JP4053125B2 - Method for synthesizing SiC single crystal - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a SiC single crystal, and more particularly to a method for producing a SiC single crystal for efficiently growing high-quality SiC suitable for semiconductor electronic components.
[0002]
[Prior art]
SiC is excellent in chemical resistance such as acid and alkali, is not easily damaged by high energy rays, has excellent durability, and is also used as a semiconductor material.
[0003]
By the way, in order to use SiC as a semiconductor material, it is necessary to obtain a high-quality single crystal having a certain size. For this reason, conventionally, a method using a chemical reaction called the Acheson method, a method using a sublimation recrystallization method called the Rayleigh method, or a SiC single crystal obtained by these methods is used as a substrate. In addition, a method of growing a SiC single crystal of a target scale by a vapor phase epitaxial growth method or a liquid phase epitaxial growth method is used.
[0004]
In particular, as a method of growing a bulk SiC single crystal, an appropriate size SiC single crystal is used as a seed crystal in a graphite crucible, and the raw SiC powder is sublimated in a reduced-pressure atmosphere. A so-called improved Rayleigh method for recrystallizing on a crystal to grow a SiC single crystal of a target scale is disclosed in, for example, Japanese Patent Publication No. 59-48792 and Japanese Patent Laid-Open No. 2-30699.
[0005]
[Problems to be solved by the invention]
However, among the conventional methods described above, the Acheson method heats a mixture of silica and coke in an electric furnace and precipitates crystals by spontaneous nucleation, so there are many impurities, and the shape and crystal plane of the crystals obtained are large. It is difficult to control.
[0006]
Further, in the vapor phase epitaxial growth method and the liquid phase epitaxial growth method, the crystal shape and crystal plane can be controlled, but the crystal growth rate is extremely slow, and it is difficult to obtain a large SiC single crystal.
[0007]
Furthermore, in the Rayleigh method, crystals grow by natural nucleation nucleation, so it is difficult to control the crystal shape and crystal plane.
[0008]
On the other hand, in the invention described in Japanese Patent Publication No. 59-48792, which is an improved Rayleigh method, it is possible to obtain a high-quality SiC single crystal. However, when an SiC single crystal is obtained by this method, the SiC crystal is naturally generated from the graphite crucible during the crystal growth period. Then, the crystal grows rapidly using it as a nucleus and inhibits crystal growth from the seed crystal, so that it is difficult to obtain a crystal of the desired scale or to obtain a crystal with high homogeneity. Therefore, it is necessary to cut out the polycrystalline portion when taking out the material, which complicates the process.
[0009]
In the invention described in JP-A-2-30699, a SiC single crystal with high homogeneity can be obtained on a target scale. However, since a manufacturing apparatus such as a crucible is complicated and precise processing is required, it takes a lot of time and is problematic in practice.
[0010]
In addition, it is known that the main gases sublimated from the raw material SiC in the Rayleigh method and the modified Rayleigh method are Si, SiC ₂ and Si ₂ C, but the vapor pressure of Si is SiC ₂ and Si ₂ C. Since Si escapes from the crucible by an order of magnitude higher than the vapor pressure, there is a problem in that the composition of raw materials gradually changes during synthesis and the synthesis conditions shift.
[0011]
In addition, the temperature of the upper surface of the raw material rises from the inside of the raw material due to the influence of heat radiation from the crucible, and the amount of sublimation is large at the initial stage of growth, and the surface becomes graphitized and gradually decreases. In order to solve this problem, Japanese Patent Application Laid-Open No. 5-105596 discloses an attempt to prevent heat radiation from the upper part of the crucible by adding carbon to the raw material and further forming a layer containing carbon on the surface of the raw material. It is shown. However, even in this method, it is difficult to control the raw material to reach the seed crystal throughout the synthesis at the same vapor pressure.
