Thin-film Resonators Research Articles

Propagation characteristics of the fundamental symmetric Lamb wave in thin aluminum nitride membranes with infinite gratings

A theoretical approach for the analysis of Lamb waves in grating configurations based on the Floquet-Bloch theorem is developed. Subsequently, it is applied for the prediction of the propagation characteristics of the fundamental symmetric Lamb wave (S0) propagating in thin c-oriented aluminum nitride (AlN) membranes with different types of aluminum grating configurations. The calculations indicate the existence of a frequency stop band exhibiting a complex behavior. The proposed analysis is to be used for the design and optimization of high velocity thin-film Lamb wave resonators. It can easily be adapted for the analysis of arbitrary Lamb waves in periodic gratings.

Effects of corner frequency on bandwidth and resonance amplitude in designing PZT thin-film actuators

In the last decade, lead zirconate titanate oxide (PZT) thin-film actuators have received increasing attention because of their high frequency bandwidth, large actuation strength, fast response, and small size. The PZT film thickness is usually less than several microns as opposed to hundreds of microns for bulk PZT patches that are commercially available. As a result, PZT thin-film actuators pose unique vibration issues that do not appear in actuators with bulk PZT. Two major issues affecting actuator performance are the frequency bandwidth and the resonance amplitude. As an electromechanical device, a PZT thin-film actuator's bandwidth and resonance amplitude depend not only on the lowest natural frequency ω n of the actuator's mechanical structure but also on the corner frequency ω c of the actuator's RC-circuit. For PZT thin-film actuators, the small film thickness implies large film capacitance C and small ω c. When the size of the actuator decreases, ω n increases dramatically. As a result, improper design of PZT thin-film actuators could lead to ω c ≪ ω n substantially reducing the actuator bandwidth and the resonance amplitude. This paper is to demonstrate this phenomenon through theoretical analyses and calibrated experiments. In the theoretical analyses, frequency response functions of a PZT thin-film actuator are obtained to predict 3 dB actuator bandwidth and resonance amplitude for cases when ω c ≪ ω n, ω c ≈ ω n and ω c ≫ ω n. In the experiments, frequency response functions of a fixed–fixed silicon beam with a 1 μm thick PZT film are measured through use of a laser Doppler vibrometer and a spectrum analyzer. The silicon beam has multiple electrodes with a wide range of resistance R and corner frequency ω c. The experimental results confirm that the actuator bandwidth and resonance amplitude are substantially reduced when ω c ≪ ω n.

Bulk acoustic wave resonator operating at 8 GHz for gravimetric sensing of organic films

The mass sensitivity of an AlN thin film resonators operated in air at 6–8 GHz has been investigated in theory and experiment. The 30 × 30 μm wide resonators included a 180–300 nm thick, highly oriented AlN thin film, bottom and top electrode, and an acoustic reflector made of AlN/SiO 2 multilayers. The top electrode was loaded by defined polymer layers to test sensitivity and simulation model. The impact of type and thickness of the top electrode was simulated. Ten to twenty nanometers thick PMMA coatings served to demonstrate sensitivity with a gettering gel in form of spontaneous adsorption of acetone from the vapour phase. Finally, a self-assembled monolayer of 11-mercaptoundecanoic acid was formed on a Pt top electrode for the demonstration of a sub-monolayer sensor in air. In both cases, pico-grams could be detected with a sensitivity up to 1000 m 2/kg.

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 today's high performance wireless applications.

Lateral-field-excited thin-film Lamb wave resonator

The basic principles and technology for the development of lateral-field-excited Lamb acoustic wave resonators on sputter-deposited c-oriented thin aluminum nitride films are presented. The experimental results demonstrate that Lamb waves can be successfully used as an alternative to high-velocity surface acoustic waves.

Analysis of thickness-extensional waves propagating in the lateral direction of solidly mounted piezoelectric thin film resonators

In a solidly mounted piezoelectric thin film resonator (SMR), acoustic waves propagate not only in the thickness direction but also in the lateral direction. In this study, we analyzed the acoustic wave in the SMR-type piezoelectric thin film resonator and derived the dispersion relation between the lateral wave number and frequency, considering wave propagation in the lateral direction. The lateral wave number was shown to be a complex number due to the leak of the acoustic energy to a substrate. It also was shown that the Q factor could be calculated from the complex cutoff frequency, and it becomes higher when the number of quarter-wave (lambda/4) layers increases. Using the dispersion relation, the trapped-energy resonant modes of an SMR were analyzed, considering the boundary conditions at the edge of the electrode.

