Vertical Scanning White-light Interferometry Research Articles

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.

Passive vibration suppression method based on frequency domain analysis in white light vertical scanning interferometer

Vibration in manufacturing environment can significantly affects the accuracy of surface measurements, especially for the white light vertical scanning interferometry (VSI), which requires mechanical scanning for full-field measurements. This paper proposes a passive vibration suppression technique that integrates a second common-path interferometer and uses frequency domain analysis to measure vibration and reconstruct surface topography. The method is feasible for suppressing vibrations with different frequencies and amplitudes, as verified by simulation and signal recovery analysis. An in-house developed interferometer with a common optical path is used to demonstrate the measurement performance experimentally. The mathematical formula relating the vibration amplitude, the vibration frequency, the frame rate of the camera and the scanning interval is presented. The experimental results show that the precision of plane measurement decreases with increasing vibration amplitude and frequency. Moreover, the step sample signals under a maximum vibration condition of 400 nm/5 Hz are recovered, with a standard deviation of 26.07 nm for the step height measurement. As the scanning interval increases, the system can suppress higher-amplitude vibrations.

Tribological properties of 1 - octyl – 3 - methylimidazolium lactate ionic liquid as a lubricant additive

This study examined the tribological properties of the IL 1 - octyl – 3 - methylimidazolium lactate (L-L108), which is used as an additive to SAE 10W-40 and PEG400, for steel–steel contacts. The ionic liquids 1 - octyl – 3 - methylimidazolium hexafluorophosphate (L-P108) and 1 - octyl – 3 - methylimidazolium tetrafluoroborate (L-B108) were used as reference lubricants because their molecular structures are similar to that of L-L108. Tribological experiments were performed on a ball-on-disk contact tribometer for 60 min (at room temperature, with a load of 5 N, sliding speed of 0.38 m/s, and relative humidity of 30%–55%). Sliding tests showed that the lubricating performance was improved when L-L108 was used as the SAE 10W-40 and PEG400 additive. After the sliding tests, the wear scar was analyzed using white light vertical scanning interferometry and X-ray photoelectron spectroscopy (XPS). The three-dimensional images and cross-sectional line profiles showed the topography of the wear surface. When L-L108 was blended into 10W-40 and PEG400, the binding energy values of C, N, O, and Fe changed compared with a neat steel surface. The XPS analysis indicated that a tribochemical reaction occurred between the mixture and metal substrate during the friction process. This formed a chemical reaction film on the steel surface, and could have been the reason for the excellent tribological performance.

Direct Exposure of Dry Enzymes to Atmospheric Pressure Non-Equilibrium Plasmas: The Case of Tyrosinase.

The direct interaction of atmospheric pressure non-equilibrium plasmas with tyrosinase (Tyr) was investigated under typical conditions used in surface processing. Specifically, Tyr dry deposits were exposed to dielectric barrier discharges (DBDs) fed with helium, helium/oxygen, and helium/ethylene mixtures, and effects on enzyme functionality were evaluated. First of all, results show that DBDs have a measurable impact on Tyr only when experiments were carried out using very low enzyme amounts. An appreciable decrease in Tyr activity was observed upon exposure to oxygen-containing DBD. Nevertheless, the combined use of X-ray photoelectron spectroscopy and white-light vertical scanning interferometry revealed that, in this reactive environment, Tyr deposits displayed remarkable etching resistance, reasonably conferred by plasma-induced changes in their surface chemical composition as well as by their coffee-ring structure. Ethylene-containing DBDs were used to coat tyrosinase with a hydrocarbon polymer film, in order to obtain its immobilization. In particular, it was found that Tyr activity can be fully retained by properly adjusting thin film deposition conditions. All these findings enlighten a high stability of dry enzymes in various plasma environments and open new opportunities for the use of atmospheric pressure non-equilibrium plasmas in enzyme immobilization strategies.

Fundamental aspects of resolution and precision in vertical scanning white-light interferometry

We discuss the height and lateral resolution that can be achieved in vertical scanning white-light interferometry (SWLI). With respect to interferometric height resolution, phase-shifting interferometry (PSI) is assumed to provide the highest accuracy. However, if the noise dependence of SWLI phase evaluation and PSI algorithms is considered, SWLI measurements can be shown to be more precise. With respect to lateral resolution, the determination of the coherence peak position of SWLI signals seems to lead to better results compared to phase based-interferometric measurements. This can be attributed to the well-known batwing effect. Since batwing is a nonlinear effect applying nonlinear filters, e.g. a median filter, it reduces them significantly. If filtering is applied prior to the fringe order determination and phase evaluation, the number of artefacts known as ghost steps can be eliminated without changing the modulus of the phase. Finally, we discuss the dependence of measured height values on surface slope. We show that in interference microscopy there are additional limitations which are more rigid compared to the maximum surface slope angle resulting from the numerical aperture of the objective lens. As a consequence, the measurement precision breaks down at slope changes of steeper flanks even if the modulation depth of the interference signals is still good enough for signal analysis.

