Three-dimensional Metrology Research Articles

Deep learning of 3D point clouds for detecting geometric defects in gears

Geometric integrity directly impacts the functionality, reliability, and safety of final manufactured products, making the qualification of parts based on measurements of their geometry a fundamental quality control activity in modern manufacturing. Recent advancements in three-dimensional (3D) metrology technologies have enabled fine-scale inspection of geometric integrity characterized by dimensional accuracy, surface quality, and shape conformity. However, the widespread adoption of high-resolution 3D metrology in manufacturing faces some significant challenges posed by the inherent data structures of 3D point clouds such as high dimensionality, unstructured nature, and sparsity in defective regions. To address these challenges, this paper first creates a “MFGNet-gear” dataset, which is a scalable and comprehensive benchmark dataset comprising 12 part designs with four quality classes for each design. Subsequently, we develop a deep learning model adapted from the PointNet++ architecture to enable automated, end-to-end analysis of 3D point clouds. The model can be configured for different decision-making tasks including part design classification and multi-class geometric defect detection. Implementations of the proposed model on the “MFGNet-gear” dataset achieve accuracies up to 100% in classifying gear designs and up to 85% in four-class quality inspection. Additionally, we systematically investigate the impacts of measurement resolution and precision on the classification performance through a series of case studies. The obtained results highlight the potential of using deep learning methods for automated analysis of 3D point clouds for a variety of quality control tasks beyond gear manufacturing. This study also proposes future research directions, including the development of new deep learning architectures specifically designed for manufacturing 3D point clouds and strategies for adaptive measurement planning.

Influence of crown shade, translucency, and scan powder application on the trueness of intraoral scanners

ObjectivesNatural teeth and dental restorations present with various shades and levels of translucency. This study aimed to determine whether these variations in ceramic crowns and scan powder application affect the trueness of intraoral scanners. MethodsEight identical premade resin typodonts, each prepared for a crown on the maxillary right second molar, were used. Eight lithium disilicate crowns, distinguished by two levels of translucency (high and low) and four shades (BL1, A2, A3, and A4), were fabricated to an identical design and cemented onto each typodont, providing eight distinct experimental groups (2 levels of translucency × 4 shades). Reference scans were acquired using a desktop scanner. Test scans were performed ten times for each experimental group using two different intraoral scanners (Medit i700 and CEREC Primescan AC), with and without the application of scan powder (n = 10). Three-dimensional metrology software was used to assess the trueness of the intraoral scan datasets. Statistical analysis involved the Kruskal–Wallis H test, Mann–Whitney U test, and independent t-test (α=0.05). ResultsFor powder-free intraoral scan datasets, the crown shade did not significantly affect trueness within each translucency group (P = 1.000). For both intraoral scanners, compared with low translucency groups, higher marked deviations were exhibited by high translucency groups (P<.001). Scan powder use largely mitigated these differences (P>.05) and enhanced the trueness of the intraoral scan (P<.01). ConclusionsShade did not significantly influence the trueness of intraoral scans. High-translucency crowns were scanned with less accuracy than were low-translucency crowns. Clinical SignificanceUnlike tooth shade, translucency significantly affected the accuracy of intraoral scans. Therefore, considering the use of scan powder when scanning objects with high translucency may be beneficial.

Modeling and algorithm of three-dimensional metrology with critical dimension scanning electron microscope

To address the challenges of accurate metrology of height and sidewall angle (SWA) of three-dimensional structures with critical dimension scanning electron microscopy, general modeling and algorithms based on tilted electron beam technology and the corresponding application skills are proposed, and the validity and error accuracy are evaluated as well. Three typical structures, the trapezoid, inverted trapezoid, and step, are investigated with Monte Carlo simulation. The maximum derivative method is used to extract key parameters from the secondary electron linescan. The efficiency of the proposed modeling, algorithms, and the accuracy of the calculated height and SWA are improved further with the application of wavelet transform.

