A method of changing the shape of a SiC substrate

Abstract

The present application relates to a kind of methods for efficiently changing SiC substrate shape, which comprises using double-sided grinder to double-sided grind wafer after cutting;After cleaning, using double-sided polishing machine to place the wafer after cleaning in polishing machine to carry out double-sided mechanical polishing, if the wafer shape after double-sided mechanical polishing is inferior to target shape, and Bow>15um, the wafer is subjected to supplementary single-sided mechanical polishing, finally the wafer after mechanical polishing, chemical mechanical polishing is carried out to silicon surface or carbon surface, and uniform recessed or convex SiC substrate is obtained.The present application breaks through the limitation of non-polar semiconductor processing by specific wafer orientation during polishing and upper and lower disc rotating speed, under the premise of ensuring high flatness and low surface roughness of SiC substrate surface, the substrate shape can be quickly processed to uniform recessed or convex, effectively reducing the cumbersome steps of traditional processing, and the polishing liquid life can be improved to save processing cost.

Description

TECHNICAL FIELD

[0001] The application relates to a method for changing the shape of a SiC substrate and belongs to the technical field of semiconductor material processing. BACKGROUND

[0002] Silicon carbide (SiC) is a typical representative of the third generation of wide band gap semiconductors and is a basic material for preparing radio frequency high power devices. For a long time, hetero-epitaxial growth of GaN on a SiC substrate is a key technology for preparing advanced radio frequency devices, but with the development of the semiconductor industry, the requirements for the substrate in the industry not only stop at the traditional sense of material and quality, but also put forward more stringent requirements for the surface shape of the substrate, including preparing a substrate with a uniform concave or convex surface shape to meet the uniformity of hetero-epitaxy or homo-epitaxy and product yield.

[0003] SiC is an ultra-hard material with a Mohs hardness of 9.2. Using a traditional processing flow for processing, the material removal rate is low, and the surface shape parameters are poor, so it is difficult to ensure that the shape of the substrate meets the epitaxial requirements. The gaseous reactants used in the epitaxial process have high requirements for the surface shape of the substrate. Even if the surface has only a micron-level deviation, it will cause uneven deposition and poor epitaxial consistency and other related problems. Generally, if the silicon surface of the substrate has a uniform concave shape, the wavelength hit rate of hetero-epitaxy can be effectively improved, and the wavelength standard deviation (PLSTD) can be reduced. If the carbon surface of the substrate has a uniform concave shape (the silicon surface has a uniform convex shape), the crystal quality of the homo-epitaxial SiC film can be improved, the number of basal plane dislocations can be reduced, and the stability of the high-voltage diode can be improved. Therefore, realizing the processing of a specific substrate shape has important significance for realizing high-quality development in the field of semiconductor materials and devices.

[0004] At present, there are few reports on SiC substrate shape in China, partly because SiC is a new type of wide bandgap semiconductor, and the related industry and technology in China are still in the exploratory stage; another reason is that SiC is a typical polar crystal, which has many differences in physical properties and processing technology from traditional non-polar crystals such as Si substrate and sapphire substrate. Taking sapphire substrate as an example, the upper and lower surfaces of the substrate have the same atomic structure, so they do not have crystal polarity, and the physical properties of the upper and lower surfaces are exactly the same. However, for SiC substrate, the upper and lower surfaces are composed of carbon atom layers and silicon atom layers respectively, and many studies have shown that the polarity of the crystal will significantly affect the properties of the substrate, for example, the carbon surface has lower hardness and faster removal rate in the polishing process than the silicon surface. In summary, due to the crystal polarity of the upper and lower surfaces of SiC, the processing of SiC substrate is more complex than that of traditional semiconductors, therefore, the processing of traditional non-polar crystal substrates cannot be applied to polar substrates, such as Chinese invention patent (CN110718450A (CN201910946953.X) which discloses a method for preparing a sapphire bowl-shaped substrate, which utilizes double-sided grinding and annealing processes to prepare a bowl-shaped wafer, but the processing flow is complicated, and only one shape of substrate can be prepared, and it cannot be applied to the processing of polar substrates. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a method for changing the shape of a SiC substrate, which takes into account and utilizes the surface polarity of the SiC substrate, breaking through the limitations of non-polar semiconductor processing, and under the premise of ensuring high flatness and low surface roughness of the SiC substrate surface, the substrate shape can be quickly processed to uniform concave or convex, effectively reducing the cumbersome steps of traditional processing, and improving the service life of polishing liquid to save processing cost.

