Micro Fabry-Perot sensor for surface measurement

摘要

A high accuracy surface inspection system for testing polished surfaces is based on a Fabry-Perot resonator. Theinspected surface serves as a relay mirror in a cat-eye retroreflector incorporated into the resonating cavity, which makesthe optical configuration insensitive to surface tilts. The laser wavelength is swept periodically over a given range, andthe local surface height is obtained by timing the resonance occurrence during each sweep. An additional highly stablereference Fabry-Perot interferometer using the same laser is employed for obtaining differential measurements, yieldingabsolute height values, distinguishing between up and down defects. Due to the finesse of the multi-beam Fabry Perotinterferometer relative to the two-beam Michelson interferometer response function, the height sensitivity is greatlyenhanced. In order to detect small contamination particles, the interferometer was supplemented by a scattering detectionchannel 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, allowsone to build a sensitive and reliable defect detection and classification procedure. The interferometer was incorporatedinto high-speed high-accuracy in-line machines for inspection of the surfaces in data storage applications. With a 0.2Angstroms resolution, the height rms repeatability at a surface scanning speed of 40 m/s is 1.5 Angstroms.