Porous metallic bond diamond grinding wheel for SiC thinning and SiC thinning method

Abstract

The invention discloses a porous metal bonding agent diamond grinding wheel for SiC thinning and a SiC thinning method, the porous metal bonding agent diamond grinding wheel for SiC thinning comprises an outer layer body and a filling cavity formed in the outer layer body, and the filling cavity is filled with microspheres. According to the porous metal bonding agent diamond grinding wheel for SiC thinning, the outer layer body is filled with the microspheres, the microspheres can be heated to expand along with temperature rising in the grinding process, the microspheres extrude and slide mutually, the filling cavities can be dynamically filled with the microspheres so as to buffer and disperse local stress, and contact pressure fluctuation can be reduced easily.

Description

Technical Field

[0001] The present invention relates to the technical field of SiC thinning, and more particularly to a porous metal-bonded diamond grinding wheel for SiC thinning and a SiC thinning method. Background Art

[0002] As a wide-bandgap semiconductor material, SiC (silicon carbide) has been widely used in the fields of power electronic devices, radio frequency devices, etc. due to its excellent properties such as high hardness, high thermal conductivity, and high breakdown electric field. In the manufacturing process of SiC devices, the thinning process is one of the key processing steps, and its purpose is to thin the SiC wafer to the required thickness to meet the performance requirements of the device. The traditional SiC thinning process mainly uses mechanical grinding methods to remove materials through the relative movement between the grinding wheel and the SiC wafer. However, during the grinding process, the contact stress between the grinding wheel and the SiC wafer is concentrated, which easily causes periodic microcracks. Under the action of the local thermal stress gradient caused by uneven penetration of the coolant, these microcracks will further expand to form so-called "sun pattern" defects. In addition, due to the high hardness and brittleness of the SiC material, the surface roughness (Ra) after thinning usually reaches 40 nm, and there is a subsurface damage layer with a depth of about 4 μm. These surface defects will seriously affect the performance and reliability of the device.

[0003] In view of this, the present invention is specifically proposed. Summary of the Invention

[0004] The purpose of the present invention is to provide a porous metal-bonded diamond grinding wheel for SiC thinning and a SiC thinning method to solve the problem of the formation and expansion of periodic microcracks caused by the concentrated contact stress between the grinding wheel and the SiC wafer during the grinding process.

[0005] The present invention is implemented as follows:

[0006] In a first aspect, the present invention provides a porous metal-bonded diamond grinding wheel for SiC thinning. The porous metal-bonded diamond grinding wheel for SiC thinning includes an outer layer body and a filling cavity provided inside the outer layer body. The filling cavity is filled with microspheres, and the thermal expansion coefficient of the microspheres is 2-3 times that of the outer layer body.

[0007] In an optional embodiment, the particle size of the microspheres is 10 μm - 50 μm;

[0008] and / or, the microspheres are at least one of shape memory alloy microspheres, silicon microspheres, graphite microspheres, and quartz glass microspheres.

[0009] In an optional embodiment, the microspheres are Ni-Ti alloy microspheres.

[0010] In an alternative embodiment, the mass ratio of the outer body to the filled microspheres is 7:(2.5 - 3.5);

[0011] and / or, the diamond particle size in the outer body is #1500 - #2500.

[0012] In a second aspect, the present invention provides a SiC thinning method, comprising: grinding the portion to be thinned of the SiC element with the porous metal-bonded diamond grinding wheel for SiC thinning according to any one of the foregoing embodiments until the thinning thickness meets the requirements.

[0013] In an alternative embodiment, the grinding is carried out in the presence of a coolant, the coolant includes nanosheets and a lubricant, and the coolant satisfies at least one of the following characteristics a - c:

[0014] a. The mass fraction of the nanosheets in the coolant is 5% - 10%;

[0015] b. The thickness of the nanosheets < 5 nm;

[0016] c. The nanosheets are boron nitride nanosheets.

