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X-ray computed tomography (CT) enables nondestructive evaluation and dimensional metrology using the reconstructed volumes of measured objects. Although the abundant applications of industrial X-ray CT exist, a high-quality reconstructed volume requires a long measurement time owing to sharp transmission images from dense views. Intensive X-rays with the exposure kept can be used to reduce the measurement time. However, the quality of the measured objects inevitably deteriorates as a result of the so-called penumbra effect caused by large focal spots in X-ray sources. This paper proposes rotational fine-tuning (RFT) for the acceleration of CT scanning with the quality kept. First, sharp transmission images from sparse views are obtained in appropriate imaging conditions. Subsequently, blurry transmission images from dense views are obtained using a larger focal spot size and a shorter exposure time. The acquired blurry images are deblurred by convolutional neural networks fine-tuned using several pairs of sharp and blurry images obtained at the corresponding projection angles, and linear interpolation integrates the deblurred images to generate the final RFT output. The proposed method indirectly reduces the measurement time because no lengthy acquisition time is required, and fine-tuning and blurring are rapid when using GPUs. In the experiments, the comparison of PSNRs between blurry and deblurred reconstructed volumes is shown. To reveal the effect of the proposed method on dimensional metrology, the improvement in the surface deviations of the stepped cylinder is also shown. Finally, the improvement of the pore spaces in a porous aluminum is shown to assess the aspect of nondestructive evaluation. Overall, experimental results demonstrate that the proposed method can perform the acceleration of X-ray CT scanning with the quality kept.

The discoveries made in the twentieth-century include attempts to understand the origin of the universe, constituents of matter, enigmas of biological reactions, etc. could be possible mainly due to the extensive development in the field of particle accelerators. All the particle accelerators, irrespective of their scientific application, need precise alignment of various components for proper functioning. Survey and alignment activities in particle accelerators comprise precise positioning of various components, mainly magnets within micrometric tolerances over a length of hundreds of meters relative to the reference position as defined by the beam physicists in an arbitrary plane. The practice of specialized techniques and instruments are compulsory to meet these requirements. Due to the advancement in optical technology, low-cost computation, and fast data acquisition, laser trackers are commonly used in the field of large-scale dimensional metrology (LSDM). The area of LSDM also includes survey and alignment of the components at their nominal value with a precision of better than ±100 μm over a distance ranging from hundreds of meters to a few kilometres in particle accelerators. In the present study, the use and applicability of laser trackers in various precision positioning activities applicable to particle accelerators are presented in detail.

Coordinate measuring machines (CMMs) are the standard displacement systems used for measurements in dimensional metrology. Since measurement with a touch probe mounted on a CMM provides high accuracy, repeatability, and reliability, it has been widely used for mechanical part inspection in manufacturing. The inspection process requires the use of several sensor orientations and optimal positioning of the part in order to measure all features. Recently, the field of probing path planning has become a huge and active research field. In this paper, various techniques aimed at generating the probe paths for part inspection are reviewed. Multiple issues related to the positioning of the part to maximise accessibility, analysis of probe accessibility to measure all inspection features, optimisation of the measurement sequence, distribution of measurement points, and collision avoidance are mentioned. The common research approaches and potential algorithms in this field are also discussed in this paper.

Estimates of the measurement uncertainty are crucial, without which precision measurements are useless from a practical point of view. Dimensional compliance with geometric product specifications (GPS) plays a key role in decision-making in manufacturing engineering. Without rigorous uncertainty assessment, manufacturers risk making incorrect decisions. Coordinate measuring machine (CMM) measurements are suitable when complex measurement tasks of a workpiece must be made, such as measurements of positional tolerances or repetitive measurements. Here, we attempted to solve problems related to the thermal influences in the assessment of uncertainty in Coordinate Metrology. Task-specific experiments based on the identification of residual errors using a hole plate standard calibrated by Physikalisch-Technische Bundesanstalt (PTB) were performed to compare the measurement uncertainty results in the different temperature ranges. The thermal issues of CMMs include measurements of the temperature of the environment and CMM errors, estimation of the uncertainties of the thermal errors of the CMM, and reduction of the uncertainties related to thermal errors of CMMs. Computation of the thermal errors must account for environmental influences and internal heat sources of the CMM. Assessment of the uncertainty of thermal errors covers almost all factors related to the uncertainty of the CMM. The state-of-the-art technique for determining the uncertainty of the measurements is discussed.

Measuring dimensional parameters (such as diameter, distance) by industrial computed tomography (CT) becomes more and more popular because of its advantages such as non-destructive method and short measurement time. However, the goodness of the measured values needs to be evaluated as a requirement of quality control. An aluminium test piece are designed and manufactured for mapping the measurement errors and uncertainties in case of dimensional CT measurements. In this article, the measurement errors are investigated based on the results of an experimental design, response surface method (RSM). Three main factors are varied systematically: the magnification of the CT, the numbers of views (NoV), and the set-up of the scanning mode. In the course of measurement evaluation several GD&T parameters are determined such as diameter of holes, distances between the holes, flatness and perpendicularity. The purpose of this research is to calculate the measurement errors and to determine the factors which have an effect on the dimensional CT measurement process.

Displacement measuring interferometers, commonly employed for traceable measurements at the nanoscale, suffer from non-linearities in the measured displacement that limit the achievable measurement uncertainty for microscopic displacements. Two closely related novel non-linearity correction methodologies are presented here that allow for the correction of non-linearities in cases where the displacement covers much less than a full optical fringe. Both corrections have been shown, under ideal conditions, to be capable of reducing all residual non-linearity harmonics to below the 10 pm level. Engineering and Physical Sciences Research Council (EPSRC) EP/R511894/1 (Project 2199198). Department for Business, Energy and Industrial Strategy; Royal Academy of Engineering Research Fellowship F\201718\174.

