Readout of micromechanical cantilever sensor arrays by Fabry-Perot interferometry

Challenges for nanomechanical sensors in biological detection

Nanomechanical biosensing relies on changes in the movement and deformation of micro- and nanoscale objects when they interact with biomolecules and other biological targets. This field of research has provided ever-increasing records in the sensitivity of label-free detection but it has not yet been established as a practical alternative for biological detection. We analyze here the latest advancements in the field, along with the challenges remaining for nanomechanical biosensors to become a commonly used tool in biology and biochemistry laboratories.

High throughput optical readout of dense arrays of nanomechanical systems for sensing applications

We present an instrument based on the scanning of a laser beam and the measurement of the reflected beam deflection that enables the readout of arrays of nanomechanical systems without limitation in the geometry of the sample, with high sensitivity and a spatial resolution of few micrometers. The measurement of nanoscale deformations on surfaces of cm(2) is performed automatically, with minimal need of user intervention for optical alignment. To exploit the capability of the instrument for high throughput biological and chemical sensing, we have designed and fabricated a two-dimensional array of 128 cantilevers. As a proof of concept, we measure the nanometer-scale bending of the 128 cantilevers, previously coated with a thin gold layer, induced by the adsorption and self-assembly on the gold surface of several self-assembled monolayers. The instrument is able to provide the static and dynamic responses of cantilevers with subnanometer resolution and at a rate of up to ten cantilevers per second. The instrumentation and the fabricated chip enable applications for the analysis of complex biological systems and for artificial olfaction.

Mechanical characterization of thin TiO2 films by means of microelectromechanical systems-based cantilevers

The measurement of mechanical parameters by means of microcantilever structures offers a reliable and accurate alternative to traditional methods, especially when dealing with thin films, which are extensively used in microfabrication technology and nanotechnology. In this work, microelectromechanical systems (MEMS)-based piezoresistive cantilevers were realized and used for the determination of Young's modulus and residual stress of thin titanium dioxide (TiO(2)) deposited by sputtering from a TiO(2) target using a rf plasma discharge. Films were deposited at different thicknesses, ranging from a few to a hundred nanometers. Dedicated silicon microcantilevers were designed through an optimization of geometrical parameters with the development of analytical as well as numerical models. Young's modulus and residual stress of sputtered TiO(2) films were assessed by using both mechanical characterization based on scanning profilometers and piezoresistive sensing elements integrated in the silicon cantilevers. Results of MEMS-based characterization were combined with the tribological and morphological properties measured by microscratch test and x-ray diffraction analysis.

Simultaneous readout of multiple microcantilever arrays with phase-shifting interferometric microscopy

A complete system for the simultaneous monitoring of multiple cantilever sensors from different sensor arrays has been developed and tested for gas- and liquid-phase applications. The cantilever sensors are operated in static-deflection mode and the readout is achieved with phase-shifting interferometric microscopy (PSIM). In contrast to existing cantilever-sensor readout methods, PSIM is not dependent on alignment and allows the monitoring of the entire displacement profiles of all cantilevers within the field of view, using just one light source. To complement the PSIM readout, we have developed a sample cell, which can hold multiple cantilever-array chips, allows for very fast and reproducible sensor-chip replacement, has very low sample-volume requirements, and allows for individual or common addressing of all chips in the sample cell. We demonstrate the functionality of our microcantilever sensor system with a setup that can monitor eight cantilevers from four different sensor chips simultaneously.

Wavelength Dependence of Photoinduced Microcantilever Bending in the UV-VIS Range

Micromechanical devices such as microcantilevers (MC) respond to irradiation with light by at least two different, photon-mediated processes, which induce MC bending as a consequence of differential surface stress. The first and slow bending is due to the absorption of photons, whose energy is transformed into heat and causes bending of bimetallic microcantilevers due to thermal expansion. The second type of deflection is fast and caused by photons of sufficient energy to promote electrons across the Schottky barrier and thus create charge carriers, resulting in photoinduced stress that causes MC bending. In this study, the MC bending response to irradiation with light of wavelengths ranging from 250 to 700 nm was investigated. Measurements of the immediate mechanical response to photoinduced stress as a function of the wavelength of incident light provide an avenue to the determination of the cut-off wavelength/energy of the Schottky barrier in the MC devices under investigation. For a gold coated Si₃Ni₄ microcantilever we measured a cutoff wavelength of 1206 nm, which lies in the range of the literature value of 1100 nm.