Quality factor enhancement in polycrystalline diamond MEMS resonators by post-deposition plasma treatment

Abstract

Polycrystalline diamond is a promising material for MEMS resonators due to its remarkable mechanical and electrical properties and compatibility with standard semiconductor manufacturing processes. However, the growth on non-diamond substrates is challenging, and a seeding process is needed to grow closed thin films. Hot filament chemical vapor deposition and reactive ion etching are powerful tools to deposit and micromachine diamond thin films on silicon substrates. In this paper, a polycrystalline diamond thin film with micrometer-sized grains is used to fabricate MEMS resonator devices, and quality factors are measured using laser Doppler vibrometry. The resonator’s bottom and top sides, as well as the cross sections, are investigated, and a substrate-near region with an elevated amount of non-diamond carbon bonds is identified with TEM in the energy-filtered transmission electron microscopy mode. By monitoring the quality factors of several resonance modes of 142 MEMS resonator devices, while a reactive ion etching treatment is performed at the bottom side of the resonators, we find an initial increase in the mean quality factor of all out-of-plane modes in the frequency spectrum from 20 to 500 kHz by nearly a factor of 3 after 28 min of back thinning of an initially 2.2 μm thick polycrystalline diamond thin film due to the reduction of the defect-rich substrate-near region. Even more, we find no correlation between surface roughness and quality factor, indicating losses at structural defects, such as grain boundaries and non-diamond carbon clusters, as dominant for energy dissipation.