Abstract
In this paper, first, the surface acoustic wave (SAW) propagation mode and a method of analyzing the propagation property are introduced briefly. Then, typical composite substrate structures ...that have been developed to obtain high-performance SAW devices are reviewed. Furthermore, the recent results obtained by the author and research colleagues on the propagation and resonance properties of leaky SAW (LSAW) and longitudinal-type LSAW on dissimilar-material bonded structures comprising a LiTaO
3
(LT) or LiNbO
3
thin plate with a thickness of less than 1
λ
(
λ
ː wavelength) and a quartz substrate are described. The control of attenuation and the cause of large coupling factor of LSAWs by utilizing layered structures were also discussed. For the bonded 4 inch wafer of 36°YX-LT/AT90°X-quartz with a thin-plate thickness of 0.3
λ
, an admittance ratio of 81 dB, a fractional bandwidth of 4.2%, and resonance and antiresonance factors of approximately 1500 with markedly improved properties compared with a single 36°YX-LT substrate were obtained experimentally at 2.2 GHz.
Piezoelectricity of YbAlN films has recently been shown to be almost as high as that of ScAlN films. YbAlN film surface acoustic wave (SAW) resonators are expected to have a high coupling factor. We ...theoretically investigated the propagation characteristics of first-mode Rayleigh SAWs (RSAWs) on Yb0.33Al0.67N film/high-velocity Si, sapphire, AlN, SiC, BN, and diamond substrates. The first-mode RSAWs on the YbAlN layered structures had high coupling factors, higher than those on ScAlN layered structures. An enhancement of the effective coupling factor of the first-mode RSAWs was observed in polarity inverted YbAlN film/BN or diamond substrate structures.
Recently, it has been reported that leaky surface acoustic waves (LSAWs) and longitudinal LSAWs on piezoelectric LiNbO3 and LiTaO3 thin plates bonded to high velocity quartz substrates have high ...phase velocity, high coupling, and low attenuation. These lead to high-frequency operation, a wide band-width, and a high Q factor for next generation SAW filters. Moreover, we theoretically demonstrated that longitudinal LSAWs on high piezoelectric ScAlN films/quartz substrates also possesses high phase velocity, high coupling, and low attenuation. In this paper, LSAW characteristics on ScAlN films/quartz substrates were investigated theoretically. When the Euler angles and the film thickness of the ScAlN film and quartz substrate were optimized for LSAW, the LSAW resonators with (0° 90° 72°) Sc0.4Al0.6N film/(0° 128.5° 0°) quartz were found to have a high Q factor of ∼21 610 and a high coupling of ∼5.8%.
Abstract
AlN film bulk acoustic wave (BAW) resonators and filters that operate at frequencies above 5 GHz are required for 5G and beyond 5G communication applications. However, the power-handling ...capability, Q factor, and electromechanical coupling in high-frequency AlN film BAW resonators will be degraded as a result of the significantly reduced thickness and volume of the devices. Higher-order mode BAW resonators with polarity inverted (PI) multilayered films can operate at high frequencies while maintaining the thickness and volume of the devices. In this study, we fabricated an eight-layered PI GeAlN/AlN film BAW resonator by alternately growing Al-polar AlN layers and N-polar GeAlN layers. The PI GeAlN/AlN film BAW resonator resonated in the 8th-order mode. The film thickness of the PI GeAlN/AlN film BAW resonator was approximately thirteen times larger than that of a single-layer AlN film BAW resonator operating in the same frequency range.
Abstract
The rapid development of mobile communications requires high performance surface acoustic wave (SAW) filters. ScAlN film SAW resonators have a high coupling factor (
K
2
) and high phase ...velocity, enabling them to function as effective SAW filters. We theoretically investigated high-order mode Rayleigh SAWs (RSAWs) on single-layered or polarity inverted two-layered ScAlN film/AlN or BN substrate structures to find the optimal structure to achieve a higher
K
2
and higher frequency. The
K
2
and phase velocity for the single-layered ScAlN film/BN substrate structure were higher than those for the corresponding AlN substrate structure. The
K
2
was enhanced in the polarity inverted ScAlN film/AlN or BN substrate structure. Finite element method analysis revealed that the effective coupling factor
K
eff
2
for the 2nd to 4th mode RSAWs on polarity inverted multilayered ScAlN films was enhanced by setting the boundary of the polarity inverted structure at the all positions where the particle displacements were concentrated.
