Processing method of silicon carbide wafer

Abstract

The invention discloses a processing method of a silicon carbide wafer, which comprises the following steps: carrying out rapid processing, grinding and polishing on the wafer by adopting an annular polishing machine to obtain sub-nanometer level surface roughness; the surface of the wafer is uniformly polished by adopting magneto-rheological polishing, and tiny deep pits on the surface after annular polishing are eliminated or broadened. In order to solve the problems of long processing period, difficulty in processing quality control and the like of the traditional single crystal SiC wafer, the invention provides a novel method for combining magneto-rheological polishing and the traditional polishing technology. According to the invention, the magneto-rheological polishing liquid suitable for polishing the single-crystal SiC is developed, and the single-crystal SiC wafer subjected to ring polishing processing is actually processed, so that the magneto-rheological polishing technology is proved to be capable of eliminating or widening wafer surface defects (such as pocks and pits) on the premise of ensuring the quality of the polished surface, and eliminating the subsurface damage of the wafer. And finally, in combination with a traditional polishing technology, high-precision and high-quality processing of the wafer is realized.

Description

Technical Field

[0001] The invention relates to the technical field of silicon carbide wafer processing, and in particular to a method for processing a silicon carbide wafer. Background Art

[0002] The development of the semiconductor industry is closely related to the progress of my country's national defense, military, aerospace, energy and other scientific and technological fields. The third-generation semiconductor material represented by single-crystal silicon carbide (SiC) is an important new wide-bandgap semiconductor material, which can be used as a substrate material for growing gallium nitride (GaN) and graphene. At the same time, single-crystal SiC materials have the characteristics of high Young's modulus, high hardness, high temperature resistance, and corrosion resistance. They can be widely used in the production of high-temperature, high-frequency, high-power semiconductor devices. They are also the core component materials of the new generation of radars, satellite communications, and communication base stations. They have very important application value and broad development prospects.

[0003] The flatness, surface roughness and subsurface damage size of single-crystal SiC wafer substrates are the core indicators for measuring wafer processing quality. Therefore, it is of great significance to study the ultra-precision processing technology of single-crystal SiC wafers with high precision, high efficiency, high quality and low cost. At present, many researchers at home and abroad have carried out research on ultra-precision processing technology of single-crystal silicon carbide substrates. The Si and C surfaces of 6H-SiC with a diameter of 5.08 cm were chemically mechanically polished with aluminum oxide abrasives to obtain a SiC single crystal substrate with a surface roughness better than 1nm. The polishing experiment of single-crystal SiC materials was carried out using chemical mechanical polishing (CMP) technology, and the polishing efficiency and surface roughness under alkaline polishing conditions were analyzed. 6H-SiC (001) was mechanically polished using diamond micropowder with a particle size of 3μm, with a size of 10μm×10μm, and the corresponding surface roughness was 2.5nm. A method for plasma-assisted polishing of single-crystal silicon carbide is proposed. The surface is modified by water vapor plasma radiation under atmospheric pressure, and polished with CeO2 abrasive to obtain a damage-free and flattened silicon carbide surface, but the surface needs to be properly modified before polishing. The root mean square (RMS) roughness of the surface of a SiC single crystal substrate with a diameter of 5.08 cm was increased to 0.6 nm by combining N ion implantation and chemical mechanical polishing. After 6H-SiC was etched with KOH, the surface roughness Ra of a SiC single crystal substrate with a diameter of 5.08 cm was measured by atomic force microscopy to be 0.5 nm. Although the above research has obtained an ultra-smooth surface of a single crystal SiC substrate, the processing efficiency and surface quality still need to be further improved. Therefore, new processing strategies and methods are urgently needed to improve the processing efficiency and processing quality of single crystal SiC wafers. Summary of the invention

[0004] In order to solve the above technical problems, a method for processing silicon carbide wafers is provided. This technical solution solves the problem raised in the above background technology that although the existing research has obtained an ultra-smooth surface of a single-crystal SiC substrate, the processing efficiency and surface quality still need to be further improved.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0006] A method for processing a silicon carbide wafer, comprising:

[0007] The wafer is quickly processed and polished using a ring polishing machine to obtain a sub-nanometer surface roughness;

[0008] Magnetorheological polishing is used to evenly polish the wafer surface, and the tiny deep pits on the surface after ring polishing are eliminated or widened;

[0009] Ring polishing combined with nano-diamond polishing liquid is used to perform final fine polishing on the wafer to obtain an ultra-smooth surface with high surface quality.

