While many other procedures are important for full determination of the performance of an AFM instrument, the Z noise floor is often used as a simple parameter to quantify instrument performance, since it indicates the lower limit of the precision that can be reached in the z axis in that instrument, and is also simple to measure.noise-floor

It can be essential to know the noise floor of the AFM instrument to assure that high resolution measurements are meaningful. This can be particularly important for measurements of very small features (i.e. < 5nm), and for high resolution force spectroscopy. Measuring the noise floor can also help in optimizing instrument setup and vibration isolation. It is important to know the noise floor when using only the z piezo in the z feedback loop, as well as the noise floor of the z calibration sensor if there is one in the instrument. In most instruments, the noise floor of the z calibration sensor will be much higher than that of the z piezo.

In order to get reproducible results, all scan parameters should be maintained the same when comparing two results. Some factors, such as the PID values vary greatly from instrument to instrument, so the specific values to use cannot be suggested here. In each case, standard values should be established such that a fair comparison can be made.

Note that the procedure below is adapted from general guidelines given in Appendix B, page 195 of Eaton and West “Atomic Force Microscopy”. For a outline of a procedure that’s generally applicable to any model of AFM, take a look at the procedure below. Click here to find a specific procedure for measuring the z noise floor on a TT-AFM from AFM workshop.

 

Measuring noise floor in the z piezo signal

a) Place a flat, clean sample in the instrument. Use a new probe.

b) Do a probe approach and scan a small image on the sample to verify cleanliness and optimize the PID parameters.

c) Set the instrument to make a zero size scan such that the probe does not move in the x and y axis. Some instruments do not seem to have the option to do this (I have found that the JPK Nanowizard software does not allow this). In this case, make the smallest size scan you are able to , such as 1nm, or even less if possible).

 

d) Measure an image without probe motion in x or y, i.e. an image with a scan size of 0 nm, at a 1 Hz scan rate. A 128 x 128 pixel image is adequate. The data from the z piezo voltage should be used. This may be labelled height, or topography. The z scale should be in nanometers.

 

e) It may be necessary to flatten the data before the measurement, e.g. by a 1st order horizontal line levelling routine.

 

f)  Calculate the RMS roughness (Rq, see chapter 5) of the image, this value is the noise floor.

If you get some transient noise in the image, from e.g. a person talking, or slamming a door, you can repeat the image.


The achievable noise floor varies from one instrument to another, as well as depending on the noise in the environment, the measurement parameters, and the vibration isolation, but typically a sub-Ångström noise floor can be achieved. An example of type of image you should get is shown in the image above. it's important that you scan a small image before doing the "zero size" image, as the instrument must be in feedback for the noise floor to be measured.

 


All text and images copyright Peter Eaton 2014-2018

TT-AFM noise floor measurement

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

1

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

 

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 1 will be very 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, in vibrating mode would be 0.3 to 0.6 angstrom.

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.


All materials on this website are copyright 2010-2018 Peter Eaton.

My group recently published a paper in the journal Ultramicroscopy reporting on direct comparison we made between different techniques that can be used to characterise the size of nanoparticles.

 

There are a wide variety of technique available to make these kind of measurements nowadays, however, microscopy is often used, because it is a direct technique (some other techniques measure properties related to size), and because it’s also possible to measure shape at the same time. The size of nanoparticles is extremely important for their properties, and ideally a technique to measure nanoparticle size will have sub-nanometer resolution.

 

Apart from microscopy, light-scattering techniques are probably the most common techniques used. The method of dynamic light scattering, or DLS; is extremely popular and used in thousands of labs worldwide. A newer method based on laser light scattering, called nanoparticle tracking analysis, or NTA, is currently growing in popularity.

 

In our project, we prepared nine samples; these we made up of nanoparticles composed of three different, and commonly used materials, a metal, an oxide (silica), and a polymer. We examined each materials in two different sizes, and also tested the ability of each method to distinguish between different populations in mixed samples.

 

A couple of new details about my book, Atomic Force Microscopy. Firstly, I just found a new (to me) review (published in German in Physik Magazine), including this great quote:

 

"Atomic Force Microscopy by Peter Eaton and Paul West is the manual that should accompany any AFM."

Prof. Othmar Marti, University of Ulm 

 

Secondly, a new paperback edition of the book, was recently published. In addition to being approximately half the price of the hardback edition, this new edition has been updated and all (known) typos corrected, so this is the version to get if you can!

 

The paperback version can be found on amazon.com here.

Atomic force microscopy

 

Bart showing his results


Our atomic force microscopy training course for 2017 ran in April, between the 10th 
and the 13th. Once again, the course was a great success, and all the places were filled. In this post, I quote some of the the feedback we got from some of me of the attendees, as well as some of the images they produced. In this edition, the highlight (for me) was the talk from Prof. Bart Hoogenboom, from UCL.

Bart demonstrated some amazing results in AFM, and gave some real inspiration as to what AFM can achieve. 

Phase image of E. coli cells

Once again, we had a good mix of students. They came from Wales, Portugal, Switzerland, the Netherlands, and Germany. We had PhD students, AFM technicians, lecturers, post-doctoral fellows and industrial researchers. It's always great to have such a wide range of opinions and nationalities!

As usual, we began with the basics of AFM, including instrumentation, modes, and fundamental concepts. Then the more fun parts, how to prepare samples, tips and tricks for running the instruments, and how to process and analyse the data.Most of the students prepared samples, and they all ran the instruments and processed and analysed image files. Based on the feed back, the students thought the course very worthwhile.

"I liked the course a lot. I think it's well-adapted to people with no AFM experience, and it seems it works well also for experienced users"

 

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