Kuo-Sheng Kao

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.

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 influence of varied sputtering condition on piezoelectric coefficients of AlN thin films

Under the consideration of clamping effect and constraint between the film and the substrate, the method of piezoelectric coefficients measurement is different between thin film and bulk material. The piezoelectric coefficients (d/sub 33/) of AlN thin films were measured using the method of periodic compression force in this study. The d/sub 33/ measurements of AlN thin films were performed on AlN/SiO/sub 2/-Si and AlN/LiNbO/sub 3/ multilayer structures, respectively. The preferred c-axis orientated AlN thin films were deposited by reactive rf magnetron sputtering. The main difference of the two substrates is the piezoelectricity of the LiNbO/sub 3/ and non-piezoelectricity of the SiO/sub 2/-Si. The correlation between growths parameters and piezoelectric coefficients will be investigated in this study. It showed that the values of d/sub 33/ are increased as the increase of X-ray diffraction intensity. From the view of full-width at half maximum (FWHM), it also showed that the values of d/sub 33/ increased as the decrease of the FWHM. The piezoelectric characteristic of thin films showed an obviously variation of AIN thin film. The microstructure controlled by the sputtering parameters has a great influence on thin film piezoelectricity.

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/).

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.

Temperature coefficients of SAW velocity for AlN thin film sputtered on ST-X quartz

Highly c-axis prefer-oriented aluminum nitride (AlN) thin films deposited on the Y-rotated, X-propagating, (ST-X), cut of quartz substrates were obtained by reactive rf magnetron sputtering to investigate the temperature coefficient of frequency (TCF) for surface acoustic wave (SAW) devices. The experimental results show that the slope of the temperature dependence of the center frequency of the SAW device on ST-X quartz substrate was doubtless zero and the TCF was calculated to be 0 ppm//spl deg/C. As compared, with AlN film on ST-X quartz, the TCF was measured to be about 16 ppm//spl deg/C with h//spl lambda/=0.05, where h was the AIN film thickness and /spl lambda/ was the wavelength of SAW. The AlN/ST-X quartz based structure could also improve the acoustic wave velocity of SAW devices.

Highly c-axis oriented AlN films deposited on LiNbO/sub 3/ substrates for surface acoustic wave devices

Highly c-axis oriented aluminum nitride (AlN) films were deposited on z-cut LiNbO/sub 3/ substrates by reactive rf magnetron sputtering. Growth behaviors of the AlN films deposited at various deposition conditions such as sputtering pressure, nitrogen concentration and substrate temperature were investigated. The crystalline orientation of the AlN film was determined by x-ray diffraction (XRD) which was sensitive to the deposition conditions. A dense pebble-like surface texture of c-axis oriented AlN film was obtained by scanning electron microscopy (SEM). The cross section of c-axis oriented AlN film showed a high degree of alignment of the columnar structure. A network analyzer was used to measure the surface acoustic wave (SAW) characteristics. The phase velocity and the electromechanical coupling coefficient were calculated to be about 4200 m/sec and 1.5%, respectively.

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.