Design of high frequency GaPO4 BAW resonators by chemical etching

GaPO4 is a piezoelectric quartz homeotype material. For bulk acoustic waves device application, chemical etching is used to obtain very thin piezoelectric membranes. This paper presents a way to design high frequency resonators by chemical etching. Before etching, GaPO4 samples were characterized by X-ray diffraction topography and infrared absorption spectroscopy to evaluate the dislocation and –OH content, respectively. The chemical etching of gallium orthophosphate in H3PO4 and NH3 based solvents are compared. The anisotropy of the etching was studied using four different crystalline orientations. For the crystalline orientation used in electronic applications (AT cut), mixed H3PO4–GaPO4 solutions are not suitable because they produce major surface defects for etching depths above 40 μm. For the etching of this orientation, basic (NH3–K3PO4) solutions were preferred as they appear to allow to dissolve, without major defects, depths of up to 250 μm. The measurement of the resulting roughness either with a profilometer or by AFM gives a roughness of 0.02 μm r.m.s. without generating important surface defects. The piezoelectric properties of the etched plates were measured by the air-gap method as a function of the etching depth. No modification of the quality factor Q was observed for depths up to 200 μm.A three-dimensional etching process, involving a mechanical mask, was developed in order to manufacture AT antimesa resonators. Up to now, according to the raw material quality, the highest frequency obtained for the fundamental mode is 26.5 MHz (QF = 1.1 × 1012) and 16.5 MHz (QF = 1.6 × 1011) after 88 μm and 150 μm etching depths in H3PO4 + GaPO4 and NH3 + K3PO4 solvents, respectively.