Industrial X-ray Computed Tomography (XCT) is one of the available choices for measuring parts’ internal and external geometries. However, there are still several challenges that limit the current XCT technologies from being widely spread as a dimensional metrology tool. In the study presented in this paper, multi-sphere standards were developed and used for the evaluation of the accuracy of dimensional measurements performed with an open-ended 450 kV XCT system of 0.4 mm focal spot. Normally, the multi-sphere standard refers to a combination of several ball rods of various heights, where a ‘ball rod’ is understood as a ruby sphere mounted on a rod that is usually made of carbon fiber or ceramic. The main purpose was to determine if the rod’s material, the spatial distribution of the ruby spheres, and/or the selection of test lengths (i.e., the center-to-center distance between two ruby spheres) influence the determination of the maximum permissible sphere distance error (MPESD) attributable to XCT data. The experimental data suggested ceramic is more suitable as a rod material than carbon fiber, and also, the spatial distribution of the ruby spheres has no obvious influence on the accuracy evaluation of dimensional measurements. However, the selection of test lengths will significantly affect the accuracy evaluation results. In particular, the results of dimensional measurements showed that the evaluation of the MPESD with multi-sphere standards for the XCT system used in this study can be expressed as MPESD = ± (159 + L/800) μm.

X-ray computed tomography (also referred to as X-ray CT, CAT scan, or simply ‘CT’) is a technological advancement with expanding applications, from medical imaging and nondestructive evaluation to, more recently, dimensional metrology. The CT technique is now used to measure a specimen's geometrical dimensions (of both internal and external features). As a result, CT presently contributes to dimensional inspection and geometric analysis for technology companies spanning a variety of industries such as aerospace, automotive, electronics, medical devices, plastic components, metalworking, and additive manufacturing (one of the main drivers presently pushing the use of CT for dimensional measurement). For medical diagnoses or other qualitative analyses that depend mainly on feature recognition dimensional accuracy is not necessary. In contrast, for precision engineering applications accurate dimensional measurement is the essence of X-ray CT metrology. This article describes the development of X-ray CT metrology beginning with a historical overview that spans the discovery of X-rays to the invention of CAT scan and focuses with greater detail on its expansion toward industrial dimensional measurements. Following this overview is a brief review of the current state of the art of the technology—specifically focused on issues of metrology—and of the present standardization efforts in the design of acceptance tests for evaluating the metrological performance of X-ray CT. As of writing, the CT metrology technique is still evolving with several technical issues yet to be resolved, in particular, to find better ways of expressing uncertainties associated with CT dimensional measurements. Supported by data indicating a growing commercial/industrial market, this technology appears to be in an ‘early adoption’ phase.

Key comparison measurements serve as an ultimate tool of quality assurance of results. Whenever the inter-comparison results indicate inconsistency, the participating laboratory needs to take the corrective actions. Practically, the systematic errors involved in the measuring system confines the achievable accuracy. Therefore, the corrective action involves either empirically determine the influences afresh or intuitively reassigns these error values. Alternatively, an analytical method based on inter laboratory comparison results is proposed. The novelty of the proposal is considering task-specific errors in the model that is used for the analysis of interlaboratory comparison results. Without accounting the uncertainties of task-specific errors, the analysis grows complicated and even sometimes it is not feasible. To supplement the proposed method, task-specific errors due to the imperfect geometry of ring gauge, practical inability in implementing the measurement, and unattended environmental influences are explored. The proposed method is demonstrated using some internal diameter key comparison data. The systematic errors responsible for the outlier in the measurement comparison are clearly distinguished.

Terrestrial Laser Scanners (TLSs) are used in a variety of large scale scanning applications such as reverse engineering, assembly of aircraft or ships and surveying. Contrast targets are used with such instruments for enabling scene registration or to establish a scale when used on a scale bar. Currently, the algorithms to calculate the centers of contrast targets (CCT) are either proprietary to the original equipment manufacturers (OEMs) or not precise and accurate. Some of these algorithms may also operate only on OEM's proprietary data format. To overcome these limitations, a novel algorithm was developed at the National Institute of Standards and Technology (NIST) to calculate the center of contrast targets. Several targets in various orientations were scanned by one TLS and their centers were calculated by both the NIST algorithm and one OEM software. The results show that the NIST algorithm is robust, addresses many data quality issues and has better precision than the OEM software in most cases.

This chapter describes the use of lasers for length measurement traceable to the definition of the metre, which is defined today via the speed of light. High-precision and traceable length measurements are especially important for production engineering with a measurement range of up to some ten metres. Today for traceable measurements the use of laser interferometers relying on frequency-stabilized lasers is state-of-the-art. Laser interferometers and grating scales with interferometric readout are also widely used in industrial applications. For machine tools and coordinate measurement machines, normally encoders are used for length measurements, because interferometers are sensitive to refractive index variations, mainly caused by temperature and pressure fluctuations, and more expensive than encoders. Frequency-stabilized lasers of both categories can be found in a list of recommended radiations for the realization of laser wavelength or frequency standards for the realization of the metre.

The precision of LPBF manufactured parts is quantified by characterizing the geometric tolerances based on the ISO 1101 standard. However, there are research gaps in the characterization of geometric tolerance of LPBF parts. A literature survey reveals three significant research gaps: (1) systematic design of benchmarks for geometric tolerance characterization with minimum experimentation, (2) holistic geometric tolerance characterization in different orientations and with varying feature sizes, and (3) a comparison of results, with and without the base plate. This research article focuses on addressing these issues by systematically designing a benchmark that can characterize geometric tolerances in three principal planar directions. The designed benchmark was simulated using the finite element method, manufactured using a commercial LPBF process using stainless steel (SS 316L) powder, and the geometric tolerances were characterized. The effect of base plate removal on the geometric tolerances was quantified. Simulation and experimental results were compared to understand tolerance variations using process variations such as base plate removal, orientation, and size. The tolerance zone variations not only validate the need for systematically designed benchmarks, but also for tri-planar characterization. Simulation and experimental result comparisons provide quantitative information about the applicability of numerical simulation for geometric tolerance prediction for the LPBF process.

