A. Ababneh

Modal optimization and filtering in piezoelectric microplate resonators

A systematic design procedure to tailor the modal response of micro-resonators based on flexible plates with piezoelectric films is demonstrated. Sensors/actuators were designed by optimizing the surface electrode shapes in the plane dimensions. A numerical finite element procedure, which considers the effective surface electrode covering the piezoelectric film as a binary function on each element, has been implemented. Two design goals are considered: (i) optimized response (actuation or sensing) in a given mode; (ii) implementation of a modal transducer by filtering specific modes. For a given mode in a plate with arbitrary boundary conditions, our calculations allowed us to predict the top electrode layout reaching higher displacement in resonance than any other electrode design for the same structure. Microcantilevers and microbridges were fabricated and their modal response characterized by laser Doppler vibrometry. In comparison to a conventional square-shaped electrode, our experiments show that the implemented designs can increase the response in any desired resonant mode and simultaneously attenuate the contributions from other unwanted modes, by simply shaping the surface electrodes. Enhancement ratios as high as 42 dB, relative to a full-size electrode case, are demonstrated. The limitations imposed by the fabrication are also discussed.

Simulation and laser vibrometry characterization of piezoelectric AlN thin films

In this paper, the electric field induced deformations of sputter-deposited piezoelectric aluminum nitride thin films sandwiched between electrodes on top of a silicon substrate are studied by numerical calculations and scanning laser interferometric measurements. In our calculations based on the finite element method, the results show the displacement of the top and bottom surfaces of both the thin film and the substrate, for either a free or a perfectly clamped structure. The confirmation that the bottom surface of the film is deformed reveals the limitations of techniques that only access the top surface, as well as the double-beam interferometric configuration, under specific conditions. In addition, the simulations demonstrate the dependence of the displacements on the size of the upper electrode and the contribution of the transverse piezoelectric coefficient d31 to the features of the displacement profiles. A laser scanning vibrometry technique was used to measure deformations on the top surface wi...

Characterization and simulation of the first extensional mode of rectangular micro-plates in liquid media

This Letter reports on the characterization of the first extensional mode of AlN-actuated mid-point supported resonant microplates in liquid media. Devices of different dimensions were fabricated and both optical and electrical measurements were performed in order to identify the modal shape under study and determine its quality factor. The dependence of the quality factor on the plate dimensions is discussed based on analytical and finite element simulation results. A quality factor of 100 was achieved in water at 3.8 MHz, and the suitability of this kind of device to work under high viscous condition (up to 51 cP) was demonstrated.

Evaluation of resonating Si cantilevers sputter-deposited with AlN piezoelectric thin films for mass sensing applications

We report on a micro-machined resonator for mass sensing applications which is based on a silicon cantilever excited with a sputter-deposited piezoelectric aluminium nitride (AlN) thin film actuator. An inductively coupled plasma (ICP) cryogenic dry etching process was applied for the micro-machining of the silicon substrate. A shift in resonance frequency was observed, which was proportional to a mass deposited in an e-beam evaporation process on top. We had a mass sensing limit of 5.2 ng. The measurements from the cantilevers of the two arrays revealed a quality factor of 155–298 and a mass sensitivity of 120.34 ng Hz−1 for the first array, and a quality factor of 130–137 and a mass sensitivity of 104.38 ng Hz−1 for the second array. Furthermore, we managed to fabricate silicon cantilevers, which can be improved for the detection in the picogram range due to a reduction of the geometrical dimensions.

Quality-factor amplification in piezoelectric MEMS resonators applying an all-electrical feedback loop

An all-electrical velocity feedback control to enhance the quality factor of piezoelectric aluminium nitride (AlN)-based microcantilevers and microbridges was implemented. Two alternatives to obtain a velocity-proportional signal were demonstrated depending on the top electrode configuration. For a straightforward electrode design in one-port configuration (i.e. self-actuation and self-sensing), a velocity signal, proportional to the piezoelectric current, was used in the feedback loop by cancelling out the dielectric current electronically. For top electrodes allowing a two-port configuration (i.e. one for actuation and one for sensing), the piezoelectric current is directly extracted and its relationship with velocity is analysed taking the symmetry of the modal shape into account. Standard operational amplifier-based configurations for the feedback circuits were implemented on a printed circuit board. Quality factors were determined from the transient electrical response of the devices. Comparable results were obtained from the displacement spectrum applying a laser Doppler vibrometer. Quality factors as high as 2 × 105, corresponding to an enhancement factor of about 200, were achieved in air for the lowest gain margin achievable before the circuit becomes unstable, making this kind of device more competitive for mass sensor applications due to enhanced spectral resolution.

