Investigation upon Mechanical Properties of Thin Film Silicon Under Cryogenic Temperature
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Investigation upon Mechanical Properties of Thin Film Silicon Under Cryogenic Temperature Yi Zhao and Xin Zhang Laboratory of Microsystems Technology, Department of Manufacturing Engineering, Boston University, 15 Saint Mary’s Street, Boston, MA 02215, USA ABSTRACT Thin film silicon material has an extensive application in cooling satellite instrumentation under cryogenic environment. The performance and reliability of the cooling system heavily depends on mechanical behavior of the thin films. In this paper, we built an experimental setup and used compressive gas to actuate a silicon thin film under both room temperature and cryogenic temperature. The elastic modulus was derived from the film’s deflection using laser Michelson interferometer. Stress distribution was obtained using Micro Raman spectroscopy. It was found that Young’s modulus derived from the deflection increases with decreasing temperature. Compressive stress concentrated at edge centers of the film and tensile stress occurred at the center. There is a good match between the theoretical predications and experimental observations. INTRODUCTION In micro satellites, a large number of dedicate instrumentations were compacted in a limited room for rapid demonstration of new concepts. The ambient temperature would rise if heat generated could not be well dissipated, which will ruin many heat sensitive devices, such like infrared cameras. However, the lack of atmosphere in outer space results in the loss of a primary cooling mode. Hence, a cooling system is required which should have the capability of “active cooling” for rapid and efficient heat dissipation, as compared to gas flowing over devices or passive atmospheric heat exchangers. Moreover, since the imaging components need to move and rotate smoothly for image capturing, the cooling system should also have the ability for “remote cooling”. That is to say, the Evaporator Preservation chamber
Cryogen container
Condenser
Cryocooler
Figure 1: The micro-pump array is employed in a heat transport loop to transport cryogen from the condenser to the evaporator, with tube coiling providing the flexibility. Individual micro-pumps consist of multi-layered silicon, making up the thin film, cryogen inlet and outlet.
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heat exchanger system (cryocooler) needs to be located remotely from the imaging components. As a solution, a two-phase loop using micro-pump was applied therein to maintain the instrumentations within a narrow cryogenic range (Figure 1). This micropump depends on elastic deformation of silicon thin film to transfer cryogen from a cryocooler to the instrumentation. The mechanical properties of the silicon thin film, especially elastic modulus, stress distribution and fatigue status, are thus critical in the performance and reliability of the cooling system. However, conventional tensile experiment was not practicable on non bulk material and in cryogenic environment.
FABRICATION Square silicon thin films were fabricated on 525-µm thick (100) silicon wafers using the standard
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