WO2017070877A1

WO2017070877A1 - METHOD FOR FORMING CVD-SiC LAYER AND CVD-SiC LAYER FORMED BY THE METHOD

Info

Publication number: WO2017070877A1
Application number: PCT/CN2015/093160
Authority: WO
Inventors: Takashi Goto; Song Zhang; Rong TU; Mingxu HAN
Original assignee: Ibiden Co., Ltd.; Wuhan University Of Technology
Priority date: 2015-10-29
Filing date: 2015-10-29
Publication date: 2017-05-04
Also published as: JP2018538436A; JP6622912B2

Abstract

A method for forming a CVD-SiC layer by a CVD method of precipitating a deposit on a substrate in a CVD furnace while supplying a raw material gas, wherein the CVD method is a photo CVD method of irradiating the substrate with a light beam and the layer-forming temperature (t [K] ) and the total pressure (p [kPa] ) satisfy the following formula (1) : p ≥ -0.04t + 72.

Description

METHOD FOR FORMING CVD-SiC LAYER AND CVD-SiC LAYER FORMED BY THE METHOD

Technical Field

The present invention relates to a method for forming a CVD-SiC layer and a the CVD-SiC layer formed by the method.

Background Art

SiC is a material having heat resistance, mechanical strength and corrosion resistance and therefore, is being used in various fields. For example, SiC is applied to a heat treatment member in the semiconductor production, such as susceptor, liner tube, process tube, wafer boat and single crystal pulling device member.

A CVD-SiC material obtained by forming SiC by a CVD method is dense and poreless and is a high-purity material and therefore, in the applications above, this material is applied onto a substrate such as graphite and SiC and used as a composite material, or a CVD-SiC layer is sometimes separated from the substrate and used as a simple substance.

JP-A-2002-003285 describes an SiC-coated graphite member having excellent thermal shock resistance against thermal shock such as rapid heating or rapid cooling and being excellent also in the corrosion resistance, in which an SiC coat is firmly adhered to a graphite substrate surface by a CVD method. Specifically, there is described an SiC-coated graphite member suitably used, for example, as a member for heat treatment in the semiconductor production, which is a graphite member coated with an SiC film deposited on a graphite substrate surface by a CVD method and is an SiC-coated graphite member where the graphite substrate has a pore profile with an average pore size of 0.4 to 3 μm and a maximum pore size of 10 to 100 μm, the population of SiC in the graphite substrate surface layer part ranging from the graphite substrate surface to a depth of 150 μm is from 15 to 50％, and the average crystal grain size of the SiC coat is from 1 to 3 μm.

In JP-A-2002-003285, it is also described that the SiC coat has crystallinity with a strong propensity to orient to SiC (111) plane and preferably has as high orientation as the diffractive peak intensity by X-ray diffraction of the SiC (111) plane being 80％or more of the intensity of all crystal faces SiC (hkl) and the SiC coat highly oriented to SiC (111) plane imparts excellent corrosion resistance.

However, according to the disclosure of JP-A-2002-003285, since the average crystal grain size is from 1 to 3 μm and moreover, a crystal oriented in a direction other than 111 is also present together, a gap is readily formed between crystal grains and in addition, a pore is likely to be formed, making it difficult to obtain a CVD layer having a sufficiently dense structure.

The CVD-SiC layer oriented in the 111 direction is a film obtainable at a low layer-forming temperature, but when the layer-forming temperature is low, a sufficiently high decomposition rate is not obtained, and a long time is required to form a thick CVD-SiC layer, making it difficult to efficiently obtain a CVD-SiC layer.

Summary of the Invention

In view of the foregoing problem, one of objects of the present disclosure is to provide a dense CVD-SiC layer with little pores, a composite material, and a method for efficiently forming the CVD-SiC layer.

According to an aspect (1) of the present disclosure, there is provided a method for forming a CVD-SiC layer by a photo CVD method of precipitating a deposit on a substrate in a CVD furnace while supplying a raw material gas,

wherein in the photo CVD method, the layer-forming temperature (t [K] ) and the total pressure (p [kPa] ) satisfy the following formula (1) :

p ≥ -0.04t + 72 (1)

According to the method for forming a CVD-SiC layer, the layer is formed at a high pressure satisfying formula (1) , so that the raw material gas can be efficiently supplied to the substrate, and the CVD-SiC layer can be formed at a high layer-forming rate. In addition, the layer is formed by a photo CVD method of irradiating the substrate with a light beam while heating the substrate at a predetermined layer-forming temperature, so that the raw material gas can readily decompose in the vicinity of the substrate due to an interaction between heat and a light beam. As a result, the raw material gas is prevented from decomposing in midair, thereby preventing disorderly precipitation of an SiC deposit, and since the raw material gas decomposes on the surface of the substrate, a dense CVD-SiC layer with a uniform crystal direction can be efficiently formed at a high layer-forming rate.

According to an aspect (2) of the present disclosure, the total pressure p may be set to be in a range from 6 to 10 kPa.

When the total pressure inside the CVD furnace during layer formation is 6 kPa or more, the raw material gas can be sufficiently supplied to the substrate, so that a dense CVD-SiC layer with a uniform crystal direction can be efficiently formed. In addition, when the total pressure inside the CVD furnace during layer formation is 10 kPa or less, absorption of the light beam by the raw material gas can be suppressed, making it possible to deter the decomposition of the raw material gas until the light beam reaches the substrate and restrain the raw material gas from decomposing in midair, and a dense CVD-SiC layer with a uniform crystal direction can be efficiently formed at a high layer-forming rate.

