New piezoelectric materials for SAW filters

Scientific research of surface acoustic wave (SAW) devices had an early start by the end of 1960s and led to the development of high frequency and small size piezo devices. A sustained effort was dedicated for these components to be transformed into many more interesting applications for telecom market. Recently the employment of new piezo materials and crystallographic orientations open new opportunities for SAW filters. New piezoelectric crystals of gallium orthophosphate (GaPO 4 ) provide higher electromechanical coupling than quartz, while maintaining temperature compensated characteristics similar to quartz. Based on this material phase transition of 970°C, development of new piezo devices to operate at higher temperatures up to 800°C can be done. SAW velocities about 30% lower than ST-X quartz, favors smaller and more compact devices. Other advantages of GaPO 4 are: stability with high resistance to stress induced twinning, 3~4 times higher electromechanical coupling than quartz and existence of SAW temperature compensated orientations. Another family of new materials of the trigonal 32 class has received much attention recently because of their temperature behavior similar to quartz and the promise of higher electromechanical coupling coefficients. It is the family of langasite (LGS, La 3 Ga 5 SiO 14 ), langatate (LGT, La 3 Ga 5.5 Ta 0.5 O 14 ) and langanite (La 3 Ga 5 .5Nb 0.5 O 14 ). Langasite crystals, easier to obtain and with the value of electromechanical coupling coefficient intermediate between quartz and lithium tantalate (k 2 =0.32% for 0°, 140°, 22.5° orientation and k 2 =0.38% for 0°, 140°, 25° orientation), enable us to design SAW filters with a relative pass band of 0.3% to 0.85%. Other piezoelectric materials are reviewed for comparison.