''' Mail til Nanosurf '''

Dear Pieter

A week ago you very kindly answered our questions regarding loose tip-clamps on our STMs here at DTU. Now I have another question, I hope you can answer.

This time it is regarding AFM. The situation is, that we have a rather flat surface, roughness of the order of 0.3mu, which we want to study. However the surface is not completely clean, it is contaminated by some spots on the order of 2-5 mu in height.

I can easily scan the surface with the z-range of 6 mu with no problems. However, to resolve the small structures I would like to scan with a z-range of 0.5 mu. This would of cause saturate the z-measurement in the contaminated spots, and I don't mind this. What I want to know, however, is if this approach is safe, or if I might destroy the tip when I saturate the z-range?

Also regarding tip-safety. Can lowering the time/line setting damage the tip by going to low, or can I safely decrease this value as long as the image stays ok?

Very best regards<<BR>>
Robert Jensen 

''' Svar fra Nanosurf '''

Dear Robert,

You can decrease the Z-range so that only the flat part of the surface remains visible. The scanner will still move over the whole Z-range. However, as you can not see how far the tip is moving, you can not be 100% sure that the large structures will not go outside the Z-Range, for example due to drift. If the dirt particles stick out so far, that they go outside the Z-range of the scanner (about 20um), the feedback system will not be able to keep the tip-sample force constant, which may lead to damage to the tip.

To make sure that this does not happen:
 * Turn off the option 'Auto.Adjust Z-Offset'
 * Make an image where you can see how far the large particles stick out of the surface.

Now you can continue with your fine measurements:
 * Decrease the Z-Range.
 * If drift is sufficiently low, keep 'Auto.Adjust Z-Offset' off, or only turn it on shortly before starting the next scan. When Auto.Adjust Z-Offset is on, the image will be deformed around the position of the large particles.
 * While imaging, make sure that the Z-Offset + the height of the large particles do not go outside the Z-Range of the scanner.

Concering scan speed, and tip damage, it is not so easy to see from the measured image. A 0.3um roughess (peak-peak, or rms ?) is rather rough, so here you may easily damage the tip.
The best way to avoid damage is to look at the LeverSig signal. Note that the force acting on the tip is LeverSig times the force constant of the cantilever. If the force becomes too large, you may damage the tip. Depending on how high requirements you have for sharpness of the tip, the force on the tip should remain below a few hundered to a few tens of nN

Best regards,

Pieter van Schendel