Results: 2021, 2020

The second stage of the SAWSAT project corresponds to Task. 2.1. SAW resonator test structure design (Run 1); Task. 2.2. SAW resonator test structures fabrication (Run 1); Task. 2.3. Structural characterization, microwave measurements and parameter extraction for Run 1; Task 2.4. Design of SAW resonators band pass filters; Task.2.5. Fabrication of SAW band pass filters (Run 2).

The main results obtained during the second stage are related to the design, fabrication, characterization and parameter extraction for the test SAW resonator structures and the design and fabrication of SAW resonator band pass filter test structures.
Test SAW structures consisting of transducers with 130 nm, 135 nm and 150 nm digit/interdigit were designed. Interdigital transducers (IDT) with and without reflectors, as well as configurations for acoustic wave transmission at different distances in symmetric and antisymmetric topologies were designed. This was followed by the design of the connecting microwave circuit, based on 50 Ohm characteristic impedance coplanar waveguide transmission line sections, with a layout geometry designed to accommodate the three-pin on wafer ground-signal-ground (G-S-G) measurement probes.

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Design of SAW resonator test structures: (a) interdigital transducer without reflectors; (b) interdigital transducer with reflectors; (c) antisymmetric and symmetric topologies
Test devices with 50 nm Ti, 5 nm Ti + 50 nm Au, 50 nm Al si 50 nm Cr IDTs were manufactured using a combination of photolithographic and electron beam lithography processes. The test structures were characterized microstructurally by means of a scanning electron microscope (SEM) and in the microwave domain, using a vector network analyzer with on wafer contacting measurement station. Calibrated scattering (S) parameter measurements were performed and the corresponding transmission and reflection parameter curves were plotted.

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SEM images of fabricated SAW resonators: (a) with 130 nm wide gold transducers; (b) with 130 nm chromium transducers; (c) with 130 nm titanium transducers; (d) with 130 nm aluminum transducers

Optimization methods were employed for the equivalent model parameter extraction for the SAW resonators placed in series with CPW transmission line segments, based on the modified Butterworth van Dyke equivalent circuit. The extracted parameters are the series resonance frequency (fser), the electromechanical coupling factor (kt2), the quality factor (Q), the static capacitance of the resonator (Co), the dielectric losses (Ro) and the conduction losses (Rs). The other parameters are calculated based on the analytical relations given below.

Modified Butterworth van Dyke (mBVD) equivalent circuit used for SAW resonator parameter extraction

Based on the SAW resonator parameters extracted from measured results, band pass filter circuits were designed for the 7.7 GHz and 8 GHz frequency bands, using IDTs with 135 nm and 130 nm digit width, respectively. The IDT metals considered for the fabrication of the filter structured were gold and aluminum. The filters were designed based on the transducer equivalent circuits, monolithically integrated with lumped element inductors placed in parallel and series connection. The circuit models of the resonators were then included in electromagnetic models and simulated in order to obtain the desired filter characteristics. The layouts of the filters monolithic integrated with planar inductors were then used for the processing mask set design. The test filter structures include IDTs with and without reflectors and were fabricated using the same manufacturing process as described for the SAW resonator test structures.

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Detail of SAW resonators from the fabricated band pass filters: (a) horizontally connected gold transducers; (b) vertically connected aluminum transducers

All objectives of the 2^nd Stage of the project were successfully accomplished.