[0017] In an alternative embodiment, it further includes chemically treating the grinding area with a treatment liquid, the treatment liquid includes potassium hydroxide and H2O2, the mass ratio of potassium hydroxide to H2O2 in the treatment liquid is (2 - 3):1, and the concentration of H2O2 in the treatment liquid is 0.4 mol / L - 0.6 mol / L.

[0018] In an alternative embodiment, the temperature of the chemical treatment is 45°C - 55°C;

[0019] and / or, the flow rate of the treatment liquid is 0.1 mL / min - 1 mL / min;

[0020] and / or, the treatment liquid is sprayed on the area to be chemically treated using a spraying device.

[0021] In an alternative embodiment, in the grinding step, the rotational speed of the porous metal-bonded diamond grinding wheel for SiC thinning is 200 rpm - 500 rpm, and the feed rate is 1 μm / s - 5 μm / s.

[0022] The present invention has the following beneficial effects:

[0023] In the present application, microspheres are filled in the outer body of the porous metal-bonded diamond grinding wheel for SiC thinning. During the grinding process, as the temperature rises, the microspheres will also thermally expand, and the microspheres will squeeze and slide against each other, and can dynamically fill the filling cavity to buffer and disperse local stress, which is beneficial to reducing the contact pressure fluctuation. Description of the Drawings

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0025] Figure 1 It is a photo of the thinned SiC element obtained in Embodiment 1 of this application;

[0026] Figure 2 It is a photo of the thinned SiC element obtained in Embodiment 5 of this application;

[0027] Figure 3 It is a photo of the thinned SiC element obtained in Comparative Example 1 of this application;

[0028] Figure 4 It is a surface profile diagram of the thinned SiC element obtained in Embodiment 1 of this application;

[0029] Figure 5 It is a surface profile diagram of the thinned SiC element obtained in Comparative Example 1 of this application. Specific Embodiments

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can all be obtained as conventional products through commercial purchase.

[0031] The embodiments of the present invention provide a porous metal-bonded diamond grinding wheel for SiC thinning. The porous metal-bonded diamond grinding wheel for SiC thinning includes an outer layer body and a filling cavity provided inside the outer layer body. The filling cavity is filled with microspheres, and the thermal expansion coefficient of the microspheres is 2-3 times that of the outer layer body.

[0032] In this application, microspheres are filled in the outer layer body of the porous metal-bonded diamond grinding wheel for SiC thinning. During the grinding process, as the temperature rises, the microspheres will also expand due to heat, and the microspheres will squeeze and slide against each other, and can dynamically fill the filling cavity to buffer and disperse local stress, which is beneficial to reducing the contact pressure fluctuation.

[0033] The coefficient of thermal expansion of the microspheres is higher than that of the outer body, which is beneficial for the microspheres to detect temperature changes in a timely manner and dynamically fill the filling cavity in a timely manner, which is more conducive to buffering and dispersing local stress in a timely manner; however, if the coefficient of thermal expansion of the microspheres is too high, it will lead to a decrease in the strength of the porous metal-bonded diamond grinding wheel and a decrease in the grinding efficiency.

[0034] In an alternative embodiment, the particle size of the microspheres is 10 μm - 50 μm; the particle size of the microspheres affects the ability of the microspheres to dynamically fill the filling cavity. If the particle size is too small, the porosity in the filling cavity is relatively low, and the difficulty of microsphere movement increases; if the particle size is too large, it is not conducive to the uniform distribution of stress between the microspheres and the outer body, and thus not conducive to the dispersion of local stress between the grinding wheel and the SiC component to be thinned.

[0035] In an alternative embodiment, the microspheres are at least one of shape memory alloy microspheres, silicon microspheres, graphite microspheres, and quartz glass microspheres. The coefficients of thermal expansion of silicon microspheres, graphite microspheres, and quartz glass microspheres meet the aforementioned requirements.

[0036] In an alternative embodiment, the microspheres are Ni-Ti alloy microspheres.

[0037] In an alternative embodiment, the diamond grit size in the outer body is #1500 - #2500;

[0038] The outer body includes a contact surface directly with the SiC component to be thinned. Reasonably selecting the diamond grit size in the outer body is beneficial for taking into account both the grinding efficiency and the surface quality of the SiC component.

