Diamond is an excellent material for Microelectromechanical Systems (MEMS) due to its superlative material properties compared to commonly used materials such as silicon. In its single crystalline form, diamond offers properties such as very high Young’s modulus, low thermal coefficient of expansion, and very high thermal conductivity as well as being chemically inert. Specifically, diamond’s high thermal conductivity offers the potential for very low thermoelastic damping (TED) in micromechanical resonators. Polycrystalline (Poly-C) diamond films deposited by Hot Filament Chemical Vapor Deposition (HFCVD) have been explored in this research. HFCVD Poly-C diamond films are easy and inexpensive to depositat wafer-scale, and retain many material properties that single crystalline diamond possesses
The first part of the study focuses on fabrication and testing of high quality factor micro-resonators such as double-ended tuning forks (DETF) and micro-cantilevers fabricated from microcrystalline and nanocrystalline diamond films with grain sizes ranging from 20 nm (nanocrystalline films) to 2.5μm(microcrystalline films)The aim of this first study was to determine whether the quality factor of HFCVD diamond resonators could reach the limits imposed by intrinsic dissipation mechanisms such as thermoelastic damping (TED). Previous studies showed large differences between measured Q-factors and the TED limit, in part because these studies assumed thermal conductivity similar to that of single crystalline diamond. Here the thermal conductivity of NCD and MCD films was measured using time-domain thermoreflectance (TDTR), and the resulting Q-factor measurements were shown to agree well with the theoretical TED limit.
The second part of this research focuses onidentifying the causes of dissipation in diamond resonators and suggesting approaches to improve the mechanical Q-factor. It is shown that, besides variations in deposition conditions such as decreasing the CH4:H2 ratio and increasing the deposition temperature, the thermal conductivity can be improvedby increasing the thickness of the film and decreasing the wafer-to-filament distance to increase the abundance of monatomic hydrogen. Applying the suggested methods, the thermal conductivity was increased threefold to ~300 Wm-1K-1. Finally, we identified the limiting dissipation mechanism of low-frequency micro-resonators to be surface loss. To reduce surface loss, step-by-step annealing was performed and the Q-factor was increased to a maximum value of 365,000, the highest reported Q for any flexural poly-crystalline resonator to date.
December 31, 2014
Motieian Najar, M. H. (2014). Design and Analysis of High Quality Factor Chemical Vapor Deposition (CVD) Diamond Micromechanical Resonators. United States: University of California, Davis.