Setpoint and Integral gain seems to be vital parameters that control the scanning of the sample surface during scanning probe microscopy. How do these parameters control the final image characteristics?
Setpoint is a value which is stabilized during scanning with an active feedback loop. Depending on the measurement mode it can be a tunneling current intensity (STM), an amplitude of a cantilever (AFM tapping mode) or a strength of the pressing force (AFM contact mode). In AFM techniques it characterizes the strength of interaction between the tip and the surface, as Janusz wrote. If it is too large the tip apex or the sample surface can be easily destroyed. Too low value makes contact with the surface too weak and topography images becomes unreliable.
Integral gain is a one of gains describing a feedback strength. Too low value makes that response from controlling electronics box is to slow. Then, the registered profile does not correspond to a real one. It can be easily noticed when you compare trace and retrace profiles. In the case of too weak gain you should see the “tails” behind the measured protrusion (thus from the left or right side of protrusion, depending on the scan direction – trace or retrace). Too strong value of the gain introduces high frequency excitations easily visible both at the topography image and the profile line.
Both parameters should be tuned by an operator just after the scan starts. A certain experience is necessary to do this properly, however the tips given in manuals are helpful.
Setpoint is the force applied via the tip to the sample's surface. Gain (both, depends of mode - integral and/or proportional) is value to reduce the noise in the image. There is no way to tell how AFM parameters You have to use. It depends of scansize (100nm - better faster scanrate), height of peeks/holes, scanrate, etc. You will have always adjust parameters for each measurement. Start at slow scan and change parameters to make lines (trace and retrace) be the same. Of course, all depends on sample, system, environment, tip, mode. Maybe, if You would let us know what material, what system, what you want to measure, we could give some recommendation.
Thankyou very much Rebis for your valuable input. I am trying to perform scanning probe microscopy (SPM) on a high carbon multiphase steel. Since phases formed are few microns in size, to get the hardness of the individual phases, I am performing nanoindenation (NI) after getting the image from SPM on a Hysitron nanomechanical system. For NI, Berchovich tip is being employed. Since the image that we are getting in not a high quality image we are stuck as we can't differentiate the individual phases.
Preparation of steel (because of corrosion) is difficult. Best results I got after electrolytic polishing or grinding sandpaper 2500. Most important thing is to measure quickly after specimen preparation. Are You sure You got flat samples w/o oxides? Another problem with AFM may be the Z scale (in Veco/Bruker/DI nanoscopes is aprox 6um). Higher differences than 6um on the sample may cause inability to study. If I understand You well, you have Hysitron nanoindenter? If so, AFM head in this device is only for 'preview' - if I remember well - in contact mode. If You got problems with obtain picture try to change gain from small to higher value. For differ materials in CM I changing gain in range 0.2 to 30 (on nanoscope). Always try to start imaging from small area - if You will be able to scan 1x1um - You will be sure SPM works ok. On the other hand, maybe using AFM (not spm head from Hysitron) in PhaseImaging, ForceModulation or MFM could show You differences of phases?
Setpoint is a value which is stabilized during scanning with an active feedback loop. Depending on the measurement mode it can be a tunneling current intensity (STM), an amplitude of a cantilever (AFM tapping mode) or a strength of the pressing force (AFM contact mode). In AFM techniques it characterizes the strength of interaction between the tip and the surface, as Janusz wrote. If it is too large the tip apex or the sample surface can be easily destroyed. Too low value makes contact with the surface too weak and topography images becomes unreliable.
Integral gain is a one of gains describing a feedback strength. Too low value makes that response from controlling electronics box is to slow. Then, the registered profile does not correspond to a real one. It can be easily noticed when you compare trace and retrace profiles. In the case of too weak gain you should see the “tails” behind the measured protrusion (thus from the left or right side of protrusion, depending on the scan direction – trace or retrace). Too strong value of the gain introduces high frequency excitations easily visible both at the topography image and the profile line.
Both parameters should be tuned by an operator just after the scan starts. A certain experience is necessary to do this properly, however the tips given in manuals are helpful.
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