Traceable dimensional measurements of step-height and lateral standards, patterned surfaces, and particles at
the nanoscale need of calibrated instruments such as the metrological AFMs. These instruments have onboard
capacitive sensors or interferometers to control the relative tip-sample movements. Interferometers
provide a direct traceability to the unit of length, at the cost of more complex set-ups. A new interferometer
set-up has been developed to monitor tip-sample z-displacements of the instrument in use at INRIM. The
interferometer is made of a compact and symmetric arrangement of the optics to fit available room and
minimize the metrology loop. Preliminary results are reported together with the analysis of the main error
sources of z-displacement measurements.