Sub-Micron Range Thickness Measurements Using a Novel Scanning Heterodyne Optical Interferometer

In this paper, we introduce a novel and highly accurate method of thickness measurement using a compact, high performance, scanning heterodyne optical interferometer. The instrument has the potential to measure thickness at an Angstrom resolution over a micron continuous thickness range. The instrument uses acousto-optic devices to implement rapid scanning for thickness measurements of the test medium. As the read optical beams scan the test medium for thickness assessment, the double Bragg diffraction makes the final interfering output beam stationary on the two high speed photo-detectors used for radio frequency signal generation via heterodyne detection. When a test medium is introduced in the read beam, an additional phase shift 2π t (n-1) / λ appears in the read beam as compared to the reference beam where t, n and λ, are the thickness, refractive index and wavelength of the laser beam, respectively. This phase difference gives the thickness of the medium. Mechanical vibrations and thermal noise are eliminated via use of common optical and rf paths. A reflective design of the interferometer results in a compact and low component count instrument. Experimental results are reported using the novel interferometer and instrument issues are discussed.