Conductive silicon carbide crystal discharge electrochemistry assisted diamond tool combined machining method

Abstract

The invention discloses a discharge electrochemistry-assisted diamond tool combined machining method for solving the machining problem of high-hardness brittle conductive silicon carbide crystals. A consolidated abrasive particle diamond tool is used, in a working medium with certain conductivity, electrochemical reaction is directly carried out on a crystal connected with a positive electrode and the diamond tool connected with a negative electrode through a combined machining power source within pulse discharge time, and bubbles and a passivation layer are formed on the surface layer of the crystal; as the inter-electrode gap is reduced, effective spark discharge to the crystal is formed, and a surface softening layer is generated. And in the pulse interval time, diamond abrasive particles are controlled to grind and remove crystal surface materials. And continuously finishing composite processing on the crystal in the next electric signal period, and repeating the steps. Through coupling control of energy fields in three forms of electrochemistry, discharge and grinding, efficient and low-tool electrode loss machining of the high-hardness brittle conductive silicon carbide crystal can be achieved, the wire bow phenomenon occurring in the machining process can be reduced, and the machining shape precision is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of composite processing of semiconductor crystal materials, and relates to a method for realizing composite processing of conductive silicon carbide crystals by discharging electrochemically assisted diamond tools. Background Art

[0002] In recent years, traditional silicon-based semiconductor materials have gradually become unable to meet the performance requirements of modern power electronic devices due to the general stability of silicon's own chemical bonds and small band gap. The third-generation wide bandgap semiconductor materials, represented by silicon carbide crystals, have become ideal substrate materials for manufacturing modern power electronic devices due to their comprehensive excellent properties such as stable chemical properties, large bandgap width, and high breakdown field strength. Among them, the conductive silicon carbide single crystal substrate is also called n-type conductive silicon carbide single crystal substrate because it mainly uses N elements as dopants to achieve n-type conductivity. Its resistivity range is (1.5~2.5)×10 -7 Ω·m (derived from the announcement of the Ministry of Industry and Information Technology on the release of the first batch of application demonstration guidance catalogues of key new materials (2024 edition)). Conductive silicon carbide single crystal substrates are particularly suitable for the manufacture of vertical power devices. Schottky diodes, MOSFETs and other power devices can be manufactured by making silicon carbide homoepitaxial wafers on conductive silicon carbide substrates. They are widely used in new energy vehicles, rail transit, and high-power power transmission and transformation equipment. However, the conductive silicon carbide single crystal substrate wafers required for the preparation of power devices are currently in short supply. This is mainly because the upstream growth rate of single crystal ingots is slow and the domestic source of large-size ingots is limited; and silicon carbide crystals themselves have high hardness (SiC Mohs hardness 9.25, close to the Mohs hardness of diamond abrasives 10), high rigidity (elastic modulus 700GPa), and low fracture toughness (3.42MPa·m 1 / 2 ) and other characteristics, and is a typical precious, hard, brittle and difficult to process material; in addition, the manufacture of single crystal substrates has strict requirements on processing surface quality and surface accuracy such as curvature, warpage, and total thickness deviation, which puts higher requirements on the processing of conductive silicon carbide crystals.

[0003] At present, the main processing methods for conductive silicon carbide crystals at home and abroad are multi-wire cutting, laser processing, water-guided laser cutting, and laser focused internal modification. Among them, multi-wire cutting has the longest application time and the widest range, but there are problems such as easy wear of diamond wire, low processing efficiency, defects on the processing surface, and low shape accuracy. Laser processing of silicon carbide crystals has a fast processing speed and narrow slits, and can achieve the cutting of any graphics. At present, picosecond or femtosecond ultrafast lasers are mainly used, but it is difficult to eliminate the incision and internal heat-affected zone on the surface of silicon carbide after processing. There are problems such as recondensation, molten chips, large thermal stress, and difficulty in achieving large thickness cutting during the processing process. Water-guided laser cutting of silicon carbide crystals can maintain good incisions and high processing efficiency, but due to the high wear rate of the focusing nozzle and the high cost of the equipment, the overall processing cost is difficult to control. Laser focused internal modification processing can ignore cutting losses, but for silicon carbide crystals with larger thicknesses, it is necessary to gradually increase the depth of the focus point to complete the cutting through multiple scans. After cutting and separation, there are defects such as large side roughness and easy edge and corner collapse.

