Liquid Crystal Thermography on Coated SAW Devices

Reliability of micro-electronic devices is one of the most important issues in modern technologies and is significantly influenced by the thermal behavior of the components. In this context, we present Liquid Crystal Thermography (LCT) not only as an easy-to-use but even comparably low-cost approach for temperature measurements. This technique is based on thermochromic liquid crystals which exhibit temperature-dependent colors by selectively reflecting incident white light. We describe and demonstrate this method in exemplary investigations of self-heating effects in a half-section ladder-type Surface Acoustic Wave (SAW) filter with silicon dioxide coating. Conventionally, mean temperature values are obtained by evaluating measured frequency shifts under load by means of the Temperature Coefficient of Frequency (TCF). Moreover, LCT provides spatially resolved measurements of the temperature distribution on the component and serves as an independent scheme for thermal characterization in contrast to TCF based evaluations. The results of LCT measurements and temperature simulations are compared and show good agreement.