[0010] Preferably, the rapid processing, grinding and polishing of the wafer by a ring polishing machine specifically comprises the following steps:

[0011] The wafer is bonded to the surface of the fused quartz glass substrate using a sealing glue;

[0012] Use diamond powder and asphalt polishing disc to quickly eliminate cutting damage on wafers;

[0013] Diamond micropowder is used in combination with asphalt polishing discs to gradually eliminate residual surface and sub-surface damage.

[0014] Preferably, the particle size of the diamond powder for eliminating wafer cutting damage is 14 to 28 μm.

[0015] Preferably, the particle size of the diamond powder for eliminating residual surface damage and sub-surface damage is 1.5 to 3.5 μm.

[0016] Preferably, the initial surface peak-to-valley value of the fused quartz glass substrate is better than 100 nm.

[0017] Preferably, the method of uniformly polishing the wafer surface by magnetorheological polishing specifically comprises the following steps:

[0018] The jet pump sprays the magnetorheological polishing liquid in the liquid storage tank onto the rotating polishing wheel through the nozzle;

[0019] The polishing fluid is driven by the polishing wheel through the tiny gap between the workpiece and the polishing disc. The high-gradient magnetic field causes the magnetorheological polishing fluid to change from a liquid state to a solid-like state, forming a flexible polishing grinding head.

[0020] The flexible polishing grinding head polishes the workpiece driven by the rotating polishing wheel;

[0021] The flexible grinding head formed by the magnetorheological polishing fluid turns into a fluid liquid after passing through the magnetic field action area and is recovered to the liquid storage tank.

[0022] Preferably, the recovery into the liquid storage tank is carried out by suction through a recovery pump.

[0023] Preferably, the final fine polishing of the wafer by ring polishing with nano-diamond polishing liquid specifically comprises the following steps:

[0024] Use a ring polishing machine combined with nano-diamond polishing liquid for fine polishing;

[0025] The surface after magnetorheological polishing is then precisely polished by a ring polishing machine.

[0026] Preferably, the central particle diameter of the nanodiamond is 50 nm.

[0027] Preferably, the fine polishing time is 2 hours.

[0028] Compared with the prior art, the present invention provides a method for processing a silicon carbide wafer, which has the following beneficial effects:

[0029] In order to solve the problems of long processing cycle and difficult to control processing quality of traditional single crystal SiC wafers, the present invention proposes a new method combining magnetorheological polishing with traditional polishing technology. A magnetorheological polishing liquid suitable for single crystal SiC polishing was developed, and single crystal SiC wafers after ring polishing were actually processed. It was proved that magnetorheological polishing technology can eliminate or widen wafer surface defects (such as pits and pits) and eliminate wafer sub-surface damage while ensuring the quality of the polished surface. Finally, combined with traditional polishing technology, high-precision and high-quality processing of wafers is achieved. This processing method shortens the processing time by about 7 hours, reduces the surface roughness PV to 9.611nm, and RMS to 0.41nm, effectively improving the processing efficiency and flatness of SiC wafers, eliminating wafer surface defects and sub-surface damage, and improving the performance of wafers. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the method flow of the present invention;

[0031] Figure 2 It is a schematic flow chart of a method for rapidly grinding and polishing a wafer using a ring polishing machine in the present invention;

[0032] Figure 3 It is a schematic flow chart of a method for uniformly polishing a wafer surface by using magnetorheological polishing in the present invention;

[0033] Figure 4 Schematic diagram of typical roughness of the machined surface in the present invention;

[0034] Figure 5 Schematic diagram of the removal function of single crystal SiC in the present invention;

[0035] Figure 6 It is a schematic diagram of the MRF processing trajectory in the present invention;

[0036] Figure 7 This is a schematic diagram of the surface error distribution after 35 minutes of MRF in the present invention;

[0037] Figure 8 Schematic diagram of the wafer surface after magnetorheological polishing in the present invention. DETAILED DESCRIPTION

[0038] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only examples, and those skilled in the art may think of other obvious variations.