Optical characterization of a homogenous photosensitive layer by thin film resonance method

Polypeptide material layers can be used for different optical applications as diffractive optical elements or diffractive storage memory. The optical parameters of the polypeptide layers must be precisely known before implementing these applications. These optical parameters are the layer thickness, layer refractive index, and layer substrate refractive index. Thin film resonance (TFR) is one of the most adapted methods to evaluate the refractive index and the thickness of a homogenous photosensitive polypeptide layer. We restrict our method to transparent films layered on a transparent substrate. The proposed method is more convenient for layers having thicknesses greater than a few microns. The photosensitive material we consider is a polypeptide layer with thickness greater than 10 µm. The measurement of these optical parameters are provided separately. A low absorption of this layer is considered. We validate the possibility of easily measuring these parameters. The excellent agreement between the calculated and measured data confirms the practical efficiency of this method. This method is applied as an example to two standard holographic emulsions PFG-04 and PFG-03M for validation. After these tests, the method is applied to characterize the coated polypeptide layer elaborated in our laboratory. We analyze the measurements performed with our proposed method on four polypeptide layers prepared with different added quantities of doping agents. The main interest of our method is to allow the measurement of the layer and substrate refractive indexes (even when they are close) jointly with the layer thickness measurement just by using a simple setup based on monochromatic light as a source and an angular scanning.

Effect of AlN Film Thickness and Top Electrode Materials on Characteristics of Thin-Film Bulk Acoustic-Wave Resonator Devices

Highly c-axis-oriented aluminum nitride (AlN) thin films were deposited on Mo electrodes by reactive RF magnetron sputtering and their thickness effects on thin-film bulk acoustic-wave resonator (FBAR) characteristics are presented. The results obtained in this study show that thick AlN films have a strong c-axis orientation and tend to promote a small rocking curve full-width at half-maximum (FWHM), but a high grain size and a high surface roughness, hence degrading the characteristics of FBARs. The characteristics of FBARs depend not only the thickness and quality of AlN films, but also the thickness of the top electrode and the type of material used. Specifically, an increase in either AlN film or top electrode thickness increases parallel resonant quality factor (QP), but reduces effective electromechanical coupling coefficient (keff2). However, the resonance provides a maximum keff2 when AlN film thickness approaches the fundamental one-half-wavelength. The effects of the type of top-electrode material and AlN film thickness on the keff2 and quality factor of the fabricated FBARs are also discussed.

The effective electromechanical coupling coefficient of piezoelectric thin-film resonators

The input electric impedance Zin for a three-layer thin-film bulk acoustic wave resonator is derived by transfer matrix method to describe the resonator behavior. Based on the impedance spectra, the effective coupling coefficient keff2 of a thin-film resonator can be evaluated with respect to the resonator structure and thin-film properties. The results for both AlN and lead zirconate titanate thin-film resonators reveal that the mechanical Q factor of the thin-film piezoelectric material has a significant effect on the effective coupling coefficient keff2 of the device. keff2 decreases with theincrease of the mechanical quality factor Q. A maximum keff2 value can be obtained at an appropriate thickness ratio of electrode/piezoelectric layers.

Electromagnetic Modeling of Thin-Film Bulk Acoustic Resonators

This paper introduces a novel technique for the electromagnetic analysis of thin-film bulk acoustic-wave resonators. The piezoelectric/acoustic linear equations are coupled to and solved together with Maxwell's equations. For the acoustic propagation, locally monodimensional behavior is assumed, while full-wave three-dimensional Maxwell's equations are rigorously solved for the complete circuit. The proposed approach is validated for single resonators by comparison with experimental data, providing very good agreement with reduced computational resources. Hence, a complete Agilent Technologies' nine-resonator passband filter is simulated in order to demonstrate a typical application to a real-world complex problem.

Interaction of magnetic-dipolar modes with microwave-cavity electromagnetic fields

We discuss the problem of magnetic-dipolar oscillations combined with microwave resonators. The energy density of magnetic-dipolar or magnetostatic (MS) oscillations in ferrite resonators is not the electromagnetic-wave density of the energy and not the exchange energy density as well. This fact reveals very special behaviors of the geometrical effects. Compared to other geometries, thin-film ferrite disk resonators exhibit very unique interactions of MS oscillations with the cavity electromagnetic fields. MS modes in a flat ferrite disk are characterized by a complete discrete spectrum of energy levels. The staircase demagnetization energy in thin-film ferrite disks may appear as noticeable resonant absorption of electromagnetic radiation. Our experiments show how the environment may cause decoherence for magnetic oscillations. Another noticeable fact is experimental evidence for eigen-electric-moment oscillations in a ferrite disk resonator.