Refractive index measurements are required when light is used as the basis of a measurement system. In dimensional metrology, refractive index measurements are used to compensate for the change in the speed of light. This is crucial because the SI unit for the metre is defined as the speed of light in a vacuum. Air refractometers are the most accurate way to measure the speed of light in air. Many research works to date have been performed to measure the refractive index of air using refractometers. This research uses a commercial prism as the vacuum etalon instead of the tube that is used most often. This novelty and newness of our research were to focus on the design, fabrication and testing of a refractometer which uses a permanent vacuum for ease of use but that will still have the same accuracy of other refractometers currently in use. Modifications to existing designs improved the long-term stability compared to other prism refractometers and are also potentially more accurate than tube refractometers. The results achieved with this permanent vacuum refractometer are accurate to 8.4 × 10–8, which compares favourably with other refractometers on accuracy. It also has the added advantage that it does not require a vacuum pump, and with added laser path improved long term stability but still portable and robust enough to use in everyday applications.

Digital manufacturing is one of the key drivers associated with the idea of flexible mass customization, which has been boosted in recent years by the substantial advances in additive manufacturing techniques. Complex free-form geometries, irregular shapes, internal and reentrant features, and variable thicknesses can be reproduced using the CAD model as a geometrical template. The dimensional quality control of additively manufactured parts, on the other hand, requires a measurement technique capable of sampling the high data density of those structures, including the hidden ones. Computed Tomography (CT) is the technology of choice to inspect additively manufactured parts. CT, however, features a complex measurement chain and the measurement accuracy relies on selecting proper scan parameters. Moreover, CT systems for dimensional metrology are expensive assets to buy and maintain. In this regard, the creation and use of the digital twin of a CT system to support these decision-making processes is presented in this work, as well as the correlation of the digital-based results with those obtained with the physical twin.

Metal additive manufacturing (AM) technologies can produce complex components with unique and controlled lattice architectures and advantageous strength-to-weight properties, for which there is increasing interest by important industrial sectors, including aerospace, automotive and biomedical. One critical open issue of metal AM products is related to dimensional and geometrical verification, not only because the dimensional accuracy of products is often poor, but also because the typical geometrical complexity of such products can be difficult to evaluate via traditional measuring instruments (e.g. tactile and optical coordinate measuring systems), especially in the case of lattice structures. X-ray computed tomography (CT) can be used as a dimensional metrology tool enabling advanced control methods that are fundamental for improving the quality of complex metal AM products and processes. However, the quantification of uncertainty of CT dimensional measurements – fundamental to effectively improve the AM process based on CT metrology – is a very complex problem due to several error sources having complex nature, the lack of well accepted models of the full measurement process and consequently the current dearth of internationally standardized procedures. This work investigates a methodology for achieving metrological traceability in CT measurements of AM lattice structures. In particular, Ti6Al4V lattice structures produced by laser powder bed fusion were measured using a metrological X-ray CT system. A new reference object was designed, produced and calibrated to be similar to the investigated AM structures and to determine the task-specific uncertainty of CT-based dimensional measurements of lattice struts. In particular, the newly developed reference object enables the implementation of the substitution method, which – although being established for determining the uncertainty of CT dimensional measurements – has never been applied before to AM lattice structures. Two different approaches were investigated for the method implementation. Results and advantages were documented through dimensional verification of lattice structure features with respect to specified tolerances. Finally, limitations of the method and future perspectives are also discussed.

The quality of parts, before they are used in space applications, is to be essentially certified for its dimensional correctness and freedom from defects to reliably ensure its intended in-flight operation. This is achieved through extensive dimensional inspection and defect detection/analysis by non-destructive evaluation. But traditional evaluation techniques have limitations that are widely known. In metrology, measurement of inaccessible inner features with surfaces that are deformable and highly reflecting by tactile and optical methods are either impossible or extremely challenging. In NDE, two-dimensional X-ray radiographic testing lacks depth information of defects and is restricted by film and source placement. Computed tomography (CT) combines the best of both worlds to overcome such limitations with the unique capability of performing simultaneous NDE and non-contact metrology in a single scan. Its superlative imaging capability allows for one-to-one reconstruction of the scanned object into a 3D model from which dimensional details of any external/internal feature including information of defects; its location and severity can be extracted. Through CT, the quality of inspection data available for interpretation can be improved substantially, thereby increasing the reliability of existing inspection activities. Unlike radiography, a reconstructed CT model can be viewed across any desired virtual plane which further facilitates faster decision making and validation. This paper describes the novel inspection activities performed with the aid of CT on launch vehicle parts and components at ISRO. Five cases of NDE, followed by two unique cases where CT was used for failure analysis (FA) of assembled components and few cases of dimensional inspection are discussed. The merits of using CT in aerospace industry are also summarized.

In dimensional metrology, the coordinate measuring machine (CMM) is the privileged tool using to control the mechanical part. To evaluate a form error of a spherical feature, discrete points are preleved on a feature to evaluates, the software of the machine fit a perfect geometric form to estimates a form error, least-squares fitting and minimum-zone fitting are two widely adopted algorithms to evaluate the form. Several methods for the evaluation of sphericity error have been proposed by different researchers. But when the feature is measured partially, the problem will be difficult, and an initial value must be introduced to the algorithm to ensure convergence to the solution. The proposed paper presents an efficient algorithm based on orthogonal distance regression (ODR) to estimates the parameters of the substitute sphere and the sphericity error. The model proposed tack into account the uncertainty of coordinates points. The obtained results are compared after to NIST reference values.

