An 8 μm epoxy film deposited on a 350 μm Si (100) Si wafer with a 0.4 μm Au transducer film deposited on top of the polymer film was used to evaluate the thermal conductivity, the modulus of the porous film, and the initiation of spalling upon laser beam irradiation on the back side of the Si wafer. The polymer films were characterized for pore microstructure using scanning electron microscopy and energy dispersive spectrometry. The polymer films were characterized using transient thermo reflectance (TTR) with laser beams illuminating the Au layer. The TTR signal from the polymer film showed only the thermal component and was characteristic of variations associated with thermal conduction into the film. To induce spalling, the back side was illuminated with a Nd-YAG laser beam with a 532 nm wavelength, pulse energy density 1.8 J/cm2, and a repetition rate of 10 Hz for 10 s in conjunction with TTR measurements on the front side. The TTR signal from the polymer film subjected to laser beam incidence from the backside of the Si wafer showed both the thermal and the acoustic components. The acoustic component was used to detect the initial stages of spalling or delamination. The acoustic oscillations were modeled using a modified wave equation to determine the velocity of sound and the modulus of the film. The results were also used to determine the effect of porosity on the modulus of the polymer film. The TTR signal was found to be very sensitive to detection of delamination without complete separation of the film.
The author acknowledges the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University for SEM characterization, which is supported by the State of North Carolina and the National Science Foundation.