Stephan Marksteiner

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.

Behavior of BAW devices at high power levels

Bulk acoustic wave (BAW) and FBAR technologies offer key advantages for transmit filters which are used between the power amplifier and antenna in cell phone applications. The classical challenge for Tx filters in duplexers is to achieve low insertion loss, high return loss, high Rx - Tx isolation and good wideband performance at the same time. The trade-offs between those parameters can be modeled in a straight- forward approach as will be briefly described. The main section of this paper discusses 2 nd order effects which become relevant at high power levels. The energy loss in each individual BAW resonator generates a shift and spread of resonance frequencies in the filter elements by self-heating effects. These effects can cause unexpected performance issues and may even lead to thermal instability. A concept for modeling of self-heating effects is presented in detail. The role of self-heating with regard to power- handling capability of a filter will be pointed out. As a consequence of extreme energy density at high power levels a certain degree of non-linear effects can be observed in BAW/FBAR filters. The origin of these effects will be discussed as well as a modeling strategy. This is in particular important for emerging W-CDMA applications of BAW and FBAR filters.

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.

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.

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.

Ohmic effects in BAW-resonators

Bulk acoustic wave (BAW) filters owe their performance advantages particularly to the excellent Q-values of the acoustic resonators. As these devices achieve very low impedances in the series resonance condition, the ohmic conductivity of the electrodes is expected to affect the device performance significantly. This paper presents a comprehensive analysis of ohmic effects including a discussion of the associated power dissipation as well as the interaction of ohmic and acoustic effects. The analysis is based on two combined techniques: first, the potential and current distribution on the electrodes and the resulting ohmic power dissipation are derived from surface vibration measured by laser interferometry and an inverse calculation of local charge distribution. The second technique is finite element modeling that has been extended by including ohmic conductivity, thus enabling a consistent electro-acoustic analysis

Bulk acoustic wave filters for GPS with extreme stopband attenuation

RF filters for GPS receivers inside phones require extreme stopband attenuation in order to protect the weak GPS signals from interference by the cellular phone bands and ISM bands. In contrast to RF filters for cellular phone bands which typically require a relative bandwidth up to 4.5% a GPS filter needs less than 0.5% relative bandwidth. The reduced bandwidth requirement can be traded in for low insertion loss and high stopband attenuation. The impact of this tradeoff on the design of the bulk acoustic wave (BAW) filters is discussed in this paper. The choice of the filter topology and acoustic layer stack forming the resonators as well as the spurious mode suppression need to be re-optimized under the new constraints. As a result of this optimization, a filter for GPS signals with a minimum insertion loss of 1.2 dB and a stopband attenuation of 50 dB is presented.

Optimization of BAW resonator performance using combined simulation techniques

Filters based upon bulk-acoustic-wave (BAW) resonators are attractive for a variety of RF applications. To master the ambitious specifications and to facilitate a fast and cost economic design, we present an efficient simulation strategy combining different modeling approaches. As the first step, a 1D transmission line model (Mason model) is used for constructing the layer stack to meet the desired resonance frequencies and bandwith. Second, the system of Newton’s equation of motion and Maxwell’s equations coupled by the piezoelectric effect is solved by FEM simulations. Thus, the lateral structure, e. g. specific border regions, can be designed to maximize the Q factor and to minimize the excitation of spurious modes. The theoretical predictions are excellently confirmed by experimental results.