Scanning White Light Interferometry Research Articles

Batwing elimination algorithm based on the Haarwavelet in white-light interferometry.

White-light scanning interferometry is a mature non-contact three-dimensional shape detection method. However, the batwing effect is an inherent problem that causes systematic error. In this paper, we proposed an algorithm for detecting and eliminating the batwing effect based on the Haar wavelet transforms without any prior knowledge of the sample surface. It is verified by simulation and experiment that the proposed algorithm can effectively eliminate the batwing effect and does not increase the time complexity significantly.

Deep learning for sub-Nyquist sampling scanning white light interferometry.

This Letter introduces sub-Nyquist sampling vertical scanning white light interferometry (SWLI) using deep learning. The method designs Envelope-Deep Residual Shrinkage Networks with channel-wise thresholds (E-DRSN-cw), a network model extracting oversampling envelopes from undersampled signals. The model improves the training efficiency, accuracy, and robustness by following the soft thresholding nonlinear layer approach, pre-padding undersampled interference signals with zeros, using LayerNorm for augmenting inputs and labels, and predicting regression envelopes. Simulation data train the network, and experiments demonstrate its superior performance over classical methods in the accuracy and the robustness. The E-DRSN-cw provides a swift measurement solution for SWLI, removing the need for prior knowledge.

A white-light interferometry method for 3D measurement of compactly spaced micro-nano structural units

Micro-nanostructures refer to technologies that operate at the nanoscale and provide materials and devices with unique mechanical, physical, and chemical properties. These structures have diverse applications in fields such as microelectronics, biotechnology, aerospace, and metamaterials. As precision machining technology advances, the demand for smaller sizes and higher accuracy of structural units has increased, leading to more compact integration. Consequently, the lossless and high-precision 3D measurement of highly integrated micro-nanostructure components has become increasingly challenging. In this study, we analyzed measurement errors in compactly spaced microstructural units using white-light scanning interferometry and proposed an algorithm to optimize this technology. By combining coherent peak-histogram processing with signal correlation analysis, we were able to improve the measurement accuracy by approximately ten times compared to current mainstream algorithms. To validate our algorithm, we performed experiments using metasurfaces with structural unit spacing at the order of sub-micrometer. The results obtained demonstrated the effectiveness of our algorithm in achieving accurate measurements.

Envelope peak detection algorithm based on the CEEMDAN in white light interferometry

White light scanning interferometry (WLSI) is a key non-contact three-dimensional (3D) morphometry method. The traditional envelope peak detection (EPD) algorithm is a typical WLSI surface recovery algorithm without phase calculation, but its accuracy is susceptible to noise and fitting errors. In this paper, we propose an improved EPD algorithm based on the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) to solve the above problem. The proposed algorithm can accurately locate the position of the envelope peak. The simulation and experiment results demonstrate that the proposed method is effective in improving the accuracy of the measurements.

P‐29: Noise reduction and defect detection algorithm in white‐light scanning interferometry for display panel inspection

White‐light scanning interferometry (WSI) has been widely used for the inspection of various display panels by 3D surface imaging from the micrometer scale to the nanometer scale. However, WSI often fails to sense light reflected from towering defects due to diffused reflection, represented by near‐zero heights, called black spot (b‐spot). It is challenging to detect the b‐spot defect especially in display pad area with varying heights. In addition, vibration of WSI facility hinders a defect spot due to harsh noise amplification. To resolve the issues, we proposed a novel and efficient image processing algorithm which consists of five steps. The first step is image denoising by median filtering with rectangle kernels. The second is pad area preprocessing to avoid misdetection. The third is to identify the presence of b‐spot defect using curvature of 1D projection array of an image. The fourth is defect detection through minimum value finding method. The last is Laplace fitting to infer the height of towering b‐spot defect. We conducted an experimental study using WSI raw data of defective display panels to evaluate the defect detection rate and the reliability of inferred height. We also calculated the correlation between the inferred height by Laplace fitting and measured height by atomic force microscopy (AFM) as a gold standard. We have shown that the proposed method can effectively detect a towering b‐spot defect position with much improved detection rate and infer the height with high precision.

