Limitations of 1D thermal model in comparison with 2D model of high power thyristor structure in thermal wave microscopy

Abstract
Thermal-wave methods are magnificent tool for visualisation and analysis of thermal non-uniformity in solids. Special case of thermal non-uniformity is lack of adhesion in multilayer structures. This very important parameter effects on efficiency of draining of the heat and the reliability of many of semiconductor devices. This paper presents possibility of detection of exfoliation in high-power thyristor structure with the help of photoacoustic microscope. Values of temperature contrasts were evaluated from thermal-wave images, which were registered in harmonic modulation and pulse modulation modes. Both methods are characterised by similar sensitiveness for lack of adhesion, however pulse modulation mode makes the time of investigation significantly shorter. Comparison of experimental data with 1-D modelling results shows differences between theoretical and experimental values of temperature contrast in range of low frequencies (harmonic modulation mode) and long times of registration (pulse modulation mode). These differences appear because the 1-D model does not take limited lateral dimensions of excitation and thermal non-uniformity into consideration. Therefore 2-D modelling process was performed in order to explain the influence of geometrical factors on temperature contrasts. On the other hand, analysis of results of 2-D modelling makes the estimation of real delamination more accurate.