[0039] Example 1

[0040] Please refer to Figure 1-3 As shown, a method for processing a silicon carbide wafer comprises:

[0041] The wafer is quickly processed and polished using a ring polishing machine to obtain a sub-nanometer surface roughness.

[0042] Magnetorheological polishing is used to evenly polish the wafer surface, and the tiny deep pits on the surface after ring polishing are eliminated or widened;

[0043] Ring polishing combined with nano-diamond polishing liquid is used to perform final fine polishing on the wafer to obtain an ultra-smooth surface with high surface quality.

[0044] The specific steps of using a ring polishing machine to quickly grind and polish a wafer include the following:

[0045] The wafer is bonded to the surface of a fused silica glass substrate with an initial surface peak-to-valley value better than 100 nm by using sealing glue;

[0046] Use diamond micro powder with a particle size of 14 to 28 μm and asphalt polishing disc to quickly eliminate wafer cutting damage;

[0047] Diamond micropowder with a particle size of 1.5 to 3.5 μm is used in combination with an asphalt polishing disc to gradually eliminate residual surface damage and sub-surface damage.

[0048] The specific steps of uniformly polishing the wafer surface using magnetorheological polishing include the following:

[0049] The jet pump sprays the magnetorheological polishing liquid in the liquid storage tank onto the rotating polishing wheel through the nozzle;

[0050] The polishing fluid is driven by the polishing wheel through the tiny gap between the workpiece and the polishing disc. The high-gradient magnetic field causes the magnetorheological polishing fluid to change from a liquid state to a solid-like state, forming a flexible polishing grinding head.

[0051] The flexible polishing grinding head polishes the workpiece driven by the rotating polishing wheel;

[0052] The flexible grinding head formed by the magnetorheological polishing fluid turns into a fluid liquid after passing through the magnetic field action area and is recovered to the liquid storage tank.

[0053] The liquid is recovered into the storage tank by suction through a recovery pump.

[0054] The final fine polishing of the wafer using ring polishing combined with nano-diamond polishing liquid includes the following steps:

[0055] A ring polishing machine combined with nano-diamond polishing liquid was used, the center particle size of the nano-diamond was 50nm, and fine polishing was performed for 2h;

[0056] The surface after magnetorheological polishing is then precisely polished by a ring polishing machine.

[0057] Example 2

[0058] Ring polishing machine processing results:

[0059] After the single crystal SiC wafer was polished for 38 hours using a ring polishing machine, the roughness of the processed surface was measured using Zygo's NewView7200 white light interferometer. The surface roughness results were as follows: Figure 3 As shown. Figure 4 It can be seen that the roughness PV value is 88.193nm, the RMS is 0.735nm, and the surface roughness Ra is 0.571nm. Although the surface roughness of the SiC wafer after ring polishing reaches the sub-nanometer level, there are pits and deep pits of different depths distributed on its surface, which produces more high-frequency components. In the traditional ring polishing process, the polishing force is perpendicular to the workpiece surface, and the mechanical force between the SiC wafer and the abrasive is large, which causes surface defects such as pits and scratches on the surface of the single crystal SiC wafer, as well as sub-surface damage. According to the theoretical analysis of the median crack depth, the sub-surface damage SSD is 150-200nm.

[0060] If the surface defects and subsurface damage on the wafer surface cannot be eliminated, the electrical properties of the single crystal will be greatly reduced or even unusable. If the traditional processing method is continued to be used to eliminate surface defects and subsurface damage, it will mean that nano-scale polishing powder and smaller weights (positive pressure) will be needed, resulting in low material removal efficiency (<0.1μm·h -1 ), if you want to completely eliminate subsurface damage and ensure surface roughness, you need nearly 10 hours of step-by-step polishing and cannot ensure the stability of flatness. Therefore, it is difficult for traditional processing methods to solve this problem. In view of this, the present invention proposes a new method that combines traditional ring polishing with magnetorheological polishing technology to achieve rapid elimination of wafer surface defects and subsurface damage.

