Seed crystal bonding method and silicon carbide crystal growth method

Abstract

The invention relates to the field of silicon carbide preparation, in particular to a seed crystal bonding method and a silicon carbide crystal growing method.The seed crystal bonding method comprises the following steps that (S.1) a graphite seed crystal cover is covered with a metal film layer; (S.2) placing a non-growth surface of a seed crystal on the surface of the metal film layer, and sequentially covering the growth surface of the seed crystal with a flexible film and a rigid pressing plate; (S.3) applying pressure above the rigid pressing plate, so that the non-growth surface of the seed crystal is tightly attached to the metal film layer; and (S.4) under vacuum and heating conditions, enabling the non-growth surface of the seed crystal and the metal film layer to be subjected to diffusion sintering reaction so as to finish the bonding of the seed crystal. The diffusion sintering reaction between the metal film layer and the seed crystal is creatively used, so that the stable bonding of the seed crystal and the graphite seed crystal cover is effectively improved, the phenomena of bubble shrinkage on the bonding surface and edge bonding stripping are avoided, and the internal dislocation density of the silicon carbide crystal is reduced.

Description

Technical Field

[0001] The invention relates to the field of silicon carbide preparation, and in particular to a seed crystal bonding method and a silicon carbide crystal growth method. Background Art

[0002] As a third-generation semiconductor material, silicon carbide single crystal has the characteristics of wide bandgap, high thermal conductivity, high breakdown electric field, high radiation resistance and high electron saturation rate, which makes silicon carbide single crystal suitable for the production of high-temperature, high-frequency, radiation-resistant and high-power devices. At present, it has been widely used in semiconductor devices, new energy vehicles and photovoltaic industries, and is in short supply worldwide.

[0003] Physical vapor transport (PVT) is the mainstream method in the industry to grow silicon carbide single crystals. It controls the external insulation conditions of the graphite crucible to achieve the regulation of the axial and radial temperature fields. The silicon carbide powder is placed at the bottom of the graphite crucible with a higher temperature, and the silicon carbide seed crystal is fixed on the top of the graphite crucible (graphite seed crystal cover) with a lower temperature. The silicon carbide powder is heated to 2000-2500℃ by induction heating, so that it sublimates and decomposes into gas phase components. These gas phase components are transported to the seed crystal end with convection, and silicon carbide crystals are crystallized on the seed crystal to achieve single crystal growth.

[0004] Among the existing silicon carbide single crystal products, 8-inch silicon carbide single crystals can significantly reduce the unit production cost and efficiency of epitaxial devices compared to 6-inch silicon carbide single crystals. Therefore, the market is currently upgrading from 6-inch silicon carbide single crystals to 8-inch silicon carbide single crystals. However, due to its larger size, the dislocation density inside the 8-inch silicon carbide single crystal is higher than that of the 6-inch silicon carbide, which seriously affects the application of 8-inch and above silicon carbide single crystals. Summary of the invention

[0005] The present invention aims to overcome the defect that the silicon carbide crystal prepared in the prior art has a high internal dislocation density, which limits its application, and provides a seed crystal bonding method and a silicon carbide crystal growth method to overcome the above defect.

[0006] To achieve the above-mentioned purpose, the present invention is implemented by the following technical solutions: In a first aspect, the present invention first provides a seed crystal bonding method, comprising the following steps: (S.1) Covering the graphite seed crystal cap with a metal film layer, wherein the melting point of the metal film layer is greater than or equal to 2500° C.; (S.2) placing the non-growth surface of the seed crystal on the surface of the metal film layer, and sequentially covering the growth surface of the seed crystal with a flexible film and a rigid pressing plate; (S.3) applying pressure on the rigid pressing plate to make the non-growth surface of the seed crystal fit closely to the metal film layer; (S.4) Under vacuum and heating conditions, the non-growth surface of the seed crystal and the metal film layer undergo diffusion sintering reaction, thereby completing the bonding of the seed crystal.

[0007] It is found in the prior art that the defects formed during the growth of silicon carbide crystals mainly include crystal cracking, micropipe defects inside the crystal, and high dislocation density inside the crystal. With the deepening of research, most of the micropipe defects in the crystal have been overcome, and the micropipe density inside the substrate has been reduced by 90%, which is sufficient to meet the needs of commercial applications. With the solution of this defect problem, the current research focus has shifted to reducing its internal dislocation density. The dislocations inside silicon carbide mainly include basal plane dislocations, edge dislocations and screw dislocations, among which basal plane dislocations are distributed on the main slip plane, that is, the (0001) plane. Basal plane dislocations have the greatest impact on the electronic and optoelectronic properties of the crystal. Therefore, the main task of reducing dislocations inside silicon carbide crystals is to reduce basal plane dislocations.

