Michio Kadota

Ultra-wideband and high frequency resonators using shear horizontal type plate wave in LiNbO3 thin plate

A cognitive radio system requires a tunable filter with a wide tunable range to compose a simple front-end. Because the tunable range is limited by the bandwidth (BW) of resonators, an ultra-wideband resonator is an important device to implement the tunable filter. The authors previously obtained low frequency resonators with a wide BWs of 26 to 29% and a large impedance ratios of 92 to 98 dB between resonance (fr) and anti-resonance frequencies (fa) using 0-th mode shear horizontal (SH0 mode) plate wave in a LiNbO3 plate. Also, a tunable filter with the tunable range of 13% was demonstrated. However, the frequency (fr =1.8 MHz and fa = 2.3 MHz) is too low for modern wireless communication. Also, the resonators suffered from ripples due to transverse mode between fr and fa. In this study, high frequency SH0 mode plate wave resonators (fr = 430–560 MHz and fa = 530–670 MHz) with a wide bandwidth and a large impedance ratio have been fabricated using an ultrathin LiNbO3 plate. The ripples were suppressed by an apodized interdigital transducer.

ScAlN Lamb wave resonator in GHz range released by XeF 2 etching

Recently, it was reported that the piezoelectric characteristic of AlN was enhanced by doping Sc, and that 40% Sc-doped AlN (Sc_0.4Al_0.6N) had approximately 5 times higher piezoelectricity than pure AlN. In this study, we designed, fabricated and evaluated Lamb wave resonators using a Sc_0.4Al_0.6N thin film. First, we examined elastic constants c₄₄^E and c₄₄^D of Sc_0.4Al_0.6N using a thickness-shear wave resonator excited by parallel electric field, and then designed Lamb wave resonators using revised material constants. The Lamb wave resonators were fabricated by electron beam lithography, reactive ion etching, XeF₂ release etching etc. The maximum and average electromechanical coupling coefficients were 8.3% and 6.8%, respectively, for 2.6 GHz devices working in S₀ mode.

Ultra-wideband ladder filter using SH 0 plate wave in thin LiNbO 3 plate and its application to tunable filter

A cognitive radio terminal using vacant frequency bands of digital TV (DTV) channels, i.e., TV white space, strongly requires a compact tunable filter covering a wide frequency range of the DTV band (470 to 710 MHz in Japan). In this study, a T-type ladder filter using ultra-wideband shear horizontal mode plate wave resonators was fabricated, and a low peak insertion loss of 0.8 dB and an ultra-large 6 dB bandwidth of 240 MHz (41%) were measured in the DTV band. In addition, bandpass filters with different center frequencies of 502 and 653 MHz at 6 dB attenuation were numerically synthesized based on the same T-type ladder filter in conjunction with band rejection filters with different frequencies. The results suggest that the combination of the wideband T-type ladder filter and the band rejection filters connected with variable capacitors enables a tunable filter with large tunability of frequency and bandwidth as well as large rejection at the adjacent channels of an available TV white space.

Bandwidth-tunable SAW filter based on wafer-level transfer-integration of BaSrTiO 3 film for wireless LAN system using TV white space

Cognitive radio technology on TV white spaces has been promoted worldwide to solve the spectrum shortage problem due to the explosive increase in personal communication systems such as smartphones. One of the most difficult challenges to utilize TV white space for personal use is the miniaturization of frequency and bandwidth tunable filters to select vacant TV channels. We have developed a one-chip bandwidth tunable filter by wafer-level transfer-integration of BaSrTiO3 (BST) film varactors and surface acoustic wave (SAW) resonators on a lithium tantalate wafer. The 3 dB bandwidth of the bandwidth tunable filter is tuned between 3.25 MHz and 6.25 MHz by applying 7V to the varactors, while the center frequency was constant at 1.004 GHz, as designed. These filters were installed in our developed prototype cognitive radio systems based on the IEEE802.11af draft and successfully demonstrated wireless LAN communication on TV frequency band.

Determination of full material constants of ScAlN thin film from bulk and leaky Lamb waves in MEMS-based samples

The full material constants of a 40% Sc-AlN thin film were systematically determined. First, 4 kinds of MEMS resonators and a Lamb wave resonator were used to determine the material constants as much as possible. The other unknown material constants together with the material constants which needed more accurate estimation were determined based on ultrasonic microspectroscopy and parameter fitting. The phase velocity versus frequency (v-f) characteristics of A0 and S0 mode leaky Lamb waves in a self-suspended ScAlN film were measured using an ultrasonic microscope, and then theoretical v-f characteristics were fitted to the measured ones by tuning the material constants as parameters.

