Juha Ellä

Bulk-acoustic-wave filters: performance optimization and volume manufacturing

Performance parameters of BAW devices are reviewed and ranked corresponding to their importance for RF-filters in mobile phone applications. The most important performance parameters - such as resonator bandwidth and Q-values - critically depend on the quality of the piezolayer and other relevant layers in the acoustic stack. The design of the complete layer stack in a Solidly Mounted Resonator (SMR) concept in combination with a proper design of lateral resonator-boundaries will be revealed to be extremely important for the suppression of spurious resonances. Challenges in manufacturing of BAW filters will be briefly reviewed. Examples of state-of-the-art in BAW filters in production and ramp-up status will be presented.

Thin film bulk acoustic wave filter

Thin film bulk acoustic wave (BAW) resonators (FBAR) are fabricated on a silicon nitride bridge using a ZnO piezolayer on a glass substrate and surface micromachining by standard thin film technology. These resonators exhibit a coupling constant k/sub t//sup 2/=7.8% at the first thickness extensional wave mode and are used as impedance elements in a ladder filter in the 1-GHz frequency band of mobile telecommunications. An electrical equivalent circuit is used to characterize the properties of the resonators and to show how the performance of the filter depends on the parameters of the resonators. 2.5% bandwidth, 2.8-dB insertion loss, and 35-dB selectivity are obtained in a filter with six resonators. The technology can be used to manufacture miniature microwave filters without any additional inductances.

Bulk acoustic wave filter with a roughened substrate bottom surface and method of fabricating same

A filter device comprises a substrate having a top surface and a bottom surface, and at least one acoustic wave situated on the top surface of the substrate, wherein the bottom surface of the substrate is roughened to reduce the reflection of an acoustic wave back to the acoustic wave filter. The effect achieved by the roughening of the bottom surface of the substrate that an acoustic wave which is generated by the acoustic wave filter and reaches the bottom surface of the substrate, is basically scattered so the acoustic wave that is actually reflected back to the acoustic wave device is reduced which, in turn, improves the performance characteristics of the acoustic wave filter.

ZnO based thin film bulk acoustic wave filters for EGSM band

We present results for a ZnO based filters for the mobile Extended GSM (EGSM) Rx band centered at 942.5 MHz. Our devices are of the SMR type. The acoustical isolation from the glass substrate is achieved by a tungsten-silicon dioxide quarter wavelength mirror. Resonators with an effective coupling coefficient of 0.236 and Q/spl sim/800 have been achieved. The filters are realized as either 3-section ladder or 2-section lattice connected FBARs without any external components. The ladder filters achieve a 3.5 dB absolute bandwidth of 39 MHz with minimum insertion loss of 1.3 dB, stop band rejection at 23 dB and VSWR of 2.2 in the pass band. The balanced filter design has a slightly larger bandwidth of 46 MHz and improved stop band behavior characteristic for this type of device.

Modelling of ZnO-based BAWs at high signal levels

This paper proposes a method to simulate the center frequency shift in bulk acoustic wave (BAW) filters, caused by high input signal levels, which heat up the devices. The results were verified by wafer level measurements and on diced devices wire bonded to jigs. In addition the temperature distribution on diced samples was imaged with an infrared camera. The measured and calculated center frequency shifts as well as the temperature distribution in filters correlate fairly well.