Measurement Techniques for Three-Dimensional Metrology of High Aspect Ratio Internal Features—A Review

Non-destructive measurements of high aspect ratio microscale features, especially those with internal geometries such as micro-holes, remain a challenging metrology problem that is increasing in difficulty due to the increasing requirement for more complexity and higher tolerances in such structures. Additionally, there is a growing use of functional surface texturing for improving characteristics such as heat transfer and wettability. As a result, measurement techniques capable of providing dimensional form and surface finish for these features are of intense interest. This review explores the state-of-the-art inspection methodologies compatible with high-aspect-ratio structures and their suitability for extracting three-dimensional surface data based on identified high-aspect ratio structure types. Here, the abilities, limitations, challenges, and future requirements for the practical implementation and acceptance of these measurement techniques are presented.

Evaluation of the functional acceptability of the ITER vacuum vessel

The International Thermonuclear Experimental Reactor (ITER) vacuum vessel (VV) is one of the critical components of the ITER tokamak fusion reactor. The first sector of the ITER VV was delivered to ITER Organization in 2020, and it is ready to assemble into the tokamak system. After manufacturing the ITER VV, an evaluation should ensure that the components are designed and manufactured to meet the functional requirements, such as vacuum leak tightness and structural integrity. The factory acceptance test (FAT) is essential for confirming acceptance in engineering and manufacturing. This paper introduces the engineering process and technical method of the FAT, which is applied explicitly to the first-of-a-kind ITER VV. We establish a visual inspection, pre-pumping assessment, pressure test, helium (vacuum) leak test, and final dimensional inspection for the FAT. The visual inspection revealed no blockages in the cooling channels of the double walls. The pre-pumping assessment conducted to check the vacuum level and residual gas condition, concluded that the inside of the VV was flawless and thus met the leak test requirements of 1 × 10−8 Pa m3 s−1. We confirmed no leakage or deformation through the pressure test under reduced pressure. The helium leak test demonstrated engineering soundness with leak tightness of 6.08 × 10−9 Pa m3 s−1, which is more stringent than the allowable limit. Furthermore, three-dimensional metrology was utilized to determine the as-built dimensions of the manufactured sector. Due to unavoidable weld deformation and tight tolerances, the as-built result does not perfectly meet the assigned tolerance level. Nevertheless, it can be considered as advanced information for assembly with in-vessel components and other sectors. Based on the conformance and suitability of the suggested FAT for the first ITER VV sector, we will determine the acceptability of the upcoming VV sectors, which will be manufactured and delivered by Korea shortly.

High-accuracy camera calibration method based on coded concentric ring center extraction

In the field of three-dimensional (3-D) metrology based on fringe projection profilometry (FPP), accurate camera calibration is an essential task and a primary requirement. In order to improve the accuracy of camera calibration, the calibration board or calibration target needs to be manufactured with high accuracy, and the marker points in calibration image require to be positioned with high accuracy. This paper presents an improved camera calibration method by simultaneously optimizing the camera parameters and target geometry. Specifically, a set of regularly distributed target markers with rich coded concentric ring pattern is first displayed on a liquid crystal display (LCD) screen. Then, the sub-pixel edges of all coded bands radial straight lines are automatically located at several positions of the LCD screen. Finally, the sub-pixel edge point set is mapped into parameter space to form a line set, and the intersection of the lines is defined as the center pixel coordinates of each target point to complete the camera calibration. The simulation and experimental results verify that the proposed camera calibration method is feasible and easy to operate, which can essentially eliminate the perspective transformation error to improve the accuracy of camera parameters and target geometry.

Surface metrology by multiple-wavelength coherent modulation imaging.

With the rapid progress of advanced manufacturing, three-dimensional metrology techniques that are able to achieve nanometer spatial resolution and to capture fast dynamics are highly desired, for which a snapshot ability and a common-light-path setup are required. Commonly used off-axis holography and phase-shifting interferometry are short in fulfilling those requirements. We studied the suitability and performance of the coherent modulation imaging (CMI) method for metrology applications. Both transparent and reflective samples are measured in visible light experiments. Thanks to its ability to retrieve separate wavefronts at different wavelengths from a single measurement, CMI allows for attaining an enlarged range of measurement free from phase wrapping by utilizing the concept of synthetic wavelength. The CMI method fulfills well the requirements for advanced metrology and can be implemented at any wavelength. We expect it would be a powerful addition to the pool of advanced metrology tools.