Nowadays, with 3D topographic features increasingly complicated and complex, scanning probe microscopes have widely been used of nano-scale dimensional measurement in the semiconductor manufacturing and space industry, which can create three-dimensional data over almost all solids in a wide range of ambient. Although scanning probe microscopes readily achieve nanometer level resolution images of measured surfaces, there are several aspects of their behavior that can cause them to yield large measurement errors. In this paper, according to possible traceability pathways for calibration of lateral axes and z axis, both pitch and step height measurements are respectively performed and axis error self-correction model is proposed. The influence of thermal stability on drift error is also discussed. Experimental results show that our method will be helpful for a fast performance evaluation test for scanning probe microscope.

A key focus of the CIRP CAT 2020 is the role of standardization in GPS specification, and the constraints and opportunities provided by standards throughout the design lifecycle, including in the verification of products. This focus is supported by two Keynote presentations from the chairmen of the ASME and ISO committees that develop standards for the specifications of geometric tolerances. The CIRP CAT conferences provide an opportunity for researchers and practitioners to exchange new ideas and discuss the implications of new and evolving research in the areas of geometrical tolerancing, assembly analysis, modeling of manufacturing processes, and standardization.

Using only two derived numbers based on a reference library, this paper shows how through-focus scanning optical microscopy (TSOM) is compatible with computational process control (CPC) for the complete 3Dshape process monitoring of nanoscale to microscale targets. This is demonstrated using three types of target switch widths (CDs) and depths ranging from 50 nm to1.0 μm, and 70 nm to 20 μm, respectively. TSOM is a high through put, low-cost and in-line capable optical dimensional metrology method ideally suited for high volume manufacturing (HVM), complementing other widely used metrology tools.

The three-dimensional architectures for field effect transistors (FETs) with vertical stacking of Gate-all-Around Nanowires provide a pathway to increased device density and superior electrical performance. However, the transition from research into manufacturing will only be successful if their feature shape, critical dimensions, and associated electrical performance are repeatable across the integrated circuit, across the wafer, and among multiple wafers. Patterning process control for these structures will require significant advances in metrology. Two techniques that are potential candidates for this purpose are Mueller Matrix Spectroscopic Ellipsometry based Scatterometry (MMSEscatterometry) and Critical Dimension Small Angle X-Ray Scattering (CDSAXS). In this work, we highlight the characterization of the Nanowire Test Structures fabricated from patterned Si/SixGe1-x/Si/SixGe1-x/Si/SixGe1-x/Si structures using CDSAXS. Preliminary experimental data shows sensitivity to the selective etching of subsurface SixGe1- x. CDSAXS diffraction data provides information in reciprocal space about line shape and periodicity as well as the amount of subsurface etching. Side lobes in the Intensity vs. Qz (structure height) data are observed around Qz positions of around 0.15 nm-1 and 0.5 nm-1 and are at the same positions in the measured and the simulated data for different amounts of SixGe1-x etch in the Nanowire Test Structures. This demonstrates its ability for successful measurement of the critical dimensions and 3D profile of the Nanowire Test Structures, which can then be extended to monitor several key process steps for Nanowire/Nanosheet FET fabrication.

Orthogonal control of both degrees of freedom of a Kerr comb soliton allows high-precision atomic spectroscopy and stabilization., Microresonator-based soliton frequency combs, microcombs, have recently emerged to offer low-noise, photonic-chip sources for applications, spanning from timekeeping to optical-frequency synthesis and ranging. Broad optical bandwidth, brightness, coherence, and frequency stability have made frequency combs important to directly probe atoms and molecules, especially in trace gas detection, multiphoton light-atom interactions, and spectroscopy in the extreme ultraviolet. Here, we explore direct microcomb atomic spectroscopy, using a cascaded, two-photon 1529-nm atomic transition in a rubidium micromachined cell. Fine and simultaneous repetition rate and carrier-envelope offset frequency control of the soliton enables direct sub-Doppler and hyperfine spectroscopy. Moreover, the entire set of microcomb modes are stabilized to this atomic transition, yielding absolute optical-frequency fluctuations at the kilohertz level over a few seconds and

Measurement is the most fundamental concept of science and technology that leads to innovations. As per the general concept, measurement is just a tool to determine quantity, whereas in reality, measurement is a fundamental aspect to control and improve various parameters associated with different technical affairs. In the present scenario, the manufacturing of extremely complexed products needs high quality control to meet the design specifications, desired functional outcomes and norm compliances. In industrial manufacturing, the main objective is quality control by eliminating errors and improving the process by precision dimensional measurement methods/devices and following standards and recommended guidelines. Precision and traceable dimensional metrology caters the industry needs ranging from macro-engineering applications to nanotechnology and helps in supporting the objective “make it right in the first time” and hence has become an inextricable part of the advance manufacturing industry.

The article describes an approach to measure child growth using a coordinate measuring arm. For this purpose, a test set up has been built to measure the lower leg length directly on the knee surface and with the help of a plate. The use of the plate resulted in the distribution of pressure on the surface to be measured. Based on the results, the LLV (a lower leg growth velocity) was determined and studies were carried out to estimate the uncertainty of this factor. The obtained LLV results at 0.4 mm/week, correspond to the results of studies conducted on specialist test set-ups for knemometric measures described in the literature. The obtained results also allow recognizing periods of good health and bad health of the child.