White-Light Interferometry for Measuring Fuel Pressure in ICF Polymer-Microsphere Targets

A new technique based on a vertical scanning white-light interferometry is developed for measuring fuel pressure in inertial confinement fusion (ICF) multiple-shell polymer-microsphere targets. Nuclear fuel pressure is an essential parameter for estimating fusion efficiency in ICF experiments. This parameter is difficult to determine because of complicated target structures, short measurement time, relatively short optical path length changes, and expansion of the target after pressurization. To reduce the effects due to changes in diameter, a model is proposed to correct for the expansion at the radial orientation for multiple-shell polymer microspheres. The model is compared to a destructive method, and D2 fill pressure accuracy is confirmed within a 10% error of uncertainty.

Reduction of chromatic aberration influences in vertical scanning white-light interferometry

Vertical scanning white-light interferometry (SWLI) is a well-established method that is widely used in high precision surface topography measurement. However, SWLI results show characteristic slope-dependent errors due to dispersion effects and lateral chromatic aberrations of the optical imaging system. In this paper, we present methods to characterize these systematic errors related to dispersion and lateral colour. Lateral colour leads to field-dependent systematic discrepancies of the topography data obtained from the envelope position of a low-coherence interference signal and the data resulting from its interference phase. Hence, an erroneous fringe order obtained from the envelope position leads to a 2π phase jump and thus to a so-called ghost step in the measured topography. Our first approach to solve this problem is based on the measurement of a surface standard of well-known geometry. By comparison of measurement results related to the envelope position and the phase of SWLI signals, the systematic error is estimated and a numerical error compensation method is proposed. Both experimental and simulation results confirm the validity of this numerical method. In addition, using an improved design of a white-light Michelson interferometer we demonstrate experimentally that lateral chromatic aberrations and dispersion influences can be reduced also in a physical way. In this context, a conventional long working distance microscope objective is used which was not originally designed for a Michelson interference microscope.

Surface topographic characterization for polyamide composite injection molds made of aluminum and copper alloys

In order to ensure flexibility and rapid new product development, the mold industry made use of soft materials for cavity inserts in injection molds. However, materials of this kind are prone to wear. This article analyzes the topographic characterization of the surface and wear processes in injection molds cavities. Two materials have been used to produce the cavities: aluminum alloy EN AW‐6082 T4 and copper alloy Cu Zn39 Pb3. The surface topography was measured with the use of optical interferometry profiling technology; roughness and surface parameters were determined according to ISO 4287, ISO 25178, and EUR 15178N. In order to complete this research, an experimental part with different thicknesses and shapes was designed, and cavity inserts of aluminum and copper were made. Polyamide PA6, with 30% fiberglass reinforcement, was employed in the experimental procedure. Measurements of cavity mold surfaces were performed after 9,200 cycles on each mold and at different locations on the mold. The surface measurement was made with a white light vertical scanning interferometry, also known as coherence scanning interferometry (ISO DIS 25178‐604). The results are analyzed and differences between the two types of cavity inserts materials are discussed.

Sensitivity analysis of thin-film thickness measurement by vertical scanning white-light interferometry

The spectral nonlinear phase method and the Fourier amplitude method have been applied to measure the thin-film thickness profile in vertical scanning white-light interferometry (VSWLI). However, both the methods have their disadvantages, and accordingly their applications are limited. In the paper we have investigated the dependence of the sensitivities of both the methods on the thin-film thickness and refractive index, the objective numerical aperture, and the incident light spectral range of VSWLI. The relation of the Fresnel reflection coefficients on the wavelength effect is also discussed. Some important research results reveal that the combination of both Fourier amplitude and nonlinear phase methods may provide a new approach to improve the VSWLI measurement sensitivity for thin-film thickness profile.

Examining Ultramicroelectrodes for Scanning Electrochemical Microscopy by White Light Vertical Scanning Interferometry and Filling Recessed Tips by Electrodeposition of Gold

In this paper, we present a technique to rapidly and directly examine ultramicroelectrodes (UMEs) by white light vertical scanning interferometry (VSI). This technique is especially useful in obtaining topographic information with nanometer resolution without destruction or modification of the UME and in recognizing tips where the metal is recessed below the insulating sheath. Two gold UMEs, one with a metal radius a = 25 μm and relative insulating sheath radius RG = 2 and the other with a = 5 μm and RG = ∼1.5, were examined, and the average depth of the gold recessions was determined to be 1.15 μm and 910 nm, respectively. Electrodeposition of gold was performed to fill the recessed hole, and the depth was reduced to ∼200 nm. With the electrodeposited gold electrode and a conventional microelectrode (a = 25 μm) as a tip and substrate, respectively, a tip/substrate distance, d, of 600 nm was achieved allowing scanning electrochemical microscopy (SECM) in positive feedback mode at a close distance, which is useful for measuring fast kinetics.