[0039] In an alternative embodiment, the mass ratio of the outer body to the filled microspheres is 7:(2.5 - 3.5); if the proportion of the microspheres is too small, it is not conducive to improving the ability to buffer and disperse local stress; however, if the proportion of the microspheres is too large, the mass of the outer body will decrease, the wall thickness corresponding to the filling cavity will decrease, the frictional force that can be provided will decrease, and the strength will decrease, which is not conducive to improving the efficiency and the service life of the grinding wheel.

[0040] In an alternative embodiment, the microspheres are Ni-Ti alloy microspheres.

[0041] Ni-Ti alloy is a kind of shape memory alloy with a reversible shape memory effect. It can be restored to its initial shape through physical stimulation, with controllable size and morphology, high strength, low hardness, a recovery strain of up to 10%, and high environmental stability; compared with brittle and easily chipping graphite and quartz glass that is easily affected by microcracks due to high hardness, Ni-Ti alloy microspheres have more advantages.

[0042] The embodiment of the present invention also provides a SiC thinning method, including: grinding the part to be thinned of the SiC element with the porous metal-bonded diamond grinding wheel for SiC thinning described in any one of the foregoing embodiments until the thinning thickness meets the requirements.

[0043] Generally, the thinning thickness is related to the grinding rate, and the requirement of the thinning thickness can be met by controlling the processing time.

[0044] In some embodiments, a laser confocal sensor can be used to monitor the surface roughness of the SiC element in real time (resolution 0.1 nm), and a 3D topography map is generated every certain time. When the topography map shows that the surface roughness of the SiC element reaches the preset value and the thinning thickness meets the requirements, the thinning of the SiC element is completed.

[0045] In an alternative embodiment, the grinding is carried out in the presence of a coolant, and the coolant includes nanosheets and a lubricating fluid. The coolant satisfies at least one of the following characteristics a-c:

[0046] a. The mass fraction of the nanosheets in the coolant is 5%-10%;

[0047] b. The thickness of the nanosheets < 5 nm;

[0048] c. The nanosheets are boron nitride nanosheets.

[0049] In the present application, the nanosheets in the coolant can form a quasi-solid lubricating film at the grinding interface, which can reduce the friction coefficient and enhance the heat conduction synchronously. Among them, choosing thinner nanosheets is beneficial to obtaining a SiC element with a smoother surface and more balanced thinning effect at each position; the nanosheets need to be selected to not react with the base fluid and have a hardness smaller than that of silicon carbide to avoid abrasion of silicon carbide. For example, boron nitride nanosheets can be selected, and boron nitride nanosheets have the following advantages: small hardness; high thermal conductivity (~2000 W / m·K), which can significantly improve the heat dissipation efficiency; strong chemical inertness, resistant to acid and alkali corrosion; low friction coefficient and small flow resistance.

[0050] The coolant also includes a base fluid, and the purpose of the base fluid is to disperse the nanosheets. Polyalphaolefin with a high viscosity index, low-temperature performance, and thermal oxidation stability can be selected; in addition, in order to improve the dispersion effect, a surfactant can also be added in some embodiments. The surfactant can be selected as dimethylamine, which has both lubricity and antistatic effects. The mass fraction of the surfactant in the coolant can be 0.4%-0.1%.

[0051] It should be noted that since the grinding wheel in this application is filled with heat-sensitive microspheres, the overall coefficient of thermal expansion of the grinding wheel increases, making it more sensitive to temperature changes. If the temperature is too high during the processing, the improved grinding wheel is prone to deformation or stress and is not stable enough. Therefore, using water in conventional grinding as the coolant cannot meet the requirement of rapid cooling of the grinding area, resulting in too high a temperature on the disk surface. Using the grinding wheel in this application in combination with the coolant in this application is more conducive to ensuring timely heat dissipation and stable temperature during the processing.

