AC/DC Thermoreflectance Imaging
Thermoreflectance imaging provides high-resolution 2D thermal distributions of electronic ICs and devices. The technique is based on the small but detectable variation of the reflectivity with the surface temperature of the sample. Our systems employ a wide-field LED for illumination and megapixel CCD camera for detection, eliminating the time-consuming need to scan the sample. By using light at visible wavelengths, submicron spatial resolutions can be easily achieved.
As the thermoreflectance coefficient of most materials is very small, lock-in techniques must be applied to resolve the thermal signal, even for steady state characterisation. We have developed a novel lock-in scheme that is able to capture Peltier and Joule distributions simultaneously. Magnitude and phase images for both effects are obtained real-time in a single measurement. The setup is schematically shown below in Fig. 1. This technique is of particular interest to the analysis of thermoelectrics, as illustrated in Fig. 2.
Figure 1 Schematic principle of quasi-static lock-in thermoreflectance for Peltier/Joule separation.
Figure 2 Characterisation structure on p-type thermoelectric material: (a) sample layout, (b) thermoreflectance images taken at 7.5Hz.
AC TRANSIENT IMAGING
Similar characterisations can be performed in AC transient regime by means of heterodyne lock-in. In this case, the illumination is pulsed at a frequency near that of the thermal field under study (Fig. 3), inducing a slow term or 'beating pattern' in the reflection that can be detected and processed. Some exemplary measurements of a high-speed microcooler are shown in Fig. 4.
Figure 3 Schematic principle of AC heterodyne thermoreflectance imaging technique.
Figure 4 Thermal images obtained for a high-speed microcooler at various electrical excitation frequencies.