Coherence Correlation Interferometry Research Articles

Advancement of Analytical Model for Hydrophobic Rectangular Pillared Array on Al-Surface and Its Experimental Validation

AbstractOver the past few decades, self-cleaning surfaces have been significantly investigated due to their commercial applications in various fields. However, the researchers are still lagging in developing better mathematical models and fabricating hydrophobic surfaces for direct espousal in industry. In this study, a force-balanced system-based mathematical model is modified for a rectangular pillared array-based micro-structure and MATLAB simulations were used to validate it theoretically. The same pattern was developed on Al-surface using a single-point diamond turning (SPDT) machine experimentally. The experimental results were validated using coherence correlation interferometry (CCI), optical microscopy, drop shape analyser (DSA), and field emission scanning electron microscopy (FESEM). The experimentally estimated and theoretically predicted contact angles of the rectangular pillared array are found in close agreement. Further, the advancement in mathematical models and models-based surface manufacturing strategies can boost the research in this domain to develop robust self-cleaning hydrophobic surfaces.

Optical design and fabrication of zinc selenide microlens array with extended depth of focus for biomedical imaging

Optical coherence tomography is a well-known technique for the optical imaging biological tissues. However, the depth scanning range of high-resolution optical coherence tomography is restricted by its depth of focus. In this study, a Zinc Selenide (ZnSe) Microlens Array (MLA) is employed to overcome the depth-of-focus limitation of optical coherence tomography. The ZnSe material with a low Abbe number and high chromatic dispersion extends the depth of focus with transverse resolution. The ZnSe MLA focused the incident light (from visible to near-infrared (NIR) region) on multiple focal planes with the uniform distribution of light over a biological tissue. The MLA is designed using Zemax OpticStudio software and fabricated via a single-point diamond-turning based on Slow Tool Servo (STS) configuration. STS machining has the unique advantage of offering larger degrees of freedom with no additional baggage, thereby reducing the setup time. The experimental results show the effectiveness of the STS machining process in fabricating ZnSe MLA with desired accuracies. The characterization of fabricated MLA using Coherence Correlation Interferometry (CCI) depicts uniform lenslets with no structural and positional distortion, with a total error of 32 nm within the tolerance limit.

Effect of Ti/Si and Ti/TiN/Si interlayers on the structure, properties, and tribological behavior of an a-C film deposited onto a C17200 copper-beryllium alloy

Reduced hardness and wear resistance may limit SAE C17200 copper‐beryllium alloy use in manufacturing applications, such as plunger tips of die casting machines and as cores and inserts for steel dies in injection molding processes. In order to improve the surface properties of Cu—Be alloys, amorphous carbon (a-C) films have been selected since these coatings may show high hardness, low wear rate and low coefficient of friction. However, the adhesion of carbonaceous films on Cu—Be alloys remains a challenge. Two interlayer compositions (Ti/Si and Ti/TiN/Si) were deposited onto Cu—Be disks and silicon wafers to assess amorphous carbon adhesion on substrates made of Cu—Be alloy. The microstructure and topography of the coatings were examined by Field Emission Scanning Electron Microscopy (FESEM) coupled with X-ray dispersive energy spectroscopy (EDS). The chemical composition depth profile was measured by glow discharge optical emission spectroscopy (GDOES), which confirmed distinct coating layers of Ti, Si, a-C (C1 and C2 conditions-pDCMS). A TiN extra layer presence was obtained for the C3 and C4 conditions-pDCMS reactive, confirmed by small angle XRD analysis. Raman scattering spectroscopy showed a higher quantity of sp3 carbon bonds for C3 and C4 coating conditions compared to C1 and C2. Instrumented indentation tests indicated a higher hardness and reduced elastic modulus for C3 and C4 coating systems, corroborating Raman results, in terms of a higher concentration of sp3 bonds. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and coherence correlation interferometry (CCI) were used to characterize the coatings' surface features and scratch tracks after the tests. In addition, ramp load scratch tests were conducted to assess coating adhesion to the Cu—Be alloy substrate by measuring critical loads and the coefficient of friction. The highest critical loads to failure (Lc2) and (Lc3) were found for the Ti/TiN/Si interlayer sample (C4 condition), indicating that this interlayer improved the coating contribution to a gradual increase in the hardness and promotion of an enhanced adhesion strength of the a-C coating.

