SAW resonators at high temperatures

SAW devices can be realized as sensors by coating a suitable sensing medium on the acoustic path of the device to detect various physical and chemical measurands. ZnO is a compound widely employed as sensing and wave propagation medium in SAW devices. In particular, high aspect ratio ZnO nanowires are a promising candidate as sensing medium for SAW gas sensors and UV detectors because of its favorable sensing properties, high surface-to-volume ratio and variety of growth techniques. In this thesis, high aspect ratio catalyst-free ZnO nanowires were directly grown as sensing medium on the surface of SAW devices for the first time through the self-seeding thermal evaporation method. Initially, growth of ZnO nanowires were conducted on blank 128°-YX LiNbO3 substrates and the alignment nature of the as-grown nanowires was studied. As-grown ZnO nanowires exhibited a crisscross aligned growth pattern due to step bunching of the polished LiNbO3 surface during the nanowire growth process. However, it was experimentally observed that nanoscale etching of the LiNbO3 surface could realize growth of well-aligned ZnO nanowires because of the added surface roughness. The thermal evaporation method was then adopted for direct growth of ZnO nanowires on SAW devices. High temperature stable Rayleigh mode SAW resonators were designed, and fabricated using Pt/Ti electrodes on 128°-YX LiNbO3 substrates to facilitate the on-chip nanowire growth conducted at 620°C. As a key part of the on-chip growth process, a mask and sleeve setup was used to protect one-half of the IDT aperture during the nanowire growth process to enhance post-growth device functionality. It was observed that the proposed device had approximately 3 times higher sensitivity to UV light of 365 nm wavelength as compared to the only previous report where 1-D ZnO nanostructures were directly grown on a LiNbO3 based SAW device using solution grown method for UV detection purposes. Further, FEM simulations were conducted to study the prospects of coupled resonance phenomenon towards enhancing the sensitivity of the ZnO nanowire integrated SAW device to conductivity changes in the sensing medium. It was observed that sensitivity increases by more than 200 times when the ZnO nanowire heights are adjusted such that the resonance frequency of the nanowires approaches the original resonance frequency of the SAW device. FEM simulations were also used to investigate the possibility of switching from Rayleigh waves to higher order Sezawa waves propagating on layered SAW devices to enhance the mass loading sensitivity of SAW sensors. As an example, it was shown that Sezawa waves propagating on a ZnO/Si layered SAW device has 2 to 8 times higher mass loading sensitivity as compared to Rayleigh waves propagating on the same device. The use of the self-seeding thermal evaporation method instead of the conventionally used solution grown method opens up possibilities towards scalable growth of catalyst-free ZnO nanowires without fusion among their roots which increases the surface area of the sensing medium. Moreover, on-chip growth of ZnO nanowires in the absence of a liquid medium has future prospects for in situ measurement of the SAW device response.

This letter presents a surface acoustic wave (SAW) resonator based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon $</tex-math> </inline-formula>-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin films deposited via metal-organic chemical vapor deposition (MOCVD). Ti/Au interdigital transducers (IDTs), with an approximate finger width of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.8~\mu $</tex-math> </inline-formula>m, were fabricated on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon $</tex-math> </inline-formula>-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film to create this SAW resonator. The SAW resonator’s series resonant frequency (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{s}$</tex-math> </inline-formula>) and parallel resonant frequency (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{p}$</tex-math> </inline-formula>) were found to be 673.5 and 673.8 MHz, respectively. Additionally, the device’s modified Butterworth-Van Dyke (MBVD) model was successfully constructed, and the elastic coefficients, piezoelectric coefficients, and dielectric constants of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon $</tex-math> </inline-formula>-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> material were obtained through first-principles calculations, forming the basis for a finite element simulation model. Finite element simulations revealed that at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{s}$</tex-math> </inline-formula>, the device exhibits Rayleigh mode. Finally, the device demonstrated a high quality factor (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q =3927$</tex-math> </inline-formula>) and a high temperature coefficient of frequency [(TCF) = -56.73 ppm/K]. These findings reveal the potential application of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon $</tex-math> </inline-formula>-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> acoustic devices in temperature sensing.