According to an aspect (3) of the present disclosure, the layer-forming temperature t may be set to be in a range from 1,600 to 1,700 K.

Since the layer-forming temperature t is 1,600 K or more, the energy of the light beam necessary for layer formation can be small, so that the raw material gas can be prevented from decomposing in the light path before reaching the surface of the substrate and the raw material can be efficiently decomposed on the surface of the substrate. In addition, since the layer-forming temperature is 1,700 K or less, a CVD-SiC layer cannot be sufficiently formed only by the action of thermal CVD, but formation of a CVD-SiC layer on the surface of the substrate can be accelerated by the combination with the action of the light beam. Therefore, the raw material gas is prevented from decomposing in midair, thereby preventing disorderly precipitation of an SiC deposit, and formation of a CVD-SiC layer on the surface of the substrate can be accelerated, so that a dense CVD-SiC layer with a uniform crystal grain size can be efficiently formed at a high layer-forming rate.

According to an aspect (4) of the present disclosure, the light beam may have a wavelength of 1,500 nm or less.

When the wavelength of the light beam is 1,500 nm or less, the light beam has a photon energy high enough to decompose the raw material gas and obtain a CVD-SiC layer, so that a dense CVD-SiC layer with a uniform crystal grain size can be efficiently obtained at a high layer-forming rate.

According to an aspect (5) of the present disclosure, there is provided a CVD-SiC layer being formed by the above-described forming method.

The CVD-SiC layer according to the present disclosure is formed by the above-described forming method, so that a dense CVD-SiC layer with a uniform crystal grain size can be efficiently formed at a high layer-forming rate.

According to an aspect (6) of the present disclosure, there is provided a CVD-SiC layer including a large number of crystal grains each extending in the layer thickness direction and being hexagonal pyramidal at its top end part, wherein F₁₁₁ (Lotgering Factor in 111 direction) in the layer thickness direction above is from 0.8 to 1.0.

The CVD-SiC layer according to the present disclosure includes a large number of crystal grains each extending in the layer thickness direction and being hexagonal pyramidal at its top end part. That is, the crystal grain extends in a direction perpendicular to the substrate surface (a surface perpendicular to the thickness direction of the CVD-SiC layer) on which a CVD-SiC layer is grown, and a large number of crystal grains are aligned, so that the gap between crystal grains can be reduced without allowing crystal grains to pile up in a disorderly manner. Therefore, a dense CVD-SiC layer with little pores can be obtained.

The top end part of the crystal grain is formed to have a regular hexagonal pyramidal shape. In the regular hexagon, a rotated figure overlaps with the original figure by each 60 degrees rotation and three times the interior angle is 360 degrees, so that columnar crystal grains can be mutually aligned to reduce the gap therebetween. As a result, a crystal grain with the crystal direction in disorder can be hardly formed in the gap, and this is considered to make it possible to obtain a dense CVD-SiC layer with little gaps.

In addition, since the top end part of the crystal grain is formed to have a regular hexagonal pyramidal shape, the distal tip is sharpened, and there is no flat plane reflecting light on the front face. That is, the surface of the CVD-SiC layer includes only an inclined plane and therefore, light illuminated is scattered, so that the effect of specular reflectance can be made to hardly occur by the ratio of the flat plane and the inclined plane. Therefore, the effect on the reflection can be reduced by the manner of growth of the crystal grain.

The Lotgering Factor is an indicator for determining the crystal orientation degree obtained by X-ray diffraction, by eliminating the effect of half-width of the peak, and the numerical value is 1 in the case of a completely oriented sample and is 0 in the case of a randomly oriented sample.

In the CVD-SiC layer according to the present disclosure, F₁₁₁ in the layer thickness direction of the CVD-SiC layer is set to be in a range from 0.8 to 1.0. Since this is a CVD-SiC layer with the 111 direction ( [111] direction perpendicular to the 111 plane) being strongly oriented in the layer thickness direction (a direction perpendicular to the surface of the substrate) , the disorder in the alignment of crystal grains is reduced, and a denser CVD-SiC layer with little pores can be obtained.

According to an aspect (7) of the present disclosure, the F₁₁₁ may be set to be in a range from 0.9 to 1.0.

In the CVD-SiC layer according to the present disclosure, it is preferable to set F₁₁₁ in the layer thickness direction to be in a range from 0.9 to 1.0, since a CVD-SiC layer with the 111 direction being more strongly oriented in the layer thickness direction, the disorder in the alignment of crystal grains is reduced, and a further denser CVD-SiC layer with little pores can be obtained.

According to an aspect (8) of the present disclosure, the maximum diameter value of the crystal grain may be set to be in a range from 50 to 300 μm.

The maximum diameter value of the crystal grain is a diameter of a largest crystal grain out of a large number of scattered crystal grains.

In the CVD-SiC layer, generally, the crystal grain gradually grows between the start and end of layer formation in a CVD furnace. The size of a crystal grain continuously grown between the start and end of layer formation is largest, and the size of a crystal grain stopped growing in the middle of layer formation or started growing in the middle of layer formation is smaller than that. Therefore, the largest crystal grain observed on the CVD-SiC layer surface is a crystal grain growing from the start of layer formation, and crystal grains having substantially the same size constitute a "large crystal grain" group in a distribution of diameters of the crystal grains, whereas a "small crystal grain" started growing in the middle of layer formation fills the gap between large crystal grains. On this account, a uniquely large crystal grain is not present. In addition, a large number of crystal grains present at the start of layer formation grow into a "large crystal grain" by natural selection and therefore, the "large crystal grain" group in a distribution of diameters of the crystal grains occupies the majority of the area in the CVD-SiC layer surface.

