[en] A hydrogen sensor based on large arrays of nanoswitches in palladium (Pd) is presented. An individual nanoswitch is realized by a suspended Pd/Ti/poly-Si trimorph electrode and a fixed Pd/Ti bottom electrode, which are separated by a vertical nanoscopic gap of approximately 10 nm in size. In hydrogen exposure, the volume expansion of Pd results in mechanical bending of the suspended electrode and the formation of an electric contact. A multitude of nanoswitches is arranged in interconnected arrays. These enable a dependence of the sensor signal on the H₂ concentration by the occurrence of percolation effects. The combined use of thin film, etching and evaporation techniques allows for the large-scale fabrication of nanoswitch arrays in arbitrary topologies by design, such as parallel linear chains or square lattices. The results of hydrogen and temperature measurements are presented and discussed. In hydrogen exposure, reversible changes in the electrical resistance of up to three orders of magnitude are obtained for H₂ concentrations below 4% and a power consumption down to a few picowatts

[en] Optical detection back-action in cantilever resonant or static detection presents a challenge when striving for state-of-the-art performance. The origin and possible routes for minimizing optical back-action have received little attention in literature. Here, we investigate the position and mode dependent optical back-action on cantilever beam resonators. A high power heating laser (100 µ W) is scanned across a silicon nitride cantilever while its effect on the first three resonance modes is detected via a low-power readout laser (1 µ W) positioned at the cantilever tip. We find that the measured effect of back-action is not only dependent on position but also the shape of the resonance mode. Relevant silicon nitride material parameters are extracted by fitting finite element (FE) simulations to the temperature-dependent frequency response of the first three modes. In a second round of simulations, using the extracted parameters, we successfully fit the FEM results with the measured mode and position dependent back-action. From the simulations, we can conclude that the observed frequency tuning is due to temperature induced changes in stress. Effects of changes in material properties and dimensions are negligible. Finally, different routes for minimizing the effect of this optical detection back-action are described, allowing further improvements of cantilever-based sensing in general. (paper)

[en] A quantitative analysis of blurring and its dependence on the stencil-substrate gap and the deposition parameters in stencil lithography, a high resolution shadow mask technique, is presented. The blurring is manifested in two ways: first, the structure directly deposited on the substrate is larger than the stencil aperture due to geometrical factors, and second, a halo of material is formed surrounding the deposited structure, presumably due to surface diffusion. The blurring is studied as a function of the gap using dedicated stencils that allow a controlled variation of the gap. Our results show a linear relationship between the gap and the blurring of the directly deposited structure. In our configuration, with a material source of ∼5 mm and a source-substrate distance of 1 m, we find that a gap size of ∼10 μm enlarges the directly deposited structures by ∼50 nm. The measured halo varies from 0.2 to 3 μm in width depending on the gap, the stencil aperture size and other deposition parameters. We also show that the blurring can be reduced by decreasing the nominal deposition thickness, the deposition rate and the substrate temperature.

[en] Fast hydrogen sensors based on discontinuous palladium (Pd) films on supporting polyimide layers, fabricated by a cost-efficient and full-wafer compatible process, are presented. The films, deposited by electron-beam evaporation with a nominal thickness of 1.5 nm, consist of isolated Pd islands that are separated by nanoscopic gaps. On hydrogenation, the volume expansion of Pd brings initially separated islands into contact which leads to the creation of new electrical pathways through the film. The supporting polyimide layer provides both sufficient elasticity for the Pd nanoclusters to expand on hydrogenation and a sufficiently high surface energy for good adhesion of both film and contacting electrodes. The novel order of the fabrication processes involves a dicing step prior to the Pd deposition and stencil lithography for the patterning of microelectrodes. This allows us to preserve the as-deposited film properties. The devices work at room temperature, show response times of a few seconds and have a low power consumption of some tens of nW.

[en] Due to low power operation, intrinsic integrability and compatibility with CMOS processing, aluminum nitride (AlN) piezoelectric (PZE) microcantilevers are a very attractive paradigm for resonant gas sensing. In this paper, we theoretically investigate their ultimate limit of detection and enunciate design rules for performance optimization. The reduction of the AlN layer thickness is found to be critical. We further report the successful development and implementation in cantilever structures with a 50 nm thick active PZE AlN layer. Material characterizations demonstrate that the PZE e_3_1 coefficient can remain as high as 0.8 C m"−"2. Electrically transduced frequency responses of the fabricated devices are in good agreement with analytical predictions. Finally, we demonstrate the excellent frequency stability with a 10"−"8 minimum Allan deviation. This exceptionally low noise operation allows us to expect a limit of detection as low as 53 zg µm"−"2 and demonstrate the strong potential of AlN PZE microcantilevers for high resolution gas detection

[en] The standard lithographic techniques to fabricate electronic components involve the use of polymers, baking steps and chemicals. This typically restricts their application to flat substrates made up of standard materials. Stencil lithography has been proposed as a stable alternative to the standard lithographic techniques. In this paper, we demonstrate the completely resistless all-through-stencil fabrication of electronic components, by performing all essential fabrication steps—implantation, etching and metallization—using stencil lithography. This is performed on a planar substrate as well as on pre-patterned 3D substrates, thus showing the potential of this technique for applications in the field of accelerometers, pressure, gas and radiation sensors. (paper)

[en] We have measured the effect of bacteria adsorption on the resonant frequency of microcantilevers as a function of the adsorption position and vibration mode. The resonant frequencies were measured from the Brownian fluctuations of the cantilever tip. We found that the sign and amount of the resonant frequency change is determined by the position and extent of the adsorption on the cantilever with regard to the shape of the vibration mode. To explain these results, a theoretical one-dimensional model is proposed. We obtain analytical expressions for the resonant frequency that accurately fit the data obtained by the finite element method. More importantly, the theory data shows a good agreement with the experiments. Our results indicate that there exist two opposite mechanisms that can produce a significant resonant frequency shift: the stiffness and the mass of the bacterial cells. Based on the thermomechanical noise, we analyse the regions of the cantilever of lowest and highest sensitivity to the attachment of bacteria. The combination of high vibration modes and the confinement of the adsorption to defined regions of the cantilever allows the detection of single bacterial cells by only measuring the Brownian fluctuations. This study can be extended to smaller cantilevers and other biological systems such as proteins and nucleic acids