Modeling and design of composite free–free beam piezoelectric resonators

A model is presented and experimentally validated for the design of asymmetrically layered piezoelectric free–free beam micromechanical resonators. The thin film resonators are composed of quad-symmetric torsional anchors connected to the nodal points of a free–free beam vibrating in its fundamental bending mode. The model provides analytical approximations for the resonance frequencies of the composite anchors and the free–free beam. Since dissipation in these devices is minimized when the mechanical impedances of the coupled structures are matched at resonance, i.e. when bending beam and torsional anchor resonance frequencies are equal, the model may be used to design resonators with optimized mechanical quality factors. The model is shown to successfully predict resonance frequencies in agreement with the results of finite element analysis and experimental results obtained for multi-layered lead zirconate titanate (PZT) piezoelectric microresonators with first natural frequencies in the range of 448 kHz to 1.1 MHz. The model is also validated as a tool for optimizing resonator quality factor.

Performance Characterization of Thin AlN Films deposited on Mo Electrode for Thin-Film Bulk Acoustic-Wave Resonators

In this paper, we discuss the implementation and testing of a film bulk acoustic-wave resonator (FBAR) comprising an AlN piezoelectric thin film sandwiched between two metal electrodes and located on a silicon substrate with a low-stress silicon nitride (Si3N4) support membrane. In the present study, we choose molybdenum as the bottom electrode rather than the conventionally used Pt/Ti, Au/Cr or Al because it has a low acoustic attenuation and a good electrical conductivity, and provides a good adhesion between the thin AlN film and the low-stress Si3N4 membrane. We investigate the influence of AlN film thickness and top-electrode thickness on the resonance frequency of the FBAR device. The results indicate that decreasing the thickness of either the AlN film or the top electrode increases the resonance frequency. This suggests the potential of tuning the performance of the FBAR device by the carefully controlling AlN film thickness. Furthermore, it is also determined that the resonance frequency of the device can be increased by specifying a higher RF power for the magnetron sputtering process used to deposit the AlN film. This study employs an X-ray diffraction (XRD) technique to investigate the influence of AlN film thickness and RF power on the c-axis orientation of the thin film. It is shown that increasing either the thickness of the AlN film or the power of the RF sputtering process improves c-axis orientation. However, scanning electron microscopy (SEM) and atomic force microscopy (AFM) results reveal that a thicker AlN film exhibits a rougher surface and a larger grain size, both of which decrease the coupling factor of the FBAR device.

High-frequency susceptibility and ferromagnetic resonance in multidomain thin films of the yttrium-iron garnet type

The field dependence of the high-frequency susceptibility and the ferromagnetic resonance were experimentally studied in a thin (d≈0.1 µm) (111)-oriented single-crystal film of substituted yttrium-iron garnet with the factor q≪1. It was shown that the anomaly in the high-frequency susceptibility observed in a magnetic field H parallel to the normal to the film surface in the magnetization saturation region (H≈Hs) has a dual nature; more specifically, this anomaly is associated with an abrupt collapse of the stripe domain structure and a ferromagnetic resonance in the experimental configuration H ∥ [111] and h ⊥ H. In this case, the film transition from the inhomogeneous multidomain state to the homogeneous (single-domain) state at the point H≈Hs has no indications of a second-order phase transition. The experimental frequency-field dependence of ferromagnetic resonance (FMR) in the sample under study, having a characteristic minimum at the point ω0=5 MHz and HFMR=Hs, agrees qualitatively and quantitatively with calculations. The influence of the cubic magnetic anisotropy and the film thickness on the FMR spectrum and the orientation of the spontaneous magnetization in domains with respect to the film plane in the zero field H was theoretically studied.

The application of the acoustic spectrophonometer to biomolecular spectrometry: a step towards acoustic "fingerprinting".

A tunable acoustic biosensor for investigating the properties of biomolecules at the solid-liquid interfaces is described. In its current, format the device can be tuned to frequencies between 6.5 MHz and 1.1 GHz in order to provide a unique detection feature: a variable evanescent wave thickness at the sensor surface. The key to its successful implementation required the careful selection of antennae designs that could induce shear acoustic waves at the solid-liquid interface. This non-contact format makes it possible to recover resonant shear acoustic waves over 100 different harmonic frequencies as a result of the electrical characteristics of the spiral coil. For testing this multifrequency sensing concept the surface of a quartz disc was exposed to solutions of immunoglobulin G (IgG) to form an adsorbed monolayer, whence protein A and IgG were added again in order to form multilayers. Spectra at frequencies between 6 and 600 MHz were generated for each successive layer and revealed two characteristic phases: an initial phase at the low megahertz frequencies consistent with the conventional Sauerbrey relation, and a possible additional phase towards the high megahertz to gigahertz frequencies, that we believe relates to the structure of the biomolecular film. This two-phase behaviour evident from differences between high and low frequencies, rather than from any distinct frequency transition, was anticipated from the reduction in evanescent wave thickness down to nanometre dimensions, and thin film resonance phenomena that are known to occur for film and fluid systems. These measurements suggested that the single element acoustic biosensor we present here may form the basis from which to generate acoustic molecular spectra, or "acoustic fingerprints", in a manner akin to optical spectroscopy.

