JP2006232669A - Low nitrogen concentration graphite material, low nitrogen concentration carbon fiber reinforced carbon composite material, low nitrogen concentration expanded graphite sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low nitrogen concentration graphite material, a low nitrogen concentration carbon fiber reinforced carbon composite material, and a low nitrogen concentration expanded graphite sheet, the nitrogen concentration of which is respectively 50 ppm or lower by the glow discharge mass spectrometry, and which are stored in such a state that the materials and sheet are kept out of the air. <P>SOLUTION: A carbon material which is highly purified in a halogen gas atmosphere is heat-treated at a pressure of 100 Pa or less at a temperature of 1,800°C or higher in an atmosphere which is not exposed to nitrogen gas so that nitrogen atoms in the carbon material are discharged. Thereafter, the carbon material is cooled to a predetermined temperature in a pressure of 100 Pa or less, and further cooled to room temperature in a noble gas atmosphere. Then the carbon material is stored in such a state that the carbon material is kept out of the air. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a low nitrogen concentration carbon-based material, and in particular, a silicon semiconductor, a compound semiconductor manufacturing jig, an epitaxial growth film manufacturing jig, and the like, a silicon carbide (hereinafter referred to as SiC) manufacturing jig. Further, the present invention relates to a low nitrogen concentration carbon-based material used for a jig for epitaxial growth of a SiC wafer and a method for manufacturing the same.

  In recent years, compound semiconductors composed of light elements such as SiC, gallium arsenide, and indium phosphide have been actively developed. Such a compound semiconductor is characterized by a large energy band gap, dielectric breakdown electric field, and thermal conductivity. And taking advantage of this feature, it has attracted attention as a material for high efficiency and high withstand voltage power devices, high frequency power devices, high temperature operation devices, or blue to ultraviolet light emitting devices. However, since the binding energy is strong, these compounds do not melt even at high temperatures at atmospheric pressure, and recrystallize the Si melt as used in silicon (hereinafter referred to as Si) semiconductors to form bulk crystals. Difficult to do.

  For example, 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, SiC single crystal pieces have been obtained by a method using a chemical reaction called the Acheson method and a method using a sublimation recrystallization method called the Rayleigh method. Recently, a SiC ingot is grown by an improved Rayleigh method in which a single crystal of SiC produced by these methods is used as a seed crystal and is sublimated and recrystallized thereon, and this SiC ingot is sliced and mirror polished. It came to be manufactured. Then, an active layer with a controlled impurity density and film thickness is formed by growing a SiC single crystal of a target scale on the substrate by vapor phase epitaxy or liquid phase epitaxy, and using this, a pn junction diode, SiC semiconductor devices such as Schottky diodes and various transistors are manufactured. In these methods, a carbon material such as a graphite material subjected to high-purity treatment in a halogen gas atmosphere or a SiC-coated graphite material in which SiC is coated on the surface of the graphite material is used.

However, even in a carbon-based material that has been subjected to high-purity treatment in a halogen gas atmosphere, about 1000 ppm of nitrogen is contained. This nitrogen is not present in the pores of the carbon-based material, but is present, for example, in a state where it is trapped between graphite layers or substituted with carbon atoms. Moreover, it combines with metal impurities contained in a trace amount in the carbon-based material to form a nitrogen compound. Conventionally, nitrogen contained in these carbon-based materials has not been particularly noticed. However, when used as a jig for manufacturing compound-based semiconductors, particularly SiC devices, the above-described SiC single crystal or SiC epitaxial growth is performed during SiC epitaxial growth. It has recently been found that it penetrates into and increases the nitrogen concentration in SiC single crystals, SiC wafers, etc., and contributes to defects in the crystals. For example, the SiC wafer contains nitrogen at 10 ¹⁷ atoms / cm ³ or more, and the epitaxial growth film contains 10 ¹⁶ atoms / cm ³ or more nitrogen. This nitrogen serves as a dopant for compound semiconductors such as SiC semiconductors, and significantly deteriorates the characteristics of the manufactured SiC device. In addition, when the nitrogen is lowered, it is necessary to maintain this for a certain period.

  An object of the present invention is to provide a low nitrogen concentration carbon-based material that maintains a nitrogen concentration of 50 ppm or less by glow discharge mass spectrometry (hereinafter referred to as GDMS).

