Jyrki Kaitila

Bulk-acoustic-wave filters: performance optimization and volume manufacturing

Performance parameters of BAW devices are reviewed and ranked corresponding to their importance for RF-filters in mobile phone applications. The most important performance parameters - such as resonator bandwidth and Q-values - critically depend on the quality of the piezolayer and other relevant layers in the acoustic stack. The design of the complete layer stack in a Solidly Mounted Resonator (SMR) concept in combination with a proper design of lateral resonator-boundaries will be revealed to be extremely important for the suppression of spurious resonances. Challenges in manufacturing of BAW filters will be briefly reviewed. Examples of state-of-the-art in BAW filters in production and ramp-up status will be presented.

Optimization of acoustic mirrors for solidly mounted BAW resonators

The overall performance of bulk acoustic wave (BAW) filters is dominated by the effective coupling coefficient and the quality factor of the constituting BAW resonators. Whereas the effective coupling coefficient and its dependency on the layer stack is quite accurately modeled with a simple one-dimensional acousto-electric model (e.g. Masonstransmission line model), the prediction and optimization of the resonators quality factor - particularly for solidly mounted resonators (SMR) - completely fails with this model: whereas a calculation of the acoustic reflectance of a standard quarter-wavelength mirror stack leads to theoretical Q-factors well above 10000, experimental SMR devices with this type of mirror show values of typically well below 1000. This discrepancy is commonly explained by either visco-elastic loss in the materials and/or laterally leaking waves leaving the active resonator area. However, we have found a new, far more important loss mechanism relating to shear waves generated in the device. These waves can be created by injection from the resonators border area as well as by reflection/refraction of longitudinal waves at non-perpendicular angle of incidence to a material interface. In this paper, a quantitative methodology for the optimization of the acoustic mirror layer stack will be proposed. The influence of the mirror structure on the trapping of both longitudinal and shear wave energy will be discussed based on this very simple approach. Trade-offs with respect to the other important device parameters, such as effective coupling coefficient, temperature coefficient of frequency (TCF) and purity of the electrical response, are analyzed. The usefulness of this approach for the optimization of resonator Q-values will be proven by experimental results demonstrating Q-factors of 1500 and higher.

Thin film bulk acoustic wave filter

Thin film bulk acoustic wave (BAW) resonators (FBAR) are fabricated on a silicon nitride bridge using a ZnO piezolayer on a glass substrate and surface micromachining by standard thin film technology. These resonators exhibit a coupling constant k/sub t//sup 2/=7.8% at the first thickness extensional wave mode and are used as impedance elements in a ladder filter in the 1-GHz frequency band of mobile telecommunications. An electrical equivalent circuit is used to characterize the properties of the resonators and to show how the performance of the filter depends on the parameters of the resonators. 2.5% bandwidth, 2.8-dB insertion loss, and 35-dB selectivity are obtained in a filter with six resonators. The technology can be used to manufacture miniature microwave filters without any additional inductances.

Acoustic reflector for a BAW resonator providing specified reflection of both shear waves and longitudinal waves

A BAW resonator includes a piezoelectric layer, a first electrode, a second electrode, a substrate, and an acoustic reflector disposed between the substrate and the second electrode. The acoustic reflector has a plurality of layers. A performance of the acoustic reflector is determined by its reflectivity for a longitudinal wave existing in the BAW resonator at the resonance frequency of the BAW resonator and by its reflectivity for a shear wave existing in the BAW resonator at the resonance frequency of the BAW resonator. The layers of the acoustic reflector and layers disposed between the acoustic reflector and the piezoelectric layer are selected, with reference to their number, material, and thickness, such that the transmissivity for the longitudinal wave and the transmissivity for the shear wave in the area of the resonance frequency is smaller than −10 dB.

