This paper presents an overview of the thin film resonator technology as it has been developed over the past few years. A brief discussion of the basic elements of wireless systems is described in order to set the stage for a description of the important enabling aspects of what is generally termed the thin film resonator (TFR) technology. Features of TFRs will be described along with specific examples of resonators and filters. The all important packaging problem is described in the context of device performance and manufacturing.

Thin Film Resonators and Filters

This letter reports on a new and unique form of acoustic bulk wave resonator composed of a thin film of ZnO sputtered onto a thin Si membrane supporting structure. The piezoelectric ZnO is used to excite a longitudinal bulk wave which reflects from the free surface of the film and membrane. The structure thus forms an acoustical cavity which exhibits parallel and series electrical resonance responses at the ZnO film electrodes for both even and odd order modes. Fundamental resonant frequencies near 500 MHz have been achieved with parallel resonant Q’s over 9000. The temperature coefficient of resonant frequency was found to be −31 ppm for a Si to ZnO thickness ratio of six.

Acoustic bulk wave composite resonators

Advances in wireless systems have placed increased demands on high performance frequency control devices for operation into the microwave range. With spectrum crowding, high bandwidth requirements, miniaturization, and low cost requirements as a background, the thin film resonator technology has evolved into the mainstream of applications. This technology has been under development for over 40 years in one form or another, but it required significant advances in integrated circuit processing to reach microwave frequencies and practical manufacturing for high-volume applications. This paper will survey the development of the thin film resonator technology and describe the core elements that give rise to resonators and filters for today's high performance wireless applications.

Thin film resonator technology

The thin film resonator technology has been under development for over forty years in one form or another. Although the basic approach is derived from the desire to reach higher frequencies than those readily achieved by thinning bulk crystals, there have always been competing technologies or fundamental material or processing problems that have impeded the development. Finally, a point was reached in the wireless market wherein competing technologies appeared unable to meet the demands of modern wireless applications and thin film approaches began to receive some emphasis. This paper will survey the thin film resonator technology. Every effort will be made to provide an objective analysis of the technology in relation to applications and competing technologies, and point out obstacles and promises, as known, for further technology advancement to high frequencies.

Novel bulk acoustic‐wave high‐Q resonators and acoustically coupled resonator filters have been fabricated and operated at their fundamental half‐wavelength mode in the 200‐500‐MHz frequency range. These structures are fabricated on thin ZnO/silicon diaphragms with dimensions small enough to be incorporated within integrated circuits. Resonator Q’s near 2600 at the fundamental mode have been obtained and strong inter‐resonator acoustic coupling has been achieved yielding filters having insertion loss (untuned) as low as 5.5 dB.

Fundamental‐mode VHF/UHF minature acoustic resonators and filters on silicon

A localised ion-beam milling has been used to increase the fundamental frequency of acoustic-bulk-wave resonators. Monolithic filters in lithium niobate (Y+37°) and lithium tantalate (X) operating, respectively, at 220 and 230 MHz have been realised. The relative bandwidth is equal to 1% and the insertion loss is about 4 dB. An a.t.-cut quartz resonator working in the fundamental mode at 270 MHz with a Q-factor of 12 000 has also been obtained. In this case, the vibrating membrane is 6 μm thick, with a centre portion of diameter 2.5 mm.

M. Berte

Papers

Acoustic Bulk Wave Resonators and Filters Operating in the Fundamental Mode at Frequencies Greater Than 100 MHz

Acoustic-bulk-wave resonators and filters operating in the fundamental mode at frequencies greater than 100 MHz