Since the maximum diameter value of the crystal grain of the present invention is 300 μm or less, a coarse crystal grain is not present, and thermal properties such as heat conduction and radiation factor can be uniformized, so that the layer can be suitably used, for example, in the applications such as semiconductor.

In addition, since the maximum diameter value of the crystal grain of the present invention is 50 μm or more, the "large crystal grain" group occupying the majority of the area in the CVD-SiC layer surface decreases the number of gaps between grains, which are formed in the surface, and a dense CVD-SiC layer can be formed.

According to an aspect (9) of the present disclosure, there is provided a composite material including a substrate and the above-described CVD-SiC layer.

The CVD-SiC layer is a high-purity material with little pores, because the raw material is a high-purity raw material gas.

According to the composite material according to the aspect (9) , a substrate is coated with a dense CVD-SiC layer, so that not only a gas can be blocked from flowing between the substrate and the outside of the CVD-SiC layer, leading to protection of the substrate against an external corrosive gas, but also release of an impurity gas from the substrate can be prevented. Therefore, a composite material resistant to a corrosive gas and assured with little release of an impurity gas can be provided.

According to an aspect (10) of the present disclosure, the substrate may be graphite.

Generally, the graphite has high heat resistance and strength and can be processed in various shapes because of its good machining properties. However, on the other hand, the graphite is disadvantageously susceptible to oxidation and rapidly consumed in a high-temperature oxidizing atmosphere. In the composite material according to the present disclosure, the substrate is coated with the above-described CVD-SiC layer and thereby can be prevented from oxidation. In addition, coating with the CVD-SiC layer makes it difficult for the graphite as a porous material to release the adsorbed gas, etc. to the outside of the CVD-SiC layer. Therefore, a composite material having excellent heat resistance and high strength and capable of conforming to various shapes can be provided.

According to the method for forming a CVD-SiC layer of the present disclosure, layer formation is performed at a high pressure and therefore, the raw material gas is efficiently supplied to the substrate, so that a CVD-SiC layer can be formed at a high layer-forming rate. In addition, layer formation is performed by a photo CVD method of irradiating the substrate with a light beam while heating the substrate, so that the raw material gas can readily decompose in the vicinity of the substrate due to an interaction between heat and a light beam. As a result, the raw material gas is prevented from decomposing in midair, thereby preventing disorderly precipitation of an SiC deposit, and the raw material gas decomposes on the surface of the substrate, so that a dense CVD-SiC layer with a uniform crystal direction can be efficiently obtained at a high layer-forming rate.

In the CVD-SiC layer according to the present disclosure, crystal grains each being regular hexagonal pyramidal at its top end part are aligned, so that the gap between crystal grains can be reduced without allowing crystal grains to pile up in a disorderly manner. Therefore, a dense CVD-SiC layer with little pores can be formed.

The composite material according to the present disclosure is coated with a dense CVD-SiC layer, so that not only a gas can be blocked from flowing between the substrate and the outside of the CVD-SiC layer, leading to protection of the substrate against an external corrosive gas, but also release of an impurity gas from the substrate can be prevented. Therefore, a composite material resistant to a corrosive gas and assured with little release of an impurity gas can be provided.

Brief Description of Drawings

Fig. 1 shows an example of the CVD apparatus for obtaining the CVD-SiC layer in an embodiment of the present disclosure.

Fig. 2 is a table showing the test conditions and results of a1 to h5 in the confirmation test of Examples of the present disclosure and Comparative Examples i1 to i4.

Fig. 3 is a schematic view showing orientation directions analyzed from the peaks obtained by the X-ray diffraction analysis of CVD-SiC layers formed in the tests of a1 to h5 in the confirmation test of Examples of the present disclosure.

Fig. 4 is a partially enlarged view of the schematic view of Fig. 3, i.e., a view enlarging the range of 4 to 10 kPa and expressing the ordinate as an actual figure axis, where the sections are divided by the orientation direction of the CVD-SiC layer.

Fig. 5 is an explanatory view showing the layer-forming rate in the tests of a1 to h5 in the confirmation test of Examples of the present disclosure.

Figs. 6A-6E are SEM photographs of CVD-SiC layers obtained in a1 to a5 where the total pressure is 2 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig. 6A shows the sample of a1, Fig. 6B shows the sample of a2, Fig. 6C shows the sample of a3, Fig. 6D is the sample of a4, and Fig. 6E is the sample of a5.

Figs. 7A-7E are SEM photographs of CVD-SiC layers obtained in b1 to b5 where the total pressure is 4 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig 7A shows the sample of b1, Fig. 7B shows the sample of b2, Fig. 7C shows the sample of b3, Fig. 7D shows the sample of b4, and Fig. 7E shows the sample of b5.

Figs. 8A-8E are SEM photographs of CVD-SiC layers obtained in c1 to c5 where the total pressure is 5 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig. 8A shows the sample of c1, Fig. 8B shows the sample of c2, Fig. 8C shows the sample of c3, Fig. 8D shows the sample of c4, and Fig. 8E shows the sample of c5.

Figs. 9A-9E are SEM photographs of CVD-SiC layers obtained in d1 to d5 where the total pressure is 6 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig. 9A shows the sample of d1, Fig. 9B shows the sample of d2, Fig. 9C shows the sample of d3, Fig. 9D shows the sample of d4, and Fig. 9E shows the sample of d5.

