Bernhard Bader

Appropriate Methods to Suppress Spurious FBAR Modes in Volume Production

Standards like CDMA in combination with the US PCS band impose challenging demands on the duplexer unit in the RF circuit of mobile phones. FBAR filters are candidates to fulfil the latest cutting-edge requirements for temperature stability, insertion loss, power durability, bandwidth, and filter skirts. Unfortunately, these filters suffer from spurious resonator modes. In this work we have investigated several suppression methods, which are capable of being applied to volume production

On the design of an FBAR PCS duplexer in LTCC chip-sized package

The paper reports about new results of a first-time attempt to utilize the existing design flow and methodology of surface acoustic wave (SAW) components to design film bulk acoustic resonator (FBAR) components. Due to the modular design of the computer aided design (CAD) environment, we could reuse many modules. In combination with new FBAR-specific modules, we obtained an FBAR-suitable CAD environment. Designing an FBAR personal communication system (PCS) duplexer comprising a low temperature co-fired ceramic (LTCC) chip sized package with integrated matching circuitry as well as the Rx- and Tx-filters we demonstrate the fitness for use of the new CAD environment. Starting from scratch the parts of the duplexer have been optimized with respect to each other. The experimental results of this design are presented indicating excellent performance especially regarding the Tx isolation. This most critical specification item has typically excelled with about 55 dB. Other specification items, such as, e.g., matching as well as pass band and stop band attenuation are also met. Measurement and simulation are in good agreement.

High performance microwave acoustic components for mobile radios

Due to their outstanding characteristics acoustic components, such as SAW and - later on - also BAW filters, have been key components in wireless communication systems from their very beginning. Regarding mobile radio handsets the primary field of application of acoustic components has moved to the RF front-end sections. Here, with the increasing demand for WCDMA-FDD capable mobile phones, duplexers, which allow to separate the TX and RX paths while being simultaneously connected to the antenna, have become indispensable. Regarding the specifications of the different operating bands the requirements on form factors, pass band characteristics, attenuations of the other signals (TX, RX, GPS, WLAN, ...), and isolations make them high performance components requiring a sophisticated acoustic as well as electromagnetic design. Due to the tough requirements imposed on the duplexers in terms of, e.g., center frequencies, spacings of RX and TX bands, relative band widths, and shape factors, they also require application-specific acoustic structures. As a consequence, in contrast to standard filters, duplexers are highly optimized components using leading-edge technologies. In order to achieve the best balance of the characteristic properties of the acoustic structures regarding, for instance, their quality factors, pole-zero distances, and temperature coefficients, the manufacturing processes of the acoustic chips are designed for the specific applications. Furthermore, optimizing the electrical characteristics a co-design of package and acoustic chip is applied. In this paper we provide an overview of the different types of duplexers that have been the result of the focused specialization and optimization efforts. We classify them according to the type of acoustic structures, matching circuitry, and package being used. So far, purely SAW-based and BAW-based duplexers as well as hybrid duplexers using both SAW and BAW chips have been reported. Most of the matching circuitry is integrated in, for instance, an LTCC package using a distributed approach based on a ?/4 line or a lumped approach comprising coils and/or capacitors. Sometimes the matching circuitry is realized externally on the PCB. Furthermore, we present recent integration successes, i.e., duplexer inserts, allowing the seamless integration of duplexers into modules without drawbacks on performance or module thickness.

Improved modeling of bulk acoustic wave resonators and filters

Duplexers and filters for mobile radio are one of the classic field of application for the BAW technology. In recent years the size of the devices has been reduced by about a half and the trend is still toward more compact designs with higher performance at the same time. This translates directly to very demanding requirements on the effective coupling coefficient and on the quality factors of the underlying BAW resonators. Measurements show that the size and the shape of the resonators have a significant influence on the overall performance. Hence, size-relevant effects need to be considered in 1D-simulation, which is the base for fast filter optimization. To evaluate the influence of the resonator size on the filter performance we analyzed several SMR-BAW resonator area and shape variations in combination with different overlap sizes in a series of experiments. Moreover, the influence of the overlap size on different resonator areas is studied with 2D-FEM simulations. In this paper we present the results of our resonator measurements. The results are basis for developing a modified 1D-model for designing BAW filters. Additionally, a new method for the extraction of an ‘equivalent acoustic impedance’ for the description of the acoustics is presented and compared to the modified 1D-model.

