Scanner calibration and certification procedures - Atomic Force Microscopy - page 195

B2.4 z noise floor

For many applications it is essential to know the noise floor of the AFM instrument to
ensure that high-resolution measurements are meaningful. This can be particularly
important for high-resolution force spectroscopy. Measuring the noise floor can also
help in optimizing instrument set-up and vibration isolation. It is important to know the
noise floor of 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.

B2.4.1 z piezo noise
This procedure is subjective, but will give a good indication of the noise floor of the AFM.
In order to get reproducible results, all scan parameters should be maintained at the same
values. Some factors, such as the PID values, vary greatly from instrument to instrument,
so they cannot be suggested here. In each case, standard values should be established such
that a fair comparison can be made.

(a) Place a flat sample in the instrument. A sample such as a silicon wafer, HOPG or mica
is appropriate. The sample must be clean.
(b) Scan an 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.
(d) Measure an image without probe motion in x or y , at a 1 Hz scan rate. A 256

256

pixel image is adequate. The data from the z piezo voltage should be used.
(e) Calculate the rms roughness ( R q , see Chapter 5) of the image, this value is the noise
floor.

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-angstrom noise floor can be achieved.

B2.4.2 z calibration sensor noise
This measurement is made in exactly the same way as described above for the z piezo
noise floor, with the exception that the data from the z calibration sensor is used.

B3 x-y-z coupling

Ideally motions of the scanner in the x , y and z axis would be orthogonal and independent.
In practice, however, there is crosstalk between the scanning in the three axes. See Chapter
2 for the explanation of these effects.

B3.1 xy orthogonality

In order to check whether there is crosstalk between the x and y axes of scanning, the
deviation from perpendicular of the features in the standard X-Y sample is measured as
shown in Figure B5.