Kinetics of the exchanges at the solid–liquid interface during the dissolution process of gallium orthophosphate

Gallium orthophosphate is a piezoelectric material with an α-quartz structure. In order to manufacture bulk acoustic wave devices (BAW), controlled chemical dissolution is often used to reduce the thickness of the piezoelectric membranes. This paper presents the kinetics of the chemical exchanges, which occur at the solid–liquid interface during the chemical dissolution of GaPO₄ in phosphoric acid. Based on chemical composition of phosphoric acid solvent, the pure dissolution rate is determined. A strong anisotropy of chemical reactivity is formed. The dissolution rate is the lowest for the crystallographic z-plane (0 0 0 1) but this orientation is the most sensitive with respect to the proton concentration and the temperature. In accordance with the crystal growth rates, the nucleation at the interface for the (1 0 2 0) plane, named X-plane, is also the most rapidly dissolved. Assuming the activation energies corresponding to dissolution and to nucleation are like standard activation energies, the different values of the standard enthalpy variation are calculated. The most important variation is obtained for the z-plane (Δ_rH=−14.3 kJ/mol) and the lowest for the X-plane (Δ_rH=−5.4 kJ/mol).