Toward realization of high-throughput hyperspectral imaging technique for semiconductor device metrology

Background: High-throughput three-dimensional metrology techniques for monitoring in-wafer uniformity (IWU) and in-cell uniformity (ICU) are critical for enhancing the yield of modern semiconductor manufacturing processes. However, owing to physical limitations, current metrology methods are not capable of enabling such measurements. For example, the optical critical dimension technique is not suitable for ICU measurement, because of its large spot size. In addition, it is excessively slow for IWU measurement. Aim: To overcome the aforementioned limitation, we demonstrate a line-scan hyperspectral imaging (LHSI) system, which combines spectroscopy and imaging techniques to provide sufficient information for spectral and spatial resolution, as well as high throughput. Approach: The proposed LHSI system has a 5-μm spatial resolution together with 0.25-nm spectral resolution in the broad-wavelength region covering 350 to 1100 nm. Results: The system enables the simultaneous collection of massive amounts of spectral and spatial information with an extremely large field of view of 13 × 0.6 mm2. Additionally, throughput improvement by a factor of 103 to 104 can be achieved when compared with standard ellipsometry and reflectometry tools. Conclusions: Owing to its high throughput and high spatial and spectral resolutions, the proposed LHSI system has considerable potential to be adopted for high-throughput ICU and IWU measurements of various semiconductor devices used in high-volume manufacturing.

Autostereoscopic-Raman Spectrometry-Based Three-Dimensional Metrology System for Measurements, Tracking and Identification in a Volume

Three-dimensional compound measurement within a volume of interest is of great importance in industrial manufacturing and the biomedical field. However, there is no current method that can simultaneously perform spatial localization and 3D measurement in a non-scanning manner as well as the identification of material in a volume. In this paper, an Autostereoscopic-Raman Spectrometry-based (ARS) three-dimensional measurement system is proposed. The target object in a large depth range is initially positioned by the autostereoscopic 3D measurement method, and then the accurate position information is cross-checked and obtained by combining the spectral signal. Meanwhile, the spectral signal at the precise excitation position guided by the autostereoscopic signal also carries the material composition information. In order to verify the proposed ARS method, an associated measurement system was developed, and experimental studies of detecting various fibers of different depths in multi-layer glass structure were conducted. The spatial locations and dimensional information of multiple different targets can be measured in a volume, and their material can also be identified at the same time. The average error between the calculated position processed by the ARS system and the actual spatial position is within sub-micron levels, and the success rate of spectrum acquisition reaches 98%.

Accuracy of Digital Impressions at Varying Implant Depths: An In Vitro Study.

Implants placed at variable depths may vary the amount of visible scannable surface of a scan body. Intraoral scanner technology uses advanced optical principles to record the surface of the scan body to accurately capture the implant position. The purpose of this study is to investigate the effect implant placement depth has on the accuracy of digital implant impressions using an intraoral scanner. A partially edentulous gypsum master model was fabricated to allow the positioning of a single implant analog at different depths. Four groups were created based on the planned implant depths of 7, 6, 3, and 0 mm and corresponding visibility of the scan body at 2, 3, 6, and 9 mm. The model was digitized with a laboratory scanner for the reference scan and with an intraoral scanner to generate 15 test scans per group, with a total of 60 scans. The test scans were superimposed onto the reference scan using the best fit algorithm to analyze and measure the positional (dXYZ) and angular deviation (d⍬) of the scan body using three-dimensional metrology software. Statistical analysis was performed using a one-way ANOVA and pairwise comparison was done with a Tukey-Kramer HSD test (α = 0.05). The one-way ANOVA of the groups for the dXYZ and dθ parameters was statistically significant (F3,56 = 11.45, p < 0.001, F3,56 = 24.04, p < 0.001). Group D (9 mm) showed the least positional deviation at 38.41 μm (95% CI 30.26; 46.56) and the least angular deviation of 0.17° (95% CI 0.12; 0.21). Group A (2 mm) showed the greatest positional deviation of 77.17 μm (95% CI 65.23; 89.11) and greatest angular deviation of 0.84° (95% CI 0.65; 1.03). The positional and angular deviation increased with increased implant depth. The accuracy of digital impressions is influenced by the implant depth and the amount of visibility of the scan body. The trueness and precision are highest when the implant is placed at 0 mm depth with complete visibility of the scan body and decreases with subgingival implant placement.