[0008] In the prior art, the commonly used method is to control the temperature distribution inside the crucible and the gas flow path and other parameters during the growth of the silicon carbide crystal, so as to achieve the growth of high-quality silicon carbide crystals. However, in the present application, the inventor of the present application unexpectedly discovered in the experiment a reason that was not previously noticed by those skilled in the art that could easily cause the increase of the internal dislocation density of the silicon carbide crystal, that is, the bonding form of the seed crystal and the graphite seed crystal cover has a great influence on the internal dislocation density of the silicon carbide crystal.

[0009] Specifically, the inventors of the present application observed in experiments that organic glue is usually used in the prior art to achieve bonding between the seed crystal and the graphite seed crystal cover. However, the traditional organic glue shrinks and carbonizes during bonding and subsequent heating, resulting in a large number of shrinkage bubbles at the bonding surface. The presence of these shrinkage bubbles leads to uneven heat dissipation on the non-growth surface of the seed crystal, thereby increasing crystal growth defects. In addition, the traditional organic glue has poor bonding strength at high temperatures and is easily etched by Si vapor, which causes the edge of the seed crystal to easily peel off from the seed crystal cover. Since the edge of the seed crystal is not effectively sealed, back evaporation occurs during the crystal growth process, resulting in large microtube void defects in the crystal.

[0010] Therefore, the conventional technical solution of using organic glue as a bonding agent is abandoned in the seed crystal bonding method of the present application.

[0011] In the present application, the applicant first covers a metal film layer on the surface of the graphite seed crystal cap, and then applies a certain pressure to the seed crystal so that the non-growth surface (Si surface) of the seed crystal is tightly fitted with the metal film layer, and performs vacuum heat treatment under this condition, so that the non-growth surface of the seed crystal and the metal film layer occur, thereby forming a diffusion sintered layer composed of metal-silicon compounds between the two, thereby achieving.

[0012] Compared with the prior art, the technical solution in this application has the following beneficial effects: first, the diffusion between the non-growth surface of the seed crystal and the metal film layer in this application is based on molecular-level fusion, so the bonding uniformity is better; second, there is no decomposition of organic matter during the bonding process, and it will not shrink during the diffusion process, thus avoiding the phenomenon of shrinkage bubbles existing in the traditional process on the bonding surface; finally, compared with the silicon carbide seed crystal, the metal film layer is more excellent, so the heat dissipation of the bonding surface is faster and more uniform. The above two points greatly reduce the defects of the silicon carbide crystals finally prepared in the silicon carbide growth process.

[0013] Therefore, in summary, the seed crystal bonding method of the present application avoids the problems of poor bonding firmness, a large number of shrinkage bubbles on the bonding surface, and uneven heat dissipation in the bonding process of traditional organic glue. Therefore, the probability of defects in the growth process of silicon carbide crystals can be greatly reduced.

[0014] Preferably, the metal film layer in step (S.1) is a single substance of three metal elements Ta, W, Re or an alloy of any several elements or an unsaturated carbide.

[0015] The metal film layer selected for use in the present application contains three metal elements, Ta, W, and Re. Since the single substances, alloys, or compounds formed by these three metal elements usually have relatively high melting points and boiling points (the melting points of the three metal elements Ta, W, and Re are all above 3000°C, and the current conventional silicon carbide growth temperature is basically below 2500°C), they can effectively meet the growth of silicon carbide crystals, thereby ensuring that the bonding stability will not be reduced due to overheating, melting, or sublimation during the growth process.

[0016] Preferably, the method for preparing the metal film layer in step (S.1) comprises any one of magnetron sputtering, vacuum evaporation and chemical vapor deposition.

[0017] Preferably, the thickness of the metal film layer is 1 μm-20 μm.

[0018] The inventors of the present application have found that, since the mass of silicon carbide crystals gradually increases with time during the growth process of silicon carbide crystals, there are important requirements for the bonding strength of the bonding surface. Therefore, the thickness of the diffusion sintered layer between the metal film layer and the non-growth surface of the seed crystal has a very important influence on the bonding strength, and the thickness of the diffusion sintered layer has an important influence on the thickness of the metal film layer. When the metal film layer is too thin, the thickness of the diffusion sintered layer will be too thin, resulting in low bonding strength. When the thickness of the metal film layer is too large, the stress of the bonding surface between the seed crystal and the metal film layer will increase, thereby greatly increasing the probability of defects in the silicon carbide crystal.

