Gernot Fattinger

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.

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

Performance and Dynamics of a RF MEMS Switch

A torsional RF-switch with low actuation voltage for use in mobile communication is presented. The device is fabricated in a BiCMOS compatible process with an additional back-end electroplating procedure. Electrical characterization from DC up to 6 GHz has been performed showing good RF characteristics. Optical measurements of the first resonance modes are in good agreement with simulated results. The transient behavior was analyzed by interferometry in order to study the damping parameter of the device. Comparison with simulation has led to the relative damping coefficients for different designs.

Technology enhancements for high performance BAW duplexer

In this article we will present the recent improvements in the performance of Solidly Mounted Resonator (SMR) high performance duplexer using Band2 duplexer for illustration purpose. Over the past 4 years insertion loss and rejection steepness were improved dramatically. Optimization of resonator intrinsic performance has played obviously a critical role but less expectedly the introduction of new building block (MIM capacitor) capitalizing on SMR technology has proven key for both performance improvement and size reduction. A new design approach based on the selective reduction of the effective coupling coefficient of certain resonators will be presented. We will discuss its impact on slope steepness improvement. The additional flexibility provided by this method enables as well to reduce further the impedance variation presented on the transmit path. This feature is critical to maintain high efficiency and linearity of any Power Amplifier upfront in the chain.

Miniaturization of BAW devices and the impact of wafer level packaging technology

As wireless mobile applications continue to drive shrinkage of the individual components, consequently a corresponding decrease in required footprint and component thickness needs to go along with it. Over the last years, the size of power amplifier duplexer (PAD) front-end modules as well as standalone duplexers has steadily decreased. A significant portion of this shrinkage has been contributed by a decrease in BAW die size. This decrease is driven by various technology improvements. In this paper we will first discuss the introduction of on-chip capacitors. These capacitors not only allow for a reduction of the required external component count, but also for an increased degree of design freedom. Second, we will discuss how the shrinkage of reflector metal dimensions leads as a direct result to an significant reduction in die sizes. Although core technology improvements can allow for radical BAW die shrinkage, the packaging technology that comes along with the BAW die needs to contribute its share to maintain this trend. Last, a novel approach for wafer-level packaging BAW die will be presented.

Full band 41 filter with high Wi-Fi rejection - design and manufacturing challenges

In this article we present a full band 41 filter using state-of-the-art BAW (bulk-acoustic wave) technology. Due to the unusually large bandwidth requirement (7.8% fractional bandwidth) low insertion loss in the passband cannot be maintained by typical design strategies used for narrower bands. We discuss how to cope with the challenges arising due to the filters wide bandwidth and Wi-Fi coexistence requirement on the design side as well as on the manufacturing side. The steepness of the filters lower edge plays a major role to fulfill the stringent rejection requirement in the Wi-Fi band, where the use of high-Q BAW technology is indispensable. Additional manufacturing challenges arise to achieve the accuracy needed and guarantee the filter selectivity as well as its low loss performance over the whole band 41.

Strategies for improved linearization in BAW multiplexer modules

Carrier aggregation (CA) and the resulting increasing number of filter chips per multiplexer modules has significantly changed linearity requirements. Accurate prediction of non-linear performance in BAW is crucial to be able to optimize a design of a filter for the required linearity specifications. Aspects for model simplification in simulation are discussed and examples given to show the sensitivity of non-linear prediction accuracy on the model. The scope of the paper is to present general approaches to improve the linearity in a single BAW filter, considering as well additional constraints and challenges applying for multiplexer modules. Factors for response variation as well as the effects of acoustic modes on the non-linearity are presented.

Impact of thermo-mechanical stress on the TCF of WLP BAW filters

For BAW filter structures, the Temperature Coefficient of Frequency (TCF) specifies the frequency shift of the filter skirts over temperature. The lions share of materials used in BAW filter stacks exhibit negative TCF characteristics, which means as the temperature rises, the filter passband shifts downwards in frequency. At the expense of performance, TCF compensation can be achieved through application of materials with positive TCF characteristics. Therefore, the extent of compensation has to be carefully increased only up to the point where it becomes sufficient for the given application. The intended TCF is usually verified through calculations based on S-parameter measurements on wafer at different temperatures. However, recent performance studies have shown a non-negligible TCF degradation of up to 5 ppm/K, which occurs after flip-chip module assembly. This paper describes the investigations that revealed the main cause for degraded TCF characteristics in a band 7 Tx filter application.

No-drift™ BAW-SMR: Over-moded reflector for temperature compensation

As more LTE bands are squeezed into the crowded global RF spectrum interference becomes a major concern. Some cases require profoundly steep transitions from passband to the tightly packed neighboring bands. The most demanding high frequency LTE bands may only be served with high-performance BAW resonators that have been temperature compensated to have essentially zero temperature drift. There is a variety of known challenges associated with the traditional temperature compensation techniques applied to BAW devices. This work will address those challenges and present a novel no-drift BAW-SMR using over-moded acoustic reflector layers. A solution for Band 30 utilizing the over-moded reflector will be presented.

Material parameter determination in BAW resonators using combined dispersion and electrical response input data

The determination of precise material parameters is indispensable for an accurate modeling of the electrical response of a BAW resonator and further filter modules. The prediction accuracy is highly dependent on the material parameters used in simulation, where small discrepancies between simulation parameters and actual parameters as achieved during production can result in prediction errors in the percent range. A fitter approach is presented using dispersion and electrical response input data to determine material parameters. An improved prediction accuracy using the fitted material parameters for the main acoustic modes as well as wideband resonances is shown to be within the expected prediction window as estimated from measurement variation.