[0061] Example 3

[0062] Removal function experiment:

[0063] To achieve efficient magnetorheological polishing of single crystal SiC, the key is to use a suitable magnetorheological polishing fluid. For this purpose, a magnetorheological polishing fluid suitable for polishing single crystal SiC materials has been developed. The main components of the polishing fluid are shown in the following table.

[0064] The iron powder used has a particle size of 4.5 μm, and the polishing powder is diamond powder with a particle size of 0.5 μm.

[0065] Ingredient Ironpowder Abrasive Water-basedcompositecarrierliquid Volume fraction / % 40 0.2 Bal.

[0066] The developed magnetorheological polishing fluid was used to carry out the removal function experiment. The diameter of the single-crystal SiC wafer used in the experiment was 100 mm, and its surface was processed by traditional ring polishing, with a surface accuracy PV less than 150 nm and a roughness RMS less than 1 nm. The polishing parameters used in the removal function experiment are consistent with those used in actual processing. The single-point dwell polishing time of the removal function is 10 s. The specific polishing parameters are shown in the following table.

[0067]

[0068] After 10 seconds of single-point dwell polishing, the Zygo interferometry test results of the removal function are as follows: Figure 5 As shown, the chromaticity bar represents the removal amount, and the unit is wavelength λ (wavelength λ = 632.8nm). Based on the test results, the peak removal efficiency (PRR) of the removal function is calculated to reach 2μm·min -1 , the size of the removal function is 14.5mm×4.8mm.

[0069] Example 4

[0070] Uniform polishing of single crystal SiC wafers:

[0071] Further experiments were carried out on full-aperture processing of single-crystal SiC wafers. The removal function obtained from the above experiment was used, and a 200nm removal amount was added to the original surface shape to perform uniform removal of the full aperture. The processing trajectory was adopted Figure 6 The grating trajectory shown in the figure, where X and Y represent the horizontal and vertical dimensions respectively. Since the diameter of the processed SiC wafer is 100mm, appropriate extension is performed during the simulation calculation path planning process, and the diameter after extension is 120mm, which will help ensure the uniform removal of wafer surface material within the 100mm effective diameter.

[0072] Based on the removal function and the uniform surface shape of the single crystal SiC, the total processing time is 35 minutes by solving the residence time matrix equation. After 35 minutes of magnetorheological uniform polishing, the surface shape error distribution of the wafer surface is as follows: Figure 7 shown.

[0073] Different from the traditional processing method, during magnetorheological polishing, the magnetorheological polishing fluid rotates with the polishing wheel to form a large shear force and act on the workpiece surface to achieve material removal. Therefore, the magnetorheological polishing force is mainly shear force, supplemented by pressure. After ring polishing, the wafer surface still has surface defects and sub-surface damage. After magnetorheological polishing, the results are as follows Figure 8 shown.

[0074] from Figure 8 It can be seen that after magnetorheological polishing, the roughness Ra of the entire surface changes little, and the surface defects left after polishing by the ring polisher are flattened, the scratch depth along the rotation direction of the polishing wheel decreases, and the width increases. At the same time, sub-surface damage is also exposed. Therefore, magnetorheological polishing technology converts pitting that cannot be eliminated by traditional polishing technology into grooves that can be processed, and exposes sub-surface damage that is difficult to eliminate, so as to achieve the purpose of elimination. The above method will provide necessary reference for the subsequent high-precision flattening of wafer surfaces through traditional CMP or ring polishing technology, and then obtain high-quality single-crystal SiC wafers.

[0075] Based on the single crystal SiC surface after magnetorheological polishing, a ring polishing machine combined with nanodiamond (center particle size of 50nm) polishing liquid was used for 2h fine polishing. The roughness of the surface after polishing was as follows: Figure 8 As shown. The surface after magnetorheological polishing is then precisely polished by a ring polisher, and the surface roughness PV is reduced to 9.611nm. At this time, the surface defects (pits, pits) have been completely eliminated, the surface roughness RMS is reduced to 0.41nm, and Ra is reduced to 0.327nm. It can be seen that combining magnetorheological polishing with traditional polishing methods can achieve ultra-fast and high-quality processing of single-crystal SiC wafers. It not only shortens the processing time by about 7h and improves the processing efficiency, but also ensures the processing quality of the processed wafers.