[0012]
Further, the region where the raw material is sublimated by being heated by heat conduction or thermal radiation from the crucible gradually spreads from the raw material in contact with the side or bottom surface of the crucible to the raw material at the center. However, as the sublimation region expands, the raw material of the crucible that was sublimated at the beginning near the bottom surface or near the side wall changes to a highly heat-insulating rod-like powder, so the heat conduction and heat radiation to the raw material in the center decrease sharply. As a result, the sublimation of the raw material in the central portion rapidly decreases or does not occur. In particular, when a large-area single crystal is synthesized, it is necessary to increase the diameter of the crucible for charging the raw material, and the deterioration of the raw material in the radial direction becomes a big problem. In the growth apparatus described in Japanese Patent Laid-Open No. 5-58774, a heat conductor is installed inside the crucible to aim for soaking of the raw material, but the SiC raw material is sublimated and changed into a bowl-shaped powder. In principle, it is impossible to avoid changes in the crystal speed over time.
[0013]
FIG. 2 is a graph showing vapor pressure curves of carbon (C) and SiC, with the vertical axis (logarithm) representing pressure (Pa) and the horizontal axis representing temperature (° C.). As can be seen from FIG. 2, the vapor pressure of Si is one digit higher than the vapor pressure of SiC ₂ or Si ₂ C as described above. In order to increase the synthesis rate, it is necessary to supply a sufficient amount of Si and C to the seed crystal, but in order to increase the partial pressure of SiC ₂ and Si ₂ C having a low vapor pressure in an attempt to sufficiently supply C. However, when the temperature of the raw material is raised, there is a problem that the partial pressure of the Si system becomes too high and the stoichiometry of the raw material and the synthetic crystal shifts.
[0014]
In addition, if SiC synthesized by the Acheson method is used as raw material SiC, there are many impurities from silica stone, and since it is usually used in the form of powder, metal contamination occurs in the raw material from the pulverizer during the pulverization process. . For example, it generally contains 1000 ppm or more of impurities such as Ti, V, Fe, and Ni.
[0015]
In order to solve this problem, Japanese Patent Application Laid-Open No. 63-50393 proposes a method for reducing the amount of impurities by sublimating and recrystallizing crystals synthesized by the Acheson method in a high vacuum. . According to this method, the raw material recrystallized in a high vacuum has an impurity concentration of about ½ or less compared to the raw material before the recrystallization process, but the impurity is reduced by two or three orders of magnitude. Must not.
[0016]
In order to solve this problem of impurities, a method of using SiC synthesized from a gas by a CVD method as a raw material or a method of mixing silicon and graphite powder at a high temperature to form a SiC raw material has been proposed. (For example, VB Shields et al., Journal of Applied Physics Letters Vo.62, No.19.1919-1921). However, these methods involve a new process for preparing raw materials, and thus the process becomes complicated.
[0017]
Furthermore, when powder is used as a raw material, the filling rate of the raw material is lowered. Therefore, when synthesizing a large SiC single crystal, it is necessary to enlarge the crucible, resulting in the enlargement of the apparatus and the associated temperature control. There is a problem.
[0018]
The present invention has been made in view of such conventional problems, and provides a method for synthesizing a SiC single crystal that can easily obtain a high-quality and large-sized SiC single crystal and can be obtained at a low cost at a high growth rate. With the goal.
[0019]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention forms _three temperature regions T ₁ , T ₂ , T ₃ in a synthesis apparatus and synthesizes the temperature of each region by T ₁ <T when synthesizing a SiC single crystal. ₂ <a T _3, solid Si in the low temperature range T _1, SiC seed crystal or the SiC single crystal substrate intermediate temperature range T _2, solid carbon each placed in a high temperature region T _3, Si from the low temperature area T ₁ The evaporated Si is allowed to react with carbon by passing through the high temperature region T ₃ to form a SiC forming gas, and the SiC forming gas reaches the seed crystal or SiC single crystal substrate in the intermediate temperature region T ₂ . It is characterized by that.
[0020]
That is, in the present invention, Si and C are individually heated and synthesized by controlling the vapor pressure at each optimum temperature. In the high temperature region T ₃ , the Si supply amount is high independently. Since the temperature can be set, sufficient C can be supplied to the seed crystal or the SiC single crystal substrate.
