Atomic-force microscopy (AFM) is a surface scanning technique that has sub-nanometer scale resolution. AFM describes a group of techniques used for non-destructive surface studies at the nanoscale. They have a resolution on the order of 103 times better than optical microscopy’s resolution limit. AFM is used widely to collect data on various mechanical, functional and electrical properties at the nanoscale as well as for topography (surface) studies.
Generally, AFM is used for three main functions:
- It is most commonly used for imagining surface topography by recording the position of the sample relative to the tip and then recording the height of the probe that corresponds to a constant probe-sample interaction as well as the variety of mechanical, functional and electrical properties.
- For surface manipulation using tip forces, for example, scanning probe lithography
- To measure the forces between the sample and the probe as a form of force spectroscopy, to examine physical and mechanical properties
How Does AFM Work?
AFM scans surfaces by raster-scanning a sharp micro-probe tip, controlled by piezoelectric elements and a feedback loop via a computer, back and forth across a small area. To gather profile and interaction data, the probe traces and contacts the surface.
The AFM probe is most commonly a sharp silicon nitrate or silicon tip on a free-moving cantilever that is mounted on a carrying chip. Cantilevers are thin arms of silicon nitrate or silicon with a rectangular or triangular shape and have well-defined mechanical properties. The tip has a radius in the nanometer range and is a very sharp projection at the end of the cantilever. Forces between the tip and the sample affect a deflection of the cantilever according to Hooke’s law when the cantilever-mounted tip is brought into proximity with a sample surface by the AFM.
Surface/tip interaction data is collected by scanning the surface with the AFM tip and this forms a spatial profile, or map, of the sample area. A laser beam is focused on the back of the cantilever and is reflected to a photodiode. This is displaced and the movement is recorded by the photodiode surface.
The way in which the tip interacts with the samples is similar to how a record stylus follows the grooves of a vinyl record. So that the surface can be effectively profiled at the highest possible resolution, it is vital to have the correct cantilever properties for the AFM operation.
Modes of AFM
According to the required application, AFM is operated in a number of modes. Imaging modes can be divided into contact or static modes and a variety of tapping or dynamic modes, where the cantilever of the probe is vibrated at a given frequency.
Where the tip is drawn across the surface in continuous contact, contact mode occurs. The feedback loop maintains constant cantilever deflection (i.e. force) in these cases. This is achieved by adjusting the relative Z position of the probe and sample as the tip scans across the surface. This motion represents the constant-force topography of the surface and is recorded. Force resulting from the contact between the tip and the sample can cause damage to the sample and wear to the tip.
The cantilever is oscillated at its resonant frequency in Tapping Mode. The oscillation amplitude is measured and then used as the input to the imaging feedback loop. The loop adjusts the relative Z position of the sample and the probe, as in contact mode. In this case, it is to maintain constant amplitude and this becomes the topography image. The tip-sample forces, especially laterally, are greatly reduced and sample damage and tip wear are greatly minimized compared to contact mode.
Furthermore, tapping mode interaction can be analyzed in greater detail, including its phase and also additional higher resonance modes. A great deal of information about tip-sample interactions is encoded in these signals.
Various different material properties can be ‘pulled out’ and quantified from this additional data, such as deformation depth, adhesion force and modulus. This means that AFM can measure and distinguish different materials present in samples, ranging from biological materials to microelectric devices and polymers.
An additional imaging technique that measures force-distance curves at high speed (up to 1000 Hz or more) whilst capturing every curve in the image scan is Fast Force Mapping Mode. Both offline and real-time analysis models can then be applied in order to calculate adhesion, modulus, and a variety of other properties as well as basic topography.
Asylum AFM History
IBM scientist Gerd Binning invented AFM in the company’s Zurich labs in 1982. By 1986 the instrument was developed to an experimental version and 1989 saw the release of the first commercial version. Asylum Research is an Oxford Instruments company which has leading expertise in atomic force microscopy in bioscience and materials research. It was founded in 1999 in order to develop AFM technology for use in industrial and academic R&D. The focus at Asylum has been to provide:
- Easier to use AFMs by eliminating surface artifacts, automating the setup process and making operation more predictable and reliable.
- Higher performance AFMs (e.g. speed, resolution)
- More useful AFMs going beyond surface topography to encompass electrical, functional and mechanical properties
Recent advances in AFM from Asylum Research have made nanomechanical, nanoelectrical and electromechanical characterization measurements much simpler and more automated to give increased productivity and consistency.
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, and 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.
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This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
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