Chien-Chuan Cheng

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO/sub 3/ substrates

The c-axis-oriented aluminum nitride (AlN) films were deposited on z-cut lithium niobate (LiNbO/sub 3/) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AlN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N/sub 2/ concentration, and sputtering pressure. Highly c-axis-oriented AlN films to fabricate the AlN/LiNbO/sub 3/-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N/sub 2/ concentration of 60%, RF power of 165 W, and substrate temperature of 400/spl deg/C. A dense pebble-like surface texture of c-axis-oriented AlN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K/sup 2/) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm//spl deg/C.

Synthesis of C-Axis-Oriented Aluminum Nitride Films by Reactive RF Magnetron Sputtering for Surface Acoustic Wave

C-axis-oriented aluminum nitride (AlN) films were deposited on SiO2-coated Si substrates by reactive rf magnetron sputtering. The crystallization of the AlN films, identified by X-ray diffraction (XRD) , was dependent on the deposition conditions. Highly c-axis-oriented AlN films, for fabricating AlN/SiO2/Si-based surface acoustic wave (SAW) devices, were obtained under an rf power of 300 W, substrate temperature of 350°C, sputtering pressure of 7.5 mTorr and N2 concentration of 75%. A dense pebblelike surface texture of the c-axis-oriented AlN films with an average grain size of about 100 nm was observed by scanning electron microscopy (SEM). The phase velocity showed a tendency to decrease with increasing kh, where k=2π/λ is the wavenumber and h is the AlN film thickness. In partucular, the phase velocity and the electromechanical coupling coefficient of the sample at kh=0.4 were calculated to be about 6080 m/s and 1.1%, respectively.

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.

The liquid sensor using thin film bulk acoustic resonator with c-axis tilted AlN films

Dual-mode thin film bulk acoustic resonator (TFBAR) devices are fabricated with c-axis tilted AlN films. To fabricate dual-mode TFBAR devices, the off-axis RF magnetron sputtering method for the growth of tilted piezoelectric AlN thin films is adopted. In this report, the AlN thin films are deposited with tilting angles of 15° and 23°. The frequency response of the TFBAR device with 23° tilted AlN thin film is measured to reveal its ability to provide dual-mode resonance. The sensitivities of the longitudinal and shear modes tomass loading are calculated to be 2295 Hz cm2/ng and 1363 Hz cm2/ng with the mechanical quality factors of 480 and 287, respectively. The sensitivities of the longitudinal and shear modes are calculated to be 0 and 15Hz cm2/µg for liquid loading.

UV detection based on a ZnO∕LiNbO3 layered surface acoustic wave oscillator circuit

This study elucidates the combination of an oscillator circuit with a high-frequency amplifier, a matching network, and a layered surface acoustic wave device for detecting ultraviolet (UV) light. The oscillator circuit shows an excellent performance with the resonance frequency of 109.34MHz and phase noise of −107.137dB at 100kHz. The frequency shift that is caused by the interaction between the acoustic wave and carriers upon excitation by UV light in the ZnO thin film is discussed. The frequency variation of the oscillator steadily increases with the intensity of the UV light. An extreme frequency shift of 63.75kHz was observed as the UV light intensity reached 1250μW∕cm2.

Design and fabrication of nanoscale IDTs using electron beam technology for high-frequency SAW devices

High-frequency Rayleigh-mode surface acoustic wave (SAW) devices were fabricated for 4G mobile telecommunications. The RF magnetron sputtering method was adopted to grow piezoelectric aluminum nitride (AlN) thin films on the Si3N4/Si substrates. The influence of sputtering parameters on the crystalline characteristics of AlN thin films was investigated. The interdigital transducer electrodes (IDTs) of aluminum (Al) were then fabricated onto the AlN surfaces by using the electron beam (e-beam) direct write lithography method to form the Al/AlN/Si3N4/Si structured SAW devices. The Al electrodes were adopted owing to its low resistivity, low cost, and low density of the material. For 4G applications in mobile telecommunications, the line widths of 937 nm, 750 nm, 562 nm, and 375 nm of IDTs were designed. Preferred orientation and crystalline properties of AlN thin films were determined by X-ray diffraction using a Siemens XRD-8 with CuKα radiation. Additionally, the cross-sectional images of AlN thin films were obtained by scanning electron microscope. Finally, the frequency responses of high-frequency SAW devices were measured using the E5071C network analyzer. The center frequencies of the high-frequency Rayleigh-mode SAW devices of 1.36 GHz, 1.81 GHz, 2.37 GHz, and 3.74 GHz are obtained. This study demonstrates that the proposed processing method significantly contributes to high-frequency SAW devices for wireless communications.

Temperature effect on the characteristics of surface acoustic wave on SiO/sub 2/ thin films

The surface acoustic wave (SAW) propagation characteristics of a layered structure consisting of a SiO/sub 2/ thin film on z-cut LiNbO/sub 3/ substrate in company with temperature dependence of frequency of this structure has been studied experimentally. The temperature coefficient of frequency (TCF) of SAW devices was obtained by a network analyzer. A SAW device on the lithium niobate (LiNbO/sub 3/) exhibits a large negative TCF. It revealed that the SiO/sub 2/ thin film was introduced as compensation layer for improving the temperature stability. The TCF of the SiO/sub 2//LiNbO/sub 3/ based structure was significantly decreased due to the SiO/sub 2/ thin film deposition. The TCF of SAW on the SiO/sub 2//LiNbO/sub 3/ device was measured to be about -51 ppm//spl deg/C at h//spl lambda/=0.12, where h was the thickness of SiO/sub 2/ film and /spl lambda/ was the wavelength of SAW. It indicated that the SiO/sub 2/ thin film deposited on LiNbO/sub 3/ substrate could improve the temperature stability, as compared with the TCF of SAW on bare LiNbO/sub 3/. Furthermore, the phase velocity (V/sub p/) of SAW on SiO/sub 2//LiNbO/sub 3/ substrate was not altered by the increase of SiO/sub 2/ thickness (h//spl lambda/).

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.