[0006] TERMS

[0007] 1>Surface terminated in carbon atom layer in <0001> direction.

[0008] Silicon surface (Si surface): refers to the surface of the silicon carbide substrate terminated in silicon atom layer in <0001> direction.

[0009] Bow: in this application, Bow refers to the curvature of the substrate;

[0010] Warp: in this application, Warp refers to the warping degree of the wafer;

[0011] Surface damage layer: a metamorphic layer with a certain thickness formed on the surface of the substrate, which is different from the normal crystal lattice structure;

[0012] Surface step: the atomic structure exposed after the surface damage layer of the substrate is completely removed.

[0013] The present application is realized by the following technical solutions:

[0014] A method for changing the shape of a SiC substrate, comprising the steps of:

[0015] 1) using a multi-wire saw, cutting a SiC single crystal rod into a wafer with a thickness of 500-700 um, the surface of the wafer after cutting being flat and crack-free;

[0016] 2) using a double-side grinding machine to grind the wafer after cutting, if the final product requires a Si surface to be concave, the wafer is placed with the carbon surface facing up and the Si surface facing down during grinding, the rotation speed of the upper plate is greater than that of the lower plate during grinding, if the product requires a carbon surface to be concave, the wafer is placed with the carbon surface facing down and the Si surface facing up, the rotation speed of the upper plate is greater than that of the lower plate during grinding, the grinding pressure is 20-100 g / cm 2 , the grinding rotation speed is 5-25 rpm, the surface of the wafer after grinding is free of scratches, the thickness is 350-550 um, and Bow<25 um and Warp<20 um;

[0017] 3) using an ultrasonic cleaning machine to clean the wafer after grinding for 40-70 min to remove surface grinding particles, and then drying and wiping the cleaned wafer;

[0018] 4) using a double-side polishing machine to polish the cleaned wafer mechanically, the wafer is placed in the same direction as in step 3), the rotation speed of the upper plate is the same as that of the lower plate, the abrasive is diamond powder, the rotation speed is 10-40 rpm, and the polishing pressure is 80-200 g / cm 2 , the surface roughness of the wafer after polishing is less than 2 nm, Bow<20 um, and Warp<10 um;

[0019] 5) if the shape of the wafer after double-side mechanical polishing is inferior to the target shape and Bow>15 um, the wafer is subjected to supplementary single-side mechanical polishing, if the final product requires a Si surface to be concave, the C surface is subjected to supplementary polishing, if the final product requires a C surface to be concave, the Si surface is subjected to supplementary polishing, the rotation speed of the single-side mechanical polishing is 30-40 rpm, and the pressure is 150-250 g / cm 2 ;

[0020] 6) using a single-side polishing machine to polish the Si surface or the C surface of the wafer after mechanical polishing in step 4) or step 5) chemically and mechanically, the abrasive is SiO2 powder, the polishing rotation speed is 30-50 rpm, and the pressure is 100-300 g / cm 2 , the surface roughness after chemical and mechanical polishing is less than 0.2 nm, Bow<20 um, and Warp<10 um;

[0021] 7) Cleaning: The chemically polished wafers are cleaned and dried using a cleaning machine and a rotary dryer to obtain a uniformly recessed or raised SiC substrate.

[0022] According to a preferred embodiment of the present invention, in step 1), the saw wire used by the multi-wire cutting machine is steel wire, and the cutting fluid is a slurry composed of diamond powder, dispersant, and water.

[0023] The proportions of diamond powder, dispersant, and water in the above cutting fluid, as well as the dispersant, are determined according to existing technology.

