Richard H. Wittstruck

Analysis of SAW properties of epitaxial ZnO films grown on R-Al/sub 2/O/sub 3/ substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112~0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al/sub 2/O/sub 3/ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h//spl lambda/). The acoustic wave properties of the material system were calculated using Adlers matrix method, and the devices were simulated using the quasi-static approximation based on Greens function analysis.

Characteristics of Mg/sub x/Zn/sub 1-x/O thin film bulk acoustic wave devices

Piezoelectric thin film zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) have broad applications in transducers, resonators, and filters. In this work, we present a new bulk acoustic wave (BAW) structure consisting of Al/Mg/sub x/Zn/sub 1-x/O/n/sup +/-ZnO/r-sapphire, where Al and n/sup +/ type ZnO serve as the top and bottom electrode, respectively. The epitaxial Mg/sub x/Zn/sub 1-x/O films have the same epitaxial relationships with the substrate as ZnO on r-Al/sub 2/O/sub 3/, resulting in the c-axis of the Mg/sub x/Zn/sub 1-x/O being in the growth plane. This relationship promotes shear bulk wave propagation that affords sensing in liquid phase media without the dampening effects found in longitudinal wave mode BAW devices. The BAW velocity and electromechanical coupling coefficient of Mg/sub x/Zn/sub 1-x/O can be tailored by varying the Mg composition, which provides an alternative and complementary method to adjust the BAW characteristics by changing the piezoelectric film thickness. This provides flexibility to design the operating frequencies of thin film bulk acoustic wave devices. Frequency responses of devices with two acoustic wave modes propagating in the specified structure are analyzed using a transmission line model. Measured results show good agreement with simulation.

Mg/sub x/Zn/sub 1-x/O: a new piezoelectric material

Piezoelectric ZnO thin films have been successfully used for multilayer surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) is a new piezoelectric material, which is formed by alloying ZnO and MgO. Mg/sub x/Zn/sub 1-x/O allows for flexibility in thin film SAW device design, as its piezoelectric properties can be tailored by controlling the Mg composition, as well as by using Mg/sub x/Zn/sub 1-x/O/ZnO multilayer structures. We report the metal-organic chemical vapor deposition (MOCVD) growth, structural characterization and SAW evaluation of piezoelectric Mg/sub x/Zn/sub 1-x/O (x<0.35) thin films grown on (011~2) r-plane sapphire substrates. The primary axis of symmetry, the c-axis, lies on the Mg/sub x/Zn/sub 1-x/O growth plane, resulting in the in-plane anisotropy of piezoelectric properties. SAW test devices for Rayleigh and Love wave modes, propagating parallel and perpendicular to the c-axis, were designed and fabricated. Their SAW properties, including velocity dispersion and piezoelectric coupling, were characterized. It has been found that the acoustic velocity increases, whereas the piezoelectric coupling decreases with increasing Mg composition in piezoelectric Mg/sub x/Zn/sub 1-x/O films.

Properties of transducers and substrates for high frequency resonators and sensors

Properties of transducers and substrates for bulk acoustic wave resonators and sensors are described. These resonators utilize one-dimensional thickness vibrations of structures consisting of a low-loss substrate crystal surmounted by a thin active piezoelectric film that drives the composite in resonant modes to achieve gigahertz frequencies. The structures considered include oblique orientations of the substrate, leading to generation of coupled elastic modes in the composite. A modified Christoffel-Bechmann (CB) formalism is presented to calculate acoustic wave speeds and displacements in the piezoelectric film transducer and the substrate. The CB method also yields the piezoelectric coupling coefficients of arbitrarily oriented piezofilms, for electric fields impressed either along the thickness or laterally. The calculations apply generally to transducer and substrate crystals of any symmetry class. The piezoelectric portion is then made specific for films of class 6mm (wurtzite structure) with arbit...