Means and effects for solving the problems

  The present invention reduces the nitrogen concentration in the carbon-based material, and uses these low nitrogen concentration carbon-based materials as a jig for manufacturing a compound-based semiconductor, thereby producing defects in the crystal of the compound-based semiconductor manufactured. As a result, the present invention has been completed. That is, the low nitrogen concentration carbon-based material of the present invention for solving the above-mentioned problems is one that is stored in a state where the nitrogen concentration by GDMS is 50 ppm or less and is shut off from the atmosphere. The same applies to a low nitrogen concentration carbon fiber reinforced carbon composite material or a low nitrogen concentration expanded graphite sheet. These low nitrogen concentration carbon-based materials are preferably used for jigs for manufacturing silicon semiconductors and compound semiconductors and jigs for manufacturing epitaxial growth films.

  The nitrogen concentration in the carbon-based material is 50 ppm or less, more preferably 10 ppm or less, and even more preferably 5 ppm or less. Thereby, for example, when used in a jig for manufacturing a silicon semiconductor or a compound semiconductor and a jig for manufacturing an epitaxial growth film, the intrusion of nitrogen into the manufactured semiconductor product can be suppressed. It becomes possible to manufacture a semiconductor having a nitrogen concentration that is less than an order of magnitude.

  The carbon-based material used in the present invention is a graphite material, a carbon fiber reinforced carbon composite material, an expanded graphite sheet, glassy carbon, pyrolytic carbon, and the like, for example, on the surface of the graphite material. It includes SiC-coated graphite material coated with SiC, pyrolytic carbon-coated graphite material coated with pyrolytic carbon, and the like.

  In addition, the jigs for manufacturing silicon semiconductors and compound semiconductors referred to in the present invention include furnace parts such as crucibles and heaters used for pulling compound semiconductors such as single crystal silicon and SiC single crystals. Including. Further, examples of the epitaxial growth film manufacturing jig include a susceptor for epitaxial growth of silicon or SiC film. In addition, furnace parts including a heat insulating material and a heater for heating are also included in the jig.

  Further, the method for producing a low nitrogen concentration carbon material of the present invention releases a nitrogen atom in the carbon material by heat-treating the carbon material which has been highly purified in a halogen gas atmosphere at a pressure of 100 Pa or less and 1800 ° C. or more. Then, it is cooled to a predetermined temperature at a pressure of 100 Pa or less, and then cooled in a rare gas atmosphere. Alternatively, after releasing nitrogen atoms, a rare gas is introduced and cooling is performed in a rare gas atmosphere.

  After high-purity treatment in a halogen gas atmosphere, heat treatment is continuously performed at a pressure of 100 Pa or less, preferably 1 Pa or less, preferably 1800 ° C. or more, preferably 2000 ° C. or more. Alternatively, the carbon-based material that has already been subjected to high-purity treatment is again heat-treated at a pressure of 100 Pa or less, preferably 1 Pa or less, 1800 ° C. or more, preferably 2000 ° C. or more. By this heat treatment, nitrogen or the like trapped between the graphite layers during the high-purity treatment or graphitization treatment in a halogen gas atmosphere is released from the carbon-based material. Thereby, a carbon-based material having a low nitrogen concentration can be obtained. At the time of cooling, after cooling to a predetermined temperature at a pressure of 100 Pa or less, preferably 1 Pa or less, a rare gas is introduced to cool to room temperature. Alternatively, a rare gas is introduced after the heat treatment for releasing the nitrogen gas, and cooling is performed to room temperature in a rare gas atmosphere. As a result, nitrogen can be prevented from entering the carbon-based material during cooling. Here, argon gas, helium gas, or the like can be used as the rare gas.

  Moreover, the manufacturing method of the low nitrogen concentration carbonaceous material of this invention is stored in the state isolate | separated from air | atmosphere after cooling to room temperature in the said rare gas atmosphere.

  After high-purity treatment or after heat treatment to remove nitrogen in the carbon-based material, store it in a state where it is cut off from the atmosphere, so that the nitrogen concentration in the carbon-based material is more reliably maintained at a low level. be able to. Here, the state cut off from the atmosphere is, for example, a state in which the carbon-based material is sealed by setting the inside of the bag excellent in confidentiality such as a resin film called a so-called vacuum pack to a reduced pressure state from the atmospheric pressure. Or the thing which made the state which sealed the carbonaceous material with the noble gas atmosphere in the bag excellent in confidentiality, such as a resin film called a gas pack. Here, a vinyl chloride film, a polyethylene film, etc. can be used as a resin film.