[0021]
FIG. 3 is a graph showing respective vapor pressure curves of Si and C. The pressure (Pa) is taken on the vertical axis (logarithm), and the temperature (° C.) is taken on the horizontal axis. As can be seen from FIG. A SiC single crystal can be synthesized with good controllability by passing through a C (graphite) region heated to 2000 ° C. or higher through Si that provides a sufficient vapor pressure from 1400 ° C.
[0022]
In addition, as the raw material solid Si and C (graphite) used in the present invention, high-purity materials can be obtained at low cost. For this reason, the impurity concentration of the SiC single crystal to be synthesized can be greatly reduced.
[0023]
Furthermore, since Si and C of the raw material are each a single element, unlike the case of using SiC powder, the composition of the raw material does not change during the synthesis, so that the synthesis conditions are stable and a high-quality SiC single crystal is produced. Obtainable.
[0024]
Moreover, since the raw material at the time of filling is not a powder but a solid (bulk) of Si and graphite, it is possible to synthesize a large and long SiC single crystal with a high filling rate.
[0025]
In the present invention, the vapor pressure of Si may be controlled by providing a shield made of carbon, quartz or SiC at the interface between the liquid phase Si and the gas phase Si in the low temperature region T ₁ . Here, glassy carbon (glassy carbon) is particularly suitable as carbon, and does not react with Si even at a high temperature, thus providing an excellent shield.
[0026]
Also, mechanically connecting the carbon of the shield and the high temperature area T _3, with a decrease of Si content by evaporation of Si in the low temperature range T _1, the carbon in the high temperature region T ₃ the position of the shield is changed to move Thus, it is preferable that the distance between the synthesized SiC single crystal and carbon be constant. Thereby, stable synthesis can be achieved for a long time.
[0027]
By increasing the surface area of the carbon by providing a thin tube or slit in the carbon in the high temperature region T ₃ , Si and carbon flying from the low temperature region T ₁ can be reacted efficiently.
[0028]
Further, at least the inner surface of the crucible for forming the low temperature region T ₁ , the medium temperature region T ₂ and the high temperature region T ₃ may be formed of diamond-like carbon or glassy carbon (glassy carbon). Thereby, the generation of natural nuclei can be suppressed, and a high-quality SiC single crystal can be synthesized.
[0029]
Furthermore, by disposing a heat shield made of graphite outside the crucibles forming the low temperature region T ₁ , the medium temperature region T _2, and the high temperature region T ₃ , it is possible to suppress heat dissipation due to thermal radiation.
[0030]
In addition, if the heat shield is formed by arranging a plurality of strip-shaped graphite plates adjacent to each other with a gap therebetween so that the overall shape becomes a substantially cylindrical shape, induction current due to high frequency heating can be suppressed, and the heat shield can be attached to the crucible. More than one in the radial direction is better in terms of suppression of heat dissipation and induction current.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a synthesis apparatus used in this embodiment. In this synthesis apparatus, a cylindrical crucible 1 made of graphite is composed of a cylindrical upper crucible 1a and a lower crucible 1b, respectively. Further, the upper end of the upper crucible 1 a is closed by a disc-shaped lid 2. On the other hand, a cylindrical Si holding crucible 3 made of graphite is inserted inside the lower crucible 1b so as to be movable in the axial direction with a slight gap from the lower crucible 1b. The lower part of the Si holding crucible 3 is fixed to the upper surface of the disc-shaped crucible support 4, and the lower side surface of the Si holding crucible 3 is provided with a plurality of through holes 3 a leading to the gap. . A cylindrical support shaft 5 connected to a drive source (not shown) is fixed at the center of the lower surface of the crucible support base 4. That is, as the support shaft 5 moves up and down, the Si holding crucible 3 is movable in the axial direction. Further, since the support shaft 5 has a hollow cylindrical shape, the temperature of the bottom surface of the Si holding crucible 3 can be measured with a two-temperature pyrometer.