[0076] For the processing of single-crystal SiC wafers, first, the defects and early thinning of the wafers need to be completed by traditional polishing methods; then, magnetorheological polishing is used to polish the wafers in the previous step, and the hidden pits, defects and SSD are exposed; finally, the high-precision fine polishing of the wafers is achieved by traditional polishing technology. This strategy can solve the sub-surface defect problem that is difficult to solve by traditional polishing methods, thereby obtaining single-crystal SiC wafers with excellent performance.

[0077] In summary, in order to solve the problems of long processing cycle and difficult to control processing quality of traditional single-crystal SiC wafers, a new method combining magnetorheological polishing and traditional polishing technology is proposed. A magnetorheological polishing fluid suitable for single-crystal SiC polishing is developed, and single-crystal SiC wafers after ring polishing are actually processed. It is proved that magnetorheological polishing technology can eliminate or widen wafer surface defects (such as pits and pits) and eliminate wafer subsurface damage while ensuring the quality of the polished surface. Finally, combined with traditional polishing technology, high-precision and high-quality processing of wafers is achieved. This processing method shortens the processing time by about 7h, reduces the surface roughness PV to 9.611nm, and RMS to 0.41nm, effectively improving the processing efficiency and flatness of SiC wafers, eliminating wafer surface defects and subsurface damage, and improving the performance of wafers.

[0078] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions only describe the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for processing a silicon carbide wafer, characterized in that: include: The wafer is quickly processed and polished using a ring polishing machine to obtain a sub-nanometer surface roughness; Magnetorheological polishing is used to evenly polish the wafer surface, and the tiny deep pits on the surface after ring polishing are eliminated or widened; Ring polishing combined with nano-diamond polishing liquid is used to perform final fine polishing on the wafer to obtain an ultra-smooth surface with high surface quality.

2. A method for processing a silicon carbide wafer according to claim 1, characterized in that: The use of a ring polishing machine to quickly grind and polish a wafer specifically includes the following steps: The wafer is bonded to the surface of the fused quartz glass substrate using a sealing glue; Use diamond powder and asphalt polishing disc to quickly eliminate cutting damage on wafers; Diamond micropowder is used in combination with asphalt polishing discs to gradually eliminate residual surface and sub-surface damage.

3. A method for processing a silicon carbide wafer according to claim 2, characterized in that: The particle size of the diamond powder for eliminating wafer cutting damage is 14-28 μm.

4. The method for processing a silicon carbide wafer according to claim 2, characterized in that: The particle size of the diamond micro powder for eliminating residual surface damage and sub-surface damage is 1.5 to 3.5 μm.

5. The method for processing a silicon carbide wafer according to claim 2, characterized in that: The initial surface peak-to-valley value of the fused quartz glass substrate is better than 100 nm.

6. The method for processing a silicon carbide wafer according to claim 1, characterized in that: The method of uniformly polishing the wafer surface by magnetorheological polishing specifically comprises the following steps: The jet pump sprays the magnetorheological polishing liquid in the liquid storage tank onto the rotating polishing wheel through the nozzle; The polishing fluid is driven by the polishing wheel through the tiny gap between the workpiece and the polishing disc. The high-gradient magnetic field causes the magnetorheological polishing fluid to change from a liquid state to a solid-like state, forming a flexible polishing grinding head. The flexible polishing grinding head polishes the workpiece driven by the rotating polishing wheel; The flexible grinding head formed by the magnetorheological polishing fluid turns into a fluid liquid after passing through the magnetic field action area and is recovered to the liquid storage tank.

7. A method for processing a silicon carbide wafer according to claim 6, characterized in that: The method of recovering the liquid to the liquid storage tank is to suck it through a recovery pump.

8. The method for processing a silicon carbide wafer according to claim 1, characterized in that: The final fine polishing of the wafer by using ring polishing and nano-diamond polishing liquid specifically includes the following steps: Use a ring polishing machine combined with nano-diamond polishing liquid for fine polishing; The surface after magnetorheological polishing is then precisely polished by a ring polishing machine.

9. A method for processing a silicon carbide wafer according to claim 8, characterized in that: The central particle diameter of the nano-diamond is 50 nm.

10. The method for processing a silicon carbide wafer according to claim 8, characterized in that: The fine polishing time is 2 hours.

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