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Thermoreflectance microscopy can deliver thermal images with high spatial resolutions by measuring the variations of the reflection coefficient with temperature. It is therefore a unique tool to measure the temperature mapping of integrated circuits with submicronic features. However, integrated circuits are usually protected by a dielectric encapsulation layer which is transparent to visible light. The optical interference which occurs in these layers strongly modifies the reflectivity and can even forbid thermoreflectance measurements at some wavelengths. For each series of circuits, it is therefore necessary to determine the illumination wavelengths for which thermoreflectance will deliver optimal signals. A sequential wavelength scan can deliver this information but it is time consuming and therefore subject to drifts. We have developed a CCD camera-based thermoreflectance microscope coupled to a grating which disperses white light directly onto the CCD. This instrument gives the complete spectra of the reflection coefficient and its temperature dependence, R(λ) and dR/dT(λ), on one or several materials with only one acquisition. The optimal wavelength for thermoreflectance measurements can therefore be measured within minutes on any sample. A model taking into account multiple reflections and the thermal expansion of the encapsulation layer has been developed to explain the spectra R(λ) and dR/dT(λ) measured on encapsulated circuits. It can be used to predict qualitatively the optimal working wavelength. [ABSTRACT FROM AUTHOR]