[0032]
Furthermore, the inner peripheral surfaces of the crucible 1 and the Si holding crucible 3 are formed of diamond-like carbon or glassy carbon (glassy carbon) having high smoothness. The surface roughness of the inner peripheral surfaces of the crucible 1 and the Si holding crucible 3 is preferably Rmax <10 μm.
[0033]
Three heat shields 6 are concentrically arranged in the radial direction of the crucible 1 outside the crucible 1. Each heat shield 6 has a plurality of strip-shaped graphite plates arranged adjacent to each other with a gap therebetween, and the overall shape is formed in a substantially cylindrical shape. The adjacent heat shields 6 do not overlap in the radial direction. Is provided. Moreover, since the heat shield 6 is not formed of carbon fiber or porous graphite that causes impurity contamination that is often used in general, there is no fear of impurity contamination. Furthermore, the upper ends of these heat shields 6 are closed by a disk-shaped lid 7 made of the same material.
[0034]
A cylindrical quartz tube 8 made of quartz is disposed outside the outermost heat shield 6 so as to be concentric with the heat shield 6. Cooling water such as water flows inside the quartz cylinder 8 so that the quartz cylinder 8 is protected. Further, although not shown, an RF work coil is disposed outside the quartz cylinder 8 so that the crucible 1 and the Si holding crucible 3 can be heated at a high frequency.
[0035]
In the crucible 1, a cylindrical seed crystal support rod 9 penetrating the center part of the lid 7 and the lid 2 is inserted into the crucible 1 so as to be movable in the axial direction. A disc-shaped seed crystal holder 10 is fixed to the lower end of the seed crystal support rod 9 so as to close the lower end opening. An SiC seed crystal 11 is fixed to the lower surface of the seed crystal holder 10 with a paste obtained by melting glucose at a high temperature. Since the seed crystal support rod 9 has a hollow cylindrical shape, the temperature of the seed crystal 11 can be measured with a two-temperature pyrometer. The seed crystal support rod 9 is provided so as to be able to rotate up to 200 rpm around the axis. The seed crystal support rod 9 is configured to be in a vacuum state within a range surrounded by the inner wall of the quartz cylinder 8 together with the crucible 1, the crucible support base 4 and the support rod 5.
[0036]
On the other hand, the Si holding crucible 3 contains a cylindrical bulk Si source 12. A disk-shaped Si vapor pressure control shield 13 made of carbon, quartz or SiC is placed on the upper surface of the Si source 12. The Si vapor pressure control shield 13 is formed with a plurality of passage holes 13a penetrating in the axial direction so that Si vapor can pass therethrough. Further, an Si vapor pressure control auxiliary shield 14 made of the same material as that of the Si vapor pressure control shield 13 is fixed at the center of the upper surface of the Si vapor pressure control shield 13. The Si vapor pressure control auxiliary shield 14 is composed of a central axis and two discs provided on the Si vapor pressure control shield 13 side. The Si vapor pressure control shield 13 and the Si vapor pressure control auxiliary shield 14 control the vapor pressure from the Si source 12, respectively. It controls the diffusion of Si in the phase.
[0037]
A cylindrical bulk carbon supply source (graphite) 15 is fixed to the upper end of the Si vapor pressure control auxiliary shield 14. The carbon supply source 15 is formed with a passage hole 15a penetrating in the axial direction so that Si vapor can pass therethrough. While passing through the passage hole 15a, Si reacts with graphite to become SiC forming gas which is an active species for SiC synthesis. The carbon supply source 15 is restricted from being lowered even when the Si holding crucible 3 is lowered by a drop stopper 16 fixed to the lower inner peripheral surface of the upper crucible 1a.
[0038]
In the synthesizing apparatus having such a configuration, the temperature of the Si source 12 is set to 1300 ° C. to 1600 ° C., the temperature of the seed crystal 11 is set to 2000 ° C. to 2400 ° C., and the temperature of the carbon supply source 15 by a work coil (not shown). Can be heated and controlled at 2300 ° C. to 3000 ° C., respectively. That is, the synthesis apparatus can form three regions in the crucible 1, that is, a low temperature region T ₁ of the Si source 12, a medium temperature region T ₂ of the seed crystal 11, and a high temperature region T ₃ of the carbon supply source 15. It is configured.