Figs. 10A-10E are SEM photographs of CVD-SiC layers obtained in e1 to e5 where the total pressure is 8 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig. 10A shows the sample of e1, Fig. 10B shows the sample of e2, Fig. 10C shows the sample of e3, Fig. 10D shows the sample of e4, and Fig. 10E shows the sample of e5.

Figs. 11A-11E are SEM photographs of CVD-SiC layers obtained in f1 to f5 where the total pressure is 10 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig. 11A shows the sample of f1, Fig. 11B shows the sample of f2, Fig. 11C shows the sample of f3, Fig. 11D shows the sample of f4, and Fig. 11E shows the sample of f5.

Figs. 12A-12E are SEM photographs of CVD-SiC layers obtained in g1 to g5 where the total pressure is 20 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig. 12A shows the sample of g1, Fig. 12B shows the sample of g2, Fig. 12C shows the sample of g3, Fig. 12D shows the sample of g4, and Fig. 12E shows the sample of g5.

Figs. 13A-13E are SEM photographs of CVD-SiC layers obtained in h1 to h5 where the total pressure is 40 kPa, in the confirmation test of Examples of the present disclosure, wherein Fig. 13A shows the sample of h1, Fig. 13B shows the sample of h2, Fig. 13C shows the sample of h3, Fig. 13D shows the sample of h4, and Fig. 13E shows the sample of h5.

Figs. 14A-14D are SEM photographs of CVD-SiC layers obtained in the layer formation test by a thermal CVD method as Comparative Examples of the present disclosure, wherein Fig. 14A shows the sample of i1, Fig. 14B shows the sample of i2, Fig. 14C shows the sample of i3, and Fig. 14D shows the sample of i4.

Description of Embodiments

The embodiments of a method for forming a CVD-SiC layer and a CVD-SiC layer formed by the method according to the present disclosure will be described below. First, the apparatus for forming a CVD-SiC layer will be described.

Fig. 1 is a schematic view of a CVD apparatus 100 as the apparatus for forming a CVD-SiC layer. The CVD apparatus 100 has a CVD furnace 10, a radiation thermometer 21, an introduction pipe 22, a light source 23, an actuator 31, a supporting member 32, a table 33, a heating device 41, and a graphite heater 42.

The CVD furnace 10 is formed of material such as stainless steel. Inside the CVD furnace 10, a substrate (graphite substrate) S as a sample is arranged. The radiation thermometer 21, the introduction pipe 22 and the light source 23 are arranged on the top surface of the CVD furnace 10. The radiation thermometer 21 can measure the temperature inside the CVD furnace 10 and thus the layer-forming temperature in the graphite substrate S, from the radiation heat inside the CVD furnace 10. The introduction pipe 22 as a raw material supply part supplies the raw material gas to the inside of the CVD furnace 10. The light source 23 is configured by, for example, a semiconductor laser, and irradiates the graphite substrate S with a laser beam L.

The actuator 31, which is configured by actuating device or motor, actuates the supporting member 32 and the table 33 in the X direction and the Y direction. The graphite substrate S is arranged on the table 33, and the table 33 connected to the actuator 31 by the supporting member 32 moves in the X direction and the Y direction, whereby the laser beam L irradiation position on the graphite substrate S also moves in the X direction and the Y direction.

The heating device 41 and the graphite heater 42 as a heater for heating are provided on the bottom surface of the CVD furnace 10, and the heating device 41 drives the graphite heater 42 to generate heat to heat the inside of the CVD furnace 10.

In the CVD apparatus 100, various controllers and other components such as vacuum pump and valve are provided, but the specific configuration of the CVD apparatus 100 is not particularly limited.

The raw material gas supplied to the CVD furnace 10 is not particularly limited. For example, a raw material gas such as methyltrichlorosilane (MTS) containing a carbon source and a silicon source at the same time, or a raw material gas obtained by mixing a carbon source and a silicon source may be used. As the carbon source, for example, methane, ethane or propane may be used. As the silicon source, for example, a halogenated silane such as tetrachlorosilane can be used, other than silane.

In the CVD method according to the present disclosure, the raw material gas is decomposed by using a light beam and heat in combination as the excitation energy. Therefore, the CVD furnace has a heater for heating (graphite heater 42) and a light source 23. The heater for heating transfers heat to the substrate (graphite substrate S) in the form of radiation heat, and the light source 23 irradiates the surface of the substrate with a light beam (laser beam L) . The light source 23 is not particularly limited, but any of an electric bulb, a discharge lamp, a laser, etc. may be used. In Fig. 1, the light source 23 is provided outside the CVD furnace but may be provided inside the CVD furnace. In the case of providing the light source outside the CVD furnace as in Fig. 1, the substrate can be irradiated with the light beam through a transparent window formed in the CVD furnace 10.

Here, in the CVD method, the layer-forming temperature (t [K] ) of the graphite substrate S and the total pressure (p [kPa] ) inside the CVD furnace 10 satisfy the following formula (1) . In other words, while the graphite substrate S inside of the CVD furnace 10, where a substrate is arranged, is maintained to have a constant relationship of temperature and pressure.

p ≥ -0.04t + 72 (1)

According to the method for forming a CVD-SiC layer of the present disclosure, layer formation is performed at a high pressure satisfying formula (1) by using, among CVD methods, a photo CVD method of irradiating a substrate with a light beam. It is considered that thanks to this layer formation, a large number of columnar crystal grains each extending in a direction perpendicular to the surface of the substrate (layer thickness direction of CVD-SiC layer) can be obtained. The mechanism thereof is as follows.

