Miniature BAW Resonators and Filters Based on Single Crystals of Strong Piezoelectrics

II. Lithium Tantalate Resonators And Filters

Ferroelectric single crystals of lithium tantalate have crystallographic orientations that allow manufacturing resonators with a zero first order temperature-frequency coefficient. That's why the crystals draw the attention of microminiature resonator and filter designers. The authors have been carrying out work aimed at setting up the production of lithium tantalate resonators that cover frequency range from 10 KHz to 100 MHz.

Low-frequency lithium tantalate resonators (10-800 KHz)
Resonators in 0.02-0.05 cm³ micro-packages have been designed at frequencies from 10 to 40 KHz with piezoelements fabricated as fork, strip and bar (pillar) plates, flexibly vibrating in Y'Z' and XY' planes. The main properties and characteristics of such resonators are listed in Table 1.

Table 1

Crystal element shape	f₀, kHz	TFC, 10^-8 C^-2	R₁, kOhm	C₀/C₁	Q, k	Mode of vibration
Fork	15 - 40	8 - 10	0.5 - 50	40 - 80	5 - 40	Flexural
Bar (pillar)	20 - 120	7 - 9	0.1 -	29 - 30	10 - 80	Flexural
Strip	350 - 800	7 - 9	0.1 - 40	20 - 40	10 - 50	Extensional

Lithium tantalate resonators with face and thickness shear at frequencies 0.500 - 100 MHz
For a frequency range of 600 to 1500 kHz resonators with face shear vibrations have been designed. The resonators have electrodes that excite "parallel" field vibration (fluctuation). For a frequency range of 5-50 MHz resonators have been designed using X-cut crystal elements, operating at fast thickness shear mode. Electrodes were round shaped. The main properties of the resonators are listed in Table 2.

Table 2

Crystal element shape	f₀, MHz	TFC, 10^-8 C^-2	R₁, Ω	C₀/C₁	Q, k	Mode of vibration
Rectangular	0.5 - 1.5	6 - 9	50 - 200	30 - 100	15 - 100	Face shear
Rectangular Round, Ø4.5 mm	8 - 50	10 - 12	10 - 80	12.5 - 20	1 - 6	Thickness shear
Rectangular Round, Ø4.5 mm	50 - 100 3-th over.	-	25 - 100	140 - 220	0.6 - 1.5	Thickness shear

The properties of microminiature vacuum dual mode lithium tantalate resonators with a volume less than 0.02 cm³ and thermodependant (at 9.4 MHz) and thermostable (at 10.6 MHz) modes are listed in Table 3. The temperature-frequency characteristics of such resonators are illustrated in Figure 1.

Table 3

Excited mode	f₀, MHz	FTC type	Thermal coefficient	R₁, Ω	C₀/C₁	Q, k	Mode of vibration
Thermostable	10.6	Parabolic	(9-10)×10^-8 C^-2	15 - 40	9.3 - 11.5	4 - 5	Thickness shear
Thermodependant	9.4	Linear	65.4 C^-1	25 - 50	97 - 107	7 - 12	Thickness shear

The temperature-frequency characteristics

Fig. 1. Temperature-frequency characteristics of a dual-mode lithium tantalate resonator

Lithium tantalate filters
At present a frequency range from 2.5 to 30 MHz has been mastered, while a relative pass bandwidth is 0.5 - 4.5% for some frequencies. The latter is achieved by using crystal elements fabricated from incongruous crystals. Moreover a high value of electromechanical coupling factor for thermostable cut allows using relatively small electrodes, that in its turn leads to wafer downsizing.

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