  Piezoresponse force-microscopy (PFM) has become the standard tool to
investigate ferroelectrics on the micro- and nanoscale. However, reliability of
PFM signals is often problematic and their quantification is challenging and
thus not widely applied. Here, we present a study of the reproducibility of PFM
signals and of the so-called PFM background signal which has been reported in
literature. We find that PFM signals are generally reproducible to certain
extents. The PFM signal difference between 180{\deg} domains on
periodically-poled lithium niobate (PPLN) is taken as the reference signal in a
large number of measurements, carried out in a low frequency regime (30-70
kHz). We show that in comparison to Pt coated tips, diamond coated tips exhibit
improved signal stability, lower background signal and less imaging artifacts
related to PFM which is reflected in the spread of measurements. This is
attributed to the improved mechanical stability of the conductive layer. The
average deviation of the mean PFM signal is 38.3%, for a diamond coated tip.
Although this deviation is relatively high, it is far better than values from
literature which showed a deviation of approx. 73.1%. Additionally, we find
that the average deviation of the background signal from 0 is 11.6% of the PPLN
domain contrast. Thus, the background signal needs to be taken into account
when quantifying PFM signals and should be subtracted from PFM signals. Those
results are important for quantification of PFM signals, since PPLN might be
used for this purpose when PFM signals measured on PPLN are related to its
macroscopic d33 coefficient. Finally, the crucial influence of sample polishing
on PFM signals is shown and we recommend to use a multistep polishing route
with a final step involving 200 nm sized colloidal silica particles.
