Markku Ylilammi

Spurious Resonance Suppression in Gigahertz-Range ZnO Thin-Film Bulk Acoustic Wave Resonators by the Boundary Frame Method: Modeling and Experiment

Zinc-oxide-based thin-film bulk acoustic wave (BAW) resonators operating at 932 MHz are investigated with respect to variation of dimensions of a boundary frame spurious mode suppression structure. A plate wave dispersion-based semi-2-D model and a 2-D finite element method are used to predict the eigenmode spectrum of the resonators to explain the detailed behavior. The models show how the boundary frame method changes the eigenmodes and their coupling to the driving electrical field via the modification of the mechanical boundary condition and leads to emergence of a flat-amplitude piston mode and suppression of spurious modes. Narrow band suppression of a single mode with a nonoptimal boundary frame is observed. Reduction of the effective electromechanical coupling coefficient keff 2 as a function of the boundary width is observed and predicted by both models. The simple semi-2-D plate model is shown to predict the device behavior very well, and the 2-D finite element method results are shown to coincide with them with some additional effects. Breaking the resonator behavior down to eigenmodes, which are not directly observable in measurements, by the models, yields insight into the physics of the device operation.

Piezoelectric coefficients and spontaneous polarization of ScAlN

We present a computational study of spontaneous polarization and piezoelectricity in Sc(x)Al(1-x)N alloys in the compositional range from x = 0 to x = 0.5, obtained in the context of density functional theory and the Berry-phase theory of electric polarization using large periodic supercells. We report composition-dependent values of piezoelectric coefficients e(ij), piezoelectric moduli d(ij) and elastic constants C(ij). The theoretical findings are complemented with experimental measurement of e33 for a series of sputtered ScAlN films carried out with a piezoelectric resonator. The rapid increase with Sc content of the piezoelectric response reported in previous studies is confirmed for the available data. A detailed description of the full methodology required to calculate the piezoelectric properties of ScAlN, with application to other complex alloys, is presented. In particular, we find that the large amount of internal strain present in ScAlN and its intricate relation with electric polarization make configurational sampling and the use of large supercells at different compositions necessary in order to accurately derive the piezoelectric response of the material.

ZnO based thin film bulk acoustic wave filters for EGSM band

We present results for a ZnO based filters for the mobile Extended GSM (EGSM) Rx band centered at 942.5 MHz. Our devices are of the SMR type. The acoustical isolation from the glass substrate is achieved by a tungsten-silicon dioxide quarter wavelength mirror. Resonators with an effective coupling coefficient of 0.236 and Q/spl sim/800 have been achieved. The filters are realized as either 3-section ladder or 2-section lattice connected FBARs without any external components. The ladder filters achieve a 3.5 dB absolute bandwidth of 39 MHz with minimum insertion loss of 1.3 dB, stop band rejection at 23 dB and VSWR of 2.2 in the pass band. The balanced filter design has a slightly larger bandwidth of 46 MHz and improved stop band behavior characteristic for this type of device.

2-D modeling of laterally acoustically coupled thin film bulk acoustic wave resonator filters

A 2-D model is developed for calculating lateral acoustical coupling between adjacent thin film BAW resonators forming an electrical N-port. The model is based on solution and superposition of lateral eigenmodes and eigenfrequencies in a structure consisting of adjacent regions with known plate wave dispersion properties. Mechanical and electrical response of the device are calculated as a superposition of eigenmodes according to voltage drive at one electrical port at a time while extracting current induced in the other ports, leading to a full Y-parameter description of the device. Exemplary cases are simulated to show the usefulness of the model in the study of the basic design rules of laterally coupled thin film BAW resonator filters. Model predictions are compared to an experimental 1.9-GHz band-pass filter based on aluminum nitride thin film technology and lateral acoustical coupling. Good agreement is obtained in prediction of passband behavior. The eigenmode-based model forms a useful tool for fast simulation of laterally coupled acoustic devices. It allows one to gain insight into basic device physics in a very intuitive fashion compared with more detailed but heavier finite element method. Shortcomings of this model and possible improvements are discussed.