[0019] Preferably, in step (S.2), the number of flexible film layers is 1-20, and the thickness of the flexible film is 0.2 mm-3 mm.

[0020] The role of the flexible film in this application The flexible film placed on the growth surface of the seed crystal plays a role in buffering and uniform force. It can prevent the rigid pressure plate from directly contacting the growth surface of the seed crystal and causing damage, because the quality of the growth surface of the seed crystal is crucial to the subsequent crystal growth. If the growth surface is scratched or damaged, it will seriously affect the initial state of crystal growth. At the same time, the flexible film can make the pressure applied to the pressure plate more evenly transferred to the contact area between the non-growth surface of the seed crystal and the metal film layer, promoting better fit between the two.

[0021] Preferably, the thickness uniformity of the rigid pressing plate in step (S.2) is within ±0.01 mm.

[0022] When bonding the seed crystal, based on the principle of mechanics, the thickness uniformity of the rigid pressing plate has a decisive influence on the pressure transmission. An ideal uniform thickness pressing plate can transmit the pressure evenly to the surface of the seed crystal according to Pascal's law. If the thickness deviates, according to the stress concentration theory, the stress and pressure are large in the thick part, and vice versa in the thin part, which destroys the consistency of the bonding between the seed crystal and the metal film layer. Microscopically, poor bonding hinders atomic diffusion sintering, affects the microstructure and chemical bond formation of the bonding interface, and reduces the bonding quality. High-precision uniform pressing plates can avoid this problem, ensure uniform and stable pressure, promote good bonding, create ideal conditions for diffusion sintering, and improve bonding quality and stability.

[0023] Preferably, the diameters of the flexible film and the rigid pressing plate are greater than or equal to the diameter of the seed crystal.

[0024] Preferably, the pressure applied to the rigid pressing plate in step (S.3) is 2 kg to 100 kg.

[0025] Preferably, in step (S.4), the vacuum degree is gas pressure <1E-5Pa, the heating temperature is 500°C-1000°C, and the heating time is 30min-5h.

[0026] High vacuum environment is conducive to the diffusion of atoms. According to Fick's law, the atomic diffusion rate is related to the concentration gradient. Under high vacuum, there are fewer impurity gases, the system is closer to the ideal state, and the diffusion of atoms between the seed crystal and the metal film layer is easier. In the diffusion sintering reaction, atomic diffusion is a key step. The high vacuum environment can provide a good condition for the atoms of the seed crystal and the metal film layer to diffuse more effectively, thereby forming a stronger bonding interface. The heating temperature range of 500℃ - 1000℃ can provide suitable energy conditions for the diffusion sintering reaction. In this temperature range, the atoms in the seed crystal and the metal film layer obtain enough thermal energy to overcome the potential barrier between atoms and begin to diffuse with each other. Different materials (seed crystal and metal film layer) have different diffusion activation energies. This temperature range can meet the diffusion sintering requirements between most high melting point metals (melting point ≥2500℃) and seed crystals, prompting atoms to rearrange at the interface, forming metallurgical bonding, and enhancing the bonding effect.

[0027] In a second aspect, the present application also provides a method for growing a silicon carbide crystal, which comprises the following steps: (1) bonding the seed crystal to the surface of the graphite seed crystal cover of the crucible according to any of the methods described above; (2) adding a silicon carbide powder source into the crucible, and performing heating and vacuuming steps on the crucible to remove impurity gases in the crucible; (3) Argon gas is introduced into the crucible for thermal field protection, and then silicon carbide crystals are deposited on the surface of the seed crystal by physical vapor deposition.

[0028] Therefore, this application has the following beneficial effects: The present application abandons the traditional technical solution of using organic glue as a bonding agent in the seed crystal bonding process, and creatively uses the diffusion sintering reaction between the metal film layer and the seed crystal, thereby effectively improving the stable bonding between the seed crystal and the graphite seed crystal cover. At the same time, during the bonding process, the shrinkage bubbles and edge bonding peeling that exist in traditional processes on the bonding surface are avoided, the internal dislocation density of the silicon carbide crystal is reduced, and the quality of the silicon carbide crystal is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the crystal dislocation diagram of Example 1.

[0030] Figure 2 This is the crystal dislocation diagram of Example 2.

[0031] Figure 3 This is the crystal dislocation diagram of Example 3.

[0032] Figure 4 This is the crystal dislocation diagram of Example 4.