X-ray computed tomography (XCT) became an important instrument for quality assurance in industry products as a non-destructive testing tool for inspection, evaluation, analysis and dimensional metrology. Thus, a high-quality image is required. Due to the polychromatic nature of X-ray energy in XCT, this leads to errors in attenuation coefficient which is generally known as beam hardening artifact. This leads to a distortion or blurring-like cupping and streak in the reconstruction images, where a significant decrease in imaging quality is observed. In this paper, recent research publications regarding common practical correction methods that were adopted to improve an imaging quality have been discussed. It was observed from the discussion and evaluation, that a problem behind beam hardening reduction for the multi-materials object, especially in the absence of prior information about X-ray spectrum and material characterizations would be a significant research contribution, if the correction could be achieved without the need to perform forward projections and multiple reconstructions.

X-ray computed tomography (CT) is uniquely suited for dimensional measurement of components having internal geometry, difficult-to-reach part features, and easy-to-deform or flexible structures. Through the development of standards it is also becoming accepted as a metrology tool. The objectives of this study are to compare dimensional measurements on artifacts containing internal structures and mechanically compliant features using current state-of-the-art CT and coordinate measure machine (CMM) techniques. It also aims to discuss some of the issues with recently developed standards for CT measurement uncertainty estimation. To illustrate the challenges of CMM and the potential of X-ray CT for reliable dimensional inspection, two different problems are presented: 1) characterization of internal geometry in a metallic artifact that has features inaccessible to tactile or vision-based measurement techniques; and 2) evaluation of dimensional measurement of diameters, form, and relative distances in the components of a flexible object with stems that bend due to mechanical contact by tactile instruments, e.g., CMM. In addition, some issues with the direct application of the ISO 15530 series for the estimation of CT measurement uncertainties are discussed, and a generalized formalism for uncertainty budgeting in CT metrology (still based on the ISO 15530 guidelines but with some differences) is applied to the measurements presented throughout this paper. For dimensions of geometric features ranging from 0.6 mm to 65 mm, a comparison between CT and CMM measurements typically resulted in differences of approximately 5 μm or less for most of the measurements, while expanded uncertainties computed for the CT measurements ranged from 1 to 20 μm. CT measurements on the internal geometries resulted in uncertainties comparable to CMM data that were obtained with the metallic artifact disassembled. However, in the particular case of measuring compliant structures, the conventional CMM strategies led to large systematic errors in the measurements. It is demonstrated that using extrapolation methods, measurements from these two systems converged to within ±2 μm.

Non-linearities in interferometric displacement measurements commonly affect both homodyne and heterodyne optical interferometers. Unwanted back reflections (ghost reflections) or polarisation leakage introduce non-linearity terms at harmonics of the illuminating wavelength that cannot be fully corrected for with standard non-linearity correction techniques. A two-wavelength interferometric approach, operating at 632.8 and 785 nm, is presented here that is capable of correcting such non-linearities. Non-linearities are separated from the difference between two displacement measurements made at differing wavelengths with a Fourier approach. Compared to a standard Heydemann ellipse fitting correction, the proposed approach reduces estimated residual non-linearities from 84 to 11 pm in the case of a linear displacement profile. In particular this approach is applicable to the correction of higher order non-linearities that are caused by multiple reflections, and that are therefore very sensitive to alignment conditions.

We review the progress made for compensation of the refractive index of air in laser-based distance measurements in the field of precision engineering. First, a comprehensive analysis is introduced to clarify how the overall measurement uncertainty is affected by the refractive index of the ambient air, particularly for dimensional metrology and geodetic survey using laser in open-air environment. Second, it is explained that the measurement uncertainty can be improved to a 10-8 level by adopting empirical dispersion formulae describing the relation of the laser wavelength with the environment parameters such as temperature, pressure, humidity and carbon dioxide concentration. Third, the principle of refractometers is given to describe that the measurement uncertainty can be enhanced to a 10-9 level by identifying the refractive index of air real time in well-controlled environmental conditions. Fourth, the two-color compensation method is discussed which enables precise laser-based distance measurement simply by using two different wavelengths without actual identification of the refractive index of open air. Finally, the recent approach of using femtosecond lasers is described with emphasis on the performance of the two-color method can be enhanced to a 10-9 level by stabilizing the pulse repetition rate as well as frequency stability of ultrashort pulse lasers with reference to the atomic clocks.

Many applications rely on scanned data, which can come from a variety of sources: optical scanners, coordinate measuring machines, or medical imaging. We assume that the data input to these applications is an unorganized point cloud or mesh of vertices. The objective may be to find particular features (medical diagnostics or reverse engineering) or comparison to some reference geometry (e.g. dimensional metrology). This paper focuses on the feature fitting of a segmented point cloud, specifically for branched, organic structures or structural frames, and targets non-monolithic geometries. In this paper, a methodology is presented for the automated recovery of cross-sectional shapes using centrally located curves. We assume a triangulated surface mesh is generated from the scanned point cloud. This surface mesh is the starting point for our methodology. We then find the curve skeleton of the part which abstractly describes the global geometry and topology. Next after segmenting the curve skeleton into non-branching segments, orthogonal planes to the curve skeleton segments, at preset or adaptive intervals, make slices through the surface mesh edges. The intersection points are extracted creating a 2D point cloud of the cross section. A number of application specific post-processing operations can be performed after obtaining the 2D point cloud cross sections and the curve skeleton paths including: calculations such as area or area moments of inertia, feature fitting or recognition, and digital shape reconstruction. Case studies are presented to demonstrate capabilities and limitations, and to provide insight into appropriate uses and adaptations for the presented methodology. Highlights Automated cross-sectional extraction for branching structures is presented. Methodology utilized skeletonization of object as reference for sampling planes. Surface mesh is sliced to extract a 2D point cloud. Filter algorithm for exclusion of peripheral slicing is presented. Several case studies demonstrate capabilities and limitations of the method.