[0052] In an alternative embodiment, it further includes chemically treating the grinding area with a treatment liquid. The treatment liquid includes potassium hydroxide and H2O2. The mass ratio of potassium hydroxide to H2O2 in the treatment liquid is (2 - 3):1, and the concentration of H2O2 in the treatment liquid is 0.4 mol / L - 0.6 mol / L. Potassium hydroxide in the treatment liquid can corrode the SiC surface, which is beneficial to eliminating the stress concentration at the tip of the microcrack; H2O2 can generate a SiO2 passivation layer on the inner wall of the oxidized crack, which is beneficial to inhibiting crack propagation.

[0053] In an alternative embodiment, the temperature of the chemical treatment is 45°C - 55°C;

[0054] and / or, the flow rate of the treatment liquid is 0.1 mL / min - 1 mL / min;

[0055] and / or, the treatment liquid is sprayed on the area to be chemically treated by a spraying device, which is beneficial to the uniform dispersion of the treatment liquid on the surface of the SiC component.

[0056] In some embodiments, the chemical treatment area is synchronized with the grinding. The treatment liquid and the coolant can be sprayed simultaneously on the grinding area for chemical treatment and cooling.

[0057] In an alternative embodiment, in the grinding step, the rotational speed of the porous metal-bonded diamond grinding wheel for SiC thinning is 200 rpm - 500 rpm, and the feed rate is 1 μm / s - 5 μm / s. This is beneficial to ensuring the grinding efficiency while ensuring that the surface smoothness, microcrack conditions, etc. of the SiC component meet the requirements.

[0058] The features and properties of the present invention will be further described in detail below in conjunction with embodiments.

[0059] Example 1

[0060] This example provides a SiC thinning method, which specifically includes the following steps:

[0061] Under the conditions of the coolant and the treatment liquid, use a porous metal-bonded diamond grinding wheel for SiC thinning to grind the area to be thinned of the SiC component;

[0062] Among them, the rotational speed of the porous metal-bonded diamond grinding wheel for SiC thinning is 300 rpm, and the feed rate is 3 μm / s; the porous metal-bonded diamond grinding wheel includes an outer body and a filling cavity arranged inside the outer body, and the filling cavity is filled with Ni-Ti alloy microspheres with an average particle size of 30 μm. The diamond grit size in the outer body is #2000, and the mass ratio of the outer body to the filled Ni-Ti alloy microspheres is 7:3;

[0063] The coolant includes boron nitride nanosheets and poly-α-olefin. The mass fraction of the nanosheets in the coolant is 7.5% and the thickness is <5 nm;

[0064] The treatment liquid is a mixed aqueous solution of 1 mol / L potassium hydroxide and 0.5 mol / L H2O2. The temperature of the chemical treatment is 50 °C, and the flow rate of the treatment liquid is 0.5 mL / min.

[0065] Example 2

[0066] This example provides a SiC thinning method, which specifically includes the following steps:

[0067] Under the conditions of the coolant and the treatment liquid, use a porous metal-bonded diamond grinding wheel for SiC thinning to grind the area to be thinned of the SiC component;

[0068] Among them, the rotational speed of the porous metal-bonded diamond grinding wheel for SiC thinning is 200 rpm, and the feed rate is 1 μm / s; the porous metal-bonded diamond grinding wheel includes an outer body and a filling cavity arranged inside the outer body, and the filling cavity is filled with Ni-Ti alloy microspheres with an average particle size of 10 μm. The diamond grit size in the outer body is #2500, and the mass ratio of the outer body to the filled Ni-Ti alloy microspheres is 7:3.5;

[0069] The coolant includes boron nitride nanosheets and poly-α-olefin. The mass fraction of the nanosheets in the coolant is 5% and the thickness is <5 nm;

[0070] The treatment liquid is a mixed aqueous solution of 1.2 mol / L potassium hydroxide and 0.4 mol / L H2O2. The temperature of the chemical treatment is 55 °C, and the flow rate of the treatment liquid is 1 mL / min.

