MEMS-Based Silicon Nitride Thin Film Materials and Devices at Cryogenic Temperatures for Space Applications
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Microshutter arrays, scheduled to be launched in 2011 as part of NASA's James Webb Space Telescope (JWST), will be the first micro-scale optical devices in outer space using MEMS technology. As the microshutter arrays consist of electrical and mechanical components and must operate in a cryogenic environment reliably over a 10 year mission lifetime, a fundamental challenge for the development of this device is to understand the mechanical behaviors of the micro-scale materials used and the possible failure mechanisms at 30 K. This thesis investigates the mechanical properties and reliability of low-stress LPCVD silicon nitride thin films, the structural materials of the microshutter arrays, at cryogenic temperatures. A helium-cooled cryogenic measurement setup installed inside a focused-ion-beam system is designed, implemented, and characterized to obtain a cryogenic environment down to 20 K. Resonating T-shaped cantilevers with different "milling masses" are used to measure the Young's modulus of silicon nitride thin films, while the fracture strength is characterized by bending tests of these beams. A passive high-sensitivity microgauge sensor based on displacement amplification is introduced to measure residual stress and coefficients of thermal expansion, which are critical for the device performance. To achieve accelerated fatigue study of the microshutter arrays, a novel mechanical-amplifier actuator is designed, fabricated, and tested to emulate their torsional operating stress. Furthermore, nano-scale tensile fatigue tests are demonstrated using similar mechanical-amplifier actuators. The research results of this thesis provide important thin film material parameters for the design, fabrication, and characterization of the microshutter arrays. Moreover, the presented test devices and experimental techniques are not limited for space applications only but can be extended for characterization of other thin film materials used in MEMS and microsystems.