Electrical and Optical Characterization of SAW Sensors Coated with Parylene C and Their Analysis Using the Coupling-of-Modes (COM) Theory.

In this paper, we present how complementary characterization techniques, such as electrical measurements with a vector network analyzer (VNA), optical measurements with a laser Doppler vibrometer (LDV), and numerical simulations with the finite element method, coupled with spectral domain analysis (FEMSDA), allow us to independently access different properties of a SAW device and fully characterize its operation using the coupling-of-modes theory (COM). A set of chemical SAW sensors coated with parylene C layers of different thicknesses (1, 1.5, and 2 µm) and an uncoated sensor were used as test samples. The sensors represent dual-channel electroacoustic delay lines operating in the vicinity of 77 MHz. The IDTs consist of split aluminum electrodes deposited on a AT-cut quartz substrate. The thickness-dependent influence of the parylene C layer was observed on the operating frequency (SAW velocity), static capacitance, attenuation, crosstalk, and reflection coefficient. COM parameters were reported for the four cases considered; measured and simulated data show good agreement. The presented approach is suitable for the design, characterization, and validation of polymer film-coated SAW sensors.

Piezoelectric aluminum nitride resonator for oscillator.

This work investigates properties of the thin film elongation acoustic resonator (TFEAR) operating at megahertz frequencies in air. This resonator is composed of a piezoelectric layer of AlN sandwiched between 2 Al electrodes. TFEAR works in the extensional mode excited via AlN d31 piezoelectric coefficient. A 3D finite element method (3D-FEM) analysis using ANSYS software has been performed to model static modal and harmonic behavior of the TFEAR. To consider insertion losses into the substrate, equivalent electrical models based on a modified Butterworth-Van Dyke (MBVD) circuit have been improved by adding extra dissipative elements. Thus, a whole model for the on-wafer characterization setup is given, allowing for automatic de-embedding of the present TFEAR equivalent circuit. Quality factors Q as high as 2500 in air have been recorded with motional resistance lower than 400 ohms. A first oscillator based on a TFEAR resonator was also designed and tested.