Energy eigenstates for magnetostatic oscillations in thin-film ferrrite disks

Abstract The confined effects for magnetostatic (MS) oscillations in normally magnetized thin-film ferrite-disk resonators demonstrate unique properties. The atomic-like character of the experimental MS spectra in such particles shows that the energy of a source of a DC bias magnetic field is absorbing “by portions”. In this paper we give the energy spectrum calculations for MS oscillations in a ferrite disk. The oscillating system can be described as a collective motion of quasi-particles—the “light magnons” (lm). Discrete energy levels of “lm” are shown. Effective masses of “lm” are analyzed.

Estimating materials parameters in thin-film BAW resonators using measured dispersion curves.

The dispersion curves of Lamb-wave modes propagating along a multilayer structure are important for the operation of thin-film bulk acoustic wave (BAW) devices. For instance, the behavior of the side resonances that may contaminate the electrical response of a thin-film BAW resonator depends on the dispersion relation of the layer stack. Because the dispersion behavior depends on the materials parameters (and thicknesses) of the layers in the structure, measurement of the dispersion curves provides a tool for determining the materials parameters of thin films. We have determined the dispersion curves for a multilayer structure through measuring the mechanical displacement profiles over the top electrode of a thin-film BAW resonator at several frequencies using a homodyne Michelson laser interferometer. The layer thicknesses are obtained using scanning electron microscope (SEM) measurements. In the numerical computation of the dispersion curves, the piezoelectricity and full anisotropy of the materials are taken into account. The materials parameters of the piezoelectric layer are determined through fitting the measured and computed dispersion curves.

Fabrication of suspended thin film resonator for application of RF bandpass filter

Characteristics of AlN thin film and thin film resonator for RF bandpass filter have been studied. AlN thin films were deposited by RF magnetron sputter system. Deposition parameters such as N 2 contents, Ar and N 2 partial pressures, and the distance between metal target and substrate were found to affect the piezoelectric response. To fabricate the suspended thin film resonator (STFR) using the piezoelectric AlN thin film, the etching of AlN and the surface micromachining process were conducted. The thickness of AlN film and membrane for the STFR are 2 and 15 μm, respectively. This membrane was fabricated by SOI technology. The device with the dimension of 160 × 160 μm 2 has a resonant frequency of 1.653 GHz, a K eff 2 of 2.4%, a bandwidth of 17 MHz, and a quality factor of 91.7. The device with the dimension of 200 × 200 μm 2 has a resonant frequency of 1.641 GHz, a K eff 2 of 1.2%, and a bandwidth of 9 MHz, and a quality factor of 50.2.

A review of thin-film resonator technology

Thin-film resonator technology has been under development for over 40 years as a means to reach higher frequencies than obtainable with conventional quartz-crystal technology. Using advances in microelectronic processing, thin films of piezoelectric materials are used to fabricate resonators and filters over a range of 500 MHz to 20 GHz. This article is a review of the thin-film resonator (TFR) technology that describes the core structures and issues and gives examples of filters and resonators that have been manufactured or demonstrated.

A novel ultra-compact resonator for superconducting thin-film filters

This paper proposes a novel thin-film resonator structure, which combines the microstrip resonator and the coplanar resonator to form an integrated resonator. This resonator structure has an extremely compact size, as compared to the thin-film resonator structures from the literature, and its resonant frequency was shown theoretically to be less sensitive to, or even insensitive to, the thickness of the substrate. An eight-pole quasi-elliptic filter based on this novel resonator was designed. The exact filter layout was simulated and optimized by full-wave electromagnetic simulation using IE3D software. The full-wave simulated filter response was in good agreement with the theoretical filter response. A filter was fabricated on a double-sided YBa/sub 2/Cu/sub 3/O/sub 7/ thin film epitaxially grown on a 2-in-diameter MgO wafer. The measured filter response showed a bandwidth of 1.5 MHz and a center frequency of 850.3 MHz at 78 K. The insertion loss at the passband center was 1 dB, corresponding to a filter Q of 28 000. Steep rejection slopes were obtained at the band edges and rejections reached over 70 dB in approximately 300 kHz from the passband edges. No pronounced changes were observed for input power levels between -20-0 dBm, indicating a relatively high power-handling capability of the filter.