Undulated bonded silicon-on-insulator structures with oxide/nitride/oxide layers are investigated nondestructively using spectroscopic ellipsometry. Optimum measured information for a wavelength range of 250 to 850 NM has been obtained by using a combination of a lens and nearly 1 mm slit width to minimize the light beam divergence. Using Marquardt regression analysis, slight vertical discrepancy is till observed between measured and calculated data, especially in the wavelength range of 250 to 285 nm due to the still unavoidable existing of beam divergence. This problem affects the accuracy of evaluated layer thicknesses from position to position on the investigated samples, especially the uppermost very thin (1 nm) oxide layer thickness. One interesting result is that because of the observed vertical error in the measured data, we have to avoid in this case making fitting on tan $Psi and cos $Delta in order to obtain best calculated data close to the nominal values.
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