Analysis of heat dissipation improvement using bonded wafer in Chip Size SAW Device structure

In the small SAW (Surface Acoustic Wave) Duplexer, improving the heat dissipation is very important. SAW Duplexer being used in RF front-end, especially Tx side, self-heating by high input power on the inter-digital transducers (IDTs) is a big issue.¹⁾ We have proposed heat dissipation improvement by Chip Size SAW Device (CSSD) structure.²⁾ In the CSSD structure, heat dissipation was improved by adding a new path that is from the die to the package through the sealing material. However, additional improvement for the heat dissipation was needed. The bonded wafer technology has improved the heat dissipation effect. Moreover, the bonded wafer technology has contributed also to the improvement of temperature compensation greatly. Finally, it has realized the big improvement, especially insertion loss in high power applied condition by using bonded wafer technology and CSSD structure. The bonded wafer consists of thinner LiTaO₃ and Sapphire substrate and these two materials are directly bonded. The temperature coefficient of frequency (TCF) of this bonded substrate is better than that of the conventional LiTaO₃ substrate by 11ppm/°C ^{3), 4)} Here, the thermal conductivity of Sapphire is 33WJ(m*K), that is more than 7 times compared with LiTaO₃ of 4.6WJ(m*K). Furthermore, it is considered that heat of the die surface is easy to diffuse to the package through the bumps or back surface of the die. The CSSD Duplexer using the bonded wafer had lower die temperature compared with conventional LiTaO₃ wafer. We confirmed that the bonded wafer technology was efficient to improve the heat dissipation besides on the better frequency characteristics.5),6) By using the bonded wafer, the die surface temperature rise at 2.85dB loss under +28dBm input power applied condition could be improved by around 36% compared with the conventional LiTaO₃ wafer. In this paper, we discuss the mechanism to- improve for heat dissipation.