F2-laser patterning of GaPO4 resonators for humidity sensing

Piezoelectric gallium orthophosphate (GaPO4) thickness – shear mode resonators were patterned by F2-laser ablation and employed for gas sensing. The thickness of GaPO4 crystals was reduced from ∼215 μm to ∼115 μm by laser ablation and the piezoelectric fundamental resonance frequency in the thinned region increased thereby from ∼6 MHz to ∼12 MHz. The Q values of laser-thinned and pristine resonators in air were Q≈7000 and Q≈95000, 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 12 MHz GaPO4 resonators, SRH≈-98 Hz/% RH, was much larger than for the pristine 6 MHz GaPO4 resonators.