Scanning White Light Interferometry Research Articles

Minimization of spectral phase errors in spectrally resolved white light interferometry by the iterative least-squared phase-shifting method

We introduce an iterative least-squared phase-shifting method, reported as inherently insensitive to any types of phase-shifting errors, to calculate the spectral phase in phase-shifting spectrally resolved white light interferometry (PS-SRI). The actual phase shifts corresponding to all wavelengths can be reversely determined from the phase-shifted spectral interference fringes through least-squares fitting and the spectral phase is calculated by using the actual phase shifts in an iterative numerical manner. Because this method reduces the phase-shifting errors for calculating the spectral phase effectively, it can contribute to improving the accuracy of measuring topographic surface profiles. Moreover, it leads to accomplishing thin film thickness measurements less than 100 nm, in which most white light scanning interferometry and spectrally resolved white light interferometry (SRI) have difficulty because of mismatching measured spectral phase with the theoretical model by the spectral phase error. In this paper, a short description of the iterative least-squared phase-shifting method is presented and verified with simulations for calculating the topographic surface and thin film thickness profiles in PS-SRI.

Leaf surface characterization of the Tabu-No-Ki tree Machilus thunbergii using electron microscopy and white light scanning interferometry

Leaf surface features were investigated in the Tabu-No-Ki tree Machilus thunbergii by electron microscopy and white light scanning interferometry. Mature leaves of the tree were air-dried at room temperature and sputter-coated with platinum. The most prominent epicuticular wax structures of M. thunbergii leaves included granules, angular rodlets and transversely ridged rodlets. Wax granules were often observed on the adaxial leaf surface. Meanwhile, the abaxial leaf surface was characterized by transversely ridged rodlets in which the ridges were perpendicular to their longitudinal axis. Having several facets, the angular rodlets were straight and grew among clusters of transversely ridged rodlets. The adaxial leaf surface of M. thunbergii appeared to be smooth and less undulating than the abaxial leaf surface. Clusters of epicuticular waxes were predominant on the abaxial leaf surface. The average heights of the adaxial and abaxial leaf surfaces were 1.79±0.58 and 3.65±0.93μm, respectively. The average roughness and root-mean-square roughness values were lower those on the adaxial surface than on the abaxial surface. These results suggest that the transversely ridged rodlets significantly contribute to the increased surface roughness on the abaxial leaf surface.

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.

Heat Treatments Modify the Tribological Properties of Nickel Boron Coatings

The effects of annealing temperature on the tribological properties of electroless nickel-boron coatings have been investigated. The coatings were annealed in a tube furnace under a flow (0.0094 N m(3)/min) of oxygen gas at temperatures of 250, 400, 550, and 700 °C for 3 h. Using scanning electron microscopy, images of the annealed coatings documented changes in surface morphology. From this it was seen that the higher annealing temperatures produced marked changes, moving from the nodular structure of nickel-boron coatings to a flaked surface morphology. The chemical effect of the annealing temperature was studied via X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS data indicated that after annealing at the temperatures of 550 and 700 °C, an accumulation of boron oxide species could be seen at the surface as well as a complete loss of nickel signal. An analysis of Raman spectra collected across the surface further identified the predominant species to be boric acid. The tribological response of the coatings was studied with a pin-on-disk tribometer with 440C stainless steel balls run against the coatings in ambient air. It was seen that the as received sample and the sample annealed at 250 °C samples exhibited modest friction properties, while the 400 °C sample had increased friction due to wear debris from the ball. The 550 and 700 °C samples showed remarkably low friction coefficients between 0.06 and 0.08, attributable to the presence of boric acid. The wear tracks were analyzed using scanning white light interferometry and from this data wear rates were obtained for the coatings ranging from 10(-8) to 10(-7) mm(3)/Nm.

Fourier domain analysis on continuously scanned structured illumination microscopy

In this note, a technique to determine the envelope peak positions of the axial intensity signal in continuously scanned structured illumination microscopy (CSSIM) for measuring surface profiles is introduced and verified with simulations. Based on the similarity between an axial intensity signal of CSSIM and a correlogram of white light scanning interferometry (WLSI), the Fourier domain analysis (FDA), widely used in WLSI, is adopted for reconstructing the height map of a sample. The continuous operation of CSSIM can significantly reduce the measurement time compared to the stop-and-measure procedure of previous structured illumination microscopy and the FDA method can improve the calculation speed because of using a single Fourier transformation as the result of simulations. It is also discussed that the FDA method can be applied to separate the upper layer and the bottom layer of the film structure, which results in overlapping axial intensity signals.

