Spectroscopic Imaging Ellipsometry Research Articles

A multi-method study of femtosecond laser modification and ablation of amorphous hydrogenated carbon coatings

We present a study on femtosecond laser treatment of amorphous hydrogen-containing carbon coatings (a-C:H). The coatings were deposited on silicon wafers by a plasma-assisted chemical vapour deposition (PA-CVD), resulting in two different types of material with distinct properties (referred to as “absorbing” and “semi-transparent” coatings in the following). The samples were laser-treated with single fs-laser pulses (800 nm center wavelength, 35 fs pulse duration) in the ablative regime. Through a multi-method approach using topometry, Raman spectroscopy, and spectroscopic imaging ellipsometry, we can identify zones and thresholds of different fluence dependent effects and have access to the local dielectric function. The two coating materials react significantly different upon laser treatment. We determined the (non-ablative) modification threshold fluence for the absorbing coating as 3.6×10-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3.6\ imes {10}^{-2}$$\\end{document} Jcm−2 and its ablation threshold as 0.22 Jcm−2. The semi-transparent coating does not show such a low-fluence modification but exhibits a characteristic interference-based intra-film ablation mechanism with two distinguishable ablation thresholds at 0.25 and 0.28 Jcm−2, respectively. The combination of tailored layer materials and correlative imaging spectroscopic methods delivers new insights into the behaviour of materials when treated with ultrashort-pulse laser radiation.Graphical abstract

Versatile fitting approach for operando spectroscopic imaging ellipsometry of HfS2 oxidation

Facile mapping of 2D heterostructures and resolving anisotropic formation kinetics down to the monolayer level are critical to developing scalable interfacing solutions and unlocking their application potential in emerging nano-optoelectronics. We adapt a Kramers–Kronig constrained variational fitting algorithm for spectroscopic imaging ellipsometry (SIE) to facilitate multi-scale heterostructure analysis comprising films with unknown complex dielectric functions and demonstrate how this enables non-destructive, scalable mapping and operando capability for the model system of HfS2 oxidation. This methodology proves highly accurate for assessing the thickness of buried HfS2 layers, oxide quality, and lateral and vertical uniformity. We capture dynamic stack evolution during thermal oxidation up to 400 ∘C, providing insights into the temperature and time-dependent nature of self-limiting oxide growth and reaction kinetics that involve the localised trapping and release of sulphur reaction products. Our methodology is versatile in material and device horizons, and advantageously agnostic to the underlying substrate. Combined with the various modes of SIE operation, it unlocks fast, high-throughput, large-area capability to accelerate process development at the atomic scale.

Scalable Synthesis of Self‐Disinfecting Polycationic Coatings for Hospital Relevant Surfaces

AbstractThe prevention of microbial infections is a global challenge. Efficient antimicrobial coatings that rapidly kill microorganisms upon contact can help minimize their transmission. However, their scalable synthesis is challenging. This work demonstrates the scalable synthesis and characterization of self‐disinfecting nanofilms for the postmodification of hospital‐relevant surfaces. Their antimicrobial action is based on charge interactions between a supercharged cationic surface film and the negatively charged bacteria membrane. Photoinitiated bulk polymerization of an air‐dried [2‐(methacryloyloxy)ethyl]trimethylammonium chloride film on cotton (gowns), nitrile rubber (protective gloves), and glass surfaces (tables, screens) is used for their supercharging, and studied with streaming potential measurements. A 6 nm thick coating dominated by cationic quarternary amine groups is shown by a combination of spectroscopic imaging ellipsometry and X‐ray photoelectron spectroscopy. Antimicrobial in vitro evaluation of the coated surfaces demonstrates up to ≈4 log reductions in bacterial populations in less than 5 min. Confocal laser scanning microscopy and live‐dead staining confirm the surface‐induced killing of bacteria. The coating's range of compatible materials and its rapid bactericidal activity can combat the surface transmission of bacteria and may help to contain the spread of infectious diseases. Its synthesis in environmental conditions is promising for integration into industrial processes.

