Ulrike Rösler

A SAW duplexer with superior temperature characteristics for US-PCS

The system requirements for US-PCS are very hard to fulfill with conventional SAW filters, since the duplex gap between Tx and Rx bands is only about 1% wide. Furthermore, current PCS CDMA systems impose strong demands on isolation and rejection levels. Typical SAW devices on rotated Y-X LiTaO/sub 3/ cannot meet these requirements over the whole operating temperature range, because of the large temperature coefficient of frequency (TC/sub f/) of the substrate. SiO/sub 2/ films can be applied to lower the effective TC/sub f/. However, since the necessary film thickness is rather high, this causes fundamental changes in the electro-acoustic properties of this new material system. Using one-port resonators and ladder-type base sections as test structures, we have conducted a series of experiments to identify interrelations between parameters defining the layer system and its electro-acoustic properties like TC/sub f/, piezoelectric coupling factor, reflectivity, and propagation loss. We have determined requirements for realisation of a SAW duplexer in the US-PCS hand and have designed a suitable layer system. We also demonstrate that the effects of a SiO/sub 2/ layer can be simulated accurately by a standard P-matrix model with suitably adjusted parameters. Finally, we present a complete SAW duplexer device for the PCS band featuring a TC/sub f/ of -17 ppm/K.

Calculation and experimental verification of the acoustic stress at GHz frequencies in SAW resonators

High power applications of surface acoustic wave (SAW) devices may lead to acoustomigration in their thin metal electrodes, which deteriorates the performance or may even destroy the SAW device. It is confirmed in this paper that the mechanism of acoustomigration is caused by the SAW-induced stress in the metal. The quantitative calculation of this stress are shown in detail, starting from the widely used P-Matrix model as a standard analysis tool. The combination with the partial wave method (PWM) yields the stress distribution inside the metal. This approach provides the flexibility to determine the stresses for any given point in a SAW device, for any input power, frequency, wavetype, device geometry, or metal layer. In order to confirm the absolute values of the stress components, we calculated and measured displacements as a function of input power and frequency.

Transversal Mode Orthogonality in SAW waveguides with transversally varying slowness anisotropy within the scalar field approximation

Within the 2D P-matrix model the scalar feld is identified with a power wave. As has been demonstrated recently, orthogonality of the transversal modes implies power conservation in the 2D P-matrix model [1]. Assuming continuity of the transversal mode profile and its derivative, however, orthogonality is only obtained if the slowness anisotropy is parabolic and constant within the waveguide. Orthogonality conditions for transversal modes within the parabolic approximation of slowness are derived for the case of transversally varying anisotropy constants. Thereto the transversal wave equations are identified with the elastic wave equations of a shear vertical bulk acoustic wave. Continuity of the displacement and the stress held at boundaries of transversal sections is required and therefrom orthogonality conditions of the modes are derived. Translating these conditions to the 2D P-matrix model implies discontinuity of the power waves at transversal section interfaces. For transversally varying anisotropy of the slowness an orthogonal mode description, fulfilling energy conservation within the 2D P-matrix model, was achieved. The new method was tested at a STW cut Quartz.

Calculation of the SAW-induced stress distributions in an electrode of a SAW-device on LiTaO/sub 3/

Acoustomigration is a well-known phenomenon, which may occur in surface acoustic wave (SAW)-devices operating at high driving levels. Based on previously published reports (Roesler, U. et al., 1996; Kubat, F. et al., 2002; 2003) about the calculation of the acoustic power, dynamic stresses and displacements in a homogenous isotropic Al-layer for a given driving condition, we expand this method for the calculation of the SAW-induced stress in an electrode of a SAW-device. The quantitative calculation of this stress is based on the combination of the widely used P-matrix based model, which yields the distribution of the potential power (Kubat et al., 2002) for a given driving level and a FEM tool (Smole, P. et al., 2002), which provides the relative stresses inside an electrode.

Investigation of GeO2 thin film properties for improvement of temperature coefficient of frequency of SAW devices

We have investigated the temperature dependent elastic constants of SiO₂ and GeO₂. For this purpose, the phase velocity of the layered system has to be calculated using surface acoustic wave (SAW) differential delay lines on LiNbO₃ substrates. Both SiO₂ and GeO₂ have a positive temperature coefficient of velocity. The GeO₂ layer shows a significant reduction of the phase velocity compared to SiO₂ which yields to an improved energy trapping of the surface acoustic wave in the overlay. However, severe stability issues arise using the GeO₂ layer.

Radio‐frequency surface acoustic wave filters for mobile communications systems

Surface acoustic wave (SAW) filters are small, rugged, crystal stable, and can be fabricated with high reproducibility. SAW filters play a key role in the rapidly growing market of mobile communication systems and are used for channel filtering as well as for RF filtering. For today’s communications systems novel types of SAW filters are being developed in order to meet the stringent requirements with respect to low insertion loss, high relative bandwidths, good stopband rejection properties, and power durability. Most SAW RF filters are fabricated on cuts of LiTaO3 and LiNbO3. To obtain very low loss filters precise knowledge of the wave propagation and loss mechanisms is a prerequisite. Using advanced analysis tools, substrate‐cut‐metallization combinations can be chosen for minimized propagation losses. To enhance power durability of SAW filters both substrate‐metallization combination as well as the design have to be optimized. Two different filter techniques are used for SAW RF filters. The working p...