Complete fringe order determination in scanning white-light interferometry using a Fourier-based technique

White-light interferometry uses a white-light source with a short coherent length that provides a narrowly localized interferogram that is used to measure three-dimensional surface profiles with possible large step heights without 2π-ambiguity. Combining coherence and phase information improves the vertical resolution. But, inconsistencies between phase and coherence occur at highly curved surfaces such as spherical and tilted surfaces, and these inconsistencies often cause what are termed ghost steps in the measurement result. In this paper, we describe a modified version of white-light interferometry for eliminating these ghost steps and improving the accuracy of white-light interferometry. Our proposed technique is verified by measuring several test samples.

Low energy helium ion texturization of titanium and relevance to biomedical applications

Helium irradiation of metals has long been studied in efforts to understand the damaging aspects associated with applications in fusion reactors and tritium storage. This work examines the possibility of using low energy helium ion bombardment as a method of producing a beneficial surface texturization to promote bone growth on orthopedic implants. Using 300eV helium ions, two unique porous titanium surfaces were created when substrates were held at temperatures of roughly 450°C and 600°C. The surfaces were physically characterized by scanning electron microscopy (SEM) and scanning white light interferometry. A week long hFOB 1.19 cell culture was performed using an untreated titanium control to evaluate the suitability of these surfaces for orthopedic implants. Cell health and viability were evaluated by calcein AM live cell staining, MTT assay, and SEM. The results show that helium texturizations promote cellular activity and have no detrimental effect on cell health.

In-situ scanning white light interferometry employing dual-sensing configuration and active fringe-locking strategy

This article presents new white light interferometry with vibration-resistant capability achieved by in-situ optical detection and closed-loop feedback strategies. Scanning white light interferometry (SWLI) has become a popular measurement method due to its long measurement range and high measurement accuracy. However, the drawback of in-situ interferometry lies mainly in unacceptable measurement errors caused by environmental disturbance such as mechanical vibration and acoustic noise. This paper describes a method employing dual-sensing configuration and active fringe-locking strategy to overcome the issue. A specially designed band-pass filter is employed to generate a high-coherent light field for real-time detecting vibratory displacement. As proven by the experimental results, the maximum measurement error caused by vibration can be effectively reduced by more than six folds.

Infrared-light interferometry and a phase-stepping algorithm for measuring the three-dimensional topography of components covered with GaAs or Si

White-light scanning interferometry is extended from the visible-light region to the infrared-light region to measure the three-dimensional (3D) topography of components. An infrared-light interference system based on a linnik-type interferometric microscope is developed according to the principle of transmission of semiconductor materials in the infrared-light region. The 3D topography is measured using the components covered with a Si or GaAs wafer based on a phase-stepping algorithm. The 3D topography can be obtained accurately and relative step heights can be determined within an error less than 3% compared with the instrument MSA 400, a well-known commercially available apparatus used in the field of 3D topography measurement.

Development of a Hybrid Atomic Force Microscopic Measurement System Combined with White Light Scanning Interferometry

A hybrid atomic force microscopic (AFM) measurement system combined with white light scanning interferometry for micro/nanometer dimensional measurement is developed. The system is based on a high precision large-range positioning platform with nanometer accuracy on which a white light scanning interferometric module and an AFM head are built. A compact AFM head is developed using a self-sensing tuning fork probe. The head need no external optical sensors to detect the deflection of the cantilever, which saves room on the head, and it can be directly fixed under an optical microscopic interferometric system. To enhance the system’s dynamic response, the frequency modulation (FM) mode is adopted for the AFM head. The measuring data can be traceable through three laser interferometers in the system. The lateral scanning range can reach 25 mm × 25 mm by using a large-range positioning platform. A hybrid method combining AFM and white light scanning interferometry is proposed to improve the AFM measurement efficiency. In this method, the sample is measured firstly by white light scanning interferometry to get an overall coarse morphology, and then, further measured with higher resolution by AFM. Several measuring experiments on standard samples demonstrate the system’s good measurement performance and feasibility of the hybrid measurement method.

Force transformation in spider strain sensors: white light interferometry.

