Stability of langasite regarding SAW applications above 800°C in air atmosphere

Y-cut langasite (LGS) crystals are annealed under air atmosphere at temperatures from 800°C to 1200°C, and for time periods between one week and one month. Surface modifications are tracked with energy-dispersive X-ray spectroscopy, secondary ion mass spectroscopy, scanning electron microscopy and atomic force microscopy. This study shows that LGS is suitable for surface acoustic wave (SAW) one-week applications in air atmosphere at temperatures up to 1100°C. Above this temperature, many rough Ga-depleted flaws appear on crystal surface. If application is extended to one month, the maximum operating temperature decreases to temperatures below 1000°C. Indeed one-month annealing at 1000°C causes the apparition of many defects at the surface of LGS crystal. This deterioration impacts the performance of SAW delay lines, increasing insertion losses of 4-5 dB. Moreover, a shift of the operating frequency is also observed, enlarging with the harmonic rank.