[0024] According to a preferred embodiment of the present invention, in step 2), the grinding mill is a double-sided cast iron disc grinding mill, and the abrasive is boron carbide micro powder with a particle size of 10-20 μm.

[0025] According to a preferred embodiment of the present invention, in step 2), if the final product requires a concave Si surface, the wafer is placed with the carbon surface facing upwards and the silicon surface downwards during grinding. The rotation speed of the grinding wheel is 15-25 rpm, and the grinding pressure is 25-50 g / cm². 2 The grinding disc rotates at 5-15 rpm, and the grinding pressure is 25-50 g / cm. 2 The loading method is cylinder pressurization, and the grinding time is 180min-240min.

[0026] According to a preferred embodiment of the present invention, in step 2), during grinding, if the product requires a concave carbon surface, the wafer is placed with the carbon surface facing down and the silicon surface facing up. The rotation speed of the grinding disc is 15-25 rpm, and the grinding pressure is 25-50 g / cm. 2 The grinding disc rotates at 5-15 rpm, and the grinding pressure is 25-50 g / cm. 2 The loading method is cylinder pressurization, and the grinding time is 180min-240min.

[0027] According to a preferred embodiment of the present invention, in step (3), the ultrasonic cleaning frequency is 50-70 kHz, and the cleaning medium is water.

[0028] According to a preferred embodiment of the present invention, in step 4), the polishing machine is a double-sided copper disc polishing machine, and the polishing material is 1-10 μm diamond micro powder.

[0029] According to a preferred embodiment of the present invention, in step 4), if the final product requires a concave Si surface during polishing, the wafer is placed with the carbon surface facing upwards and the silicon surface facing downwards during grinding, the rotation speed of the polishing plate is 30-40 rpm, and the grinding pressure is 25-50 g / cm. 2 Polishing time is 6-9 hours.

[0030] According to a preferred embodiment of the present invention, in step 4), during polishing, if the product requires a concave carbon surface, the wafer is placed with the carbon surface facing down and the silicon surface facing up. The rotation speed of the polishing disc is 30-40 rpm, and the polishing pressure is 25-50 g / cm. 2 Polishing time is 6-9 hours.

[0031] According to a preferred embodiment of the present invention, in step 5), the single-sided mechanical polishing is performed using a single-sided copper disc polishing machine, and the polishing material is diamond micro powder with a particle size of 10-15 μm.

[0032] According to a preferred embodiment of the present invention, in step 6), the chemical mechanical polishing pad is a polyurethane polishing pad, and the polishing time is 10-16 hours; the polishing liquid is an acidic slurry composed of hydrogen peroxide, nano-silica particles with an average particle size of 200 nm, and a stabilizer.

[0033] The proportions of hydrogen peroxide, nano-silica particles with an average particle size of 200 nm, and stabilizer in the above polishing solution, as well as the stabilizer itself, are determined according to existing technologies.

[0034] The SiC substrate with depressions or convexities is obtained, and the wafer's surface shape parameters are tested using a surface shape analyzer. Surface imaging and recording are performed on the polished surface. The wafers obtained through these steps have a uniform, concave bowl shape, with minimal fluctuation in key surface shape parameters such as Bow and Warp of the substrate. The surface is bright and scratch-free, and the surface shape parameters of wafers from the same batch tend to be consistent. Compared with previous semiconductor shape processing technologies (including annealing, chamfering, etc.), this process reduces the processing steps and saves more than 5 hours of time.

[0035] In the method of this invention, all equipment and raw materials are well-known, and those not specifically limited can refer to currently widely used semiconductor wafer processing equipment.

[0036] The superior effects of this invention are as follows:

[0037] 1. This invention overcomes the limitations of non-polar semiconductor processing by adjusting the wafer orientation and rotation speed of the upper and lower disks during polishing with a specific grinding machine. While ensuring high flatness and low surface roughness of the SiC substrate, the substrate shape can be quickly processed into uniform depressions or protrusions, effectively reducing the cumbersome steps of traditional processing and improving the polishing slurry life, thereby saving processing costs.

[0038] 2. The present invention produces wafer products with more uniform Bow values, making the shapes of wafers more similar and improving processing consistency.