Scanning white light interferometry and micro-force measurements were applied to analyse stimulus transformation in strain sensors in the spider exoskeleton. Two compound or 'lyriform' organs consisting of arrays of closely neighbouring, roughly parallel sensory slits of different lengths were examined. Forces applied to the exoskeleton entail strains in the cuticle, which compress and thereby stimulate the individual slits of the lyriform organs. (i) For the proprioreceptive lyriform organ HS-8 close to the distal joint of the tibia, the compression of the slits at the sensory threshold was as small as 1.4 nm and hardly more than 30 nm, depending on the slit in the array. The corresponding stimulus forces were as small as 0.01 mN. The linearity of the loading curve seems reasonable considering the sensor's relatively narrow biological intensity range of operation. The slits' mechanical sensitivity (slit compression/force) ranged from 106 down to 13 nm mN(-1), and gradually decreased with decreasing slit length. (ii) Remarkably, in the vibration-sensitive lyriform organ HS-10 on the metatarsus, the loading curve was exponential. The organ is thus adapted to the detection of a wide range of vibration amplitudes, as they are found under natural conditions. The mechanical sensitivities of the two slits examined in this organ in detail differed roughly threefold (522 and 195 nm mN(-1)) in the biologically most relevant range, again reflecting stimulus range fractionation among the slits composing the array.

Visually guided 3D micro positioning and alignment system

This paper presents a Visually Guided Micro Positioning and Alignment System that employs Scanning White Light Interferometry (SWLI). An Interferometry microscope is integrated with a magnetically suspended motion stage to achieve alignment of micro objects in 3D. Based on the SWLI measurement an integrated control scheme is presented which enables the alignment of micro objects in 3D space. A demonstration of 3D micro alignment and manipulation was designed to show the capabilities of the integrated system. Alignment and docking of two silicon micro cantilevers is demonstrated using the magnetically suspended stage and the 3D sensing ability of the SWLI. Experimental results show that cantilever docking can be achieved with less than 30nN force sensitivity.

Effects of deformation mode on surface roughening of austenitic stainless steels

Free surface roughening of polycrystalline austenitic stainless steels during tensile and deep drawing deformation is investigated using scanning white light interferometry. A fractal analysis method is applied to analyse the surface profile data in two directions which are perpendicular to each other. It is shown that the surface profile is fractal within the length and is on the same order of magnitude as the grain size. The Hurst exponent H of the two deformation modes in different directions stabilises at H≈0·85. While the correlation lengths are different, the value in the longitudinal direction is about two times larger than the one in the transverse direction. The relationship between root mean square roughness and strain appears to be linear, and the roughening rate in tensile deformation is larger than the one in deep drawing deformation.

Surface profile measurement in white-light scanning interferometry using a three-chip color CCD

White-light scanning interferometry (WLSI) is a useful technique to measure surface profile when a test object contains discontinuous structures or microstructures. A black and white CCD camera is usually utilized to capture interferograms, and a series of corresponding algorithms is used to achieve the profile measurement. However, the color information in the interferograms is lost. A novel profile measurement method that uses phase information in different color channels (red-green-blue) of an interferogram obtained using a three-chip color CCD in WLSI is proposed. The phase values are extracted by a windowed Fourier transform algorithm. Simulation and experimental results are presented to demonstrate the validity of the proposed method.

3d surface profiling and high resolution imaging for refining the florin rings and epicuticular wax crystals of Pinus koraiensis needles

3D surface profiling and high resolution imaging were performed to refine the Florin rings and epicuticular wax crystals of Pinus koraiensis needles. Needles were collected from four-year-old seedlings and air-dried for surface observations. Field emission scanning electron microscopy revealed that stomata were found on the abaxial (lower) surface of needles. Measured as ca. 40 μm long, they were largely elliptical or oval-shaped. Epicuticular wax crystals were present in the epistomatal chambers as well as on the surrounding epidermis. Rodlets were prevalently found on the stomatal bands and furrows as well as within the epistomatal chambers. The presence of wax tubules was ascertained by the distinct terminal openings at their ends. The occurrence of wax ridges was evident on the epidermis near the saw-tooth margins (nonstomatal areas). No distinct wax ridges were detected on the dewaxed needles. Raised Florin rings were distinct on the stomata. White light scanning interferometry showed that the diameter and width of stomata were ca. 44.02 ± 3.33 μm and 32.10 ± 3.30 μm, respectively. Measured from the neighboring epidermis to the stomatal aperture, the mean height of the stoma reached ca. 6.23 ± 1.28 μm. Focus variation metrology allowed measuring the mean elevation angle of the stoma, reaching ca. 41.41 ± 11.25°. This is the first report on a novel approach to the establishment of quantitative criteria of Florin ring classification by nontactile 3D surface profiling beyond the previous qualitative descriptions of Florin rings of coniferous species.

