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TT-AFM noise floor measurement

 

This procedure can also be applied to other AFMWorkshop instruments. For tip-scanning instruments such as the LS-AFM or NP-AFM, the results will show larger noise floors, while for the HR-AFM, extremely lownoise floors can be achieved. The TT2-AFM is practically identical to the first-gen TT-AFM in terms of noise floor.

 

 

Procedure

 

1. Place a clean silicon calibration sample on the scanner.
2. Place a new probe in the instrument.
3. For this comparison, use vibrating mode. Setup the optical alignment and Tune Frequency as normal.
4. Select parameters to test are listed below. note that for a fair comparison, you can use parameters relevant to your typical measurements. The parameters described here, will give you an “ideal” value, i.e the best result possible.

 

Suggested parameters 

Parameter

Suggested value

X Gain %

0

Y Gain %

0

XY HV Gain

Initially 1 (note it should be 0 for the actual measurement)

Z HV Gain

3*

Image Add

Off

Z Feedback values

Values you typically  use, for example Gain 1.5, Proportional 150, Integral 1500

Lines

128

speed

1 Hz

Left Image

Z_DRIVE

Samples/Pixel

25

 

* Z HV Gain is the most important parameter controlling the noise,  if the measurement is being made under low-noise conditions. Lower values will give lower values. However, lower values also decrease the overall z range of the scanner, making approaching problematic in non-ideal conditions. For a 17 microns scanner, decreasing this value may reduce noise further. Below a value of 3, it's unlikely to improve noise much. 

Note that you can use any different parameters you like for this, and they can and will alter the results that you get. Also, the instrument should be properly calibrated in z to allow comparison with any other values. Be aware that tip approach with Z HV gain at 5 will be slow, and some post-approach adjustments might be necessary.(i.e Jog Down).

 

5. Go into feedback, and scan an image of a small area of the surface (XY HV Gain of 1). The image should be clean, with very few features visible. Ensure you end the scan with the probe on a flat part of the sample.

6. Withdraw probe.

7. Set scan size to 0, by using XY HV Gain = 0.

8. Approach surface again.

 

NOTE: It is important that you go into feedback on the surface in the same way as you did when you scanned an image. If you go into false feedback (probe almost but not quite, on the surface), you will not make a valid noise floor measurement.

9. Scan another image. No sample features should appear, as scan size is zero. The image may look something like the image below, or have some regular patterns in it.

noise-floor
1
0. Save the file, and open the Left image -  z piezo drive file (height image) in gwyddion.
11. Apply a 1st order polynomial levelling (fit linear).
12. Use “Statistical Quantities” and record the RMS average roughness (Sq). This is the noise floor

You should get a value of <1 angstrom (Gwyddion reports this in picometers, typically, so you expect to see a value of <100 pm).

If you do not get a satisfactory value, try removing sources of external vibration (other machinery, acoustic noise, unnecessary cables) from the instrument. Ensure probe and sample are grounded. Ensure that the vibration isolation system is setup properly.

Typical values found in a TT-AFM in an acoustically shielding box, with bungee-cord type isolation, would be 0.3 to 0.6 angstrom, 300 to 600 pm. Significant external noise will increase this.

You should be able to achieve at least the value specified by AFM workshop on the instrument specification sheet which was delivered with your instrument.

For an AFMWorkshop HR-AFM, achievable nosie floors can be less than 100 pm.


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