This paper reports the fabrication of transparent SAW resonators using AZO as the transparent electrodes. Transparent SAW resonators exhibited two types of wave modes: Rayleigh and Sezawa waves, and signal amplitudes up to 25 dB were obtained with the transparency above 80%. The series resistance effect of the AZO film electrode on the performance of transparent SAW devices have been systematically investigated and results show that the series resistance should be lower than 10 Ω/sq. Temperature sensing showed the transparent SAW device has a temperature coefficient of frequency of ∼45 ppm/K. The transparent devices have also demonstrated their ability to induce a strong acoustic streaming with a streaming velocity up to 2.27 cm/s. This research opened a door for further exploration of the SAW devices in transparent electronics such as transparent microfluidics and sensors.

A 45/spl deg/ rotated X cut, Z propagation Li/sub 2/B/sub 4/O/sub 7/ substrate has attracted attention as a piezoelectric substrate for SAW devices. The substrate is advantageous not for SAW transversal filter but for SAW resonator and resonator filter applications. It is especially suitable for SAW resonator filters which have the relative bandwidth of 0.3% to 0.6% considering coupling coefficient and temperature coefficient of delay. Also, the chip size of SAW resonator filters can be reduced because of the large reflection coefficient for aluminum strips on Li/sub 2/B/sub 4/O/sub 7/ substrates. SAW resonator filters which have low insertion loss, high rejection level and small size are reviewed and described for pager, CTI, GSM and cordless telephone receiver applications. Moreover, the fabrication process for Li/sub 2/B/sub 4/O/sub 7/ substrates is also mentioned. >

The response mechanism for temperature sensors based on surface acoustic wave (SAW) resonators was analyzed by using the finite element method (FEM). Modeling the periodic electrodes on a piezoelectric substrate in COMSOL, the material parameters and the thermal expansion of the periodic structure at different temperatures were considered. By using the modal analysis and the harmonic frequency response analysis, the relative frequency change of the SAW temperature sensor was deduced. The numerical results of frequency-temperature behavior for Y+33°-X quartz and Y+36°-X quartz agree well with the reported experimental results, confirming the efficiency of the proposed theoretical analysis. The FEM analysis can provide an effective guide for the application of SAW temperature sensors.

Lithium tetraborate (Li2B4O7) single crystal has attracted attention as a piezoelectric substrate for SAW devices. This paper focuses on its application to a SAW resonator. Reflection characteristics for aluminum strips and configuration suppressing surface-to-bulk-wave conversion are discussed. Results prove that the strips on Li2B4O7 substrate reflect efficiently, compared with those on the other substrates, and that a newly developed configuration improves the resonator Q at least twice. Further, a low insertion loss of 2.5 dB is achieved for a paging receiver filter.

The surface acoustic wave (SAW) resonator response of ZnO/diamond/Si layered structures are calculated and discussed including velocity dispersion. Smith's second equivalent circuit including energy storage effects is used to calculate the resonator response. The effect of velocity dispersion appears for the shift of resonant frequency to the expected center frequency, and appears for the bandwidth of resonator becoming narrower than that of the nondispersive case. The coupling of modes equation is modified for the velocity dispersion to explain these effects. The reflection coefficients of gratings are calculated for velocity dispersive case and the non velocity dispersive case. These results agree with the response calculated by the equivalent circuit model. Assuming the deviation of velocity dispersion is small neighboring the resonance area, the effect of velocity dispersion is explained theoretically. The ZnO/diamond/Si SAW resonators provide high frequency operation and high quality factor (Q) SAW resonators due to the velocity dispersion.

A theoretical analysis for the SAW wireless tag for wireless instrumentation was proposed. Employing the developed wireless tag, a telemetry instrument was also developed. Other SAW device based systems such as SAW match filter based optical time-domain reflectometer and high Q-value SAW resonator based analytical instruments are also reported. These state-of-the-art instruments can enlarge the application scope of SAW technology.

