Abstract:
The present invention provides a process for depositing diamond by chemical vapor deposition without using conventionally-used hydrogen, which is an explosive gas, as a reaction gases. The process includes contacting a substrate with a two-component gas mixture, under the conditions of a substrate temperature of 150° C.-900° C., a pressure of 1-50 torr, an input microwave power of 250-450 W. The two-component gas mixture is a hydrocarbon (C x  H y ) plus CO 2  with a flow rate ratio of the C x  H y  to CO 2  of 0.2-0.8, or a gasified liquid state oxygen-containing hydrocarbon (C x  H y  O z ) plus CO 2  with a flow rate ratio of the C x  H y  O z  to CO 2  of 12-17. High quality diamond can be obtained even at low temperature of 180° C.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for depositing diamond by chemical vapor deposition, more particularly to a process for depositing diamond without using conventionally-used hydrogen, which is an explosive gas, as a reaction gas, and to a process for depositing diamond at temperature below 400° C. 
     2. Description of the Prior Art 
     Diamond, which is the hardest substance (about Hv 10,000 kg/mm 2 ) in the world, has very high thermal conductivity (about 24 watt/°C.-cm), and good insulating properties (about 10 13  -10 16  Ωcm/15° C.). In addition, it is transparent within the ultra-violet, visible and infra-red spectrum. Diamond is also resistant to chemical corrosion, low in friction, and high in sonic propagation speed. Therefore, diamond is a valuable material with many special characterstics not found in other materials. For example, diamond may become semi-conductive if a small amount of phosphorols or boron is added to a diamond film. As a result, diamond is regarded as an important material in electronic and defense industry application. 
     Synthetic diamond was previously used in the manufacture of tools. In the past few years, techniques for producing artificial diamond have improved greatly, expanding the range of applicability of synthetic diamond into such industries as optical instruments, semi-conductor elements, VLSI integrated circuit heat sinks, horn-like vibrating plates, the reflection resistant film of solar batteries, and various other mechanical tools. 
     Diamond films are usually synthesized from gas mixtures that include a small amount of hydrocarbon in hydrogen which is an explosive gas, and the typical growth temperature is 700°-1000° C. For many applications, it is important to grow diamond films at low temperatures (below 500° C.), because many substrate materials, such as GaAs, plastics, ZnS and MgF 2 , are unstable at high temperature. 
     Deposition of diamond films at low temperature can be achieved by altering deposition techniques and reaction gases. 
     For example, Liou et al. use microwave plasma chemical vapor deposition (MPCVD) to deposit diamond. The reaction gases are H 2  /CH 4  /O 2 . Diamond can be deposited at temperatures above 400° C. (Appl. Phys. Lett. 56(5), p. 437(1990)). 
     Yasuchi et al. also synthesize diamond by MPCVD. The reaction gases are CO/O 2  /H 2 . It is confirmed that diamond can be grown at temperatures above 400° C. (Surface and Coatings Technology, 47(1991), p. 1). 
     Wei et al. disclose that diamond films can be grown by magneto-microwave plasma CVD with CH 4  +CO 2  /H 2  gas mixture at temperature above 500° C. (Journal of Crystal Growth, 99(1990), p. 1201). 
     All the above processes for depositing diamond have the common problem that a large amount of H 2  which is an explosive gas is required, thus causing possible danger. Another problem is that the deposition must be carried out at temperatures higher than 400° C., thus requiring high energy and limiting the substrates which can be used. 
     SUMMARY OF THE INVENTION 
     An object of this invention is therefore to solve the above-mentioned problems and to provide a process for depositing high quality diamond without the use of an explosive gas i.e., hydrogen. 
     Another object of this invention is to provide a process for depositing high quality diamond at a much lower temperature, i.e., at a temperature below 400° C. Also, the growth rate of diamond is relatively fast. 
     To achieve the above object, the process for depositing diamond of the present invention uses a chemical vapor deposition process, and includes contacting a substrate with a two-component gas mixture, under the conditions of a substrate temperature of 150° C.-900° C., a pressure of 1-50 torr, an input microwave power of 250-450 W. The two-component gas mixture can be a hydrocarbon (C x  H y ) plus CO 2  with a flow rate ratio of the C x  H y  to CO 2  of 0.2-0.8, or a gasified liquid state oxygen-containing hydrocarbon (C x  H y  O z ) plus CO 2  with a flow rate ratio of the C x  H y  O z  to CO 2  of 12-17. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of the microwave plasma CVD apparatus of the present invention. 
     FIGS. 2a and 2b are the SEM micrograph of the diamond film obtained from Example 1 of the present invention. 
     FIGS. 3a and 3b are the SEM micrograph of the diamond film obtained from Example 2 of the present invention. 
     FIGS. 4a and 4b are the SEM micrograph of the diamond film obtained from Example 3 of the present invention. 
