This book gives the fundamental principles and device design techniques for surface acoustic wave filters. It covers the devices in widespread use today: bandpass and pulse compression filters, correlators and non-linear convolvers and resonators. The newest technologies for low bandpass filters are fully covered such as unidirectional transducers, resonators in impedance element filters, resonators in double-mode surface acoustic wave filters and transverse-coupled resonators using waveguides.The book covers the theory of acoustic wave physics, the piezoelectric effect, electrostatics at a surface, effective permittivity, piezoelectric SAW excitation and reception, and the SAW element factor. These are the main requirements for developing quasi-static theory, which gives a basis for the non-reflective transducers in transversal bandpass filters and interdigital pulse compression filters. It is also needed for the reflective transducers used in the newer devices. It is a thorough revision of a classic on surface acoustic wave filters first published in 1985 and still in print. It uniquely combines easy-to-understand principles with practical design techniques for all the devices in widespread use today. It includes complete coverage of all the latest devices which are key to mobile phones, TVs and radar systems; and, a new foreword by Sir Eric Albert Ash.

Surface Acoustic Wave Filters: With Applications to Electronic Communications and Signal Processing

World Scientific Publishing Company

Today, acoustic filters are the filter technology to meet the requirements with respect to performance dictated by the cellular phone standards and their form factor. Around two billion cellular phones are sold every year, and smart phones are of a very high percentage of approximately two-thirds. Smart phones require a very high number of filter functions ranging from the low double-digit range up to almost triple digit numbers in the near future. In the frequency range up to 1 GHz, surface acoustic wave (SAW) filters are almost exclusively employed, while in the higher frequency range, bulk acoustic wave (BAW) and SAW filters are competing for their shares. Prerequisites for the success of acoustic filters were the availability of high-quality substrates, advanced and highly reproducible fabrication technologies, optimum filter techniques, precise simulation software, and advanced design tools that allow the fast and efficient design according to customer specifications. This paper will try to focus on innovations leading to high volume applications of intermediate frequency (IF) and radio frequency (RF) acoustic filters, e.g., TV IF filters, IF filters for cellular phones, and SAW/BAW RF filters for the RF front-end of cellular phones.

Acoustic Wave Filter Technology–A Review

This paper describes a highly sensitive, film bulk acoustic resonator (FBAR) mass sensor (built on a micromachined silicon-nitride diaphragm with a piezoelectric thin film and Al electrodes) that can operate in vapor and liquid. The sensitivity of the device to mass change on its surface has been investigated by having various thicknesses of silicon-nitride support layer and also of Al layer. The sensor is measured to have a mass sensitivity of 726 cm/sup 2//g, which is about 50 times that of a typical quartz crystal microbalance (QCM). In vapor, the sensor (operating at around 1 GHz and having a relatively high quality (Q) factor of 200-300) shows a minimum detectable frequency shift of about 400 Hz, which corresponds to a mass change of 10/sup -9/ g/cm/sup 2/ on the sensor surface, comparable with that detectable by a QCM. In liquid, though the Q usually drops more than an order of magnitude, we obtain a Q of 40 at 2 GHz by using a second harmonic resonance of the resonator. And with the Q, a minimum 5 ppm resonant frequency shift can be detected, which corresponds to 10/sup -8/ g/cm/sup 2/ change on the sensor surface.

Micromachined acoustic resonant mass sensor

The coupling-of-modes approach for modeling and analyzing surface-acoustic wave devices is reviewed. We discuss the established formalism and survey the modifications introduced to account for phenomena such as resistivity, dispersion and in particular, the effects related to surface transverse wave and leaky surface-acoustic wave devices. The extraction of the COM parameters from experiments and theoretical simulations are considered. The design of various SAW devices such as resonators and resonator filters as well as practical aspects are discussed. Finally, the unresolved modeling problems are addressed.

Coupling-of-modes analysis of saw devices

We have developed a new electrode material, suitable for AlN-based FBARs. The necessary conditions for FBAR electrode materials are low density, low resistivity, and high acoustic impedance. We investigated the influence of electrode materials on the resonant characteristics of FBAR in FEM simulations and experiments. We found the properties of ruthenium (Ru) to be suitable for use as electrode materials for AlN-based FBARs. In addition, by smoothing the surface of the electrodes, full width at half-maximum (FWHM) of the X-ray rocking curves of the AlN films is found to be 2.9/spl deg/ on Ru electrodes. The resonant characteristics of FBAR using Ru electrodes were an unloaded resonant quality factor, Q/sub r/, of 3585 and an effective electromechanical coupling coefficient, k/sup 2/, of 6.6% at 5 GHz. Furthermore, we applied these resonators to ladder-type FBAR filters for 5 and 2 GHz. As a result, we achieved low-loss and high-Q FBAR filters with Ru electrodes.