Single-to-balanced filters for mobile phones using coupled resonator BAW technology

The coupled resonator filter (CRF) is a new type of bulk-acoustic-wave (BAW) device in which two piezoresonators are stacked on top of each other in a way that a certain degree of acoustic interaction occurs. Experimental results of single-ended and mode-converting CRF for GSM applications also featuring impedance conversion, operating at 1.8 GHz and manufactured at Infineon Technologies are presented. For the balanced port, those results demonstrate excellent amplitude- and phase-imbalance, obtained by a very symmetrical design. Also, a method for suppression of spurious resonances is demonstrated. Moreover, the latest results confirm that unwanted passbands can be overcome by modification of the acoustic mirror. Furthermore, the temperature coefficient of frequency (TCF) has been measured. Due to the compensating effect of SiO/sub 2/ in the CRF stack the values obtained are exceptionally small.

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.

Prediction of BAW resonator performance using experimental and numerical methods

Filters based upon bulk-acoustic-wave (BAW) resonators are attractive for a variety of RF applications. To satisfy the ambitious specifications and to facilitate a fast and cost economic design, we present an efficient simulation strategy combining different modeling approaches. First, a 1D transmission line model (Mason model) is used to construct the layer stack to meet the desired resonance frequencies and bandwidth. Second, the system of Newtons equation of motion and Maxwells equations coupled by the piezoelectric effect is solved by FEM simulations. Thus, the lateral structure, e.g., a specific border region, can be designed to maximize the Q-value and to minimize the excitation of spurious modes. The theoretical predictions are excellently confirmed by electrical measurements and laser interferometry. Typical technological features, such as processing-related non-uniform thicknesses, and their impact on the resonance characteristics are analyzed by numerical simulations.

4E-3 Spurious Mode Suppression in BAW Resonators

Designing bulk acoustic wave resonators for RF filter applications is governed by the need for a high quality factor (Q-value), a large piezoelectric coupling and the purity of the main resonance. The most powerful method to suppress spurious modes is terminating the edges of the resonators by a specific border region with a different eigenresonance. In this paper, we demonstrate the design constraints according to the dispersion diagrams and discuss specific realizations of this border regions, particularly for resonators of dispersion type II. Contrary to mirror type resonators with monotonic dispersion branches, membrane resonators comprising AlN as the piezoelectric layer show a negative slope in the main dispersion branch. Consequently, a border region of higher eigenresonance frequency is required for spurious mode suppression. Based on dispersion calculations and FEM simulations, we demonstrate how it has to be designed and discuss the sensitivity to processing induced layer thickness variations. The additional modes emerging between series and parallel resonance are analyzed and related to the respective branches of the dispersion diagram

Experimental investigation of acoustic substrate losses in 1850-MHz thin film BAW resonators

After optimizing for electromechanical coupling coefficient K2, the main performance improvement in the thin film bulk acoustic wave resonators and filters can be achieved by improving the Q value, i.e., minimizing the losses. In Bragg-reflector-based solidly mounted resonator technology, a significant improvement of Q has been achieved by optimizing the reflector not only for longitudinal wave, the intended operation mode, but also for shear waves. We have investigated the remaining acoustic radiation losses to the substrate in so-optimized 1850-MHz AlN resonators by removing the substrate underneath the resonators and comparing the devices with and without substrate by electrical characterization before and after the substrate removal. Several methods to extract Q-values of the resonators are compared. Changes caused by substrate removal are observed in resonator behavior, but no significant improvement in Q-values can be confirmed. Loss mechanisms other than substrate leakage are concluded to dominate the resonator Q-value. Difficulties of detecting small changes in the Q-values of the resonators are also discussed.

Temperature compensated solidly mounted bulk acoustic wave resonators with optimum piezoelectric coupling coefficient

We propose a new way to temperature compensate solidly mounted bulk acoustic wave resonators (SMRs). With the proposed process high Q, high electromechanical coupling coefficient, fully temperature compensated resonators have been successfully fabricated with TCF less than 1ppm/°C and total thermal drift of less than 35 ppm across the temperature range from 0–100°C.