Morphology of Foliar Trichomes of the Chinese Cork OakQuercus variabilisby Electron Microscopy and Three-Dimensional Surface Profiling

Morphology of foliar trichomes was analyzed in Quercus variabilis by electron microscopy and three-dimensional surface profiling. Leaves from suppressed or dominant sprouts of the oak species were collected after a forest fire to unravel the effects of the disturbance factor on sprouting of the oak species. Scanning electron microscopy revealed two types of trichomes depending on the leaf surface. The trichomes on the adaxial surface were branched and constricted, and possessed a single row of thin-walled cells with a collapsed morphology (glandular branched uniseriate trichomes). Meanwhile, the trichomes on the abaxial surface were star-shaped, unfused with each other, and had 6 to 10 rays (nonglandular simple stellate trichomes). An apparent proliferation of trichomes was evident on the adaxial surface of the dominant sprouts. Uniseriate trichomes could be discernable as an elevation from the surface by white light scanning interferometry. By transmission electron microscopy, thin and convoluted cell wall, degenerated cytoplasm, and a single row of cells were characteristic of the trichomes on the adaxial surface. The thick cell walls of the mature trichomes on the abaxial surface represented the nonglandular nature. This is the first report on the morphological and ultrastructural characterization of foliar trichomes of the oak species.

Simultaneous normal and torsional force measurement by cantilever surface contour analysis

This study presents an alternative to the current Hooke's law-based force relation between rectangular cantilever deflection and applied force. In the new approach, a transduction constant is presented that (1) includes no cross-talk between torsion and normal force components, (2) is independent of the load application point, and (3) does not depend on the cantilever beam length. Rather than measuring the cantilever deformation at a single point (such as the tip location), it is measured at multiple adjacent points using scanning white light interferometry to provide a three-dimensional description of the cantilever deformation during loading. This measurement, processed by a force relation based on a superposition of deflections derived from Euler–Bernoulli bending theory and St Venant's torsion theory, provides the vertical, axial, and torsional force components simultaneously. Experimental results are compared to force predictions for the vertical and torsional components using macro-scale cantilevers under mass loading. An uncertainty analysis is also provided.

Profile measurement system based on linnik-type interferometric microscope for visible-light region and infrared-light region

A profile measurement system consisting of linnik-type interferometric microscope, light source and cameras is developed based on white-light scanning interferometry extending from the visible-light region to the infrared-light region. Three-dimensional profiles of microstructures are obtained via a phase-stepping algorithm using a CMOS array camera for the visible-light region and an InGaAs CCD array camera for the infrared-light region. Errors of the measurement system along with extensive applications in MEMS device profile reconstruction of top surface, sidewall and internal structures are discussed in detail.

Transmission measurement of three-dimension profile based on infrared-light interference technology with wafer compensation

A new method of three-dimension profile measurement is proposed based on infrared-light interference technology, which is extended from visible-light region to infrared-light region for white-light scanning interferometry. Microstructure with two steps of different heights is used to be demonstrative sample to prove the property of designed system based on linnik-type interferometric microscope with phase-stepping algorithm. Three-dimension profile and step height can be obtained accurately with GaAs wafer transmission compensation and are highly consistent with the results that obtained from the famous commercial instrument MSA 400. The novel method has widely potential applications in the field of MEMS, such as reconstructing the internal profile of encapsulation devices and analyzing the quality evaluation of bonding interface based on semiconductor materials.

Application of least-square estimation in white-light scanning interferometry

White-light scanning interferometry (WLSI) is a powerful tool for investigating the profile of a test object that contains sharp steps. Due to the light source used in WLSI system, it is able to overcome phase ambiguity problem, which is often encountered in monochromatic interferometry. In this paper, a new algorithm based on least-square estimation using short-time Fourier transform (STFT) is proposed to measure the profile of a test object. STFT is used to extract the peak position of the coherence envelope of a white-light interference signal and retrieve the corresponding phase values simultaneously at first. A complex phasor (CP) method is introduced to further reduce the phase noise. Then, the phase values at several positions around are utilized to achieve a more accurate peak position based on least-square line fitting. Both simulated and experimental results show that the proposed algorithm is able to accurately measure the profile of a test object.