Inline Characterization of Polysilicon Layers in TOPCon Solar Cell Precursors With Reflectance Spectroscopy

This study presents an inline-compatible technique based on reflectance spectroscopy for characterizing both the thickness and crystalline volume fraction of polysilicon (poly-Si) layers in tunnel oxide passivated contact solar cell precursors, serving as an intermediate characterization step suitable for production and process control. An optical model based on the Fresnel equations and the transfer-matrix method is used to simulate the reflectance of a poly-Si layer on a planar silicon substrate quantitatively. The Bruggeman effective medium approximation is used to define poly-Si as crystalline silicon particles dispersed in an amorphous silicon matrix. By treating the poly-Si layer thickness and crystalline volume fraction as fit parameters, estimates of these values can be obtained from the measured inline reflectance spectra using the developed optical model. The estimated thicknesses and the crystalline volume fractions show a good correlation with the reference thickness values measured from scanning electron microscopy (SEM) and reference crystalline volume fraction values estimated with Raman spectroscopy, respectively. The maximum relative difference in thickness values obtained from reflectance spectra and SEM measurements is only 3%. Moreover, the maximum relative difference in crystalline volume fractions derived from reflectance and Raman spectra is just 1.8%.