Scanning White Light Interferometer Research Articles

Exact surface measurement based on phase error insensitive method for white-light scanning interferometer

Abstract Phase error induced by the scanning error of piezoelectric transducer and vibration in white-light interference signal (WLIS) impose limitations on measurement accuracy. In this paper, two techniques are investigated to eliminate phase error cooperatively. A monochromatic interferometer owning the same optical path as the white-light scanning interferometer (WLSI) is employed to detect and compensate the phase error. Due to the incomplete consistency between the detected phase error and its actual distribution, an algorithm is introduced to further enhance the measurement accuracy. The simulation is given to prove the proposed method is available in different magnitude of noise. In the experiment, the front surface of wedged window is measured, whose maximum fluctuations are reduced to 28.8 nm and 3.0 nm from 62.7 nm with the gradual application of two technologies, and the repeatability reach 5.0 nm and 0.4 nm from 16.8 nm, respectively. The root mean square height Sq and maximum height Sz ultimately decrease to 1.3 nm and 1.6 nm. The measurement of step with height of about 3.0 m also has the good accuracy and repeatability after phase error elimination. Since the different reflectivity of measured surfaces in the wavelength range of light source will also bring the measurement error, its influences are analyzed by simulation. The proposed techniques provide a phase error insensitive method for WLSI that has potential on measurement in the complex environment.

Polarization-multiplexed snapshot lateral shearing interferometric sensor for surface roughness measurements

A compact optical roughness measurement sensor based on lateral shearing interferometry is theoretically described and experimentally verified. By adopting the advanced surface roughness parameter extraction methods based on statistical machine learning technique and the partial integration approach, the sensor using a polarization grating and a polarization camera can instantaneously obtain a surface gradient map and provide reliable surface roughness parameters. To verify the operation and the performance of the sensor, a flat mirror and roughness standard specimens were measured, and the results were compared with those of a white light scanning interferometer as the reference values. As a result, the roughness parameters were very close to the reference values with <5 nm accuracy and <0.1 nm repeatability.

Simultaneous Measurement of Group Refractive Index Dispersion and Thickness of Fused Silica Using a Scanning White Light Interferometer.

In this study, we simultaneously measured the group refractive index dispersion and thickness of fused silica using a scanning white light interferometer on a spectral range from 800 to 1050 nm. A delay error correction was performed using a He-Ne laser. The accuracy of the measured group refractive index dispersion of fused silica, when compared to the temperature-dependent Sellmeier equation, is within 4 × 10-5.

Coarse-to-fine auto-focus method using a white-and-red light source for white-light scanning interferometers

To realize high-efficiency automation for white-light scanning interferometers, auto-focusing is a critical technique, which is expected to have a large focusing range and high repeatability. In this paper, a coarse-to-fine auto-focusing method using a white-and-red LED light source is proposed. The approximate focus position is firstly searched efficiently in the coarse focusing using a re-blur sharpness algorithm, and then the optimal focusing position is determined with a full range scanning in the fine focusing. By combining the long coherence length of the red LED with the short coherence length of the white LED, the proposed auto-focusing method can realize a large focusing range up to 40 µm and achieve high repeatability within several nanometers level. The comparison experiments of sharpness algorithms and auto-focus experiments are carried out. The results show that the proposed method is robust, low-cost, and insensitive to noise, surface textures, and light intensity, which is highly suitable for the optical surface measurement with White-light scanning interferometer.

Robust unbalanced Gires-Tournois mirror for group delay dispersion measurement evaluation by white light interferometer

A robust unbalanced Gires-Tournois interferometer (GTI) mirror for evaluating group delay dispersion (GDD) measurement by scanning white light interferometer is presented, and a novel processing algorithm based on scanning white light interferometer (WLI) is proposed to extract group delay and group delay dispersion characteristics with improved accuracy and low level of noise. The GDD measurement evaluation method is based on error analysis of manufacturing errors and spectral measurement. Spectral measurement is carried out to determine the resonance wavelength of GTI mirror. The sample was prepared by ion beam sputtering deposition. The relative errors of refractive index and layer thickness for the sample are estimated through reverse engineering based on multi-angle transmittance measurements. A novel processing algorithm the extrema searching method (ESM) is proposed that search the positions of multi-order extrema from reconstructed interferogram intensity to exact the GDD characteristics of the sample. GDD measurements performed by WLI with different processing methods are added to comparison including windowed Fourier transform, wavelet transform and time-domain scanning method. The results show that our method is reliable and effective for GDD measurement with a good accuracy and low level of noise.

