Height scaling and calibration - scanner sensitivity, force curves and offset deratings

Luis posted on Sat, Feb 22 2014 7:46 AM

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Hello, I've been trying to understand how the NanoScope software scales and calibrates the height data.

This has become particularly important to me in order to understand the hysteresis observed in force curves obtained with a Dimension 3100 microscope and also shown in the microscope's manual (http://nanoscaleworld.bruker-axs.com/nanoscaleworld/forums/p/1505/4527.aspx#4527)

A typicall FC plot may show deflection vs piezo height. The height or Z signal (not the height sensor) is obtained by assuming a linear applied voltage ramp (of slope: HardValue of Ramp size Zsweep/ samples per line) and then multiplying by the sensitivity value (Sens. ZSens) to scale from volts to nm.

I thought I understood this scaling procedure, but when I read the calibration chapter of the manual I got confused.

There it is explained how the intrinsic hysteresis and nonlinearity of the piezo is corrected by appliyng a nonlinear voltage curve to the piezo, differently when extending and retracting. This correction is supposed to make the piezo move linearly with time. 

1) If a nonlinear voltage is applied to the piezo, why does the software assumes a linear voltage ramp?

It also says that the scanner's sensitivity changes with the applied voltage, scan rate and scan size. 

2) How is this taken into account? 

3) Does it have to do with the offset derating parameters?

4) Shouldn't the sensitivity (Sens. ZSens) change for different scan rates and scan sizes?

5) If Sens. ZSens is fixed after the calibration, is this just the correct value for the specific scan rate and scan size used the day of the calibration?

I am really thankfull to anyone that can give their opinion on this issues. Maybe after some first comments I'll ask about some details of the offset deratings.

Thanks again,

Luis