Force Volume Procedures

Tip Movement

The force volume data set height image is collected in a slightly different manner than one taken in Image Mode. In Image Mode, the tip is scanned along a line in the XY plane, deflecting to and from the surface as it encounters features. The heights of features are determined by the piezoelectric actuator moving the probe to minimize the error between the setpoint and tip deflection. In Force Volume Mode, the piezoelectric actuator closes the distance between sample and tip until a (trigger) threshold tip deflection is reached. The piezo position at the deflection trigger is recorded as the height of the feature. Tip deflection or another signal type during this approach is recorded as the extending portion of a force curve. Once the height measurement has been made, the piezoelectric actuator retracts one Ramp size and the AFM records the retracting portion of the force curve. The forces measured reflect the Z component of the sum of forces acting on the tip at that location. This process is then repeated at the next XY position in the area.

Force volume imaging always includes executing tip motions as follows:

• The tip is lowered and pressed against the sample surface until the cantilever is deflected to the Trig[ger] threshold value. If the surface is not contacted within one Ramp size distance, the tip is extended one additional Ramp size (for a total distance of two times the Ramp size), then retracted one Ramp size. This means that the tip is incrementally lowered (or “ratcheted”) one Ramp size for each extension-retraction cycle until the surface is contacted, or the Z piezoelectric actuator reaches its maximum extension at 220 volts.
  
• Once the tip contacts the surface and deflection attains the Trig[ger] threshold value, sample height is recorded for that Z-axis threshold. A force curve is recorded for the (X,Y) coordinates after the piezoelectric actuator retracts. The tip is lifted clear of the surface and translated to the next sample (X,Y) coordinates. The entire process is then repeated.

General Force Volume Procedure

The general sequence for obtaining a force volume image consists of the following:

	Obtain a height, deflection, amplitude or phase image of the sample in Image Mode. In the Feedback panel, select the same Data Type intended for force calibrating and for triggering.
	NOTE: A trigger is required to obtain a height image during force volume imaging.
	Obtain a force plot of the sample surface (any portion of the surface suffices). Using the force plot, determine the sample’s general force characteristics and set the following parameters: Setpoint, Ramp size, Z scan start, and Trigger mode.
	NOTE: While not required, using Image and Force Plot Modes to set parameters can quicken the set-up process. Making parameter adjustments in Force Volume Mode can take longer since the SPM responds to the new settings only when it begins scanning a new line.
	Return to Image Mode.
	Obtain a Force Volume image by clicking the Force Volume icon in the Workflow Toolbar. Set the following parameters in the Z Scan panel: Ramp rate, FV scan rate, Number of samples, Display Mode. Set the following parameter in the Force Volume panel: Force per line. Set the following parameter in the Image Scan panel: Samples per line.
	Fine tune Image Scan parameters to obtain a height image. Based upon the experimental design, set parameters in the Force Volume and Force Curve panels.
	Capture the Force Volume image.
	NOTE: This takes longer than a normal image capture—the force volume image may consist of up to 512 samples taken 512 times per line at the Ramp Rate.
	Interpret the Force Volume image using NanoScope Analysis.

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