Mechanical characterization and stress engineering of nanocrystalline diamonds films for MEMS applications

In this work we discuss growth and properties of nanocrystalline diamond films grown with a HFCVD system on 4 in. silicon (100) wafers. Nucleation was performed by an in situ bias pretreatment with nucleation densities of more than 10(10) cm 2. Growth of nanocrystalline films has been accomplished by reduction of the concentration of atomic hydrogen. This is obtained by an increase in the recombination of hydrogen radicals either by addition of nitrogen to the gas phase or by an increase of the total pressure during growth. The concentration of nitrogen to carbon atoms in the gas phase can reach values of 10 and more and pressures varies from 1 to 9.5 kPa. In this nanocrystalline morphology Young's moduli of the diamond films between 800 and 980 GPa as well as fracture strengths of more than 3 GPa can be obtained. The renucleation process which produces this nanosized diamond material with a grain size of less than 60 nm creates also a homogeneous growth pattern, which leads to an almost vanishing vertical stress gradient across the film thickness. Variations of growth parameters such as pressure and substrate temperature result in different compressive stress values between 0 and 390 MPa. The possibility to control and adjust the absolute value of the stress inside the diamond film with respect to the silicon substrate allows to use the built-in stress as design parameter of MEMS devices and engineer for example bistable membrane configurations.