for example, marking the lattice orientation in a semiconductor crystal.

[0075] Reference is made to Figure 5 , the spatial coordinates of the defects 4, 4’ can also be used to optimize the selection of the cut offset, and subsequently orient the chip according to one of the equivalent crystal directions C2.1, C2.2 (e.g. parallel or orthogonal to the crystal axis), by rotating the surface area covered by the chip 12, the chip can be geometrically oriented from this direction, so as to find as many inclusions as possible in the waste area 11 between the outer circumferential edge of the chip and the rectangular profile. In the embodiment shown, it is preferable to orient the chip 12 according to the crystal direction C2.2 rather than the crystal axis C2.1, since in the first angular orientation aligned with the crystal direction C22, there are fewer defects 4 in the area covered by the chip 12 than in the second angular orientation C2.1, in which the area covered by the chip 12’ has more defects 4’. Figure 4

[0076] If the crystal is used for an application of extracting chips as shown in Figure 5 , for example, extracting semiconductor chips from a silicon carbide crystal, the offset can also be adjusted to take into account the inclusions of the periphery and the inclusions found in the waste volume 11 between the circular periphery of the disc wafer and the actual chip.

[0077] Sequence Listing:

[0078] Core extractor

[0079] Core cutting tool 6

[0080] Slicer

[0081] Cutting line 7

[0082] Crystal holder 8

[0083] 3D detector

[0084] Original crystal 1

[0085] Outer profile / surface 9

[0086] Useful volume 10 ​

[0087] Waste volume 11

[0088] Defect 4

[0089] Flat cut 13

[0090] Cylindrical crystal core 2, 2a, 2b, 2c

[0091] End face 5

[0092] Reference face disc / wafer 3

[0093] Chip 12

[0094] Crystal axis C1, C2, C3

[0095] Optical axis, for example

[0096] Equivalent crystal directions C1.1, C1.2, C1.3 and C2.1, C2.2, for example crystal axes Core diameter Dn

[0097] Disc or wafer slice thickness S

[0098] Cutting blade thickness B

[0099] Offset O

Claims

1. A method for producing wafers from a crystal produced by industrial means, said method comprising the steps of: - scanning the volume of the crystal (1) to form a 3D volume digital model of the crystal; - recording the 3D spatial coordinates of the defects (4, 4') detected during the scanning; - measuring one or more crystallographic axes (C1, C2, C3) provided by the crystallographic structure of the crystal and recording said crystallographic axes in the 3D volume digital model of the crystal; - extracting one or more ingots (2) from the crystal (1) in a selected crystallographic direction, said direction being parallel to one of said measured crystallographic axes or at an angle to one of said measured crystallographic axes; - slicing the ingot with a wafer slicer orthogonal to the selected crystallographic direction, said wafer slicer comprising a slicing tool comprising a plurality of cutting lines or blades spaced apart at a regular slicing pitch (G) for cutting wafers of the same thickness (S) from the ingot; - wherein the method comprises calculating an offset position (O) of the slicing tool along the selected crystallographic direction, said offset position being configured to have a minimum number of defective wafers, and - adjusting the position of the slicing tool relative to the ingot along the selected crystallographic direction according to the calculated offset position.

2. The method of claim 1, wherein, The magnitude of the offset position varies in the range from 0 to the thickness of the regular slicing pitch (G) of the wafer.

3. The method of claim 1, wherein, The scanning of the crystal comprises an optical scanning of the crystal.

4. The method of claim 1, wherein, The ingot (2) is placed on a holder (8), said holder and ingot being placed in the slicer.

5. The method of claim 4, wherein, After the ingot has been cut from the crystal, the crystallographic axis of the ingot corresponding to the selected crystallographic direction is measured again and the position of the crystallographic axis of the ingot is adjusted by the holder so that the slicing tool cuts the wafers orthogonal to the adjusted crystallographic axis.

6. The method of claim 1, wherein, The maximum diameter (Dmax) of the ingot cut from the crystal is such that, by calculation using the 3D volume digital model, the minimum number of defects is found in the wafers to be cut from the crystal.

7. The method of claim 1, wherein, The absolute value of the offset position is calculated from a reference position on the surface of the crystal before the ingot extraction operation.

8. The method of claim 1, wherein, The steps of scanning the defects and geometry of the crystal are performed after the ingot extraction operation on the top and / or bottom end of the original crystal.

9. The method of claim 1, wherein, The crystal comprises a plurality of equivalent crystallographic axes or a plurality of equivalent crystallographic directions (C1.1, C1.2, C1.3) which are equivalent to each other due to the symmetry of the crystal, said method comprising generating a plane (13a, 13b, 13c) intersecting the ingot or the wafer, said plane being parallel to an axis of the ingot and orthogonal to a crystallographic axis, calculating the number of defects between said plane and the outer contour of the ingot or wafer, selecting one of said planes for slicing, the plane having the maximum number of defects (4') in the waste area (11) between the plane and the outer contour.

10. The method of claim 1, wherein, The non-circular pattern is a pattern for cutting semiconductor dies (12) from a wafer (3), the non-circular pattern being oriented according to one of the calculated plurality of crystal directions (C2.1, C2.2) so that the number of defects (4) found within the area of the dies or within the minimum number of dies is the least and the number of defects found within the waste area (11) between the outer contour of the die pattern and the outer circumference of the wafer is the most.

11. The method of claim 10, wherein, The die pattern is contained in the 3D volume digital model of the crystal for calculating the offset position to take into account the defects in the waste area (11) between the wafer outer diameter and the dies to be cut from the wafer.

12. A system for producing wafers from industrially produced crystals, the system comprising: a scanner for scanning the volume of a crystal (1) and a program module for forming a 3D volume digital model of the crystal from the output of the scanner; the program module being further configured to record the 3D spatial coordinates of the defects (4, 4') detected by the scanner; wherein the program module is configured to calculate the offset position (O) of the slicing tool along the selected crystal axis, the selected crystal axis being configured to have the minimum number of defective wafers; the system further comprising a measuring device for measuring one or more crystal axes (C1, C2, C3) provided by the crystal structure of the crystal and recording the one or more crystal axes in the 3D volume digital model of the crystal; the system further comprising a crystal core taking device for cutting one or more cylindrical crystal cores (2) from the crystal (1) in the direction of the selected crystal axis; and a wafer slicer comprising a slicing tool comprising a plurality of cutting lines or blades spaced apart at a regular slicing pitch (G) for cutting wafers of the same thickness (S) from the crystal core according to the offset position (O); the system being configured to perform the method of any one of claims 1-11.

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