Evaluation of bone growth around bioactive glass S53P4 by scanning acoustic microscopy co-registered with optical interferometry and elemental analysis

Bioactive glass (BAG) is a bone substitute that can be used in orthopaedic surgery. Following implantation, the BAG is expected to be replaced by bone via bone growth and gradual degradation of the BAG. However, the hydroxyapatite mineral forming on BAG resembles bone mineral, not providing sufficient contrast to distinguish the two in X-ray images. In this study, we co-registered coded-excitation scanning acoustic microscopy (CESAM), scanning white light interferometry (SWLI), and scanning electron microscopy with elemental analysis (Energy Dispersive X-ray Spectroscopy) (SEM–EDX) to investigate the bone growth and BAG reactions on a micron scale in a rabbit bone ex vivo. The acoustic impedance map recorded by the CESAM provides high elasticity-associated contrast to study materials and their combinations, while simultaneously producing a topography map of the sample. The acoustic impedance map correlated with the elemental analysis from SEM–EDX. SWLI also produces a topography map, but with higher resolution than CESAM. The two topography maps (CESAM and SWLI) were in good agreement. Furthermore, using information from both maps simultaneously produced by the CESAM (acoustic impedance and topography) allowed determining regions-of-interest related to bone formation around the BAG with greater ease than from either map alone. CESAM is therefore a promising tool for evaluating the degradation of bone substitutes and the bone healing process ex vivo.

Face Turning of Single Crystal (111)Ge: Cutting Mechanics and Surface/Subsurface Characteristics

Abstract Single crystal Ge is a semiconductor that has broad applications, especially in manipulation of infrared light. Diamond machining enables the efficient production of surfaces with tolerances required by the optical industry. During machining of anisotropic single crystals, the cutting direction with respect to the in-plane lattice orientation plays a fundamental role in the final quality of the surface and subsurface. In this study, on-axis face turning experiments were performed on an undoped (111)Ge wafer to investigate the effects of crystal anisotropy and feedrate on the surface and subsurface conditions. Atomic force microscopy and scanning white light interferometry were used to characterize the presence of brittle fracture on the machined surfaces and to evaluate the resultant surface roughness. Raman spectroscopy was performed to evaluate the residual stresses and lattice disorder induced by the tool during machining. Nanoindentation with Berkovich and cube corner indenter tips was performed to evaluate elastic modulus, hardness, and fracture toughness of the machined surfaces and to study their variations with feedrate and cutting direction. Post-indentation studies of selected indentations were also performed to characterize the corresponding quasi-plasticity mechanisms. It was found that an increase of feedrate produced a rotation of the resultant force imparted by the tool indicating a shift from indentation-dominant to cutting-dominant behavior. Fracture increased with the feedrate and showed a higher propensity when the cutting direction belonged to the <112¯> family.

Surface Properties of Concrete and Bonding Strength of Pine Resin-Based Adhesives to Concrete Surfaces

This article reports a quantitative investigation of the surface characteristics of normal concrete manufactured with different types of formworks and bonding strength between pine resin-based adhesives and concrete surfaces. Three types of formworks and adhesives with different ratios of pine resin to wax were prepared. Scanning electron microscopy, energy-dispersiveX-ray spectroscopy, white-light scanning interferometry, and electrophoretic light scattering were utilized for the surface characterization of concrete, and a bonding test was performed. Test results showed that the surface roughness of concrete was significantly influenced by the type of formwork; all techniques including visual observation provided similar results. On the contrary, chemical compositions and electrical characteristics were not influenced by the type of formwork. Bonding test results showed that bonding strength increased as the ratio of wax increased and the degree of the concrete surface roughness increased. The results of this study indicate the feasibility of pine resin-based adhesives for concrete structures as green construction materials.

Kinematically engaged yoke system for segmented lens-based space telescope integration and testing

One of the most profound and philosophically captivating foci of modern astronomy is the study of Earth-like exoplanets in the search for life in the Universe. The paradigm-shifting investigation described here calls for a new type of scalable space telescope that redefines the available light-collecting area in space. The Nautilus Space Observatory, enabled by multiple-order diffractive optics (the MODE lens), is ushering in the advent of large space telescope lenses designed to search for biosignatures on a thousand exo-earths. The Kinematically Engaged Yoke System (KEYS) was developed to align a segmented version of the MODE lens. A technology demonstration prototype of KEYS was built and tested using scanning white light interferometry and deflectometry. A deflectometry system was also developed to monitor the closed-loop alignment of the segmented MODE lens during its UV (i.e., Ultraviolet) curing.