[0039]
In this synthesizing apparatus, the through hole 3a of the Si holding crucible 3 is blocked by melting the Si source 12 due to high temperature, partly flowing out of the through hole 3a, and solidifying with the lower crucible 1b. The steam is efficiently guided upward.
[0040]
A method for synthesizing a SiC single crystal using the synthesizer having the above configuration will be described below.
[0041]
First, after setting the seed crystal 11, the Si source 12, the carbon supply source 15 and the like at predetermined positions, the seed crystal support rod 9 is moved upward to raise the seed crystal 11, and the Si source together with the Si holding crucible 3. After moving 12 downward, evacuation was performed for 1 hour in the space formed inside the inner wall of the quartz cylinder 8. Next, Ar gas was flowed into the apparatus to normal pressure (760 Torr), and the crucible 1 was heated to 2800 ° C. while flowing cooling water into the quartz cylinder 8 and baked for 1 hour to degas. . At this time, the graphite of the carbon supply source 15 remains in the crucible 1 due to the drop stopper 16 of the carbon supply source 15, so that it can be baked simultaneously.
[0042]
Next, the Si source 12 is moved upward together with the Si holding crucible 3 to the state shown in FIG. 1, the seed crystal support rod 9 and the seed crystal 11 are lowered to predetermined positions, and then the seed crystal support rod 9 is moved to 100 rpm. The work coil for high-frequency heating (not shown) was adjusted while rotating at a temperature of 2300 ° C. for the seed crystal 11, 2500 ° C. for the graphite of the carbon supply source 15, and 1600 ° C. for the Si source 12. By setting the temperature at such a normal pressure, it is possible to prevent a crystal having poor crystallinity from growing.
[0043]
Thereafter, the pressure inside the inner wall of the quartz cylinder 8 including the inside of the crucible 1 is lowered to 5 Torr in an Ar gas atmosphere, and this state is maintained, so that the Si vapor is passed through the passage hole 13a of the Si vapor pressure control shield 13. The SiC forming gas is formed by passing through the passage hole 15a of the carbon supply source 15, and the SiC forming gas reaches the seed crystal 11, and the surface of the seed crystal 11 has a speed of 1 to 2 mm / h. Then, a SiC single crystal was grown and finally a bulk of a 2 inch φ SiC single crystal was synthesized.
[0044]
When the photoluminescence characteristics of the SiC single crystal thus obtained were examined, the peak wavelength was about 490 nm, and it was found to be a 6H type SiC single crystal.
[0045]
In addition, as a result of Hall measurement, it was possible to synthesize a SiC single crystal having a specific resistance of 8 Ωcm, a carrier density of about 3 × 10 ¹⁶ cm ⁻³ , a conductivity type n-type, a high resistance, and a low carrier density. I understood.
[0046]
Furthermore, this SiC single crystal bulk was sliced into a wafer having a thickness of 400 μm, polished with a diamond grindstone, and double-sided mirror finished. As a result, it was visually uniform over the entire surface of 2 inches, and when polycrystallized from the end and the light transmittance of the crystal were examined, it was good for a wavelength of 2 to 5 μm. It was found that there were few good crystals.
[0047]
As described above, according to this embodiment, the temperature of the Si source 12 and carbon supply source 15 individually heated, so synthesized by controlling the vapor pressure at the respective optimum temperatures, Si in the high temperature region T ₃ Since a high temperature can be set independently of the supply amount of SiC, a SiC single crystal can be synthesized efficiently. In addition, the Si source 12 and the carbon supply source 15 can be obtained in a bulky and high-purity material at a low cost, greatly reduce the impurity concentration, and synthesize a large and long SiC single crystal. can do. Moreover, since the graphite of the Si source 12 and the carbon source 15 is a single element, the synthesis conditions are stable, and a high-quality SiC single crystal can be obtained.
