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.
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.
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.
Increasing capabilities in precision manufacturing and micro technology are accompanied by increasing demands of high
precision industrial metrology systems. Especially for measuring functional surfaces, areal optical principles are widely
used. If, in addition, nanometer height resolution is needed interferometers seem to be the most promising instruments.
First, this contribution focuses on the transfer characteristics of white-light interferometers with microscopic field of
view. In general, microscopic instruments suffer from their limited lateral resolution capabilities. Hence, the transfer
function of these instruments is typically assumed to show a linear low-pass characteristic. We studied the transfer
characteristics of white-light interferometers by theoretical simulations and experimental investigations. Our results show
that in most practical cases these instruments behave nonlinear, i.e. the output surface profile cannot be obtained from the
input profile by a simple linear filter operation.
Although they are well-established, there are some further limitations of white-light interferometers if they are used to
measure micro or even sub-microstructures. If edges, steeper slopes or abrupt slope changes are present on a measuring
object characteristic errors such as batwings occur. Furthermore, a high effort concerning the correction of chromatic
aberration is necessary in order to avoid dispersion effects. Otherwise, there will be systematic discrepancies between
profiles obtained from evaluation of the coherence peak and those resulting from the phase of the interference signals.
These may lead to 2π phase jumps if the fringe order is obtained from the position of the coherence peak. Finally,
measurement artifacts may also result if the measured micro-structure shows discontinuities of the surface slope.
This contribution analyses the different phenomena and discusses approaches to overcome existing limitations.
Scanning white-light interferometry (SWLI) provides the capability of fast and high-precision three-dimensional
measurement of surface topography. Nevertheless, it is well-known that white-light interferometers more than imaging
microscopes suffer from chromatic aberration caused by the influence of dispersion. In this paper several interferometric
measurement systems are used for surface topography measurement. A Linnik interferometer and two Michelson
interferometers of different aberration correction are compared. A correction system designed using the ray tracing
software “Zemax” aims at an optimization the modulation transfer function (MTF). Although the MTF is optimized the
resulting spot diagrams are blurred due to chromatic aberration. Finally, a doubly corrected Michelson interferometer
will be presented. For this interferometer a nearly optimal MTF as well as minimized spot diagrams are achieved.
Scanning white-light interferometry (SWLI) provides the capability of fast and high-precision three-dimensional
measurement of surface topography. Nevertheless, it is well-known that white-light interferometers more than imaging
microscopes suffer from chromatic aberrations caused by the influence of dispersion. Chromatic aberrations lead to
systematic measuring errors in SWLI, especially on micro-structures with curved or tilted surface areas. For example, the
plane glass plates used in a Mirau-interferometer are a potential source of dispersion. If this influence is not completely
corrected for, errors in height measurement occur. In addition, the magnitude of these errors strongly depends on whether
the coherence peak's position or the phase of an interference signal is evaluated. This study is intended to overcome
these difficulties by a dispersion optimized white-light interferometer. The design corresponds to a Mirau-interferometer,
but in order to reduce dispersion phenomena, a reflective Schwarzschild microscope objective is used.
For beam splitting a so-called pellicle is positioned in-between the objective and the measuring object. The dominant
effect, which limits the accuracy of the interferometer is supposed to depend on multiple reflections from the front and
the back side of the pellicle beam splitter. As a consequence, ghost signals were measured in addition to the typical
white-light interference signals. This indicates that multiple reflections influence the results and finally limit the accuracy
of the interferometer.
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