  The reliable operation of micro and nanomechanical devices necessitates a
thorough knowledge of the water film thickness present on the surfaces of these
devices with an accuracy in the nm range. In this work, the thickness of an
ultra-thin water layer was measured by distance tunnelling spectroscopy and
distance dynamic force spectroscopy during desorption in an ultra-high vacuum
system, from about 2.5 nm up to complete desorption at 1E-8 mbar. The
tunnelling current as well as the amplitude of vibration and the normal force
were detected as a function of the probe-sample distance. In these experiments,
a direct conversion of the results of both methods is possible. From the
standpoint of surface science, taking the state-of-the-art concerning
adsorbates on surfaces into consideration, dynamic force spectroscopy provides
the most accurate values. The previously reported tunnelling spectroscopy,
requiring the application of significantly high voltages, generally leads to
values that are 25 times higher than values determined by dynamic force
spectroscopy.