[0004] From the above research status of various processing methods for conductive silicon carbide crystals at home and abroad, it can be seen that due to its special precious, hard and brittle material characteristics, it is extremely difficult to use traditional processing or special processing. At present, only simple slicing processing can be carried out, and the investment in equipment and diamond tools is huge, the processing cost is very high, and the processing yield is less than 50%, which greatly limits the application of conductive silicon carbide crystals in modern power device substrates. Therefore, it is urgent to obtain a high-efficiency, precise, and low-tool-loss processing method for conductive silicon carbide crystals. Summary of the invention

[0005] In order to solve the problems of diamond tool wear, easy surface defects, low processing efficiency, low processing shape accuracy and high processing cost in the existing conductive silicon carbide crystal processing, the present invention provides a discharge electrochemical assisted diamond tool processing method, which can realize the efficient, accurate and low diamond tool loss processing of this type of difficult-to-process materials.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] A composite processing power source capable of realizing discharge and electrochemical reaction is adopted. By using a diamond tool with an outer layer coated with a conductive metal element as a composite electrode, in a working medium with a certain conductivity, the control system realizes discharge processing and electrochemical processing of the conductive silicon carbide crystal within the pulse width maintenance time, and grinding of the conductive silicon carbide crystal within the pulse interval time, and repeats this cycle to complete efficient, precise, and low diamond tool loss processing of the conductive silicon carbide crystal.

[0008] The composite processing power source can realize discharge and electrochemical action when the discharge pulse width is maintained, and diamond abrasive grinding can be performed when the discharge pulse interval is maintained. The discharge parameters and electrochemical reaction parameters can be independently adjusted during processing. The composite electrode has a core material of a metal wire of a certain diameter, and its surface is coated with a conductive metal layer (such as nickel, copper, zinc, etc.). Its surface may contain diamond abrasives deposited in the conductive metal layer. Diamond wire can also be used instead of the composite electrode to perform discharge electrochemical assisted diamond tool grinding. The working medium with a certain conductivity can ensure that the conductive silicon carbide material undergoes an electrochemical reaction, and can also ensure that it plays a role in cooling, chip removal and deionization during the discharge machining stage. The anions contained in its components have a relatively positive standard electrode potential, and the metal cations contained have a relatively negative standard electrode potential (E0<-2V). Its conductivity range is (0.3~2.5)S·m -1 The discharge electrochemical assisted diamond tool composite processing method can achieve the control and optimization of the coupling effects of the three energy fields of discharge, electrochemical and grinding by adjusting the parameters of the electrical processing and grinding processing. By using diamond tools of different processing types, such as various wire saws and rope saws, grinding wheels and grinding discs, and drill bits, it can achieve the corresponding cutting, surface processing, and hole processing of conductive silicon carbide crystal materials.

[0009] Beneficial effects of the present invention:

[0010] The present invention aims at the problem that conductive silicon carbide crystals are difficult to process, and proposes a processing method of electrochemically assisted grinding by discharge in a working medium with a certain conductivity in combination with a composite processing power supply, which overcomes the problems of large diamond tool loss, easy generation of edge collapse and micro-crack surface defects, low processing efficiency, low processing shape accuracy and high processing cost in the existing multi-wire cutting method of conductive silicon carbide crystals. The present invention has a significant role in promoting the efficient, accurate and low diamond tool loss processing of conductive silicon carbide crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Schematic diagram of conventional diamond wire cutting to process conductive silicon carbide crystals;

[0012] Figure 2 Edge collapse and cleavage photos of conductive silicon carbide crystal slices produced by traditional diamond wire cutting;

[0013] Figure 3 The relationship between the discharge probability and time when cutting conductive silicon carbide crystals with traditional reciprocating wire discharge;

[0014] Figure 4 Schematic diagram of discharge electrochemical assisted diamond wire composite cutting of conductive silicon carbide crystals;

[0015] Figure 5 Comparison photos of the cut surfaces of conductive silicon carbide crystals during traditional cutting and composite cutting;

[0016] Figure 6 Single set of electrical waveforms for discharge electrochemical assisted diamond wire composite cutting of conductive silicon carbide crystals

[0017] Note: In the figure, 1-diamond abrasive, 2-emery wire, 3-conductive silicon carbide crystal, 4-composite processing power supply, 5-working medium, 6-anode bubble, 7-cathode bubble DETAILED DESCRIPTION

[0018] The present invention will be further described below with reference to the accompanying drawings and implementation examples.

[0019] Implementation Cases:

[0020] like Figure 1 As shown, it is a schematic diagram of machining conductive silicon carbide crystals using traditional diamond wire cutting. The material removal process of the silicon carbide crystal can be understood as a series of two-body contact abrasive grinding and removal processes of the conductive silicon carbide crystals by the consolidated diamond abrasives.