Bandwidth-tunable filter consisting of SAW resonators and BaSrTiO 3 varactors directly fabricated on a LiTaO 3 wafer

A prototyped bandwidth-tunable filter consisting of SAW resonators and BaSrTiO3 (BST) varactors is discussed in this paper. In general, the BST varactors require high deposition temperature (>600°C) in order to obtain good varactor performances. Thus special cared wafer integration process of BST varactors onto a 42°YX-LiTaO3 wafer was newly developed to prevent performance deterioration in SAW resonators. The bandwidth-tunable filter composed of ten pieces of both surface acoustic wave (SAW) resonators and BST varactors on the 42°YX-LiTaO3 substrate was successfully fabricated using the integration process. The measured center frequency of the filter is 1GHz, and the 3dB bandwidth is electrically tuned from 4.5 to 9.6MHz by adjusting a DC supply voltage from 0 to 10V.

Improvement of insertion loss of band pass tunable filter using SAW resonators and GaAs diode variable capacitors

Tunable filters with a wide band tunable range are required for mobile phones with multi-bands and cognitive radio systems to simplify their circuits. Previously, the authors prototyped a band pass tunable filter, which continuously changes frequency from 1734 to 1844 MHz (6.1%) using surface acoustic wave (SAW) resonators and Si diode variable capacitors (varicap). However, the insertion loss was large compared with calculated one because the Q factor of the Si diode varicap was as low as 1 at 1.8 GHz. In this study, the authors improved the insertion loss of the tunable filter using GaAs varicaps. Compared with the Si varicap, the GaAs varicap has a higher Q factor of about 41 at 1.8 GHz. The authors fabricated a tunable filter composed of two SAW resonators and the GaAs varicaps instead of the Si varicaps on a printed circuit board (PCB). As a result, the insertion loss was drastically improved by 12 dB at the maximum, and the frequency characteristic approached the calculated one. The tunable range was also improved from 6.1% to 6.8%.

Solidly mounted ladder filter using shear horizontal wave in LiNbO 3

The 0-th shear horizontal (SH0) mode plate wave has an electromechanical coupling factor k2 larger than 50% in a (0°, 120°, 0°) LiNbO3 (LN) plate thinner than 0.1λ, where λ is a pitch of an intergidital transducer (IDT). In the previous study, ultra-wide-band ladder filters composed of cavity type resonators recorded 6 dB bandwidth from 41% to 51% in a digital TV band. However, the structure is fragile, because the LN plate as thin as 0.5-0.6 μm is self-suspended. Inspired by solidly mounted (SM) type film bulk acoustic resonators (FBAR), the authors have come up with SM type SH wave devices. The SM type SH wave resonator has an ultra-wide bandwidth of 20% similar to that of SH0 plate wave resonators. In this study, the SM type ladder filter with an insertion loss of 0.5 dB was developed.

Ultrawide band ladder filter using SH 0 plate wave in thin LiNbO 3 plate and its application

Currently, a cognitive radio using vacant frequency band of digital TV (DTV) channels (TV white space: 470 to 710 MHz in Japan) is receiving a lot of attention. The cognitive radio strongly requires a tunable filter covering a wide frequency range of DTV band and bandwidth tunability. Authors previously fabricated ultra-wide band 0-th mode shear horizontal (SH0) plate wave resonators for the DTV band. A T type ladder filter, which has a low minimum insertion loss and an ultra-wide bandwidth covering the DTV channels, was numerically synthesized from the measured characteristics of the resonators. Combining the wideband filter and a band rejection filter composed of several resonators with different frequency, the band pass filters with center frequencies of 530 and 660 MHz, which are corresponding with lowest and highest frequencies in DTV band, were obtained. This paper shows that the combination of the wideband filter and both type band rejection filters connected with variable capacitors enables a tunable filter with large tunable range of frequency and bandwidth, and a large rejection of the outside of DTV band.

Solidly mounted resonator using shear horizontal mode plate wave in LiNbO 3 plate

A 0-th shear horizontal (SHo) mode plate wave has an electromechanical coupling factor k2 larger than 50% in a (0°, 120°, 0°) LiNbOs (LN) plate thinner than 0.1λ (λ is a pitch of interdigital transducer). However, the structure is fragile, because the plate thickness is as thin as 0.5~0.6 μm for devices working in several hundred MHz range. To address this issue, a solidly mounted resonator (SMR) structure was applied to SH mode plate resonators. The materials and layer thicknesses of a Bragg reflector, the Euler angle and thickness of a LN plate, and the thickness and material of electrodes were investigated to obtain large k2 and spurious-free response by finite element method (FEM) simulation. The designed Bragg reflector is composed of 6 layers of SiO2 and AlN. The best Euler angle is (0°, 90°, 0°) and the suitable thickness of a LN plate is 0.3 to 1 λ. Also, heavy and thick electrodes are better for interdigital transducers (IDT). This design is significantly different from that of the cavity type. A SMR type plate wave resonator of several hundred MHz was fabricated using 2 μm thick LN, and a wide bandwidth of 20% was measured.