[0033] Figure 5This is the crystal dislocation diagram of comparative example 1. DETAILED DESCRIPTION

[0034] The present invention is further described below in conjunction with specific embodiments. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are generally only embodiments of a part of the present invention, rather than all embodiments. Therefore, based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work should fall within the scope of protection of the present invention.

[0035] Example 1 A seed crystal bonding method comprises the following steps: (S.1) magnetron sputtering a 5 μm thick Ta film on the graphite seed crystal cap; (S.2) Place the Si surface of the 205 mm diameter seed crystal on the upper surface of the Ta film, and cover the seed crystal growth surface with three layers of 0.35 mm thick 210 mm diameter SGL graphite paper discs and one 5 mm thick 210 mm diameter graphite sheet with a thickness uniformity within ±0.01 mm; (S.3) A graphite cylinder with a diameter of 160 mm and a height of 100 mm is placed in the center above the rigid pressing plate so that the non-growth surface of the seed crystal is closely attached to the metal film layer; (S.4) Place the entire stacked structure in a vertical tubular heating furnace, evacuate to 5E-6 Pa, heat to 600 °C for 1 hour, and maintain the temperature at 600 °C for 2 hours, so that the non-growth surface of the seed crystal and the metal film layer undergo diffusion sintering reaction, and then cool to room temperature, thereby completing the bonding of the seed crystal.

[0036] Example 2 A seed crystal bonding method comprises the following steps: (S.1) magnetron sputtering a 10 μm thick W film on the graphite seed crystal cap; (S.2) Place the Si surface of a 205 mm diameter seed crystal on the upper surface of the W film, and sequentially cover the seed crystal growth surface with 10 layers of 0.1 mm thick 210 mm diameter silica nanofiber membranes and a 5 mm thick 210 mm diameter graphite sheet with a thickness uniformity within ±0.01 mm; (S.3) A graphite cylinder with a diameter of 160 mm and a height of 100 mm is placed in the center above the rigid pressing plate so that the non-growth surface of the seed crystal is closely attached to the metal film layer; (S.4) Place the entire stacked structure in a vertical tubular heating furnace, evacuate to 5E-6 Pa, heat to 500 °C in 1 hour, and maintain the temperature at 500 °C for 5 hours, so that the non-growth surface of the seed crystal and the metal film layer undergo diffusion sintering reaction, and then cool to room temperature, thereby completing the bonding of the seed crystal.

[0037] Example 3 A seed crystal bonding method comprises the following steps: (S.1) magnetron sputtering a 20 μm thick Re film on the graphite seed crystal cap; (S.2) Place the Si surface of the 205 mm diameter seed crystal on the upper surface of the Re film, and cover the seed crystal growth surface with a layer of 3 mm thick carbon fiber cloth with a diameter of 210 mm and a 5 mm thick graphite sheet with a diameter of 210 mm and a thickness uniformity within ±0.01 mm; (S.3) A graphite cylinder with a diameter of 160 mm and a height of 100 mm is placed in the center above the rigid pressing plate so that the non-growth surface of the seed crystal is closely attached to the metal film layer; (S.4) Place the entire stacked structure in a vertical tubular heating furnace, evacuate to 5E-6 Pa, heat to 1000 °C for 1 hour, and maintain the temperature at 1000 °C for 0.5 hour, so that the non-growth surface of the seed crystal and the metal film layer undergo diffusion sintering reaction, and then cool to room temperature, thereby completing the bonding of the seed crystal.

[0038] Example 4 A seed crystal bonding method comprises the following steps: (S.1) magnetron sputtering a Ta film with a thickness of 1 μm on the graphite seed crystal cap; (S.2) Place the Si surface of the 205 mm diameter seed crystal on the upper surface of the Ta film, and cover the seed crystal growth surface with 20 layers of 0.2 mm thick 210 mm diameter SGL graphite paper discs and 1 5 mm thick 210 mm diameter graphite sheet with a thickness uniformity within ±0.01 mm; (S.3) A graphite cylinder with a diameter of 160 mm and a height of 100 mm is placed in the center above the rigid pressing plate so that the non-growth surface of the seed crystal is closely attached to the metal film layer; (S.4) Place the entire stacked structure in a vertical tubular heating furnace, evacuate to 5E-6 Pa, heat to 800 °C for 1 hour, and maintain the temperature at 800 °C for 1.5 hours, so that the non-growth surface of the seed crystal and the metal film layer undergo diffusion sintering reaction, and then cool to room temperature, thereby completing the bonding of the seed crystal.

