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Atomic force microscopy: Choosing an AFM

Atomic force microscopy (AFM) is based on a simple measuring principle. In order to measure reliably on a nanometer scale a sophisticated instrument design is required. This article shows which aspects to consider before using an AFM.

Cantilever handling

Before it is possible to perform a measurement with an AFM, a (micro)cantilever needs to be mounted. The cantilevers in an optical lever AFM are very small – commonly between 50 µm and 300 µm long, about 20 µm to 60 µm wide, and 2 µm to 8 µm thick.

Cantilever handling normally requires experience: Not only can cantilever mounting be quite tricky in practice (and costly if several cantilevers are used for mounting), it can also be frustrating if the laser alignment does not work because the cantilever was not mounted properly. In this case, the user has to go back to the cantilever exchange step, re-mount the cantilever (or mount a new one), and re-start the laser alignment procedure.

Therefore, modern AFM instruments need to provide a solution to this problem.

Automatic laser alignment

Fig. 1: Automatic laser alignment

After placing the probe in the instrument, the alignment of the optical lever is carried out. In order to perform AFM measurements correctly, proper laser alignment is crucial. Poor alignment can reduce the sensitivity of the optical lever, introduce imaging artifacts, or even prevent imaging[1].

However, manual laser alignment can be time-consuming and difficult to perform. Modern AFM instruments typically carry out the laser alignment automatically.

Coarse approach with side-view camera

Fig. 2: Coarse approach with side-view camera

One of the major challenges in AFM design is building a motion control system that permits the approach of the probe to the surface before scanning. This must be done in such a way that the probe does not crash into the surface and break. However, once the motion control system is successfully designed and integrated into the AFM, the user has to control how the probe approaches the surface of the sample.

This is particularly challenging for transparent samples, dark samples, and samples with complex geometry, as it is difficult to estimate the distance between the probe and the sample. This process carries a substantial risk of crashing the probe into the sample.

New generations of AFMs are equipped with a side-view camera so that the user can see the exact position of the cantilever in relation to the surface and safely move the cantilever close to the surface. The automatic engagement procedure can then be started in the software – and within seconds the cantilever is ready for scanning.

Time-to-measure

When working with an AFM, the time-to-measure – the time it actually takes to start a measurement – is essential. The user needs to perform five steps:

  1. Load the cantilever
  2. Load the sample
  3. Align the laser
  4. Navigate to the feature
  5. Approach and start the measurement

For AFM instruments with difficult cantilever exchange, manual laser alignment, no side-view camera, and lack of automation these five steps take time to be performed. In addition, if users are not very experienced in AFM or the samples are not always of the same type, it can take even longer to carry out these steps. Therefore, AFMs with a high level of automation can help reduce the time required to perform measurements.

Decoupled xy- and z-scanners

Fig. 3: Decoupled xy- and z-scanners

Piezoelectric ceramics must be configured in such a way that they can move the probe (or sample) in the direction of the x-, y-, and z-axes. One of the most commonly used three-dimensional scanner configurations is the tube scanner – mainly because it is easily fabricated.

However, it has some disadvantages: Due to their geometry, tube scanners are subject to a lot of non-linearity, particularly bow, when using the full range of the scanner. Scanner bow comes from the fact that the trajectory of the tube scanner is curved, so the result is an apparent curvature or “bow” in the height of measured samples.

In AFMs with decoupled xy- and z-scanners, the xy-scanner is placed under the sample while the z-scanner is placed in the head in order to reduce the cross-talk between the scanners (Fig. 3).

References

  1. Eaton, P. and West, P. (2010): Atomic Force Microscopy. New York: Oxford University Press.