Emissivtty Studies on Polycrystalline Silicon and a-Si/SiO 2 Si/Si
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with a micro-processor controlled chopper and a high resolution Bomem FTIR spectrometer as a sensor for measuring the optical properties. Ge and HgCdTe detectors are used to acquire the measured optical properties via a Pentium computer. The reflectivity of the material under consideration is obtained by comparing it with that of a gold mirror, whose reflectivity equals 1. The transmissivity is measured directly using the optics of the emissometer. The emissivity is extracted from the measured reflectivity and transmissivity. The emissivity can- also be derived by taking the ratio of the radiance of the sample, which is measured directly using the emissometer, to that of the blackbody at the same temperature. EXPERIMENTAL RESULTS ON POLYSILICON The results of the measurements of the wavelength dependent optical properties of two
types of polysilicon are presented in Figs. l(a) and l(b). The samples considered here comprise of (a) mechanically textured and (b) diffused polysiliconr As can be seen in these figures, the diffused polysilicon is characterized by higher emissivity in the long wavelength, range, while the mechanically textured polysilicon exhibits a comparatively lower emissivity. The measured values of reflectance and transmittance lead us to evaluate the refractive index and extinction coefficient. THEORETICAL BACKGROUND The deconvolution of the measured reflectance and transmittance into fundamental optical constants is performed using optical property calculator, OPCalc, which is a data analysis program. Reflectance and transmittance data points collected by a data acquisition system, such as SpectraCalc, are statistically analyzed at various points. Selected optical properties are calculated based on the acquired data, and the results are posted in a database type file. OPCalc inspects two data files, one containing the reflectance data as a function of the wavelength ?,, and the other containing transmittance data as a function of X. These experimental values permit the calculation of several other related optical properties, such as emissivity (s(,)), the absorption coefficient (ct(,)), the extinction coefficient (k(X)), and the refractive index (n(X)). In theory, all of the data in the reflectance and transmittance files can be processed, producing a complete list of optical properties as a function of X. In actual practice, all of the data collected is not of significance. These include characteristic absorption peaks due to IR sensitive molecules such as H 20 and CO 2. Only specific wavelengths are of importance for a particular application. These wavelengths, however, may or may not coincide with the acquired data, and the experimental data inherently has deviations from theoretical values. Multiple internal reflections of doublesided polished wafers also contribute to deviations of the data from the accepted, theoretical values. Using various curve-fitting and statistical methods, it is theoretically possible to eliminate or reduce most of these error factors and predict data for values of X
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