Suppression of spurious modes in mirror-type thin film BAW resonators using an appropriate shape of the active area

F ilm bulk acoustic resonators (FBARs) have emerged as an important new technology for realization of GHz filter components. Two different technologies are commonly used: bridge-type and mirror-type resonator layer stacks. Both resonator configurations are susceptible to unwanted lateral modes beside the main resonance. Filters composed of FBARs, e.g. bandpass ladder-type filters, suffer from those spurious modes: they show a higher insertion loss, a lower bandwidth and several ripples in their passbands. Therefore, a practical method is needed to suppress spurious modes in FBARs. In the past, using membrane-type resonators with non-parallel edges of the active area (apodization) has been successfully applied to suppress lateral spurious modes. In contrast, for mirror-type FBARs the focus has been on mode suppression by means of a lateral edge design (frame-like resonators). In this paper we investigate in detail, if apodization works for acoustically more complicated mirror-type FBARs. We have processed a lot of eight 200-mm-wafers using a special test reticle set comprising resonators and filters with both distinct shapes of the active area, lateral edge design as well as combinations thereof. Our measurement results clearly single out optimum electrode shapes for apodization, but also manifest the superior performance of the lateral edge design. I. INTRODUCTION

Comparison of different electromagnetic models of bulk acoustic wave resonators and filters

High-performance and miniatuarized bulk acoustic wave (BAW) devices require accurate and sophisticated computer aided design methods. Different modeling and simulation methods have to be applied depending on the simulated device, its complexity, available computational resources, and required accuracy. In order to model all relevant effects that are important to accurately characterize a device, it is important to know what details to model, how to model them, and to what extent. This paper presents and compares different electromagnetic models of mirror-type BAW resonators that are simulated with a 3D finite element method electromagnetic solver. The impact of meshing effects of the 3D electromagnetic solver on the simulated resonator and filter performances is discussed. Additionally, a simulation method is presented in which the acoustic and electromagnetic effects are simulated together. The combined simulation of the acoustic and electromagnetic effects is accomplished by incorporating the acoustic effects into the electromagnetic simulation by applying a frequency-dependent, complex-valued, equivalent permittivity for the piezo material. For the examined electromagnetic BAW resonator models the electromagnetic effects, computational effort, accuracy, and applicability are discussed. By considering the different enhancements introduced in the proposed electromagnetic BAW models, it is possible to improve the agreement of measurement and simulation.

Low-loss BAW filters on high resistivity silicon for mobile radio

Integration of bulk acoustic wave (BAW) filters into W-CDMA mobile communication applications is of high interest, as the BAW technology allows for low-loss and high Q signal forming in these 2 GHz devices. During the design phase of low-loss BAW filters it is very important that we not only control the most major loss mechanisms, like metal conductivity and viscosity of piezo material. We also have to focus the less well known losses.

Influence of mobile charges in the substrate of BAW filters on high resistivity silicon

BAW (bulk acoustic wave) filters have emerged as an important technology for GHz filtering components. Especially the high quality factor and the good temperature coefficient make BAW filters well suited for W-CDMA band 2 and band 3 devices. During the design phase it is essential to analyze and minimize possible loss mechanisms to meet the high requirements of these devices. Well known loss mechanisms are metal conductivity, viscosity and acoustic losses. A less well known loss mechanism is the existence of mobile charges at the surface of high resistivity silicon. This loss effect appears between the silicon substrate and the first silicon dioxide layer of the BAW structure. Induced charges increase the insertion loss and influence the performance of BAW filters. The two methods that are most promising for BAW applications to minimize the effect are ion implantation and the use of low charge silicon-dioxide. The aim of this work is the investigation and comparison of the mentioned methods in case of mirror-type BAW filters. We describe the loss mechanism, its counter measures and the impact on the filter performance. Therefore we designed test filters and test structures, manufactured on several test wafers. Additionally we used EM-simulations to evaluate the measurements of our test structures. An extended filter simulation model reflects the measured insertion attenuation of the filter performance pretty well.

Enhanced electromagnetic modeling of bulk acoustic wave resonators and filters

With the rapid miniaturization and increasing performance demands of bulk acoustic wave (BAW) devices, more accurate and sophisticated design and modeling methods are required. Accurate simulation results and appropriate software tools as well as their correct application are essential for a precise characterization of BAW devices. Depending on the simulated device, its complexity, required accuracy and available computational resources, different modeling and simulation methods have to be applied. For this modeling task it is important to know what details to model, how to model them and at what extent. In order to capture all relevant effects that are important to characterize the device with high accuracy, deep knowledge of the simulation softwares working principles as well as its computational limits and capabilities are necessary. This paper presents different enhanced electromagnetic models of mirror-type BAW resonators that are simulated with a 3D electromagnetic solver, whereas the acoustic effects are computed with a 1D solver. The electromagnetic effects of different resonator models are analyzed by fitting the simulated resonator admittance to an equivalent lumped circuit and comparing the fitted values. Further, the effects of the electromagnetic mesh-cell-density in simulations for the different resonator models are analyzed. The computational costs for these resonator models are shortly discussed by comparing mesh size, required memory and computational time. With the understanding of the model and electromagnetic mesh properties from the resonator simulations, an enhanced electromagnetic BAW duplexer model is simulated and compared to a measurement. The simulations of BAW resonators and duplexers can be improved by appropriate electromagnetic modeling and specific knowledge about the simulated device.