Liquid crystal and Infrared Thermography on coated SAW devices

Reliability of micro-electronic devices is one of the most important issues in mobile communication systems and is significantly influenced by the thermal behavior of the components. The present contribution demonstrates Liquid Crystal Thermography (LCT) and Infrared Thermography (IRT) in exemplary investigations of self-heating effects in a half-section ladder-type Surface Acoustic Wave (SAW) filter with thick SiO₂ coating. Conventionally, mean temperature values are obtained indirectly by evaluating measured frequency shifts under load using the Temperature Coefficient of Frequency (TCF). In contrast to TCF based evaluations, LCT and IRT provide spatially resolved measurements of the temperature distribution on the component and serve as an independent and direct scheme for thermal characterizations. The results of LCT and IRT measurements are compared with simulations of the temperature distribution and show good agreement.

I. Introduction

Modern UMTS and LTE mobile communication systems make high demands on RF filtering applications. Devices based on Surface Acoustic Wave (SAW) technology meet these requirements very well. However, despite many years of research and development, domains such as reliability and lifetime are still subject of current investigations [1]. They need to be thoroughly studied especially since mobile applications require continuous reduction in component size. One of the important issues deteriorating the device reliability is the increase of internal temperature due to power dissipation in the component [2]. This self-heating causes crucial frequency shifts of the filter characteristics at high electrical power levels as well. Therefore, investigating the temperature behavior of SAW resonators is an important task regarding reliability and is also necessary to optimize the filter design.