Quality factor (Q) and electro-mechanical coupling factor (k 2 ) of acoustic resonators are keys to achieve low-loss and wideband filters. Transverse modes (TMs) need to be suppressed to avoid spurious responses with minimal degradation of Q and k 2 . Guided SAW (e.g. LT/SiO2/Si) using a shear horizontal (SH) mode makes it difficult to use piston mode configurations to suppress TMs because of a large scattering loss on the edge of electrodes (slow region). Apodization is a well-known technique to reduce TMs. However, without a proper design, it reduces Q and k 2 as well as resonator capacitance resulting in a larger resonator size. This paper demonstrates a new apodization scheme able to suppress TMs with minimal reduction in Q, k 2 and capacitance. First, a simulator combining coupling-of-modes (COM) and scalar potential theory is developed to accurately predict TM responses as well as SAW distributions in resonators. Second, a systematic optimization of an apodization scheme maintaining high Q, k 2 and large capacitance is conducted using the developed simulator. Finally, guided SAW resonators with the optimized apodization and a piston mode configuration are fabricated on LT/SiO2/Si and the performances are compared. The measurement confirms that the proposed apodization has a similar k 2 , capacitance density and TM suppression to the piston mode configuration, while its Bode Q is higher. This is probably because the proposed apodization without a slow region has lower scattering losses than the piston mode configuration.

Thanks to improved wafer bonding technologies it is now possible to manufacture wafers with thin single crystal lithium niobate (LN) films with arbitrary orientation. Previous research has focused on interdigitated transducer (IDT) resonators on an LN membrane and cavity. In this work a solidly mounted LN IDT resonator is demonstrated. Hafnium oxide is chosen as a high acoustic impedance reflector material to reduce the parasitic capacitance caused by conductive layers. To ensure the modeling accuracy of hafnium oxide, delay lines have been used to extract a shear stiffness coefficient of 94.4 GPa. To maximize resonator coupling, the LN orientation is chosen to be YX 1 120°• Solidly mounted LN IDT resonators with coupling larger than 20% are demonstrated.

The layered SAW with a thin piezoelectric layer on a support substrate is a promising technology for its high quality factor (Q), high electromechanical coupling factor $k^{2}$ and small temperature coefficient of frequency (TCF). A large $\Delta\mathrm{TCF}$ (TCF difference between resonance and anti-resonance) is one of the biggest challenges for this technology. Last year, we proposed a new LT/quartz substrate providing a near-zero ATCF [1]. Another challenge for layered SAW is transverse mode suppression. This paper analyzes the transverse modes on layered SAW and proposes a LT/quartz substrate which drastically suppresses the transverse mode generation. Dispersion/slowness curves for SAW propagating on metal gratings on layered SAW substrates are calculated by FEM to analyze the mechanism of transverse mode generation. It is revealed that most layered SAW substrates such as LT/Si, LT/sapphire or LT/quartz have convex slowness curves which cause transverse mode generation. One way to suppress the transverse modes is to have a vertical slowness curve. Based on careful slowness calculations, a quartz orientation is optimized to allow a vertical slowness curve in LT/quartz near the propagation angle $\theta=0$ . The optimized quartz orientation is 69°Y90°X which is the same orientation as the one we proposed in [1]. The LT/69°Y90°X quartz bonded wafer is fabricated and a transverse mode free response is experimentally confirmed without any apodization or piston mode configuration for 1 port resonators. In addition, both TCF and $\Delta \mathrm{TCF}$ are measured near 0 ppm/°C, and a spurious free out-of-band response is achieved while maintaining a high Q and high $k^{2}$ .

A layered SAW configuration consisting of a thin piezoelectric layer bonded onto a support substrate has been drawing attention for its superior performances—a higher quality factor (Q), higher electromechanical coupling factor (k 2 ) and better temperature coefficient of frequency (TCF)—than conventional SAW technologies. However, one of the biggest challenges on the layered SAW is a much worse ΔTCF than conventional acoustic resonator technologies such as LiTaO3 (LT) SAW, temperature compensated (TC) SAW with SiO2 over coat and BAW, where ΔTCF is defined as TCF at anti-resonance (TCFp) minus TCF at resonance (TCFs). The worse ΔTCF causes a serious return loss/VSWR degradation as well as a significant impedance shift of the filters with temperature change. First, this paper analyzes the cause of ΔTCF degradation on layered SAW considering linear and nonlinear effects. Based on the analyses, a new cut of quartz substrate allowing a near-zero ΔTCF as well as near-zero TCFs and TCFp is proposed. The bonded wafer with thin LT on the new quartz substrate is fabricated and resonator performances on the new layered SAW substrate are evaluated. Both TCFs and TCFp are measured near 0 ppm/°C. The ΔTCF is only –2 ppm/°C, which is the best ΔTCF of all the conventional SAW/BAW technologies. In addition, the experimental results confirm a very high Q of over 5,000 and high k 2 of 8.8% at 1 GHz. Moreover, a spurious free out-of-band response is achieved.

