Robert Thalhammer

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

Improved coupled resonator filter performance using a carbon-doped oxide de-coupling layer

We describe a newly developed de-coupling material SiOCH for coupled resonator filter applications. The SiOCH films belong to a general class of low-k dielectrics often referred to as carbon-doped oxides (CDO). In this work, CDO replaces SiLK, significantly improving the performance of the resulting filters. In contrast to the spin-on and curing process used to deposit SiLK, the CDO films are deposited using plasma enhanced chemical vapor deposition. The resulting films possess a low acoustic impedance that can be varied over a range greater than 2∶1 through a choice of deposition conditions. The new filters possess several key advantages over the SiLK-based devices reported previously, including decreased filter insertion loss, a passband free of spurious notches, and a dramatically lower temperature coefficient of frequency.

FBAR laterally Coupled Resonator Filter

A very attractive CRF (Coupled Resonator Filter) topology is to laterally couple thin film bulk acoustic resonators (FBAR). However, the filter band width is narrow because the coupling between conventional FBAR resonators is very weak. The coupling, and thus the bandwidth, may be increased by forming pairs of FBAR with an inter-digital coupling configuration. Both narrow electrode finger width and multiple coupled finger pairs enhance lateral acoustical coupling, resulting in filters with wider pass band. The finite element simulation and experimental results on LCRF structures show that the multiple acoustical modes can be combined to form a wider range of pass bands than available in a conventional ladder filter.

Finite-Element Analysis of Bulk-Acoustic-Wave Devices: A Review of Model Setup and Applications

In this paper, the principles of finite-element modeling for the electroacoustic simulation of bulk-acoustic-wave devices will be summarized. We will outline the model setup including governing equations and boundary conditions, as well as its efficient computer implementation. Particular emphasis will be given to tailoring the model dimension to the specific requirements of the desired investigation. As 3-D simulations still require a major effort, it will be illustrated that various aspects of device physics and design can already be addressed by fast and efficient 2-D simulations. Multiple theoretical and experimental evidence will be presented to demonstrate the validity of the modeling concepts. Based on various examples, it will be sketched how to benefit from numerical simulations for understanding fundamental effects, designing devices for actual products, and exploring novel technologies.

Finite element analysis of BAW devices: Principles and perspectives

This work summarizes the principles of Finite-Element-Modeling for bulk-acoustic-wave devices. It illustrates the model setup in terms of the governing system of equations and boundary conditions, as well as the efficient computer implementation. While some problems require the significant effort of 3D models, various effects can be addressed by fast and efficient 2D simulations. Multiple examples will be demonstrated of how numerical simulations can improve our understanding of the underlying physics, support the design of actual products, and help to pioneer the way into new technologies by facilitating to explore novel device concepts.

An Investigation of Lateral Modes in FBAR Resonators

Using first principles and the constitutive equations for a piezoelectric, we solve the 2-D acoustic wave inside a single, infinite, piezoelectric membrane to study the dispersion of thin film bulk acoustic resonator (FBAR) lateral modes, with and without infinitely thin electrodes. The acoustic eigenfunction is a dual wave, composed of longitudinal and shear components, able to satisfy the 2-D acoustic boundary conditions at the vacuum interfaces. For the single piezoelectric slab, we obtain analytical expressions of the dispersion for frequencies near the longitudinal resonant frequency (Fs) of the resonator. These expressions are more useful for the understanding of dispersion in FBARs and more elegant than numerical methods like finite-element modeling and various matrix methods. We additionally find that the interaction between the resonators electrodes and the acoustic wave modifies the lateral-mode dispersion when compared to the case with no electrodes. When correctly accounting for these interactions, the dispersion zero is placed clearly at Fs, unlike what is calculated from a 2-D model without electrodes where the dispersion zero is placed at Fp. This is important since all experimental evidence of measures FBAR resonators shows that the dispersion zero is at Fs. Furthermore, we introduce an electrical current-flow model for the propagating acoustic wave inside the electroded piezoelectric, and based on this model, we can discuss an electrode-loss mechanism for FBAR lateral modes which depends on dispersion. From our model, it results that lateral modes with real kx have higher electrode dissipation if they are closer to the resonant frequency. This is consistent with the typical behavior of measured FBAR filters where the maximum lateral mode damage on the insertion loss takes place for frequencies immediately below Fs.