Wavelength-modulated interferometric thermometry for improved substrate temperature measurement

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Abstract
Interferometric thermometry is a promising noncontact technique for measuring the temperature of transparent substrates (with polished front and back surfaces) from thermally induced changes in sample thickness and refractive index. However, for substrates of uniform thickness, the technique is not sensitive to the direction of temperature change, thus restricting its use to situations in which the temperature variation is monotonic. In this paper, we present some new schemes for interferometric thermometry based on the wavelength modulation capabilities of the distributed feedback laser diode. These schemes allow both the magnitude and direction of temperature change to be determined. One scheme utilized to measure temperature changes in a silicon wafer during thermal cycling is described in detail. In addition, the calibration factors required to convert the thermally induced reflectance oscillations (‘‘fringes’’) of known‐thickness samples into temperature change are provided for Si and GaAs at wavelengths near 1.5 μm.