Keiji Onishi

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.

Direct bonding of piezoelectric materials and its applications

The needs for piezoelectric devices such as SAW filters has been greatly increasing with the rapid growth of mobile communication systems. The requirements for the devices have been widely spreading and becoming more critical. In order to satisfy these requirements not only new designing methods but also new piezoelectric materials have to be brought into practice. New technique of direct bonding of piezoelectric materials is very promising to give us new piezoelectric characteristics. The direct bonding can be carried out by contacting the hydrated surfaces of the piezoelectric materials and by heat-treating at relatively low temperatures. Authors found that the direct bonding can be applied to wide variety of materials such as quartz, LiNbO/sub 3/, LiTaO/sub 3/, PLZT, Si, glass and so on. Some combinations of them showed new preferable piezoelectric characteristics. In this paper the possibility of the direct bonding of piezoelectric materials and its applications such as SAW filters are discussed.

A new balanced-unbalanced type RF-band SAW filter

A new balanced-unbalanced type RF-band (950 MHz) surface acoustic wave (SAW) filter for use in portable telephones is presented. This SAW filter has unbalanced input and balanced output terminals. Balanced-unbalanced type SAW filters having a pass-band at 950 MHz were fabricated. The obtained results showed excellent characteristics such as low insertion loss, wide bandwidth, and small VSWR. Using the SAW filter, one can realize balanced-type RF circuits in portable telephones.

A new balanced-type RF-band SAW filter using SAW resonators

A new balanced-type RF-band (950 MHz) surface acoustic wave (SAW) filter using SAW resonators for use in portable telephones is presented. This SAW filter has balanced input and output terminals. Based on the computer simulation, balanced-type SAW filters having a pass-band at 950 MHz were fabricated. The obtained characteristics showed excellent characteristics such as low insertion loss, wide bandwidth, high attenuation at stopbands, and small VSWR. Using the SAW filter, one can realize balanced-type RF circuits in portable telephones.<<ETX>>

RF-MEMS switching devices using vertical comb-drive actuation in the CMOS process

Radio frequency micro-electro-mechanical system (RF-MEMS) switching devices using vertical comb-drive actuation toward low-voltage actuation, fast response are presented in this paper. The switching devices, which comprise comb-drive electrodes, are actuated entirely by the electrostatic forces applied not only for the down-state but also for the up-state. The cost-effective MEMS process compatible with the complementary metal oxide semiconductor (CMOS) process is presented in this paper as well. The fabrication process is composed by adapting the CMOS 0.18 µm back end of line (BEOL) process on 200 mm wafers. The MEMS process in the CMOS process enables the realization of passive devices integrated with active devices, which is effective for size and cost reduction. Two metal interconnection layers in the BEOL process are used for the MEMS process. Interconnection aluminum and inter-layer dielectric tetraethoxysilane (TEOS) are used as MEMS structural material and sacrificial material, respectively. The chemical mechanical polishing (CMP) process is implemented to planarize the sacrificial material surface. The structures were fabricated using a simple low-cost two-mask process. The characteristics of switching capacitors, C-V, RF performance, switching speed and continuous drive cycles are measured on the fabricated devices. The capacitance ratio for the down-state/up-state is Cdown/Cup = 5.4. The characteristics of switching speed response/actuation voltage in the down-state and up-state are 4.5 µs/5 V and 8.0 µs/5 V, respectively. The switching speed is stable up to 107 cycles in spite of the fact that the unipolar voltage speed is stable up to 107 cycles.