Temperature effect on oxidized silicon reflectivity: Experimental determination of the relative sensitivity; Application to temperature non-contact measurements on the surface of a GTO thyristor in commutation.

Temperature effect on the absolute reflectivity of oxidized silicon is experimentally studied in the spectral range 300 nm < lambda < 500 nm. R(lambda) spectra are measured for 25 degrees C < T < 225 degrees C with the precision Delta R/R = 2 x 10(-3). The largest relative variations of R are observed near direct interband transition: E(1) = 3.4 eV. In this case, the relative sensitivity R(-1)(dR/dT) reaches the maximum value (2.4 +/- 0.4) x 10(-4) K-1 which is about two times higher than in the case of bare silicon. This result is used to make a temperature contactless measurement, by a technique based on reflectometry, along the gate-cathode junction on the upper face of a 1,200 volts gate turn-off thyristor operating at 400 Hz in the switching mode. The size of the optical probe is 20 micrometers, and the smallest temperature variation that can be detected is 10 degrees C. The measurements show a noticeable variation of heating coefficient dT/dE according to the probe's position, dissipated energy E at turn-off being constant (1 mJ < E < 25 mJ). Maximum values of the junction temperature were found as high as 275 degrees C and 350 degrees C, according to the considered area on the chip.