Ramp Panel

The Ramp size sets the total range for the Z piezo movement which is scaled along the horizontal axis of the force plot. The units are volts or nanometers. The Ramp size is limited by the full range of the Z piezo.

If the Ramp size is set too small, the indentation may be incomplete because of the scanner’s limited vertical movement. Be sure to select a value which best displays the force plot along the horizontal axis—this scaling cannot be changed after indentation. If in doubt, set the Auto ramp size parameter in the Indent panel to Enabled. This will automatically adjust the Ramp size parameter according to the value of the Trigger threshold (in the Indent Panel). If the Auto ramp size is Enabled, the Ramp size 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 is automatically adjusted to 50 V. Note that the Ramp size remains at 10 V for all Trigger threshold values below 0.2 V.

The Scan rate is the speed, in Hertz, at which the cantilever/sample is loaded and unloaded during indentation. If set to 1Hz, an indentation will take 1 second to execute.

Although the Scan rate is limited to a range of 0.01 Hz to 200 Hz, values from 0.5 Hz to 10 Hz are typically used for indentation.

The number of data points collected during a downward (extension) or an upward (retraction) travel cycle of the piezo when capturing a force plot during indentation. This parameter defines the resolution of the force plot. The Number of samples can be set to discrete values from 16 to 64000. The higher the value, the greater the detail shown on the force plot. Typically, it is set to 256 or 512. Higher values will increase the amount of disk space necessary to store the force plot and vice versa.

The Spring constant parameter is used to record the spring constant of the indentation cantilever that is currently being used. This parameter is input by the user and is recorded along with each force plot captured. It is used for off-line analysis of the force plot only. This parameter does not effect the real-time indentation process. It is not critical to set the Spring constant, since it can be altered in the off-line analysis of the captured force plot.

The radius of the rounded portion at the apex of the probe tip.

Poisson's ratio is a measurement of a body under longitudinal stress, specifically the ratio of the transverse strain to the longitudinal strain in the direction of stretching force.

The angle formed by the center line of the probe tip (cone) and the exterior edge of the probe tip.

X Rotate allows the user to move the tip laterally, in the X direction, during indentation. This is useful since the cantilever is at an angle relative to the surface. One purpose of X Rotate is to prevent the cantilever from plowing the surface laterally, typically along the X direction, while it indents in the sample surface in the Z direction. Plowing can occur due to cantilever bending during indentation or due to X movement caused by coupling of the Z and X axes of the piezo scanner. When indenting in the Z direction, the X Rotate parameter allows the user to add movement to the scanner in the X direction. X Rotate causes movement of the scanner opposite to the direction in which the cantilever points. Without X Rotate control, the tip may be prone to pitch forward during indentation. Normally, it is set to about 12.0 degrees. This parameter typically ranges between 7 and 25 degrees.

Figure 1: The tip engages the surface in TappingMode, then begins indentation. At engagement (1) the tip is oriented normally; however, as the tip is pressed into the surface, it tends to pitch forward (2). By applying a slight X-axis offset (3), the tip is brought normal again.

The images shown in Figure 2 all use the same Trigger threshold value and demonstrate the effect of X Rotate at various settings. Notice that the indentation is larger for a value of 0.0 degrees, and less for an X Rotate value of 20.0 degrees. Notice also that there is material deposition (pileup) on the left, outboard side of each indentation depending upon the amount of correction. The pitching forward of the cantilever during nanoindentation tends to move the laser spot in a direction opposite to normal deflection. This produces a counter-effect that may result in less deflection at the photodetector but higher forces. The end result is deeper, larger dents for lower X Rotate values.

Figure 2: The effect of various X Rotate values for dents made in the same material (1 mm gold film sample) with the same force.

The approach (Forward) and retract (Reverse) speed of the probe as it performs a complete ramp cycle.

The X offset and Y offset are used to adjust the position of the tip in the X-Y plane. Units are volts or microns. In Indent mode, the X and Y offsets are used to offset the tip to execute a indentation at a different location. Their values are limited to a range of –220 to 220 Volts. The X and Y offsets are further limited by the Scan size, since they are used to offset the X-Y scan in image mode. A large Scan size will reduce the range of the X-Y offsets, since they define the center of the scan. Similarly, large X-Y offsets will reduce the maximum Scan size possible.

The Units parameter allows the user to switch the units displayed for certain parameters. The Units parameter has two settings:

• Volts: Displays parameters in Volts, where applicable.
• Metric: Displays parameters in microns or nanometers, where applicable.
• Force: Displays the parameters in metric force units, where applicable.

Changes the force plot to display only the Extend portion of the plot, the Retract portion of the plot, or Both the extend and retract portions of the plot (default).

Turns the closed loop for the Z ramp On (default) or Off.