Real-time displacement measurements with a Fabry-Perot cavity and a diode laser

We present the basic operating principles of a traceable measurement system suitable for use with atomic force microscopes (AFMs) and nanometer-resolu tion displacement sensors. Our method is based upon a tunable external-cavi ty diode laser system which is servo-locked via a phase-modulated heterodyn e locking technique to a Fabry-Perot interferometer cavity. We discuss mech anical considerations for the use of this cavity as a displacement metrolog y system and we describe methods for making real-time (sub 10 ms sampling p eriod) measurements of the optical heterodyne signals. Our interferometer s ystem produces a root-mean-squared (RMS) displacement measurement resolutio n of 20 pm. Two applications of the system are described. First, the system was used to examine known optical mixing errors in a heterodyne Michelson interferometer. Second, the Fabry-Perot interferometer was integrated into the Z axis of a commercial AFM scanning stage and used to produce interfero meter-based images of a 17 nm step height specimen. We also demonstrate ato mic resolution interferometer-based images of a 0.3 nm silicon single atomi c step-terrace specimen. (C) 2001 Elsevier Science Inc. All rights reserved .