AlN Resonators Research Articles

Nanomechanical Crystalline AlN Resonators with High Quality Factors for Quantum Optoelectromechanics.

High-quality factor (Qm) mechanical resonators are crucial for applications where low noise and long coherence time are required, as mirror suspensions, quantum cavity optomechanical devices, or nanomechanical sensors. Tensile strain in the material enables the use of dissipation dilution and strain engineering techniques, which increase the mechanical quality factor. These techniques have been employed for high-Qm mechanical resonators made from amorphous materials and, recently, from crystalline materials such as InGaP, SiC, and Si. A strained crystalline film exhibiting substantial piezoelectricity expands the capability of high-Qm nanomechanical resonators to directly utilize electronic degrees of freedom. In this work, nanomechanical resonators with Qm up to 2.9 × 107 made from tensile-strained 290 nm-thick AlN are realized. AlN is an epitaxially-grown crystalline material offering strong piezoelectricity. Nanomechanical resonators that exploit dissipation dilution and strain engineering to reach a Qm × fm-product approaching 1013 Hz at room temperature are demonstrated. A novel resonator geometry is realized, triangline, whose shape follows the Al-N bonds and offers a central pad patterned with a photonic crystal. This allows to reach an optical reflectivity above 80% for efficient coupling to out-of-plane light. The presented results pave the way for quantum optoelectromechanical devices at room temperature based on tensile-strainedAlN.

An A0 mode Lamb-wave AlN resonator on SOI substrate with vertically arranged double-electrodes

In this paper, a novel vertically arranged double-electrodes A0 mode Lamb-wave AlN resonator on SOI substrate with a high electromechanical coupling coefficient and high figure of merit (FOM) is reported. The AlN resonator has a sandwich structure with aluminum and N-type doped silicon as electrode layers and a 500 nm thick AlN film as piezoelectric layer. The resonator has only two electrodes vertically arranged rather than horizontal interdigitated (IDT) electrodes which is common in conventional Lamb-wave resonators. The electrode gaps for the vertically arranged double-electrodes resonators are defined by AlN layer thickness rather than by photolithography for lateral field excitation resonators, which results in higher electric field strength and higher electromechanical coupling efficient (). Compared with conventional thickness field excitation (TFE) resonators with floating bottom electrodes, the vertically arranged double-electrodes resonators have higher electric field strength as the potential difference is larger between the top electrode and bottom electrode than that between the IDT electrodes and floating electrode. As a result, a higher electromechanical coupling coefficient is achieved. Furthermore, the resonant frequency of the vertically arranged double-electrodes resonator presented in this work can be defined by photolithography by controlling the width of the silicon layer. The of the vertically arranged double-electrodes resonator calculated from the measurement results of admittance versus frequency by numerically fitting with the Butterworth Van Dyke model shows an increase by 3.85 times, from 0.073% to 0.281% compared with conventional TFE resonators, and the FOM also increases by three times, from 2.66 to 7.99. This work provides a new structure to design future AlN Lamb-wave resonators on SOI substrate.

Ion-beam-induced in-plane a-axis oriented (0001) AlN and ScAlN thin film BAW resonators

High-Q RF bulk acoustic wave filters are still required for mobile communication applications. However, the in-plane orientation of commercial AlN films is random, despite their excellent out-of-plane c-axis orientation. We proposed combining two different orientation control techniques: self-texture for out-of-plane alignment and ion beam irradiation for in-plane alignment. To demonstrate the effectiveness of this approach, both in-plane a-axis and out-of-plane c-axis aligned AlN and ScAlN films were grown on Ti/silica glass substrates using 0.3–0.5 keV grazing ion-beam-assisted RF sputtering technique without using epitaxial growth. The clear six symmetry in the pole-figure observed in AlN and ScAlN thin films indicates the single-crystal-like in-plane a-axis orientation. The ion beam direction corresponded to the in-plane 〈101̄0〉 axis. In-plane x-ray diffraction ϕ-scan curve and ω-scan curve FWHM were measured to be 25° and 2.2°.

