Atomic Force Microscopy
AFM
This microscope called by this name because the use of atomic forces such as van der Waals force for surface analysis.
For expressing the importance of this mode, it is enough that we are reminded that many of people know SPM microscopes only by AFM mode. AFM is the most widely used scanning electron microscopy. It can be explored by its assistance, the surface topography (stereoscope), surface roughness, phase image, friction picture, magnetic properties, size and thickness of each layer.
In AFM microscope, as shown in Figure 1, use of a Cantilever Probe that is connected to it usually, a special needle tip of silicon to scan (Figure 2). While the needle tip is scanning the specimen surface, the force between them is controlled. For evaluating the probing movement, use of a laser mechanism.
Such that, according to the shape, a laser beam is hitting constantly to the cantilever that is scanning and the returned beam, which is affected from the probing movement of the cantilever, is analyzed by a detector. Usually, to increase the efficiency of reflection, they cover the cantilever with a layer of gold or aluminum.
Figure 1: The different parts of the SPM system.
Figure 2: The actual shape of the cantilever tip, which has been obtained by electron microscope.
Generally, AFM microscopes work in two ways:
1. The contact mode (static)
2. The non-contact mode (Dynamic)
It should be noted that the contact word is different in the world of nanotechnology with micro and macro worlds. Particularly, in the world of nanotechnology, the purpose of contact mode isn't physical contact, and a few angstrom distance known as contact mode.
1. In the contact mode, which is called the DC mode also, Cantilever is placed at a distance of a few Angstroms from the sample surface and inter atomic force between the cantilever and sample (which should be from the type of Van Der Waals repulsive force), is about 100-10 nano-newton.
By keeping constant the force between them, the cantilever move up and down and thus the vertical movement of cantilever influenced by the ups and downs of the surface, by returned light from the cantilever, is measured by the detector. Computer shows this vertical movement as a topographic image. In other cases, the distance between the sample and the surface is kept constant, and the force between the needle tip and sample surface is variable, and thus, the force map is obtained. Figure 3 shows several kinds of different scanning modes.
2. In non-contact mode (which is called AC mode also) cantilever is located in distance of a few dozen to several hundred angstroms from the sample surface, and in this case, with regard to the long distance between needle tip and the sample surface, Van der Waals force between the cantilever and samples is the type of attraction.
This mode works well for samples with very smooth surfaces. AFM can, in addition to surface topography, measures the friction between the needle tip and the sample. It also determines the electrical and roughness properties of the sample surface.
Figure 3: The method of imaging using scanning probe.
In Figure 4, two obtained different images from the deposited silver layer on a glass substrate, using the mode AFM, is shown within the range of one micron.
Figure 4: The image of deposited silver layer on a glass substrate within the range of one micron.