  Below, a graphite material is demonstrated as an example of the embodiment example of the carbonaceous material used by this invention.

  As the graphite material used in the present embodiment, one manufactured by a general manufacturing method can be used. As an example of a general production method, first, a carbon molded body is heated to 800 to 1000 ° C. in a firing furnace, and a readily volatile component contained in a binder or the like is dispersed and evaporated to be fired (step A), then The fired body is taken out and graphitized by heating to 3000 ° C. in a graphitization furnace such as an Acheson furnace, a Kaston furnace, or an induction heating furnace (for example, JP-A 57-166305, 166306, 166307, 166308). In addition, the graphite material thus obtained is heated in a halogen gas atmosphere in another reactor to change the impurities in the graphite material to a substance having a high vapor pressure. It is manufactured through a process consisting of a process (process C) which volatilizes from the material and purifies the graphite material.

  At the time of these graphitization and high-purification treatments, it is generally performed to flow a nitrogen gas around a heater or the like required for heating to prevent oxidation of the heater or the like. This nitrogen gas is trapped between the graphite layers during the graphitization and high-purification treatment, is replaced with carbon atoms, or reacts with metal impurities slightly remaining in the graphite material to form nitrogen compounds. And remain in the graphite material.

  In general, at the time of cooling after the high-purification treatment and the graphitization treatment, cooling in a nitrogen atmosphere is performed in order to increase the cooling rate and prevent oxidation of the graphite material. Nitrogen gas introduced into the furnace during this cooling is also trapped between the layers of the graphite material, replaced with carbon atoms, or reacts with metal impurities slightly remaining in the graphite material to form nitrogen compounds. .

  As described above, when the nitrogen remaining in the graphite material is used as a jig for manufacturing a silicon semiconductor or a compound semiconductor and a jig for manufacturing an epitaxial growth film, the nitrogen penetrates into these semiconductors to increase the nitrogen concentration in the semiconductor. It will be. Nitrogen in these semiconductors is thought to contribute to defects in the crystal.

  In the present embodiment, the graphite material is prevented from being exposed to nitrogen (atmosphere) as much as possible during the graphitization treatment and the high-purification treatment or after the high-purity treatment, or by releasing nitrogen in the post-treatment. Nitrogen trapped between the graphite layers of the material, nitrogen atoms replacing carbon atoms, and nitrogen reacting with slight metal impurities remaining in the graphite material are reduced.

  That is, in producing the graphite material according to the present embodiment, a rare gas such as argon gas or helium gas can be used instead of the nitrogen gas flowing around the heater during the graphitization or the purification process. Furthermore, it is preferable to use a rare gas such as argon gas or helium gas as the atmospheric gas during the graphitization treatment (step B). Further, at the time of the high-purification treatment (step C), after the treatment with the halogen gas, the pressure is 100 Pa or less, preferably 1 Pa or less, or 1800 ° C. or more, preferably in a rare gas atmosphere such as argon gas or helium gas. Heat treatment is performed at 2000 ° C. or higher to release nitrogen existing in some state in the graphite material. Further, at the time of cooling, the cooling treatment is performed under a pressure of 100 Pa or less, preferably 1 Pa or less, or a rare gas atmosphere such as argon gas or helium gas without using nitrogen gas as much as possible. This series of steps can be performed continuously in the same furnace, or each step can be performed in a separate furnace. In addition, the gas that flows around the heater during the high-purity treatment (Step C) is a nitrogen gas during the high-purity treatment with the halogen gas, and a rare gas such as argon gas or helium gas when the high-purity treatment with the halogen gas is completed. May be introduced to perform heat treatment and cooling treatment to release nitrogen gas.

  Further, a graphite material subjected to graphitization treatment and high-purity treatment by a conventional method is also subjected to a pressure of 100 Pa or less, preferably 1 Pa or less, or 1800 ° C. in a rare gas atmosphere such as argon gas or helium gas. As described above, the nitrogen concentration in the graphite material can be reduced by reheating to 2000 ° C. or higher.

