PRODUCTION OF SILICON CARBIDE

Abstract: Crystalline beta silicon carbide is deposited on a compatible substract by subjecting a mixture of hydrocarbon vapour and a silane to a glow discharge at a partial pressure of hydrocarbon vapour and silane of not more than 1 Pa while maintaining the substrate at 600 DEG to 1000 DEG C.

Description: H- PRODUCTION OF SILICON CARBIDE The present invention relates to the production of beta silicon carbide. Beta silicon carbide is a potentially valuable semiconductive material. The virtues of beta silicon carbide are discussed by J.D. Parsons et al in Solid State Technology, November 1985 pages 133 - 138. However, Parson et al point out that the problem with beta silicon carbide is how to grow satisfactory material. Beta silicon carbide has been made by vapour phase epitaxy. However, difficulties have been found in depositing beta silicon carbide particularly on substrates other than beta silicon carbide itself. Parsons et al disclose the chemical vapour deposition of SiC on TiC using ion plating with Si which has been evaporated by an electron beam, and acetylene gas as a source of carbon. Substrate temperatures in excess of 1250°C were required " and good quality layers ø only obtained at temperatures over 1400°C. The necessity to use these high temperatures and a special substrate namely TiC is an undesirable restriction of the process. Beta silicon carbide is a crystalline material. It is possible to produce amorphous alloys of silicon and carbon. US 4 459 163 (McDiarmid and Kiss) disclose the production of amorphous silicon by pyrolitic decomposition of silanes on a substrate at a temperature of 390°C. It is stated that methane may be added to the silane to produce the carbon containing alloy. EP 115 125 discloses the production of an amorphous silicon carbon alloy by subjecting a mixture of silane, methane, and hydrogen to a glow discharge in a chamber at 10" 2 torr and with a substrate at 270*C. Although amorphous silicon carbon alloys can be deposited at much lower temperatures than are required to make crystalline silicon carbide the resulting alloys do not have the advantageous electronic properties of the crystalline material. FR 2 159 592 states that a monocrystalline layer of beta-silicon carbide may be deposited on monocrystalline silicon by heating a silicon substrate in a mixture of hydrocarbon, silane, and hydrogen fed over the silicon. No details are given of the temperature or pressure conditions used. We have not found it possible to deposit monocrystalline beta-silicon carbide directly on monocrystalline silicon at the relatively low temperatures which are desirable for a commercial process. DE 34 15 799 discloses the deposition of polycrystalline SiC on monocrystalline silicon by chemical vapour deposition followed by deposition of single crystal SiC. The first deposition step is carried out at 800-1200 β C, preferably 1000-1100°C atmospheric pressure and the second stage is at 1200-1400°C atmospheric pressure. The possibility of using reduced pressures e.g. 0.01-100 torr (corresponding to a minimum of 1.33 Pa) is mentioned but no examples are given. Chemical vapour deposition has the disadvantage that it is necessary to use high temperatures to obtain satisfactory levels of deposition. At high tempβ-ratures close to the melting, point of silicon there will be a greater tendency for contaminants in the substrate to be released and to adversely affect the properties of the silicon carbide layer. The difference in thermal expansion coefficients between the silicon substrate and the SiC layer will also be larger leading to poorer quality. We have now found an improved process for making crystalline beta silicon carbide. According to the present invention a process for the deposition of crystalline beta silicon carbide on a compatible substrate comprises subjecting a mixture of hydrocarbon vapour and a silane to a glow discharge at a partial pressure of hydrocarbon vapour and silane of not more than 1 Pa while maintaining the substrate at a temperature in the range 600 to 1000 β C. The substrate is a compatible substrate, ie it must be capable of surviving the conditions used and its lattice structure must be compatible with that of beta silicon carbide. An example of a suitable compatible substrate is TiC. Crystalline silicon is not a compatible substrate. However a compatible substrate can be produced by producing a layer on the surface of the crystalline silicon with a composition varying from pure silicon to a composition corresponding to SiC. This may be done in two ways. One method is to introduce carbon into the upper layer of a crystalline silicon substrate by bringing the heated substrate into contact with a hydrocarbon gas while subjecting the gas to a glow discharge. Another method is to deposit an epitaxial silicon layer on the crystalline silicon substrate by glow discharge in a gas mixture containing silane while progressively introducing a hydrocarbon into the reactor to raise- the Si:C ratio in successive portions in the layer deposited on the crystalline silicon until it reaches a value corresponding to SiC. The glow discharge process of the present invention is a PECVD process (Plasma Enhanced Chemical Vapour Deposition). An alternative method is to bring the silicon substrate into contact with a hydrocarbon vapour at elevated temperatures e.g. 800°C-1000°C in the absence- of a glow discharge and at low pressures e.g. 10~ 4 Pa to 10 -2 Pa. The substrate is preferably thoroughly cleaned before use. If the substrate has any tendency to form an oxide film on its surface, which would tend to adversely effect the deposition of the crystalline layer, then the oxide may be removed by heating the substrate in the presence of hydrogen subjected to a glow discharge before any buffer layer, and then the silicon carbide, is deposited. The hydrocarbon vapour is preferably provided by hydrocarbon containing 1 to 7 carbon atoms in the molecule for example methane or ethylene. The silane may for example be SiH . The silicon carbide deposition process may be carried out in the presence of hydrogen. The carbon:silicon ratio of the hydrocarbon/silane mixture are preferably maintained -at a value very slightly greater than 1:1 preferably at a value corresponding to a C:(C+Si) ratio in the range 0.5 to 0.7, during the deposition in order to give the desired 1:1 atomic ratio in the SiC. The substrate temperature is preferably in the range 800"C to 1000 β C. The total partial pressure of the silane and hydrocarbon must be below 1 Pa, preferably below 0.5 Pa. The total pressure must not of course be sufficiently high to inhibit the initiation of a glow discharge, and is preferably not greater than 1000 Pa. The glow discharge is produced by the application of radio frequency energy. Relatively low frequency radio frequency energy (e.g. about 100 kHz) is used to produce induction heating of substrate holders in some techniques of preparing semi-conductor materials. The frequencies required to produce glow discharges will generally be considerably higher than those used for induction heating e.g. above 1 MHz, preferably above 10 MHz. In general it is preferred to use higher powers than are conventional in glow discharge deposition techniques, e.g. powers in the range 0.1 to 10 watts7_m 2 . Lower pressures enable optimum results to be obtained at lower temperatures. However for a given deposition method a very low pressure will lead to low deposition rates. The duration of the deposition step is dependent on the thickness of the layer required. The invention will now be described with reference to the following examples. Example 1 A pre-cleaned single crystal silicon substrate was clamped on a substrate heater (capable of reaching 1000°C) inside a vertical tube vacuum chamber. The sample was de-gassed at pressures below 10~" torr (1.3 x lO -1 * Pa) and at a silicon surface temperature approximately 850°C for 45 minutes. Hydrogen was then introduced into the chamber, bringing the pressure up to 8 x 10-^ torr (106 Pa) . A glow discharge was initiated within the chamber for approximately 30 minutes, to remove the native oxide from the silicon surface. The glow discharge was initiated using high frequency radio frequency energy at a frequency of 13.56 MHz and a power of 30 watts. With the hydrogen discharge still operating the hydrogen flow rate was reduced and a low flow rate of C2H was introduced into the reactor. Finally the flow rate of hydrogen was reduced to zero. A pure C2H discharge was maintained for a further 10 minutes at a pressure of 5 x 10"^ torr. Subsequently the C2H4 flow rate was- reduced and a small quantity of SiH was added to the gas phase, maintaining the total pressure at 5 x 10"^ torr (0.7 Pa). The discharge was run for at least a further 100 minutes, at which point the run was terminated. The material deposited on the silicon was shown by X-ray diffraction to be crystalline beta silicon carbide with a preferred orientation. Example 2 An experiment was carried out as in Example 1 but using methane in place of ethylene. Crystalline beta silicon carbide was produced as in Example 1. '' - Example 3 An experiment was carried out as in Example 1 but with the following differences. The hydrogen discharge was terminated after the oxide removal step. The ethylene vapour was then introduced into the chamber at a pressure of 2 x 10~* Pa and the silicon maintained at a temperature of 900°C for 10 minutes. A mixture of ethylene, silane vapour and hydrogen was then introduced into the chamber at a total pressure of 20 Pa (the ratio of ethylene to silane was such as to give a carbon silicon atom ratio of 1:0.9. The total partial pressure of ethylene and silane was 0.5 Pa. The discharge was continued for at least 100 minutes as in Example 1. The material deposited on the silicon was shown to be crystalline beta silicon carbide with a preferred orientation.

