Characterization of Mechanical Properties of Silicon Nitride Thin Films for Space Applications
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A5.21.1
Characterization of Mechanical Properties of Silicon Nitride Thin Films for Space Applications Wen-Hsien Chuang1, Thomas Luger1, Rainer K. Fettig, and Reza Ghodssi1 1 Department of Electrical and Computer Engineering and Institution for Systems Research University of Maryland, College Park, MD 20742, U.S.A. ABSTRACT Mechanical 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. INTRODUCTION Advancements in MEMS technologies have made it possible to develop MEMS devices for space applications [1]. Currently, two dimensional microshutter arrays, programmable field selectors for a Multi-Object Spectrometer (MOS) in the James Webb Space Telescope (JWST), are being developed at NASA Goddard Space Flight Center [2,3]. The microshutter arrays, made of low-stress LPCVD silicon nitride thin films, require cryogenic operation at 30 K to reduce thermal emission into the instrument. Since the JWST operates in outer space without the possibility of human intervention, a complete understating of mechanical properties and reliability issues at cryogenic temperatures is crucial for the development of the microshutter arrays. This paper presents the design of a measurement setup with a PZT translator and a microneedle installed inside a focused-ion-beam system. Instead of using conventional cantilevers, beams with T-shape structures are utilized in the study of mechanical properties. A milling mass approach is introduced for the first time to obtain an effective thickness of a thin film material and to increase accuracy in the measurement of the Young’s modulus. An ANSYS finite element analysis (FEA) model is also developed to determine the fracture strength of silicon nitride thin films in bending tests. The presented measurement setup and experimental techniques are suitable for measurements through a large temperature range (from room temperature to 23 K) and can be easily extended to characterize a series of MEMS materials.
A5.21.2
CRYOGENIC MEASUREMENT SETUP The measurement
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