-Jen Chung

Influence of surface roughness of Bragg reflectors on resonance characteristics of solidly-mounted resonators

The solidly mounted resonator (SMR) is fabricated using planar processes from a piezoelectric layer sandwiched between two electrodes upon Bragg reflectors, which then are attached to a substrate. To transform the effective acoustic impedance of the substrate to a near zero value, the Bragg reflectors are composed of alternating high and low acoustic impedance layers of quarter-wavelength thickness. This paper presents the influence of Bragg reflector surface roughness on the resonance characteristics of a SMR. Originally, an AlN/Al multilayer is used as the Bragg reflector. The poor surface roughness of this Bragg reflector results in a poor SMR frequency response. To improve the surface roughness of Bragg reflectors, a molybdenum (Mo)/titanium (Ti) multilayer with a similar coefficient of thermal expansion is adopted. By controlling deposition parameters, the surface roughness of the Bragg reflector is improved, and better resonance characteristics of SMR are obtained

Synthesis and bulk acoustic wave properties on the dual mode frequency shift of solidly mounted resonators

This study focused on the fabrication and the theoretical analysis of solidly mounted resonators (SMR) concerning dual-mode frequency responses and their frequency shift of bulk acoustic wave (BAW) resonance. For this device fabrication, RF/DC magnetron sputtering and photolithography were employed to constitute the required multilayer structure. For the theoretical analysis, the dual- mode frequency shift was characterized by the Sauerbreys formula, and a modified formula was carried out following the trend for the large frequency shift. In the fabrication of the SMR device, Mo/SiO2 was chosen to construct the Bragg reflector as the high/low acoustic impedance materials, respectively, and aluminum nitride (AlN) was used as a piezoelectric layer. To investigate the characteristics of BAW on the dual-mode frequency shift, the c-axis tilted angle of AlN was altered as well as the various mass loading on the SMR. Based on the experimental results, the dual-resonance frequencies showed a nonlinear decreasing trend with a linear increase of the mass loading. Therefore, a modified formula was carried out. Furthermore, the ratio of the longitudinal-resonant frequency to the shear-resonant frequency remained at a range around 1.76 despite the various c-axis tilted angles of AlN and gradual mass loading on the SMR. The electromechanical coupling coefficient, keff 2, of the shear resonance rose with the increase of the c-axis tilted angle of AlN.

Superior dual mode resonances for 1/4 λ solidly mounted resonators

The concept of the solidly mounted resonator (SMR) structure was introduced in 1965. A SMR consists of a multi-layered structure and requires material interfaces that confine waves to resonate as standing waves. Thin piezoelectric films such as AlN and ZnO with tilted texture have the capability to excite the dual mode resonance, namely, the longitudinal and shear mode resonance. To grow the tilted AlN, the substrate is placed at a variable distance from the substrate holder center in a reactive magnetron sputtering system. In addition, we tilt the off-center substrates toward the sputtering source in order to reduce the acoustic energy loss of the longitudinal wave and preserve the shear mode resonance at the same time. In this study, the 1/4 lambda mode SMR devices made with a seven-layer Mo/SiO2 Bragg reflector and the c-axis tilted AlN are carried out. The Bragg reflector is optimized deposited with 2.28 nm RMS surface roughness, and the AlN is sputtered in appropriate sputtering pressure and appropriate substrate temperature to promote the growth of both the highly c-axis orientated and tilted AlN. The off-center deposition method evolves in a competitive growth bringing about an AlN growth pivoted in the ion-flux direction. The outcome frequency responses show dual resonant characteristics around 1.4 GHz and 2.5 GHz resulted from the shear and longitudinal resonances, respectively. We successfully improve the longitudinal resonance by tilting the substrate toward the sputtering source. Not only the shear resonance for the liquid media sensing application, but also an outstanding longitudinal resonance could be obtained. The superior dual mode resonances are realized.

Surface acoustic wave properties of proton-exchanged LiNbO/sub 3/ waveguides with SiO/sub 2/ film

