Atsushi Isobe

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.

Rigorous treatment of leaky SAWs and new equivalent circuit representation for interdigital transducers

Leaky surface acoustic wave (SAW) interdigital transducers (LDTs) formed on 36/spl deg/ YX-LiTaO/sub 3/ and 41/spl deg/ and 64/spl deg/ YX-LiNbO/sub 3/ are theoretically analyzed, and a new equivalent circuit representation is proposed. The influence of the attenuation constant due to leakage as well as conductance caused by bulk wave radiation are taken into account. A new treatment based on an integral equation approach provides all circuit parameters. Fundamental experiments show fairly good agreement between theoretical and experimental results, confirming that this treatment provides very accurate tools for designing leaky-SAW devices.

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.

Recent and future RF SAW technology for mobile communications

Surface acoustic wave (SAW) technology has been widely applied to VHF/UHF radio communications to reduce the volume of transceivers. In this paper, not only recent new SAW devices but also future SAW technology to achieve much higher-performance devices are investigated. First, the latest status of 0.8-1.5-GHz SAW antenna duplexers for cellular radios and a miniature SAW-VCO are presented. Second, new SAWs with extremely high velocities and fine submicron process techniques are examined. Finally, a future SAW-based chip-type receiver is discussed.

New rigorous treatment of leaky SAWs and its application to high-performance filters used in mobile pocket phones

Leaky-SAW IDTs formed on 36° YX-LiTaO3, and 41° and 64° YX-LiNbO3 are theoretically analyzed, providing a new equivalent circuit. Fundamental experiments show fairly good agreement between theoretical and experimental results. New high-performance SAW filters using this equivalent circuit satisfy all the specifications for cellular pocket phones, incorporating the characteristics of a SAW antenna duplexer also proposed recently

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

A miniature high-Q grating-mode-type SAW resonator and a wide-band 1-GHz SAW-VCO for mobile communications

We have designed and tested a compact high-Q SAW resonator for wide-band VCOs that oscillate in the 1-GHz band. This was achieved by optimizing the SAW propagation in the Al-grating structure, especially by changing the shape of the structure. The resonator chip size was 0.8 /spl times/ 0.8 mm and the Q value of the SAW resonator exceeded 200. Its spurious response originating from Rayleigh waves was almost negligible when the Al-grating thickness was optimized for the cut angle of the substrate. A 1-GHz VCO that used the SAW resonator showed a high control-voltage sensitivity of 25.0 MHz/V (2.3%/V) and a high phase-noise performance of <-106.0 dBc/Hz when offset 25 kHz from the carrier.

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.

Quality Factor of Contour-Mode AlN Resonator

We present contour-mode AlN resonator structure and fabrication process for high quality factor (Q-factor). The vibrational energy should be confined to the resonant part of the radial extensional (RE) resonator in order to obtain a high Q-factor and high coupling factor (k2) from devices made using micro electro mechanical systems (MEMS) technology. In this paper, we discuss how to form a support for the resonant part in a high-precision process. We fabricated simple RE resonators and clarified through experimentation that it is difficult to avoid deterioration of the Q-factor because the RE vibration energy leaks from the resonant part into the silicon substrate through the supporting points. Next, we fabricated tuning-fork RE resonators by using the three-etching-process method and clarified through experimentation that this method is useful for forming the resonant part into the correct circular shape and avoiding energy leakage.