Transient Thermal Imaging Using Thermoreflectance

Lock-in thermoreflectance imaging has proven effective in obtaining thermal images of active electronic and optoelectronic devices with submicron spatial resolution and 10- 50 mK temperature resolution. Thermoreflectance systems that use a lock-in method capture the steady state thermal signal but provide limited information about the thermal transient. We present a simple time series thermoreflectance method based on pulsed box-car averaging and a novel differencing technique to obtain transient thermal images with millisecond and microsecond time resolution and submicron spatial resolution. The technique relies on precise adjustment of the phase between the pulsed thermal excitation of the device and the illumination pulse used to measure the thermoreflectance change on the device. The full thermal transient pattern is reconstructed and captured in a charge coupled device (CCD) camera in a matter of minutes. Images are presented of the time evolution of the thermal signals on 40times40, and 100times100 micron square gold heaters.