New electrode material for low-loss and high-Q FBAR filters

The paper describes how the propagation loss /spl alpha/ of leaky surface acoustic wave (LSAW) is dependent upon the Al thickness h of grating electrodes and rotation angle /spl theta/ for rotated Y-X LiTaO/sub 3/. Since the energy leak of LSAW originates from the coupling of the slow-shear field component with other components through the surface boundary conditions, zero /spl alpha/ can be realised when the slow-shear component is decoupled. This means that the optimal /spl theta/ giving zero /spl alpha/ should change with h as well as the surface electrical boundary conditions. Since relatively large h of 0.07-0.1 wavelengths is needed for present LSAW devices, the optimal /spl theta/ shifts to 40-42/spl deg/ from the conventional value of 36/spl deg/. Thanks to the minimised propagation loss, experimental results showed that device performances are markedly improved only by increasing /spl theta/ to circa 42/spl deg/ from 36/spl deg/. We also describe the state-of-the-art of the performances obtainable in commercially available LSAW devices based on 42/spl deg/YX-LiTaO/sub 3/.

Optimum leaky-SAW cut of LiTaO/sub 3/ for minimised insertion loss devices

A novel temperature-compensated SAW substrate was developed based on bonded LiTaO/sub 3//sapphire. Because of its small TEC (thermal expansion coefficient) and large Young's modulus, sapphire is an ideal material for a support substrate. The frequency and temperature characteristics of the bonded LiTaO/sub 3//sapphire SAW substrate and its application to a US-PCS duplexer are described.

Temperature compensated LiTaO/sub 3//sapphire bonded SAW substrate with low loss and high coupling factor suitable for US-PCS application

The improvement on power durability of SAW filters in order to realize a SAW duplexer is described in this study on IDT metallization. Compared with the conventional single layer metallizations such as Al-Cu, the laminated films are found to show the higher power durability. Many kinds of laminated film were tested changing the material of the middle layer for three layered films of which the top and bottom layers were made of Al-Cu or pure Al. Among them, two types of films such as Al-Cu/Cu/Al-Cu and Al/Mg/Al showed the best power durability. Al-Cu/Cu/Al-Cu had a lifetime 250 times longer than that of current Al-Cu single film. Al/Mg/Al further had a longer lifetime than Al-Cu/Cu/Al-Cu. These three layered films are being successfully applied to SAW antenna duplexers of cellular phones, indicating sufficient power durability for practical use.

SAW duplexer metallizations for high power durability

The paper describes how leaky surface acoustic wave (LSAW) propagation characteristics on LiTaO/sub 3/ depend upon its rotation angle when the Al electrode thickness is increased. It is shown that the propagation loss arising from the leaky nature changes parabolically with both the film thickness and rotation angle, and becomes zero when they are properly chosen. This means that even when relatively thick electrodes are needed, zero propagation loss is always achieved by properly determining the rotation angle for LiTO/sub 3/. When the electrode thickness is 0.07/spl sim/0.1 wavelengths, for example, LSAWs on the 40/spl sim/42/spl deg/ Y-X LiTaO/sub 3/ offer zero propagation loss without deteriorating other characteristics. Ladder filters for 800 MHz range were fabricated, which essentially need thick Al electrodes. As is predicted by theoretical calculation, experimental results showed that if the rotation angle a is increased to circa 42/spl deg/ from the conventional value of 36/spl deg/, the insertion loss and shape factor are improved by 0.7 dB and 0.26, respectively. The result gives a valuable guideline for determining the optimal rotation angle of LiTaO/sub 3/ to be used for high performance LSAW filters, which are one of the key components in RF sections of mobile communication systems.

M. Ueda

Papers

New electrode material for low-loss and high-Q FBAR filters

Optimum leaky-SAW cut of LiTaO/sub 3/ for minimised insertion loss devices

Temperature compensated LiTaO/sub 3//sapphire bonded SAW substrate with low loss and high coupling factor suitable for US-PCS application

SAW duplexer metallizations for high power durability

Optimum cut of LiTaO/sub 3/ for high performance leaky surface acoustic wave filters