[0071] Example 3

[0072] This example provides a SiC thinning method, which specifically includes the following steps:

[0073] Under the conditions of the coolant and the treatment liquid, use a porous metal-bonded diamond grinding wheel for SiC thinning to grind the area to be thinned of the SiC component;

[0074] Among them, the rotational speed of the porous metal-bonded diamond grinding wheel for SiC thinning is 500 rpm, and the feed rate is 5 μm / s; the porous metal-bonded diamond grinding wheel includes an outer body and a filling cavity arranged inside the outer body, and the filling cavity is filled with Ni-Ti alloy microspheres with an average particle size of 50 μm. The diamond grit size in the outer body is #1500, and the mass ratio of the outer body to the filled Ni-Ti alloy microspheres is 7:2.5;

[0075] The coolant includes boron nitride nanosheets and poly-α-olefin, and the mass fraction of the nanosheets in the coolant is 10% and the thickness < 5 nm;

[0076] The treatment liquid is a mixed aqueous solution of 1.2 mol / L potassium hydroxide and 0.6 mol / L H2O2. The temperature of the chemical treatment is 45 °C, and the flow rate of the treatment liquid is 0.1 mL / min.

[0077] Comparative Example 1

[0078] This comparative example provides a SiC thinning method, which specifically includes the following steps:

[0079] Under the condition of using water as the coolant, the SiC element to be thinned is ground with a grinding wheel;

[0080] Among them, the rotational speed and feed rate of the grinding wheel are the same as those in Example 1. The grinding wheel is a solid structure without a filling cavity and is not filled with Ni-Ti alloy microspheres.

[0081] Example 4

[0082] This example provides a SiC thinning method, which is mainly different from Example 1 in that the grinding wheel is different, the coolant is water, and no treatment liquid is used for chemical treatment.

[0083] Example 5

[0084] This comparative example provides a SiC thinning method, which is mainly different from Example 1 in that no treatment liquid is used for chemical treatment.

[0085] Example 6

[0086] This comparative example provides a SiC thinning method, which is mainly different from Example 1 in that the microsphere material is quartz glass.

[0087] Example 7

[0088] This comparative example provides a SiC thinning method, which is mainly different from Example 1 in that the average particle size of the microspheres is 100 μm.

[0089] Example 8

[0090] This comparative example provides a SiC thinning method, which is mainly different from Example 1 in that: the mass ratio of the outer body to the filled Ni-Ti alloy microspheres is 9:1.

[0091] The surface roughness and the depth of the subsurface damage layer of the thinned SiC components obtained in the above-mentioned examples and comparative examples were tested, and the test methods are as follows:

[0092] For the surface roughness test, 5 points were selected on the surface of the thinned SiC component, and the surface roughness of the selected points was measured using an Atomic Force Microscope (AFM) and the average value was calculated;

[0093] The method for testing the depth of the subsurface damage layer is: the thinned SiC component was polished until the sun pattern was removed.

[0094] The test results are shown in Table 1.

[0095] Table 1

[0096] Surface roughness nm Depth of subsurface damage layer μm Example 1 10 2.0 Example 2 12 2.5 Example 3 14 2.6 Example 5 24 3.2 Example 6 37 3.8 Example 7 30 3.3 Example 8 28 3.5 Comparative Example 1 40 4.0

[0097] As can be seen from Table 1, compared with Comparative Example 1 through Examples 1 and 5, using the grinding wheel of the present application is beneficial to reducing the surface roughness and the depth of the subsurface damage layer, and increasing the treatment liquid can further reduce the surface roughness and the depth of the subsurface damage layer. Comparing Example 1 with Example 8, it can be seen that the Ni-Ti alloy is more beneficial to reducing the surface roughness and the depth of the subsurface damage layer, because the Ni-Ti alloy has high strength, low hardness and a recovery strain of up to 10%, while quartz glass has high hardness and is easily affected by microcracks, which may cause new scratches, resulting in an increase in the surface roughness and the depth of the subsurface damage layer. Comparing Example 1 with Example 9, it can be seen that too high a microsphere particle size will lead to an increase in the surface roughness and the depth of the subsurface damage layer. The reason may be that the volume increase of the large-particle-size microspheres due to thermal expansion is relatively large, resulting in a decrease in the contact area between diamond and silicon carbide, and less material is removed from the wafer within the same processing time, resulting in residues in the pre-stage damage layer. Comparing Example 1 with Example 10, it can be seen that if the diamond proportion is too large, the role of the microspheres is too small, resulting in a relatively large surface roughness and a relatively deep subsurface damage layer depth.

