S. Thakur

Spectroscopic ellipsometry of TiO2 layers prepared by ion-assisted electron-beam evaporation

Abstract Single layer TiO 2 films deposited on BK7 glass substrates by the ion-assisted electron-beam evaporation technique have been characterized by phase modulated spectroscopic ellipsometry. The ellipsometry spectra were recorded in the wavelength range of 300–1000 nm. The measured spectra were then fitted with theoretically simulated curves generated assuming different model sample structures. Analyses were carried out separately in three different wavelength regimes, the transparent regime with no dispersion of refractive index (650–1000 nm), the transparent regime with dispersion of refractive index (400–650 nm) and the absorbing regime (300–450 nm). Modeling has been attempted with both homogeneous and inhomogeneous sample structures. For the inhomogeneous structure, both linear and non-linear variation of refractive index along the depth of the sample were used. Refractive indices of the samples were determined separately from the best-fit ellipsometric data in the above three wavelength regimes. Finally, variation of refractive index with the variation in ion beam currents has been studied.

Reactive electron beam evaporation of gadolinium oxide optical thin films for ultraviolet and deep ultraviolet laser wavelengths

Availability of ultraviolet optical thin film materials, especially high index refractory oxides that transmit down to deep ultraviolet (approx. 0.2 mm) is very much limited. The present article discusses some of the research optimization studies on gadolinium oxide (Gd O ), a novel lanthanide sesquioxide material, for such challenging spectral requirements. Optical and 23 topographical properties of single layer films have been studied using phase modulated spectroscopic ellipsometry, spectrophotometry and multimode atomic force microscopy. Films deposited at lower substrate temperatures have shown higher band gaps and higher substrate temperatures yielded higher refractive indices. Multilayer high reflection filters have been developed for ultraviolet laser wavelengths such as ArF (193 nm), KrCl (222 nm), KrF (248 nm) and Nd-Yag-III (355 nm), using this material at lower substrate temperature conditions and successfully tested for their performances. 2003 Elsevier Science B.V. All rights reserved.

Correlation of optical and microstructural properties of Gd2O3 thin films through phase-modulated ellipsometry and multi-mode atomic force microscopy

Gadolinium oxide thin films have been prepared by electron beam evaporation with different reactive oxygen pressures at low ambient substrate temperature. These thin films have been analyzed with phase-modulated spectroscopic ellipsometry and multi-mode atomic force microscopy (AFM). The distinct influences of oxygen pressure on surface topographies, microstructures and refractive indices of the thin films have been observed from the results of these advanced measurements. Both these techniques have displayed very strong co-relationships in the characterisation results acquired through respective modes of measurements and data analysis. Unequivocally, these two techniques indicated an optimum value of the oxygen pressure leading to best optical and structural properties of such a novel optical coating material.

Achieving superior band gap, refractive index and morphology in composite oxide thin film systems violating the Moss rule

The interrelation between energy gap and high frequency refractive index in semiconductors and dielectrics is manifested by an inverse law which is popularly known as the Moss rule. This semi-empirical relationship is based on the fundamental principle that in a dielectric medium all energy levels are scaled down by a factor of the square of the dielectric constant. Such a rule is obeyed by most pure semiconductors and dielectrics with a few rare violations in composite materials which display several interesting parametric and microstructural evolutions. The present results are based on some specific oxide composite thin films involving Gd2O3/SiO2 and ZrO2/SiO2 codeposited systems that have displayed a superior refractive index and energy gaps violating the semi-empirical Moss rule. Also, morphological supremacy is also distinctly noticed in these composites. The novel microstructural and polarizability properties of such composite systems were probed through multi-mode atomic force microscopy and phase modulated spectroscopic ellipsometry using refractive index modelling, autocorrelation and height–height correlation functional analyses. These binary composite thin films have shown their potential as well as the possibility of meeting expectations in satisfying the challenging optical coating requirements of the deep ultraviolet spectral region.

Characterization of a multilayer highly reflecting mirror by spectroscopic phase-modulated ellipsometry

The characterization of optical multilayer coatings has been a challenging task for thin-film scientists and engineers because of the various complex, interdependent layer parameters that exist in the system. Spectroscopic phase-modulated ellipsometry has some advantages in the postanalysis of the layer parameters of such multilayer coatings because it suitably models the layer structure with respect to the ellipsometric measurements. An algorithm to characterize multilayer optical coatings with large numbers of layers has been described by spectroscopic ellipsometry by use of a discrete spectral zone fitting approach. A 23-layer multilayer highly reflecting mirror has been characterized by this technique in the wavelength range 280-1000 nm. The ellipsometric spectra (? and D versus wavelength) have been fitted separately in three wavelength regimes. Fitting the ellipsometric spectra in the wavelength regime of 700-1000 nm permitted the sample structure to be determined. The data were then fitted in the wavelength range 280-340 nm, i.e., near the fundamental absorption edge of TiO(2), to yield the dispersion relation for the optical constants of TiO(2). Finally, the data were fitted in the wavelength range 340-700 nm, and the true dispersion of the refractive index of TiO(2), along with the best-fitting sample structure, was obtained.