[0048]
Further, since the seed crystal support rod 9 can be rotated up to 200 rpm, it can improve the in-plane uniformity, and can promote the diffusion and increase the growth rate.
[0049]
Further, since the Si vapor pressure control shield 13 and the carbon supply source 15 are mechanically connected, the carbon supply source 15 is lowered as the Si source 12 decreases, and the synthesized SiC single crystal and carbon The distance to the supply source 15 is constant, and stable synthesis can be performed for a long time.
[0050]
Furthermore, since the inner surface of the crucible 1 or the like is made of diamond-like carbon or glassy carbon, generation of natural nuclei is suppressed, and a high-quality SiC single crystal can be obtained.
[0051]
The heat shield 6 suppresses heat radiation due to heat radiation and suppresses induced current due to high frequency heating.
[0052]
【The invention's effect】
As described above, according to the method for synthesizing a SiC single crystal according to the present invention, since single Si and C, which are single elements, are individually heated, a high-quality and large-sized SiC single crystal can be easily obtained. And can be obtained inexpensively at a high growth rate.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a synthesis apparatus used in an embodiment of the present invention.
FIG. 2 is a graph showing vapor pressure curves of C and SiC.
FIG. 3 is a graph showing vapor pressure curves of Si and C.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Crucible, 3 ... Si holding crucible, 6 ... Heat shield, 9 ... Seed crystal support rod, 10 ... Seed crystal holder, 11 ... Seed crystal, 12 ... Si source, 13 ... Si vapor pressure control shield, 14 ... Si vapor pressure control auxiliary shield, 15 ... carbon source, T ₁ ... a low-temperature region, T ₂ ... intermediate temperature range, T ₃ ... high-temperature region.

Claims (6)

Three temperature regions T ₁ , T ₂ , T ₃ are formed in the synthesizer, and the temperature of each region is set to T ₁ <T ₂ <T ₃ , so that the low temperature region T ₁ is solid Si and the medium temperature region T ₂ is Solid carbon is placed in each of a SiC seed crystal or a SiC single crystal substrate and the high temperature region T ₃ , and carbon, quartz or SiC is formed on the interface between the liquid phase Si and the gas phase Si in the low temperature region T ₁ . the composed shield is provided to evaporate the Si from the low temperature region T ₁ while controlling the vapor pressure of the Si, is reacted with the carbon SiC forming gas by a Si obtained by its evaporation passing said high temperature region T ₃ to form a method of synthesizing SiC single crystal, characterized in that said the SiC-forming gas to reach the seed crystal or the SiC single crystal substrate of the intermediate temperature range T ₂ to synthesize SiC single crystal. And said shield and said high temperature region T ₃ carbon mechanically coupled, the with the decrease of the Si amount due to evaporation of the low-temperature region T ₁ of the Si, the carbon of the high temperature region T ₃ which is connected to the shield move, synthesized SiC synthetic methods SiC single crystal according to claim 1, wherein the distance between the single crystal and the carbon is characterized by a constant. Claim 1, wherein the increasing the surface area of the carbon by providing a capillary or slit to the carbon of a high temperature region T _3, characterized in that to efficiently react with Si and the carbon coming from the low temperature region T ₁ Or the synthesis method of the SiC single crystal of 2 . The low temperature region T _1, at least the inner surface of the crucible to form the intermediate temperature range T ₂ and the high-temperature region T ₃ is, in any one of claims 1 to 3, characterized in that it consists of diamond-like carbon or glassy carbon A method for synthesizing a SiC single crystal. The low temperature region T _1, the outside of the crucible to form the intermediate temperature range T ₂ and the high-temperature region T _3, SiC single of any one of claims 1 to 4, characterized in that a heat shield made of graphite Crystal synthesis method. The heat shield is formed by arranging a plurality of strip-shaped graphite plates adjacent to each other with a gap therebetween so that the overall shape becomes a substantially cylindrical shape, and a plurality of the heat shields are arranged in a radial direction of the crucible. 5. A method for synthesizing a SiC single crystal according to 5 .

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