Combustion assisted CVD of diamond films

Abstract

Diamond films have been grown at low pressures on non-diamond substrates for over two decades using a number of different plasma based techniques. This has attracted extensive research investigating the mechanism of how diamond can nucleate and grow in the thermodynamically hindered zone under metastable conditions. The outstanding mechanical, thermal, optical and electrical properties of diamonds and the new and novel uses of materials such as diamond films have promoted considerable research in this area. In this work, diamond films were grown, largely by combustion assisted chemical vapour deposition using an oxy-acetylene combustion flame producing high quality, polycrystalline diamond films at growth rates between 50 and 100µm/hr. It is distinguished from other methods by its high growth rates, low operating costs and easy modification of growth conditions, making it a versatile method for the study of diamond deposition. The use of electrical biasing with diamond deposition has been proposed to enhance diamond nucleation. This was initially investigated here using the more conventional microwave assisted CVD method. The application of an electrical bias applied to silicon substrates in conjunction with a relatively high carbon/hydrogen gas ratio resulted in significant changes to the surface of silicon wafers over those treated without the application of an electrical bias. Conical structures with a maximum density of 6.8 x 1014/cm2 formed on the surface and are similar to those found by other researchers and identified as α and β-SiC, well-known precursors to the nucleation and growth of diamond. Electrical biasing was then applied to the CACVD setup. The effects of this treatment were examined, particularly the impact on improving the formation of silicon carbide and subsequent diamond nucleation. XPS analysis indicates that the initial removal of the native oxide from silicon substrates with HF facilitated the formation of silicon carbide, increasing the amount of the substrate converted to carbide from l0% to nearly 20%. Application of electrical biasing further improved this figure to a maximum of 30% and importantly, reduced the competing formation of silicon dioxide. The optimum bias treatment time was between 5 and l0 minutes. Changes in the substrate surface composition during the different stages of diamond deposition were monitored by the amount of electron emission from the surface while undergoing the negative electrical biasing treatment. Emission currents varied between 5 and 30 mA and are attributed to the growth of diamond and changes in the diamond surface morphology, rather than changes in the gas composition of the flame. Thus, this provides an in situ monitoring of the deposition process and was used as a monitor to indicate the state of the substrate surface and to allow control of the deposition process. Scanning of substrates with single and double flame burners to coat larger surfaces was shown to be possible. Pretreatment of the silicon substrates involved producing a carburised layer to enhance nucleation and ensure formation of a complete diamond film on the following over-scan by the combustion flame. The tail gas etching effects of the retreating carburising flame and the advancing diamond depositing flame significantly eroded this layer when it was exposed for more than 30 minutes, thus causing a reduction in diamond nucleation and poor film coverage. The diamond films themselves were able to withstand the etching effects of the tail gases with no notable changes to the surfaces. Haynes alloy is an example of a nickel-based material that exhibits high resistance to carbide formation and thus was used as a challenging test case for diamond deposition as the alloy forms a mixed oxide scale consisting of chromium and aluminium oxides at high temperatures. Segregated regions within the alloy surface were shown by EDX and Raman spectroscopy to contain significant levels of aluminium that reacted with oxygen creating a barrier to carbon diffusion. This discouraged the formation of sp2 carbon, allowing diamond nucleation to occur rapidly, directly on the metal surface. Furthermore, the nickel/iron rich surface area surrounding the aluminium rich segregated areas catalysed the formation of graphite and amorphous carbon. This produced a weakened diamond/metal interfacial bond that consequently led to the easy delamination of the deposited diamond film. These segregated areas also produced the unusual patterning seen in the diamond films.