Advances in Scanning White Light Interferometry for Surface Topography Measurement

Advances in Scanning White Light Interferometry for Surface Topography Measurement

Effect of the micro-textured piston on the performance of a hermetic reciprocating compressor

A hermetic reciprocating compressor is one of the most critical parts for the energy efficiency of a household refrigerator. Piston-cylinder contact in the hermetic compressor accounts for most of the energy loss. The tribological performance of the piston-cylinder pair can be enhanced by introducing micro-texturing on the piston surface. In this research, an experimental study is presented to tribologically assess the effect of the micro-textured piston on the performance of the hermetically sealed reciprocating compressor. The micro-texture on the piston surface was prepared by the laser surface texturing method. Four different micro-textures were studied: radial micro-grooves, axial micro-grooves, mesh micro-grooves, and micro-dimples. The textures’ size, shape, and depth were studied using scanning electron microscopy (SEM) and white light interferometry (WLI) techniques. The results were compared with the non-textured piston compressor. It was found that the radial, axial, and mesh micro-grooves pistons have a negative effect on the coefficient of performance of the hermetic reciprocating compressor. However, the piston with the micro-dimples texture increased the compressor's coefficient of performance by 1%. Refrigerant leakage from the piston-cylinder clearance was also investigated and it was observed that micro-dimples on the piston surface decrease the refrigerant leakage by 35% due to the presence of a continuous oil film between piston and cylinder. The compressor's cooling capacity (Qc) was observed to be increased by 1 W in the case of a micro-dimpled piston.

Performance and efficacy of a recently introduced diamond rotary instrument: Cutting, surface preparation, and cleanability

Statement of problemEvaluation of the cutting efficiency and effectiveness, surface roughness, and cleanability of a novel rotary instrument is lacking. PurposeThe purpose of this in vitro study was to compare the cutting efficiency and effectiveness of a recently introduced diamond rotary instrument containing corundum microspheres with conventional instruments by evaluating the heat generated, surface roughness, and cleanability of each instrument after tooth preparations. Material and methodsSound molars (n=225) were used to evaluate cutting efficiency and effectiveness by measuring the heat generated by 3 diamond dental rotary instruments: test instrument (TI), reference instrument (RI), and NTI instrument (NI). Thirty cavity preparations (27 mm3) were prepared, and the thermal change (ΔT) was determined from a thermocouple inserted in the pulp chamber. The surface roughness of the dentin substrate was determined after veneer preparations using scanning white-light interferometry and scanning electron microscope imaging. The cleanability of TI and RI was also determined by comparing the efficacy of 3 conventional disinfection protocols after contaminating the instrument with Gram-positive or Gram-negative oral pathogens. The mean and standard deviation values for thermal change, surface roughness, and colony forming units were calculated at a 95% confidence level, and 1-way ANOVA was used to determine statistical significance (α=.05). ResultsThe NI instrument had the lowest mean ΔT (1.47 °C). The TI (1.77 °C) and RI (1.85 °C) groups showed statistically similar means (P>.05). The TI presented the lowest surface roughness (1.68 μm), followed by the RI (1.87 μm) (P<.001). The NI resulted in the highest surface roughness (2.17 μm) (P<.001). The disinfection protocols used were more effective on the TI group than on the RI group regardless of organisms and time exposed to the cleaning solution (P<.001). ConclusionsThe novel diamond instrument demonstrated similar cutting efficiency and effectiveness when compared with conventional diamond instruments. However, the novel instrument produced smoother tooth preparations and was easier to clean than the conventional diamond rotary instruments.

Pseudo Wigner-Ville distribution for 3D white light scanning interferometric measurement.