Fabrication and estimation of optical constants of hydrophobic Ta2O5 thin films

Present study investigates the optical properties and film thickness estimation of tantalum oxide (Ta2O5) thin films of various thicknesses, deposited by physical evaporation technique. The optical constants and thickness of deposited films were estimated by using envelope method. The thickness value obtained from this method has been compared with in-situ (quartz crystal thickness monitor) as well as Ex-situ (Coherence Correlation Interferometry) thickness estimation techniques. Refractive index calculated by envelope method was also validated using Reflectometer. These thickness values were used for the estimation of other optical constants such as refractive index, absorption and extinction coefficients. Contact angle measurement studies are also performed on uncoated as well as Ta2O5 coated glass substrates, which indicate the hydrophobic nature of these films.

Scratch test of pearlitic steels: Influence of normal load and number of passes on the sub-superficial layer formation

Abstract The formation of sub-superficial deformed layer in pearlitic steels under dry sliding conditions has been widely researched, especially for the rail/wheel system. The RCF defects are considerate the most important feature to be analyze on the rail service life determination, being common practices, the employ of rail grinding procedure to remove the surface defects. In this work, the multi-pass scratch was used as the laboratory test simulating the material removal mechanism under wear-controlled conditions, observed in maintenance procedures on heavy railways (which correspond to multi-pass abrasive wear events). It was analyzed the microstructural transformation beneath the scratched surface as a function of the numbers of passes (i.e. one-pass to five-passes) and the constant normal load (i.e. 4 N and 8 N). The material of interest consisted in samples of pearlitic steel without prior work hardening. The features of the grooves were characterized by Coherence Correlation Interferometry (CCI) and Scanning Electron Microscopy (SEM). The White Etching layer (WEL) formation was characterized by Focused Ion Beam (FIB-SEM) and the crystallographic orientation was determined by Electron Backscatter Diffraction (EBSD). The characterization of the grooves features shows a small difference in the friction coefficient between test conditions not correlated by the geometrical aspects (variation up to 30% in depth and width). The SEM and FIB-SEM micrographs revels the WEL formation, consisted in a sub-superficial and transitional layer, characterized by ultra-fine grains and mixture of deformed pearlite colonies with undeformed colonies, respectively. Crystallographic texture can be found in samples tested with the lower normal load (4 N) and its relationship with the number of passes was determined as a function of the reference plane selected. The crystallographic texture to scratch direction (RD) was found in the less severe condition of passes, while in the transversal direction (TD) as a reference plane, a high concentration of pearlite colonies oriented at {111} was determined.

Comprehensive study to evaluate the lifespan of flexible polishing pads by 3D surface characterization technique

The topography of a polishing pad plays a significant role to maintain the effectiveness of the finishing process and to preserve the work surface integrity. 3D surface characterization is an efficient tool to reveal the micro-geometry of the pad. A sequential finishing operation of WC-Co coating is performed by shape adaptive grinding (SAG), and chemical assisted SAG (CA-SAG) using zirconia–alumina polishing pads with mean particle sizes of 125 µm, 80 µm and 60 µm. Coherence correlation interferometry (CCI) is employed to investigate the topography of the polishing pads and to assess the microscopic wear and the lifespan of the pads. A new criterion for selecting the optimum sampling interval is proposed to ascertain the peak density precisely. A comprehensive study is undertaken to quantify and analyse the abrasive wear of the polishing pads after both SAG and CA-SAG by employing 13 advanced 3D surface texture parameters. The density and sharpness of the abrasive particles, the coarseness of the polishing pad, the area and volume of the individual motifs and the bearing and fluid retention properties of the pads are characterized. In the case of CA-SAG, the chemical reagent attacks the backing material of the pads and reduces its effective lifespan as compared to SAG.

The assessment of the surface texture of friction node components after tribological tests

At present, a number of methods can be applied to measure the surface structure of elements. In this paper, following methods were selected to measure the surface texture of friction node components (a ball-disc couple) after tribological tests: contact profilometry, coherence correlation interferometry and atomic force microscopy. The specimens under investigation were: a disc coated by DLC (diamond-like carbon) layer and a ball made of bearing steel.

