J. Hernando

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...

Control of MEMS Vibration Modes With Pulsed Digital Oscillators—Part II: Simulation and Experimental Results

This paper extends our previous work on the selective excitation of mechanical vibration modes in MEMS devices using pulsed digital oscillators (PDOs). It begins by presenting extensive simulation results using the set of iterative maps that model the system and showing that it is possible to activate two or three spatial modes (resonances) of the mechanical structure with a PDO. The second part of this paper presents experimental results corroborating the theory and simulation results. It is shown that it is possible to separately excite vibration modes of the device by setting a few parameters of the PDO structure such as the sampling frequency and the sign of the feedback loop.

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.

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.

Pressure dependence of the quality factor of piezoelectrically driven AlN/Si-microcantilevers

In this work, the fabrication process of piezoelectric AlN cantilevers 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 5x10-3 mbar. The quality factor (Q factor) is an important figure of merit to evaluate the performance of micro-resonators. In particular, two different modes were detected and analyzed. The first bending mode detected at 19.5 kHz has a quality factor of 470 at atmospheric pressure which increases continuously to 985 at 1x10-1 mbar. The corresponding resonant frequency shifted from 19.500 kHz at atmospheric pressure to 19.573 kHz at 5 mbar. Below this pressure level, the resonance frequency stays unaffected within the measurement accuracy. The second bending mode detected at 117.264 kHz exhibits a quality factor of about 570 at atmospheric pressure increasing continuously to 1275 at 1x10-1 mbar. In agreement with the other resonant frequency under investigation the corresponding resonant frequency decreased from 117.264 kHz at atmospheric pressure to 117.630 kHz at 5 mbar.

Analysis of the Quality Factor of Piezoelectric-Actuated Micro- Resonators

The behaviour of AlN self-actuated beams for potential applications in the field of resonant sensors is analyzed focusing on the characterization of the quality factor. This study is extended to high-order modes up to 7 MHz. Laser Doppler vibrometry and impedance analysis were the measurement techniques used. For the former, the quality factor (Q factor) is deduced from both, the frequency response and the transient response. The impedance measurement is not possible for all the modes due to the symmetry in the modal shape, but when it can be measured, the Q factor may be deduced either from the characteristic frequencies of the resonance or from the equivalent resonant circuit. All the four methods yielded comparable magnitudes for the Q factor in air. In order to validate the quality of the devices, and for comparison purposes, calculations based on finite element method were utilized, and a good agreement was found with measured data, regarding modal shapes and resonance frequencies of each mode.

Sputtered polycrystalline AlN as a platform for biofunctionalized devices

Here we present a comparison between polycrystalline AlN and (100) silicon as a support for the development of an immunosensor. A covalent approach was followed for the modification of the initially oxidized surfaces. First a layer of epoxy-based silane organic molecules was deposited. Next, protein A was immobilized with the purpose of taking advantage of its ability to properly orient the antigen binding sites of IgG antibody molecules. Finally the antibodyantigen reaction was accomplished using rabbit IgG and a corresponding antigen, such as anti-rabbit goat IgG. The antirabbit goat IgG was labelled with HRP. This allowed us to quantify the quantity of immobilized antigen. Our results demonstrate the reliability of polycrystalline AlN as a platform for immunosensing, with results comparable to those of silicon.