White light scanning interferometry is a commonly used optical measurement method for three-dimensional (3D) surface profiles. In the case of large phase errors, accurate height values can generally be obtained indirectly from the interferometric signal envelope information derived using various envelope extraction methods. However, the current envelope extraction algorithms have the disadvantages of low robustness, increasing the half-width of the envelope information, and requiring correct parameter settings in advance. In this study, the pseudo Wigner-Ville distribution is modified and applied to white light scanning interferometric 3D measurement to avoid the above-mentioned drawbacks. Simulations and experiments are performed in a single-frequency mode (only the approximate central wave-number is used to guide both the proposed and wavelet transform methods). The simulation results prove that the proposed method has a 31.7% higher reconstruction accuracy than the wavelet transform method under a 25 dB signal to noise ratio condition. Concurrently, the proposed method is insensitive to the change in the central wavelength with a constant central wave-number parameter and has a good extraction effect for a long coherent length. The experiments measure standard step objects (VLSI standard, 1.761 ± 0.01 µm), and the reconstruction height error of the proposed method is 0.0035 µm. Simulations and experiments show that the proposed method can adaptively provide accurate envelope information after half-width reduction under the condition that only the approximate central wave-number a priori knowledge is used. Simultaneously, the proposed method is shown to be robust and effective.

Composite wavelet decomposition algorithm combined with correlation analysis in white-light scanning interferometry

White-light scanning interferometry (WLSI) is a mature and efficient technique that is widely used in the field of high-precision three-dimensional measurements. However, coherence and phase information in the WLSI technique are inconsistent, false height jumps, such as batwings artifact and ghost steps, are typical obstacles in applying WLSI, which distort measurement results and reduce the measurement accuracy. In this study, we proposed a composite wavelet decomposition algorithm combined with a correlation analysis to solve the above two obstacles without having any prior knowledge about the tested sample. Two calibration step plates with heights of 459.8 ± 3.0 nm and 90.5 ± 2.8 nm were used to evaluate the accuracy of the proposed algorithm, and the results were compared with those of Centroid and Fourier transform algorithms. Consequently, the proposed algorithm exhibited a higher correction effect, and the accuracy was improved by more than two times, which proved that our approach could effectively suppress the batwings effect and eliminate the ghost steps.

Characterization of gap-plasmon-based metasurfaces with scanning white-light interferometry

Subject of study. The possibility of using the well-established method of scanning white-light interferometry (SWLI), implemented as a commercially available device, is studied for characterizing promising metasurfaces based on third-order plasmon resonance. The characteristics of the samples were determined for two types of experimental metasurfaces, namely, for a binary grating with a period of 12.6 µm and a phase gradient grating with a period of 6.75 µm. Objective. The main goals of this study were to test the fundamental possibility of using a commercially available white-light interferometer to determine the phase characteristics of plasmonic metasurfaces and to estimate experimentally the lateral optical resolution in the phase image of metasurfaces. Main results. Although the method of SWLI was originally intended for measuring a geometric microrelief, it was found that it can be used to determine the phase increment of the optical radiation reflected from the studied metasurfaces in the range of 10°–100° with a relative accuracy of approximately 50% without any modification of the signal processing algorithm. The possibility of obtaining a phase mapping of the surface with such accuracy in this configuration is not obvious because the metasurface is a matrix of nanoelements with the same height of 50 nm, and the phase shift in the reflected optical radiation is created due to third-order gap plasmon resonance. On the obtained optical phase images of metasurfaces, unit (single) cells of the metasurface are reliably resolved owing to a sufficiently high lateral resolution (approximately 450 nm). The results of the characterization of metasurfaces obtained using SWLI are compared with those obtained in a laboratory setup using scanning differential heterodyne microscopy (SDHM). Although the SWLI method demonstrates better lateral resolution compared with the SDHM method, it is less accurate in restoring the phase characteristics of metasurfaces with a phase gradient. Practical significance. The results obtained allow us to conclude that the evaluation of the phase optical characteristics of nanostructured plasmonic metasurfaces by the method of SWLI is possible and requires further experimental and theoretical studies.

Residual Stress Characterization in Cross-Sections of Small Parts By Combining the Contour Method and Scanning White-Light Interferometry

Residual Stress Characterization in Cross-Sections of Small Parts By Combining the Contour Method and Scanning White-Light Interferometry

Maintaining low friction coefficient and ultralow wear in metal-filled PTFE composites

