Bryan Clark

Micro Fabry-Perot sensor for surface measurement

A high accuracy surface inspection system for testing polished surfaces is based on a Fabry-Perot resonator. The inspected surface serves as a relay mirror in a cat-eye retroreflector incorporated into the resonating cavity, which makes the optical configuration insensitive to surface tilts. The laser wavelength is swept periodically over a given range, and the local surface height is obtained by timing the resonance occurrence during each sweep. An additional highly stable reference Fabry-Perot interferometer using the same laser is employed for obtaining differential measurements, yielding absolute height values, distinguishing between up and down defects. Due to the finesse of the multi-beam Fabry Perot interferometer relative to the two-beam Michelson interferometer response function, the height sensitivity is greatly enhanced. In order to detect small contamination particles, the interferometer was supplemented by a scattering detection channel integrated into the same compact optical head. The combination of the bright-field interferometric signal, yielding both the phase (surface height) and amplitude (surface reflectivity), and the dark-field scattering channel, allows one to build a sensitive and reliable defect detection and classification procedure. The interferometer was incorporated into high-speed high-accuracy in-line machines for inspection of the surfaces in data storage applications. With a 0.2 Angstroms resolution, the height rms repeatability at a surface scanning speed of 40 m/s is 1.5 Angstroms.