Spectral microthermography for component discrimination and hot spot identification in integrated circuits

We demonstrate an efficient and versatile spectral microthermography technique for identifying hot and cold spots in the active layer of a biased integrated circuit. Hot (cold) spots are regions where heat accumulates more rapidly (slowly) than the average rate of the entire active layer. Knowledge of the hot and cold spot locations is crucial in assessing the thermal integrity of a layer structure because hot spots are locations were defects are more likely to develop. The active layer is uniformly illuminated with light from a tungsten lamp and its reflectance image r(x, y) is scanned across (x-direction) the entrance slit of a grating-prism pair (GRISM) spectrometer to produce a spectral map R(lambda; x, y) where lambda is the wavelength [450 </= lambda (nm) </= 650]. For a particular slit position x = x(1), the GRISM spectrometer outputs a one-dimensional spectral map R (lambda; x(1), y). A pair of maps R(ub) (lambda; x, y) and R(b)(lambda; x, y) are obtained from the active layer in the absence and presence of voltage bias, respectively. A reflectance gradient map R(lambda; x, y) = R(b)(lambda; x, y) - R(ub)(lambda; x, y), is derived and used to locate possible hot and cold spots because R(lambda; x, y) is proportional to the temperature gradient T(lambda; x, y). We use the technique to generate gradient maps of a photodiode array and the emitting surface of a biased light emitting diode. Two different semiconductor materials could be distinguished easily from their dissimilar reflectance spectra.