Detection of Residual Stress in Silicon Carbide MEMS by μ-Raman Spectroscopy

Abstract

Micro-Raman (μ-Raman) spectroscopy is used to measure residual stress in singlecrystal, 6H-silicon carbide (SiC) used in micro-electro-mechanical-systems (MEMS) devices. These structures are bulk micro-machined by back etching a 250-μm-thick, single-crystal 6H-SiC wafer (p-type, 7 Ω-cm, 3.5º off-axis) to form a 50-μm thick diaphragm. A Wheatstone bridge, patterned of piezoresistive elements, is formed across the membrane from a 5-μm, 6H-SiC (n-type, doped 3.8 x 10 18 cm -3 ) epilayer; the output of the bridge is proportional to the flexure of the MEMS diaphragm. For these samples, the μ-Raman spectroscopy is performed using a Renishaw InVia Raman spectrometer with an argon-ion excitation source (λ = 514.5 nm, hν = 2.41 eV) with an approximate 1-μm 2 spot size through the 50X objective. 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 MEMS structures that contain residual stress. Shifts in the transverse optical (TO) Stokes peaks of up to 2 cm -1 along the edge of the diaphragm and through the piezoresistors indicate significant material strain induced by the MEMS fabrication process.The phonon deformation potential is measured to quantify the material stress as a function of the shift in the Raman peak position. The line center of the TO Stokes peak is shifted by applying a uniform stress to the sample and monitoring the applied stress using a strain gauge, while the μ-Raman spectrum is being measured. A spectral analysis code tracks the shift of the Raman peak position with respect to the line center of the Rayleigh peak to account for any thermal drift of the spectrometer during the time of the area scan.

Topic Review: Application of Raman Spectroscopy Characterization in Micro/Nano-Machining

“…They claimed that the asymmetry downshift of the Raman band might be originated from stacking faults and/or residual strain, and the broadening indicated the generation of defects (including dislocations) during scratching. Not only the above machining methods, but also lapping [ 17 ], ion implantation [ 82 , 83 ], and others had also adopted Raman spectroscopy to identify phase transformation successfully.…”

“…Silicon carbide materials had also been widely studied for quantitative calculation of residual stress using Raman spectroscopy [ 88 , 89 , 90 , 91 , 92 ]. Figure 14 shows typical Raman spectra of the LO and TO modes at different pressures obtained from a single crystal 6H-silicon carbide [ 88 ].…”

“…For 3C-SiC ( β -SiC), the TO mode and LO mode for the unstressed state are 796 cm −1 and 973 cm −1 , respectively [ 96 ]. For single crystal 6H-SiC, the residual stress measurement was realized using Raman spectroscopy by Zingarelli et al [ 89 ] and they concluded that the Raman shift ω of TO1 (766 cm −1 ) and TO2 (788 cm −1 ) mode could be determined by [ 89 ]: where σ was a positive tensile pressure measured in GPa.…”

“…As one kind of spectral analysis technique, the Raman effect is based on the inelastic scattering process between the incident photons and the phonons in materials [ 16 ]. When the photons of incident light interact with the vibration phonons of the materials, the light is scattered at a frequency and the difference in frequency between the incident and the scattered light provides the information of the lattice vibrations [ 17 ]. Therefore, Raman vibration spectra is widely used for providing a structural fingerprint for molecules identification.…”

Invited Article: Simultaneous mapping of temperature and stress in microdevices using micro-Raman spectroscopy

“…In this technique, the anharmonic characteristics of the lattice under strain provide a method by which stress can be estimated as has been implemented in a number of materials and devices including single crystal silicon, diamond, SiC, GaN, MEMS, and complementary metal-oxide semiconductor ͑CMOS͒ circuit elements. [9][10][11][12][13] In addition, Raman spectroscopy is sensitive to both temperature and stress fields providing a unique capability for probing combined thermomechanical responses in materials.…”