K.M. Lakin

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.

Acoustic bulk wave composite resonators

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.

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.

Microwave acoustic materials, devices, and applications

This paper surveys applications of acoustic waves in microwave devices. After a general and historical introduction to bulk acoustic waves (BAWs), surface acoustic waves (SAWs), practical wave types, and acoustoelectric transducers, a review is given of technologically important materials for microwave acoustic applications. Following this, we discuss BAW and SAW microwave devices and their technologies. Specifically reviewed are thin-film resonators and filters, transversal filters, and filters for correlative analog signal processing. Finally, an overview of the most important microwave applications is given, along with manufacturing and packaging issues.

Coupled resonator filters

Coupled Resonator Filters (CRF) are a new form of bulk acoustic wave device that involves the vertical stacking of resonators. In that regard, the CRF can be thought of as a variation on the better known Stacked Crystal Filter (SCF). This paper will review the SCF and expand on the basic concepts of the CRF. Experimental results will be shown for SCFs operating to 12 GHz and CRFs near 3 GHz. Manufacturing issues associated with both filter types will be reviewed with greater emphasis on the CRF.

Solidly mounted resonators and filters

Acoustic resonators require material interfaces that confine waves to a finite volume in an efficient manner. Conventionally this is achieved by using air or vacuum interfaces at the electrodes. Another technique is to fabricate the resonator onto a set of quarter wavelength thick layers attached to a substrate to form a solidly mounted resonator (SMR). The SMR concept has been used to fabricate low insertion loss filters for GPS and other applications. Filters for GPS having less than 3 dB insertion loss and 40 dB out-of-band rejection have been demonstrated. These filters are composed of ladder networks with series and shunt resonators.

Temperature compensated bulk acoustic thin film resonators

Thin film resonators have been made that exhibit a high degree of temperature compensation. These resonators are composed of piezoelectric aluminum nitride films, aluminum top and bottom electrodes, and are compensated with layers of silicon dioxide within the resonator. The resonators are fabricated with the solidly mounted resonator (SMR) configuration using a sequence of aluminum nitride and silicon dioxide reflector layers. Silicon dioxide has a positive temperature coefficient and can be used to offset the -25 ppm per degree C coefficient of aluminum nitride. Results are reported on hermetic packaging, temperature cycle testing, temperature coefficient measurements, and preliminary ageing.

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.

Growth morphology and surface‐acoustic‐wave measurements of AIN films on sapphire

Aluminum nitride epitaxial films of thicknesses greater than 10 μ with excellent mechanical and chemical stability have been grown on R‐plane sapphire substrates. The orientation relationship of single‐crystal AlN on sapphire is discussed. Examination of the as‐grown AlN film surface with a scanning electron microscope identifies the growth morphology and enables differentiation between good and poor quality films. The surface‐acoustic‐wave coupling constant K2 and propagation velocity Vs are measured out to a thickness‐to‐wavelength ratio t/λ=0.75 with typical K2 values of 0.8% and Vs of 6.1 km/sec. Preliminary results also indicate that AlN‐sapphire interfacial strain extends about 1 μ into the AlN film.