Systematic analysis of the Rayleigh–Wood anomalies and the symmetry relations of 2D‐periodic nanostructures by imaging spectroscopic ellipsometry

AbstractPeriodic arrays of metallic nanostructures were fabricated by electron beam lithography and studied by means of spectroscopic imaging ellipsometry in the near infrared to ultraviolet spectral ranges. The sample consists of gold nanodisc and nanorod gratings on a silicon substrate. Spectroscopic imaging ellipsometry, allowing for the simultaneous observation of all gratings with microscopic resolution, was used to systematically analyze the varying grating and nanostructure parameters. The ellipsometric spectra obtained for a full in‐plane sample rotation proved to be a highly sensitive measure of the gratings’ Rayleigh–Wood anomalies. Their dependence on the sample azimuth was in excellent agreement with the Rayleigh lines calculated from the grating parameters and could be tracked for both the ambient‐to‐grating and the grating–substrate interfaces. Contrary to other studies on similarly sized nanoparticle arrays, we found no indication of localized plasmon resonance in the visible range of the spectrum, but only a weak NIR response truncated by the Rayleigh lines. Finally, the symmetry of the structures was investigated by imaging Mueller‐matrix measurements.

Characterizing optical phase-change materials with spectroscopic ellipsometry and polarimetry

This paper discusses the fundamentals, applications, potential and limitations of polarized light reflection techniques for the characterization of phase-change materials (PCMs). These techniques include spectroscopic ellipsometry, time-resolved ellipsometry and imaging ellipsometry as well as polarimetry. We explore the capabilities of spectroscopic ellipsometry in the determination of the extinction coefficient of PCMs and the capabilities of imaging ellipsometry to characterize PCMs. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about crystallization/amorphization kinetics and mapping anisotropies.

Exciton Manifolds in Highly Ambipolar Doped WS2.

The disentanglement of single and many particle properties in 2D semiconductors and their dependencies on high carrier concentration is challenging to experimentally study by pure optical means. We establish an electrolyte gated WS monolayer field-effect structure capable of shifting the Fermi level from the valence into the conduction band that is suitable to optically trace exciton binding as well as the single-particle band gap energies in the weakly doped regime. Combined spectroscopic imaging ellipsometry and photoluminescence spectroscopies spanning large n- and p-type doping with charge carrier densities up to 10 cm enable to study screening phenomena and doping dependent evolution of the rich exciton manifold whose origin is controversially discussed in literature. We show that the two most prominent emission bands in photoluminescence experiments are due to the recombination of spin-forbidden and momentum-forbidden charge neutral excitons activated by phonons. The observed interband transitions are redshifted and drastically weakened under electron or hole doping. This field-effect platform is not only suitable for studying exciton manifold but is also suitable for combined optical and transport measurements on degenerately doped atomically thin quantum materials at cryogenic temperatures.

Layered gallium sulfide optical properties from monolayer to CVD crystalline thin films.

Interest in layered van der Waals semiconductor gallium monosulfide (GaS) is growing rapidly because of its wide band gap value between those of two-dimensional transition metal dichalcogenides and of insulating layered materials such as hexagonal boron nitride. For the design of envisaged optoelectronic, photocatalytic and photonic applications of GaS, the knowledge of its dielectric function is fundamental. Here we present a combined theoretical and experimental investigation of the dielectric function of crystalline 2H-GaS from monolayer to bulk. Spectroscopic imaging ellipsometry with micron resolution measurements are corroborated by first principle calculations of the electronic structure and dielectric function. We further demonstrate and validate the applicability of the established dielectric function to the analysis of the optical response of c-axis oriented GaS layers grown by chemical vapor deposition (CVD). These optical results can guide the design of novel, to our knowledge, optoelectronic and photonic devices based on low-dimensional GaS.

Spectroscopic imaging ellipsometry for two-dimensional thin film thickness measurement using a digital light processing projector

An improved version of spectroscopic imaging ellipsometry is described for accurate reconstruction of two-dimensional thin film thickness. A digital light processing projector enables a selected area of the back focal plane of the objective lens to be illuminated so that the angle of incidence and the polarization state of the light source vary depending on the area of the back focal plane being illuminated. By combining multiple images of the object plane obtained at different polarization states, every pixel in the field of view has its own ellipsometric parameters; therefore a reconstruction of two-dimensional thin film thickness is possible. Because the proposed ellipsometry system has a co-axial optical structure in which the objective lens is arranged in the normal direction to the measurement target, the spatial resolution is improved due to the application of a high-magnitude objective lens. In addition, spectroscopic analysis can be conducted using a number of band-pass filters which each have a central wavelength. The effect of the proposed method on thin film thickness measurement was evaluated by comparing the experimental results with a topographic profile obtained using a commercial atomic force microscope.

