Jenny Le, Jinchi Sun and David G. Cahill
NSF CBET 24-28927

Time-domain thermoreflectance and frequency-domain thermoreflectance (FDTR) have been widely used for non contact measurement of anisotropic thermal conductivity of materials with high spatial resolution. However, the requirement of a high thermoreflectance coefficient restricts the choice of metal coating and laser wavelength. We are developing alternative frequency-domain pump-probe techniques based on probe beam deflection. We refer to these methods as thermo-optic phase spectroscopy (TOPS) with two main varieties: 1) displacement-TOPS (D-TOPS) where the beam deflection is primarily caused by thermoelastic deformation of the sample surface, with a magnitude determined by the thermal expansion coefficient of the material ; and 2) immersion-TOPS (I-TOPS) where the beam deflection is created by a gradient in the index of a transparent liquid or elastomer. We are developing an approach using I-TOPS to characterizing the thermal resistance of microelectronic devices, e.g., high bandwidth memory, that manufactured by stacking of several Si dies that are separated by interconnect layers. This requires greatly increasing lateral size of the pump and probe beams while maintaining adequate signal-to-noise ratio.