Calibration of z-axis linearity for arbitrary optical topography measuring instruments

Abstract

The calibration of the height axis of optical topography measurement instruments is essential for reliable topography measurements. A state of the art technology for the calibration of the linearity and amplification of the z-axis is the use of step height artefacts. However, a proper calibration requires numerous step heights at different positions within the measurement range. The procedure is extensive and uses artificial surface structures that are not related to real measurement tasks. Concerning these limitations, approaches should to be developed that work for arbitrary topography measurement devices and require little effort. Hence, we propose calibration artefacts which are based on the 3D-Abbott-Curve and image desired surface characteristics. Further, real geometric structure s are used as an initial point of the calibration artefact. Based on these considerations, an algorithm is introduced which transforms an arbitrary measured surface into a measurement artefact for the z-axis linearity. The method works both for profiles and topographies. For considering effects of manufacturing, measuring, and evaluation an iterative approach is chosen. The mathematical impact of these processes can be calculated with morphological signal processing. The artefact is manufactured with 3D laser lithography and characterized with different optical measurement devices. An introduced calibration routine can calibrate the entire z-axis-range within one measurement and minimizes the required effort. With the results it is possible to locate potential linearity deviations an d to adjust the z-axis. Results of different optical measurement principles are compared in order to evaluate the capabilities of the new artefact

Calibration sample for arbitrary metrological characteristics of optical topography measuring instruments

“…Previously, we showed that this allows for a similar calibration of the linearity z l and the amplification coefficient z α of the height axis [18,27]. Additionally, an integral calibration of the device based on the 3D roughness parameters is possible with the type AIR geometry [18]. The surface topography is based on an actual engineering surface and thus enables a practical calibration of multiple properties of surface topography measuring instruments.…”

“…The surface topography is based on an actual engineering surface and thus enables a practical calibration of multiple properties of surface topography measuring instruments. The aforementioned linear Abbott-curve allows to achieve an almost stepless calibration of the height axis [18,27]. Additionally to specific linearity criteria that compare the n m ⋅ topography heights of the measured Abbott-curve ( ) C Mr with the target Abbott-curve, we use an alternative analysis to derive z α and z l , the parameters described in the ISO 25178-60x series [24].…”

“…The proposed method uses a large number of topography points for a determination of the z-axis linearity instead of only using a small number of step heights for the response function estimation. Additionally to the parameters based on the response function, a S and q S of the sample can be evaluated for the integral calibration [18,27]. The resulting "universal calibration artefact" is shown in Fig.…”

“…The results are more or less independent from the coating material with a small offset only when the linearity deviation for the Cr-coating is examined. For the evaluation, we calculate these parameters as suggested in previous work [18,27] (see section 3): all height values of the measurement data are compared with the linear target Abbott-curve and the inner 80% of the height values serve for the response function estimation. This evaluation method leads to a high correlation of the amplification coefficient and the a S -value.…”

“…We use direct laser writing (DLW) to fabricate the according samples [17]. The general suitability of DLW for the manufacturing of calibration geometries has already been proven in previous studies [18][19][20].…”