Mounted Bulk Acoustic Wave Resonators Research Articles

9.5–12 GHz solidly mounted bulk acoustic wave resonators utilizing overtone of thickness extension mode

To realize HF bulk acoustic wave (BAW) devices, we have explored the application of integer multiples of overtones (3rd and 6th) of the thickness extension mode in solidly-mounted resonators (SM-BAWRs). When the average thickness of the acoustic films in the Bragg reflector of the SM-BAW device is wavelength (λ)/4, the fundamental mode is strongly excited, but overtone excitation is weak. However, it was found that 0.035λ–0.075λ acoustic layer thickness of a Bragg reflector was suitable for 3rd overtone excitation using a single LiNbO3 (LN) layer and 0.01λ–0.045λ for 6th overtone excitation using a polarization-flipped double LN layer. By using a 1 μm thick LN single thin layer, we successfully excited 3rd overtone at 9.5 GHz. Furthermore, overtones at 9.5 and 12 GHz were excited by bonding 1 μm and 0.9 μm thick LN plates with different polarities.

Graphene as an active virtually massless top electrode for RF solidly mounted bulk acoustic wave (SMR-BAW) resonators

Mechanical and electrical losses induced by an electrode material greatly influence the performance of bulk acoustic wave (BAW) resonators. Graphene as a conducting and virtually massless 2D material is a suitable candidate as an alternative electrode material for BAW resonators which reduces electrode induced mechanical losses. In this publication we show that graphene acts as an active top electrode for solidly mounted BAW resonators (BAW-SMR) at 2.1 GHz resonance frequency. Due to a strong decrease of mass loading and its remarkable electronic properties, graphene demonstrates its ability as an ultrathin conductive layer. In our experiments we used an optimized graphene wet transfer on aluminum nitride-based solidly mounted resonator devices. We achieved more than a triplication of the resonator’s quality factor Q and a resonance frequency close to an ‘unloaded’ resonator without metallization. Our results reveal the direct influence of both, the graphene quality and the graphene contacting via metal structures, on the performance characteristic of a BAW resonator. These findings clearly show the potential of graphene in minimizing mechanical losses due to its virtually massless character. Moreover, they highlight the advantages of graphene and other 2D conductive materials for alternative electrodes in electroacoustic resonators for radio frequency applications.

Frequency response of AlN-based solidly mounted resonators under mechanical stress

We analyse the performance of solidly mounted bulk acoustic wave resonators under induced mechanical stress to explore their viability as strain sensors. The resonators are made of polycrystalline AlN piezoelectric thin films containing uniformly tilted microcrystals to excite both longitudinal and shear modes. The resonators are grown on top of silicon bars that are fixed at one or two edges using two home-made apparatus specifically designed to induce deformations of several hundreds of microstrains. The induced strain causes frequency shifts of tenths of MHz, yielding strain coefficients of the resonant frequency (SCF) up to −71% per unit strain (−0.71ppm/μe). The influence of the nature of the resonant mode (shear of longitudinal), the electromechanical coupling factor and the operation frequency on the SCF is analysed.

Carbon nanotube forests as top electrode in electroacoustic resonators

We grow carbon nanotube forests on piezoelectric AlN films and fabricate and characterize nanotube-based solidly mounted bulk acoustic wave resonators employing the forests as the top electrode material. The devices show values for quality factor at anti-resonance of ∼430, and at resonance of ∼100. The effective coupling coefficient is of ∼6%, and the resonant frequencies are up to ∼800 MHz above those observed with metallic top electrodes. AlN promotes a strong catalyst-support interaction, which reduces Fe catalyst mobility, and thus enforces the growth of forests by the base growth mechanism.

Optimized reflector stacks for solidly mounted bulk acoustic wave resonators

The quality factor (Q) of a solidly mounted bulk acoustic wave resonator (SMR) is limited by substrate losses, because the acoustic mirror is traditionally optimized to reflect longitudinal waves only. We propose two different design approaches derived from optics to tailor the acoustic mirror for effective reflection of both longitudinal and shear waves. The first one employs the stopband theory in optics; the second one takes advantage of the periodic nature of reflection spectra in a Bragg reflector: the diffraction grating design approach. The optimized design using stopband theory reaches a calculated minimum transmission of 25 dB and 20 dB at resonance frequency for longitudinal and shear waves, respectively, for various practical reflector material combinations. Using the diffraction grating approach, a near quarter-wave performance is maintained for longitudinal waves, whereas shear waves reach minimum transmission below 26 dB. However, this design does necessitate relatively thick layers. The experimental results show good agreement with finite element models (FEM). The extracted 1-D Q for the realized shear optimized devices was increased to around 3300.

