DIAMOND SYNTHESIS

Abstract: The synthesis of diamond (both individual crystallites and continuous films) at enhanced rates by the use of chemical vapour deposition in which carbon containing precursor have an enriched level of isotopic carbon 13 (<13>C).

Description: D I A M O N D S Y N T H E S I S Introduction: The isotopic composition of natural diamond, or synthetic diamond produced at high pressure and temperature, is typically 1.1% 13 C and 98.9% 12 C. This ratio is the same as occurs naturally in the environment. Diamond can also be produced at low temperature (<1200°C ) and pressure (<760torr) using chemical vapour deposition (CVD) techniques in laboratory apparatus. One such technique is plasma assisted CVD (PACVD) where a mixture of carbon containing precursor (hydrocarbon, alcohol or ketone) is diluted in H2 to typically 0.1 - 10% by volume. A plasma is used to dissociate the hydrocarbon into CHx radicals (where x = 1 , 2 or 3) which form diamond on contact with a substrate held at temperatures between 400° and 1200°C. The plasma also dissociates H2 into 2H° which etches away any non-diamond deposits. Another CVD technique is thermal CVD where a tungsten filament (held at _ 2000°C) replaces the plasma in the process described above. The deposition pressure in both cases can be varied between 5 and 750torr and higher diamond growth rates are achieved at higher pressures, higher carbon precursor concentrations and higher substrate temperatures. The diamond deposits achieved from these two different techniques are very similar. Isotopic analysis of these low pressure CVD diamond posits surprisingly show a significant increase in the 13 C content of the deposit over that measured in the precursor input gas. This implies SUBSTITUTE SHEET that the diamond synthesised at low pressures and temperatures, by CVD techniques, has a different crystal growth mechanism to that seen in diamond synthesised in the high pressure, high temperature regime, and that the carbon isotope concentrations are important to the CVD synthesis process in general. SUBSTITUTE SHEET The fundamental difference between 13 C and 12 C is the nuclear mass and nuclear spin. 12 C is a boson and has spin 0 whilst 13 C is a fermion and has a spin 1/2 and thus possesses a magnetic moment. During the synthesis of diamond by CVD techniques that incorporate an electric field (both thermal CVD and PACVD have such -a field) it is probable that the difference in nuclear spin, between 13 C and 12 C, is significant to the growth process. It is also possible that the increased mass of 13 C over that of 12 C increases the residence time of the 13 C atom on the growing diamond surface and therefore increases the likelihood of incorporating the 13 C atom on that surface. The nuclear spin and magnetic moment (present in 13 C) could also couple into the electric or magnetic field present in the discharge and hence cause rotation of the 13 C atom. This could increase the energy of the atomic species and thus allow a reduction in synthesis temperature as the 13 C atoms are at a higher excitation level, that 12 C atoms. Invention : The point of this invention is to enhance the growth rate of diamond deposits produced by a low pressure CVD techniques, by increasing the 13 C: 12 C ratio in the precursor gas ( 13 C enrichment). In natural carbon containing compounds, this ratio is typically 1.1% 13 C to 98.9% 12 C; however, the ratio can be increased from 1.1% to a fully enriched level of 100% 13 C by isotope separation techniques. SUBSTITUTE SHEET By increasing the 13 C: 12 C ratio ( 13 C enrichment), in the carbon containing precursor gases, an increase in diamond deposition rate would be achieved (at a similar temperature and pressure) that when using an un-enriched precursor. Similarly, the deposition temperature could be reduced when using l3 C enriched precursor to achieve the same deposition rate as a higher temperature with an un-enriched precursor input gas. The increase in rate can be realised by using either fully 13 C enriched precursor or mixtures of fully enriched with un-enriched (to achieve partial enrichment of 13 C) to any desired level, as input gas. SUBSTITUTE SHEET

Claims: CLAIMS: 1 . A method of synthesising diamond by chemical vapour deposition comprising the step of feeding, as precursors, carbon containing compounds in which the carbon 13 ( 13 C) content is τ_ r» τ» _-* o -'^ 2. A method as claimed in claim 1 wherein the carbon 13 ( 1 C ) content is greater than 1.1 %. 3. A method as claimed in claim 1 or 2 wherein the substrate temperature is below 1200°C. 4. A method as claimed in claim 1 , 2 or 3 wherein the pressure is maintained at less than 760 ton-. 5. A method as claimed in any preceding claim wherein the precursors are deposited in a plasma. 6. A method as claimed in any of claims 1 to 4 wherein the precursors are deposited by a heated tungsten filament. 7. A method of synthesising diamond substantially as hereinbefore described. 8. Synthetic diamonds in which the carbon 13 ( 1 3 C) isotope content has been increased by enrichment of the level of carbon 13 ( 13 C) in precursive carbon containing compounds. SUBSTITUTE SHEET

Inventors: WORT CHRISTOPHER JOHN, WORT, CHRISTOPHER, JOHN
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1990
Main classification: C30B29
IPC codes: C23C16, C30B25, C30B29