Surface Acoustic Wave Resonators With Hetero Acoustic Layer (HAL) Structure Using Lithium Tantalate and Quartz

This article reports a new concept of surface-acoustic-wave (SAW) resonator, which uses shear horizontal (SH) wave confined in a thin LiTaO₃ (LT) layer supported by a quartz (Qz) substrate. The LT layer is 35-50°YX LT, and the quartz substrate is 35-60°Y90°X Qz. A negative temperature coefficient of frequency (TCF) of the SH SAW in the LT layer is compensated by the quartz substrate, which shows a wide range of positive TCF depending on the crystalline orientation. Excellent TCFs of 2 and -10 ppm/°C were measured for the series and parallel resonance frequencies, respectively. The strong confinement of the SH SAW in the LT layer results in the best level of resonance characteristics ever reported. The measured impedance ratio reached 84 dB. On the other hand, spurious waves other than the SH SAW are not confined in the LT layer due to the unique properties of quartz, which results in spurious-free characteristics throughout a wide frequency range.