[0098] It should be noted that for Examples 1-3 and Examples 5-8, the temperature of the grinding disk during the grinding process can be maintained between 35-40 °C, while in Example 4, due to the inappropriate selection of the coolant, the temperature of the grinding disk exceeds 45 °C, which causes greater damage to the equipment.

[0099] In addition, the surface pictures of the thinned SiC components obtained in Example 1, Example 5 and Comparative Example 1 are as Figures 1-3As shown, it can be seen that there is no "sun pattern" observed on the surface of the thinned SiC component obtained in Example 1. Only at the position within the red frame on the surface of the thinned SiC component obtained in Example 5 is there a "sun pattern" and it is not obvious. However, the "sun pattern" on the surface of the thinned SiC component obtained in Comparative Example 1 is obvious. Further, as Figures 4-5 Figure showing the surface profile of the thinned SiC components obtained in Example 1 and Comparative Example 1 measured by the Tropel method. It can be seen that compared with Comparative Example 1, the "sun pattern" in Example 1 almost disappears.

[0100] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A porous metal bond diamond grinding wheel for SiC thinning, characterized in that: The porous metal bond diamond grinding wheel for SiC thinning comprises an outer layer body and a filling cavity arranged inside the outer layer body, wherein the filling cavity is filled with microspheres, and the thermal expansion coefficient of the microspheres is 2-3 times the thermal expansion coefficient of the outer layer body.

2. The porous metal bond diamond grinding wheel for SiC thinning according to claim 1, characterized in that: The particle size of the microspheres is 10 μm-50 μm.

3. The porous metal bond diamond grinding wheel for SiC thinning according to claim 1, characterized in that: The microspheres are at least one of memory alloy microspheres, silicon microspheres, graphite microspheres, and quartz glass microspheres.

4. The porous metal bond diamond grinding wheel for SiC thinning according to claim 1, characterized in that: The microspheres are Ni-Ti alloy microspheres.

5. The porous metal bond diamond grinding wheel for SiC thinning according to claim 1, characterized in that: The mass ratio of the outer layer body to the filled microspheres is 7:(2.5-3.5); And / or, the diamond grain size in the outer layer body is #1500-#2500.

6. A SiC thinning method, characterized in that: include: The porous metal bond diamond grinding wheel for SiC thinning according to any one of claims 1 to 5 is used to grind the portion of the SiC element to be thinned until the thinning thickness meets the requirements.

7. The SiC thinning method according to claim 6, characterized in that: The grinding is performed in the presence of a coolant, the coolant comprising nanosheets and a lubricating liquid, and the coolant satisfies at least one of the following features ac: a. The mass fraction of the nanosheets in the coolant is 5%-10%; b. The thickness of the nanosheet is less than 5 nm; c. The nanosheets are boron nitride nanosheets.

8. The SiC thinning method according to claim 6, characterized in that: It also includes using a treatment liquid to chemically treat the grinding area, wherein the treatment liquid includes potassium hydroxide and H2O2, the mass ratio of potassium hydroxide to H2O2 in the treatment liquid is (2-3):1, and the concentration of H2O2 in the treatment liquid is 0.4mol / L-0.6mol / L.

9. The SiC thinning method according to claim 8, characterized in that: The temperature of the chemical treatment is 45°C-55°C; And / or, the treatment liquid flow rate is 0.1mL / min-1mL / min; And / or, the treatment liquid is sprayed onto the area to be chemically treated using a spray device.

10. The SiC thinning method according to claim 6, characterized in that: In the grinding step, the rotation speed of the porous metal bond diamond grinding wheel for SiC thinning is 200rpm-500rpm, and the feed speed is 1μm / s-5μm / s.

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