Photothermal microscopy: Thermal contrast at grain interface in sintered metallic materials

Abstract

A theoretical and experimental study of the influence of planar thermal barriers on photothermal reflectance microscopy signals is presented. An analytical solution is developed for the problem of vertical barriers in a semi-infinite solid and the signal contrast obtained when scanning through the barrier is discussed as a function of the thermal resistance, the thermal diffusion length, and the pump and probe beam dimensions. The shape and the width of the signal perturbation introduced by the barrier is also analyzed. For the case of slanted barriers results of finite-element calculations are presented, and the main feature of the signal when going from vertical to slanted barriers is thus shown. Finally, the theoretical predictions are compared with measurements made on Fe sintered samples. Scanning through grain interfaces revealed different signal shapes and contrast. Good agreement between theory and experiment was found when the optical contrast at the interface is negligible. Examples are shown where the thermal barrier model is no longer valid and an extended model seems to be necessary.

Contrast enhancement in the detection of defects in transparent layered structures: The use of optothermal interference technique in solar cell investigation

“…4. A laser diode operating at 670 nm is used as probe beam, while an Ar ϩ laser ͑514.5 nm͒ is used as excitation.…”

“…, ␣ a being the thermal diffusivity 4 ͒ is a ϭ20 m, equal to the layer thickness. In this situation, the thermal wave generated mainly at the silicon/coating interface is not strongly attenuated at the adhesive/glass interface.…”

“…The reflectance ͑refraction indexes͒ coefficients were considered temperature dependent as well as the adhesive thickness. The temperature distribution was obtained by solving the three-dimensional ͑3D͒ heat diffusion equation 4 for the three media: Si substrate, adhesive, and glass. The p -n junction and the coating were disregarded in the calculation of the temperature; for the optical reflection, however, the coating was taken into account.…”

“…2 The second advantage is explored in photothermal microscopy investigations through reflectance measurements. [3][4][5][6][7][8][9][10][11][12] Despite the high spatial resolution achieved by this technique, its sensitivity is usually ordinary and strongly dependent on the material optical properties and probe wavelength, as discussed in Ref. 9.…”

“…Photothermal reflectance microscopy has been extensively used in the characterization of local thermal and electronic properties of materials and structures [3][4][5] as well as in the investigation of heat source distribution in operating microelectronics devices. [6][7][8][9] The main thermal parameter obtained with these measurements is the thermal diffusivity.…”