Matthias Eifler

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

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 structures 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 and to adjust the z-axis. Results of different optical measurement principles are compared in order to evaluate the capabilities of the new artefact.

Two-photon laser lithography in optical metrology

Two-photon laser lithography has become one of the most promising additive manufacturing techniques on the micron scale and is applied, e.g., in fields of micro-optics and -robotics as well as optical and mechanical metamaterials. Here, we report on the feasibility, limits and general benefits of this method to fabricate material measures for the calibration of industrial optical topography measuring devices. Since calibration procedures are essential in the scientific and industrial application of those measuring instruments, appropriate material measures are highly required. In contrast to traditional manufacturing technologies, we show that two-photon laser lithography allows a highly resolved fabrication of multiple, almost arbitrary standardized calibration geometries on a micron length scale. Hereby, all structures are fabricated on only one single substrate, therefore enabling a mapping of a broad range of metrological characteristics for topography characterization. The most required calibration geometries are manufactured and analyzed regarding their aging behavior, their quality improvement by a post-UV development and the resolution limits within the manufacturing as well as the calibration process. Thus, the general industrial and scientific relevance of manufacturing material measures with two-photon laser lithography is demonstrated.

Calibration of areal surface topography measuring instruments

The ISO standards which are related to the calibration of areal surface topography measuring instruments are the ISO 25178-6xx series which defines the relevant metrological characteristics for the calibration of different measuring principles and the ISO 25178-7xx series which defines the actual calibration procedures. As the field of areal measurement is however not yet fully standardized, there are still open questions to be addressed which are subject to current research. Based on this, selected research results of the authors in this area are presented. This includes the design and fabrication of areal material measures. For this topic, two examples are presented with the direct laser writing of a stepless material measure for the calibration of the height axis which is based on the Abbott- Curve and the manufacturing of a Siemens star for the determination of the lateral resolution limit. Based on these results, as well a new definition for the resolution criterion, the small scale fidelity, which is still under discussion, is presented. Additionally, a software solution for automated calibration procedures is outlined.

A model-based approach for the calibration and traceability of the angle resolved scattering light sensor

Within the field of geometric product specification there is a growing need for the application of inline measurement systems. The use of inline measurement requires robust and fast measurement principles. A very robust optical measurement principle is the angle resolved scattering light (ARS) sensor. The ARS sensor provides high precision and high resolution measurement data of technical surfaces because the surface angles are measured as an intensity distribution on a detector instead of measuring a series of discrete height values. However, until now, there were no specific measurement standards for the calibration of the ARS sensor and no traceability was ensured. In this paper, new strategies for the calibration of an ARS sensor are proposed. A new mathematical model for the ARS sensor is introduced. Based on this, two new measurement standards for the calibration of the sensor parameters are derived. These standards are designed with a model-based approach and can calibrate sensor-specific properties of the ARS sensor. The manufacturing of the standards is described and measurement results are provided.

3D printing of material measures for areal surface texture (Conference Presentation)

