Troubleshooting

If the indent force is not sufficiently large, an indentation will not be visible on the sample surface. Try indenting the surface using a larger Trigger threshold; increase the Trigger threshold by 0.1–0.2 V increments until an indent is visible on the sample. In general, indentations will not be visible on most samples for Trigger threshold values < 0.2 V.

A Trigger threshold of 1.0 V should almost certainly result in an indentation on the sample surface. If not, see below for other causes of this problem. Note that successful indentations should be visible when imaged using scan sizes from 1–3 μm.

It is not uncommon for the diamond tips to pick up debris from the sample when indenting and, in particular, when scratching. Like a dull tip, a dirty tip may require more force to successfully indent or scratch. Also, a dirty tip can cause irregular (not triangular) indents to be made. Possibly, no indentation can be made, even using the maximum force available.

If the tip is dirty, the user should also notice a loss in the image resolution. A dirty tip, like a dull tip, will not resolve fine features. This becomes most apparent when comparing images before and after a nanoindentation operation; if the image has degraded noticeably, it is probably contaminated. To clean the tip, try the procedures below (listed in the order shown they should be attempted).

Tip Cleaning Procedures

1. Indent on the Sample

Position the tip over a new section of the sample and perform multiple (3–5) indentations at the same location on the sample.

On harder samples, such as DLC (diamond-like carbon) films, limit the Trigger threshold to about 0.5 V. Hopefully, these moderate force indentations will knock the debris off the tip. Do not use larger forces than are necessary; large forces on very hard samples may cause damage to the diamond tip.

On softer samples, such as the gold film provided with the nanoindentation package, use a larger Trigger threshold such as 2.0 V. Performing indentations with large forces on a very soft sample may knock off the tip debris.

If the hardness of the sample is not known, use the gold sample provided. The gold sample is soft enough to insure that the diamond tip is not damaged, even with the largest indent force, during the tip cleaning procedure. The gold sample also has good topographic features to determine if the image resolution has improved.

If the tip has been successfully cleaned, the image resolution should improve.

2. Indent on the Soft Gold Sample

In general, cleaning the tip by indenting is more successful on a soft sample with large forces than on a harder sample using moderate forces. Thus, if the sample is relatively hard, it may be helpful to engage on a soft sample, such as the gold film provided with the nanoindentation package, and try indenting as described above. Use a large Trigger threshold, about 2.0 V, and indent multiple times in the same location.

If the tip has been successfully cleaned, the image resolution should improve.

If the first attempt fails to clean the tip, offset to a new location on the sample and repeat above. This method is not always successful on the first attempt.

3. Indent on the Soft Gold Sample with Increased Force

If methods A and B are unsuccessful, it may prove useful to make an indentation on the gold sample with even more force than possible in Indent mode. This should only be attempted if the previous two methods fail to clean the tip, and should only be carried out on the gold sample provided with the nanoindentation package, or a similarly soft sample.

4. Indent on Soft Gold Sample with Maximum Force

If method C above is not successful, it may be helpful to further increase the indent force used for cleaning the tip. This procedure is similar to method C, except that the vertical stepper motor is used to maximize the range of the Z piezo prior to the tip cleaning indentation. Do the following in conjunction with method C, above:

1. Engage the microscope on the gold sample using TappingMode.
2. Select Motor > Step Motor from the Real-time menu to open the Motor Control panel.
3. Set the SPM step size to ~0.5 μm.
4. Click on the Tip Down button until the scanner is almost retracted.

The motor may not move the SPM until multiple steps of the motor are executed. Continue stepping the motor until the line on the Z Center Position bar moves to a position near the Retracted side. The Z Center Voltage, displayed near the Z Center Position bar, should decrease to approximately –200 V.

5. Perform steps #2–5 in method C, above.

If the photo-detector does not have enough range to allow for the specified cantilever deflection, the indentation will fail. This commonly occurs because the zero vertical deflection level has drifted within the photo-detector’s range. The level of zero vertical deflection is the level of the flat/horizontal portion of the curve in the force plot. Since the Trigger threshold is measured relative to this zero deflection level, drift can cause decreased range for indenting.