Performance and durability of thin film solar cells via testing the abrasion resistance of broadband anti-reflection coatings

Reflection from the front glass of solar modules causes over 4% optical loss leading to a significant decrease in module efficiency. Single layer solution gelation (sol-gel) anti-reflective (AR) coatings are effective over a narrow range of wavelengths, whereas reflection losses can be reduced over a broader wavelength when multilayer broadband AR coatings are applied. In this work, three different multilayer AR coatings including 4-layer SiO2/ZrO2, 4-layer SiO2/ITO, and 6-layer SiO2/ZrO2 were deposited using magnetron sputtering. The abrasion resistance is important because the coatings will be subject to regular cleaning cycles. A variety of abraders including Felt pad, CS-10 and CS-8 under different loads are used. The optical performance and durability of these coatings were analyzed using a spectrophotometer, optical microscope, scanning electron microscope, and scanning white light interferometer. No damage was observed after abrasion of the coatings with a felt pad under 1 and 2 N loads. However, there was a slight increase in Weighted Average Reflection. When coatings were tested with CS-10 and CS-8 abraders, coatings with ZrO2 resulted in higher scratch resistance in comparison to coating with ITO. However, all-dielectric broadband AR coatings are more durable and have better optical performance compared to single layer sol-gel coatings.

Effects of polishing pressure and sliding speed on the material removal mechanism of single crystal diamond in plasma-assisted polishing

Plasma-assisted polishing (PAP) was confirmed to be high-efficiency and high-quality when applied to single crystal diamond (SCD) substrates. The effects of polishing pressure applied to the SCD substrate and sliding speed between the polishing plate and SCD substrate in PAP were systematically investigated in this study. Higher polishing pressures or sliding speeds resulted in a higher material removal rate (MRR) of SCD substrate, and the highest MRR achieved 5.3 μm/h. In the case of PAP conducted at low polishing pressures such as 62.5, 81.3 kPa, scanning white light interferometer (SWLI) (84-μm square) and atomic force microscope (AFM) (5-μm square) measurements revealed an atomic-scale smooth surface without the surface texture depending on the crystal direction and the lowest Sq roughness of 0.3 nm (84-μm square) was achieved. Correspondingly, at low polishing pressure of 62.5 kPa, MRRs in PAP along the <100> and <110> directions were nearly identical, suggesting that isotropic removals occurred. In the case of PAP performed at high polishing pressures such as 143.8, 246.9, and 350.0 kPa, SWLI and AFM measurements revealed a rough surface with bands of grooves along <100> direction. Correspondingly, at high polishing pressures of 246.9 and 350.0 kPa, MRRs in PAP along <100> direction were ~11 and ~12 times faster than that in <110> direction, respectively, suggesting that anisotropic removals occurred. Moreover, scanning transmission electron microscopy and angle-resolved X-ray photoelectron spectroscopy measurements confirmed that no damage or non-diamond layer was present on the PAP-processed SCD substrate. However, as the sliding speed varied, both SWLI and AFM measurements demonstrated that Sq roughness remained nearly constant with the same polishing pressure applied to the SCD substrate.

Surface Characteristics and Microbiological Analysis of a Vat-Photopolymerization Additive-Manufacturing Dental Resin.

The wide application of additive manufacturing in dentistry implies the further investigation into oral micro-organism adhesion and biofilm formation on vat-photopolymerization (VP) dental resins. The surface characteristics and microbiological analysis of a VP dental resin, printed at resolutions of 50 μm (EG-50) and 100 μm (EG-100), were evaluated against an auto-polymerizing acrylic resin (CG). Samples were evaluated using a scanning electron microscope, a scanning white-light interferometer, and analyzed for Candida albicans (CA) and Streptococcus mutans (SM) biofilm, as well as antifungal and antimicrobial activity. EG-50 and EG-100 exhibited more irregular surfaces and statistically higher mean (Ra) and root-mean-square (rms) roughness (EG-50-Ra: 2.96 ± 0.32 µm; rms: 4.05 ± 0.43 µm/EG-100-Ra: 3.76 ± 0.58 µm; rms: 4.79 ± 0.74 µm) compared to the CG (Ra: 0.52 ± 0.36 µm; rms: 0.84 ± 0.54 µm) (p < 0.05). The biomass and extracellular matrix production by CA and SM and the metabolic activity of SM were significantly decreased in EG-50 and EG-100 compared to CG (p < 0.05). CA and SM growth was inhibited by the pure unpolymerized VP resin (48 h). EG-50 and EG-100 recorded a greater irregularity, higher surface roughness, and decreased CA and SM biofilm formation over the CG.