Probing the local dielectric function of WS2 on an Au substrate by near field optical microscopy operating in the visible spectral range

The optoelectronic properties of nanoscale systems such as carbon nanotubes (CNTs), graphene nanoribbons and transition metal dichalcogenides (TMDCs) are determined by their dielectric function. This complex, frequency dependent function is affected by excitonic resonances, charge transfer effects, doping, sample stress and strain, and surface roughness. Knowledge of the dielectric function grants access to a material’s transmissive and absorptive characteristics. Here we use the dual scanning near field optical microscope (dual s-SNOM) for imaging local dielectric variations and extracting dielectric function values using a pre-established mathematical inversion method. To demonstrate our approach, we studied a monolayer of WS2 on bulk Au and identified two areas with differing levels of charge transfer. The experiments highlight a further advantage of the technique: the dielectric function of contaminated samples can be measured, as dirty areas can be easily identified and excluded for the calculation, being important especially for exfoliated 2D materials (Rodriguez et al., 2021). Our measurements are corroborated by atomic force microscopy (AFM), Kelvin force probe microscopy (KPFM), photoluminescence (PL) intensity mapping, and tip enhanced photoluminescence (TEPL). We extracted local dielectric variations from s-SNOM images and confirmed the reliability of the obtained values with spectroscopic imaging ellipsometry (SIE) measurements.

Single Femtosecond Laser-Pulse-Induced Superficial Amorphization and Re-Crystallization of Silicon.

Superficial amorphization and re-crystallization of silicon in <111> and <100> orientation after irradiation by femtosecond laser pulses (790 nm, 30 fs) are studied using optical imaging and transmission electron microscopy. Spectroscopic imaging ellipsometry (SIE) allows fast data acquisition at multiple wavelengths and provides experimental data for calculating nanometric amorphous layer thickness profiles with micrometric lateral resolution based on a thin-film layer model. For a radially Gaussian laser beam and at moderate peak fluences above the melting and below the ablation thresholds, laterally parabolic amorphous layer profiles with maximum thicknesses of several tens of nanometers were quantitatively attained. The accuracy of the calculations is verified experimentally by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (STEM-EDX). Along with topographic information obtained by atomic force microscopy (AFM), a comprehensive picture of the superficial re-solidification of silicon after local melting by femtosecond laser pulses is drawn.

Coaxial spectroscopic imaging ellipsometry for volumetric thickness measurement.

We propose spectroscopic imaging ellipsometry that can measure spectral ellipsometric signals in the entire field of view simultaneously without areal scanning or operation of polarization devices. The proposed imaging ellipsometry is configured in a coaxial optical structure so that the high magnification objective lens is applicable and the spatial resolution is highly increased. Without the operation of polarization components and to efficiently obtain the spectral data in the object plane, the ellipsometric parameters are encoded into the high frequency in the spectral domain and are measured by an imaging Michelson interferometer. The volumetric thickness measurement by the proposed method was verified by comparing the thickness results of the SiO2/Si sample that has four different thicknesses with commercial ellipsometer results.

Gold nanoparticles growing in a polymer matrix: What can we learn from spectroscopic imaging ellipsometry?

Despite an increasing number of studies and the apparent simplicity of these model systems, the mechanisms of the growth of metal nanoparticles in a polymer matrix and, in particular, that of gold nanoparticles is still not fully understood. Usually, reported results concern global (ca. surface-averaged) measurements. Furthermore, the optical properties of plasmonic nanocomposites are difficult to investigate when the metal volume fraction is very low, typically less than 1%. This is especially true in the case of gold for which the localized plasmon resonance is less sharp than in the case of silver and, therefore, less easy to probe. In this article, the optical properties of gold-doped nanocomposites have been studied at the (sub)micrometer scale using spatially resolved spectroscopic ellipsometry. At low gold volume fraction, the thermal annealing of the composite leading to the in situ growth of the gold nanoparticles induces a local inhomogeneity of the Ψ and Δ ellipsometric images that can be analyzed in terms of heterogeneity of the gold fraction. Spectroscopic imaging ellipsometry confirms the existence of gold-depleted regions in the vicinity of the largest gold particles.

