Optimization of SiO2 Films and Application of Fluorine Doped SiO2 Films for Temperature Compensated Surface Acoustic Wave Devices SAW SiO2 SiOF

Recently, temperature compensation of surface and bulk acoustic wave (SAW/BAW) technologies has become a topic of interest due to strong demand on wideband filters with excellent temperature stability. Two types of techniques are studied extensively. One is the wafer bonding of a piezoelectric substrate with a support substrate, which have large Young's modulus and small thermal expansion coefficient. Another one is the deposition of the SiO2 film on the piezoelectric substrate. In this type, the TCF improvement is owed to an anomalous property of SiO2, i.e., SiO2 becomes stiff with an increase in temperature. In other words, SiO2 possesses positive temperature coefficient of elasticity (TCE). In the latter type, it is known that the TCF (or TCE) considerably depends on the preparation technique and deposition condition of the SiO2 film . Nevertheless no systematic investigation has been reported on the relation between the TCE and SiO2 properties, to the authors’ best knowledge. Therefore, a significant amount of try and error is necessary to find the adequate preparation technique and deposition condition even though determined ones may not be optimal. This paper shows the preparation technique with FT-IR measurement. From this measurement, the optimized SiO2 films increased the TCF of the device. Furthermore, it can be applied to find the most appropriate dopant for developing high performance temperature compensated SAW devices. As a results, we found that the fluorine doped SiO2 (SiOF) has larger TCE than that of SiO2.