Crystal quality of LGT samples: Influence on BAW resonators

Materials of the Langasite family are promising to design high quality bulk acoustic waves resonators. But, as for quartz crystal, LGS and LGT "boules" exist with different quality grades (not yet normalized). So, to analyze the quality of a given crystal, we have used the same tools allowing the characterization of the quartz crystal: essentially the IR / visible / UV spectrometry coupled to chemical analyzes of impurities content. These experiments and analyzes are performed on LGS and LGT samples coming from different suppliers (in USA, Russia, Japan or France). After that, LGT crystals have been processed into Y-cut planoconvex resonators working at 10 MHz on its 5th overtone. We have observed that these resonators present better results than those of SC-cut quartz resonators (particularly on their Q.f product). But, unlike quartz crystals, the asymptotic approximation of the analytical model of Tiersten allowing optimizing the geometrical parameters for good energy trapping is not verified. If it is possible, for quartz resonators, to maintain the Q.f product constant in the range [1 MHz, 1 GHz], it is not the same case for LGT ones. Nevertheless, we have obtained LGT resonators, manufactured in a best quality crystal sample, with a Q-factor of about 1.8 million on their 10 MHz 5^th overtones, measured at working temperatures for which we do not observed activity dips.