Development and evaluation of three-dimensional metrology of nanopatterns using electron microscopy

The semiconductor industry continuously moves toward smaller features and more complex, three-dimensional (3D) structures to enable next-generation devices. Thus there is a high need for 3D metrology techniques that would provide feedback to the advancing nanoscale fabrication processes. We introduce an approach for evaluating, calibrating, and developing new metrology of nanopatterns through comparing the new metrology data to 3D ground truth data, obtained by accurate scanning transmission electron microscopy (STEM) tomography 3D characterization. We demonstrate this approach by evaluating 3D height maps of sub-20-nm patterns obtained using multi-secondary electron detector scanning electron microscopy (multi-perspective SEM) and photometric stereo algorithm. We demonstrate the importance of full 3D characterization, including cross-sectional structure, height fluctuations, and line edge roughness to truly probe the pattern’s average structures and their 3D variations. Although STEM tomography is a high-resolution and accurate method, it is considered demanding in terms of sample preparation and cannot be used as an efficient in-line metrology method. However, we show that it can be used as a reliable 3D characterization technique to obtain the necessary 3D data for the development of new 3D metrology tools and demonstrate this approach by evaluating the accessible, in-line, multi-perspective SEM.

The Digitalisation in Cobalt-Chromium Framework Fabrication. Surface Roughness Analysis: A Pilot Study

Selective laser melting (SLM) is a new technique in fabricating cobalt-chromium denture framework. However, the surface properties of cobalt-chromium denture framework fabricated using the aforementioned technique have not been widely investigated. The aim of this paper was to investigate the surface roughness of cobalt-chromium alloy in removable partial denture fabricated with SLM technique. Cobalt-chromium denture frameworks were fabricated with two techniques (n = 10); the conventional lost-wax casting (conventional group) and SLM techniques (SLM group). Specimens from the conventional group were subjected to the standard cobalt-chromium denture polishing protocols. No treatment was employed for specimens from the SLM group. All specimens were subjected to surface roughness measurement on polished and fitting surfaces using non-contact optical three-dimensional metrology and surface roughness analysis machine (Infinite Focus Real 3D Alicona). Statistical analysis showed no significant difference in surface roughness between the specimens from conventional and SLM groups (p &gt; 0.05). There was no statistically significant difference in surface roughness between the polished and fitting surfaces of SLM specimens (p &gt; 0.05). Surface roughness quality of the cobalt-chromium denture framework fabricated with the SLM technique is comparable to that fabricated with the conventional lost-wax casting technique. The surface roughness of SLM fabricated cobalt-chromium denture frameworks carries the same surface roughness quality between the polished and fitting surfaces.

Explore3DM—A Directory and More for 3D Metrology

Explore3DM will be an online resource to explore the diverse interests behind three-dimensional measurement and three-dimensional metrology (3DM). The motivation has been the development of large-volume and portable 3D methods and systems for applications in manufacturing, an activity which has been growing for the past 40 years. However, the measurement spectrum in Explore3DM will be wider and include, for example, as-built process plant at the large-object end and X-Ray CT inspection at the small-object end. This wider spectrum will support cross-sector research at University College London (UCL) to transfer 3DM developments from one sector to another. Initially, Explore3DM will have a core directory incorporating systems manufacturers, service suppliers, research groups and disseminators of metrology knowledge. Mechanisms for solving end users’ measurement tasks will add to further growth of 3DM. The resource is intended to be free to use and the directory free to join at a basic level. Premium directory sponsorship by commercial companies is expected to provide revenue to sustain and develop the resource and support 3DM development. With regard to standards, LVM and PCM systems and techniques can be difficult to assess with a standardized approach because of the highly flexible ways they can be applied. However, some standards have been developed and there is scope for more, for example in the terminology used. A dictionary will be a component of Explore3DM’s future knowledge base. By presenting a first version in a centralized resource, standardized terminology will be encouraged.