Longitudinal leaky surface acoustic wave (LLSAW) modes with a high phase velocity are suitable for high-frequency SAW filter applications. However, their attenuation is higher than that of other SAW ...modes, causing a small Q factor. On the other hand, a low attenuation and high coupling factor K2 in LLSAWs on a piezoelectric thin plate/high-velocity sapphire or quartz substrate have been demonstrated theoretically and experimentally. We expected that LLSAWs propagating on a highly piezoelectric ScAlN layer/sapphire or quartz substrate also have a high phase velocity, high K2 and low attenuation. In this study, the characteristics of LLSAWs on ScAlN layer/sapphire or quartz substrates were theoretically investigated. The cut angle and SAW propagation direction of quartz substrates strongly affect the attenuation of LLSAWs on the layered structure. We found that a LLSAW resonator with (0° 90° 90°) Sc0.4Al0.6N layer/X-cut 30° Y propagation quartz has a high Q factor and high K2, which is adequate for SAW filter applications.
Abstract
The enhancement of high-order surface acoustic wave (SAW) harmonics by increasing the metallization ratio was theoretically and experimentally investigated in high-performance bonded ...dissimilar-material structures. The simulation using a finite element method for 36°Y-cut X-propagating LiTaO
3
(36°YX-LT)/AT-cut 90°X-propagating quartz (AT90°X-quartz) and 27°YX-LiNbO
3
/AT90°X-quartz was performed by setting the metallization ratio of the interdigital transducer to
a/p
= 0.8 (
a
: electrode width,
p
: pitch), and the LT and LN thin-plate thicknesses to
h
/
λ
= 0.05 (
λ
: wavelength) and 0.1, respectively. The fractional bandwidths for 36°YX-LT/AT90°X-quartz and 27°YX-LN/AT90°X-quartz were obtained to be 1.6% and 4.0%, respectively, which were larger than those for single LT and LN. For a leaky SAW (LSAW) resonator with
a
/
p
= 0.8 fabricated on 36°YX-LT/AT0°X-quartz, the resonance properties and temperature coefficient of frequency of LSAW fundamental waves and harmonics were measured. The measured fractional bandwidth increased from 0.8% to 1.4% for single LT.
Abstract
The propagation and resonance properties of a leaky surface acoustic wave (LSAW) on quartz thin plates bonded to a similar-material substrate are investigated theoretically. The ...electromechanical coupling factor
K
2
on Z-cut quartz (Z–Q) thin plates bonded to an AT-cut 0°X-propagating quartz (AT0°X-Q) support substrate is calculated to be 0.43%, which is approximately three times larger than the maximum value of a single quartz substrate. A positive temperature coefficient of frequency for LSAW can be produced on a quartz thin plate bonded to a quartz substrate with a different cut angle. By the finite element method, the aluminum thin-film thickness dependence of the resonance properties of LSAW on LST-cut quartz (LST-Q) and LST-Q/AT0°X-Q is analyzed. In the simulation with the optimal Al thin-film thickness, the admittance ratio on the LST-Q/AT0°X-Q is found to be larger than that for the single LST-Q.
To obtain a bonded structure with low attenuation for leaky surface acoustic waves (LSAWs), the propagation and resonance properties of LSAWs on a similar-material thin plate bonded to a LiTaO3 (LT) ...or LiNbO3 (LN) support substrate were theoretically analyzed. It was found that zero attenuation can be simultaneously obtained for free and metallized surfaces by utilizing the similar-material bonded structure but not when using a single structure. For a 10°YX-LN/66°YX-LN structure, the antiresonance Qa and the admittance ratio of the bonded structure were 1570 and 99 dB, respectively, even taking into consideration a mechanical loss of 1/Qm = 10−3, and were higher than those of a single LN structure. For a YX-LT/36°YX-LT structure, we observed the same or less attenuation and an approximately 1.3 times higher coupling factor than those for a single 36°YX-LT structure. The LT similar-material bonded structure had slightly superior resonance properties to the single structure.
Piezoelectric AlN films possess high bulk acoustic wave velocity, low acoustic attenuation, and good temperature characteristics. However, AlN film bulk acoustic wave resonators (FBARs) have a ...relatively small electromechanical coupling coefficient k eff 2. It was recently reported that Cr doping in AlN films increased the piezoelectric constant d 33. The k t 2 of AlN FBARs may thus be enhanced by Cr doping. In this study, we investigated the effect of Cr doping in (0001)-oriented AlN films on k t 2 from the frequency characteristics of high overtone bulk acoustic wave resonators. k t 2 of the Cr-doped AlN films was increased for Cr contents of less than 3%. The maximum k t 2 observed for the Cr0.012Al0.088N film was 5.9%, which was approximately 1.4 times higher than that of pure AlN film (k t 2 = 4.4%).
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