How to Use an AFM to Measure the Noise Floor
While many other procedures are required to fully determine an AFM instrument's performance, the Z noise floor is frequently used as a simple parameter to quantify instrument performance because it indicates the lower limit of precision that can be achieved in the z-axis in that instrument and is also simple to measure.
It is critical to understand the AFM instrument's noise floor in order to ensure that high resolution measurements are relevant. This is especially crucial for measuring extremely small features (such as 5nm) and for high resolution force spectroscopy. The noise floor can also be used to optimize instrument setup and vibration isolation. When employing simply the z piezo in the z feedback loop, it is critical to understand the noise floor, as well as the noise floor of the z calibration sensor, if one is present in the instrument. The noise level of the z calibration sensor will be substantially greater than that of the z piezo in most instruments.
To obtain reliable findings, all scan settings should be kept constant while comparing two outcomes. Some parameters, such as PID values, vary widely between instruments, therefore particular values cannot be provided here. Standard values should be set in each circumstance so that a fair comparison may be made.
Noise floor measurement in the z piezo signal
a) Insert a clean, flat sample into the instrument. Replace the probe.
b) Use a probe technique to test the cleanliness and improve the PID settings by scanning a tiny picture on the sample.
c) Program the instrument to do a zero-size scan in which the probe does not move along the x and y axes. Some instruments do not appear to have this option, unlike NMI ezAFM Software. In this scenario, make the lowest scan size feasible, such as 1nm (or even less if possible).
d) At a scan rate of 1 Hz, measure an image with no probe motion in x or y, i.e. an image with a scan size of 0 nm. A 128 × 128 pixel picture is sufficient. The z piezo voltage data should be utilized. This might be labeled as height or topography. The z scale should be expressed in nanometers.
e) Prior to measurement, the data may need to be flattened, for example, using a first order horizontal line leveling process.
f) Determine the image's RMS roughness (Rq, see Chapter 5); this value represents the noise floor.
You can replay the image if there is any transitory noise in it, such as a person talking or slamming a door.
The attainable noise floor varies based on the instrument, as well as the noise in the surroundings, measurement settings, and vibration isolation, but often a sub-ngström noise floor may be reached. The image above is an example of the sort of image you should receive. It is critical to scan a little picture before doing the "zero size" image since the noise floor cannot be assessed until the device is in feedback mode.