Al0.68Sc0.32N Lamb wave resonators with electromechanical coupling coefficients near 10.28%

Aluminum scandium nitride (Al1−xScxN/AlScN) (x = 0.32) Lamb wave resonators (LWR) have been fabricated and tested to demonstrate electromechanical coupling coefficients (kt2) in excess of 10%. The resonators exhibited an average kt2 and unloaded quality factor (Qu) of 10.28% and 711, respectively, when calculated from the measured data. Applying the Butterworth Van-Dyke (BVD) model to the measured data enabled the extraction of the resonator's lumped element parameters to calculate the motional quality factor (Qm), which neglects the contributions of the electrical traces. For the best measured resonator response, results from the BVD model showed a Qm of 1184 and a resulting figure-of-merit (FOM = K2 · Qm) of 100. Comparing the response of similar AlScN and AlN resonators shows that the AlScN LWR has a significantly lower motional resistance (Rm), suggesting that AlScN has a strong potential for use in piezoelectric microelectromechanical oscillators.

Determination of etching parameters for pulsed XeF2 etching of silicon using chamber pressure data

A technique is presented for determination of the depletion of the etchant, etched depth, and instantaneous etch rate for Si etching with XeF2 in a pulsed etching system in real time. The only experimental data required is the pressure data collected temporally. Coupling the pressure data with the knowledge of the chemical reactions allows for the determination of the etching parameters of interest. Using this technique, it is revealed that pulsed etching processes are nonlinear, with the initial etch rate being the highest and monotonically decreasing as the etchant is depleted. With the pulsed etching system introduced in this paper, the highest instantaneous etch rate of silicon was recorded to be 19.5 µm min−1 for an initial pressure of 1.2 Torr for XeF2. Additionally, the same data is used to determine the rate constant for the reaction of XeF2 with Si; the reaction is determined to be second order in nature. The effect of varying the exposed surface area of Si as well as the effect that pressure has on the instantaneous etch rate as a function of time is shown applying the same technique. As a proof of concept, an AlN resonator is released using XeF2 pulses to remove a sacrificial poly-Si layer.

The Influence of the Acoustic Reflector Design on the Temperature Coefficient of Frequency for Shear and Longitudinal Mode AlN Resonators

Most of the applications of solidly mounted resonator need the compensation of temperature coefficient of resonant frequency. The presence of materials with different properties in the stack makes it complicated to explain the influence of each material individually on temperature coefficient. To reduce design and fabrication costs, an accurate and precise modeling of the resonators is required. In this paper, Mason model and finite-element analysis are used to design the resonators and results for both models, which are almost similar, are presented. Different kinds of configuration (symmetric and asymmetric, according to the thickness of the layers) are designed to achieve the temperature coefficient of frequency as near-zero as possible with an optimized response. Such designs are fabricated and characterized obtaining measurements of the temperature coefficient of frequency. The result shows that useful and reliable information on the performance of resonators can be achieved with accurate modeling. [2017-0107]

(Invited) AlN and ScAlN Contour Mode Resonators for RF Filters

Piezoelectric acoustic resonators are widely utilized for RF devices but most can only utilize a few different resonant frequencies on the same substrate. Contour mode resonators (CMR) have resonant frequencies defined lithographically offering the advantage of an extensive frequency range on the same film, wafer, or die. This work will discuss our efforts in developing ScAlN for CMRs to achieve RF performance suitable for use in RF filters. First we will discuss our RF sputter deposition process for creating high-performance Sc0.12Al0.88N piezoelectric films and compare it with an established sputtered piezoelectric AlN. Fabrication and integration techniques for fabricating CMRs will be then detailed. Finally, electrical design and resonator performance is then discussed. These early investigations into ScAlN films suggest that performance metrics such as k2 eff can substantially improve while simultaneously ensuring good resonator Q factors over a wide frequency range of devices.

Quantized acoustic-phonon shear horizontal modes in an unbounded hexagonal nitride-based piezoelectric nanostructure

For the first time a quantum mechanics based theoretical description of acoustic-phonon shear horizontal modes in a class of piezoelectric media is proposed. The quantized acoustic modes that are needed in the transition from the microscale to the nanoscale are derived for a resonator with geometries of interest in optoelectronics and nanoelectronics. The acoustic-phonon frequency dispersion relations are obtained quantum mechanically for odd and even symmetry shear horizontal modes. It is shown that the derived dispersion relations are identical to the previously reported dispersion relations obtained classically as is expected. For each symmetry, the phonon-mode amplitude is derived in terms of the energy of the quantized vibrational mode, which is of great importance for modelling carrier-acoustic phonon interactions. Moreover, the product of quality factor and frequency (Q.f) have been estimated for AlN and GaN resonators by using the anharmonic phonon scattering theory. Furthermore, the electrical surface perturbation in the piezoelectric nanoresonator is studied and the resulting resonance frequency shift is determined.

