How do you use the standards mentioned?

Once one becomes proficient enough, mounting a new tip, aligning the laser, engaging the surface and obtaining an image should take less than five to ten minutes. With experience, an AFM operator should then be able to quickly assess the quality of the tip simply based on the image of such a known standard.

This can assist in establishing a good laboratory practice of keeping high quality tips on hand. For example, one could spend an hour whenever necessary to assess and maintain a reserve stock of good tips. Once the AFM tip in use becomes suspect, it can be set aside to be cleaned or discarded, and imaging can continue with one of the good tips without too much interuption in the work schedule.

The most important aspect of using a standard is knowing what a good image should look like. Here are some examples of the standards used. Click on the small picture to get a full view of the images mentioned, then the BACK key on your browser to restore this page.


Titanium Thin Films

The titanium thin films are very good for assessing the sharpness of an AFM tip. I find that a 2x2 micron image can easily be done in air without the aid of vibration isolation at a scan rate of around 2 lines per second, so at 256 lines per image this takes very little time. Remember, we just want a quick idea of the tip quality, so we can forgoe the need for high resolution images. Good Ti sample

For these Titanium thin films, a good image should have sharp edges to the features. It is important to look for features which might repeat themselves (indicating a double tip or worse). Also, one should be able to use scan parameters they find typical, such as the scan rate and gain settings for that particular mode of operation.
Good Ti sample

Not all the features in the film have to be sharp - the film may have picked up some contamination (such as dust) or some minor damage to the surface was caused while engaging the surface or during the scan. Look for sharp features elsewhere in the scan to determine if the blotches are due to the tip.
Good Ti sample

Of course, nothing is perfect - even using the thin film can cause damage to the tip, especially if the tip is very fragile. Here, the tip broke or had a large contaminant dislodged while scanning down. The new image looks better than before, but should be set aside as it is now suspect.
Good Ti sample Here's an example of a tip which isn't too bad, it just isn't very sharp, as indicated by the rounded edges of the film's structures.
Good Ti sample This is a subtle example of a double tip. Note that many of the features have twin vertical components, while some don't. If there is any question about the tip qulity, best set is aside.
Good Ti sample Here is an image taken by a multiple tip. Note how all the features have what looks like two or three vertical partners.
Good Ti sample This is an image of a triple tip which is so huge it might be hard to spot the defect if one focuses on a small area for their tip evaluation. This is a good example of where the AFM operator should be able to immediately identify a bad tip and move on.
Good Ti sample Sometimes you just can't figure out what is wrong with the image - it just doesn't look right! Best to set this one aside and evaluate another.


"TGT" Spikes

The "TGT" standards made by NT-MDT are neat spikes around 1 micron high spaced about 1.5 microns apart. These represent a "delta-like function", where the sample is so sharp the image is only of the inverted tip. Although great in theory, imaging these requires a very slow scan rate and high gain settings. A very good tip will show sharp spikes - too sharp and the image could be of the sample itself. An isometric view of one of these shows the body of the tip, ideal here for samples with sharp, deep holes.

The images can look very interesting, especially for bad tips......

A formation of B2 bombers?

A school of fish, maybe? Note that the "eyes" are rounded tips about 100 nm in radius.

AYEEEEEE. The return of Space Invaders!!!!


Spheres

Spheres, either colloids, polystyrene, or other material, represent a favourite calibration standard. Unfortunately, their images are fairly boring.

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Last revised on 03-02-1999 by Peter Markiewicz