Spurious resonance suppression in ZnO based thin-film baw resonators: FEM modeling and experiment

Spurious resonance suppression in ZnO based thin film BAW resonators by the boundary ring method of (1) is studied. Electrical responses of resonators with varying width of the boundary ring structure are measured. Very clean resonator response is reached with optimum dimensions. Emergence of the piston mode and simultaneous spurious resonance suppression is demonstrated by 2D FEM simulation with a model corresponding to the measured devices. Good correlation between measurement and simulation is found. I. INTRODUCTION Thin film bulk acoustic wave resonators and filters for the GHz range have been intensively developed during the last ca. 10 years due to great demand in the mobile communications industry. Main application of the thin film BAW devices is in the band pass filters of the antenna circuit of mobile phones. With the thin film BAW technology, low insertion loss, high power handling, steep pass band skirts and small footprint are sought together with low manufacturing cost. The BAW filters are also inherently better applicable for higher frequencies (> 2 GHz) than SAW filters due to the lack of critical dimensions. Also as BAW resonators do not rely on a special crystalline substrate, possibility to integrate them with silicon IC:s exists. In order to create high performance filters, the BAW resonators they consist of, must fulfill certain performance criteria. Most importantly the effective coupling coefficient K 2 determines the attainable bandwidth of the filter and the Q- values affect the insertion loss and pass band skirt steepness. In addition, the resonator should not have ripple in or near the inductive region of its electrical response. In a low insertion loss filter this is only attained by suppressing the spurious resonances in some manner.

Modelling of ZnO-based BAWs at high signal levels

This paper proposes a method to simulate the center frequency shift in bulk acoustic wave (BAW) filters, caused by high input signal levels, which heat up the devices. The results were verified by wafer level measurements and on diced devices wire bonded to jigs. In addition the temperature distribution on diced samples was imaged with an infrared camera. The measured and calculated center frequency shifts as well as the temperature distribution in filters correlate fairly well.

An eigenmode superposition model for lateral acoustic coupling between thin film BAW resonators

A simple model is developed for calculating lateral acoustical coupling between adjacent thin film BAW resonators forming an electrical N-port. The model is based on plate wave dispersion properties of different regions of the device and solution of lateral eigenmodes and eigenfrequencies. Mechanical and electrical response of the device is calculated as a superposition of eigenmodes according to voltage drive at one electrical port at a time while extracting current induced in the other ports, leading to a full Y-parameter description of the device. Comparison of model prediction with experimental results from adjacent large square resonators and narrow finger-like resonators is performed. Large resonators exhibit comb-shaped oscillating transmission characteristics over wide frequency band, while narrow finger create a single pass-band. Fair qualitative agreement is obtained.

Lamb-wave resonator for microphone application

We study the pressure sensitivity of AlN thin-film resonators on a poly-Si membrane sublayer. The studied resonators operate on the S0-mode Lamb wave at 750 MHz. The resonators are fabricated on a released membrane which consists of a polysilicon sublayer, molybdenum electrodes, and AlN as the piezoelectric layer. Operation of the test structures is simulated using 2D and 3D finite element method models. Simulated pressure sensitivity at pressures below 1 Pa is 0.035 ppm/Pa. Pressure sensitivity of the fabricated test devices was measured by probing the devices on-wafer and measuring their electric frequency response while applying a variable differential pressure over the wafer. To remove the noise and the outliers, the data was filtered with a 3-point median filter prior to analysis. The linearized pressure sensitivity of frequency below 1 Pa is around 1 ppm/Pa. With minimized residual stress, optimal materials and modified membrane design, the pressure sensitivity can be improved.

P2G-5 Area and Dispersion Dependence of Vibration Shape and Coupling Coefficient in Thin Film BAW Resonators

Size dependence of resonator performance is studied in AlN and ZnO resonators with FEM simulation, electrical measurements, and laser interferometry. Eigenmode spectrum compression due to increased size of the resonator leads to smoothening of the electrical response and flattening of the vibrational at-resonance response as several modes can be simultaneously excited near the device resonance frequency. This is seen both in simulations and in measurements.

Nonlinear effects in solidly-mounted ZnO BAW resonators

The mechanical nonlinearity due to high power level in zinc oxide thin film bulk acoustic wave resonators has been studied in this work. The nonlinear behavior of these resonators was investigated using transmission response measurements. The resonator was decoupled from the 50 Ohm line impedance by 20 pF shunting capacitors in order to allow the excitation of high mechanical vibration levels at the series resonance frequency. The power levels ranged from +5 dBm to +29 dBm at the resonator input. At low excitation power (+5 dBm) the measured series and parallel resonance frequencies of the studied device were 941 MHz and 969 MHz, respectively. When increasing the excitation power level the transmission response exhibits a Duffing-behavior with tilting direction towards lower frequencies. The transmission shows a two-valued response starting at +15 dBm.