ZnO on nickel RF micromechanical resonators for monolithic wireless communication applications

On-chip integrability of high-Q RF passives alongside CMOS transistors is crucial for the implementation of monolithic radio transceivers. One of the most significant bottlenecks in back-end-of-line (BEoL) integration of MEMS devices on CMOS processed wafers is their relatively low thermal budget, which is less than that required for typical MEMS material deposition processes. This paper investigates electroplated nickel as a structural material for piezoelectrically-transduced resonators to demonstrate ZnO-on-nickel resonators with a CMOS-compatible low temperature process for the first time. Aside from the obvious manufacturing cost benefit, electroplated nickel is a reasonable substitute for polycrystalline or single crystal silicon and thin-film microcrystalline diamond device layers, while realizing decent acoustic velocity and moderate Q. Electroplated nickel has been already adopted by MEMSCAP, a multi-user MEMS process foundry, in its MetalMUMPs process. Furthermore, it is observed that a localized annealing process through Joule heating can be exploited to significantly improve the effective mechanical quality factor for the ZnO-on-nickel resonators, which is still lower than the reported AlN resonators. This work demonstrates ZnO-on-nickel piezoelectrically-actuated MEMS resonators and resonator arrays by using an IC compatible low temperature process. There is room for performance improvement by lowering the acoustic energy losses in the ZnO and nickel layers.

Selection of aptamers to Neisseria meningitidis and Streptococcus pneumoniae surface specific proteins and affinity assay using thin film AlN resonators

The first steps in the development of an aptasensor for bacterial meningitis diagnosis in real time are described: slow off-rate modified aptamers (SOMAmer) selection, gravimetric sensor fabrication and functionalization of its active surface. SOMAmers polyclonal populations were generated to surface specific proteins PavA (S. pneumoniae) and FHbp (N. meningitidis) by systematic evolution of ligands by exponential enrichment method (SELEX). After eight rounds, they were tested by direct enzyme-linked oligonucleotide assays (ELONA) and by high frequency (1.4GHz) gravimetric sensors based on thin film AlN resonators operating in shear mode. The sensing surface of the resonators was functionalized using a silane-glutaraldehyde based protocol and the binding of PavA and FHbp was measured in real time. The SOMAmers polyclonal populations showing the best ELONA results have been also tested using gravimetric sensors and the binding to the protein functionalized surface was measured in real time showing positive results. These first steps towards the development of an aptasensor for the targeted bacteria demonstrate the potential of the method for sensitive, rapid, and cost effective detection of bacterial meningitis.

AlN-based surface acoustic wave resonators on platinum bottom electrodes for high-temperature sensing applications

Surface acoustic wave (SAW) resonators with Pt/AlN/Si and Pt/AlN/Pt/Si configurations were fabricated by lift-off photolithography techniques. High-temperature performances of both configurations were investigated for temperature sensor applications. AlN films grown on Pt-coated Si substrates exhibit a lower (002) preferred orientation than those grown on Si substrates. The center frequencies of Pt/AlN/Si and Pt/AlN/Pt/Si configurations at room temperature are 424.1 and 456.4 MHz, respectively. The SAW was limited by Pt bottom electrodes to propagate in AlN layer. The temperature coefficient of frequency (TCF) values of Pt/AlN/Si and Pt/AlN/Pt/Si configurations are −51.6 × 10−6 and −69.2 × 10−6 °C−1, respectively. Compared with that of Pt/AlN/Si configuration, the TCF value of Pt/AlN/Pt/Si configuration decreases by 34.1 %. AlN resonator with the Pt floating bottom electrodes provides a large, quasi-constant temperature sensitivity which is suitable for temperature sensor applications.

