Characterization of Mechanical Properties of Silicon Nitride Thin Films for Space Applications

Abstract

ABSTRACTMechanical properties of micro-electro-mechanical systems (MEMS) materials at cryogenic temperatures are investigated to extend MEMS devices into space applications. A helium-cooled measurement setup mimicking the outer space environment is developed and installed inside a focused-ion-beam (FIB) system. T-shape, low-stress LPCVD silicon nitride cantilevers suspended on a silicon substrate are fabricated as the test structures using bulk micromachining technique. A lead-zirconate-titanate (PZT) translator and a silicon diode are utilized as an actuator and a temperature sensor in the measurement setup, respectively. The resonant frequencies of an identical cantilever with different “milling masses” are measured to obtain the thickness and the Young's modulus. Additionally, a bending test is performed to determine the fracture strength. From the experiments, the Young's modulus of LPCVD silicon nitride thin films varies from 260.5 GPa ± 5.4 GPa at room temperature (298 K) to 266.6 GPa ± 4.1 GPa at 30 K, while the fracture strength ranges from 6.9 GPa ± 0.6 GPa at room temperature to 7.9 GPa ± 0.7 GPa at 30 K.