Design and characterization of AlN-based in-plane microplate resonators

In this paper, a design procedure to perform an efficient actuation of in-plane modes in piezoelectric resonators is presented. This procedure is applied to different microplate structures, paying attention to two in-plane mode families: contour modes and flexure-actuated modes. A representative set of devices from both families were used as illustrative examples. These devices were characterized electrically by measuring the impedance and their in-plane modal shapes were measured with a novel technique based on speckle-pattern interferometry. Figures of merit such as the quality factor or the motional resistance were obtained and used to evaluate the different design approaches.

Piezoelectric micro-scale tuning fork resonators for sensing applications

MEMS resonators have stimulated an increasing interest within the last years, and many applications of this technology in fields such as telecommunications, inertial sensors, as well as biosensors, have been demonstrated. The achievement of a high quality factor (Q factor) for these devices is a key issue to maximize their performance. Therefore, there is a continuous search for the structure and the corresponding mode of vibration that enhances the Q factor [1]. Among the wide range of possibilities, the combination of tuning fork-shaped structures and the first anti-phase in-plane mode of such structures has already demonstrated its potential in the macroscopic scale for the fabrication of oscillators [2]. So, here, we pay attention to piezoelectric-actuated tuning forks, but in the micro-scale. The obtained results are very promising, achieving a Q factor in air as high as 4344 for the first anti-phase in-plane mode.

Characterization and simulation of high-quality AlN-actuated resonant suspended beams

Micro-cantilevers and micro-bridges actuated by sputter-deposited aluminium nitride (AlN) thin films were measured with a scanning laser Doppler vibrometer up to 6 MHz, covering more than 10 resonance modes of different nature. A finite element model (FEM) was used to simulate the modal response of the micromachined structures. The comparison between experiment and simulation, regarding modal shapes and frequencies, resulted in an excellent agreement, what confirmed the quality of the structures. Finally, we point out, and illustrate with the help of micro-bridges, the importance for a locally tailored distribution of electrical excitation on the top surface of the device, in order to either optimize or cancel out the displacement of a given mode.

Investigations on the high temperature compatibility of various adhesion layers for platinum

In this paper we report on the high temperature compatibility of various adhesion layers for plat inum (Pt ) thin films. We investigated different adhesion layers, such as titanium (Ti), tantalum (Ta), aluminium nitride (AlN), aluminium oxide (Al2O3) and titanium oxide (TiO2). All films were deposited on SiO2/Si substrate by using the sputter technique. After deposition the films were annealed in air at 800°C for different time lengths up to 16 h ours. After annealing, Al2O3 and TiO2 showed a dense oxide layer between Pt and SiO2/Si and they seem to be suitable as adhesion layers for Pt at high temperatures. AlN is not suitable as adhesion layer for Pt at high temperatures. Ti and Ta are also not suitable for high temperatures, diffusing strongly into Pt layers and leading to the format ion of oxide precipitates (TiOx or TaOx) in the Pt grain boundaries. In addition, the format ion of Pt-crystallites (hillocks) on the surface was common in all the films.

Piezoelectric modal sensors/actuators based on microplates applying surface electrode patterning

We demonstrate an advanced design procedure to fabricate resonators based on flexible plates with piezoelectric films. Two design strategies are considered: i) optimized response (actuation or sensing) in a given mode; ii) implementation of a modal transducer by filtering specific modes. Sensors/actuators were designed by optimizing the surface electrode shape in both dimensions. A numerical finite element procedure, which considers the effective surface electrode covering the piezoelectric film as a binary function on each element, has been developed. Microcantilevers and microbridges were fabricated and their modal response characterized by laser Doppler vibrometry. Our calculations allowed us to predict, for a given mode in a plate with arbitrary boundary condition, the top electrode layout with higher displacement in resonance than any other electrode design for the same structure. Our experiments show that the implemented designs can suppress the contributions of different modes simply by shaping the surface electrodes.