Optimization and calibration of two-wavelength transmission for absolute thickness measurements of InGaAsP/InP layers

The calibration and optimization of a two-wavelength transmission method for nondestructive thickness mapping of InGaAsP layers are described. In two-wavelength transmission, two beams are focused simultaneously to the same sample point: the absorption beam for which the substrate is transparent and only the layer under investigation is absorbing, and the reference beam that exhibits no absorption but nearly identical parasitic losses. The derivation of the absorption coefficient of the InGaAsP layers from measurements on layers with known thicknesses is discussed. The compensation of parasitic losses, optimized by using a reference wafer without an epitaxial layer during the balancing procedure, is also discussed