Langasite, langatate - new piezoelectric crystals
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Miniature BAW Resonators and Filters Based on Single Crystals of Strong Piezoelectrics

V.B. Gruzinenco*, A.V. Medvedev**, A.N. Matsak*, O.A. Buzanov**

*Piezo, 16, Buzheninova, 107023 Moscow, Russia
**Fomos, 16, Buzheninova, 107023 Moscow, Russia

Using single crystals of "strong" piezoelectrics allows sizable reduction of BAW-resonators and filters. This paper reviews new information about development of microminiature BAW-resonators and monolithic filters fabricated from a single piece of quartz, LGS, LGT and lithium tantalate. The investigation results of wideband high-frequency (40-100 MHz) monolithic crystals filters using inverted mesa-technology are given. It is shown that these miniature resonators have considerable smaller values of aging, motional resistance and capacitance ratio in comparison with these parameters of quartz crystal units. The possibility of application of microminiature resonators (volume less than 0.02cm3) of different new types TCXO's, VCXO's, wide-band filters and other devices is proved.

I. Introduction

Recent improvements in the technology of "strong" piezoelectric crystal growing have considerably cut production costs and increased single crystal's quality. On the other hand new design solutions and technologies have caused device microminiaturization. Moreover less piezoelectric material is spent on a resonator or a monolithic filter. It has made "strong" piezoelectric devices more competitive, their production has been increasing from year to year, new radioelectronic devices with properties and characteristics that couldn't be realized on quartz have appeared.

High Q factor, chemical stability, a high temperature of phase shift (or its absence) allow realizing BAW piezoelectric resonators at a wide frequency range from 10 kHz to 100 MHz excited at fundamental mode. Recently crystal blank orientations have been found that allow realizing resonators with zero values of temperature-frequency coefficients or low steepness coefficients.

If a piezoelectric designer needs to microminiaturize devices, increase long-term frequency stability and performance temperature, reduce motional resistance R1 and capacitance ratio C0/C1 in resonators or widen a passband and lower load impedance in monolithic filters then he should consider LT, LGS or langatate as ideal candidates for wafers. These materials will allow him to design wideband filters and widen the tuning range of voltage controlled oscillators.

Recently Russian scientists have achieved a great success in the field of "strong" piezoelectric growth. A major contribution was made by the research of physical properties and the improvement of growing technologies that have been carrying out by Fomos together with faculties of the Institute of Crystallography of the RAS (IC RAS), Moscow State Institute of Steel and Alloys (MISA), Moscow State Academy of Information and Instrument-making, the International Academy of Information.

The high reproducibility of resonator electrical properties is achieved thanks to the quality of used single crystals. The research of growing technology effect on physical properties of single crystals has enabled to establish the reliable production of 3″ langasite and langatate crystals. The main results were achieved due to raw material quality control, the optimization of initial charge synthesis and heat assembler design, etc.

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