In this way, a rare gas such as argon gas or helium gas is used in place of nitrogen gas during the graphitization process or the purification process. Alternatively, if nitrogen is used, the graphite material should be kept as little as possible in the graphite material. Alternatively, a graphite material produced through graphitization treatment or high-purification treatment is separately used at a pressure of 100 Pa or less, preferably 1 Pa or less, or 1800 ° C. or more, preferably 2000 ° C. in a rare gas atmosphere such as argon gas or helium gas. By performing the heat treatment as described above, the nitrogen concentration in the graphite material can be reduced, and the nitrogen concentration by GDMS is 100 ppm or less, preferably 50 ppm or less, more preferably 10 ppm or less, and further preferably 5 ppm or less. it can.
Here, the measurement of the nitrogen concentration by GDMS is performed using a glow discharge mass spectrometer (VG9000, manufactured by VG Elemental) in a vessel in which the ultimate pressure is maintained at 10 ⁻⁴ Pa or less, in the surface of the graphite material or in the pores. The nitrogen gas contained in was exhausted sufficiently.

  Even if the graphite material according to the present embodiment is left as it is, it can be used as a jig for manufacturing a silicon semiconductor or a compound semiconductor and a jig for manufacturing an epitaxial growth film. It can also be used as a base material for SiC-coated graphite material. Thus, when used as a base material for a SiC-coated graphite material, for example, even when used as a jig for producing an epitaxial film of SiC, it is possible to suppress the penetration of nitrogen into the SiC wafer.

  In addition, the graphite material according to the present embodiment is not limited to a silicon semiconductor, a compound semiconductor manufacturing jig and an epitaxial growth film manufacturing jig, or an SiC-coated graphite material used for them. It can also be used for a portion that comes into contact with a neutron or the like of a reactor or a fusion reactor. At this time, if the nitrogen concentration in the graphite material is low, the contact cross-sectional area with neutrons can be kept small. For this reason, it can suppress that the utilization efficiency of a neutron falls. This increases the accuracy of nuclear design and increases efficiency.

  Further, the present invention is not limited to graphite materials, but also carbon fiber reinforced carbon composite materials, expanded graphite sheets, glassy carbon, etc., for example, SiC-coated graphite materials in which SiC is coated on the surface of graphite materials, pyrolytic carbon Can be applied to all carbon-based materials such as pyrolytic carbon-coated graphite material coated with, and the same method as the above graphite material can be applied to materials other than graphite materials.

  Next, the present invention will be described specifically by way of examples.

Example 1
The present invention will be described below step by step. First, a carbon material is installed in the furnace. Next, nitrogen gas is introduced into the furnace, the air inside the container is replaced with nitrogen gas, and then the pressure in the furnace is reduced. And after applying a voltage gradually to a heater and heating the inside of a furnace and keeping the to-be-heated carbon material at 800-1000 degreeC for about 5 hours by the radiant heat (baking process), temperature rising is continued gradually, 2450- The temperature was maintained at 2500 ° C. for 15 hours (graphitization step). Since the inside of the container is maintained at about 0.1 Pa from the start of heating, it is convenient for discharging degass that is slightly volatilized at this stage. During graphitization, or at the stage where graphitization has progressed slightly, a halogen or its compound gas, such as dichlorodifluoromethane (flow rate is in the vessel) in a reduced pressure state (about 0.1 Pa). It is increased or decreased depending on the amount of the carbon material to be heated, but is supplied for about 8 hours (for example, about 1 to 7 lNTP / kg). During the process, argon gas is always flowed around the heater for the purpose of protecting the heater. The graphitization and purification steps are completed by the above method. Subsequently, the continuously graphitized material is maintained at 2200 ° C., and heat treatment is performed for 5 hours while the internal pressure of the container is strongly reduced to 0.1 Pa (denitrification gas process). At this time, argon gas is also used as the gas around the heater to prevent nitrogen gas from entering the graphite material. Thereby, a low nitrogen concentration graphite material can be obtained. And if it heat-processes for a predetermined time, it will cool to 200 degreeC, keeping the pressure in a container at 0.1 Pa. When the temperature reaches 200 ° C., argon gas is introduced into the container as a rare gas and cooled to room temperature. After cooling to room temperature, it was sealed and stored with argon gas in a bag made of a resin film so as not to be exposed to the atmosphere.