Flattening of surface roughness in ultrashort pulsed laser micro-milling

Micro-milling of AISI 316L was attempted by pulsed (800fs) laser ablation using a scan strategy based on spiral trajectories. The aim is twofold: reducing the areal roughness of the ablated surface with respect to those obtainable by simply flanking parallel grooves and ensuring an uniform depth of cut per layer. The influence of the laser parameters (pulse energy, repetition rate) and the trajectory parameters (beam speed, number of turns) on the areal roughness Sa were investigated by means of experiments removing circular layers with a diameter of 300μm. Ablated morphologies were investigated by coherence correlation interferometry and surface metrology was carried out according to ISO 25178-2.Guidelines were derived to remove larger layers with a diameter of 600μm and the dependence of areal roughness and depth of cut was studied as a function of the energy density parameter, to include laser parameters in a simplified term. The removal process of layers having a depth of cut ranging between 1 and 2μm and areal roughness of about 100nm was ascertained and used for a straightforward micro-machining case study.

Surface topographical analysis of geomembranes and sands using a 3D optical profilometer

Understanding the stress-displacement response of granular materials and their interfaces with geosynthetics through surface topographical analysis is evolving as a new area of potential interest. Recent developments in optical techniques coupled with digital image recording have enabled the measurement of the surface topographical features of sand particles and geosynthetics, which can be applied to measure the shear-induced surface changes in geomembranes. This paper presents one of the latest, advanced surface measurement techniques based on coherence correlation interferometry using a 3D optical profilometer. An experimental technique was developed to measure the surface topography of sand particles, and the shear-induced surface changes in a smooth geomembrane caused by sand particles of different mean sizes, in interface direct shear tests. As the measured surfaces consisted of form and waviness along with roughness, appropriate filters were applied to remove form and waviness and measure the roughness of the surfaces. The selection of these filters for different cases and the spatial variability of surface roughness measurements are discussed. The results of the surface topographical analysis of sand particles of three different sizes, geomembranes sheared by sands of three different mean sizes, and their repeatability, are presented. The stress-displacement response of the sand–geomembrane interfaces is correlated to the surface changes on sheared geomembranes through visual observations and roughness quantifications.

Wpływ materiału i rodzaju obróbki na możliwość pomiaru stereometrii powierzchni metodą koherentnej interferometrii korelacyjnej

Wpływ materiału i rodzaju obróbki na możliwość pomiaru stereometrii powierzchni metodą koherentnej interferometrii korelacyjnej

Investigation of Surface Roughness of Single Point Diamond Turned Germanium Substrate by Coherence Correlation Interferometry and Image Processing

Germanium is a widely used material in the infrared range. Single crystal germanium is used as semiconductor and optical material due to its salient features like high refractive index and proper working in cryogenic conditions. Thus, germanium is an important substrate for infrared lens having many applications in thermal imaging cameras, optical telescopes and miniaturization of infrared optical elements. These applications require optical elements of excellent surface quality and high dimensional accuracy. In addition to fulfil the demands, ultraprecision machine is used to fabricate the optical components. In this work, single crystal germanium (111) mirror is fabricated by using single point diamond tool with, negative rake angle. A large number of experiments are performed to achieve the surface finish of nanometric range. The best and worst combinations of process parameters are found on the basis of surface roughness with the help of coherence correlation interferometry(CCI) measurement and image processing using Canny, Prewitt, Roberts and Sobel edge filters and histogram. These results can be used for fabrication of diffractive optical elements and aspheric lenses of germanium.

Characterisation of Thin Films Using a Coherence Scanning Interferometry

Accurate measurement of film thickness and its uniformity are very important to the performance of many coated surfaces and for many applications are critical. Effective inspection of the film thickness and uniformity is the key to high performance. Conventionally, film thickness is meas- ured using a spectrophotometer/reflectometer, ellipsometer or a physical step measurement; however, these techniques all have limitations. Coherence Scanning Interferometry (CSI) is an established method to measure surface topography as this technique offers many advantages such as speed, ease of use and accuracy. The measurement of “thick” films (exceeding ~1.5 μm) which gives rise to clearly separate fringe bunches is a well-established CSI capability. However, a methodology known as the ‘helical complex field’ (HCF) [1] [2] function allows film thickness to be measured down to ~25 nm. This new method, combined with Coherence Correlation Interferometry (CCI) [3] offers film thickness measurements with sub-nanometre vertical resolution and ~1 μm lateral resolution. It is ideally suited for detailed analysis of coated optical surfaces. In this paper, the fundamentals of the techniques are described and some case studies are presented.

