Design, manufacturing, and characterization of high-frequency thickness-shear mode resonators

AT-cut quartz crystals vibrating in the thickness-shear mode are well known as mass sensitive devices. After deposition of a sensitive layer on one or both surfaces of the quartz discs these resonators are suitable for the application as chemical sensors for analysis in gaseous and liquid media, which are known as quartz crystal microbalances (QCM). Up to now the resonant frequencies of these resonators are 5 to 30 MHz. The application of combined photolithographic and etching processes offers new promising approaches for the manufacturing of quartz resonators with higher resonant frequencies, up to 75 MHz, and smaller diameters. Resonators were fabricated and subsequently characterized optically and mechanically by means of light microscopy, SEM, and surface profiling and electrically by means of impedance analysis. The etched surfaces are very smooth and parallel, leading to high Q-factors up to 5·10⁴, which is excellent for high frequency resonators. A high mechanical stability due to the 128 μm thick quartz frame was proofed. The influences of surface roughness and etch channels on the resonators´ performance were examined. The behaviour under acoustic load was investigated experimentally. The results showed that 50 MHz resonators operated in water reach zero phase of impedance. In liquid media, changes in the viscosity (η_L) and density (ρ_L) lead to a decrease of the resonant frequency (Δf) of the QCM. A linear relationship between (η_Lρ_L)^1/2 and Δf was observed, in agreement with theory, while the frequency shifts are much higher than reported before