Effects of biologically compatible buffers on the electrical response of gravimetric sensors operating at GHz frequencies

Gravimetric biosensors based on piezoelectric resonators have their active area just atop an electrical contact. Sometimes both contacts are located on the same surface (coplanar electrodes). The use of bio-buffers with high ionic content makes often electrical isolation of the electrodes from the liquid mandatory. This restricts the use of surfaces for functionalization and worsens the response of the resonator. In this paper we demonstrate that, if operating frequencies are high enough, there is little effect of the liquid ionic conductivity on the performance of the resonators with both electrodes in contact with the latter. We test the response of 1.32GHz AlN shear mode resonators in NaCl solutions of different concentrations to show the effects of the ionic conductivity on their performance. The conclusion is that, even the presence of a parallel conductance due to the ionic conductivity into the liquid, the shear mode resonance is not significantly disturbed. To prove the viability of non-isolated AlN resonators as biosensors, we present measurements of thrombin detection using devices with aptamer-functionalized iridium electrode surfaces.

Electrical characterization of micromachined AlN resonators at various back pressures

In this work, the fabrication process of piezoelectric cantilevers based on aluminum nitride films is presented. The cantilevers were electrically characterized in a vacuum chamber offering the possibility to close-loop control the back pressure from atmospheric conditions down to 10−2 mbar. The quality factor is an important figure of merit to evaluate the performance of micro resonators. In particular, three different modes were detected and analyzed: the first flexural mode, the second flexural mode and the first torsional mode. Among these three investigated modes, the first torsional mode has a higher quality factor compared to the others, which makes this mode more suitable for mass change detection applications as higher sensitivities are feasible. With the first torsional mode, quality factors of up to 4,557 are achievable at low chamber pressure, where the intrinsic damping is the dominating loss mechanism. The quality factor decreased to 1,050 at atmospheric pressure and is limited by viscous losses.

Thermal Nonlinearities in Contour Mode AlN Resonators

In this paper, we analyze the origin of elastic nonlinearities in aluminium nitride contour mode resonators (CMRs). Our study highlights that the nonlinear behavior is due to thermal effects when the resonators are electrically excited and the input is slowly (slow with respect to the device thermal time constant) swept through the excitation frequencies close to the main resonance. An analytical expression that relates the nonlinear behavior of the device to its geometry and material properties is derived. Amplitude-frequency (A-f) and third-order intermodulation (IMD3) measurements on 1-GHz AlN CMRs are employed to demonstrate the theoretical reasoning. The two experiments confirm the validity of the analytical derivation when the system is dominated by thermally induced nonlinearities. In the case of large frequency difference between the modulation frequencies, purely elastic nonlinearity can also be extracted from the IMD3 measurements.

Quality Factor of Contour-Mode AlN Resonator

We present contour-mode AlN resonator structure and fabrication process for high quality factor (Q-factor). The vibrational energy should be confined to the resonant part of the radial extensional (RE) resonator in order to obtain a high Q-factor and high coupling factor (k2) from devices made using micro electro mechanical systems (MEMS) technology. In this paper, we discuss how to form a support for the resonant part in a high-precision process. We fabricated simple RE resonators and clarified through experimentation that it is difficult to avoid deterioration of the Q-factor because the RE vibration energy leaks from the resonant part into the silicon substrate through the supporting points. Next, we fabricated tuning-fork RE resonators by using the three-etching-process method and clarified through experimentation that this method is useful for forming the resonant part into the correct circular shape and avoiding energy leakage.

Quality Factor of Contour-Mode AlN Resonator

We present contour-mode AlN resonator structure and fabrication process for high quality factor (Q-factor). The vibrational energy should be confined to the resonant part of the radial extensional (RE) resonator in order to obtain a high Q-factor and high coupling factor (k2) from devices made using micro electro mechanical systems (MEMS) technology. In this paper, we discuss how to form a support for the resonant part in a high-precision process. We fabricated simple RE resonators and clarified through experimentation that it is difficult to avoid deterioration of the Q-factor because the RE vibration energy leaks from the resonant part into the silicon substrate through the supporting points. Next, we fabricated tuning-fork RE resonators by using the three-etching-process method and clarified through experimentation that this method is useful for forming the resonant part into the correct circular shape and avoiding energy leakage.