Comparison of surface and subsurface damage of mosaic single-crystal diamond substrate processed by mechanical and plasma-assisted polishing

The surface and subsurface damage of a mosaic single-crystal diamond (SCD) (100) substrate processed by mechanical polishing (MP) and plasma-assisted polishing (PAP) were studied. Scanning white-light interferometer (SWLI) (84-μm square) observation for different local positions showed that the surface damage such as scratches and deep pits were observed on the mosaic SCD substrate after MP, while such damage was completely removed after PAP. Scanning cathodoluminescence (CL) measurement of band-A emission for different local positions showed that scratches with high luminescence intensity attributed to subsurface damage were observed on the mosaic SCD substrate after MP, while such specific luminescence was not observed after PAP. Micro-Raman spectroscopy measurement showed that peak position shift of diamond Raman lines attributed to residual stress and full width at half maximum (FWHM) broadening attributed to destruction of the crystalline structure were observed on the mosaic SCD substrate after MP, while such a peak position shift and FWHM broadening were not observed after PAP.

Shape measurement of large thickness glass plates with a white-light scanning interferometer using a compensation glass and a fixed reference surface

Conventional white-light scanning interferometers cannot measure exactly large thickness of glass plates because of a great dispersion effect. In this paper a compensation glass plate is used to reduce the dispersion effect. Moreover, a fixed reference surface is added behind the object and four-step measurements are performed. By this method a front surface profile and a thickness distribution of the object can be measured exactly without knowing the refractive index of the object, the thickness of the compensation glass, and dispersion effect containing in the interferometer. Due to the difficulty in scanning the reference surface at a constant speed over a long distance for a large thickness of the object, a piezoelectric transducer stage with a high positioning resolution are utilized together with an additional interferometer which measures the movement of this stage. Experimental results show that the error in thickness measurement is less than 60 nm and 170 nm for 1 mm and 5 mm-thickness glass plates, respectively.

Combination of Plasma Electrolytic Processing and Mechanical Polishing for Single-Crystal 4H-SiC.

Single-crystal 4H-SiC is a typical third-generation semiconductor power-device material because of its excellent electronic and thermal properties. A novel polishing technique that combines plasma electrolytic processing and mechanical polishing (PEP-MP) was proposed in order to polish single-crystal 4H-SiC surfaces effectively. In the PEP-MP process, the single-crystal 4H-SiC surface is modified into a soft oxide layer, which is mainly made of SiO2 and a small amount of silicon oxycarbide by plasma electrolytic processing. Then, the modified oxide layer is easily removed by soft abrasives such as CeO2, whose hardness is much lower than that of single-crystal 4H-SiC. Finally a scratch-free and damage-free surface can be obtained. The hardness of the single-crystal 4H-SiC surface is greatly decreased from 2891.03 to 72.61 HV after plasma electrolytic processing. By scanning electron microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) observation, the plasma electrolytic processing behaviors of single-crystal 4H-SiC are investigated. The scanning white light interferometer (SWLI) images of 4H-SiC surface processed by PEP-MP for 30 s shows that an ultra-smooth surface is obtained and the surface roughness decreased from Sz 607 nm, Ra 64.5 nm to Sz 60.1 nm, Ra 8.1 nm and the material removal rate (MRR) of PEP-MP is about 21.8 μm/h.

Optical phasing method based on scanning white-light interferometry for multi-aperture optical telescopes.

A novel, to the best of our knowledge, sensing approach based on scanning white-light interferometry (SWLI) is proposed to detect piston errors of the multi-aperture optical telescope. The scanning white-light interferometer is composed of a Mach-Zehnder interferometer (MZI) and an optical path modulator (OPM). A lenslet array is used to image the interferometric wavefront between the reference and the other individual apertures. The piston errors can be estimated from these SWLI signals focused by the lenslet array. The measurement range of the proposed method depends on the modulation range of the OPM and can reach millimeter order, and its accuracy is better than a 1/20 wavelength. In addition, the proposed method's amplitude-splitting interferometry of the MZI makes it fit for a multi-aperture optical telescope. We demonstrate a proof of concept and validate the feasibility of our proposed phasing method.

