Atomic Force Microscope (AFM)

Atomic force microscopy is arguably the most versatile and powerful microscopy technology for studying samples at nanoscale. It is versatile because an atomic force microscope can not only image in three-dimensional topography, but it also provides various types of surface measurements to the needs of scientists and engineers. It is powerful because an AFM can generate images at atomic resolution with angstrom scale resolution height information, with minimum sample preparation.

So, how does an AFM work? In this article, I introduce you to the principles of an AFM with an easy to understand video animations. Feel free to share this article with others, and to email me if you have any questions.

What is AFM?

Since the introduction of the Nobel Prize-winning scanning tunneling microscope (STM) and then the invention of the atomic force microscopy (AFM), a one kind of scanning probe microscopes (SPM), from the landmark publication by Binnig, Quate, and Gerber, the field of scanning probe microscopy has exploded well beyond using interatomic forces to image topography on the nanometer scale.

SPMs are designed to measure local properties, such as height, friction, magnetism, with a probe. Now, many material properties can be determined with AFM techniques, including friction, electrical forces, capacitance, magnetic forces, conductivity, viscoelasticity, surface potential, and resistance. To acquire an image, the SPM raster-scans the probe over a small area of the sample, measuring the local property simultaneously.

AFM Principle

- Surface Sensing

An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip cause the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, increasingly repulsive force takes over and causes the cantilever to deflect away from the surface.

- Detection Method

A laser beam is used to detect cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever deflection (and the subsequent change in direction of reflected beam) is recorded by the PSPD.

- Imaging

An AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface—thus maintaining constant laser position—the AFM can generate an accurate topographic map of the surface features.

To further your understanding, you are welcome to follow this straight forward visual tutorial available online.

AFM Operating Modes

Applications and Uses of AFM

Currently, AFM is one of the most effective imaging techniques being used at the nanoscale and subnanoscale level. This technique has been applied to multiple problems across the field of natural sciences and can record a range of material surface properties in both liquid media and in air. Disciplines where AFM is used include:

• Semiconductor science and technology
• Thin film and coatings
• Tribology (surface and friction interactions)
• Surface chemistry
• Polymer chemistry and physics
• Cell biology
• Molecular biology
• Energy storage (batter) and energy generation (photovoltaic) materials
• Piezoelectric and ferroelectric materials

AFM is a powerful imaging and measurement technique that has become critical to nanoscale research and to industrial R&D in all of its possible forms. A good example of this is in the semiconductor industry, where AFM is used in quality control and imaging for silicon integrated circuits.

It is also used in the imaging and development of graphene. AFM has allowed the study and characterization of graphene composite materials. Industries such as the aerospace and automotive industries rely highly upon AFM in the development of materials. AFM is so versatile that it can determine a variety of mechanical properties at the nanoscale level and also characterize a material test sample completely in hours, rather than days. AFM can be used in biological research to distinguish cancer cells from normal cells, based on their stiffness. There are new applications for AFM appearing every day and there is an almost unlimited number of research fields.

AFM Advantages

The atomic force microscope is a powerful tool that is invaluable if you want to measure incredibly small samples with a great degree of accuracy.

Unlike rival technologies it does not require either a vacuum or the sample to undergo treatment that might damage it.

At the limits of operation however, researchers have demonstrated atomic resolution in high vacuum and even liquid environments.

AFM Disadvantages

One of the major downsides is the single scan image size, which is of the order of 150x150 micrometers, compared with millimeters for a scanning electron microscope.

Another disadvantage is the relatively slow scan time, which can lead to thermal drift on the sample.

As the technology matures, researchers are relying on there being progress instrumentally, requiring improved signal-to-noise ratio, decreased thermal drift, and better detection and control of tip-sample forces, including the use of sharp probes.

Novel solutions are steadily improving these performance issues.