STMs are driven by piezo linear elements in most cases, and the drive
voltage and any error feedback are used to locate the tip. Also, as the piezo
element acts like a capacitor, it will store a charge that is proportional to
its extension. Thermal noise can be a real problem, because even the energy of
room temperature molecules can add up to a significant error signal. Cooling is
the best way to get rid of this.
The largest sources of noise are a) thermal noise in the piezo elements
themselves, b) the amplifiers that drive the piezo elements, and c) vibration in
the room that the STM is being used in.
The resolution in an STM can be sub-angstrom with proper noise control and
very stable power supplies for the driver amplifiers.
Cheers!
Chip Shults
My robotics, space and CGI web page -
http://home.cfl.rr.com/aichip