Atomic Force Microscopes (AFM)

 

Atomic Microscopes (AFM)

What is an atomic force microscope?

An atomic force microscope is able to image surfaces to molecular accuracy by mechanically probing their surface contours. Measuring the atomic force acting on its tip as it moves along the surface of the sample. Atomic force microscopy is a technique for analysing the surface of a rigid material all the way down to the level of the atom.

AFM uses a mechanical probe to magnify surface features up to 100 000 000 times, and produces 3D images of the surface. The technique is derived from a related technology, called scanning tunnelling microscopy (STM). The difference is that AFM does not require the sample to conduct electricity, whereas STM does.

AFM also works in regular room temperatures, while STM requires special temperature and other conditions. AFM is being used to understand materials problems in many areas including data storage, telecommunications, biomedicine, chemistry, and aerospace. The atomic force microscope was invented in 1986. It uses various forces that occur when two objects are brought within nanometres of each other.

An AFM can work either when the probe is in contact with a surface, causing a repulsive force, or when it is a few nanometres away, where the force is attractive. Because the prototype STM relied on electrical flow between tip and sample, it could only be used to examine materials that would conduct at least a small electric current. Since the early 1980s, STMs have evolved into Atomic Force Microscopes (AFMs) that are able to see a wider range of nano-scale samples.

The process resembles the original one where a needle-like tip scans across a surface whose topography is "read" and then translated into a graphic image, but the AFM is able to see samples that are not highly conductive, such as biological samples. Rather than maintaining a constant distance between tip and sample, the tip of an AFM is attached to the end of a highly sensitive cantilevered arm and actually touches the surface of the sample to trace it and generate an image.

The idea is to combine magnetic-resonance imaging (MRI) with an AFM � the resulting tool is called a magnetic-resonance force microscope (MRFM). The technology is in the earliest stages, but there are already six granted patents referring to MRF Microscopy. What is an Atomic Force Microscope (AFM)? Leading semiconductor device companies have pushed their chip technology beyond the limits of optical microscopy-based testing techniques.

This turnkey system enables the rapid development of future generations of nano-scale geometry devices. An AFM uses nanometer resolution actuators to scan a tiny probe on a surface while maintaining force control. Force-feedback-control loops maintain a constant deflection and therefore a constant force between the probe and the surface. Tools that do not use force feedback are not AFMs and risk increased contact force as the probe deflects over features, which can cause damage to both the sample and probe. The AFM is part of the scanning probe microscope family and is often used in biology related research.

The AFM uses a very small sensor to measure the amount of force between the tip of the sensor and the research sample. The basic idea of the AFM is to measure the amount of bending that occurs when the tip of the sensor is close to the sample. The AFM uses a small laser to measure the amount of bending and then a small picture is created that displays the results. According to the timeline, the basic microscope was created before the atomic force microscope. There are similarities between the two; however, the function is very different. The AFM is used to research and detect things on a very small scale, smaller than the samples that are viewed through a regular microscope.

Both microscopes have similar parts, such as the eyepiece, head, and base. In fact, both microscopes have parts that do almost the same job. For example, on a basic microscope the stage clips hold the sample in place and on the AFM microscope the tipholder is what holds the small sensor in place. However, the AFM has many different ways that it can detect information while the basic microscope does not. The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit.

The precursor to the AFM, the scanning tunneling microscope, was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first AFM in 1986. The AFM is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning.

An atomic force microscope (AFM) is an extremely precise microscope that images a sample by rapidly moving a probe with a nanometer-sized tip across its surface. This is quite different than an optical microscope which uses reflected light to image a sample. An AFM probe offers a much higher degree of resolution than an optical microscope because the size of the probe is much smaller than the finest wavelength of visible light. In an ultra-high vacuum, an atomic force microscope can image individual atoms. Its extremely high resolution capabilities have made the AFM popular with researchers working in the field of nanotechnology.

The AFM uses a microscale cantilever with a probe tip whose size is measured in nanometers. An AFM operates in one of two modes: contact (static) mode and dynamic (oscillating) mode. In static mode, the probe is kept still, while in dynamic mode it oscillates. When the AFM is brought close to or contacts the surface, the cantilever deflects. Usually, on top of the cantilever is a mirror which reflects a laser. The laser reflects onto a photodiode, which precisely measures its deflection.

When the oscillation or position of the AFM tip changes, it is registered in the photodiode and an image is built up. Sometimes more exotic alternatives are used, such as optical interferometry, capacitive sensing or piezoresistive (electromechanical) probe tips. Why are atomic force microscope (AFM) standards needed? Atomic force microscopes (AFM's) are being increasingly used as metrology tools in a variety of industrial applications, thus driving an increasing demand for accuracy in these instruments.

Some properties commonly measured in the industrial setting are feature spacing (pitch), feature height (or depth), feature width (critical dimension), and surface roughness. To achieve high accuracy in AFM measurements, the scales of an instrument should be calibrated. The use of a calibration standard is normally the most straightforward and appropriate means of doing this.

An example of a popular AFM standard is a three dimensional "grid" or "waffle" pattern which can be used as a three axis magnification standard. Many presently available AFM standards are calibrated using stylus instruments and optical techniques. The effectiveness of this approach, however, is limited by the differences in the working ranges of the various techniques and by questions of methods divergence (i.e., difference in instrumental response to a sample for different measurement techniques).