Atomic force microscope

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Image:Atomic force microscope block diagram.png Image:AFMimageRoughGlass20x20.png

The atomic force microscope (AFM) is a very powerful microscope invented by Binnig, Quate and Gerber in 1985. Besides imaging it is also one of the foremost tools for the manipulation of matter at the nanoscale.

The AFM consists of a cantilever with a sharp tip at its end, typically composed of silicon or silicon nitride with tip sizes on the order of nanometers. The tip is brought into close proximity of a sample surface. The Van der Waals force between the tip and the sample leads to a deflection of the cantilever according to Hooke's law, where the spring constant of the cantilever is known. Typically, the deflection is measured using a laser spot reflected from the top of the cantilever into an array of photodiodes. However a laser detection system can be expensive and bulky; an alternative method in determining cantilever deflection is by using piezoresistive AFM probes. These probes are fabricated with piezoresistive elements that act as a strain gage. Using a Wheatstone bridge, strain in the AFM probe due to deflection can be measured, but this method is not as sensitive as the laser deflection method.

If the tip were scanned at constant height, there would be a risk that the tip would collide with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to keep the force between the tip and the sample constant. Generally, the sample is mounted on a piezoelectric tube, which can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample. The resulting map of s(x,y) represents the topography of the sample.

Over the years several modes of operation have been developed for the AFM. The primary modes of operation are contact mode, non-contact mode, and dynamic contact mode. In the contact mode operation, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection. In the non-contact mode, the cantilever is externally oscillated at or close to its resonance frequency. The oscillation gets modified by the tip-sample interaction forces; these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics. Because most samples develop a liquid meniscus layer, keeping the probe tip close enough to the sample for these inter-atomic forces to become detectable while preventing the tip from sticking to the surface presents a major hurdle for non-contact mode in ambient conditions. Dynamic contact mode was developed to bypass this problem (Zhong et al). In dynamic contact mode, the cantilever is oscillated such that it comes in contact with the sample with each cycle, and then enough force is applied to detach the tip from the sample.

Schemes for non-contact and dynamic contact mode operation include frequency modulation and the more common amplitude modulation. In frequency modulation, changes in the oscillation frequency provide information about a sample's characteristics. In amplitude modulation (better known as intermittent contact or tapping mode), changes in the oscillation amplitude yield topographic information about the sample. Additionally, changes in the phase of oscillation under tapping mode can be used to discriminate between different types of materials on the surface.

The AFM has several advantages over the electron microscope. Unlike the electron microscope which provides a two-dimensional projection or a two-dimensional image of a sample, the AFM provides a true three-dimensional surface profile. Additionally, samples viewed by an AFM do not require any special treatment that would actually destroy the sample and prevent its reuse. While an electron microscope needs an expensive vacuum environment for proper operation, most AFM modes can work perfectly well in an ambient or even liquid environment. This makes it an excellent tool for studying live biological samples. In addition, the AFM can produce images of materials as small as 1nm, while the SEM is limited to around 100nm.

The main disadvantage that the AFM has compared to the scanning electron microscope (SEM) is the image size. The SEM can show an area on the order of millimetres by millimetres and a depth of field on the order of millimetres. The AFM can only show a maximum height on the order of micrometres and a maximum area of around 150 by 150 micrometres. Additionally, the AFM cannot scan images as fast as an SEM. It may take several minutes for a typical region to be scanned with the AFM, however an SEM is capable of scanning at near real-time (although at relatively low quality).

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