The precise production of micro- or micro-structured components of increasingly different materials requires ever more precise and flexibly adjustable geometry measurement methods. Today’s optical metrology offers various innovative approaches for this purpose. A major shortcoming is, however, that not all surfaces and structures to be measured are optically cooperative and return too little light or information to the measurement system for the signal analysis. Therefore, indirect optical geometry measurements are introduced as a new approach: Instead of directly detecting the outer boundary layer of the measuring object, the shape of the object’s imprint in the surrounding medium is examined. For this purpose, the surrounding medium is enriched with fluorescent substances and a confocal microscope scans the space surrounding the measured object. The spanned area above which the fluorescence signal disappears is then determined as the boundary layer between the measurement object and the surrounding medium. As a result, the object geometry is obtained completely independently of the optical response behavior of the object. While first realizations studied measurements in a liquid environment, this work demonstrates for the first time the feasibility of indirect optical geometry measurements in air environments with the aid of fluorescent microparticles. In order to maximize the measurement accuracy, different model-based signal evaluation approaches for determining the interface geometry from the fluorescence signals are investigated and compared, taking both cases (liquid and air environment) into account. Finally, indirect optical measurements are performed on a step geometry, reconstructing the height profiles using the theoretically derived model function.
For digital image speckle correlation (DISC), a novel approach is introduced where an ensemble average over multiple different speckle patterns is calculated. As a result, the measurement uncertainty of the displacement is reduced by an order of magnitude without deteriorating the spatial resolution. This enables precise surface displacement field measurements in the micrometer range with a measurement uncertainty lower than 100 nm at a spatial resolution below 20 µm. By using a digital micromirror device (DMD) for illumination modulation, measuring rates in the range of 25 Hz are possible while each measurement is based on 80 images.
The concept of process signature uses the relationship between a material load and the resulting modification remaining in the workpiece to better understand and optimize manufacturing processes. The metrological recording of the loads occurring during the machining process in the form of mechanical deformations is the basic prerequisite for this approach. An appropriate characterization method is speckle photography, which is already applied for in-plane deformation measurements in various manufacturing processes. A shortcoming of this fast and robust measurement technique based on image correlation techniques is that deformations in the direction of the measurement system are not detected and that they increase the error of measurement for in-plane deformations. Therefore, this work investigates a method that infers local out-of-plane motions of the workpiece surface from the decorrelation of speckle patterns and thus is able to reconstruct three-dimensional deformation fields. The implementation of the evaluation method in existing sub-pixel interpolation algorithms enables a fast reconstruction of 3D deformation fields, so that the desirable in-process capability remains given. Using a deep rolling process, first measurements show that dynamic 3D-deformations below the tool can be detected, which confirms the suitability of the speckle photography not only for the 2D- but also the 3D-analysis of deformations in manufacturing processes.
The challenging environment of in situ micro-geometry measurements in fluids (e.g. for laser- or electrochemical machining), such as refractive index fluctuations, small dimensions and high surface gradients, hinder many conventional measurement techniques. Confocal microscopy remains most suitable with uncertainties < 1μm, but complex micro-geometries with edge slopes > 75° often produce unwanted artifacts. To prevent the formation of artifacts, the isotropically emitting fluorescence light of a fluid layer covering the specimen is measured instead. The geometry reconstruction for in situ-relevant fluid depths >100 μm is not trivial and requires a signal model that includes the contributions of light absorption and the confocal volume shape. For model validation, the surface position of a reference step-object (nominal height: 250 μm), submerged in a fluid layer > 1 mm, is determined using a fluorescence signal model that is fitted to the measured data. First experiments yield a step height uncertainty of 8.8 μm, about one order of magnitude above the requirement. In order to identify optimization potential, the minimum achievable measurement uncertainty is estimated for both a signal with experiment-equivalent variance and a shot noise limited signal. The estimated uncertainties are 3.8 μm and 0.1 μm, respectively, and decrease with lower fluorophore concentration and fluid thickness. The differing experimental and estimated uncertainties result from model simplifications such as the missing contribution of reflected light at the specimen surface where the current model assumes that the confocal volume is cut off. Expanding the model promises to reduce the measurement uncertainty and to converge estimation and experiment, enabling geometry measurements of complex micro-geometries with different surface reflectivities under challenging in situ conditions in fluids.
The goal of this work is to describe a simulatively designed scatterometry approach for the in-line characterization of
sub-wavelength sinusoidal gratings, which are formed on a transparent foil in a roll-to-roll procedure. The challenge is to
acquire the 3D information of the workpiece, i.e., to measure the grating height in addition to the grating period with nm
precision. The grating period is obtained straightforward from the position of the first order diffraction maxima in the
reflection and the transmission region. For determining the grating height, the inverse problem is solved, i.e., the relation
between the scattered intensities of the diffraction maxima and the grating height is extracted from light scattering
simulations. The measurement uncertainty is evaluated for different instrumentation and simulation parameters, such as
the detection and incidence angle, the laser wavelength as well as the input parameters of the simulation. As a result, the
measurement uncertainty for the grating period and the height is estimated to 0.3 nm and ≤8 nm, respectively, when
using laser light in the visible wavelength range. Large area scanning measurements performed offline using the setup
parameters derived from simulations verify the sensitivity of the presented measurement approach for identifying local
variations of the spatial surface properties. Depending on the chosen detection system, sampling rates up to the MHz
range are feasible meeting the requirements of in-line process control of the roll-to-roll production procedure.
Surface micro topography measurement (e.g., form, waviness, roughness) is a precondition to assess the surface quality
of technical components with regard to their applications. Well defined, standardized measuring devices measure and
specify geometrical surface textures only under laboratory conditions. Laser speckle-based roughness measurement is a
parametric optical scattered light measuring technique that overcomes this confinement. Field of view dimensions of
some square millimeters and measuring frequencies in the kHz domain enable in-process roughness characterization of
even moving part surfaces. However, camera exposure times of microseconds or less and a high detector pixel density
mean less light energy per pixel due to the limited laser power. This affects the achievable measurement uncertainty
according to the Heisenberg uncertainty principle. The influence of fundamental, inevitable noise sources such as the
laser shot noise and the detector noise is not quantified yet. Therefore, the uncertainty for speckle roughness
measurements is analytically estimated. The result confirms the expected inverse proportionality of the measurement
uncertainty to the square root of the illuminating light power and the direct proportionality to the detector readout noise,
quantization noise and dark current noise, respectively. For the first time it is possible to quantify the achievable
measurement uncertainty u(Sa) < 1 nm for the scattered light measuring system. The low uncertainty offers ideal
preconditions for in-process roughness measurements in an industrial environment with an aspired resolution of 1 nm.
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