Anja Boisen

Cantilever-like micromechanical sensors

The field of cantilever-based sensing emerged in the mid-1990s and is today a well-known technology for label-free sensing which holds promise as a technique for cheap, portable, sensitive and highly parallel analysis systems. The research in sensor realization as well as sensor applications has increased significantly over the past 10 years. In this review we will present the basic modes of operation in cantilever-like micromechanical sensors and discuss optical and electrical means for signal transduction. The fundamental processes for realizing miniaturized cantilevers are described with focus on silicon- and polymer-based technologies. Examples of recent sensor applications are given covering such diverse fields as drug discovery, food diagnostics, material characterizations and explosives detection.

Environmental sensors based on micromachined cantilevers with integrated read-out

An AFM probe with integrated piezoresistive read-out has been developed and applied as a cantilever-based environmental sensor. The probe has a built-in reference cantilever, which makes it possible to subtract background drift directly in the measurement. Moreover, the integrated read-out facilitates measurements in liquid. The probe has been successfully implemented in gaseous as well as in liquid experiments. For example, the probe has been used as an accurate and minute thermal sensor and as a humidity sensor. In liquid, the probe has been used to detect the presence of alcohol in water.

Enhanced functionality of cantilever based mass sensors using higher modes

By positioning a single gold particle at different locations along the length axis on a cantilever based mass sensor, we have investigated the effect of mass position on the mass responsivity and compared the results to simulations. A significant improvement in quality factor and responsivity was achieved by operating the cantilever in the fourth bending mode thereby increasing the intrinsic sensitivity. It is shown that the use of higher bending modes grants a spatial resolution and thereby enhances the functionality of the cantilever based mass sensor.

A microcantilever-based alcohol vapor sensor-application and response model

A recently developed microcantilever probe with integrated piezoresistive readout has been applied as a gas sensor. Resistors, sensitive to stress changes, are integrated on the flexible cantilevers. This makes it possible to monitor the cantilever deflection electrically and with an integrated reference cantilever background noise is subtracted directly in the measurement. A polymer coated cantilever has been exposed to vapors of various alcohols and the resulting cantilever response has been interpreted using a simple evaporation model. The model indicates that the cantilever response is a direct measure of the molecular concentration of alcohol vapor. On the basis of the model the detection limit of this cantilever-based sensor is determined to be below 10 ppm for alcohol vapor measurements. Furthermore, the time response of the cantilever can be used to distinguish between different alcohols due to a difference in the evaporation rates.

Atomic force microscopy probe with piezoresistive read-out and a highly symmetrical Wheatstone bridge arrangement

Abstract We have developed a new generic platform for the fabrication of multipurpose microprobes with integrated piezoresistive read-out, built-in background filter and silicon tip. The probe fabrication is based on SOI wafers with buried boron etch-stop layers, which allow us to realize probes with fully encapsulated resistors and integrated silicon tips. The dimensions of the resistors are well defined and leak-current is eliminated. Probes with a force constant in the range of 0.8–4 N/m and with resonant frequencies in the range of 40–80 kHz have been fabricated. The probes typically display a deflection sensitivity of (ΔR/R)z−1=2.4×10−7 A−1, and a force sensitivity (ΔR/R)F−1=2.7×10−6 nN−1. The change in resistance of the piezoresistors is detected by a highly symmetrical on-chip Wheatstone bridge arrangement. The measured noise level in the Wheatstone bridge is in good agreement with the calculated noise limit and a minimum detectable cantilever deflection of 0.3 A has been predicted for a measurement bandwidth of 10 Hz. The symmetrical bridge configuration has been compared with a nonsymmetrical setup, and it is concluded that the symmetrical Wheatstone bridge significantly decreases nonlinearities in the output response. Finally, the probe has successfully been used for atomic force microscopy (AFM) imaging.

Optimised cantilever biosensor with piezoresistive read-out

We present a cantilever-based biochemical sensor with piezoresistive read-out which has been optimised for measuring surface stress. The resistors and the electrical wiring on the chip are encapsulated in low-pressure chemical vapor deposition (LPCVD) silicon nitride, so that the chip is well suited for operation in liquids. The wiring is titanium silicide which-in contrast to conventional metal wiring-is compatible with the high-temperature LPCVD coating process.

Mass and position determination of attached particles on cantilever based mass sensors

An analytical expression relating mass and position of a particle attached on a cantilever to the resulting change in cantilever resonant frequency is derived. Theoretically, the position and mass of the attached particle can be deduced by combining measured resonant frequencies of several bending modes. This finding is verified experimentally using a microscale cantilever with and without an attached gold bead. The resonant frequencies of several bending modes are measured as a function of the bead position. The bead mass and position calculated from the measured resonant frequencies are in good agreement with the expected mass and the position measured.

Optimization of sensitivity and noise in piezoresistive cantilevers

In this article, the sensitivity and the noise of piezoresistive cantilevers were systematically investigated with respect to the piezoresistor geometry, the piezoresistive materials, the doping dose, the annealing temperature, and the operating biased voltage. With the noise optimization results, dimension optimized array cantilevers were designed and fabricated by using single-crystal silicon, low-pressure chemical-vapor deposition (LPCVD) amorphous silicon and microcrystalline silicon as piezoresistive layers. Measurement results have shown that the smallest Hooge factor (α) was 3.2×10−6, the biggest gauge factors was 95, and the minimum detectable deflection (MDD) at 6 V and 200 Hz-measurement bandwidth was 0.3 nm for a single-crystal silicon cantilever. Of the two LPCVD silicon piezoresistive cantilevers, amorphous silicon piezoresistors had relatively lower 1/f noise. The MDD for a LPCVD silicon cantilever at a 200 Hz-measurement bandwidth was 0.4 nm. For all kinds of piezoresistive cantilevers, the...

Large Area Fabrication of Leaning Silicon Nanopillars for Surface Enhanced Raman Spectroscopy

Using a simple two step fabrication process substrates with a large and uniform Raman enhancement, based on flexible free standing nanopillars can be manufactured over large areas using readily available silicon processing equipment.

Adsorption kinetics and mechanical properties of thiol-modified DNA-oligos on gold investigated by microcantilever sensors

Immobilised DNA-oligo layers are scientifically and technologically appealing for a wide range of sensor applications such as DNA chips. Using microcantilever-based sensors with integrated readout, we demonstrate in situ quantitative studies of surface-stress formation during self-assembly of a 25-mer thiol-modified DNA-oligo layer. The self-assembly induces a surface-stress change, which closely follows Langmuir adsorption model. The adsorption results in compressive surface-stress formation, which might be due to intermolecular repulsive forces in the oligo layer. The rate constant of the adsorption depends on the concentration of the oligo solution. Based on the calculated rate constants a surface free energy of the thiol-modified DNA-oligo adsorption on gold is found to be -32.4 kJ mol(-1). The adsorption experiments also indicate that first a single layer of DNA-oligos is assembled on the gold surface after which a significant unspecific adsorption takes place on top of the first DNA-oligo layer. The cantilever-based sensor principle has a wide range of applications in real-time local monitoring of chemical and biological interactions as well as in the detection of specific DNA sequences, proteins and particles.