Yosihiro Tomita

A novel temperature compensation method for SAW devices using direct bonding techniques

We have developed a novel temperature compensation method for SAW devices using a direct bonding technique. The SAW device applied this method was composed of a conventional piezoelectric substrate such as LiTaO/sub 3/ and LiNbO/sub 3/, directly bonded without any bonding agents to a glass substrate having a relatively low thermal expansion coefficient (TEC). The piezoelectric substrate and the glass substrate were bonded in an atomic scale and the interface was very uniform. Therefore, the thermal strain at the surface of the bonded substrate caused by the difference between the TECs of the substrates was quite uniform and stable. With this structure, the thermal expansion of the piezoelectric substrate was restrained and the elastic constant of the piezoelectric substrate was changed by the thermal strain. Using this technique, we have succeeded to improve the temperature coefficient of frequency (TCF) of the SAW devices without causing any deterioration in the frequency response. This novel temperature compensation method is very promising for RF-SAW device applications.

Temperature stable SAW devices using directly bonded LiTaO/sub 3//glass substrates

We have developed temperature stable SAW devices by using a direct-bonding technique. Temperature stability approximating that of quartz was achieved by thinning a 36/spl deg/ Y-X LiTaO/sub 3/ substrate to 30 /spl mu/m on a glass substrate. The temperature compensated SAW devices were fabricated using a stacking procedure based on the direct bonding of a piezoelectric single crystalline layer onto a glass substrate. Because the thermal expansion coefficient (TEC) of the glass substrate is smaller than that of the piezoelectric layer, the piezoelectric layer experiences thermal stress. As a result, the temperature dependence of the SAW devices could be improved. By using a glass substrate with small TEC and minimizing the LiTaO/sub 3//glass thickness ratio, the temperature coefficient of frequency (TCF) of the SAW devices was maintained at approximately -6 ppm//spl deg/C from -30/spl deg/C to 80/spl deg/C. The SAW propagation characteristics (velocity and electromechanical coupling factor) and the frequency response of the SAW devices were almost the same as those of SAW devices using a 36/spl deg/ Y-X LiTaO/sub 3/ substrate.

Shock sensors using direct bonding of LiNbO/sub 3/ crystals

We have developed a shock sensor made with a bimorph type cantilever using by a technique of directly bonding piezoelectric single crystals. The cantilevers polarization-inverted structure was achieved by directly bonding LiNbO/sub 3/ single-crystal wafers having reverse polarization. This technique did not require any bonding agent. The basic characteristics of the shock sensor were evaluated. The resonance frequency of the cantilever having a length of 2 mm was 20 kHz. The sensor made from 140/spl deg/ rotated Y cut LiNbO/sub 3/ wafers had a high sensitivity of 6.4 mV/G, and excellent linearity.

Variable property crystal resonators by direct bonding techniques

Novel single-crystal bulk acoustic wave (BAW) resonators have been developed. These resonators realize desired electrical properties by directly bonding piezoelectric single crystals such as LiNbO/sub 3/ and LiTaO/sub 3/. The direct bonding technique fabricates composite piezoelectric substrates with new electrical properties depend on the combination of bonded wafers. For the first trial, polarization-inverted substrates have been fabricated by directly bonding two wafers of Z-cut LiNbO/sub 3/ single crystals having opposite polarization-direction. Second overtone resonators using these substrates provide excellent performance. Typically, the resonator has a high Q-value over 5,000, a figure of merit of 138, a capacitance ratio of 40, a resonant resistance of 30 /spl Omega/ and a frequency deviation of -50 ppm//spl deg/C.

Even-order thickness-shear mode resonators using X-cut LiTaO/sub 3/ plates realized by a direct bonding technique

Even-order thickness-shear mode resonators with good temperature characteristics have been achieved by applying direct-bonding techniques to X-cut LiTaO/sub 3/ single crystals. The variations in the resonance frequency of their main fast thickness shear mode was less than 500 ppm over the temperature range of -10 to 60/spl deg/C. Moreover, the results of studies on strip type chip resonators using direct-bonded x-cut LiTaO/sub 3/ wafers suggest that fabrication of small sized resonators with no spurious response is feasible. Their typical Q-value was between 3000 and 5000, and the resonant resistance was 10-20 /spl Omega/ at 13 MHz. These values an ideal for a range of applications. We confirmed that the direct bonding technique was particularly suitable for realizing high frequency resonators because the thickness of the resonator could be double that of a conventional fundamental resonator.