Spectroscopic ellipsometry of multilayer dielectric coatings

Abstract Multilayer dielectric coatings deposited by e-beam evaporation have been characterised by the phase modulated spectroscopic ellipsometer (PMSE). Measurements have been done on various multilayer thin films devices e.g., high reflectivity mirror, narrow band filter, beam combiner, beam splitter, etc. consisting of several bilayers of TiO 2 /SiO 2 . Results have been shown here for the first two samples. The measured Ellipsometry spectra are fitted with theoretical spectra generated assuming appropriate models regarding the sample structures. Optical constants of the substrates and the SiO 2 films have been supplied and trial dispersion relations have been used for the optical constants of the TiO 2 layers. The fittings have been done by minimising the squared difference ( χ 2 ) between the measured and calculated values of the ellipsometric parameters ( ψ and Δ) and accurate information have been derived regarding the thickness and refractive indices of the different layers.

Optical properties and laser damage threshold of HfO 2 –SiO 2 mixed composite thin films

HfO(2)-SiO(2) mixed composite thin films have been deposited on fused silica substrate by co-evaporation of HfO(2) and SiO(2) through the reactive electron-beam evaporation technique. The composition-dependent refractive index and the absorption coefficient have been analyzed using different effective medium approximation (EMA) models in order to evaluate the suitability of these models for such mixed composite thin films. The discrepancies between experimentally determined and EMA-computed values are explained through microstructural and morphological evolutions observed in these mixed composite films. Finally, the dependence of the laser damage threshold as a function of silica content has been investigated, and the improved laser-induced damage threshold for films having more than 80% silica content has been explained through the defect-assisted multiphoton ionization process.

Deposition and characterization of titania-silica optical multilayers by asymmetric bipolar pulsed dc sputtering of oxide targets

Titania?silica (TiO2/SiO2) optical multilayer structures have been conventionally deposited by reactive sputtering of metallic targets. In order to overcome the problems of arcing, target poisoning and low deposition rates encountered there, the application of oxide targets was investigated in this work with asymmetric bipolar pulsed dc magnetron sputtering. In order to evaluate the usefulness of this deposition methodology, an electric field optimized Fabry Perot mirror for He?Cd laser (? = 441.6?nm) spectroscopy was deposited and characterized. For comparison, this mirror was also deposited by the reactive electron beam (EB) evaporation technique. The mirrors developed by the two complementary techniques were investigated for their microstructural and optical reflection properties invoking atomic force microscopy, ellipsometry, grazing incidence reflectometry and spectrophotometry. From these measurements the layer geometry, optical constants, mass density, topography, surface and interface roughness and disorder parameters were evaluated. The microstructural properties and spectral functional characteristics of the pulsed dc sputtered multilayer mirror were found to be distinctively superior to the EB deposited mirror. The knowledge gathered during this study has been utilized to develop a 21-layer high-pass edge filter for radio photoluminescence dosimetry.

Phase modulated spectroscopic ellipsometry of dielectric multilayer beam combiner

Abstract An algorithm to characterise multilayer optical coatings with large numbers of layers by spectroscopic ellipsometry, using a discrete spectral-zone fitting approach, has been demonstrated by characterising a 27-layer TiO2/SiO2 multilayer beam combiner in the wavelength range of 280–1200 nm. The ellipsometric spectra are fitted first in the wavelength regime of 700–1200 nm and the sample structure was determined. TiO2 and SiO2 layers have been assumed to be transparent in this wavelength regime with dispersion-less refractive indices. The data were then fitted in the wavelength range of 280–340 nm to find the dispersion relation for the optical constants of TiO2. Finally, the fitting has been done in the wavelength range of 340–700 nm and the true dispersion of refractive index of TiO2 along with the best fit sample structure have been obtained.

Annealing effects on microstructure and laser-induced damage threshold of HfO 2 /SiO 2 multilayer mirrors.

HfO2/SiO2 periodic multilayer high reflection mirrors have been prepared by a reactive electron-beam evaporation technique. The deposited mirrors were annealed in the temperature range from 300°C to 500°C. The effects of annealing on optical, microstructural, and laser-induced damage characteristics of the mirrors have been investigated. The high reflection band of the mirror shifts toward a shorter wavelength with increasing annealing temperature. As-deposited and annealed mirrors show polycrystalline structure with a monoclinic phase of HfO2. Crystalinity and grain size increase upon annealing. The laser-induced damage threshold (LIDT) has been assessed using a 532 nm pulsed laser at a pulse width of 7 ns. The LIDT value of the multilayer mirror increases from 44.1  J/cm2 to 77.6  J/cm2 with annealing up to 400°C. The improvement of LIDT with annealing is explained through oxygen vacancy defects as well as grain-size-dependent thermal conductivity. Finally, the observed laser damage morphology, such as circular scalds and ablated multilayer stacks with terrace structure, are analyzed.