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Hello everyone,
I'm trying to perform nanoindentation with a Dimension 3100 SPM we have here at the University of Delaware on a range of substrates (thin PTFE films, steel, silicon, glass, etc.) using a TESP tip.
The procedure I follow is to engage in contact mode, then move to the 'Ramp' window. I set a threshold voltage, turn the z-closed loop on and perform a single indent. Most of the times, this seems to work ok and I get a typical voltage-piezo curve. However, many times I run into a situation where upon doing a single-ramp, instead of getting the characteristic indentation curve, I end up with a flat-line on the deflection error versus z (or z_sensor) plots. I've tried increasing set-point deflection (thinking I might actually be out of contact), increasing the 'ramp size' to it's maximum allowable value and I still end up with a flat line (which looks like I'm not even making contact with the sample). Even when I get the flat curves on the ramp plots, if I do a raster scan on the scan window, I am able to pick up topography like I should. I never get this problem if the Z-closed loop is OFF, but with that setting, I get piezo drift which makes the measurement unreliable (please see image).
What has worked for me as a fix is to disengage, move the stage to a new spot on the susbstrate, re-focus the surface and engage (while changing no other parameter). This seems to often fix the problem, but many times it doesn't. I haven't been able to figure out why simple changing to a new location on the substrate would randomly fix the problem, as a flip-side of that, I usually need to move around quite a bit before the flat-lines disappear and I start getting normal curves. Also, I've noticed that sometimes if my x and y offsets are 0, I will get normal curves but as soon as I increase the offset significantly, say to 30 microns or so, I run into the problem again.
The only thing I could think of was not making contact with the surface while I'm in z closed-loop, but I'm not sure the scanner can ramp anything beyond 6 microns (that's the maximum ramp size I'm able to set).
I would really appreciate any suggestions on what you think might be causing this.
http://i933.photobucket.com/albums/ad175/rokmeamadeus/z-closedloopoff.jpg
http://i933.photobucket.com/albums/ad175/rokmeamadeus/rampparameters.jpg
http://i933.photobucket.com/albums/ad175/rokmeamadeus/normalcurve.jpg
Thanks,
Harman.
PS. The software we're using is "Research NanoScope 7.30 (Build R1Sr3.55790)"
Hi Harman,
Thanks for your question and for posting the photos. One concern that I have from looking at the photos is that it looks like the Z range of your scanner is only about 2um, although the software says 5um. This could be causing your problem. If you look at the "Z sensor" vs "Z" curve in normalcurve.jpg, you can see that the Z scale is showing 5um, but the Z Sensor is only measuring 2um. The Z scale numbers are coming from a nm/V conversion number in the calibration file. If the scanner has lost range, it would still show the old values even though the actual movement may be reduced. This is not the case with the Z sensor which is more likely showing the actual range of movement.
One way to check this is to the scan the calibration grid and check the depth of the pits, which should be either 180 or 200nm, depending on the grid. When you do this, you want to bring up "Height" in Channel 1, and "Z Sensor" in channel 2 (or it may be called "Height Sensor") and collect the images of one of the pits. If the depth of the pit is significantly off in the "Height" channel, then you may find that once the scanner is recalibrated that overall Z range is reduced.
My thought is that if you have a significantly reduced Z range, you may be limiting out in Z when creating your force curves during indentation, which would produce the flat line that you are describing.
Let me know if I can be of further help with this. You are welcome to contact me directly at john.thornton@bruker-nano.com.
Thanks,John