SAW resonators with frequencies ranging from 199 MHz to 746 MHz were fabricated and exposed to sequentially increasing total doses of various types of ionizing radiation, and the effects were observed. The substrates used were air-swept and as-grown (nonswept) 40 degrees Y-rotated quartz, LST-quartz, and nonswept 40 degrees Y-rotated quartz coated with a 400-AA-thick layer of SiO/sub x/. The total doses used range from 1 kRad(Si) to 10 MRad(Si), and the radiation types tested were Co-60 gamma, high energy electron ( beta ), proton, and heavy ion (Si-28, Cl-35, Ge-74, and Au-197) radiation. The measurement results show a distinct difference in the radiation sensitivity of SAW device performance parameters based on the substrate used. The effects of the passivating SiO/sub x/ layer were also interesting, since the coated devices exhibited significantly smaller radiation-induced changes than those without the coating. >

15 - SAW Oscillators and Frequency Synthesizers

Time domain analysis of the nonlinear second-order harmonic response generated in the RF-SAW resonator is discussed. At first, how to pick up the very weak complex amplitude, which consists of the magnitude and phase, from the SAW resonators, is demonstrated. Then, the time domain response is calculated by the inverse Fourier transform of the complex amplitude in the frequency domain. It is confirmed that the time domain response consists of a few components with different delay times. Two components are extracted individually by the time-gating technique and transformed into the frequency responses. The results support the prediction about the generation mechanism of the second-order harmonic response in the SAW resonator. Thus, it is verified that the time domain analysis should be a powerful tool for understanding the nonlinearity of SAW devices.

DC magnetron sputtering and Ti addition for Al electrodes are studied to achieve higher durability of SAW devices operating at high frequency and high power. Experiments on SAW resonators as a test medium show the following: the sputtering is better than conventional e-gun evaporation for both the reproducibility of film composition and for durability; 0.6 wt% to 0.9 wt% Ti addition to the sputtered Al films leads to durability for 3 times larger power than 0.7 wt% Cu addition to evaporated Al films, while less loss is experienced if resistivity is the same in each film.

Communication technologies develop very fast in recent years, especially mobile communication technology. One of the key parts of communication system is the SAW device, which development directions are high frequency, low insert loss and miniaturization. As the frequency is higher, requirement of optical lithography technology increases and the conventional contact lithography technology is inefficient. Three kind of lithography is ideal: UV projection lithography, Electron beam direct writing technology and X-ray lithography. In order to reduce fabrication cost and development cycle, a variably shaped electron beam lithography system (JBX-6A□, JOEL, 1980) was adopted. This system was mainly used for optical lithography reticle fabrication, and it had a low resolution of 1µm. In order to develop the potential capability of the electron beam direct writing system, specially designed substrate exposure clamp, electron beam proximity effect correction graphic design technology and different shaped beam size combination exposure technology were adopted, and the parameter of resist coating, pre-bake, exposure dose, hard bake, etching temperature and time were optimized. As a result, this system got a resolution of 0.5µm. In this paper, research and batch fabrication task of SAW filter (900MHz, 1272MHz, 1575.42MHz and 2300MHz, etc) and SAW resonator (1320MHz and 1440MHz, etc) were successfully completed.

Fourier analysis of SAW resonance frequency variations for improved detection

SAW characteristics of various layer structures of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /IDT/AlN/diamond substrates that employ 2nd mode of wave (Sezawa mode) were studied both theoretically and experimentally. It was found that the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /IDT/AlN/diamond substrate would allow using thicker metal IDT in SAW device designs than other SAW substrates and also would have zero TCF characteristic at room temperature. SAW phase velocity and electromechanical coupling coefficient of the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /IDT/AlN/diamond substrate are 11150m/s and 0.5%, respectively. Fabricated 1-port SAW resonators using the diamond substrate showed Q of 660 at 5.4 GHz anti-resonance frequency. Frequency drift over temperature range of -25°C to 80°C was about 90 ppm that is even less than the ST-Quartz SAW substrate. In the end of paper, a 5 GHz band stop type SAW filter design is presented. It is shown that 30 MHz wide stopband at 6 dB rejection level can be achieved while keeping the passband insertion loss just in -0.76 dB.