Attempt to Detect Nano Oil Film on the Surface of Polished KDP Crystal

Abstract—The KDP (Potassium Dihydrogen Phosphate) crystal is important in high-energy optical system as a high quality non-linear optical material. The residual oil film which even is of nanometer thickness has great adverse effect on the KDP optical performance, such as laser absorption and damage threshold. In this paper, spectroscopic imaging ellipsometry (SIE) is applied to investigate the residual oil film on KDP crystal surface during the processing, and atomic force microscopy (AFM) is applied as a reference of the SIE results. Two modes of SIE are applied to detect oil film thickness. A rotating substrate measurement scheme is proposed because KDP crystal is a uniaxial anisotropic material. An optical model with thin oil film and KDP substrate is built including a top surface roughness layer, a flat oil film layer and KDP substrate, which is used to fit the detection data. Then, the single-point measurement results and thickness maps of residual oil film are established to describe the oil film distribution quantitatively. The AM-FM mode of AFM is applied to detect the oil film distribution on KDP crystal after surface cleaning, and the oil film thickness established in AFM measurement corresponds well with the SIE results. The nano oil film on KDP crystal can be measured by SIE technology accurately and the measurement scheme can also provide a reference for the similar detection.

Novel combined measurement system to characterize film structures by spectral interferometry and ellipsometry.

We propose a novel measurement system to simultaneously measure surface and thickness profiles of thin film structures, which cannot be realized in typical measurement techniques. This measurement is accomplished by combining spectral interferometry and ellipsometry. These two distinct measurement techniques are involved in a single system by the abnormal optical configuration. Further, the measurement results are complementary in order to characterize film structures. Film thickness profiles are measured by spectroscopic imaging ellipsometry and surface profiles are obtained from the spectral phase by spectrally resolved interferometry. This method eliminates the theoretical spectral phase by film thicknesses. The proposed system can determine the dimensional film structures at once, even though they have multi-layered films, substrate textures, and even thin film layers. In the experiments, each measurement principle was fundamentally verified with standard specimens. Further, a 4-layered film specimen was measured in order to reconstruct its 3D film structure. As the result, the repeatability of spectroscopic imaging ellipsometry was less than 1 nm and that of spectrally resolved interferometry was a few nanometers, which dominantly affected the performance of the whole system. Several issues for improving accuracy and precision of the proposed system are also discussed in this paper.

Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography.

Adequate characterization and quality control of atomically thin layered materials (2DM) has become a serious challenge particularly given the rapid advancements in their large area manufacturing and numerous emerging industrial applications with different substrate requirements. Here, we focus on ellipsometric contrast micrography (ECM), a fast intensity mode within spectroscopic imaging ellipsometry, and show that it can be effectively used for noncontact, large area characterization of 2DM to map coverage, layer number, defects and contamination. We demonstrate atomic layer resolved, quantitative mapping of chemical vapor deposited graphene layers on Si/SiO2-wafers, but also on rough Cu catalyst foils, highlighting that ECM is applicable to all application relevant substrates. We discuss the optimization of ECM parameters for high throughput characterization. While the lateral resolution can be less than 1 μm, we particularly explore fast scanning and demonstrate imaging of a 4″ graphene wafer in 47 min at 10 μm lateral resolution, i.e., an imaging speed of 1.7 cm2/min. Furthermore, we show ECM of monolayer hexagonal BN (h-BN) and of h-BN/graphene bilayers, highlighting that ECM is applicable to a wide range of 2D layered structures that have previously been very challenging to characterize and thereby fills an important gap in 2DM metrology.

LASIE: Large Area Spectroscopic Imaging Ellipsometry for Characterizing Multi-Layered Film Structures

In this investigation, we describe a spectroscopic imaging ellipsometry for large area measurements, named as large area spectroscopic ellipsometry (LASIE). LASIE uses a broadband light source and an imaging spectrometer in order to obtain the spectral-spatial intensity images corresponding to a measurement line of a specimen and it can characterize the 3D multi-film structures with the aid of lateral scanning. Opposed to the typical high resolution imaging ellipsometry with the small field of view (FOV), LASIE uses a low magnification imaging lens to enlarge the measurement area and line profiles of multi-layered film structure can be reconstructed at a single operation based on the operation of the spectroscopic imaging spectrometer. In the experiment, 3- and 4-layered film specimen were measured after the system calibration and the 3D film thickness profiles of all film layers were obtained with 1 nm repeatability.