[0039] 3. The present invention provides a method for performing supplementary single-sided mechanical polishing on wafers after double-sided mechanical polishing. The wafer shape is inferior to the target shape, and 15um < Bow < 20um. This method greatly improves the success rate and the quality of the product.

[0040] 4. The process steps of this invention are simple and effective, reducing environmental pollution. Attached Figure Description

[0041] Figure 1 In Example 1, the shape of the 6-inch N-type substrate after processing by the method of the present invention is imaged, and the silicon surface is a uniformly concave shape after processing;

[0042] Figure 2 In Example 2, the shape of the 6-inch seed substrate after being processed by the method of the present invention is imaged. Before polishing, the shape is irregular. After processing, the silicon surface is a uniformly convex shape, that is, the carbon surface is a uniformly concave shape.

[0043] Figure 3 In Example 3, the shape of the 4-inch semi-insulating substrate after being processed by the method of the present invention is imaged. Before polishing, the shape is irregular, and after processing, the silicon surface has a uniform concave shape.

[0044] Figure 4 The image shows the surface roughness after CMP in Example 1, with a surface roughness of less than 0.1 nm.

[0045] Figure 5 The image shows the surface roughness after CMP in Example 2, with a surface roughness of less than 0.1 nm.

[0046] Figure 6 The image shows the surface roughness after CMP in Example 3, with a surface roughness of less than 0.1 nm. Detailed Implementation

[0047] The growth method of the present invention will be further described below with reference to embodiments and accompanying drawings, but is not limited thereto.

[0048] Example 1:

[0049] A method for preparing a 6-inch N-type 4H-SiC substrate with a uniformly recessed silicon surface, comprising the following steps:

[0050] 1) The 6-inch 4H-SiC ingot obtained after high-temperature growth is multi-wire cut, and the cut wafers are cleaned after cutting to obtain wafers with an average thickness of 550um.

[0051] 2) Use a double-sided grinder to grind the cutting disc, with the C-side facing upwards and the Si-side downwards. Check that the grinding disc is flat and that there are no blockages in the grooves. For the first 3 hours, the grinding pressure should be 100 g / cm². 2 The grinding speed was 15 rpm for the lower plate and 25 rpm for the upper plate; after removing a certain thickness (in the last 3 hours), a grinding pressure of 20 g / cm was applied. 2The grinding speed is 5 rpm for the lower plate and 15 rpm for the upper plate, and the grinding time is 3 hours until the average thickness of the wafer is 350 μm.

[0052] 3) Use an ultrasonic cleaner to ultrasonically clean the polished wafers for 50 minutes to remove surface abrasive particles. After cleaning, dry and wipe the wafers.

[0053] 4) Place the cleaned grinding disc in a double-sided polishing machine for mechanical polishing. Position the disc with the C-side facing up and the Si-side facing down. Polishing speed: upper disc 30 rpm, lower disc 20 rpm, pressure 180 g / cm². 2 Polishing time is 6 hours. After polishing, the wafer is cleaned and inspected.

[0054] 5) After double-sided mechanical polishing, the Si surface of the wafer is chemically mechanically polished using a single-sided polishing machine at a polishing speed of 40 rpm and a pressure of 250 g / cm. 2 The polishing time was 10 hours, the abrasive was SiO2 micro powder, and the surface roughness of Si decreased to below 0.2 nm after the polishing was completed.

[0055] 6) Remove the wafer, perform final cleaning, testing and packaging to obtain a commercial substrate with an atomically smooth surface and a uniformly recessed silicon surface.

[0056] The shape image processed according to Example 1 is shown below. Figure 1 As shown, through Figure 1 As can be seen, the silicon surface is uniformly concave after processing. This shows that the present invention has overcome the limitations of non-polar semiconductor processing and successfully obtained a 4H-SiC substrate with a concave shape.

[0057] Atomic force microscopy roughness after CMP is shown in [reference needed]. Figure 4 As can be seen from the figure, the surface roughness is less than 0.1 nm.