Claims: -I- Claims: 1. A process for the deposition of crystalline beta-silicon carbide on a compatible substrate which comprises subjecting a mixture of hydrocarbon vapour and a silane to a glow discharge at a total pressure of hydrocarbon and silane of not more than 1 Pa while maintaining the substrate at a temperature in the range 600-1000 I, C. 2. A process according to claim 1 wherein the total pressure is not more than 1 Pa. 3. A process according to claim 1 wherein the compatible substrate is crystalline silicon having an upper layer containing carbon. 4. A process according to claim 2 wherein the crystalline silicon having an upper layer containing carbon is produced by depositing an epitaxial silicon layer on the crystalline silicon substrate by subjecting a silane to a glow discharge and progressively introducing a hydrocarbon into the gas mixture to raise the Si:C ratio in successive portions of the layer deposited on the crystalline silicon until it reaches a value corresponding to SiC. 5. A process according to claim 3 wherein the crystalline silicon having an upper layer containing carbon is produced by bringing the silicon substrate into contact with a hydrocarbon vapour at elevated temperature and low pressure in the absence of a glow discharge. 6. A process according to any one of the preceding claims wherein the hydrocarbon vapour contains 1 to 7 carbon atoms in the molecule. 7. A process according to claim 4 wherein the hydrocarbon is ethylene. 8. A process according to any one of the preceding claims wherein the silane is SiH . 9. A process according to any one of the preceding claims wherein the deposition of silicon carbide is carried out in the presence of hydrogen. 10. A process according to any one of the preceding claims wherein the carbon:silicon carbide ratio of the hydrocarboπ/silane mixture are maintained at a value very slightly greater than 1:1 during the deposition of the silicon carbide. 11. A process according to any one of the preceding claims wherein the substrate temperature is in the range 800-1000*C. 12. A process according to any one of the preceding claims wherein the pressure is below 0.5 Pa.

Inventors: THWAITES MICHAEL JOHN, THWAITES, MICHAEL, JOHN
Applicants: BRITISH PETROLEUM CO PLC, THE BRITISH PETROLEUM COMPANY P.L.C.
Application year: 1987
Main classification: C23C16
IPC codes: C01B31, C23C16, C30B25, C30B29, H01L21, H01L33