Thin film thickness measurements using Scanning White Light Interferometry

Scanning White Light Interferometry is a well-established technique for providing accurate surface roughness measurements and three dimensional topographical images. Here we report on the use of a variant of Scanning White Light Interferometry called coherence correlation interferometry which is now capable of providing accurate thickness measurements from transparent and semi-transparent thin films with thickness below 1μm. This capability will have many important applications which include measurements on optical coatings, displays, semiconductor devices, transparent conducting oxides and thin film photovoltaics. In this paper we report measurements of thin film thickness made using coherence correlation interferometry on a variety of materials including metal-oxides (Nb2O5 and ZrO2), a metal-nitride (SiNx:H), a carbon-nitride (SiCxNy:H) and indium tin oxide, a transparent conducting oxide. The measurements are compared with those obtained using spectroscopic ellipsometry and in all cases excellent correlation is obtained between the techniques. A key advantage of this capability is the combination of thin film thickness and surface roughness and other three-dimensional metrology measurements from the same sample area.

Laser Interference Microscopy for Nanobiotechnologies

Interference microscopy is widely used in different areas of science for comprehensive study of optical and geometric properties of various microand nanoobjects. The fact that phase imaging makes it possible to overcome the classical resolution threshold has been well known since the 1950s (1). However, only recently the progress in electronics and computer technology has allowed the capacities of phase microscopy to be used to the full extent. Presently, many models of interference micro� scopes are available on the market. The specifications of the interference microscopes are consistent with the area of application. Optical profilometers based on whitelight interferometers are most commonly used. Optical pro� filometers are used in semiconductor technology for quality control. In recent years, interference microscopy finds con� tinuously increasing application for studying biological objects. The advantages of interference microscopy are high spatial resolution, noninvasive monitoring, and lack of special requirements for the medium in which the measurement is taking place (vacuum or chemical dyes are not required). Interference microscopy, which is a strong alternative to conventional optic and confocal microscopy, is based on modern methods of interfero� gram processing. These methods include: coherence phase microscopy (2), optical coherence microtomogra� phy (3), coherence correlation interferometry (4), and digital holographic microscopy (5). The main problem of interference microscopy is insufficient correlation between spatial resolution and efficiency. The method of coherence phase microscopy provides lateral resolution up to 100 nm, but a frame 128 × 128 pixels is recorded within 14 sec, which is too long to test the fast processes in biological objects. On the other hand, the method of digital holographic microscopy allows the images to be obtained at a rate 200 frame/sec, but the spatial resolution of the method is below 200 nm, which is inappropriate for visualization of cell organelles or intracellular structures. The goal of this work was to describe a new modifi� cation of modulation interference microscope (MIM) for biomedical research. The MIM detects the distribution of optical and material parameters of the microscopic object of interest (refraction coefficient, reflection coefficient, anisotropy, and polarization) with high spatial and tem� poral resolution.

Advanced metrology of Laser Groove Buried Contact processing for silicon CPV

ABSTRACTThe solar cells employed in low to medium (50 to 200 suns) concentration photovoltaic (CPV) are usually mono-crystalline silicon. Laser Groove Buried Contacts (LGBC) are preferred to screen printing in these cells due to the high currents generated in the system. In this paper, we report on the use of Coherence Correlation Interferometry (CCI) to accurately measure the width and depth of the laser-ablated grooves. In addition, the technique is also used to measure the surface roughness at the bottom of the trenches, since this can determine the success of the subsequent plating process, and at the top surface to optimize the debris control and obtain clean surfaces and well-shaped groove edges. The laser ablation process was also optimized to obtain the groove aspect ratio and surface quality required. Process parameters to be controlled include laser power, pulse energy, stage speed and focal length. The CCI technique is capable of providing all the groove and surface metrology required for this process optimization.