Department of Mechanical Engineering
Kenneth E. Goodson
The nanosecond thermoreflectance technique (sometimes called transient thermoreflectance, TTR) has been a fixture in our laboratory for decades. This method characterizes the thermal properties of sample structures by optically measuring the transient thermal response to nanosecond heating pulses. The technique is particularly useful for measuring the thermal properties and boundary resistances of thin films ranging in thickness from hundreds of nanometers up to microns. The technique uses 6 ns pulses from a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 10 Hz to heat the top metal film with a spot diameter of ~3 mm, creating a transient temperature field in the sample. The reflectivity of the metal film is proportional to the surface temperature and is measured through the reflected intensity of a continuous wave probe laser. Because the heating diameter is much larger than the maximum thermal diffusion depth, data interpretation can rely on solutions of the one-dimensional heat diffusion equation for a multilayer stack. A multi-parameter least-squares fit of the transient thermal response data to the solution of the thermal model extracts the effective thermal properties of the film and its interfaces. The confining of heat to the film due to the short timescales, and unique time-dependent sensitivity of the thermal response to the thermal properties of the film enable profiling of thermal resistances within the film and at its interfaces from a single thermal trace.