[0021] like Figure 2 As shown in the figure, the conventional reciprocating wire diamond wire slicing test was carried out on the n-type conductive silicon carbide crystal. The diameter of the diamond wire was Φ0.20mm, the average particle size of the diamond abrasive was 30μm, the feeding speed of the diamond wire was 20μm / s, the reciprocating wire speed was 7m / s, and the tension of the diamond wire was 20N. The working medium used JR1A composite working fluid (1:20). The test results show that after the conventional diamond wire was used to slice the conductive silicon carbide crystal, as the diamond wire was fed to the workpiece, the diamond wire was gradually bent to form a "wire bow", and the actual feeding trajectory of the diamond wire deviated from the theoretical cutting trajectory. The theoretical feeding length of the conductive silicon carbide crystal was 17.5mm, while the actual slicing feed length was 18.9mm, and the processing time was 18 minutes and 05 seconds. At the same time, it was easy to have edge collapse and cleavage surface defects.

[0022] The conventional reciprocating wire EDM test was conducted on n-type conductive silicon carbide crystals. The diameter of the molybdenum wire was Φ0.18mm, the reciprocating wire speed was 11.2m / s, and the working medium was JR1A composite working fluid (1:20). The open circuit voltage was 110V, and the pulse width maintenance time (T on =20μs), pulse interval (T off = 200μs), the relationship between the discharge probability and time is obtained about 2 hours after processing. Figure 3 As shown by Figure 3It can be seen that the discharge probability value during EDM of n-type conductive silicon carbide crystals is about 30%, which is much lower than the discharge probability value of 90% during EDM of metal materials. It can be seen that the processing efficiency of EDM of conductive silicon carbide crystals alone is low.

[0023] like Figure 4 The figure shows a schematic diagram of the composite cutting of conductive silicon carbide crystals by discharge electrochemical assisted diamond wire. Through the composite processing power supply, in the alkaline conductive working medium, within the pulse width maintenance time, when the inter-electrode gap is large, the conductive silicon carbide crystal connected to the positive electrode and the conductive diamond wire connected to the negative electrode in the processing area directly undergo electrochemical reactions, and bubbles and a passivation layer are formed locally on the surface of the silicon carbide crystal; as the inter-electrode gap decreases, the inter-electrode voltage forms an effective discharge on one side of the conductive silicon carbide crystal, and local high temperature forms a softening layer on the surface of the crystal. During the pulse interval, the diamond abrasive on the diamond wire is controlled to contact and grind to remove the softening layer, passivation layer, and surface metamorphic layer on the surface of the crystal material. In the next electrical signal cycle, the discharge electrochemical assisted diamond wire grinding composite processing is continued for the crystal material, and this cycle is repeated. In this case, under the conditions of the alkaline conductive working medium, the following electrochemical reaction occurs between the two electrodes. The reaction equation for the formation of bubbles on the anode surface of the silicon carbide crystal is as follows:

[0024] 4(OH) - -4e - →2H2O+O2↑

[0025] The reaction equation for the formation of bubbles on the cathode surface of the diamond wire is as follows:

[0026] 2H2O+2e - →2(OH) - +H2↑

[0027] The discharge electrochemical assisted diamond wire composite slicing test was carried out on n-type conductive silicon carbide crystals, and the working medium conductivity was 0.49S·m -1 . Use a composite machining power supply, during the pulse width maintenance period (T on =30μs) for discharge electrochemical assisted machining of n-type conductive silicon carbide crystals. During the pulse interval (T off=240μs) to perform abrasive grinding on the inner surface of the slit, and the other processing parameters are the same as the above-mentioned traditional reciprocating wire diamond wire slicing test. The test results show that as the diamond wire feeds to the workpiece, the diamond wire bow in the discharge electrochemical assisted diamond wire composite cutting is significantly reduced, and the deviation value between the actual feeding trajectory of the diamond wire and the theoretical cutting trajectory is also reduced. The theoretical feed length of the silicon carbide crystal material is 17.5mm, while the actual slicing feed length is 18.0mm, and the processing time is 17 minutes and 42 seconds. Therefore, compared with traditional cutting, the composite processing slicing feed length is reduced by 4.8% and the processing time is reduced by 2.1%. It can be seen that the discharge electrochemical assisted diamond wire composite cutting can improve the processing efficiency, reduce the diamond wire bow, and thus improve the shape accuracy.

