Properties of SiAlO2N protective coatings on surface acoustic wave devices

• SiAlO2N protective films have been deposited on surface acoustic wave devices.
• The SiAlO2N films were structurally, chemically, and electrically characterized.
• SiAlO2N elastic moduli were determined using differential acoustic delay measurements.
• Thicker SiAlO2N films have large effect on quartz SAW device temperature behavior.

The use of a protective wear-resistant amorphous SiAlO2N thin film overlayer (amorphous SiO2–AlN alloy) on top of surface acoustic wave (SAW) devices is demonstrated on both quartz and langatate substrates. SiAlO2N films were deposited by RF magnetron sputtering onto sapphire substrates, quartz SAW devices, and langatate SAW devices. The SiAlO2N layer had an amorphous structure, a density of 2.8 ± 0.1 g/cm3, a roughness less than 1 nm as measured by X-ray reflectivity, and a dielectric permittivity of 7.5 ± 0.05 as determined from microfabricated SiAlO2N capacitors. SiAlO2N elastic constants C11 and C44 were extracted using a numerical implementation of the matrix method for SAWs traveling in multilayer structures, and were found to be C11 = 160 ± 30 GPa and C44 = 55 ± 5 GPa. The operating frequencies of quartz SAW devices covered with SiAlO2N coatings were only slightly perturbed, but the temperature coefficient of delay (TCD) near 100 °C increased significantly by 250 ppm/°C. For langatate SAW devices, the SiAlO2N coating contributed an additional 8.5 dB to device transmission loss but the TCDs were minimally affected for SiAlO2N thicknesses up to 800 nm. This result suggests that langatate SAW devices for which temperature-frequency characteristics are important can be designed without consideration of the multi-layer structure, which greatly simplifies device design and modeling.