CANTILEVER TECHNIQUE IN NANOTECHNOLOGY
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26-02-2011, 10:14 AM
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CANTILEVER TECHNIQUE IN NANOTECHNOLOGY
What is a cantilever ??
A cantilever is a beam supported on only one end. The beam carries the load to the support where it is resisted by moment and shear stress
Cantilevered beams are the most ubiquitous structures in the field of micro electromechanical systems (MEMS
The MEMS cantilevers are commonly made as unimorphs or bimorphs:
UNIMORPH: A unimorph is a cantilever that consists of one active layer and one inactive layer. In the case where active layer is piezoelectric, deformation in that layer may be induced by the application of an electric field. This deformation induces a bending displacement in the cantilever. The inactive layer may be fabricated from a non-piezoelectric material.
• BIMORPH : A bimorph is a cantilever that consists of two active layers: piezoelectric and metal. These layers produce a displacement via:
Electrical activation as in a piezoelectric bimorph
What is Nanotechnology ??
The definition of nanotechnology is based on the prefix “nano” which is from the Greek word meaning “dwarf”. It is the study and use of structures between 1 nanometer and 100 nanometers in size.
Nanotechnology is the manipulation or self-assembly of individual atoms, molecules, or molecular clusters into structures to create materials and devices with new or vastly different properties.
The AFM (center) has inspired a variety of other scanning probe techniques. Originally the AFM was used to image the topography of surfaces, but by modifying the tip it is possible to measure other quantities (for example, electric and magnetic properties, chemical potentials, friction and so on), and also to perform various types of spectroscopy and analysis.
Atomic force microscopy:
Atomic force microscopy (AFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, developed by Gerd Binnig and Heinrich Rohrer.
The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface.
When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law.
Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes.
Imaging mode :The AFM can be operated in a number of modes, depending on the application.
The primary modes of operation for an AFM are static mode and dynamic mode. In static mode, the cantilever is "dragged" across the surface of the sample and the contours of the surface are measured directly using the deflection of the cantilever. In the dynamic mode, the cantilever is externally oscillated at or close to its fundamental resonance frequency or a harmonic. These changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics.
In contact mode, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection.
The static tip deflection is used as a feedback signal. Because the measurement of a static signal is prone to noise and drift, low stiffness cantilevers are used to boost the deflection signal.
However, close to the surface of the sample, attractive forces can be quite strong, causing the tip to "snap-in" to the surface. Thus static mode AFM is almost always done in contact where the overall force is repulsive.
In this mode, the tip of the cantilever does not contact the sample surface. The cantilever is instead oscillated at a frequency slightly above its resonance frequency where the amplitude of oscillation is typically a few nanometers.
The van der Waals forces acts to decrease the resonance frequency of the cantilever. This decrease in resonance frequency combined with the feedback loop system maintains a constant oscillation amplitude or frequency by adjusting the average tip-to-sample distance. Measuring the tip-to-sample distance at each (x,y) data point allows the scanning software to construct a topographic image of the sample surface.