Electrical characterisation of high-frequency thickness-shear-mode resonators by impedance analysis

Increasing the frequency of coated AT-cut quartz resonators should result in an increase of the measuring signal of these devices in the application as chemical sensors. A combined photolithographic/etching process allows a reliable fabrication of mechanically stable resonators with frequencies up to 75 MHz. The results of the electrical characterisation of the high-frequency resonators by means of impedance analysis are presented. The measured Q-values are about 5×104, which is an excellent figure for high-frequency resonators. The separation of unwanted inharmonic from harmonic modes is improved by reducing the diameters of the deposited electrodes. The dependence of the equivalent circuit values on frequency and electrode diameters are investigated. The results reveal good agreement with theoretical values obtained from a one-dimensional transmission-line model for L and C1 and quite poor agreement in R and C0. Investigations on the impedance behaviour under liquid load show an increase in damping for high-frequency resonators as expected. The possibility for operating the high-frequency resonators in an oscillator circuit under liquid load depends on various influence parameters. A general estimation is difficult to be made and should be tested experimentally for certain applications. However, the use of resonators with frequencies higher than 50 MHz in liquids seems not to be advisable.