An optical method for measuring the temperature of a substrate material with a temperature dependent band edge. In this method both the position and the width of the knee of the band edge spectrum of the substrate are used to determine temperature. The width of the knee is used to correct for the spurious shifts in the position of the knee caused by: (i)
thin film interference in a deposited layer on the substrate; (ii)
anisotropic scattering at the back of the substrate; (iii) the
spectral variation in the
absorptance of deposited
layers that absorb in the vicinity of the band edge of the substrate; and (iv) the spectral dependence in the optical response of the
wavelength selective detection
system used to obtain the band edge spectrum of the substrate. The adjusted position of the knee is used to calculate the substrate temperature from a predetermined
calibration curve. This
algorithm is suitable for real-time applications as the information needed to correct the knee position is obtained from the spectrum itself. Using a model for the temperature dependent shape of the
absorption edge in GaAs and InP, the effect of substrate thickness and the optical geometry of the method used to determine the band edge spectrum, are incorporated into the
calibration curve.