Optical EngineeringMeasurement of physical thicknesses in micromachined structures consisting of glass and c-Si, by Fourier transform infrared
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The measurement of thicknesses of micromachined structures consisting of etched glass bonded to c-Si and multilayer structures of both etched c-Si and etched glass is presented. The peaks within the recorded interferograms are identified by inspecting the equation for the reflection coefficient and the corresponding equation for the reflectivity. The amplitudes of the various peaks resulting from the 3-D structure underneath the spot of the beam are evaluated to be able to perform an unambiguous choice of the correct peak when performing automatic process monitoring. Furthermore, the peaks in the interferogram may be a strong function of wafer position (depending on device layout). In the latter case, it is demonstrated that information obtained directly from the interferogram can be used in wafer imaging. The dielectric dispersion functions for the glass and the c-Si wafer under investigation are evaluated from the reflectance measurements. An anomalous shift is observed in the interferogram peak corresponding to the glass thickness. The shift is found to be due to the strong dispersion in the refractive index of glass resulting from the SiOSi asymmetric stretching frequency around 1100 cm-1. Both simulations of the interferogram, and analysis provide a 5% correction for the thickness of the glass under investigation.