Micro-Raman Evaluation of Polycrystalline Silicon MEMS Devices
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0890-Y08-37.1
Micro-Raman Evaluation of Polycrystalline Silicon MEMS Devices Justin R. Serrano1, Leslie M. Phinney, and Sean P. Kearney Engineering Sciences Center, Sandia National Laboratories, Albuquerque, NM 87185, USA 1 PO Box 5800, MS 1310, Albuquerque, NM 87185, USA; ABSTRACT Micro-Raman spectroscopy is a valuable tool for thermometry of operational polysilicon MEMS devices. By using the temperature-calibrated response of the optical phonon peak of the polysilicon Raman signature and the micron-scale spatial resolution achieved with a 488 nm Ar+ laser Raman probe, we have obtained spatially resolved steady-state thermal profiles of Jouleheated thermal flexure actuators. The measured thermal profiles are further compared to onedimensional numerical models of the thermal response of the electrically heated devices. INTRODUCTION Surface micromachined electro-thermal actuators have become a ubiquitous building block of MEMS design in recent years. MEMS actuators based on thermal expansion offer numerous benefits over electrostatically-based actuators, including lower operating voltages and improved force and displacement characteristics [1]. In general, two types of electro-thermal actuators exist: bent beam or linear thermal actuators [2-5] and the U-shaped or flexure actuators [6-11]. Both types amplify the motion resulting from the thermal expansion of the heated structure but produce motion in different directions. Because of their versatility, many numerical and analytical thermomechanical models have been developed over the years to assess and predict actuator performance [9-12]. However, until recently the thermal models of powered electrothermal actuators could only be validated indirectly through measurements of displacement, force, voltage, or other performance characteristics [9-11]. In this paper we present spatially resolved temperature measurements of active thermal flexure actuators by using micro-Raman spectroscopy and compare them to numerical thermal models of the actuator temperature profile. The measurements serve as a validation tool for models of thermal actuators. To obtain the temperature measurements, we use the temperature dependence of the Raman spectrum for polycrystalline silicon, which has been thoroughly studied over the past 20 years [13-19]. In micro-Raman thermometry, a surface is illuminated by a focused laser beam and the Raman spectra resulting from the inelastically scattered laser light is evaluated. The spectral location and width of the observed Raman peaks of polysilicon are used as indicators of the temperature under the probed spot. An advantage of micro-Raman scattering with a visible wavelength probe is that spatial resolution is on the order of the diffraction limit of the probe laser, and the shallower penetration depth of visible light in the polysilicon allows for probing of relatively thin samples. This compares favorably against other thermometry techniques, such as IR imaging, which has poor spatial resolution and can be hindered by material transparency issues
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