An image taken with a scanning electron microscope of an ant. (Image Credits: shutterstock.com/lafayette-picture)
Scientists looking to investigate the smallest of physical structures have a range of tools at their disposal. Two of the most common are Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM).
While both techniques are capable of providing nanometer-scale information about a material, they are very different methods, and can be used to reveal different kinds of information. Often, they are found side-by-side in a number of research institutions.
Atomic Force Microscopy
Developed in the 1980s, AFM uses a sharp tip on a cantilever which reads across a surface. It can be thought of as a sort of phonograph, capable of sensing the smallest variations on the surface of a material.
As the AFM probe scans, a laser aimed at the non-contact side of the cantilever measures the amount of interaction between the tip and the sample. With this setup, AFM is able to determine the surface topography of a material on the atomic scale vertically, and the nanometer scale horizontally.
AFM Principle - How AFM works. (Video Credit: Park AFM / YouTube)
The two most frequently used methods are contact mode AFM and TappingMode AFM, which can be carried out in a number of environments. Contact mode AFM involves scanning the probe or sample in a rectangular manner while checking the shift in cantilever deflection. TappingMode AFM involves gently “tapping” the tip on the sample surface during scanning.
Both techniques allow AFM to generate a high-resolution image of a nanometer-scale surface.
Scanning Electron Microscopy
Put into practice in the 1960s, SEM uses a focused beam of electrons, rather than a laser beam, to generate an image.
Using a Scanning Electron Microscope (Video Credit: University of Leicester/YouTube)
An electron gun at the top of the microscope generates the beam of electrons, which travels down through a vacuum. As it travels toward the sample, the electron beam is focused by a series of lenses and electromagnetic fields. When the beam strikes the sample, it releases X-rays and electrons, which are detected and converted into a three-dimensional image.
AFM/SEM Differences: Surface Structure
Significant differences between AFM and SEM arise when it comes to creating a representation of a sample surface.
On atomically smooth surfaces, AFM is capable of producing a three-dimensional topography using just a single scan. AFM also provides a greater level of detail for these surfaces, as SEM is not as efficient in resolving the subtle changes on a highly smooth surface.
When scanning thin films, like those used as capacitors in memory devices, SEM and AFM can produce very similar results. However, with images generated by SEM, it can sometimes be difficult to determine the slope of a surface. Conversely, AFM provides height information, making it easy to determine if surface features on a thin film are rising or falling.
On relatively rough surfaces, SEM’s large depth of field gives it a significant advantage over AFM. If a sample has details that are millimeters high, the penetration of the electron beam used in SEM makes it possible to images those details.
An AFM image of a layer of tin. (Image Credits: Piret Pikma/Wikipedia Commons)
AFM/SEM Differences: Composition
SEM also has a distinct advantage over AFM when it comes to determining the composition of a material. When the electron beam strikes a sample, a number of different signals are emitted, including back-scattered electrons, secondary electrons, X-rays, light and electrons. All of these signals provide different kinds of information.
X-rays and back-scattered electrons are the two most commonly used signals. X-ray emissions are created when electrons in the SEM beam strike electrons in the sample atoms. Using a spectrometer, scientists can use X-ray emissions to determine the elemental composition of a sample.
Back-scattered electrons are electrons from the beam being scattered out of the back of the sample. The number of back-scattered electrons from a sample has been determined to be based on the atomic number of the material. A detector can pick up these electrons and determine the make-up of a sample.
AFM may not be able to provide any information on the elemental makeup of a sample, but it can determine a number of other compositional properties – including stiffness, elasticity, adhesion, magnetic and electrostatic fields, temperature distribution, spreading resistance, and conductivity. Some of these techniques involve the use of a magnetic or conductive probe tip, which can detect the attractive and repulsive forces between the tip and the sample.
Using a diamond tip on the AFM probe can also reveal details about a sample’s composition. The diamond tip can be used to scratch or make small indentations in the sample, revealing information about its properties.
AFM/SEM Differences: Environment
One of the biggest differences in the way that the two techniques are carried out is the environment in which the tests are performed. SEM must be conducted in a vacuum environment, whereas AFM can be performed in a vacuum, ambient, gas or liquid environment.
This fact becomes particularly relevant in certain aspect of biology, where hydrated samples need to be tested. A special chamber can be set up to use SEM in these situations; however, this results in a loss of resolution.
In instances where a sample must be tested in a particular environment, AFM provides a distinct advantage. The technique is frequently used to image samples in liquid in an enclosed environment. AFM can also be used to test in gaseous environments, such as those involved in the development of polymers.
Some similarities and a word of warning
Even though these two techniques have a number of significant differences, SEM and AFM are quite similar in a number of ways. Both tools involve interacting with a surface to generate an image: one using electrons and one using light. Both produce similar lateral resolutions.
There are a few systems that combine the two techniques, and some have argued that a combination setup compromises the integrity of each individual technique. For example, operating AFM in a vacuum lowers its flexibility and increases the amount of operating time necessary to generate an image.
When used side-by-side, on the other hand, SEM and AFM can complement each other very well and provide a wealth of information about a material.
Sources and Further Reading
- SEM and AFM: Complementary Techniques for High Resolution Surface Investigations - Bruker
- Scanning Electron Microscope - Purdue University