Finite element calculations and fabrication of cantilever sensors for nanoscale detection

Influence of Poisson's ratio variation on lateral spring constant of atomic force microscopy cantilevers

Atomic force microscopy (AFM) can be used to measure the surface morphologies and the mechanical properties of nanostructures. The force acting on the AFM cantilever can be obtained by multiplying the spring constant of AFM cantilever and the corresponding deformation. To improve the accuracy of force experiments, the spring constant of AFM cantilever must be calibrated carefully. Many methods, such as theoretical equations, the finite element method, and the use of reference cantilever, were reported to obtain the spring constant of AFM cantilevers. For the cantilever made of single crystal, the Poisson's ratio varies with different cantilever-crystal angles. In this paper, the influences of Poisson's ratio variation on the lateral spring constant and axial spring constant of rectangular and V-shaped AFM cantilevers, with different tilt angles and normal forces, were investigated by the finite element analysis. When the cantilever's tilt angle is 20 degrees and the Poisson's ratio varies from 0.02 to 0.4, the finite element results show that the lateral spring constants decrease 11.75% for the rectangular cantilever with 1 microN landing force and decrease 18.60% for the V-shaped cantilever with 50 nN landing force, respectively. The influence of Poisson's ratio variation on axial spring constant is less than 3% for both rectangular and V-shaped cantilevers. As the tilt angle increases, the axial spring constants for rectangular and V-shaped cantilevers decrease substantially. The results obtained can be used to improve the accuracy of the lateral force measurement when using atomic force microscopy.

Enhancing chemi-mechanical transduction in microcantilever chemical sensing by surface modification

The use of chemically selective thin-film coatings has been shown to enhance both the chemical selectivity and sensitivity of microcantilever (MC) chemical sensors. As an analyte absorbs into the coating, the coating can swell or contract causing an in-plane stress at the associated MC surface. However, much of the stress upon absorption of an analyte may be lost through slippage of the chemical coatings on the MC surface, or through relaxation of the coating in a manner that minimizes stress to the cantilever. Structural modification of MC chemical sensors can improve the stress transduction between the chemical coating and the MC. Surfaces of silicon MC were modified with focused ion beam milling. Sub-micron channels were milled across the width of the MC. Responses of the nanostructured, coated MCs to 2,3-dihydroxynaphthalene and a series of volatile organic compounds (VOCs) were compared to smooth, coated MCs. The analytical figures of merit for the nanostructured, coated MCs in the sensing of VOCs were found to be better than the unstructured MCs. A comparison is made with a previously reported method of creating disordered nanostructured MC surfaces.

Finite element analysis of nanostructures with roughness and scratches

Finite element analysis facilitates optimal design of MEMS/NEMS devices for reliability. A finite element method (FEM) was developed to analyze the effect of types of surface roughness and scratches on stresses of nanostructures. Beams with surface roughness in the form of semicircular and grooved asperities in varying numbers in the longitudinal direction were considered. In the transverse direction semicircular asperities and scratches in varying numbers and with different pitch were modeled. Furthermore semicircular asperities were truncated both in longitudinal and transverse direction and analyzed. It was observed that the asperities and scratches increase the bending tensile stresses which could lead to failure of MEMS/NEMS devices. The beam material was assumed to be purely elastic, elastic-plastic and elastic-perfectly plastic to observe the variations in the bending stresses and displacements for both smooth nanobeam and nanobeam with defined roughness. The results of the analysis can be useful to designers to develop the most suitable geometry for nanostructures.