[0058] Example 2:

[0059] A method for preparing a 6-inch 4H-SiC seed substrate with a uniformly raised silicon surface is disclosed. The method steps are the same as in Example 1, except that...

[0060] In step 2), the placement orientation is: C-face down, Si-face up. Initially, the lower plate rotates at 15 rpm and the upper plate at 25 rpm; thereafter, the lower plate rotates at 5 rpm and the upper plate at 15 rpm.

[0061] In step 4), the placement orientation is: C-face down, Si-face up, lower plate rotation speed is 20 rpm, and upper plate rotation speed is 30 rpm.

[0062] In step 5), the mechanically polished wafer needs to undergo C-surface chemical mechanical polishing at a speed of 30 rpm and a pressure of 100 g / cm², reducing the C-surface roughness to below 0.2 nm. After polishing, the silicon surface of the substrate has a uniform convex shape.

[0063] The shape image after processing in Example 2 is shown below. Figure 2 As shown, through Figure 2 As can be seen, the silicon surface has a uniform convex shape after processing. This shows that the present invention has overcome the limitations of non-polar semiconductor processing and successfully obtained a 4H-SiC seed substrate with a uniform convex shape.

[0064] Atomic force microscopy roughness after CMP is shown in [reference needed]. Figure 5 As can be seen from the figure, the surface roughness is less than 0.1 nm.

[0065] Example 3

[0066] A method for preparing a 4-inch semi-insulating 4H-SiC substrate with a uniformly recessed silicon surface includes the following steps:

[0067] 1) A 4-inch 4H-SiC ingot is cut into multiple wires. After cutting, the cut pieces are cleaned to obtain wafers with an average thickness of 700um.

[0068] 2) Use a double-sided grinder to grind the cut pieces, with the C-side facing upwards and the Si-side downwards, at a grinding pressure of 80 g / cm². 2 The grinding wheel rotates at 20 rpm on the upper plate and 15 rpm on the lower plate for 6 hours, reducing the wafer thickness to 550 μm.

[0069] 3) Perform double-sided mechanical polishing on the grinding disc, with the C-side facing upwards and the Si-side facing downwards. The upper polishing disc rotates at 40 rpm, the lower disc at 30 rpm, and the pressure is 200 g / cm². 2 After polishing, the wafers are cleaned and inspected.

[0070] 4) Testing revealed that the Bow value of the Si side of the wafer after double-sided mechanical polishing was +20µm. Supplementary mechanical polishing was then performed on the C side at a polishing speed of 40rpm and a pressure of 250g / cm². 2 After polishing, the Bow value of the Si surface decreased to +5µm.

[0071] 5) Perform chemical mechanical polishing (CMP) on the Si side of the wafer after double-sided mechanical polishing at a polishing speed of 50 rpm and a pressure of 300 g / cm. 2 After polishing, the surface roughness is less than 0.2 nm;

[0072] 6) Remove the wafer and perform final cleaning and inspection to obtain a substrate with a uniformly concave silicon surface.

[0073] The shape image after processing in Example 3 is shown below. Figure 3 As shown, through Figure 3 As can be seen, the silicon surface is uniformly concave after processing. This shows that the present invention has overcome the limitations of non-polar semiconductor processing and successfully obtained a 4-inch semi-insulating 4H-SiC substrate with a concave shape.

[0074] Atomic force microscopy roughness after CMP is shown in [reference needed]. Figure 6 As can be seen from the figure, the surface roughness is less than 0.1 nm.