Viscosity sensor using ZnO and AlN thin film bulk acoustic resonators with tilted polar c-axis orientations

We present a theoretical analysis of liquid viscosity sensors using ZnO and AlN thin film bulk acoustic wave resonators (FBARs) with tilted polar c-axis orientations. Besides the thickness longitudinal mode, the tilted c-axis orientation induces thickness shear mode in the resonator, which allows resonators operated in liquid medium without significant damping for sensory application. The equation for predicting electric impedance of shear mode film bulk acoustic wave resonators (FBARs) with a viscous liquid loading was derived from the basic piezoelectric constitutive equations. The viscosity sensitivity of shear mode ZnO and AlN resonators was achieved by calculation of resonant frequency shift due to viscous liquid loading. In the simulation, it is assumed that all the resonators have 2 μm thickness and 300 μm×300 μm electrode area; three different liquids (water, acetone, and olive oil) were chosen as the liquid loadings; and different tilt c-axis angles for both ZnO FBARs and AlN FBARs have been examined. It was found that the sensitivities of shear mode resonators to the three liquid loading are very close, and do not change much with the c-axis tilt angle with a value rang from 0.91e-3 to 0.97e-3 (kg m−3 Pa·S)−0.5 for ZnO FBARs and from 1.12e−3 to 1.13 e−3 (kg m−3 Pa·S)−0.5 for AlN FBARs. When the resonator’s mechanical quality factor (Q) is changed from 50 to 10 000, viscosity sensitivities are almost same. However, Q has a great effect on resonator impedance; and if Q is too low or the viscosity of the liquid is high, the maximum phase angle of the resonator will be less than 0, which makes excitation of the oscillation difficult if an oscillator circuit is used for sensor measurement. The results can be used for design and application of ZnO or AlN FBARs to monitor liquid viscosity.

Micro‐ and nano‐electromechanical resonators based on SiC and group III‐nitrides for sensor applications

AbstractWide‐bandgap semiconductors represent an attractive option to meet the increasing demands of micro‐ and nano‐electromechanical systems (MEMS/NEMS) by offering new functionalities, high stability, biocompatibility and the potential for miniaturization and integration. Here, we report on resonant MEMS and NEMS devices with functional layers of SiC, AlN and AlGaN/GaN heterostructures on different substrates, which have been investigated and analysed in the course of an interdisciplinary research focus programme of the German Research Foundation (DFG). The specific deposition and etching technologies necessary for the three‐dimensional micro‐structuring are explained. Further, the implementation of appropriate electromechanical transduction schemes is discussed. In case of SiC and AlN resonators, actuation and sensing was achieved by a magnetomotive scheme. A piezoelectric coupling scheme where the counter electrode is formed by the two‐dimensional electron gas at the interface of the III/V heterostructure was realized for the AlGaN/GaN resonators.Thus, flexural and longitudinal vibration modes were excited and characterized using electrical and optical techniques. The measured key parameters of resonant frequency and quality factor are related to geometry, material and environmental parameters using analytical and finite element (FE) models. Finally, potential sensor applications are experimentally investigated.

Modal analysis and harmonic response of resonators: An extension of a mapped orthogonal functions technique

A mapped orthogonal functions technique is extended for solving three-dimensional acoustical wave problems with taking into account electrical sources. For validation and illustration purposes, it is applied to the determination of the frequency spectrum of a piezoelectric disc resonator. Formulation is given for the calculation of the electric input admittance along with specializations for open- and short-circuit boundary conditions. In view of validation, normal frequencies and electric input admittance are obtained for specific geometries where a one-dimensional analytical model is working and compared against results from the analytical approach. Illustrative results, dispersion curves for a PZT5A resonator and electromechanical coupling coefficients as a function of the diameter to thickness ratio for the first radial mode of a respectively PZT5A, AlN and SiC resonator are also given.

Contour-Mode AlN Resonator with High Coupling Factor

A micro electro mechanical systems (MEMS) type contour-mode resonator with a high coupling factor of several percent is proposed. The fabricated 60-MHz-band radial-extensional resonators using MEMS process technology of AlN film exhibited a high coupling factor of 2.8% and high Q values of 3000 at the series resonant frequency and 4000 at the parallel resonant frequency. We demonstrated a Pierce type oscillator using the radial-extensional resonators and achieved low power consumption and stable oscillation. These radial-extensional resonators were suitable for filter and oscillator applications because the figure of merit, defined as the ratio of the Q value and the capacitance ratio, was 71 at the series resonant frequency and 94 at the parallel resonant frequency, which were the highest values of the contour-mode AlN resonators.