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Hi,
Sorry to be back on the forum so soon, but I'm having a few issues on the caliber scans that I wanted some help with:
1. I've been noticing for several days that when I engage my tip to the sample (the tip is MLCT-B and the sample is a sputtered coating on steel substrate), as soon as the tip is engaged, the laser spot starts jumping in the vertical direction rapidly. This doesn't visibly affect my scan (or that I can see), but I also noticed that once this 'jumpiness' sets in, the AFM head starts making a continuous but faint 'squeek' sound. It is very faint, but I observe it only when my laser (when engaged) is unstable. I tried leveling the head better, and sometimes it seems to fix the problem, other times (such as today), leveling the head does nothing. Further, the laser is unstable with a range around zero (say, plus/minus 1-2volts around zero). I also have my substrate grounded.
2. A second problem I noticed today was with some of the topographic scans I did on the same substrate. I seem to be getting specks on the image, which show up as spikes in the line profiles. These aren't contamination because I noticed they don't scale with scan size. I'm not sure if this is related to the first problem or if it is independent of it. I started noticing this problem only after I went to scans equal or smaller than 10um by 10um; this did not show up in the 100 or 50 squared micron scans. I have a jpeg of a sample scan I did where this problem showed up:
http://i933.photobucket.com/albums/ad175/rokmeamadeus/Untitled.jpg
Any help with these problems is greatly appreciated!
Best,
Harman.
Hi Haman,
1. The cantilever movement you see is probably either coming from a PI force-feedback gain that is too high or from your xy closed loop feedback being to high. For the xy closed loop: Please open your closed loop feedback dialog and reduce your I gain by 0.1. You PI force feedback gain can be changed in the SCNCTRL dialog.
Another way determining if your xy closed-loop feedback setting is the culprit: Open the Closed-loop feedback dialog. Simply turn one axis (x or y) off by unchecking that box with your mouse. If the noise disappears it is your closed-loop thats causing it. Now reduce the feedback gain accordingly.
2. The "specs" you see are probably related to the noise you see. When you hear noise it means something is vibrating and that has to be adressed first before proceeding.
Stefan
Hi Stefan,
Thanks again. Adjusting the PI does get rid of the instability.
I do have a related question about the gains. Since I am primarily interested in the lateral (friction) signals, suppose from one scan to another (or within a single raster scan), I were to change the gains, would that change manifest in the raw voltage signals I get for the lateral signal (specifically, for the loop-width of the forward-reverse signals)?
The idea is that I want to keep the laser stable, but at the same time not let the gains affect what I perceive to be the value of friction I'm measuring.
Thanks,
Hi Harman,
Which PI gains did you change? The force feedback gains or the closed-loop settings? If the closed-loop settings were wrong and you changed them it is advisable to write them down and apply them also to the "Tapping Mode" workspace.
Your frictional signal is of course dependent on the (normal) load that you apply to the cantilever-tip. Thus it is imporntant the keep the force-feedback loop well adjusted, scan slower if needed, to ensure your setpoint does not fluctuate too much. Another good idea may be that if you record "friction loop" to temporarily reduce the "overscan" to "zero". Set it back to approx 10% for Caliber for imaging. If you haven't done so Rene Overney has performed a ton of great work on friction using AFMs. his website is here: http://www.washington.edu/research/energy/researcher/ren%C3%A9-m-overney
best,