Many reinforcing fillers (particles, fibers, and platelets) can reduce the high wear rate of polytetrafluoroethylene (PTFE) (K∼10−4 to 10−3mm3/Nm) by 10–1000 times. However, certain α-alumina particles can produce up to 10,000 times lower wear rate when composited with PTFE (K∼1.3 × 10−7- 6.7 × 10−8 mm3/Nm). This ultralow wear behavior was attributed to two mechanisms: tribofilm generation and tribochemistry. However, while reducing the wear rate of PTFE, α-Al2O3 can also increase its friction coefficient up to two times (μ∼ 0.1 vs 0.2–0.25). This study revealed that the friction coefficient of PTFE-Cr against Brass 260 counterbody is up to three times lower than the ultralow wear PTFE α-alumina against 304 Stainless Steel (μ = 0.08–0.12 vs μ = 0.2–0.25) with up to 200 times lower steady-state wear rate (K∼1.4-34 × 10−9 mm3/Nm) than PTFE α-alumina against 304 SS (K∼6.7-13 × 10−8), and ∼3 × 105 times lower than unfilled PTFE. PTFE-Ti (μ = 0.09, Kss= 2.9 × 10−8 mm3/Nm) and PTFE-Mn (μ = 0.12, Kss= 1.2 × 10−8 mm3/Nm) against Brass 260 also performed better than PTFE-α-Al2O3 against 304 SS both in reducing wear rate and friction coefficient. The tribofilms created by the PTFE-Cr, PTFE-Mn, and PTFE-Ti composites were evaluated using Scanning White Light Interferometry (SWLI) and Infrared (IR) Spectroscopy. These analyses showed that these composites created thin (500–1500 nm) films on the Brass 260 counterbody and their worn surfaces exhibited new chemical groups (carboxylate salts, hydroxyl groups, and acetate ions). These new chemical groups signal the formation of bonds between the PTFE matrix, filler particles, and the metal counterbody, which protects both the composite and counterbody from wear.

Phase noise estimation based white light scanning interferometry for high-accuracy surface profiling.

White light scanning interferometry (WLSI) has been an extremely powerful technique in precision measurements. In this work, a phase noise estimation based surface recovery algorithm is proposed, which can significantly improve the measurement accuracy by decreasing the noise level in phase map coming from the systemic and environmental disturbances. The noise existed in phase map is firstly researched in spectrum domain and defined as the linear combination of complex terms at each angular wavenumber. Afterwards, based on the theoretical linearity of the phase distribution, the surface features can be redefined through establishing the function with respect to phase noise. By applying least square estimation (LSE), a spectral coefficient is defined to determine the optimal estimation of phase noise that represents the best statistical consistency with the actual case, from which a more accurate surface after removing most phase noise will then be generated. In order to testify the noise elimination ability of the proposed method, a nano-scale step height standard (9.5nm±1.0nm) is scanned, and the measurement result 9.49nm with repeatability 0.17nm is successfully achieved. Moreover, a leading edge of an aero-engine blade is also tested to investigate the potential of this method in industrial inspections. The measurement comparison with AFM is also displayed.

Measurement of Film Structure Using Time-Frequency-Domain Fitting and White-Light Scanning Interferometry

A new technique is proposed for measuring film structure based on the combination of time- and frequency-domain fitting and white-light scanning interferometry. The approach requires only single scanning and employs a fitting method to obtain the film thickness and the upper surface height in the frequency and time domains, respectively. The cross-correlation function is applied to obtain the initial value of the upper surface height, thereby making the fitting process more accurate. Standard films (SiO2) with different thicknesses were measured to verify the accuracy and reliability of the proposed method, and the three-dimensional topographies of the upper and lower surfaces of the films were reconstructed.

Three-dimensional reconstruction of super-resolved white-light interferograms based on deep learning

White-light scanning interferometry is an effective and widely used technology for measuring the microscopic three-dimensional morphology of an object. Its vertical resolution can reach the sub-nanometer level, and its lateral resolution reaches submicron level. However, for the samples containing complex structure or high-density periodic distribution structural units, the measurement results are strongly restricted by magnification and numerical aperture (NA) of the microscopic objective. In this paper, we proposed a three-dimensional reconstruction algorithm for white-light interferograms after super resolution processing, using fast super-resolution convolutional neural networks (FSRCNN) to improve the detailed information of the interferograms, and then we used centroid method combined with the five-step phase-shift method to extract the zero optical path difference (ZOPD) position of the interference signal after super resolution processing. After processed by the proposed method, the interferograms collected by the 10X microscope objective (NA=0.3) recovered the 3D surface is the same as that measured by the 100X microscope objective (NA=0.7), which is proved by the experiment results.