In general, a CVD-SiC layer by a thermal CVD method is known to be readily oriented in the 111 direction (a direction perpendicular to 111 plane and a [111] direction) at a low temperature and oriented in other directions at a high temperature. In JP-A-1994-092761, which discloses thermal CVD method, it is disclosed that under the conditions of a temperature range from 1,323 K to 1,473K and a pressure range from 1.3 kPa to 13 kPa, a 111 orientation with a high orientation degree in the 111 direction is obtained, and as the temperature rises and the pressure lowers, the orientation degree in other directions increases, making it difficult to obtain a 111 orientation, and that under the conditions of a temperature range from 1,473K to 1,673K and a pressure range from 0.13 kPa to 1.3 kPa, a 220 orientation with a high orientation degree in 220 direction is obtained. This is based on the difference in the growth rate of SiC crystal grains and is considered to occur because the growth rate in the 111 direction is high. In the thermal CVD method at a low layer-forming temperature, growth in directions except for the 111 direction having a high growth rate is culled in the process of a CVD-SiC layer growing from the surface of the substrate, and a CVD-SiC layer oriented in the 111 direction is readily obtained. In the thermal CVD method, it is considered that when the layer-forming temperature is raised, the raw material gas decomposes in midair to precipitate a decomposition product as a deposit and sideway buckling takes place in the 111 direction having a high growth rate, making it difficult to obtain a CVD-SiC layer oriented in the 111 direction.

On the other hand, according to the method for forming a CVD-SiC layer by a photo CVD method according to the present disclosure, the raw material gas decomposes on the surface of the substrate by an interaction between light and heat. Therefore, the growth rate on the surface of the substrate irradiated with light is relatively higher than the growth rate of the crystal grain in midair, hardly causing sideway buckling of the crystal grain, and a CVD-SiC layer oriented in the 111 direction having a high growth rate can be grown at a high rate.

In addition, the crystal grain oriented in the 111 direction is obtained in the shape of being sharpened at the distal tip and therefore, the irradiated light repeats the reflection while repeatedly acting on the decomposition of the raw material gas by repeating the reflection. It is considered that thanks to this repeated action, the raw material gas can be efficiently decomposed on the surface of the substrate and a CVD-SiC layer having a high orientation degree in the 111 direction can be obtained with good efficiency. Furthermore, the irradiated light is absorbed between sharp crystal grains while repeating the reflection. It is considered that because of this absorption, growth of the CVD-SiC layer can be accelerated also in the gap formed between crystal grains and a dense CVD-SiC layer can be obtained.

The above-described total pressure p is preferably set to a range of 6 to 10 kPa. When the total pressure inside the CVD furnace 10 during layer formation is 6 kPa or more, the raw material gas can be sufficiently supplied to the graphite substrate S, so that a dense CVD-SiC layer with a uniform crystal direction can be efficiently formed. In addition, when the total pressure inside the CVD furnace 10 during layer formation is 10 kPa or less, absorption of the light beam by the raw material gas can be suppressed, making it possible to deter the decomposition of the raw material gas until the light beam L reaches the graphite substrate S and restrain the raw material gas from decomposing in midair, and a dense CVD-SiC layer with a uniform crystal direction can be efficiently obtained at a high layer-forming rate.

The above-described layer-forming temperature t is preferably in a range of 1,600 to 1,700 K. Accordingly, the layer-forming temperature t may be set to 1,600 K or more, and the energy of the laser beam L necessary for layer formation can be small, so that the raw material gas can be prevented from decomposing in the light path before reaching the surface of the graphite substrate S and the raw material can be efficiently decomposed on the surface of the graphite substrate S. In addition, a combination of condition in which the layer-forming temperature is 1,700 K or less and usage of thermal CVD may not be sufficient in forming a CVD-SiC layer. The formation of a CVD-SiC layer on the surface of the graphite substrate S may be accelerated by additional usage of the light beam L. Therefore, the raw material gas is prevented from decomposing in midair, thereby preventing disorderly precipitation of an SiC deposit, and formation of a CVD-SiC layer on the surface of the graphite substrate S can be accelerated, so that a dense CVD-SiC layer with a uniform crystal grain size can be efficiently obtained at a high layer-forming rate.

It is preferably to set a wavelength of the laser beam L to be 1,500 nm or less. When the wavelength of the laser beam L is 1,500 nm or less, the laser beam has photon energy high enough to decompose the raw material gas and obtain a CVD-SiC layer, so that a dense CVD-SiC layer with a uniform crystal grain size can be efficiently obtained at a high layer-forming rate.

The CVD-SiC layer formed is described below in terms of an indicator called Lotgering Factor (sometimes referred to as Lotgering orientation degree) . The Lotgering Factor is an indicator for evaluating the crystal orientation degree by eliminating the effects of half-width in the X-ray diffraction, analyzer performance, etc. and can be obtained by analyzing the diffraction pattern of X-ray diffraction. The numerical value is 1 in the case of a completely oriented sample and is 0 in the case of a randomly oriented sample. In the description of the present disclosure, the Lotgering Factor in the hkl direction is denoted by the symbol F_hkl and, for example, the Lotgering Factor in the 111 direction is denoted by F₁₁₁.

In the CVD-SiC layer of the present disclosure, F₁₁₁ in the layer thickness direction (a direction perpendicular to the surface of the substrate) of the CVD-SiC layer is from 0.8 to 1.0, and since the layer includes a CVD-SiC layer with the strongly oriented 111 direction being oriented in the layer thickness direction, the disorder in the alignment of crystal grains is reduced, so that a denser CVD-SiC layer with little pores can be obtained.

