Thin film interferometer using a light source with spectrally nonequidistantly distributed sampling points

Abstract

ABSTRACT Numerous processes, e.g. in semiconductor and optics producing industries require “lm thickness observation.These measuring systems depend on dierent working principles, e.g. spectral re”ectometry or ellipsometry. Thespectral re”ectometry interrogation method can be evaluated by various algorithms depending on resolution andmeasuring range demanded. All methods require a broad spectral distribution of the light source in order tosample the signal suciently for parameter extraction. Spectral sampling is often realized using a spectroscope,which produces equidistant sampling points in frequency space. In contrast to conventional spectrally broadlight sources, the one employed here emits several spectral lines, which are non-equidistantly distributed. It alsointroduces problems like variations of intensity in the output spectrum and narrow wavelength bands, in whichthe re”ected spectrum can be investigated. Non-equidistant sampling points additionally imply problems inconventional analysis algorithms, e.g. a FFT anticipates equidistant sampling points. Narrow wavelength bandsimply little information to interrogate at the same spectral resolution of the interrogator. Strong variationsof intensity lead to high noise levels at wavelengths with low intensities. Therewith, accuracy, resolution andmeasuring range are limited. An interrogator based on a Hg-Ar light source, a “ber coupler and a commercialspectroscope is described in this work. Both, accuracy and measuring range, are investigated by simulation andare experimentally proven on a glass on silicon demonstrator. Introducing an advanced algorithm, uncertaintiesinvoked by the sources spectral and intensity distribution are minimized and resolution as well as measuringrange are increased.Keywords: thin “lm interferometry, TFI, re”ectometry, spectrally non-equidistantly distributed light source