Detection of Residual Oil Film on Polished KDP Crystal by Atomic Force Microscope

In this paper, the bimodal amplitude-and frequency modulation method of atomic force microscopy (AFM) is applied to investigate the residual oil film on KDP crystal surface after polishing and surface cleaning. The thickness map of residual oil film is obtained based on the relationship between the cantilever resonant frequency shift and the surface stiffness. The thickness maps measured by AFM and spectroscopic imaging ellipsometry methods are compared, and the results obtained by two methods are consistent. The measurement scheme extends the application of AFM technology to characterize the residual oil on the KDP surface quality quantitatively after polishing and surface cleaning.

Manifold Coupling Mechanisms of Transition Metal Dichalcogenides to Plasmonic Gold Nanoparticle Arrays

We reveal the manifold interaction mechanisms between monolayers of MoS2 and single layers of plasmon-active gold nanoparticles. The MoS2 shows a 10- to 20-fold enhanced photoluminescence when covered with the gold nanoparticles. Surprisingly, we detect this enhancement also for excitation energies that are not resonant to the surface plasmon polaritons of the nanoparticles. Complementary Kelvin probe force measurements indicate a lowering of the work function when the MoS2 is decorated with the gold nanoparticles. This is in agreement with a reduced band gap for the decorated MoS2 as determined from absorbance measurements. We furthermore demonstrate a dielectric coupling between the two layers by spectroscopic imaging ellipsometry as well as Raman spectroscopy. Combining the various results, we discuss the enhanced photoluminescence in terms of a modified emission pattern of the radiative dipole in the MoS2 monolayers in the presence of the gold nanoparticles. In particular, the studied systems elucidat...

Characterization of Free-Standing Nano-Membranes by Using Ellipsometry

The thickness of the pellicle is only a few tens of microns in extreme ultraviolet lithography (EUVL). This is because the absorption loss by the pellicle is high. Thus, the thickness and contamination on the surface of the EUVL pellicle are important factors for controlling the transmission of EUV light. In this work, we fabricate ultra-thin silicon-nitride membranes for EUVL pellicles and use micro-spot spectroscopic ellipsometry and imaging ellipsometry for characterization. We successfully deduce not only the thickness but also the optical function of the membrane. However, we found that some precautions were required for accurate measurement of the free-standing thin membranes by using ellipsometry. Issues related to the vibration of the membrane and the sensitivity of the measurement are discussed.

Electron beam induced changes in optical properties of glassy As35S65 chalcogenide thin films studied by imaging ellipsometry

Spectroscopic imaging ellipsometry was used to study 50 μm × 50 μm squares recorded into glassy As35S65 thin film by electron beam with different exposure doses. Optical constants (refractive index n and extinction coefficient k) of electron exposed areas were determined from imaging ellipsometric data recorded over a spectral range 390–800 nm. Complex dielectric permittivity of each of electron exposed square (glassy As35S65 with different electron exposure dose) was parametrized by a Tauc-Lorentz oscillator. Gradient optical model with the refractive index changing linearly within the layer thickness was used.Evaluation of parameters of Tauc-Lorentz oscillator revealed decrease of energy bandgap, increase of the refractive index in whole recorded spectra (more pronounced at the top of the film) and increase of extinction coefficient in the part of spectra where the material absorbs with the increasing electron exposure dose used for exposure of the squares.The decrease in local disorder with increasing electron exposure dose is quantified from the short wavelength absorption edge onset using a Mott-Davis model. The dependence of the parameters of Wemple-DiDomenico model for the transparent part of the optical constants spectra on electron exposure dose is shown.Thin film with recorded image was subsequently etched in amine based solution in aprotic solvent. While native film was fully etched off the remaining height of the exposed areas is increasing with the increasing electron exposure dose used.Knowledge of the changes of optical properties and etching rates induced by electron beam of different exposure doses can be used for fabricating of optical elements by high resolution electron beam lithography.