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Hallo everybody,
I am using a multimode afm with a nanoscope IIIa controller, AS-130VLR scanner and nanoscope 5.31r1 software.
When recording force-distance-curves at different ramp-sizes for determination of deflection-sensitivity i observe a dependence of deflection-sensitivity on ramp-size.
This dependence is not an effect of scanner calibration. I tested this by calibrating the scanner at different heights (52 nm & 200 nm) and recording force-distance-curves with the proper z-sensitivity. The deflection-sensitivity in this case differs about a factor of 3!
To determine the "real" deflection-sensitivity i scanned a calibration-grating in "constant-height-mode" (I-Gain = 0.0001; P-Gain = 0) and calculated the deflection-sensitivity from the deflection-image.
An example of this dependence is shown in the graph. Especially confusing are the two occurring jumps at 700 nm and 1400 nm.
Please could you tell me how to determine the deflection-sensitivity in the right way?
Thanks a lot in advance :)
Best regards
Jonas
Jonas,
This idea about a bad Z board makes a lot of sense. NanoScope controllers use three digital-analog converters (DACs) per axis. One controls offset, another controls the scale, and another performs the actual scan. The concept is that the full bit resolution of the DACs is available no matter if you are doing a very large scan or a very small scan. If the scaling DAC were not functioning then it would affect the calibration for that axis. The less obvious point is that this scaling DAC is used in ramp mode, set to something a bit larger than the selected ramp size. So as you change the ramp size the scaling DAC changes too. This theory also explains why a second scanner didn't fix the issue.
This is actually pretty easy to test. If you scan the calibration grating with different "Z limit" values you would expect to measure the same pit depth at each Z limit value. If the values are off considerably then it indicates a problem with the Z scaling DAC. Be sure to set gains to track the pits, not zero gains like you did before.
If it's Tech Support suggesting this cause then they've probably seen it before.
Regards,
-Ben
Hi Jonas,
To calibrate deflection sensitivity calibration reliably here are a few suggestions:
1) Always calibrate on a clean sample that won't be indented significantly by your probe. The NSC36A probe has a nominal spring constant of 0.95 N/m. It should be fine to calibrate this on glass or silicon. For stiffer probes you might need to use a harder sample like sapphire. Note that this might mean you need to calibrate on a different sample than the one you want to measure. You need to be especially careful with biological samples. Proteins and other compounds can absorb on substrates and cause errors in deflection sensitivity calibration.
2) It's good practice to try to keep the non-contact deflection value near 0V. This is where the detector is most linear. So let any thermal drift settle and adjust the vertical deflection to 0V before engaging. Also engage near 0V on the scanner Z voltage (i.e. center of the Z piezo range).
3) Calibrate the deflection sensitivity over a 1-2V region of the deflection signal in the contact portion of the curve. Split photodetectors become non-linear over large deflections. So it's best to keep the calibration and the actual force measurements within a smaller range of the full detector range.
4) Be sure to collect enough points per curve. You want to have plenty of points in the region of the curve where you calibrate. There's really no reason not to collect at least 2048 points per curve.
5) Probably obvious, but be sure that you are getting good "normal" force curves with a flat baseline and a linear contact region. The only feature should be some modest adhesion, especially if done in air. If the curve is wavy, a huge adhesion distorts the whole thing, the contact region is obviously non-linear or the curve otherwise looks odd then it's a good idea to sort out that problem before continuing to calibration.
These are the main points to consider. The AS-130VLR scanner is an open-loop scanner. So there might be some additional error due piezo non-linearity and some hysteresis between extend and retract curves. You can compare deflection sensitivity on the extend and retract curves to get a sense of the size of this error. It should not be very large, certainly not factors of 3 or anything near that. Calibrating the same lever over and over I might expect scatter on the order of maybe 5% or so when following these guidelines.
It's hard to guess what is causing the wide range of values that you are seeing here. It would help to see the actual data files. You can send those to me if you like at ben.ohler(at)bruker-nano.com and I'll review them and respond back.
Hallo Ben,
thanks for your help.
I will send the actual data files to you and would be great pleased if you could take a look at those.
Hi Jonas,We looked at your files and indeed the behavior is strange. We think there may be an equipment problem. Do you have an extra scanner? If so can you repeat the measurement with the different scanner, using a different probe, on a silicon sample? If you still have a problem, we can set up a TeamViewer session to take the next step.Thanks,Steve
Hi Steve,
I don't have another scanner at the moment but i will try to organize one.
Unfortunately in the next three weeks i will not have the possibility to do new measurements.
OK. Keep us posted. It sounds like you are doing everything right, so this may be a service issue. If you will be out for 3 weeks you may want to consider contacting tech support, so the repair can be underway while you are gone.
http://www.bruker-axs.com/nano_surfaces_support.html
Best,
Steve
Thanks a lot for your help, I will keep you posted. Unfortunately time is too short to contact tech support...
Hallo,
I repeated the measurement with a D-scanner and a NSC36B(Si3N4) cantilever on a Si-substrate. The data is shown in the graph. The same strange behaviour occurs like in the measurement before. So I think the scanner should be OK...
What do you think is the reason for this behaviour and what should I do next?
Thanks a lot for your help.
is it possible that the z-board of the controller is responsible for this?
It is difficult to diagnose your issue via the forum. We suggest to contact your closest Bruker service center, you can find the contact info here: http://www.bruker-axs.com/nano_surfaces_support.html
Stefan
Hallo Stefan,
thanks a lot, I already did.
Hi Ben,
you are right. I recorded a calibration grid with different z-limits and observed a change in height of about 20nm! The jump occurs between the z-limits 716nm and 717nm. I also observed a jump in deflection-sensitivity when recording force-distance-curves without trigger at a ramp-size of 716nm and 717nm.
Is there a chance to fix this on my own?
Ben Ohler: 5) Probably obvious, but be sure that you are getting good "normal" force curves with a flat baseline and a linear contact region. The only feature should be some modest adhesion, especially if done in air. If the curve is wavy, a huge adhesion distorts the whole thing, the contact region is obviously non-linear or the curve otherwise looks odd then it's a good idea to sort out that problem before continuing to calibration.
If my baseline is really wavy, what could this be caused by? Is there any way to troubleshoot it?
Hi Rachelle,
I think it's caused by interference.
May be it helps when you rotate or tilt your sample by a few degrees so that the light deflected from the sample surface doesn't hit the detector.
Best regard