Shape measurement of a thin glass plate through analyzing dispersion effects in a white-light scanning interferometer

• Phase position Z p of π nearest to a maximum amplitude position Z a in a complex-valued interference signal (CVIS) is analyzed for measuring thickness T of thin glass plates by simulations. • Coefficients that relate the Z a and Z p values to the physical thickness T are obtained. • Periodical position jumps occurred in Z p are made clear, and the number of occurrences of the jump is calculated from the Z a and Z p values. • In experiments, a shape of 20 μm-thickness glass plate is measured exactly with an accuracy of 6 nm from the Z a and Z p values. Utilization of a complex-valued interference signal (CVIS) in a white-light scanning interferometer is a powerful method to measure exactly surface positions of a glass plate. In this method a zero phase position nearest to a maximum amplitude position in the CVIS is a measured surface position. Proportional coefficients that relate the maximum amplitude position and the zero phase position of a rear surface to the thickness of a glass plate are required for measuring exactly the thickness. These coefficients and periodical position jump occurring on the zero phase position are investigated by simulations. The number of occurrences of the position jump is calculated from the maximum amplitude and zero phase positions. Shape measurement of a glass plate with about 20 μm-thickness is achieved by using the coefficients and the number of occurrences of the position jump.

Multi-sensor optical profilometer for measurement of large freeforms at nm-level uncertainty

Optical measurement of large freeform samples is often limited by the resolution, size and slope limits of measurement devices. This trade-off can be solved using stitching—which, however, creates several difficulties often linked to accuracy of movement of the sample or objective. We present a stitching multisensor freeform topography instrument based on scanning white light interferometer, confocal sensor and accurate movements of the sample tracked using laser interferometers. The interferometers track the sample in 2D at an accuracy of a few nm over a 10 cm × 10 cm area. The instrument is thoroughly characterized and uncertainty is estimated to ensure traceable results. Based on the characterization results the instrument allows topography measurement of freeform sample with 54 nm standard uncertainty for datasets of a few hundred sub-images.

Damage-free highly efficient plasma-assisted polishing of a 20-mm square large mosaic single crystal diamond substrate

Plasma-assisted polishing (PAP) as a damage-free and highly efficient polishing technique has been widely applied to difficult-to-machine wide-gap semiconductor materials such as 4H-SiC (0001) and GaN (0001). In this study, a 20-mm square large mosaic single crystal diamond (SCD) substrate synthesized by microwave plasma chemical vapor deposition (CVD) was polished by PAP. Argon-based plasma containing oxygen was used in PAP to modify the surface of quartz glass polishing plate, and a high material removal rate (MRR) of 13.3 μm/h was obtained. The flatness of SCD polished by PAP measured by an interferometer was 0.5 μm. The surface roughness measured by both scanning white light interferometer (SWLI) (84-μm square) and atomic force microscope (AFM) (5-μm square) was less than 0.5 nm Sq. The micro-Raman spectroscopy measurement results of mosaic SCD substrate processed by PAP showed that residual stress and non-diamond components on the surface after PAP processing were below the detection limit.

A dual-scanning white-light interferometer for exact thickness measurement of a large-thickness glass plate

Conventional scanning white-light interferometers (SWLIs) cannot measure the exact shape of a thick glass plate because of the large dispersion effect. In this paper, a novel dual SWLI is proposed where the thicknesses of both an object and a compensation glass plate can be measured exactly in four-step measurements by decreasing the dispersion effect in both arms of the dual SWLI. The compensation glass plate is made of the same material as the object and has a similar thickness. The peak positions of the interference signals in the SWLI can be exactly measured using a piezoelectric transducer stage and an additional interference signal which detects the time-varying optical path difference in the SWLI. Experimental results show that the measurement error is less than 60 nm for a glass plate of thickness 1 mm.