A thorough understanding about the AFM tip geometry dependent artifacts and tip-sample convolution effect is essential for reliable AFM topographic characterization and dimensional metrology. Using rigid sapphire cylinder protrusions (diameter: 2.25 μm, height: 575 nm) as the model system, a systematic and quantitative study about the imaging artifacts of four types of tips—two different pyramidal tips, one tetrahedral tip and one super sharp whisker tip—is carried out through comparing tip geometry dependent variations in AFM topography of cylinders and constructing the rigid tip-cylinder convolution models. We found that the imaging artifacts and the tip-sample convolution effect are critically related to the actual inclination of the working cantilever, the tip geometry, and the obstructive contacts between the working tip’s planes/edges and the cylinder. Artifact-free images can only be obtained provided that all planes and edges of the working tip are steeper than the cylinder sidewalls. The findings reported here will contribute to reliable AFM characterization of surface features of micron or hundreds of nanometers in height that are frequently met in semiconductor, biology and materials fields.

The article presents a new method of air and material separation for X-ray tomography for engineering applications. It consists of executing additional calibration measurements and determining certain correction for air-material threshold values on the greyscale of the volumetric file obtained as a result of a tomographic measurement. Usability and effectiveness of this method has been experimentally verified on five different tomographs for industrial application. The proposed method performs better than existing well-known, automated algorithm ISO50% when measuring the volume as well as linear dimensions of workpieces made of different materials. Experiments shown considerable improvements of measurements for the tested conditions.

To gain a maximum benefit of high-speed optical scanning in the metrological calibration of emerging AM (Additive Manufacturing) fixtures it is necessary to investigate the metrology performance of laser line scanning in particular on coordinate measuring machines (CMMs) based on new generation of Geometrical Product Specification standard. A set of designed experiments was set up both on an actual fixture and artefacts manufactured by the Additive Manufacturing company, Materialise-RapidFit+. It enables to identify the influence of a fixture’s geometrical characteristic, material, colour, transparency and roughness, as well as of the alignment of the part coordinate system (PCS) mathematically relating to the machine coordinate system (MCS). Performance measures that were tracked included inspection accuracy, measuring repeatability and time consumption. Results demonstrate that the laser line scanning strategy enables to lower the time consumption in the order of 49% for a fixture with six locators mounted on it and a PC with 128GB RAM, especially providing a superior accuracy performance for complex geometries compared to a tactile measuring strategy.

Dimensional metrology for micro structure plays an important role in addressing quality issues and observing the performance of micro-fabricated products. In white light interferometry, the proposed method is expected to measure three-dimensional topography through modulation depth in spatial frequency domain. A normalized modulation depth is first obtained in the xy plane (image plane) for each CCD image individually. After that, the modulation depth of each pixel is analyzed along the scanning direction (z-axis) to reshape the topography of micro samples. Owing to the characteristics of modulation depth in broadband light interferometry, the method could effectively suppress the negative influences caused by light fluctuations and external irradiance disturbance. Both theory and experiments are elaborated in detail to verify that the modulation depth-based method can greatly level up the stability and sensitivity with satisfied precision in the measurement system. This technique can achieve an improved robustness in a complex measurement environment with the potential to be applied in online topography measurement such as chemistry and medical domains.

Accurate identification and measurement of internal voids and porosity is an important step towards improvement of production processes to obtain high quality materials and products. Recently, the importance of knowing the exact size, shape, volume and location of defects has become even higher as tighter requirements and new standards have been introduced in industry. There are several well-established methods for defects evaluation based on various principles (both destructive and non-destructive). However, all conventional methods have various deficiencies and the information about internal voids/porosity that can be extracted is limited. Most of these drawbacks can be overcome by using X-ray computed tomography (CT). Unlike other methods, CT provides full three-dimensional information about shape, size and distribution of internal voids and porosity; however, the accuracy of measurements is still under investigation. Hence, further evaluations on CT porosity measurements must be performed in order to consider X-ray computed tomography a reliable instrument for the assessment and detection of internal defects. A reference object with artificial defects was used in this research work in order to evaluate the accuracy of porosity measurements by CT. The reference object was manufactured by ultra-precision micro-milling. The object contains dismountable components with embedded internal hemispherical features that simulate internal porosity. The artificial porosity was micro-milled on top surfaces of dismountable cylindrical inserts. The hemispherical calottes were thereafter calibrated by traceable coordinate measuring systems and calibrated values were compared to actual values measured by a CT system. The accuracy of CT porosity measurements was then evaluated based on results obtained on various measurands, using different software tools and measuring procedures, comparing real scans to numerical simulations and investigating the influence of CT system parameters settings on measurement results.

Virtual metrology involves the estimation of metrology values using a prediction model instead of metrological equipment, thereby providing an efficient means for wafer-to-wafer quality control. Because wafer characteristics change over time according to the influence of several factors in the manufacturing process, the prediction model should be suitably updated in view of recent actual metrology results. This gives rise to a trade-off relationship, as more frequent updates result in a higher accuracy for virtual metrology, while also incurring a heavier cost in actual metrology. In this paper, we propose an intelligent virtual metrology system to achieve a superior metrology performance with lower costs. By employing an ensemble of artificial neural networks as the prediction model, the prediction, reliability estimation, and model update are successfully integrated into the proposed virtual metrology system. In this system, actual metrology is only performed for those wafers where the current prediction model cannot perform reliable predictions. When actual metrology is performed, the prediction model is instantly updated to incorporate the results. Consequently, the actual metrology ratio is automatically adjusted according to the corresponding circumstances. We demonstrate the effectiveness of the method through experimental validation on actual datasets.

