Chemical Controlled Dissolution of LGS Samples: Comparison with Quartz and GaPO4
Though the use of LGS family crystals for filtering applications is now well known, we present here one of the best way to realize antimesa structures working at higher frequencies than a few tens of MHz. Generally, this kind of realizations requires dissolution depths higher than 100 mum. Consequently, the rough surface state does not remain constant: it becomes smoother or its roughness parameter can increase drastically. For that, we have compared the chemical dissolution of the crystal in four different etchants (HCl, HNO3, H3PO4 and NH4F.HF) for three crystal orientations: X-, Y-and Z-cuts (in LGS, Y-cut is similar to the AT-cut in quartz crystal). The parameters chosen to compare the efficiency of our selected solvents are the dissolution rate and the average roughness Ra. Furthermore, we have worked at 2 different temperatures: 60 and 80degC. The experiences show firstly that a X-cut plate is etched in HCl rapidly, compared to the Z-cut and so reveal the anisotropy of the material structure. Secondly, we observe that the dissolution rate of LGS in HCl solution is 8 to 10 times higher than in other solvents, at a given temperature. Unfortunately, it tends to reveal deep dissolution figures on the surface. From this set of experiments, we conclude that the best compromise is obtained with the ammonium bifluorure solvent which allows us to obtain a smoother surface state than initially.
Investigations on LGS and LGT crystals to realize BAW resonators
The LGS family are promising materials for the design of high quality bulk acoustic wave resonators. We have manufactured many plano-convex 10 MHz 5th overtone Y-cut resonators using langasite (LGS, La3Ga5SiO14) and langatate (LGT, La3Ga5.5Ta0.5O14) crystals. We observed that the quality factor strongly depends on the polishing method, the supplier of the material, and on the energy trapping. For quartz crystals, we have found that resulting IR spectra exhibit absorption peaks more or less deep, linked to defects. These predominant criteria are not surprising, but they have to be defined in manner similar to that used for quartz crystal. A satisfying machining and polishing method has been first applied to elaborate high Q resonators, and a comparison between samples of LGS and LGT materials from different suppliers is established. In addition, LGT resonators are characterized by their motional parameters and frequency-temperature curves. Nevertheless, one of the main results is that the measured Q times f product is not the expected one. We present results of Q-factor versus radius of curvature: it appears that an optimization should be performed and that this last one cannot be directly transposed from that of quartz crystal resonator. Currently, the best resonator that we have made has a Q times f product of 1.4 times 1013 on its 5th overtone (1.7 times 1013 on its 9th overtone). This result is slightly higher than the similar parameter obtained on a state-of-the-art SC-cut quartz crystal resonator working at the same frequency.
Growth and characterization of La3Nb0.5Ga5.5O14 single crystals
Abstract We have successfully grown piezoelectric La 3 Nb 0.5 Ga 5.5 O 14 (LNG) single crystals with dimensions of 1 and 2 inches in diameter, using the Czochralski technique. The congruency of LNG was examined by measuring the lattice parameters and chemical composition of the grown crystals. Device properties of LNG filters were investigated. LNG single crystals showed superior properties for piezoelectric applications.