     FIG. 5 is the SEM micrograph of the diamond film obtained from Example 4 of the present invention. 
     FIGS. 6a and 6b are the SEM image of surface morphology and cross-sectional view of the diamond film obtained from Example 5 of the present invention respectively. 
     FIG. 7 is the SEM image of surface morphology of the diamond film obtained from Example 6 of the present invention. 
     FIG. 8 is the SEM image of surface morphology of the diamond film obtained from Example 7 of the present invention. 
     FIG. 9 is the SEM image of surface morphology of the diamond film obtained from Example 8 of the present invention. 
     FIG. 10 is the cross-sectional view of the diamond film obtained from Example 9 of the present invention. 
     FIGS. 11a and 11b are the SEM image of surface morphology and cross-sectional view of the diamond film obtained from Example 10 of the present invention respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, the process for depositing diamond can be a chemical vapor deposition (CVD) process, such as microwave plasma CVD process, R.F. plasma CVD process and hot filament CVD process. 
     In the following examples, the microwave plasma CVD process is used to deposit diamond. A schematic diagram of the apparatus for performing this process is shown in FIG. 1. In this figure, a reaction chamber 1 is connected to a microwave generator 3 with a waveguide 2, and is also connected at the bottom to a vacuum system 4 and at the top to gas supply systems 8 and 9 with a gas supply line 5. A substrate holder 6 is disposed in the reaction chamber 1 to hold a substrate. The substrate holder 6 is cooled by a cooling system 7 connected to the reaction chamber 1. The reaction gases to be deposited, for example, CO 2  and CH 4 , are supplied from their respective gas supply systems 8 and 9 via flow controllers 10 into the reaction chamber 1. 
     The reaction gases suitable for use in the present invention are a two-component gas mixture. The two-component gas mixture can be a hydrocarbon (C x  H y ) plus CO 2  or a gasified liquid state oxygen-containing hydrocarbon (C x  H y  O z ) plus CO 2 . Examples of the C x  H y  are CH 4 , C 2  H 2 , C 2  H 4 , C 3  H 8 , C 4  H 10  and C 6  H 6 . Examples of the C x  H y  O z  are C 3  H 6  O 3 , CH 3  OH, CH 3  COCH 3 , CH 3  COOCH 3 , C 2  H 5  OH and C 2  H 5  OC 2  H 5 . 
     According to the present invention, the substrate temperature is controlled in the range of 150°-900° C., the pressure in the range of 1-50 torr, the input microwave power in the range of 250-450 W, the flow rate of the C x  H y  to CO 2  in the range of 0.2-0.8, and the flow rate of the C x  H y  O z  to CO 2  in the range of 12-17. 
     The substrate temperature is measured with an optical pyrometer end a K-type thermocouple in direct contact with the graphite substrate holder. The exact temperature of the growth surface is calibrated by the melting point method, such as lead (m.p.=335° C.) and tin (m.p.=243° C.). Thus, we can be sure that the exact substrate surface temperature, is 65°±10° C. higher than the thermocouple readout. The values for the temperature shown in the present invention are the true substrate surface temperature. 
     The following specific examples are intended to demonstrate this invention more fully without acting as a limitation upon its scope, since numerous modifications and variations will be apparent to those skilled in the art. 
     EXAMPLE 1 
     The apparatus as illustrated in FIG. 1 was employed in this example. A silicon wafer substrate (10 mm×10 mm) was scratched with diamond powder and then was placed on the substrate holder 6, having a diameter of 15 mm, located at the center of the reaction chamber 1 with an outer diameter of 55 mm. During the deposition process, the substrate was heated up to 850° C. by microwave plasma. The pressure in the reaction chamber 1 was maintained at 25 torr and the input microwave power was 400 W. The reaction gases were CO 2  --C 2  H 2  and the flow rates of CO 2  and C 2  H 2  were 30.0 ml/min and 14 ml/min respectively. After a total deposition time of 3 hours, a 2.7 μm thick diamond film of good crystallinity was obtained. The growth rate of the diamond film was 0.9 μm/hr. The SEM micrograph of the diamond film is shown in FIG. 2. To obtain a high quality diamond film, when the flow rate of CO 2  is kept at 30.0 ml/min, the flow rate of C 2  H 2  is preferably maintained in the 13.8-14.5 ml/min range. 
     EXAMPLE 2 
     The same apparatus and process as described in Example 1 were employed except that the reaction gases used were CO 2  --CH 4  with flow rates of 30.0 ml/min and 21.1 ml/min respectively. After a total deposition time of 3 hours, a 6 μm thick diamond film of good crystallinity was obtained as shown in FIG. 3. The growth rate of the diamond film was 2 μm/hr. To obtain a high quality diamond film, when the flow rate of CO 2  is kept at 30.0 ml/min, the flow rate of CH 4  is preferably maintained in the 20.6-21.7 ml/min range. 