Component surfaces feature more and more complex functional properties and deterministic geometric structures. The result is that an areal characterization of surfaces is more often necessary. The increasing incidence of areal surface topography measuring instruments in geometrical product specification enables the acquisition of more information about a surface topography. However, also more complex calibration procedures are required as an increasing number of metrological characteristics need to be verified. This verification is achieved with areal material measures which are described in the standard ISO 25178-70. State of the art is a manufacturing of the proposed geometries with many different principles because there is a broad range of geometries whose structure size is usually in the micrometer-range. Typically applied principles include lithography, etching and ultra-precision cutting. The application of an ultra-precise 3D-printing technology, two-photon laser lithography alias direct laser writing, exhibits enormous potential for areal material measures as arbitrary 3D-freeform surfaces can be manufactured with a high repeatability in the nanometer-range. Hence, a feasibility study of the application of direct laser writing for the manufacturing of areal material measures is conducted. In doing so, different standardized material measures are manufactured and the resulting surface topographies are compared to their target geometries in order to qualify the manufacturing process. The measurements are performed with different surface topography measuring instruments in order to examine the overall suitability of the principle for the manufacturing of areal material measures. The standardized measurands of the ISO 25178-70 serve as evaluation criteria just as recently defined new parameters for the verification of surface topography measuring instruments. As a new resolution criterion, for example the small scale fidelity limit is evaluated. The enhancement of the resolution of the manufacturing process with stimulated emission depletion is examined and the resolution limits of the manufacturing and the measuring processes are compared. The samples that are manufactured with direct laser writing are further examined regarding their practical abilities. An important property of material measures is their stable provision of constant evaluation parameters. In order to examine this relevant characteristic of the samples, different studies which describe the aging behavior of varying coating materials are conducted. Based on the results, a suitable coating material with suitable optical characteristics can be chosen and the time-dependent behavior of the geometries can be evaluated. Because optical surface topography measuring instruments which are calibrated with the proposed material measures may feature varying magnifications and fields of view, in another study scaling effects are examined and material measures with different structure sizes are manufactured in order to evaluate the scalability of the different types of material measures. It can be concluded that almost any standardized areal material measure can be manufactured reliably with direct laser writing. Due to the scalability of the structures, a calibration of optical surface topography measuring instruments with varying fields of view can be ensured.

Application of GPU-based, highly parallelized algorithms for the estimation of electromagnetic multi-layer interactions

In nowadays industry, complex surfaces with material contrasts or surface coatings are present and represent a challenge for optical topography measuring instruments. The reason is that varying optical properties lead to phase jumps and to topography deviations when the surface height is evaluated. Thus, Ellipso-Height-Topometry as a measurement technique which can measure both topography and material properties of technical surfaces was proposed in order to achieve a correction of the occurring topographic artefacts. The height correction value can be obtained for the compensation of material-induced height deviations and the thickness of surface layers can be evaluated. Currently, it is possible to calculate the surface characteristics from ellipsometric parameters for at most two layers. However, the described height corrections are only possible when well-defined and realistic models of surface layers can be utilized, e.g. a given set of homogeneous oxide layers. Oxidation effects however describe statistical processes which can be predicted with underlying material distribution models. This leads to an uncertainty in ellipsometry, which is considered with a new approach that will be discussed in this publication. Therefore, an extended multi-layer approach which is capable of handling additional layers based on a parallelized algorithm using graphic processing units and the commonly known CUDA technology is proposed. This algorithm can also be used to consider material proportions which result from oxidation effects in z direction. The new approach for the Ellipso-Height-topometry measurement technique is compared with the current procedures which often neglect the existence of an oxide layer for the basic material. To experimentally verify the approach and according algorithm, it is applied for the evaluation of actual surfaces with multiple plane layers and different materials. Test samples with different materials are used in order to evaluate the complex refractive index, the distribution of identified materials and the layer thicknesses with actual Ellipso-Height- Topometry measurements. The results of the measurements are compared to the predicted theoretical results.

Modellbasierte Entwicklung eines Geometrienormals für pneumatische Abstandssensoren

Zusammenfassung Aufgrund ihrer Robustheit ist die pneumatische Messtechnik gut für die Inprozess-Messtechnik einsetzbar. Moderne Rechnersysteme liefern neue Ansätze, bisher noch unzureichend beschriebene Grundlagen zur Physik des Verfahrens zu betrachten. Ein Beispiel hierfür sind Kalibrierstrategien, zu denen diese Veröffentlichung Ergebnisse präsentiert. Dabei werden die Auslegung zwei neuer Geometrienormale auf Basis verschiedener Entwicklungsansätze der virtuellen pneumatischen Messung sowie zugehörige Messergebnisse vorgestellt. Die Beurteilung erfolgt durch den Vergleich der verschiedenen Abtastergebnisse.