Information depths of analytical techniques assessing whitefish otolith chemistry

The elemental composition of otoliths provides historical information on migration and provenance of fish. Due to the complex structure of the otoliths, the depth from which the elemental information originates has to be considered. In this work, three analytical methods often used to assess otolith chemistry are compared. The information depths are calculated for µ-XRF and PIXE and measured for LA-ICP-MS. The information depth in PIXE depends on the energy of the incident particles and on the element to be analysed while in XRF it depends mostly on the element to be analysed as the energy of the incident X-rays usually is high enough to excite atoms at depths of several hundreds of micrometres. If we assume that the otolith is exposed to X-rays from a Rh-tube (20.16 keV) about 50% of the detectable Sr(Kα) X-rays will be emitted from a depth ranging from 0 to 114 µm. In the case of PIXE with 3 MeV protons the corresponding range is 0–15 µm. The information depths in LA-ICP-MS were determined by measuring the depth of the laser-ablated craters or trenches that remained on the otolith surface. The depths measured with a scanning white light interferometer (SWLI) were found to be about 40 µm (spots) and 11 µm (trenches).The correlation between strontium concentrations obtained by spot analysis of whitefish otoliths with PIXE and LA-ICP-MS was excellent (92%), although different spot sizes were used. A comparison of strontium concentration profiles measured with µ-XRF and LA-ICP-MS showed that the higher information depth of µ-XRF in combination with the 52 degree angle of the incident X-rays smoothens out the sharp edges seen in the LA-ICP-MS profiles. Otoliths from whitefish captured in the Baltic Sea (n = 30) were analysed in this comparison of methods. Among these whitefish there are sea spawners, river spawners and stocked fish with different life history and different otolith chemistry.

Selective laser ablation of CFRP composite to enhance adhesion bonding

ABSTRACTIn this work, the effect of picosecond laser radiation on ablating carbon fiber-reinforced polymer (CFRP) composite surface to increase the adhesive bond strength was analyzed. The experiment was conducted using a picosecond laser of 3 W average power with 532 nm wavelength, working at the 20kHz frequency. Effect of process parameters (scanning speed, beam overlap, and number of passes) on surface treatment of CFRP was analyzed and optimized for maximum shear strength using response surface methodology (RSM). Single lap shear test was performed on adhesively bonded laser-treated specimens using the optimized process parameter (for maximum shear strength) and maximum roughness. Acoustic emission data was collected during the shear test to investigate and characterize the failure mechanism. The morphology of laser-treated surface and shear tested surface was explored using scanning electron microscopy (SEM) and white light interferometer (WLI).

Nanosecond laser texturing with hexagonal honeycomb micro-structure on Titanium for improved wettability and optical properties

Nanosecond laser provided an efficient and flexible solution for surface texturing on different metals. In this work, a hexagonal honeycomb model was designed on Titanium by laser processing. The surface texturing experiment was carried out by a nanosecond pulsed fiber laser with different scanning speeds. The Scanning Electron Microscopy and White Light Interferometer were used to characterize the surface morphology and roughness. Besides, the contact angle instrument and the Fourier Transform Infrared Spectrometer were used to obtain the effects of surface modification on wettability and optical properties of the Titanium. The wettability and optical reflectivity of the surface changed by controlling the side length of the hexagonal honeycomb model and the laser scanning parameters. The contact angle reached a maximum of 147.4°, the contact angle hysteresis was found to lie between 1.6° and 7.1°, and the optical reflectance for titanium surface decreased by more than 90% after laser processing. The approach presented in this study provided a feasible and efficient method to improve the wettability and optical absorption of titanium surface.

Vibration Compensated, Scanning White Light Interferometer for In Situ Depth Measurements in a Deep Reactive Ion Etcher

A scanning white light interferometer with a long working distance and active vibration cancellation was designed and tested to enable in situ depth measurements in a deep reactive ion etcher. A superluminescent diode is used as the light source, enabling high-intensity illumination of the sample inside the etcher’s vacuum chamber. An additional laser beam co-propagates with the white light beam and is used to sense changes in the optical path length difference by observing the beam’s interference signal on a photodiode. The photodiode signal is used as feedback to control the position of the interferometer’s reference mirror which is mounted on a piezo actuator. By locking the position of the reference mirror to a zero crossing of the laser interference signal, during a scan, the reference mirror moves in discrete multiples of the laser wavelength. This design choice has the added advantage of tying the stability of the scan distance and step size to the laser wavelength stability. The prototype instrument is capable of imaging over a 1 cm $\times1$ cm field of view with a 1- ${\boldsymbol \mu }\text{m}$ depth resolution, 20- ${\boldsymbol \mu }\text{m}$ lateral resolution, and 30-cm working distance. The feedback system reduces the amplitude of vibrations by more than 10 dB out to 1 kHz. [2018-0263]