Studies on Mg/sub x/Zn/sub 1-x/O thin film resonator for mass sensor application

Zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg _xZn_1-xO) are piezoelectric materials for high quality factor bulk acoustic wave (BAW) resonators operating at GHz frequencies. Mg_xZn_1-xO thin film BAW devices built on Si substrates are particularly attractive for integrating piezoelectric Mg_xZn_1-xO with the main stream semiconductor devices and circuits. In this paper, we report single-mode Mg_xZn_1-xO based thin film resonators (TFRs) built on Si substrates. An acoustic mirror, composed of alternating quarter-wavelength silicon dioxide (SiO₂) and tungsten (W) layers, is used to isolate the resonator from the Si substrate. High quality and well c-axis oriented Mg_xZn_1-xO thin films are deposited on Si substrates using RF sputtering technology. X-ray diffraction (XRD) and field emission electron microscopy (FESEM) are used to characterize the Mg_xZn_1-xO layers. The theoretical analysis of the TFR, based on the transmission line model, is presented. The BAW velocity and effective coupling coefficient of Mg _xZn_1-xO can be tailored by varying the Mg composition in the films. The acoustic velocity increases with increasing Mg composition. The feasibility to use this structure to build ultra-high-sensitive mass BAW TFR sensor is analyzed. A mass sensitivity higher than 103 Hz cm₂/ng is demonstrated

Lateral field excitation of bulk acoustic waves from an IC-compliant low voltage source

An Interdigital Bulk Acoustic-Wave Transducer (IBAT) device is provided with pairs of exciting electrode fingers disposed sufficiently close together on the piezoelectric substrate and dielectric coating over the exciting electrode fingers to generate an IC-compatible voltage at relatively high electric field strength, resulting in a reduced region of excitation and uniform electric field strength distribution. The IBAT advantageously produces a lateral electric field substantially uniform over a substantial portion of the active BAW structure area, reducing, or virtually eliminating sharp voltage spikes, an electrical field produced by the low voltages resident on integrated circuit (IC) chips, usually of a magnitude of 10 volts, or lower, the planar electrode structure being compatible with IC processing techniques, such as photolithography and the BAWs produced thereby being essentially plane waves, with propagation away from, but with phase progression substantially parallel to, the substrate surface. Numerous IBAT structural arrangements are possible by advantageously over-coating the IBAT electrode finger stripes with an insulating dielectric in different configurations, and any possible configuration achieved through over-coating is considered to be within the contemplation of the devices and methods of the present invention. Interdigital bulk acoustic wave transducers and methods for exciting bulk acoustic waves with interdigital electrode fingers are also provided.

Analysis of BAW responses in ZnO multi-layer structures using transmission line method

With the advent of epitaxial semiconductor growth technology, piezoelectric multilayer materials became available for broad application. Piezoelectric ZnO has large electromechanical coupling coefficients making it a promising candidate for multilayer thin film resonant filter devices. A family of methodologies has been developed to explain the analogous behavior of such multi-layer structures in terms of solutions to the acoustic wave differential equation. In the work encompassed by this paper, the model using transmission line representations to simulate the resonant behaviors in BAW devices is demonstrated. The model shows the electrical admittance response as a function of frequency.

Analysis of bulk acoustic wave response in ZnO based structures using transmission-line method

Piezoelectric thin film ZnO and its ternary alloy MgZnO have broad applications in transducers, resonators, filters and sensors. In ZnO based multi-layer piezoelectric device structures, such as ZnO on r-Al/sub 2/O/sub 3/, multi-acoustic wave mode coupling between each layer in the multilayer structure must be considered. In this paper, we use transmission line representation to analyze frequency response of a structure based on piezoelectric Mg/sub x/Zn/sub 1-x/O. The structure is Al/Mg/sub x/Zn/sub 1-x/O/n/sup ++/ ZnO/r-sapphire, where the Al and n/sup ++/ ZnO layers serve as top and bottom electrodes, respectively. Due to the crystalline orientation of the n/sup ++/ ZnO electrode and r-plane sapphire used in this work, there exist quasi shear and longitudinal wave modes in the device. The simulation results and measured device response show general agreement.