Showing papers by "Joanna Schmit published in 2004"

Temporal correlation vortices and topological dispersion.

Abstract: Interference measurements of a polychromatic partially coherent light source verify the existence of a temporal correlation vortex. Topological dispersion is found to destabilize this singularity.

Film thickness and boundary characterization by interferometric profilometry

Abstract: Two threshold parameters are used to identify the intensity modulation peaks corresponding to the interfaces of the two sides of a thin film with the adjacent media. The first parameter is used to distinguish modulation data from noise and is set on the basis of actual background noise data measured during the interferometric scan. The second parameter is used to separate actual contrast data from signals of relatively high modulation that satisfy the first parameter but do not in fact result from interference fringes. Data that satisfy both parameters are considered valid modulation data and the peak of each modulation envelope is then calculated using conventional means. The thickness of the film at each pixel is obtained by dividing the scanning distance corresponding to the two peaks by the group index of refraction of the film material.

Measurement of object deformation with optical profiler

Abstract: An interferometric profiler is used to measure object motion by modifying the motion of the scanner so that the phase variation at each scanning step is kept within the acceptable limits of the algorithm used to calculate phase changes. The scanner motion is so manipulated on the basis of prior knowledge about the nature of the object motion, or knowledge obtained by pre-calibration, or by real-time feedback based on current measurements. The object motion is recovered from the scanning information by subtracting the scanner position from the object position as it evolves during the scan.

Effects of source shape on the numerical aperture factor with a geometrical-optics model.

Abstract: We study the effects of an extended light source on the calibration of an interference microscope, also referred to as an optical profiler. Theoretical and experimental numerical aperture (NA) factors for circular and linear light sources along with collimated laser illumination demonstrate that the shape of the light source or effective aperture cone is critical for a correct NA factor calculation. In practice, more-accurate results for the NA factor are obtained when a linear approximation to the filament light source shape is used in a geometric model. We show that previously measured and derived NA factors show some discrepancies because a circular rather than linear approximation to the filament source was used in the modeling.

Fringe spacing in white-light interferometry

Abstract: Interference microscopes use quasi-monochromatic, broad band, and white light illumination for surface topography measurement. Fringe spacing for quasi-monochromatic illumination changes with the numerical aperture of system, and these changes have been previously examined by others. In this article we compare changes in fringe spacing for white light and broad band illumination for objectives with a numerical aperture of 0.13 and 0.55. We find that white light fringe spacing changes with tilt of the object much faster than for monochromatic illumination. We also investigate the influence of reference mirror tilt on changes in white light fringe spacing.

Calibration of high-speed optical profiler

Abstract: Optical profilers that employ white-light interferometry (WLI) techniques are widely used in industry and research to map surface structures with precise sub-nanometer resolution. The accuracy, repeatability and consistency of these instruments depend mainly on the proper calibration of two parameters: the mean wavelength and the scanner speed. Equally important in industrial applications is high throughput of measuring devices; high throughput can be achieved by increasing the speed of the measurement, which in turn reduces the sampling rate of the interference signal. Again, exact calibration of the system is the key to obtaining accurate measurements. Our solution for calibrating the system involves inserting into the optical profiler a second, laser-based, reference signal interferometer that creates a primary length standard for calibration. This innovation allows for substantially improved performance and repeatability at different measurement speeds up to 100 microns per second and over an 8 millimeter scanning range.