Detection of Residual Stress in SiC MEMS Using micro-Raman Spectroscopy
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH SCHOOL OF ENGINEERING AND MANAGEMENT/DEPT OF ENGINEERING PHYSICS
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Micro-Raman mu-Raman spectroscopy is used to measure residual stress in two silicon carbide SiC poly-types single-crystal, hexagonally symmetric 6H-SiC, and polycrystalline, cubic 3C-SiC thin films deposited on Si substrates. Both are used in micro-electrical-mechanical systems MEMS devices. By employing an incorporated piezoelectric stage with submicron positioning capabilities along with the Raman spectral acquisition, spatial scans are performed to reveal areas in the 6H-SiC MEMS structures that contain residual stress. Shifts in the transverse optical TO Stokes peaks of up to 2 cm-1 are correlated to the material strain induced by the MEMS fabrication process through the development of phonon deformation potential curves for this material. The 3C-SiC films, with thicknesses ranging from 1.5-5 microns, are deposited by CVD on 100 Si substrates and are also investigated to determine their residual stress. An ultraviolet excitation source lambda 325 nm, hv 3.82 eV was determined to be more effective for the detection of Raman shifts in these thin films than the 514-nm source, since the absorption coefficient in SiC at 300 K at 325 nm is 3660 cm-1, while that at 514 nm is less than 100 cm-1.
- Electrical and Electronic Equipment
- Atomic and Molecular Physics and Spectroscopy