Technological process and resonator design optimization of Ir/LGS high temperature SAW devices
Photolithography together with ion beam etching was used for fabrication of high temperature SAW devices. Ir thin film of 0.3 μm thick was deposited by magnetron sputtering without additional adhesion layers and than Ir film was annealed after electrode patterning in different conditions. The resistivity of magnetron sputtered thick Ir films drops noticeably after annealing. However, this process requires special care in order to avoid delaminating of the film due to developing high stress during such procedures. We have annealed the substrates with Ir films in different regimes and in different gas/vacuum conditions. The results of these studies have shown that annealing in air up to about 500 °C decreases the Ir film resistivity 1.5-2 times. Vacuum annealing did not show much improvement in comparison to open air annealing. Magnetron sputtered thin Ir films have somewhat porous structure allowing oxygen to diffuse from the substrate surface through Ir films. Resonator structures with thick Ir electrodes were prepared and tested. Examples of the resonator structures show very promising properties, such as high conductance and high Q-factor.
Investigation of the CTGS single crystals potential for high temperature SAW devices
SAW resonators were patterned on CTGS wafers with different orientations at different propagation angles. Ir was used as a metal without additional adhesion layers. Resonator responses were acquired with a network analyzer. Resonance frequencies of the responses were measured and processed to obtain temperature behavior. For the cut with Euler angles (0, 90°, 0) almost linear behavior was observed with TCF close to -35 ppm/°C. The turnover point of the fitted parabolic curve gradually changed with propagation angle ψ (0, 90°, ψ) from negative temperatures up to about +550°C at (0, 90°, 90°). This also means that this material gives orientations (close to (0, 90°, 40°)) with the turnover point near the room temperature. Surrounding orientations should probably be same useful (the turnover point change is about 0.5°C with ψ angle change by 1' for these orientations). The coefficient at the quadratic term (with Ir metal) has a low value of about -30 ppb/°C2. This value is several times lower than that of most langasite cuts and is close to that of ST-quartz. The material seems to be chemically stable at high temperatures. CTGS shows great potential and useful properties for devices operating in a wide temperature range. Similar to ST-quartz, CTGS can serve for temperature compensated resonators and filters at room temperature. It can also work in devices operating at temperatures up to several hundred degrees C.
Optimization of Quartz SAW Resonator Structure with Groove Gratings
Abstruct-A new surface acoustic wave (SAW) resonator optimum design method is proposed to reduce resonator sue. The fundamental design feature is optimization of the interdigital transducer (IDT) electrode period to reflector electrode period ratio, in order to adjust the frequency for the maximum IDT admittance to the maximum reflector reflection point. As a result of the optimized electrode period ratio, both reflector and IDT frequency responses are utilized to advantage, and good resonant Characteristics are obtained with a reduced number of reflector electrode fingers. This design method can be effectively applied for SAW resonators with groove gratings, both on reflectors and transducer, that are formed by a reactive sputter etching process. 145 MHz SAW resonators with groove gratings fabricated on a small 6 mm X 3 mm ST-cut quartz chip have provided good electrical performances, such as high Q values above 2 X lo4 and low resonance resistance in the 20-30 range.