The method for calculating the Lotgering orientation degree F_hkl in the hkl direction is described below. First, the X-ray diffraction pattern of the target sample is measured, and the orientation degree is evaluated by the comparison with the X-ray diffraction pattern of a non-oriented sample.

Formula (2) below is a calculation formula for calculating F_hkl, and formula (3) is a calculation formula for calculating the value of P used in formula (2) .

P_hkl is a sum total of peaks regarding orientations measured, relative to the sum total of peaks of the target sample. ΣI _(hkl) is, for example, in the case of ΣI (111) , the sum total of I₍₁₁₁₎ , I ₍₂₂₂₎ , ... , I _(nnn) , which are integral multiples of the (111) direction, and in the case of ΣI₍₀₀₂₎ , the sum total of I ₍₀₀₂₎ , I ₍₀₀₄₎ , ... , I _(00n) , which are integral multiples of the (002) direction. In addition, ΣI is the sum total of all peaks of the target sample.

P₀ is the sum total of peaks regarding orientations measured, relative to the sum total of peaks in a non-oriented sample, and is calculated in the same manner as P.

In the CVD-SiC layer of the present disclosure, as described above, the Lotgering Factor F₁₁₁ in the 111 direction is from 0.8 to 1.0. The crystal grain extends in the layer thickness direction, and its top end part takes on a regular hexagonal pyramidal shape. That is, the crystal grain extends in a direction perpendicular to the substrate surface (a surface perpendicular to the thickness direction of the CVD-SiC layer) on which a CVD-SiC layer is grown, and a large number of crystal grains are aligned, so that the gap between crystal grains can be reduced without allowing crystal grains to pile up in a disorderly manner. Therefore, a dense CVD-SiC layer with little pores can be obtained.

The top end part of the crystal grain is formed to have a regular hexagonal pyramidal shape. In the hexagon, a rotated figure overlaps with the original figure by each 60 degrees rotation and three times the interior angle is 360 degrees, so that columnar crystal grains can be mutually aligned to reduce the gap therebetween. As a result, a crystal grain with the crystal direction in disorder can be hardly formed in the gap, and this is considered to make it possible to obtain a dense CVD-SiC layer with little gaps.

The F₁₁₁ is preferably from 0.9 to 1.00. In this range, since the layer is a CVD-SiC layer with the 111 direction being more strongly oriented in the layer thickness direction, the disorder in the alignment of crystal grains is reduced, and a further denser CVD-SiC layer with little pores can be obtained.

The maximum diameter value of the crystal grain is preferably from 50 to 300 μm. Here, the maximum diameter value of the crystal grain is a diameter of a largest crystal grain out of a large number of scattered crystal grains.

When the maximum diameter value of the crystal grain is 300 μm or less, a coarse crystal grain is not present, and thermal properties such as heat conduction and radiation factor can be uniformized, so that the layer can be suitably used, for example, in the applications such as semiconductor.

When the maximum diameter value of the crystal grain is 50 μm or more, the "large crystal grain" group occupying the majority of the area in the CVD-SiC layer surface decreases the number of gaps between grains, which are formed in the surface, and a dense CVD-SiC layer can be formed.

Furthermore, a composite material is formed from a substrate and the above-described CVD-SiC layer.

According to the composite material of the present disclosure, a substrate is coated with a dense CVD-SiC layer, so that not only a gas can be blocked from flowing between the substrate and the outside of the CVD-SiC layer, leading to protection of the substrate against an external corrosive gas, but also release of an impurity gas from the substrate can be prevented. Therefore, a composite material resistant to a corrosive gas and assured with little release of an impurity gas can be provided.

The substrate may preferably be graphite.

Generally, the graphite has high heat resistance and strength and can be processed in various shapes because of its good machining properties. . On the other hand, graphite is disadvantageously susceptible to oxidation and rapidly consumed in a high-temperature oxidizing atmosphere. In the composite material of the present disclosure, the substrate is coated with the above-described CVD-SiC layer and thereby can be prevented from oxidation. In addition, coating with the CVD-SiC layer makes it difficult for the graphite as a porous material to release the adsorbed gas, etc. to the outside of the CVD-SiC layer. Therefore, a composite material having excellent heat resistance and high strength and capable of conforming to various shapes can be provided.

The CVD-SiC layer of the present disclosure and the forming method thereof are specifically described by referring to Fig. 1.

First, a graphite substrate S is put in a CVD furnace 10 of a CVD apparatus 100. A graphite heater 42 is provided under the graphite substrate S. An external heating device 41 applies an electrical current to the graphite heater 42, and heat can be generated by resistance heat generation of the graphite heater 42. The heat generating method is not limited thereto, and other methods such as induction heating and high-frequency heating can be used without any particular limitation.

The top of the CVD furnace 10 is irradiated with a laser beam L from a light source 23 through, for example, a quartz glass window. The quartz glass has a small thermal expansion coefficient and heat resistance as well as a high transmittance in the range from an ultraviolet region to an infrared region and therefore, can be suitably used as a material constituting the window. Since the laser beam has high energy output, it is preferable to provide an antireflection coating on an incident surface of the quartz glass window, which is configured to prevent the laser beam having specific wavelength from reflecting on the incident surface, to prevent damage of optical components by the heat of reflected laser beam and to prevent injury of a user. For example, as for such antireflection coating, a fluorine optical coating may be used.