[0028] like Figure 5 As shown in the figure, the cross-section comparison photos of the traditional cutting and the composite cutting of the conductive silicon carbide crystal are compared. By comparing and observing the surface photos of the two methods of slicing, it is judged that there are two forms of processing of silicon carbide crystals: one is the high-temperature material etching by electric sparks and the etching by electrochemical reaction. Since the silicon carbide crystal material has high hardness and high thermal conductivity, and the discharge pulse width is in the microsecond level, a large part of the heat generated by the discharge will be transferred to the silicon carbide matrix; at the same time, due to the electrochemical reaction under the working medium, the high-temperature etching and collapse area formed by the discharge is not large, and the thermal damage and tool electrode loss can be reduced. The other is the abrasive grinding of the surface and subsurface layer of the silicon carbide crystal by diamond abrasives.

[0029] like Figure 6 As shown in the figure, the analysis of a single set of electrical waveforms of discharge electrochemical assisted diamond wire composite cutting of conductive silicon carbide crystals can be divided into two main processes within the composite processing pulse width maintenance time of 30μs. The first process is about 10μs, in which electrochemical reaction occurs first and mainly, and the subsequent 20μs mainly produces pulse discharge. Through the single set of electrical waveform signals, it can be judged that in the processing assistance at this time, discharge is the main effect, and electrochemistry is a relatively minor effect. During the subsequent pulse interval, the silicon carbide crystal is ground and removed by diamond abrasives.

[0030] The discharge electrochemically assisted diamond wire slicing test on conductive silicon carbide crystals shows that the discharge electrochemically assisted diamond wire processing method described in the present invention can achieve efficient, precise and low tool loss processing of conductive silicon carbide crystal materials.

[0031] The above contents are further detailed descriptions of the present invention in combination with specific preferred implementations, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For the technical field to which the present invention belongs, without departing from the control method proposed by the present invention based on the research, several simple deductions or substitutions can be made, which should be regarded as belonging to the protection scope of the present invention.

Claims

1. A conductive silicon carbide crystal discharge electrochemical assisted diamond tool composite processing method, characterized in that: A composite processing power source capable of realizing discharge and electrochemical reaction is adopted. By using a diamond tool with an outer layer coated with a conductive metal element as a composite electrode, in a working medium with a certain conductivity, the control system realizes discharge processing and electrochemical processing of the conductive silicon carbide crystal within the pulse width maintenance time, and grinding of the conductive silicon carbide crystal within the pulse interval time, and repeats this cycle to complete efficient, precise, and low tool electrode loss processing of the conductive silicon carbide crystal.

2. The composite processing power supply according to claim 1, characterized in that: The composite processing power source can realize discharge and electrochemical action when the discharge pulse width is maintained, and can perform diamond abrasive grinding when the discharge pulse interval is maintained.

3. The composite processing power supply according to claim 1, characterized in that: The composite machining power supply can realize independent adjustment of discharge parameters and electrochemical reaction parameters during machining.

4. The composite electrode according to claim 1, characterized in that: The composite electrode has a core material of a metal wire with a certain diameter, and a conductive metal layer (such as nickel, copper, zinc, etc.) is coated on the surface.

5. The composite electrode according to claim 1, characterized in that: The surface layer of the composite electrode may contain diamond abrasive grains electrodeposited in the conductive metal layer, and diamond wire may be used instead of the composite electrode to perform discharge electrochemical assisted diamond tool grinding.

6. The working medium with a certain conductivity according to claim 1, characterized in that: The working medium component can ensure that the conductive silicon carbide material undergoes an electrochemical reaction and can also ensure cooling, chip removal and deionization during the electrical discharge machining stage.

7. The working medium with a certain conductivity according to claim 1, characterized in that: The anions contained in the working medium component have a relatively positive standard electrode potential, and the metal cations contained have a relatively negative standard electrode potential (E0<-2V). The working medium conductivity range is (0.3~2.5)S·m -1 .

8. The discharge electrochemical assisted diamond tool composite machining method according to claim 1, characterized in that: By adjusting the parameters of electrical machining and grinding, the coupling effects of the three energy fields of discharge, electrochemistry and grinding can be regulated and optimized.

9. The discharge electrochemical assisted diamond tool composite machining method according to claim 1, characterized in that: The composite processing method can realize the cutting, surface processing and hole processing of the conductive silicon carbide crystal material by adopting diamond tools of different processing types, such as various wire saws and rope saws, grinding wheels and grinding discs, and drill bits.

Images