Nondestructive characterization of nanoscale subsurface features fabricated by selective etching of multilayered nanowire test structures using Mueller matrix spectroscopic ellipsometry based scatterometry

Nondestructive measurement of three-dimensional subsurface features remains one of the most difficult and unmet challenges faced during the fabrication of three-dimensional transistor architectures, especially nanosheet and nanowire based field effect transistors. The most critical fabrication step is the selective etching of silicon-germanium subsurface layers. The resulting shape and dimensions of the remaining Si(1 − x)Gex structure strongly impacts further processing steps and ultimately the electrical performance of gate-all-around transistors, thus creating the need for accurate inline metrology. In order to demonstrate the ability to characterize this etch, nanowire test structures made from Si(1 − x)Gex/Si/Si(1 − x)Gex/Si/Si(1 − x)Gex/Si multilayers have been characterized using Mueller matrix spectroscopic ellipsometry based scatterometry. Transmission electron microscopy images were used to corroborate the authors’ scatterometry measurements. Here, they successfully demonstrate the ability to measure the Si(1 − x)Gex etch, providing an industrially viable technique for inline three-dimensional metrology.

X-ray characterization of contact holes for block copolymer lithography

The directed self-assembly (DSA) of block copolymers (BCPs) is a promising low-cost approach to patterning structures with critical dimensions (CDs) which are smaller than can be achieved by traditional photolithography. The CD of contact holes can be reduced by assembling a cylindrical BCP inside a patterned template and utilizing the native size of the cylinder to dictate the reduced dimensions of the hole. This is a particularly promising application of the DSA technique, but in order for this technology to be realized there is a need for three-dimensional metrology of the internal structure of the patterned BCP in order to understand how template properties and processing conditions impact BCP assembly. This is a particularly challenging problem for traditional metrologies owing to the three-dimensional nature of the structure and the buried features. By utilizing small-angle X-ray scattering and changing the angle between the incident beam and sample we can reconstruct the three-dimensional shape profile of the empty template and the residual polymer after self-assembly and removal of one of the phases. A two-dimensional square grid pattern of the holes results in scattering in both in-plane directions, which is simplified by converting to a radial geometry. The shape is then determined by simulating the scattering from a model and iterating that model until the simulated and experimental scattering profiles show a satisfactory match. Samples with two different processing conditions are characterized in order to demonstrate the ability of the technique to evaluate critical features such as residual layer thickness and sidewall height. It was found that the samples had residual layer thicknesses of 15.9 ± 3.2 nm and 4.5 ± 2.2 nm, which were clearly distinguished between the two different DSA processes and in good agreement with focused ion beam scanning transmission electron microscopy (FIBSTEM) observations. The advantage of the X-ray measurements is that FIBSTEM characterizes around ten holes, while there are of the order of 800 000 holes illuminated by the X-ray beam.

Advanced damage detection techniques in historical buildings using digital photogrammetry and 3D surface anlysis

Abstract In the last twenty years, advances in technology led to a progressive digitalization in photography and photogrammetry and to the development of a considerable number of image processing software. In several fields, Digital Image Processing techniques began to spread. For example, in Civil Engineering there are many methodologies for the monitoring of reinforced concrete structures or road pavements. In most cases they involve the application of mathematical and morphological filters to two-dimensional images, to obtain quantitative information about the decay of the analyzed structures. Instead in Architectural Restoration there are still few researches focused on these methodologies, because of the great complexity and uniqueness of historical buildings. Furthermore, until now architectural photogrammetry mainly concerned geometric survey and it was not widely used to diagnose the presence of alterations on buildings, despite the great potential of a non-invasive, contactless survey technique. Therefore, the aim of this research is to create an analysis approach, to detect damages on three-dimensional models, richer in information about depth and volume. The analysis can be carried out through a specific set of spatial and morphological filters for advanced surface analysis, adopting software tools mostly used for three-dimensional metrology and surface topography. A sequence of operations can be executed, allowing to obtain quantitative information about some kinds of alterations (cracks or features induced by material loss) from three-dimensional models like point clouds or polygonal meshes. The procedure was tested and validated on a case study (Palazzo Palmieri, Monopoli – Italy). The result of the research is a low interaction approach, through which it is possible to identify and quantify damages on the surfaces.