As the light source 23, for example, a laser light source can be used. A semiconductor laser, a gas laser, etc. can be used, and the laser light source is not particularly limited. The laser beam L is illuminated onto the graphite substrate S through a window from the light source. In addition, the temperature on the surface of the graphite substrate S can be measured through the window by using a radiation thermometer 21.

The apparatus has a raw material gas introduction pipe 22, for example, at the top of the CVD furnace 10. A raw material gas can be supplied to the inside of the CVD furnace 10 through the raw material introduction pipe 22.

In advance of layer formation of the CVD-SiC layer, the gas inside the CVD furnace 10 is discharged by a vacuum pump to reduce the pressure and at the same time, the interior of the furnace is heated to a layer-forming temperature. The pressure (total pressure) inside the CVD furnace and the layer-forming temperature are set, for example, to 8 kPa and 1,623 K, respectively. The layer-forming temperature is a temperature on the surface of the graphite substrate S. Heating and pressure reduction may be performed at the same time or may be performed in a reverse order, and the order is not particularly limited.

Next, the surface of the graphite substrate S is irradiated with a laser beam L from the light source 23 through a quartz glass window. For example, an AlGaAs semiconductor laser can be used for the light source 23.

A CVD-SiC layer is grown on the graphite substrate S while moving the graphite substrate S by use of an actuator 31, a supporting member and a table 33 and thermally decomposing the raw material gas by a laser beam L. Incidentally, in the example of Fig. 1, the graphite substrate S is moved at the time of forming a CVD-SiC layer, but the laser beam L may be moved by moving the light source 23 while keeping the graphite substrate S fixed.

In this way, a CVD-SiC layer can be formed on the surface of the graphite substrate S. The obtained CVD-SiC layer may be used with the graphite substrate S or may be used as a CVD-SiC layer alone by separating the graphite substrate S. For example, when the substrate is graphite as in this example, the CVD-SiC layer can be separated by cutting work, oxidation in an oxidizing atmosphere, or mechanical separation. In the case of mechanically separating the CVD-SiC layer from the substrate, pores on the surface of the substrate are preferably sealed so as to prevent the CVD-SiC layer from intruding into the pore of the substrate. As for the sealing method, coating with glassy carbon, pyrolytic carbon, etc. makes it possible to seal the pores and facilitate the separation. In this way, the CVD-SiC layer can be separated from the substrate.

(Examples)

In order to confirm the range in which the CVD-SiC layer of the present disclosure can be formed, a CVD-SiC layer was formed by a photo CVD method using a laser beam while heating the substrate, and the properties of the obtained CVD-SiC layer were verified.

As for the test level, the pressure condition is set to one selected from 8 levels, and the temperature condition is set to one selected from 5 levels.

The test conditions were marked with marks a1 to a5, b1 to b5, c1 to c5, d1 to d5, e1 to e5, f1 to f5, g1 to g5, and h1 to h5 and thereby distinguished.

The alphabetic prefix indicates the total pressure inside the CVD furnace, i.e., "a" is 2 kPa, "b" is 4 kPa, "c" is 5 kPa, "d" is 6 kPa, "e" is 8 kPa, "f" is 10 kPa, "g" is 20 kPa, and "h" is 40 kPa.

The numeral suffix indicates the layer-forming temperature inside the CVD furnace, i.e., "1" is 1,473 K, "2" is 1,523 K, "3" is 1,573 K, "4" is 1,623 K, and "5" is 1,673 K.

Common production conditions are shown below.

Vaporizer: bubbling of liquid raw material

CVD Furnace: horizontal furnace

Light source: semiconductor laser (AlGaAs)

Beam diameter: 20 mm

Wavelength: 1,064 nm

Heating: graphite heater

Exhaust: dry vacuum pump

Exhaust treatment: filter, cold trap, scrubber

Control: Labview

Substrate: graphite substrate

Temperature measurement: radiation thermometer

Raw material gas: SiCl₄, CH₄, H₂

Gas ratio: SiCl₄ : CH₄ : H₂ ＝ 1 : 0.5 : 3.5

The graphite substrate was placed on the table inside the CVD furnace, and the furnace was closed and vacuumized. The pressure was sufficiently reduced by a vacuum pump and then, the interior of the furnace was heated by the graphite heater as an auxiliary heating source. After heating such that the temperature of the graphite substrate reaches 1,273 K over 60 minutes, irradiation with the laser beam was applied and when the temperature of the graphite substrate was stabilized, the raw material gases were flowed to grow a CVD-SiC layer.

After flowing raw material gases for 30 minutes to form the CVD-SiC layer, supply of the raw material gases was stopped to terminate the layer formation.

The output of the laser beam was set to 350 W at levels of a1 to h1 where the layer-forming temperature is 1,473 K, to 400 W at levels of a2 to h2 where the layer-forming temperature is 1,523 K, to 450 W at levels of a3 to h3 where the layer-forming temperature is 1,573 K, to 500 W at levels of a4 to h4 where the layer-forming temperature is 1,623 K, and to 550 W at levels of a5 to h5 where the layer-forming temperature is 1,673K, and the temperature was finely adjusted with the heating by a laser beam.

The total pressure inside the CVD furnace is controlled by adjusting the opening of an exhaust valve. The exhaust valve is provided between the vacuum pump and the CVD furnace.