(Example 2)
By the same method as in Example 1, the graphite material that had undergone the graphitization and purification steps was once taken out of the processing furnace. At this time, it was sealed and stored together with argon gas in a bag made of a resin film so as not to be exposed to the atmosphere as much as possible. Then, the graphite material is taken out from the bag made of a resin film, placed in the furnace again, reheated to 2200 ° C., and the pressure in the container is strongly reduced to 0.1 Pa, followed by heat treatment for 5 hours (denitrogenation). Gas process). And if it heat-processes for a predetermined time, it will cool to 200 degreeC, keeping the pressure in a container at 0.1 Pa. When the temperature reaches 200 ° C., argon gas is introduced into the container as a rare gas and cooled to room temperature. After cooling to room temperature, it was sealed and stored with argon gas in a bag made of a resin film so as not to be exposed to the atmosphere.

(Example 3)
By the same method as in Example 1, the graphite material that has undergone the graphitization and purification step is reheated to 2200 ° C., and the internal pressure of the container is strongly reduced to 0.1 Pa, followed by heat treatment for 5 hours (denitrogenation). Gas process). When heat treatment is performed for a predetermined time, argon gas is introduced into the container as a rare gas and cooled to room temperature. After cooling to room temperature, it was sealed and stored with argon gas in a bag made of a resin film so as not to be exposed to the atmosphere.

(Comparative Example 1)
A graphite material that has been graphitized and highly purified by the same method as in Example 1 was cooled with nitrogen gas without performing a denitrification gas step, and a material stored in the atmosphere was compared with Comparative Example 1. It was set as the sample of this.

(Comparative Example 2)
As the denitrification gas step, the same operation as in Example 1 was performed, except that the internal pressure of the container was not reduced to 0.1 Pa, but an argon gas atmosphere was set at 2000 ° C. under normal pressure.

(Comparative Example 3)
As the denitrification gas step, the same operation as in Example 1 was performed, except that the internal pressure of the container was not reduced to 0.1 Pa and the argon gas atmosphere was set at normal pressure and 1800 ° C.

In the following samples (1) to (3), the nitrogen concentration contained was measured by GDMS.
(1) Nitrogen concentration in the graphite materials of Examples 1 to 3 and Comparative Examples 1 to 3,
(2) Using the graphite materials of Examples 1 to 3 and Comparative Examples 1 to 3 as a base material, the nitrogen concentration in CVD-SiC when CVD-SiC was formed on the surface of these graphite materials,
(3) Nitrogen concentration in the epitaxially grown film when the CVD-SiC coated graphite material according to (2) is used as a susceptor and used as a jig when an epitaxially grown film is formed on a SiC wafer.

  Table 1 summarizes the nitrogen concentration of each sample.

  From Table 1, it can be seen that the graphite material that has undergone the denitrification gas process has a low nitrogen concentration at each stage. Accordingly, by using the low nitrogen concentration graphite material according to Examples 1 to 3 as a jig for manufacturing a SiC semiconductor, the generation of crystal defects in the SiC semiconductor device can be suppressed.

  From the above, at the time of graphitization treatment or high purity treatment, a rare gas is used, or a low nitrogen concentration carbon-based material, a low nitrogen concentration carbon fiber reinforced carbon composite material by passing through a denitrification gas step after the high purity treatment, A low nitrogen concentration expanded graphite sheet can be used. These low nitrogen concentration carbon-based materials and the like are stored in a state where they are cut off from the atmosphere by using a resin so-called vacuum packing. As a result, when the resin is removed on-site and the low nitrogen concentration carbon-based material is used as a jig for manufacturing a compound semiconductor or the like in a clean state, nitrogen can be prevented from entering the semiconductor device. It has the effect of suppressing crystal defects in the device.

Claims (3)

  A low nitrogen concentration graphite material used for a jig for producing an SiC epitaxial growth film or a jig for producing an SiC single crystal, which has a nitrogen concentration by glow discharge mass spectrometry of 50 ppm or less and is stored in a state of being cut off from the atmosphere.   Low nitrogen concentration carbon fiber reinforced carbon composite material used for SiC epitaxial growth film manufacturing jig or SiC single crystal manufacturing jig stored in a state where the nitrogen concentration by glow discharge mass spectrometry is 50 ppm or less and cut off from the atmosphere .   A low nitrogen concentration expanded graphite sheet used for a jig for producing an SiC epitaxial growth film or a jig for producing a SiC single crystal, which has a nitrogen concentration by glow discharge mass spectrometry of 50 ppm or less and is stored in a state of being cut off from the atmosphere.