[0039] Comparative Example 1 A seed crystal bonding method comprises the following steps: (S.1) Dip-coat the graphite seed crystal cap with organic glue and dip-coat the silicon surface of the seed crystal with organic glue; (S.2) Bonding the bonding surface of the graphite seed crystal cap and the silicon surface of the seed crystal to obtain a bonding member, and hot-pressing and sintering the bonding member to carbonize the glue, thereby completing the bonding of the seed crystal.

[0040]

Application examples

[0041] Table 1 Dislocation Type TSD TED BPD Example 1 <![CDATA[17 per cm 2 > <![CDATA[1,564 per cm 2 > <![CDATA[49 per cm 2 > Example 2 <![CDATA[14 per cm 2 > <![CDATA[1159 per cm 2 > <![CDATA[55 per cm 2 > Example 3 <![CDATA[5 per cm 2 > <![CDATA[1728 per cm 2 > <![CDATA[143 per cm 2 > Example 4 <![CDATA[47 per cm 2 > <![CDATA[1652 pieces / cm 2 > <![CDATA[50 per cm 2 > Comparative Example 1 <![CDATA[179 pieces / cm 2 > <![CDATA[3289 pieces / cm 2 > <![CDATA[1056 per cm 2 > Figure 1 , Figure 2 , Figure 3 , Figure 4 as well as Figure 5 The crystal dislocation diagrams obtained by testing Examples 1-4 of the present invention and Comparative Example 1 are shown in Table 1 and the accompanying drawings. It can be seen that the bonding method of the present application has a more uniform composition of the bonding layer and a more uniform thermal conductivity, thereby making the crystal dislocation lower. In particular, the phenomenon of large microtube void defects in the crystal due to the edge of the seed crystal not being effectively sealed, which causes back evaporation during the crystal growth process, is avoided.

[0042] The specific embodiments described herein are merely examples of the spirit of the present invention. A person skilled in the art of the present invention may make various modifications or additions to the specific embodiments described or replace them in a similar manner, but this will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A seed crystal bonding method, characterized in that: The following steps are involved: (S.1) Covering the graphite seed crystal cap with a metal film layer, wherein the melting point of the metal film layer is greater than or equal to 2500° C.; (S.2) placing the non-growth surface of the seed crystal on the surface of the metal film layer, and sequentially covering the growth surface of the seed crystal with a flexible film and a rigid pressing plate; (S.3) applying pressure on the rigid pressing plate to make the non-growth surface of the seed crystal fit closely to the metal film layer; (S.4) Under vacuum and heating conditions, the non-growth surface of the seed crystal and the metal film layer undergo diffusion sintering reaction, thereby completing the bonding of the seed crystal.

2. A seed crystal bonding method according to claim 1, characterized in that: The metal film layer in step (S.1) is a single substance of three metal elements Ta, W, and Re or an alloy of any several elements or an unsaturated carbide.

3. A seed crystal bonding method according to claim 1 or 2, characterized in that: The method for preparing the metal film layer in step (S.1) includes any one of magnetron sputtering, vacuum evaporation, and chemical vapor deposition.

4. A seed crystal bonding method according to claim 3, characterized in that: The thickness of the metal film layer is 1 μm-20 μm.

5. A seed crystal bonding method according to claim 1, characterized in that: The number of layers of the flexible film in step (S.2) is 1-20, and the thickness of the flexible film is 0.2 mm-3 mm.

6. A seed crystal bonding method according to claim 1, characterized in that: The thickness uniformity of the rigid pressing plate described in step (S.2) is within ±0.01 mm.

7. A seed crystal bonding method according to claim 1, 5 or 6, characterized in that: The diameters of the flexible film and the rigid pressing plate are greater than or equal to the diameter of the seed crystal.

8. A seed crystal bonding method according to claim 1, characterized in that: The pressure applied on the rigid platen in step (S.3) is 2 kg -100 kg.

9. A seed crystal bonding method according to claim 1, characterized in that: In step (S.4), the vacuum degree is gas pressure <1E-5Pa, the heating temperature is 500°C-1000°C, and the heating time is 30min-5h.

10. A method for growing silicon carbide crystals, characterized in that: The following steps are involved: (1) bonding the seed crystal to the surface of the graphite seed crystal cover of the crucible according to the method described in any one of claims 1 to 9; (2) adding a silicon carbide powder source into the crucible, and performing heating and vacuuming steps on the crucible to remove impurity gases in the crucible; (3) Argon gas is introduced into the crucible for thermal field protection, and then silicon carbide crystals are deposited on the surface of the seed crystal by physical vapor deposition.

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