A new type of surface acoustic wave (SAW)configuration consisting of a very thin single crystal piezoelectric film bonded onto a support substrate (layered substrate)has been actively developed in recent years. This layered SAW structure exhibits superior performance compared to conventional SAW or temperature compensated (TC)SAW, with SiO 2 over coat, on traditional piezoelectric substrates through a higher quality factor (Q), higher electromechanical coupling factor (k 2 ) and smaller temperature coefficient of frequency (TCF). However, without a careful design of the substrate, the layered SAW potentially has spurious responses in the out-of-band frequencies due to higher order modes guided in the layer. This paper focuses on the design of layered substrate not only to optimize its narrow band characteristics (Q, k 2 , TCF)but also for eliminating the out-of-band spurious responses. By using a finite element method / boundary element method (FEM/BEM)approach, requirements for the substrate velocity and piezoelectric layer thickness are derived to avoid the presence of spurious modes. Based on the analyses, a new orientation of quartz is proposed as a spurious free support substrate. Sapphire as well as new quartz is selected as a demonstration support substrate and bonded wafers are fabricated using LiTaO 3 (LT)piezoelectric thin film. Fabricated SAW resonators on LT/sapphire confirm ultra-high Q (>7,000), high k 2 (9.0%)and small TCF (-2 ppm/K)at 1 GHz, but have several out-of-band spurious responses. LT/new quartz SAW resonators also show ultra-high Q (>6,000), high k 2 (9.9%)and small TCF (-23 ppm/K)at 1 GHz, and achieve a spurious free out-of-band response as expected.

Nonlinear generation of harmonics is measured on the wafer level on temperature compensated surface-acoustic-wave resonators on lithium niobate. The second-harmonic response features strong peaks looking like multiple resonant modes. The product of the frequency difference between the successive peaks to the substrate thickness is found to remain almost constant. The results also show that the level of the harmonic signal strongly depends on the roughness of the back side of the substrate. Due to these findings, it was assumed that the signal is due to nonlinear generation of a bulk mode. This assumption is confirmed experimentally by measuring the second-harmonic wave field on the back side of the substrate with an interferometer. Possible nonlinear mechanisms explaining these results are discussed.

With the introduction of tactile smart phones, and the mobile web, the cumulative annual growth rate (CAGR) for mobile data traffic surged to over 100%. With the CAGR remaining near 50% [1], global mobile data demand is expected remain strong. To meet the demand, greater efficiencies for existing spectrum, and new spectrum, will be required. Therefore, the growth of mobile data traffic drives a demand for larger quantities and improved performance of acoustic filters. By using analytic solutions for Chebyshev filters, the performance specifications for filters may be translated into required acoustic resonator performance metrics. Temperature compensated SAW (TCSAW) filters with high quality factors (Q) are well suited to meet the stringent requirements of low frequency LTE bands. The technology innovations, essential to high-Q TCSAW, are summarized and their performance metrics of TCSAW are compared to those required to meet the LTE band specifications. Finally, some representative TCSAW duplexer responses are presented.

Finite element methods are often used to simulate surface acoustic wave (SAW) resonators. Usually, only bi-dimensional or periodic models are practical on long resonators. The hierarchical cascading method proposed in 2016 uses the repetitive structure of a SAW resonator to obtain a computation time proportional to the logarithm of the electrode number. Using this approach, a full 3D simulation of SAW resonators becomes possible. The full 3D simulation of leaky SAW and temperature compensated SAW resonators is demonstrated.

