F2-laser patterning of GaPO4 resonators for humidity sensing

Piezoelectric gallium orthophosphate (GaPO4) thickness - shear mode resonators were patterned by F-2-laser ablation and employed for gas sensing. The thickness of GaPO4 crystals was reduced from similar to 215 mu m to similar to 115 mu m by laser ablation and the piezoelectric fundamental resonance frequency in the thinned region increased thereby from similar to 6MHz to similar to 12MHz. The Q values of laser-thinned and pristine resonators in air were Q approximate to 7000 and Q approximate to 95 000, respectively. The GaPO4 crystals were coated by thin polyimide layers that served as receptor for water vapour. The resonance frequency of coated crystals decreased linearly with increasing level of relative humidity (RH) and the sensitivity for laser-patterned 12MHz GaPO4 resonators, S-RH approximate to - 98 Hz/% RH, was much larger than for the pristine 6MHz GaPO4 resonators.