We present a high-resolution Linnik scanning white-light interferometer (SWLI) with integrated distance measuring
interferometer (DMI) for close-to-machine applications in the presence of environmental vibrations. The distance,
measured by DMI during the depth-scan, is used for vibration compensation of SWLI signals. The reconstruction of the
white-light interference signals takes place after measurement by reordering the captured images in accordance with their
real positions obtained by the DMI and subsequent trigonometrical approximation. This system is the further development
of our previously presented Michelson-interferometer. We are able to compensate for arbitrary vibrations with frequencies
up to several kilohertz and amplitudes in the lower micrometer range. Completely distorted SWLI signals can be
reconstructed and the surface topography can be obtained with high accuracy. We demonstrate the feasibility of the method
by examples of practical measurements with and without vibrational disturbances.
The investigation of transparent optical layers is a growing field of application of white-light interferometry. Robust algorithms exist that extract the signal components from different layers inside a transparent structure. The separated signal contributions are then evaluated individually. Two contradicting situations have to be accounted for when low-coherence interferometry is used to measure layer structures. First, with a low NA system and a short coherence light source, the optical path difference between the layers is measured. Second, if a high NA interferometer and a long coherence light source is used, the limited depth of focus limits the correlogram width. In this case, the layer thickness is underestimated. In this paper a 2.2 μm thick reference layer is studied. This layer was measured with different interferometric systems: Michelson and Mirau interferometers with magnifications from 5x to 100x. Furthermore, light sources with different temporal coherence length were used. If lateral resolution is unimportant, the combination of a low NA measuring system and a low coherence length light source provides a larger distance between the signal contributions from different boundary layers and therefore better separation, bias correction, and higher accuracy, compared to a high NA system. The interferometer system can be calibrated by measuring the layer thickness of a small structure with respect to a substrate. Such a calibration permits performing measurements with a high NA interferometer and a low coherence light source. The main contribution of this paper is to compare and discuss results of these different options of layer thickness measurement with respect to measurement accuracy and uncertainty influences.
We present a scanning white-light interferometer (SWLI) for close-to-machine applications in the presence of environmental vibrations. It combines an area measuring white-light interferometer and a punctual measuring laser distance interferometer (LDI) in one device. The measurement spot of the LDI is within the field of view of SWLI. The LDI measures any distance change during the white-light measurement with a high temporal resolution. With the knowledge of the real distance changes during the measurement we can compensate for the influence of environmental vibrations on the white-light correlograms. The reconstruction of the white-light interference signals takes place after measurement by reordering the captured images in accordance with their real positions obtained by the LDI. With this system we are able to reconstruct completely distorted and unusable SWLI signals and to determine the 3D topography of the measurement specimen from these reconstructed signals with high accuracy. We demonstrate the feasibility of the method by examples of practical measurements with and without vibrational disturbances.