G.R. Kline

High-Q microwave acoustic resonators and filters

The authors present recent experimental and modeling results for high-Q microwave acoustic resonators and filters for use in oscillators and other frequency control applications. Overmoded resonators have exhibited an FQ product greater than 1*10/sup 14/ Hz (e.g., Q=68,000 at 1.6 GHz) with a strong inductive response suitable for one-port and two-port oscillator feedback circuits. Ladder filters fabricated with overmoded resonators have loaded Qs greater than 40,000 with 76-kHz bandwidth at 1.6 GHz. Aluminum nitride films were used for transduction on Z-cut sapphire and lithium niobate substrates. >This paper presents recent experimental results and modeling obtained on high Q microwave acoustic resonators and filters for use in oscillators and other frequency control applications. Overmoded resonators have exhibited an FQ greater than 1 x 1014 Hz ( e.g. Q=68000 at 1.6 GHz) with a strong inductive response suitable for one-port and two-port oscillator feedback circuits. Ladder filters fabricated with overmoded resonators have loaded Q’s greater than 40000 with 76 KHz bandwidth at 1.6 GHz. Aluminum nitride films were use for transduction on Z-cut sapphire substrates.

Thin Film Resonators and Filters

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.

Development of miniature filters for wireless applications

Miniature filters have been under development for wireless applications from 500 MHz to over 6 GHz using thin piezoelectric films on common substrates. This paper discusses recent results in the development of miniature filters using a solidly mounted resonator (SMR) concept wherein the acoustic resonator is isolated from the substrate with a sequence of quarter wavelength thick layers that form a reflector. The SMR concept is discussed in detail and applications to filters is presented. Ladder filters have been demonstrated with insertion losses in the 3 dB range using aluminum nitride films for the piezoelectric and appropriate substrates such as silicon, sapphire, and glass. The ladder filters reported consist of interconnected series and shunt resonators forming a monolithic structure on a single die of comparable size to an integrated circuit.

Thin film resonator technology

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 todays high performance wireless applications.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.

Thin film bulk acoustic wave filters for GPS

The design and analysis of ladder filters are discussed, and reports on progress to date in the implementation of these devices for Global Positioning System (GPS) applications are summarized. Results are described for a two-dimensional plate wave analysis that illustrates the effects of resonator size on spurious resonances that degrade filter performance. Ladder filters in the series/shunt configuration are fabricated with 5-12 resonators on a common membrane of AlN supported by a silicon substrate. Preliminary experimental results show that the ladder filters have a clean out-of-band response with some spurious responses near the passband region. The filter insertion loss in the experimental devices requires improvement.<<ETX>>

1.0‐GHz thin‐film bulk acoustic wave resonators on GaAs

This letter reports on a new fabrication technique and experimental results obtained on bulk acoustic wave resonators using thin piezoelectric composite films of A1N on GaAs insulating substrates. The fabrication involves only a wafer top side planar processing compatible with integrated circuit technology. Resonators have been made in the frequency range UHF to 1 GHz in order to demonstrate the fabrication technique and evaluate material performance in resonator devices. Both longitudinal and shear wave resonators have been measured with temperature coefficients of −24 and −26.5 ppm/°C, respectively. Shear wave results were obtained from tilted c‐axis films grown in a dc planar magnetron sputtering system.

Stacked crystal filters implemented with thin films

Low-insertion-loss 1-GHz stacked crystal filters have been implemented with thin films of piezoelectric AlN. Two-pole filters having insertion losses of less than 2 dB have been fabricated on Si and GaAs substrates using microelectronic processing techniques. These devices can serve as high-performance front-end filters for receivers or for other frequency-control applications. The over-moded stacked crystal shows considerable promise as a two-port 50 Omega high-Q feedback element for oscillators or for other narrowband applications.<<ETX>>

Performance of TFR filters under elevated power conditions

Research into the application of thin-film piezoelectric resonator (TFR) technology in novel monolithic bandpass filtering devices is reported. Research into the performance of TFR-based filters under elevated power levels was initiated to assess the suitability of this technology in systems requiring ultraminiature transmit and transceiver functions. Characteristics of particular interest include the 1-dB compression point and the two-tone third-order intercept point. The results of research conducted on silicon-based TFR bandpass filter devices under elevated power conditions are presented.<<ETX>>

Overmoded high Q resonators for microwave oscillators

High-overtone bulk acoustic wave resonators operating in the 1-6 GHz range have been fabricated on sapphire, lithium niobate, dilithium tetraborate, and quartz substrates in an effort to obtain high-Q C-axis normal aluminum nitride films were used for transduction for all the substrates, yielding longitudinal modes. The resonators on Z-cut sapphire exhibited an unloaded fQ product in excess of 1 /spl times/ 10/sup 14/ at frequencies between 1.6 and 4.0 GHz. The resonators on Z-cut sapphire exhibited Qs as high as 68,000 at 1.6 GHz with highly inductive response. Lithium niobate exhibited a Q of 54,000 at 1.6 GHz. When used in combination with a mode-select filter these resonators would make good feedback elements for low phase noise oscillators where inherent high temperature stability is not necessary.<<ETX>>

Thin film microwave acoustic filters on GaAs

The authors report on progress made in ultraminiature resonators and bandpass filters fabricated on GaAs substrates stimulated by significant advances made in the research and development of thin-film resonator (TFR) technology on Si and other substrates and a growing need for high-performance integrated passive frequency control elements on GaAs MMICs (monolithic microwave integrated circuits). It is shown that significant size reductions and performance characteristics can be obtained by using microwave acoustic waves whose wavelength and, hence physical device size, is approximately 100000 times smaller than that of electromagnetic counterparts. Monolithic multisection SCF (stacked crystal filter) devices, exhibiting higher selectivity, have been achieved on GaAs by cascading single SCF sections, without interstage coupling elements, and have demonstrated center frequencies above 1 GHz.<<ETX>>