The full range of the photodetector is –10 V to +10 V. An increase in cantilever deflection causes a positive voltage change. If the zero deflection level has drifted to a value of 1.0 V, then the cantilever will only be able to deflect by 1.5 V (2.5 minus 1.0 V), before the laser beam moves out of the photo-detector’s range. Hence, even though the Trigger threshold can be set to a maximum of 2.50 V, this amount of deflection may not be possible if the zero deflection level has drifted.

If the Trigger threshold is out of the photo-detector’s range the software will not perform an indentation and the force plot will appear as a flat horizontal line. A message should appear on the screen, near the force plot, showing the word “Limit” in red. If this occurs, it is necessary to adjust the zero deflection level to a position lower on the photo-detector, preferably in the center.

Withdraw the probe before set the Vertical deflection to zero. This is equivalent to setting the zero deflection level to zero on the photo-detector. Typically, the zero deflection level will be near zero after engaging but can drift after scanning for a period of time.

An indentation attempt may fail if the Ramp size is too small. The Ramp size needs to be sufficiently large to provide enough Z movement for the cantilever to deflect the required amount. Indenting on a soft sample will require more Z travel than indenting on a hard sample, since the tip will penetrate deeper into the soft sample before the Trigger threshold is reached. Also, larger Trigger threshold values will require larger Ramp size values. Thus, the Ramp size should be set to different values, depending on the sample and the indent force used.

If the Ramp size is insufficient, the force plot will appear as seen in Figure 1. Simply increase the Ramp size until a typical force plot is obtained. If necessary, disable the Auto ramp size parameter, which sets the Ramp size to a value 50 times the Trigger threshold.

Figure 1: Force plot having an insufficient Ramp size.

It is not uncommon for the diamond tips to pick up debris from the sample when indenting and, in particular, when scratching. A dirty tip can cause not only problems with indenting the surface, but also imaging the surface. In particular, a dirty tip will not resolve fine features as well as a clean tip.

See Diamond Tip is Contaminated, above, for a description of this problem and a variety of solutions.

In some cases, indentation cantilevers can have unfavorable amplitude response, which may cause poor imaging quality. The signs of poor cantilever response can be seen in the cantilever tune or frequency sweep, some of which are listed below:

• Amplitude peak is not well defined.
  1. The frequency sweep shows multiple resonance peaks.
  2. The resonant peak is not as steep and sharp as usual.

The optimum frequency sweep consists of a single symmetric amplitude peak, which is only a few kHz wide.

• Requires unusually high Drive amplitude to obtain the required 0.25 V RMS amplitude (typically, the Drive amplitude is below 500 mV).

If any of the above is true, the cantilever is not responding optimally. It is still possible to engage the microscope and image the sample, but the image quality and resolution may or may not be as good as usual. Try the following to improve the cantilever response:

1. Re-position the indentation probe within the cantilever holder.

After moving the probe, it will be necessary to re-align the laser and to adjust the laser spot on the photodetector. Check the cantilever tune for an improved response.

2. Remove the indentation probe and clean the cantilever holder using one or more of the following methods:
   1. Clean the surface using a cotton swab and alcohol.
   2. Blow the holder clean with an available air source.

After cleaning the cantilever holder, re-install the indentation tip carefully. It is necessary to re-align the laser and adjust the laser position on the photodetector before checking if the cantilever response has improved.

If the tip is not tracking the surface topography well, the resonance peak may have shifted. This is a common problem with indentation probes during prolonged operation of the microscope. If the resonant frequency shifts enough, the Drive frequency, set prior to engaging the surface, may not coincide with the resonance of the cantilever, causing imaging problems.

One possible solution is to slightly lower the Amplitude setpoint. While adjusting the Amplitude setpoint, look at the scope traces to determine how well the tip is tracking the surface.

If lowering the Amplitude setpoint does not improve the image, withdraw the probe and view the frequency sweep in cantilever tune. If necessary, adjust the Drive frequency to center it on the shifted amplitude peak. Then, adjust the Drive amplitude, if necessary, to obtain the appropriate RMS amplitude for the cantilever. Re-engage the microscope and check if image quality has improved.

This procedure may be repeated periodically if the tip has problems tracking the surface features. Alternatively, perform a surface tune while engaged.