Stitched open-loop measurements with a focal-distance-modulated confocal sensor

Abstract

The article presents measurements with a high-speed focal-distance modulated fibre-coupled confocal sensor integrated into a nano measuring machine (NMM-1). Combined with an appropriate signal processing, this axial beam modulating sensor yields a linear characteristic curve within a range of about 600 nm. This characteristic curve enables scans in closed-loop constant distance mode, where the controller of the NMM-1 keeps the distance between the measured surface and the sensor constant. Therefore, only one lateral scan is necessary to measure a given topography. Nevertheless, it is also possible to conduct open-loop constant height measurements on discrete heights. The axial offset between the heights needs to be only slightly less than the measuring range of the sensor, leading to a great reduction of the number of needed lateral scans to measure a given topography compared to a conventional confocal sensor, where the axial offset for a similar optical system is typically about 50 nm. To combine the scans taken at different heights, it is possible to stitch them using overlapping measuring points taken at two adjacent heights. Within this article, measurements are conducted on a roughness standard in closed-loop constant distance and in open-loop constant height mode, where the latter are evaluated with and without the application of the stitching algorithm.

Effect of a Misidentified Centre of a Type ASG Material Measure on the Determined Topographic Spatial Resolution of an Optical Point Sensor

“…The setup consisted of a fibre-coupled confocal sensor firstly described in [67] and in its latest design, as it was applied for the investigations shown within this paper, in [68]. A thorough description of the sensor, including the theory of the signal formation based on general works on fibre-coupled confocal sensors of Gu et al [69], may be found in [70].…”

Investigations on the accuracy and evaluation of the uncertainty of stitching in optical surface metrology

“…The single measurements are deliberately taken with overlapping areas to conduct the fine registration, which in optical surface metrology is known as sub-aperture stitching. 1 Most usual is the combination of an areal-topography measuring instrument, like a disc scanning confocal microscope or a coherence scanning interferometer, and a translational positioning system, [1][2][3][4][5][6] but also combinations with a rotatory positioning system, 7-9 a positioning system with more than three degrees of freedom [10][11][12][13][14] or a one-dimensional point distance sensor 15 have been shown. [16][17][18][19] Various algorithms are applied to conduct the fine registration or stitching with the correction of positioning errors in a variable number of degrees of freedom.…”