Sputter deposition of undoped and doped SiO2 films for temperature compensated SAW components

Silica (SiO2) has the unique property of increasing stiffness with temperature. This makes it suitable for compensating the temperature drift of frequency in SAW components based on Lithiumniobat (LN) or Lithiumtantalat (LT) wafers. SiO2 films with 1EL2µm thickness on top of the interdigital transducer are typically used for this purpose. To reduce insert losses due to the SiO2 film, mechanical properties of the film should be closed to fused silica. Therefore sputtering has established as dominant technology for the deposition of the SiO2 films for temperature compensated (TC-) SAW, as it leads to films with very good mechanical properties within the temperature limitations of the typical SAW devices.Recent publications showed that doping of the silica films with Fluorine leads to an even stronger stiffness increase with temperature. Such films will be denoted as SiOF throughout the paper. SiOF films have the potential of achieving temperature compensation at reduced film thickness. This makes it feasible to reduce insertion losses and to improve temperature compensation when using SiOF rather than SiO2 films. Most of the work on SiOF films has been done up to now using PECVD deposition techniques. PECVD processes typically require an elevated substrate temperature that may exceed the typical temperature limitations of SAW devices in the range of 250EL300°C. Consequently sputter deposition is an attractive candidate to deposit SiOF films with good mechanical properties while observing the substrate temperature limit of SAW devices. Up to now there are only few publications on sputter deposition of SiOF films [e.g. 4], probably mainly due to the fact that using fluorine containing gases requires special measures regarding personal and equipment safety that are not implemented at a standard sputter tool. A detailed investigation on the sputter deposition of SiO2 and SiOF films onto Si and LN substrates by reactive sputtering of Si targets in a mixture of the gases Ar, O2 and NF3 is presented in this paper. Compositional analysis, infrared spectroscopy (FTIR), measurement of refractive index and Young's modulus of the films are reported in dependence on process parameters like sputtering power, pulse mode, gas flows, substrate temperature, substrate bias and pressure. Furthermore, stability of the films upon annealing is investigated.