The noise of the frequency-shift signal Δf in noncontact atomic force microscopy (NC-AFM) consists of cantilever thermal noise, tip–surface-interaction noise and instrumental noise from the detection and signal processing systems. We investigate how the displacement-noise spectral density d
z
at the input of the frequency demodulator propagates to the frequency-shift-noise spectral density d
Δ
f
at the demodulator output in dependence of cantilever properties and settings of the signal processing electronics in the limit of a negligible tip–surface interaction and a measurement under ultrahigh-vacuum conditions. For a quantification of the noise figures, we calibrate the cantilever displacement signal and determine the transfer function of the signal-processing electronics. From the transfer function and the measured d
z
, we predict d
Δ
f
for specific filter settings, a given level of detection-system noise spectral density d
z
ds and the cantilever-thermal-noise spectral density d
z
th. We find an excellent agreement between the calculated and measured values for d
Δ
f
. Furthermore, we demonstrate that thermal noise in d
Δ
f
, defining the ultimate limit in NC-AFM signal detection, can be kept low by a proper choice of the cantilever whereby its Q-factor should be given most attention. A system with a low-noise signal detection and a suitable cantilever, operated with appropriate filter and feedback-loop settings allows room temperature NC-AFM measurements at a low thermal-noise limit with a significant bandwidth.
Thermal noise limit for ultra-high vacuum noncontact atomic force microscopy