This presentation will introduce how we have utilized a frequency comb for multi-scale high-precision dimensional measurements, ranging from sub-nanometer-scale thermo-plasmonic motions to kilometer-scale inter-satellite-distances.

This study proposes an experimental method for the performance evaluation of a commercial combined probing system equipped with a contacting probe, an imaging probe, and a line laser scanner developed by our cooperation. According to its application purpose, this method first obtains the circular characteristics of our novel designed 3D artefact composed of 2D rings, cone and cylinders. It is suitable to measure the touch probe, imaging probe as well as optical sensor. Next, detailed measuring procedures were defined for this artifact, in order to achieve comparable results and make the measurements feasible for the different sensors involved. A series of representative experiments were carried out on a commercial combined probing system produced by our partner. The experimental results show that multiple probing system form error and location error were less than 4.0μm, and multiple probing system size error was less than 2.0μm in three different registration positions. System combination errors were less than its maximum permissible errors (MPEs). On the other hand, its root mean errors (RMSE) were less than 0.5μm. Therefore, we can conclude that the designed artifact is suitable to be used to assess the performance of CMMs with multiple probing systems. These conclusions are helpful for further use of this combined probing system and can be utilized to optimize those combined parameters further.

Laser interferometry serves a fundamental role in science and technology, assisting precision metrology and dimensional length measurement. During the past decade, laser frequency combs---a coherent optical-microwave frequency ruler over a broad spectral range with traceability to time-frequency standards---have contributed pivotal roles in laser dimensional metrology with ever-growing demands in measurement precision. Here we report spectrally resolved laser dimensional metrology via a free-running soliton frequency microcomb, with nanometric-scale precision. Spectral interferometry provides information on the optical time-of-flight signature, and the large free-spectral range and high coherence of the microcomb enable tooth-resolved and high-visibility interferograms that can be directly read out with optical spectrum instrumentation. We employ a hybrid timing signal from comb-line homodyne, microcomb, and background amplified spontaneous emission spectrally resolved interferometry---all from the same spectral interferogram. Our combined soliton and homodyne architecture demonstrates a 3-nm repeatability over a 23-mm nonambiguity range achieved via homodyne interferometry and over 1000-s stability in the long-term precision metrology at the white noise limits.

International audience; This paper reports on progress in the analysis of time-domain optical coherence tomography (OCT) applied to the dimensional metrology of through-silicon vias (TSVs), which are vertical interconnect accesses in silicon, enabling three-dimensional (3D) integration in microelectronics, and estimates the deviations from earlier, simpler models. The considered TSV structures are 1D trenches and circular holes etched into silicon with a large aspect ratio. As a prerequisite for a realistic modeling, we work with spectra obtained from reference interferograms measured at a planar substrate, which fully includes the dispersion of the OCT apparatus. Applying a rigorous modal approach, we estimate the differences to a pure ray tracing technique. Accelerating our computations, we focus on the relevant fundamental modes and apply a Fabry–Perot model as an efficient approximation. Exploiting our results, we construct and present an iterative procedure based on the minimization of a merit function, which concludes TSV heights reliably, accurately, and rapidly from measured interferograms.

Optical scatterfield imaging microscopy technique which has the capability of controlling scattered fields in the imaging mode is useful for quantitative nanoscale dimensional metrology that yields precise characterization of nanoscale features for semiconductor device manufacturing process control. To increase the sensitivity in the metrology using this method, it is required to optimize illumination and collection optics that enhance scatterfield signals from the nanoscale targets. Partial coherence of the optical imaging system is used not only for enhancing image quality in the traditional microscopy or lithography but also for increasing the sensitivity of the scatterfield imaging microscopy. This paper presents an empirical investigation of the effect of partial coherence on measurement sensitivity using a deep ultraviolet scatterfield imaging microscope platform that uses a 193 nm excimer laser as a source and a conjugate back focal plane as a unit for controlling partial coherence. Dimensional measurement sensitivity is assessed through analyzing scatterfield images measured at the edge area of periodic multiline structures with nominal linewidths ranging 44–80 nm on a Molybdenum Silicide (MoSi) photomask. Intensities scattered from the targets under the illuminations with various partial coherence factors and two orthogonal polarizations are assessed with respect to sensitivity coefficient. The optimization of partial coherence factor for the target dimension is discussed through the sensitivity coefficient maps.

As the modern technology becomes increasingly nanoscale, microscopy and metrology methods relying on visible and UV wavelengths can no longer fully satisfy scientific and industrial needs. Reduction of the working wavelength to the extreme ultraviolet (EUV, 5 to 50 nm) region is a promising way to extend the applicability of optical techniques to cutting-edge nanotechnology. Short wavelengths and efficient interaction of EUV radiation with matter provides spatial and depth resolution on the nanoscale, which enables not only ultra-high-resolution lithography, but also microscopy, high-precision thin film analysis and dimensional metrology of nanostructures. The presentation is focusing on laboratory-based applications of EUV radiation enabled by compact plasma-based EUV radiation sources. The topics of nanoscale imaging, defect detection, thin film analysis and critical dimensions metrology will be discussed. The developed laboratory tools, such as multi-angle spectroscopic EUV reflectometer and EUV dark-field microscope, will be presented together with latest obtained experimental results. Thin film analysis with sub-nm precision will be demonstrated and benchmarked against state-of-the-art X-ray reflectometry. Specific modelling challenges in the EUV region will be outlined and possible solutions and opportunities will be presented. At the end general limits of applicability of the EUV radiation for industrial and scientific metrology will be discussed together with an outlook to potential perspectives of the EUV-based optical metrology.

