Nanocrystalline diamond films for mechanical applications

Abstract

Nanocrystalline diamond films have been grown in a hot filament CVD (HFCVD) system allowing BEN, textured growth and HOD growth of diamond on 4 silicon (100). The growth process of nanocrystalline diamond discussed here consists of a BEN process for diamond nucleation on silicon, with a density of more than 10 10 cm -2 across the entire wafer surface. The subsequent growth of the nanocrystalline diamond films is achieved by addition of nitrogen into the gas phase with a relative concentration to carbon of 1:1. At substrate temperatures between 680 °C and 740 °C, closed diamond films are grown with thicknesses between 1 pm and 20 pm. At low nitrogen concentrations the nanocrystalline diamond films exhibit still a columnar structure with a lateral grain size below 200 nm whereas the vertical grain size can reach several microns. Higher nitrogen concentrations lead to a nanocrystalline diamond growth mode with no columnar structure and a grain size is no longer detectable by SEM. The advantages of diamond in MEMS technology rely on its outstanding mechanical and thermal properties. Additionally, the stress distribution inside the diamond films due to thermal expansion as well as the polycrystalline nature of the film has to be taken into account. Applying different test structures we could measure fracture strength of more than 4.0 GPa and elasticity moduli of up to 1020 GPa for these nanocrystalline films. The intrinsic vertical and horizontal stress remains in most cases below the detection limit of 5 MPa, so that freestanding structures with a length of several millimeters can be fabricated without noticeable bending.