Chemical-Mechanical Polishing and Rapid Thermal Annealing of SiC: Raman Spectroscopy and ESCA (XPS) Studies
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Chemical-Mechanical Polishing and Rapid Thermal Annealing of SiC: Raman Spectroscopy and ESCA (XPS) Studies. Bahram Roughani1, Uma Ramabadran1, Diana Phillips1, W.C. Mitchel2, and C.L. Neslen2 Science and Mathematics Department, Kettering University 1700 W. third Ave, Flint, MI, 48504-4898 USA 2 Air Force Research Laboratory, Materials and Manufacturing Directorate Wright Patterson AFB, OH, 45433 USA 1
ABSTRACT: The effects of Chem-Mechanical Polishing (CMP) and Rapid Thermal Annealing (RTA) on n-type 4H:SiC samples doped with nitrogen were investigated using Raman scattering and X-ray Phtoelectron Spectroscopy (XPS a.k.a. ESCA) measurements. A comparison of the Raman spectra from Mechanically Polished (MP) SiC annealed at 600°C and 800°C displays a frequency shift in the coupled plasmon LO-phonon mode. Since the coupled mode frequency is a direct measure of the free carrier concentration, this observation may suggest the removal of polishing induced carrier traps with increasing annealing temperature. The CMP samples did not show this frequency shift, thereby indicating that such polishing traps were not created in that process. The Si-peak observed in the XPS spectra of the unannealed CMP sample indicates primarily a Si-C bonding, while that for the MP sample is more complex, indicating other bonds beside Si-C. Drastic changes in O, C, Si surface content were observed for annealing between 1000°C and 1100°C. The peaks in the XPS spectra associated with the chemical environment for C, O, and N are complex and may be explained as silicon oxycarbide type structures near the surface or possibly around the interface of the SiC substrate with a thin surface oxide layer. INTRODUCTION: Investigations and improved understanding of surface and bulk properties of the Silicon Carbide (SiC) under various processes can enhance the material’s applications as high power, high temperature, high frequency, and high radiation tolerance devices [1]. Interest in SiC is based on its excellent thermal and chemical stability, high thermal conductivity, high electric field breakdown, and wide band gap characteristics [1]. With advances in growth of SiC, various device applications using 4H- SiC and other poly-types of SiC have emerged. The improved quality of the substrates will positively affect the quality of the epi layer and the device performance. Major challenges in preparing SiC surfaces for device fabrication include chemical inertness and the high hardness of this material. Wafer surfaces may be subjected to mechanical polishing (MP), chemical-mechanical polishing (CMP), or etching [2-5]. High temperature hydrogen etch has been reported to produce defect-free surfaces [2]. However, since etching removes layers of material at the same rate over the whole wafer it cannot solve the problem of long-range roughness over the wafer surface. Etching treatment has also shown no improvement in terms of the dislocation density for SiC substrates [3]. The effects of surface polishing and thermal annealing, both of which can directly affect
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