Investigation of the coefficient of thermal expansion in nanocrystalline diamond films

Abstract Nanocrystalline diamond films are a promising class of nanomaterials with tunable properties. An especially appealing field of application for NCD are nitrogen doped semiconducting films. The residual stress in the films is an important film property directly influencing the adhesion of the film on the substrate and thus the film performance. The residual stress consists of two components: a thermal part due to the different coefficients of thermal expansion of film and substrate and an intrinsic part. The residual stress in most films that are deposited at high temperatures is dominated by an effect arising from the difference in coefficients of thermal expansion in film and substrate. By measuring the residual stress in the film at different temperatures it is possible to calculate the coefficient of thermal expansion of the films. For this purpose an ex-situ optical device was used to measure the residual stress of the film. Nanocrystalline diamond films were deposited by microwave-plasma CVD at a pressure of 200 mbar from an Ar/H2/CH4 plasma while the hydrogen fraction in the process gas and the substrate temperature were varied between 3 to 6% to 600 to 800 °C respectively. To investigate the influence of the nitrogen admixture in the plasma on the thermal expansion coefficient more films were deposited at a pressure of 200 mbar with admixtures of nitrogen of 2.5% and 7.5%. It is shown that by controlling the process parameters the coefficient of thermal expansion in the NCD films can be matched with the silicon substrate for insulating as well as for conductive films and therefore the thermal stress component be minimized. The results are important for the development of MEMS where silicon as a substrate is widely used.