  Piezoelectric quartz tuning forks have been employed as the force sensor in a
dynamic mode scanning force microscope operating at temperatures down to 1.7 K
at He-gas pressures of typically 5 mbar. An electrochemically etched tungsten
tip glued to one of the tuning fork prongs acts as the local force sensor. Its
oscillation amplitude can be tuned between a few angstroms and tens of
nanometers. Quality factors of up to 120000 allow a very accurate measurement
of small frequency shifts. Three calibration procedures are compared which
allow the determination of the proportionality constant between frequency shift
and local force gradient based on the harmonic oscillator model and on
electrostatic forces. The calibrated sensor is then used for a study of the
interaction between the tip and a HOPG substrate. Force gradient and dissipated
power can be recorded simultane-ously. It is found that during approaching the
tip to the sample considerable power starts to be dissipated although the force
gradient is still negative, i.e. the tip is still in the attractive regime.
This observation concurs with experiments with true atomic resolution which
seem to require the same tip-sample separation.