Solidly mounted BAW resonators with layer-transferred AlN using sacrificial Si surfaces

We present a new method to manufacture solidly mounted bulk acoustic wave resonators. This new process introduces the use of wafer bonding techniques and sacrificial surface removal to manufacture solidly mounted resonators having special properties. With the proposed process, Aluminum Nitride (AlN) thin films are obtained having exceptional c-axis crystal orientation with XRD rocking curve FWHM of 1.36° and material electromechanical coupling constant of 6.8% exceeding that of the epitaxial AlN electromechanical coupling constant. Fully functional single-mask resonators were successfully fabricated with this process working around 2.35 GHz and enjoying Q-values as high as 1300.

Bandpass filters for 8 GHz using solidly mounted bulk acoustic wave resonators

Frequency shift, design, and fabrication issues have been investigated for the realization of 8 GHz bandpass filters based on AlN thin film bulk acoustic wave resonators. Fabrication includes well-textured AlN thin films on Pt (111) electrodes and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /AlN Bragg gratings for the solidly mounted resonators. The chosen ladder filter design requires the tuning of the shunt resonators with respect to the series one. For this purpose, mass loading of the shunt resonators with aluminum (Al) and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were studied. Design simulations showed that the channel bandwidth can be doubled by shifting more than the difference of resonance and antiresonance frequency. Bandpass filters at 8 GHz were successfully fabricated with -5.5 dB insertion loss, -26 dB out-of-band rejection, 99 MHz (1.2%) ±0.2 dB channel bandwidth, and 224 MHz (2.8%) 3 dB bandwidth. The group delay variations within any 30 MHz channel inside the channel bandwidth amounts to < 0.2 ns. Comparisons with simulation calculations and single resonator characteristics show that each π-section includes a parasitic series resistance and inductance.

Bandpass filters for 8 GHz using solidly mounted bulk acoustic wave resonators

Frequency shift, design, and fabrication issues have been investigated for the realization of 8 GHz bandpass filters based on AlN thin film bulk acoustic wave resonators. Fabrication includes well-textured AIN thin films on Pt (111) electrodes and SiO2/AlN Bragg gratings for the solidly mounted resonators. The chosen ladder filter design requires the tuning of the shunt resonators with respect to the series one. For this purpose, mass loading of the shunt resonators with aluminum (Al) and SiO2 were studied. Design simulations showed that the channel bandwidth can be doubled by shifting more than the difference of resonance and antiresonance frequency. Bandpass filters at 8 GHz were successfully fabricated with -5.5 dB insertion loss, -26 dB out-of-band rejection, 99 MHz (1.2%) +/- 0.2 dB channel bandwidth, and 224 MHz (2.8%) 3 dB bandwidth. The group delay variations within any 30 MHz channel inside the channel bandwidth amounts to < 0.2 ns. Comparisons with simulation calculations and single resonator characteristics show that each pi-section includes a parasitic series resistance and inductance.

Determination of ZnO temperature coefficients using thin film bulk acoustic wave resonators

Thin film bulk acoustic wave (BAW) resonators have been the subject of research in RF microelectronics for some time. Much of the interest lay in utilizing the resonators to design filters for wireless applications. Some of the major advantages BAW devices present over other filter technologies in use today include size reduction and the possibility of on-chip integration. As the technology matures, the necessity to more fully characterize the performance of the devices and to develop more accurate models describing their behavior is apparent. In this investigation, the effects that temperature variations have on 1.8-2.0 GHz zinc oxide (ZnO)-based solidly mounted BAW resonators (SMRs) are studied. The average temperature coefficients of the series and parallel resonant frequencies of the fabricated devices are found to be -31.5 ppm/degrees C and -35.3 ppm/degrees C, respectively. The slight decrease in separation between the two resonant frequencies with temperature implies there is slightly less effective coupling with increased temperature. No definite trend is found describing the behavior of the quality factor (Q) of the resonator with temperature variations. With little temperature coefficient data for thin film ZnO available in the literature, the importance of an accurate model is evident. The resonator device performance is simulated using Ballato's electronic circuit model for acoustic devices on a SPICE-based platform. By virtue of the comparison between the predicted and measured device response, various material parameters are extracted.

Influence of electrode configurations on the quality factor and piezoelectric coupling constant of solidly mounted bulk acoustic wave resonators

This article presents an electrode effect on the quality factor and effective coupling coefficient empirically and theoretically. The possible quality factor detractors in film bulk acoustic wave resonator devices are the acoustic wave attenuation in the metal and piezoelectric films, the wave scattering loss due to the surface roughness, and the electrode electrical loss. Due to the small acoustic wave attenuation of the Mo electrode, the composite Mo–AlN–Mo resonator with the thick electrode provides higher Q values than the resonator with the good quality AlN film and the thin Mo electrode. In such Mo electroded resonators, the effective coupling coefficient is kept relatively high. It is found that the Q value of solidly mounted resonators (SMRs) is not determined by only the piezoelectric film quality, but by a weighted average of the electrode and piezoelectric film quality. Of Q value detractors, the scattering loss originating from surface roughness is thought to be very important factor in SMR quality value.