Surface acoustic wave (SAW) properties of proton-exchanged (PE) z-cut lithium niobate (LiNbO/sub 3/) waveguides with silicon dioxide (SiO/sub 2/) film layers were investigated using octanoic acid. The distribution of hydrogen measured by secondary ion mass spectrometry (SIMS) showed a step-like profile, which was assumed to be equal to the waveguide depth (d). The SiO/sub 2/ film was deposited on z-cut LiNbO/sub 3/ waveguide by radio frequency (rf) magnetron sputtering. We investigated the important parameters for the design of SAW devices such as phase velocity (V/sub p/), insertion loss (IL) and temperature coefficient of frequency (TCF) by a network analyzer using thin-film aluminum interdigital transducer electrodes on the upper SiO/sub 2/ film surface. The experimental results showed that the V/sub p/ of SAW decreased slightly with the increase of h//spl lambda/, where h was the thickness of SiO/sub 2/ films and /spl lambda/ was the wavelength. The IL of SAW increased with increased h//spl lambda/. The TCF of SAW calculated from the frequency change of the output of SAW delay line showed an evident decrease with the increase of h//spl lambda/. The TCF for PE z-cut LiNbO/sub 3/ was measured to be about -54.72 ppm//spl deg/C at h//spl lambda/ = 0.08. It revealed that the SiO/sub 2/ films could compensate and improve the temperature stability as compared with the TCF of SAW on PE samples without SiO/sub 2/ film.

Proton-exchanged 36/spl deg/ Y-X LiTaO/sub 3/ waveguides for surface acoustic wave

A nontoxic proton source, octanoic acid, was adopted to fabricate proton-exchanged (PE) waveguides in 36/spl deg/ Y-X lithium tantalate (LiTaO/sub 3/) substrates. The PE ability of octanoic acid on LiTaO/sub 3/, the penetration depth, was investigated by secondary-ion mass spectrometry (SIMS). The penetration depth of hydrogen ion exhibited an obviously step-like profile, which will be excellent for waveguide application. The relationship between waveguide depth (d) and exchanging time (t) was represented by d = 0.0653 /spl times/ /spl radic/t at T = 200/spl deg/C. To deserve to be mentioned, the octanoic acid has a slight dissociation coefficient and low activation energy, thus the accurate waveguide depth control can be obtained. For the application of acoustic wave guided acousto-optic devices, the leaky surface acoustic wave (LSAW) properties of PE 36/spl deg/ Y-X LiTaO/sub 3/ waveguides were investigated. The phase velocity slightly decreased with the increase of kd, where k was wavenumber. An indispensable parameter of acoustic wave device, the temperature coefficient of frequency (TCF), calculated from the frequency change of the output of LSAW delay line showed an increase with increased kd.

Phase Tunable SAW Device on LiNbO3 Substrate

The interdigital transducers (IDTs) were fabricated on the surface of the z-cut LiNbO3 substrates. Top and bottom electrodes are evaporated on the propagation path of surface acoustic wave. In order to investigate the effects of electric field on SAW signal, the measurement in time domain using a network analyzer is carried out, in which, the center frequency of the device is fixed. From the experimental results, it reveals that the phase of signal shifts linearly with the variation of bias voltage. The phase shift of SAW could be controlled by the biased electric field, and thus, the various time delays could be obtained.

Temperature Coefficient of SAW Device on SiO2/Proton Exchanged LiNbO3 Substrate

Proton exchange (PE) is an attractive method for the fabrication of optical and acoustic waveguides in lithium niobate (LiNbO3). LiNbO3 substrate has large electromechanical coupling coefficient (K2) and moderate SAW velocity, but its temperature coefficient of frequency (TCF) is as large as -73 ppm/°C. Silicon dioxide (SiO2) is a well-known non-piezoelectric material having the positive TCF for SAW. The SiO2 thin films are deposited on the PE-LiNbO3 substrates to improve the TCF of the SAW devices. The results show that the temperature stability of SAW on SiO2/PE-LiNbO3 substrate is improved to be about -55 ppm/°C.

The characteristics of leaky surface acoustic wave of proton-exchanged lithium tantalite

Proton exchange (PE) is an attractive method for the fabrication of optical and acoustic waveguides in lithium tantalite (LiTaO/sub 3/) substrate. Octanoic acid was used for the fabrication of PE LiTaO/sub 3/ waveguides. After the PE process, the resultant substrate showed a damage-free surface from observations by scanning electron microscopy (SEM). And X-ray diffraction (XRD) was adopted to identify the crystalline structure of LiTaO/sub 3/. From the X-ray diffraction (XRD) results, the reflection peak of (306) was shifted after the PE process. The influences of the exchanging time on properties of leaky surface acoustic wave (LSAW) devices were discussed. The single electrode geometry of interdigital transducers (IDT) was adopted to excite and receive the acoustic wave. The wavelength (/spl lambda/) of the LSAW is 32 /spl mu/m. The frequency response of the LSAW is measured using an HP 8720ET network analyzer. The velocity of LSAW was decreased as the increase of PE time; nevertheless, the range of variation was quite small. Measurements of the temperature dependence of LSAW velocity were carried in the temperature range from 0/spl deg/C to 80/spl deg/C. The temperature coefficient of frequency (TCF) of LSAW devices was significantly increased by the increase of PE time. The PE process changed the characteristics of material and specific property can be obtained.