Indent Panel

The Trigger threshold (abbreviated as Trig threshold) is the value of the cantilever deflection (as measured by the photodetector) desired for the indentation. The Trigger threshold defines the maximum force applied to the sample corresponding to the upper rightmost point on the force plot (see the example force plot shown in Figure 1).

Figure 1: Force Plot Showing the Location of the Trig[ger] Threshold

The Trigger threshold controls the amount of force applied to the sample. When indenting, the tip is pushed into the sample surface until the cantilever deflection equals the Trigger threshold. At this point, the tip is lifted up away from the sample to remove the load. The cantilever deflection is measured relative to the value of the deflection at the surface contact point, located somewhere on the flat portion of the force plot where the force is zero. The Trigger threshold is specified only in volts, because the nanometer representation is incorrect unless the cantilever sensitivity is measured. The cantilever sensitivity (labeled TM Deflection Sens. on software panels), located in the Channel 1 panel, is the conversion from volts to nanometers for the cantilever deflection. Refer to Interpreting Hardness Data for more details on cantilever sensitivity.

The Indent setpoint parameter serves as a multiplier to the Amplitude setpoint and affects the point at which the indentation or force plot is triggered. It is in effect only for nanoindentation and is disabled at all other times.

Nanoindentation uses the TappingMode function to find the surface when executing an indentation. Before penetrating the surface, the cantilever is oscillated using the drive frequencyFrequency of the signal used to oscillate a tip in TappingMode. and drive amplitudeAmplitude of the signal used to oscillate a tip in TappingMode. previously set in the Scan view. Then, the tip is moved toward the sample, until the amplitude of oscillation of the cantilever has been reduced to a predetermined amount. This predetermined amount is specified by the Amplitude setpoint, also previously set in the Scan view, and by the Indent setpoint, and is equal to the Indent setpoint times the Amplitude setpoint.

For example, if the Amplitude setpoint is set to 200 mV and the Indent setpoint is set to 0.9, then the predetermined amount would be 180 mV (90% of the Amplitude setpoint). The Z position at which the amplitude is reduced to this predetermined amount is the surface contact point. Finally, with the oscillation off, the tip is forced into the sample until the cantilever deflection, measured relative to the deflection at the surface contact point, is equal to the Trigger threshold (Figure 2).

The Indent setpoint is useful in cases where the free-air (pre-contact) part of the deflection curve is not flat. If this is the case, the maximum deflection and force during indenting will vary depending on where the indentation was triggered. The Indent setpoint allows the user to move the surface contact point closer to the point where cantilever deflection begins (see Figure 2). A typical range for Indent setpoint is 0.5–1.0; a good default value to use is 0.9.

During the tip’s descent to the surface, the graph will reveal the surface contact point as a vertical yellow line near the ramped (sloped) portion of the plot (see below).

Changes to the Indent setpoint will cause shifts in the surface contact point on the force plot: increasing the Indent setpoint will shift the surface contact point rightward on the plot; decreasing the Indent setpoint will shift the surface contact point leftward (Figure 2). Generally an Indent setpoint of 0.9 is recommended.

Figure 2: Changes in Indent Setpoint will shift the surface contact point left or right

Auto ramp size affects the ramp size (Z distance) during ramping. This parameter has two settings:

• Enabled: The Ramp size parameter is automatically adjusted to a value 50 times the value of the Trigger threshold. For example, if the Trigger threshold is set to 1.0 V then the Ramp size will be 50 V. Note that the Ramp size remains at 10 V for all Trigger threshold values below 0.2 V.
• Off: Turns off automatic adjustment of the Ramp size parameter. In this case, the Ramp size must be adjusted by the user, depending on the indentation force or Trigger threshold used, in order to optimize the force plot.

The Lift height parameter should always be set to 200 nm. A value less than 200 nm may cause the diamond tip to crash into the surface during the “fast approach” process.

The Lift height parameter is used to speed up the indentation process. Its definition will become clear through an explanation of the indent process:

• Prior to executing an indentation, the indentation probe is held above the surface at a height proportional to the current Z scan size value.
• During indentation, the tip is then moved quickly towards the sample surface (a fast approach), until the distance between the tip and sample is equal to the Lift height.
• When the tip reaches the Lift height the amplitude feedback is enabled, causing the probe to approach the surface (a slower approach).
• The Z position, at which the oscillation amplitude of the cantilever reaches a predetermined setpoint (Amplitude setpoint x Indent setpoint), is used as a reference for the surface for the upcoming indentation. The Lift height is the height at which the “fast approach”, changes to the “slower approach”.

The Surface Delay setting causes the probe to pause for a specified time after an indentation before withdrawing. Without a surface delay, the tip is retracted immediately after the indentation has been made. For samples which exhibit “flow”, surface delay may be appropriate.