In the microelectronics industry, most of the dimensional metrology relies on critical dimension (CD) estimation. These measurements are mainly performed by critical dimension scanning electron microscopy, because it is a very fast, mainly nondestructive method and enables direct measurements on wafers. To measure CDs, the distance is estimated between the edges of the observed pattern on an SEM image. As the CD becomes smaller and smaller, the needs for more reliable metrology techniques emerge. In order to obtain more meaningful and reproducible CD measurements regardless of the pattern type (line, space, contact, hole, etc.), one needs to perform a CD measurement at a known and constant height due to a methodology that determines the topographic shape of the pattern from SEM images. An SEM capable of bending the electron beam (up to 12 deg in our case) allows images to be caught at different angles, giving access to more information. From the analysis of such images, pattern height and sidewall angles can be determined using geometric considerations. Understanding interaction between three-dimensional (3-D) shapes, pattern materials, and the electron beam becomes essential to correlate topography information. A preliminary work based on Monte–Carlo simulations was conducted using JMONSEL, a software developed by the National Institute of Standards and Technology. With this analysis, it is possible to determine theoretical trends for different topographies and beam tilt conditions. Due to the effects highlighted by simulations, the processing of the tilted beam SEM images will be presented, as well as the method used to create a mathematical model allowing topographic reconstruction from these images. Finally some reconstruction using this model will be shown and compared to reference measurements. The overall flow used to process images is presented. First, images are transformed into grayscale profiles. After a smoothing procedure, positional descriptors are computed for specific profile derivatives values. Then, from these descriptors coming from two images of the same pattern taken at different tilt angles, we use a low-complexity linear model in order to obtain the geometrical parameters of the structure. This model is created and initially calibrated using JMONSEL simulations and then recalibrated on real silicon patterns. We demonstrate that the use of real SEM images coming from real silicon patterns with our model leads to results that are coherent with conventional 3-D measurements techniques taken as reference. Moreover, we are able to make reliable reconstructions on patterns of various heights with a single calibrated model. Our batch of experiment shows a three-sigma standard deviation of 10 nm on the estimated height for heights ranging from 50 nm to more than 200 nm. Based on simulations, we are able to reconstruct the corner rounding (CR) from SEM images. However, because our wafer has no CR variability, measurements still need to be assessed on real wafer.

In this work, using our range-resolved interferometry (RRI) signal processing technique, we present a novel approach to multidimensional displacement measurements using only a single optical access port and very simple optical setup. By utilising surface reflections from a stage-mounted moving beamsplitter and two orthogonal stationary reference mirrors, two interferometers for the two Cartesian measurement directions are formed. With RRI, the interferometric phase signals of both interferometers can be independently demodulated, allowing simultaneous measurements of displacement in both dimensions using a single continuous-wave laser diode source and a single photodetector. In this paper, the capabilities of this approach are demonstrated using a proof-of-concept experiment with a multidimensional Piezoelectric stage performing a variety of stage movements. Measurements of displacements over a nominal stage working range of ±50μm are presented, demonstrating independent, simultaneous displacement measurements of two dimensions. The presented measurements show nanometer-level displacement resolutions with typical noise densities of 0.02 nm/√Hz over a 21 kHz bandwidth. It is thought that this approach could offer an interesting alternative to existing interferometric techniques for multi-dimensional metrology, benefiting from both simplicity and cost-effectiveness whilst maintaining the advantages that make optical techniques attractive to scientific and industrial applications.

The successful combination of electromagnetic scattering simulations and optical measurements allows for the quantification of deep-subwavelength features, including thicknesses via ellipsometry and parameterized geometries via scatterometry. Although feature size reduction has slowed in recent years, nanoelectronics still yields ever-smaller structures, thus optical measurement capabilities are ever-challenged. The critical problem is that the optical properties of materials often become thickness dependent at sub-5 nm, greatly complicating accurate fitting. These optical properties can be characterized empirically using ellipsometry and used with other prior information to reduce uncertainties via hybrid metrology, but atomistic modeling offers a unique perspective on the macroscopic optical response from features with dimensions only a few atoms in width. To illustrate the potential of such modeling, we have performed a series of density-functional theory (DFT) calculations for an ultrathin film, Si with hydrogen-terminated Si(111) surfaces. Kohn-Sham wavefunctions determined in DFT are instrumental in solving for the dielectric tensor of these configurations, as the in-plane and out-of-plane components can differ greatly with respect to incident wavelength and Si thickness. Techniques for DFT and dielectric tensor determination are reviewed, highlighting both their limitations and potential for improving optics-based metrology. The thickness- and wavelength-dependence of the resulting tensor components are parameterized using Tauc-Lorentz and Lorentz oscillators. Using an illustration from goniometric reflectometry, the quantitative effects upon dimensional metrology of employing the full thickness-dependent dielectric tensor are compared against simpler approximations of these optical properties. Reductions in parametric uncertainty in the thickness and optical constants are evaluated with a prior knowledge of the ultrathin film’s thickness with uncertainties.

We demonstrated that an iodine stabilized Distributed Bragg Reflector (DBR) diode based laser system lasing at a wavelength in close proximity to $\lambda = 633\,$nm could be used as an alternative laser source to the He-Ne lasers in both scientific and industrial metrology. This yields additional advantages besides the optical frequency stability and coherence: inherent traceability, wider optical frequency tuning range, higher output power and high frequency modulation capability. We experimentally investigated the characteristics of the laser source in two major steps: first using a wavelength meter referenced to a frequency comb controlled with a hydrogen maser and then on a interferometric optical bench testbed where we compared the performance of the laser system with that of a traditional frequency stabilized He-Ne laser. The results indicate that DBR diode laser system provides a good laser source for applications in dimensional (nano)metrology, especially in conjunction with novel interferometric detection methods exploiting high frequency modulation or multiaxis measurement systems., Comment: Published 16 February 2017 copyright 2017 IOP Publishing Ltd