Kengo Asai

Influence of Underlayer Materials on Preferred Orientations of Sputter-Deposited AlN/Mo Bilayers for Film Bulk Acoustic Wave Resonators

Highly c-axis-oriented aluminum nitride (AlN) films were deposited by reactive rf magnetron sputtering on molybdenum (Mo) films for film bulk acoustic wave resonators (FBARs). We investigated the effect of underlayer materials on the crystalline qualities of AlN/Mo bilayers. AlN/Mo bilayers were sputter-deposited on silicon, silicon dioxide and AlN underlayers under the same sputtering conditions. The results revealed that the AlN underlayer markedly improves Mo film quality, and this greatly affects the orientation of the upper AlN film. By applying the AlN underlayer, we obtained a full width at half maximum of 2.3° from the X-ray diffraction rocking curve of the AlN film grown on a Mo film. We propose that the large surface energy of AlN is the reason the AlN underlayer improves the quality of the Mo films. In the FBAR with the AlN underlayer, a quality factor of 800 and an effective electromechanical coupling coefficient of 5.7% were achieved.

Experimental and theoretical investigation for temperature characteristics and propagation losses of SAWs on SiO/sub 2//Al/LiTaO/sub 3/

Improvement of temperature coefficients of frequency (TCFs) for LiTaO/sub 3/ substrate was investigated. SAW resonators formed on SiO/sub 2//Al/36/spl deg/Y-X LiTaO/sub 3/ substrate with various kinds of SiO/sub 2/ relative thickness were examined experimentally and theoretically. A zero TCF at fr was obtained for h//spl lambda//spl cong/0.31, while that at fa was obtained for h//spl lambda//spl cong/0.36 from experiments. Moreover, the experimental propagation losses revealed also different characteristics between fr and fa with regard to h//spl lambda/. These results can not be obtained by the conventional simulation procedures assuming the uniform SiO/sub 2/ film and uniform Al film on LiTaO/sub 3/ substrate. We not only applied the coupled finite-element and analytical-solution method to SiO/sub 2//Al/36/spl deg/Y-X LiTaO/sub 3/ substrate but also assumed same periodic unevenness for the surface of SiO/sub 2/ as that determined from the Al electrodes. Very good agreements between experimental and the theoretical results were achieved.

Grating-mode-type wide-band SAW resonators for VCOs

Grating-mode-type surface acoustic waves (SAWs) that are particularly suitable for wide-band SAW resonators were found. Their bandwidths are larger than 13%. Their spurious responses originating from Rayleigh waves are almost negligible when the Al-grating thickness is optimized for the cut angle of substrate. For example, the ratio of the Al-grating thickness to the SAW wavelength must be required within 0.125/spl plusmn/0.007 for 15/spl deg/-rotated Y-cut X-propagating lithium niobate. These SAW resonators were applied to UHF-band voltage-controlled oscillators. They achieved a high control-voltage sensitivity of 13.5 MHz/V (3.2%/V) and a high phase-noise performance of <-115 dBc/Hz offset 25 kHz from the carrier.

Multilayer Film Piezoelectric Lamb Wave Resonator for Several GHz Applications

We present a film piezoelectric Lamb wave (PLW) resonator based on a newly contrived multilayer film structure for several GHz applications. The multilayer film consists of two piezoelectric AlN layers that are excited by an intermediate electrode layer. Using numerical and experimental methods, we investigated the PLWs characteristics propagating in the multilayer film. Both the simulation and fabrication results indicated that a PLW mode with a phase velocity of above 25000 m/s can be excited in the multilayer film. These results show that our multilayer film structure is suitable for PLW resonators in the GHz range

Contour-Mode AlN Resonator with High Coupling Factor

A micro electro mechanical systems (MEMS) type contour-mode resonator with a high coupling factor of several percent is proposed. The fabricated 60-MHz-band radial-extensional resonators using MEMS process technology of AlN film exhibited a high coupling factor of 2.8% and high Q values of 3000 at the series resonant frequency and 4000 at the parallel resonant frequency. We demonstrated a Pierce type oscillator using the radial-extensional resonators and achieved low power consumption and stable oscillation. These radial-extensional resonators were suitable for filter and oscillator applications because the figure of merit, defined as the ratio of the Q value and the capacitance ratio, was 71 at the series resonant frequency and 94 at the parallel resonant frequency, which were the highest values of the contour-mode AlN resonators.

Q values of longitudinal leaky SAWs propagating on rotated Y-cut LN substrates along the perpendicular to the X axis

Longitudinal leaky SAWs (LLSAWs), which can be excited in a thick aluminum (Al) grating on a rotated Y-cut lithium-niobate substrate, were analyzed in a simulation that combined both FEM and BEM. The Q-value dependence of an LLSAW on the rotated angle and the Al-grating thickness was clear. The simulation results were confirmed experimentally. These SAWs have potential applications in high-frequency SAW devices because their velocities range from 6100 to 6500 m/s and their optimum Q values are theoretically larger than 4000.

New high performance SAW convolvers used in high bit rate and wideband spread spectrum CDMA communications system

New surface acoustic wave (SAW) convolver structures with high conversion efficiency and self-temperature compensation characteristics have been developed. Strong piezoelectric substrates, regardless of temperature coefficients of delay (TCD), can be used in these convolvers. New demodulation techniques using the developed SAW convolver for high bit rate and wideband spread spectrum code division multiple access (CDMA) communications have also been developed. I- and Q-channel demodulation data can be derived directly from binary phase shift keying (BPSK) or quadri-phase shift keying (QPSK) CDMA signals. In an experiment using a 128/spl deg/ YX-LiNbO/sub 3/ substrate, CDMA signals of 9 Mbps (megabits per second) with 60 Mcps (megachips per second) spread by 13-chip Barker code and 11 Mbps with 140 Mcps spread by 25-chip Shibas code were clearly demodulated, demonstrating the effectiveness of these techniques for use in future CDMA communications.

New miniature SAW antenna duplexer used in GHz-band digital mobile cellular radios

New filters used in an antenna duplexer which is applied to GHz-band digital cellular radios were developed. Taking 1.5 GHz PDC, we have achieved low insertion losses of 0.8 and 1.6 dB for T1 and R1 filters, respectively. The developed duplexer is 1.4/spl times/0.6/spl times/0.2 cm/sup 3/, several-time smaller than a conventional dielectric-filter duplexer. It also satisfied all required specifications and provided sufficient power-handling capabilities.

New high-performance SAW convolvers and their fundamental experiments for high-bit-rate CDMA communications system

New SAW-convolver structures with high-performance characteristics were proposed. Strong piezoelectric substrates which provide high-conversion efficiencies can be used. Fundamental experiments showed their complete temperature-compensation effects unrelated to TCDs of the substrates and applicabilities of high-bit-rate CDMA transmissions with more than several Mbps.

SAW timing-extraction filter and investigation of submicron process technology for Gb/s optical communications system

A new SAW filter that consists of 3 /spl lambda//8-wide meander-type electrodes and 3 /spl lambda//8-wide excitation electrodes on ST-cut quartz was designed for use in Gb/s optical communications systems. New submicron process techniques using the phase shifting method were used to fabricate this filter, which had an insertion loss of less than 14 dB and a Q of about 700, and satisfied all required specifications over a temperature range from 0 to 60/spl deg/C.