Design of three-dimensional structured-light sensory systems for microscale measurements

Recent advances in precision manufacturing have generated an increasing demand for accurate microscale three-dimensional metrology approaches. Structured light (SL) sensory systems can be used to successfully measure objects in the microscale. However, there are two main challenges in designing SL systems to measure complex microscale objects: (1) the limited measurement volume defined by the system triangulation and microscope optics and (2) the increased random noise in the measurements introduced by the microscope magnification of the noise from the fringe patterns. In a paper, a methodology is proposed for the design of SL systems using image focus fusion for microscale applications, maximizing the measurement volume and minimizing measurement noise for a given set of hardware components. An empirical calibration procedure that relies on a global model for the entire measurement volume to reduce measurement errors is also proposed. Experiments conducted with a variety of microscale objects validate the effectiveness of the proposed design methodology.

Inverse conversion algorithm for an all-optical depth coloring camera.

Three-dimensional (3D) metrology has received a lot of attention from academic and industrial communities due to its broad applications, such as 3D contents, 3D printing, and autonomous driving. The all-optical depth coloring (AODC) camera has some benefits in computation load since it extracts depth information of an object fully optically. The AODC camera represents the depth of the object as a variation of wavelength, and spectroscopy is generally required to measure the wavelength. However, in the AODC camera, the color vector in RGB color space is convertible inversely into the wavelength after projection on the normalized rgb plane because the detected spectrum through the gating part has a narrow bandwidth as a result of the width of the slit in the projection part. In this paper, we propose an inverse conversion algorithm from RGB color to depth without spectroscopy. Experimental results are presented to confirm its feasibility. Also, some practical limitations are discussed, resulting from the nonlinearity of the response of the image sensor and the widths of the slits in the projection part and the gating part.

Evaluating the effects of modeling errors for isolated finite three-dimensional targets

Optical three-dimensional (3-D) nanostructure metrology utilizes a model-based metrology approach to determine critical dimensions (CDs) that are well below the inspection wavelength. Our project at the National Institute of Standards and Technology is evaluating how to attain key CD and shape parameters from engineered in-die capable metrology targets. More specifically, the quantities of interest are determined by varying the input parameters for a physical model until the simulations agree with the actual measurements within acceptable error bounds. As in most applications, establishing a reasonable balance between model accuracy and time efficiency is a complicated task. A well-established simplification is to model the intrinsically finite 3-D nanostructures as either periodic or infinite in one direction, reducing the computationally expensive 3-D simulations to usually less complex two-dimensional (2-D) problems. Systematic errors caused by this simplified model can directly influence the fitting of the model to the measurement data and are expected to become more apparent with decreasing lengths of the structures. We identify these effects using selected simulation results and present experimental setups, e.g., illumination numerical apertures and focal ranges, that can increase the validity of the 2-D approach.

Secondary signal imaging (SSI) electron tomography (SSI-ET): A new three-dimensional metrology for mesoscale specimens in transmission electron microscope

We have demonstrated a new electron tomography technique utilizing the secondary signals (secondary electrons and backscattered electrons) for ultra thick (a few μm) specimens. The Monte Carlo electron scattering simulations reveal that the amount of backscattered electrons generated by 200 and 300keV incident electrons is a monotonic function of the sample thickness and this causes the thickness contrast satisfying the projection requirement for the tomographic reconstruction. Additional contribution of the secondary electrons emitted from the edges of the specimens enhances the visibility of the surface features. The acquired SSI tilt series of the specimen having mesoscopic dimensions are successfully reconstructed verifying that this new technique, so called the secondary signal imaging electron tomography (SSI-ET), can directly be utilized for 3D structural analysis of mesoscale structures.