Optimal Design of Optical Filter for White Light Interferometry based on Sampling Theory

Vertical-scanning white light interferometry is a technique of profiling the surface topography of objects such as semiconductors, liquid crystal displays (LCDs), and so on. The profile is given by the “squared-envelope function” of the interference signal, which is called the interferogram. The world’s fastest surface profiling algorithm exploits a generalized sampling theorem that directly reconstructs the squared-envelope function from an infinite number of samples of the interferogram. The optical system of the white light interferometer is equipped with an optical filter. So far there were no guidelines for designing the optical filter’s characteristics. Field engineers empirically chose some combination of commercial optical filters by checking the corresponding interferograms. The main purpose of this paper is to provide a guideline for designing the optical filter’s characteristics. That is, we devise the optimal characteristics of the optical filter in the sense that, with a fixed passband of the optical filter, the second moment of the squared interferogram is minimized. Simulation results confirm the effectiveness of the optimal characteristics.

A nanomeasuring machine based white light tilt scanning interferometer for large scale optical array structure measurement

White light vertical scanning interferometry (VSI) has been well developed during the past two decades and it is well suited for optical component evaluation due to its non-contact nature. However, because of its inherent limits in the field of view, the VSI is incapable of performing large range measurements. For large surfaces, stitching must be employed. In this paper, a nanomeasuring machine (NMM) based white light tilt scanning interferometer (TSI) is described, which can evaluate an optical array component in one round, without the need for stitching. By integrating a NMM, the tilt angle in the TSI no longer will require to be calibrated, easing in this way the realization of the TSI. With a tilt stage mounted on the NMM, the proposed system has two angles related to the measurement, which can provide excellent “fringes slipping out” correction ability. The analysis and the experiments are given.

Stress gradient of a micro-optoelectromechanical systems Fabry–Perot cavity based on InP

Because of the gradient distribution of arsenic through the thickness of an InP layer, stress gradient in the structural layer of an InP-based Fabry-Perot (FP) cavity structure could be introduced during the fabrication process. This stress gradient, usually tensile at the upper surface and compressive at the lower surface, could induce a significant out-of-plane deformation, which may eventually affect its optical performance. White-light vertical scanning interferometry is employed to measure the stress-induced deflection of InP-based cantilever and membrane components used in a FP cavity structure. Deformation patterns caused by stress gradient in various cantilever and membrane structures with different configurations and geometries are investigated through experiments and simulations. The results indicate that the stress gradient induced during the fabrication process results in varying degrees of the FP structural deformation, which is further influenced by the configurations and geometries of the structural membranes and supporting beams. Four types of membrane structures of a FP cavity device are studied, and the results are compared to that obtained using a finite element analysis.

Investigation of a dual-layer structure using vertical scanning interferometry

This paper proposes a method based on white light vertical scanning interferometry (VSI) to investigate a dual-layer structure. The optical arrangement is based on a modified Michelson interferometer that utilizes a reference beam and two object beams. Each object beam interferes with the reference beam and produces an interferogram. A series of interferograms are obtained on a dual-layer structure and the thickness of each layer is obtained. A continuous wavelet transform (CWT) is used to extract the envelope of each interferogram in order to determine the peak intensity that provides an indication of each layer's boundary. Tests are conducted on a semiconductor wafer and a micro-gear made of polymeric material deposited on a metal substrate. Results show that the proposed method has a good potential for investigating a dual-layer micro-structure.

Lateral scanning white-light interferometer

White-light vertical scanning interferometry is a well-established technique for retrieving the three-dimensional shapes of small objects, but it can measure only areas as big as the field of view of the instrument. For bigger fields a stitching algorithm must be applied, which often can be a source of errors. A technique in which the object is scanned laterally in front of an instrument with a tilted coherence plane is described. It permits measurements at higher speeds while measurement accuracy is retained and eliminates the need for stitching in one direction. Experimental confirmation is provided.

Fringe modulation skewing effect in white-light vertical scanning interferometry

An interference fringe modulation skewing effect in white-light vertical scanning interferometry that can produce a batwings artifact in a step height measurement is described. The skewing occurs at a position on or close to the edge of a step in the sample under measurement when the step height is less than the coherence length of the light source used. A diffraction model is used to explain the effect.