     EXAMPLE 3 
     The same apparatus and process as described in Example 1 were employed except that the reaction gases used were CO 2  --C 3  H 8  with flow rates of 30.0 ml/min and 8.6 ml/min respectively. After a total deposition time of 3 hours, a 6 μm thick diamond film of good crystallinity was obtained as shown in FIG. 4. The growth rate of the diamond film was 2 μm/hr. To obtain a high quality diamond film, when the flow rate of CO 2  is kept at 30.0 ml/min, the flow rate of C 3  H 8  is preferably maintained in the 7.9-9 ml/min range. 
     EXAMPLE 4 
     The same apparatus and process as described in Example 1 were employed except that the reaction gases used were CO 2  --C 3  H 6  O 3  with flow rates of 1.35 ml/min and 20 ml/min respectively. After a total deposition time of 3 hours, a 11 μm thick diamond film of good crystallinity was obtained as shown in FIG. 5. The growth rate of the diamond film was 3.67 μm/hr. To obtain a high quality diamond film, when the flow rate of C 3  H 6  O 3  is kept at 20 ml/min, the flow rate of CO 2  is preferably maintained in the 1.2-1.6 ml/min range. 
     EXAMPLE 5 
     The same apparatus and process as described in Example 1 were employed except that the reaction gases used were CO 2  --CH 4  with flow rates of 30.0 ml/min and 18.0 ml/min respectively, the pressure was 25 torr, the input microwave power was 290 W, the substrate temperature was 340° C., and the total deposition time was 24 hours. The growth rate of the diamond film was 0.3 μm/hr, and that of the diamond granule as 0.8 μm/hr. The SEM image of surface morphology and cross-sectional view of the obtained diamond film were shown in FIGS. 6a and 6b respectively. To obtain a high quality diamond film, when the flow rate of CO 2  is kept at 30.0 ml/min, the flow rate of CH 4  is preferably maintained in the 17-21 ml/min range. 
     EXAMPLE 6 
     The same apparatus and process as described in Example 5 were employed except that the total deposition time was 4 hours. The SEM image of surface morphology of the obtained diamond film was shown in FIG. 7. It proves that, at such a condition, only 4 hours are needed to obtain a high quality and continuous diamond film. 
     EXAMPLE 7 
     The same apparatus and process as described in Example 1 were employed except that the reaction gases used were CO 2  --CH 4  with flow rates of 30.0 ml/min and 20.5 ml/min respectively, the pressure was 25 torr, the input microwave power was 290 W, the substrate temperature was 340° C., and the total deposition time was 3 hours. The growth rate of the diamond film was 0.3 μm/hr. The SEM image of surface morphology of the obtained diamond film was shown in FIG. 8. 
     EXAMPLE 8 
     The same apparatus and process as described in Example 1 were employed except that the reaction gases used were CO 2  --CH 4  with flow rates of 30.0 ml/min and 17.5 ml/min respectively, the pressure was 25 torr, the input microwave power was 290 W, the substrate temperature was 300° C., and the total deposition time was 24 hours. The growth rate of the diamond film was 0.25 μm/hr. The SEM image of surface morphology of the obtained diamond films was shown in FIG. 9. To obtain in a high quality diamond film, when the flow rate of CO 2  is kept at 30.0 ml/min, the flow rate of CH 4  is preferably maintained in the 16-20 ml/min range. 
     EXAMPLE 9 
     The same apparatus and process as described in Example 1 were employed except that the reaction gases used were CO 2  --CH 4  with flow rates of 30.0 ml/min and 17.0 ml/min respectively, the pressure was 10 torr, the input microwave power was 290 W, the substrate temperature was 270° C., and the total deposition time was 24 hours. The growth rate of the diamond film was 0.24 μm/hr. The cross-sectional view of the obtained diamond film was shown in FIG. 10. To obtain a high quality diamond film, when the flow rate of CO 2  is kept at 30.0 ml/min, the flow rate of CH 4  is preferably maintained in the 16-20 ml/min range. 
     EXAMPLE 10 
     The sane apparatus and process as described in Example 1 were employed except that the reaction species used were CO 2  --CH 4  with flow rates of 30.0 ml/min and 16.9 ml/min respectively, the pressure was 4 torr, the input microwave power was 290 W, the substrate temperature was 180° C., and the total deposition time was 24 hours. The growth rate of the diamond film was 0.13 μm/hr. The SEM image of surface morphology and cross-sectional view of the obtained diamond film were shown in FIGS. 11a and 11b respectively. To obtain a high quality diamond film, when the flow rate of CO 2  is kept at 30.0 ml/min, the flow rate of CH 4  is preferably maintained in the 16-19 ml/min range.