RF spectrum is becoming a very sparse resource and several systems have to coexist while separated by small frequency gaps. For example, public safety band NS07 is 2.5 MHz below the LTE band 13 TX band. Standard SAW and typical Temperature Compensated SAW (TC-SAW) filters do not provide enough rejection in this band, especially within the required operating temperature range. The only way, when not using an NS07 compliant duplexer, to avoid noise radiation into the public safety band is to reduce their transmitted power when the public safety network is activated. To be compliant with NS07, the band 13 TX filter needs to have a very steep transition. Therefore, high quality factors as well as low temperature coefficient of frequency (TCF) are required. To reduce the TCF, the current standard approach is to bury the electrodes inside a silicon oxide overlay. Since silicon oxide has a positive TCF while piezoelectric substrates have negative TCFs, compensation is possible. Usually, TC-SAW filters on lithium niobate substrates have a TCF in the range of -20 to -25 ppm/°C. This corresponds to a frequency shift of 2 MHz at 800 MHz for a temperature range of 100°C. This is too large for applications requiring steep transitions bands. A band 13 duplexer meeting the NS07 rejection requirement is presented. The substrate utilized is lithium niobate with an orientation close to Y+128°. The oxide thickness is increased enough to obtain full temperature compensation. Piston mode transducers are used in order to suppress the transverse modes and to reduce the losses. By increasing the silicon oxide thickness, it is possible to reduce TCF toward some targeted ppm/degC value. The drawback is a reduction by about 40% of the coupling factor and thus, of the possible relative bandwidth. However, band 13 is narrow enough to be compatible with this approach. The resulting temperature compensation produces a frequency shift smaller than 200 kHz p.p. within the 100°C temperature range. Quality factors in the 1500 range are obtained. Typical results are a rejection of 25dB in the NS07 band within the temperature range with an insertion loss better than 3dB. It is understood that this duplexer is the first band 13 SAW duplexer meeting the NS07 rejection specification.

It is well known that transverse effects contribute significantly to the loss of SAW resonators on lithium tantalate. In particular, for frequencies above resonance, the surface wave is not guided inside the transducer and radiates into the busbars. In addition, because bulk modes can also be excited, scalar models are not sufficient to accurately predict transverse effects. It is also known that the layout of a SAW resonator (electrode gaps and dummy electrodes) has a strong impact on the transverse effects. In this paper, a periodic FEM/BEM model is presented and is used to simulate the transverse effects for various SAW resonator layouts. Test devices matching those simulated are fabricated and measured; the measured results are compared with the simulated results and show good agreement. By analyzing the dispersion curves produced from the FEM/BEM model in the different regions of the device, several frequency bands corresponding to different transverse behaviors are identified. These results are consistent with the elastic displacements, also computed by the FEM/BEM model. It is further shown that guided conditions in the gap between the transducer and the busbar occur for a frequency range above resonance. This result is in agreement with measurements showing that resonators with smaller gaps exhibit smaller spurious responses in their admittance.

Nonlinearity distortion measurements and accurate modeling techniques for surface acoustic wave (SAW) devices are desired for third generation (3G) and fourth generation (4G) telecommunication systems. In this paper, a novel nonlinear Mason equivalent circuit model including a 3rd order nonlinear coefficient in the wave propagation is presented. The model is able to accurately simulate both small signal (S parameters) and large signal (3rd harmonic and 3rd order intermodulation) responses of a SAW duplexer. Harmonic and intermodulation measurement techniques are also discussed.

Nonlinear intermodulation distortion inside duplexers is a limitation for the sensitivity of mobile devices receivers. A perturbation approach for non linear harmonic acoustic generation along the propagation of surface acoustic wave is derived. It is used to extend the P matrix model to non linear generation of harmonics and mixing products. Interface of the new model to harmonic balance simulators is discussed. Results are presented showing the validity of the approach.

Buried electrodes SAW resonators on lithium niobate suffer from transverse mode spurii even if the acoustic velocity inside the resonator is faster than the acoustic velocity outside the resonator (i.e. when a `waveguiding' condition is not established). In practical devices, the electrode end gaps play an important role since they constitute a very fast region and reflect the acoustic waves creating multiple transverse modes. Guiding conditions need to be created in resonators to obtain good quality factors. This can be done by using sufficiently large gaps sizes. Unfortunately, this leads to even stronger transverse modes. The proposed solution consists in tailoring the shape of the main mode. A “piston mode” shape is obtained by adding a slow region at the edge of the active region. The mode shape is matched to the rectangular excitation and therefore, the coupling to the higher order modes becomes negligible. Results exhibit low spurious, lower losses and better effective coupling coefficients than apodized devices. Quality factors in the 1500 range are measured on resonators while CRF losses are significantly reduced.