Resonant microcantilevers are widely spread for different applications, such as atomic force microscopy or biosensor
devices. The use of piezoelectric films on top of the cantilevers is a very promising technology due to the possibility of
implementing an all electrical actuation/detection scheme. Among the different piezoelectric materials, polycrystalline
aluminium nitride (AlN) is a very attractive material as, in contrast to typical alternatives such as lead zirconate titanate
or zinc oxide, it is fully compatible with standard silicon technology, what facilitates its integration in silicon-based
In this study, our objective is to use sputter-deposited polycrystalline AlN as the actuation material for cantilever-based
biosensors and to study the performance of high frequency modes for bio-detection in liquid media. Our results
demonstrate a limit of mass detection of 0.07 ng for a 300x200 μm<sup>2</sup> cantilever in liquid media, which represents a value
below the mass of a monolayer of IgG on a 300x200 μm<sup>2</sup> area.
Surface micromachining requires the use of easily-removable sacrificial layers fully compatible with all the materials
and technological processes involved. Silicon dioxide films, thermally grown on silicon substrates or deposited by CVD,
are commonly used as sacrificial layers in surface micromachining technologies, despite their low lateral etch rate in
conventional fluorinate solutions. The development of silicon oxide layers with high etch rates poses a great
technological challenge. In this work we have investigated the possibility of obtaining easily removable silicon oxide
layers by pulsed-DC magnetron reactive sputtering. We have carried out a comprehensive study of the influence of the
deposition parameters (total pressure and gas composition) on the composition, residual stress and lateral etch rate in
fluorine wet solutions of the films. This study has allowed to determine the sputtering conditions to deposit, at very high
rates (up to 0.1 μm/min), silicon oxide films with excellent characteristics for their use as sacrificial layers. Films with
roughness around 5 nm rms, residual stress below 100 MPa and very high etch rate (up to 5 μm/min in the lateral
directions), around 70 times greater than for thermal silicon oxide, have been achieved. The structural characteristics of
these easily removable silicon oxide layers have been assessed by infrared spectroscopy and atomic force microscopy,
which have revealed that the films exhibit some kind of porous structure, related to very specific sputter conditions.
Finally, the viability of these films has been demonstrated by using them as sacrificial layer in the fabrication process of
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
The electromechanical response of piezoelectrically-actuated AlN micromachined bridge resonators has been characterized using laser interferometry and electrical admittance measurements. We compare the response of microbridges with different dimensions and buckling (induced by the initial residual stress of the layers). The resonance frequencies are in good agreement with numerical simulations of the electromechanical behavior of the structures. We
show that it is possible to perform a rough tuning of the resonance frequencies by allowing a determined amount of built-in stress in the microbridge during its fabrication. Once the resonator is made, a DC bias added to the AC excitation signal allows to fine-tune the frequency. Our microbridges yield a tuning factor of around 88 Hz/V for a 500 μm-long microbridge.
Aluminum nitride is lately being considered as a promising candidate for its use as the actuator in piezoelectrically-actuated MEMS due to its good piezoelectric and mechanical properties, high chemical stability and full compatibility with conventional silicon technologies. In this work we present the mechanical response of doubly-clamped microbridges with piezoelectric actuation by a sputtered AlN film. A complete technology for the fabrication of the microbridges on silicon substrates using surface micromachining has been developed. The mechanical response of the microbridges under electrical excitation has been measured. Finite element method (FEM) computations have been carried out in order to analyze the static and dynamic response of devices with several configurations and to determine their optimum design. These simulations include the influence of the properties of the materials, the initial residual stress and the different geometries on the device operation. Although the qualitative behavior of the microbridges is well predicted, a significant discrepancy is observed between the measured displacement of the beam and the simulated response. The measured values of the out-of-plane displacement of the bridge are near ten times greater than those obtained in the simulations. Some of the possible causes of these discrepancies are widely discussed.
In this article we present a study of deposition and etching techniques of germanium (Ge) and amorphous oxygen germanium (GeO<sub>x</sub>) films, with the aim of using them as sacrificial layer in the fabrication of AlN-based MEMS by surface micromachining processes. The Ge and GeO<sub>x</sub> layers were deposited by RF magnetron sputtering in Ar and Ar/O<sub>2</sub> atmospheres. By controlling the process parameters we were able to set the final composition of the GeO<sub>x</sub> films, which was assessed by FTIR measurements. We have studied the etch rates of GeO<sub>x</sub> films with x ranging from 0 to 1 in H<sub>2</sub>O<sub>2</sub> and H<sub>2</sub>O<sub>2</sub>/acid solutions. Depending on the etching temperature and the oxygen content in the layers, etch rates ranging from 0.2 to 2 μm/min were obtained. Nearly stoichiometric germanium oxide (GeO2) was etched in pure H<sub>2</sub>O at very high rate (>1 μm/min at room temperature). We have also developed a chemomechanical polishing (CMP) process for the planarization of Ge and GeO<sub>x</sub>. The influence of the slurries containing diverse powders (CeO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>) and chemical agents (NH<sub>4</sub>OH, HCl), the different pads, and the various process parameters on the removal rate and the final sample topography has been studied. Finally, we have analysed the compatibility of the materials involved in the process flow with the processes of planarization and removal of the sacrificial layers.