In order to compare the total pressure "kPa" with the value "-0.04t + 72" calculated from the layer-forming temperature [K] , the value of the following formula (4) was calculated. When the value obtained is "positive" or "0" , the conditions of formula (1) are satisfied, and when the value is "negative" , the conditions of formula (1) are not satisfied.

p- (-0.04t + 72) (4)

The CVD-SiC layer sample formed was then analyzed by an X-ray diffraction method, and the obtained chart was analyzed to confirm the orientation direction. Furthermore, the Lotgering Factor in the 111 direction was determined. In addition, the layer-forming rate was calculated from the thickness of the SiC film formed and the layer-forming time.

Fig. 2 is a table showing the test conditions and the results obtained. Fig. 3 is a schematic view showing orientation directions analyzed from the peaks obtained by the X-ray diffraction analysis of CVD-SiC layers formed in the tests of a1 to h5 in the confirmation test of Examples. Fig. 4 is a partially enlarged view of the schematic view of Fig. 3, i.e., a view enlarging the range of 4 to 10 kPa and expressing the ordinate as an actual figure axis, where the sections are divided by the orientation direction of the CVD-SiC layer. Fig. 5 is an explanatory view showing the layer-forming rate in the tests of a1 to h5 in the confirmation test of Examples of the present disclosure. Furthermore, the CVD-SiC layer formed was photographed by SEM. Fig. 6A to 13E show SEM photographs obtained by the photographing.

The "total pressure" is the pressure [kPa] inside the CVD furnace, the layer-forming temperature is the temperature [K] on the surface of the substrate, the layer-forming rate is the layer-forming rate [μm/h] of the CVD-SiC layer obtained, and "p- (-0.04t + 72) " is a formula corresponding to formula (4) for determining formula (1) and is pertinent when "positive" or "0" . F₁₁₁ is the calculated Lotgering Factor in the 111 direction. In addition, formation or no formation of "crystal grains each extending in a direction perpendicular to the surface of the substrate and being regular hexagonal pyramidal at its top end part" was determined from the SEM photograph and when formed, rating "○" was assigned. Furthermore, at the level where "crystal grains each extending in a direction perpendicular to the surface of the substrate and being regular hexagonal pyramidal at its top end part" are formed, the "maximum diameter value of the crystal grain" was measured from the SEM photograph.

In the samples of d5, e4, e5, f3, f4, f5, g1 to g5 and h1 to h5, where the value of "p- (-0.04t + 72) " is positive, a high layer-forming rate is obtained and moreover, the F₁₁₁ value of the CVD-SiC layer obtained is from 0.8 to 1.0, revealing that the layer is strongly oriented in the 111 direction. In addition, in the samples of d5, e4, e5, f3, f4 and f5, where the pressure is 10 kPa or less, it was confirmed that "crystal grains each extending in a direction perpendicular to the surface of the substrate and being regular hexagonal pyramidal at its top end part" are formed and the "maximum diameter value of the crystal grain" is from 50 to 300 μm.

Figs. 14A-14D show SEM photographs of CVD-SiC layers by a thermal CVD method of heating the substrate only by means of an auxiliary heating source without applying irradiation with a laser beam. This forming method is not a photo CVD method and does not fall under the method for forming a CVD-SiC layer of the present disclosure. In the sample of i1 where the layer-forming temperature is low and 1,473 K (Fig. 14A) , a 111-oriented CVD-SiC layer is obtained, but the layer-forming rate is low. In the sample of i1 where the layer-forming temperature is high and 1,773 K (Fig. 14D) , a high layer-forming rate is obtained, but the orientation is a 110 orientation, and a 111 orientation is not obtained. In the samples where the layer-forming temperature is intermediate therebetween, i.e., the samples of i2 of 1,573 K (Fig. 14B) and i3 of 1,673 K (Fig. 14C) , a CVD-SiC layer with a mixture of a 111 orientation and a 110 orientation is formed, and a 111-oriented CVD-SiC layer is not obtained. Thus, it was confirmed that in the thermal CVD method, at a low temperature, a 111 orientation is obtained but the layer-forming rate is low, and at a high temperature, the layer-forming rate is high but a 111 orientation is not obtained.

Claims

A method for forming a CVD-SiC layer by a CVD method of precipitating a deposit on a substrate in a CVD furnace while supplying a raw material gas,

wherein the CVD method is a photo CVD method of irradiating the substrate with a light beam and the layer-forming temperature (t [K] ) and the total pressure (p [kPa] ) satisfy the following formula (1) :

p≥-0.04t+72 (1)
The method for forming the CVD-SiC layer as claimed in claim 1,

wherein the total pressure p is set to be in a range from 6 kPa to 10 kPa.
The method for forming the CVD-SiC layer as claimed in claims 1 or 2,

wherein the layer-forming temperature t is set to be in a range from 1,600 K to 1,700 K.
The method for forming the CVD-SiC layer as claimed in any one of claims 1 to 3, wherein the light beam has a wavelength of 1,500 nm or less.
CVD-SiC layer formed by the forming method as claimed in any one of claims 1 to 4.
A CVD-SiC layer containing crystal grains each extending in the layer thickness direction and being regular hexagonal pyramidal at its top end part,

wherein F₁₁₁ (Lotgering Factor in 111 direction) in the layer thickness direction is set to be in a range from 0.8 to 1.0.
The CVD-SiC layer as claimed in claim 6,

wherein the F₁₁₁ is in a range from 0.9 to 1.0.
The CVD-SiC layer as claimed in claims 6 or 7,

wherein the maximum diameter value of the crystal grain in the CVD-SiC layer is in a range from 50 μm to 300 μm.
A composite material comprising a substrate and the CVD-SiC layer claimed in any one of claims 6 to 8.
The composite material as claimed in claim 9,

wherein the substrate includes graphite.