Claims

1. A method for changing the shape of a SiC substrate, comprising the following steps: 1) Using a multi-wire dicing machine, SiC single crystal rods are diced into wafers with a thickness of 500~700 μm. After dicing, the wafer surface is flat and free of cracks. 2) The cut wafers are polished on both sides using a double-sided polishing machine; If the final product requires a concave Si surface, the wafer should be placed with the carbon surface facing up and the silicon surface facing down during polishing. The rotation speeds of the upper and lower grinding plates should be controlled during polishing, with the upper grinding plate rotating at a higher speed than the lower grinding plate. If the product requires a concave carbon surface, the wafer should be placed with the carbon surface facing down and the silicon surface facing up. The grinding wheel speed should be greater than the grinding wheel speed; the grinding pressure should be 20-100 g / cm. 2 The grinding speed is 5~25 rpm. After grinding, there are no tool marks on the wafer surface, the thickness is 350~550 um, and the bow is <25 um and the warp is <20 um. The grinding mill adopts a double-sided cast iron disc grinding mill, and the abrasive is boron carbide micro powder with a particle size of 10-20um; During polishing, if the final product requires a concave Si surface, the wafer should be placed with the carbon surface facing up and the silicon surface facing down. The polishing wheel should rotate at 15-25 rpm, and the polishing pressure should be 25-50 g / cm². 2 The grinding disc rotates at 5-15 rpm, and the grinding pressure is 25-50 g / cm. 2 The loading method is cylinder pressurization, and the grinding time is 180min-240min; During grinding, if the product requires a concave carbon surface, the wafer should be placed with the carbon surface facing down and the silicon surface facing up. The grinding wheel speed should be 15-25 rpm, and the grinding pressure should be 25-50 g / cm. 2 The grinding disc rotates at 5-15 rpm, and the grinding pressure is 25-50 g / cm. 2 The loading method is cylinder pressurization, and the grinding time is 180min-240min; 3) Use an ultrasonic cleaner to ultrasonically clean the polished wafers for 40-70 minutes to remove surface polishing particles. After cleaning, dry and wipe the wafers. The ultrasonic cleaning frequency is 50-70 kHz, and the cleaning medium is water. 4) Place the cleaned wafer in a double-sided polishing machine for double-sided mechanical polishing. The wafer orientation is the same as in step 3). The rotational speeds of the upper and lower polishing discs are the same as those of the upper and lower grinding discs. The abrasive is diamond powder, the rotational speed is 10-40 rpm, and the polishing pressure is 80-200 g / cm. 2 After polishing, the roughness of both sides of the wafer is less than 2nm, Bow < 20um, Warp < 10um; The polishing machine uses a double-sided copper disc polishing machine, and the polishing material is 1-10um diamond micro powder; During polishing, if the final product requires a concave Si surface, the wafer should be placed with the carbon surface facing up and the silicon surface facing down. The polishing wheel should rotate at 30-40 rpm, and the polishing pressure should be 25-50 g / cm². 2 Polishing time: 6-9 hours; During polishing, if the product requires a concave carbon surface, the wafer should be placed with the carbon surface facing down and the silicon surface facing up. The polishing pad should rotate at 30-40 rpm, and the polishing pressure should be 25-50 g / cm. 2 Polishing time: 6-9 hours; 5) If the wafer shape after double-sided mechanical polishing is inferior to the target shape, and the bow > 15µm, perform supplementary single-sided mechanical polishing. If the final product requires a recessed Si surface, perform supplementary polishing on the C surface; if the final product requires a recessed C surface, perform supplementary polishing on the Si surface. Single-sided mechanical polishing speed is 30-40 rpm, and pressure is 150-250 g / cm. 2 ; 6) Using a single-sided polishing machine, perform chemical mechanical polishing on the silicon or carbon surface of the wafer after mechanical polishing in step 4) or 5). The abrasive is SiO2 micro powder, the polishing speed is 30-50 rpm, and the pressure is 100-300 g / cm. 2 After chemical mechanical polishing, the surface roughness is less than 0.2 nm, Bow < 20 μm, and Warp < 10 μm. The chemical mechanical polishing pad is a polyurethane polishing pad, and the polishing time is 10-16 h. The polishing liquid is an acidic slurry composed of hydrogen peroxide, nano-silica particles with an average particle size of 200 nm, and stabilizers. 7) Cleaning: The chemically polished wafers are cleaned and dried using a cleaning machine and a rotary dryer to obtain a uniformly recessed or raised SiC substrate.

2. The method according to claim 1, characterized in that, In step 5), single-sided mechanical polishing is performed using a single-sided copper disc polishing machine, and the polishing material is diamond micro powder with a particle size of 10-15um.

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