Semiconductor Laboratory

Nanosurf Easyscan 2 - Atomic Force Microscope (AFM)

Atomic force microscopy (AFM) is a scanning probe microscopy technique that can provide topographical images with a very high resolution in the sub-nanometer range.¹

Theoretical background

In atomic force microscopy, a sharp tip on a cantilever is raster-scanned over a surface and its deflection due to atomic force interactions with the sample surface is measured. This is usually achieved by a laser reflecting from the top of the cantilever onto a position-sensitive detector. The deflection depends on the force and the stiffness of the cantilever according to Hooke's law

where $F$ is the force between tip and surface, $z$ is the distance the cantilever is displaced and $k=3EI/L^3$ is the effective spring constant of the cantilever. Here $E$ is the elastic modulus $L$ is the length of the beam, and $I$ is the second moment of the area of the cantilever's cross section. See: Beam vibrations.

Tapping mode

An AFM can be operated in different imaging modes of which the so-called dynamic tapping mode is the most frequently used mode when measuring in ambient conditions. In the tapping mode, the cantilever is oscillated while scanning across the sample surface. When the tip gets close to the surface, the interaction forces with the sample lead to dampening of the oscillation. These changes in resonant frequency or amplitude of the cantilever can be measured and used to construct the topographical image. See: AFM Working Principle for more information.

Step-by-step guide

Measurement
1. Turn on the anti-vibration table and press (E) to activate isolation. Display reads: 'Isolation enabled'.
2. Choose scan head (small head: 10×10×2 μm, big head: 70×70×14 μm), mount it onto AFM stage and connect it to all cables.
3. Switch on the controller.
4. Clean sample from dust (e.g. using air stream) and mount it onto AFM stage using double-sided adhesive tape.
5. On the computer, start the software 'Nanosurf easyscan 2'.
6. Make sure that the correct .hed file is loaded (status bar on the bottom right of software window). If necessary, load correct .hed file. Settings tab -> Calibration
7. Bring cantilever close to the sample surface. For a rough approach, press and hold 'Advance' button in the acquisition tab. Observe shadow of the cantilever either with the camera or the eye.
8. Press 'Approach' for automatic, final approach.
9. Start data acquistion and adjust image and $z$-controller parameters until the scans look reasonable.
10. To save an image, right click on the image in the gallery (on the right) and select 'Save as...'.

Image parameters
11. Image size [μm]: maximum image size depends on scan head.
12. Time per line [s]: 0.5-2.0 s for image acquistion, sometimes shorter times work better.
13. Points per line: ≥ 512 for image acquisiton.
14. Rotation: usually 0°.

Z-controller parameters
15. Setpoint [% of free vibration amplitude]: the lower the value, the closer the tip moves to the surface. Always use highest value that gives a good image.
16. P(roportional) Gain: reaction proportional to the error signal; ideal value proportional to feature and image size.
17. I(ntegral) Gain: reaction proportional to the integral of the error signal; I-Gain least sensitive to noise; rule of thumb: ratio P:I should be approx. 10:1.
18. D(ifferential) Gain: reaction proportional to ther derivative of the error signal; can usually be kept at zero.

¹For the Nanosurf Easyscan 2, the nominal drive resolution depends on the used scan head. For the small scan head, XY resolution is 0.15 nm and Z resolution 0.027 nm. For the big scan head, XY resolution is 1.1 nm and Z resolution 0.21 nm. However, during actual measurements noise levels might be significantly higher.

1. Atomic Force Microscopy
2. Wikipedia article: Atomic Force Microscopy
3. Nanosurf Easyscan manual (pdf)