Optical characterization of inhomogeneous thin films
Date
2010-06-01T12:58:36Z
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Abstract
Töö tegeleb õhukeste väheneelavate kilede optiliste parameetrite määramisega olukorras, kus kile murdumisnäitajat läbi materjali ei saa lugeda konstantseks. Näidatakse, et mõõdetud läbilaskvusspektri alusel on võimalik määrata murdumisnäitaja struktuuri, kirjeldades kilet Lorentzi dispersioonsõltuvusega, kasutades kahest alakihist koosnevat struktuuri ja mitmeparameetrilist globaalset optimeerimist. Seejuures peab ühe alakihi paksus olema väiksem, kui optiline veerandlaine kasutatavas mõõtepiirkonnas. Demonstreeritakse, et sel viisil arvutuslikult saadud struktuurid on korduvad, kui aurustustingimused pole muutunud. Veel parem lähendus saadakse, kui arvesse on võetud ka sujuv üleminek alakihtide vahel, kuid sel juhul pole kindel, et tulemus kirjeldab kihi struktuuri, aga mitte mõõtmisega kaasnevat müra.
Vaakumaurustatud kilede jaoks (MgF2, Y2O3 jt.) annab modelleerimine tulemuseks negatiivse mittehomogeensusega struktuuri, mille korral ülemisel, õhuga kokkupuutes oleval alakihil, on väiksem murdumisnäitaja, kui alumisel, alusega kokkupuutuval alakihil. Kumbal pool paikneb õhuke alakiht, pole võimalik määrata spektrist, kuid neid kaht paigutust ja lineaarse mittehomogeensuse juhtu on võimalik eristada, kui võtta juurde kihi füüsilise paksuse mõõtmise tulemus.
Aatomkihtsadestatud kilede jaoks on näidatud, et neil on protsessitingimustest sõltuvalt võimalik nii positiivne kui ka negatiivne mittehomogeensus. TiO2 kilede jaoks on tõestatud, et modelleerimisega on võimalik kihi struktuurist informatsiooni saada ka juhul, kui kile paksusest ei piisa interferentsiekstreemumide tekkeks. Kasvuaegse minimaalse kandegaasi voo korral kujuneb TiO2 kihi korral õhuke, väiksema murdumisnäitajaga, alakiht kontaktis sulakvartsist alusega.
Läbilaskvusspektri kasutamine annab võimaluse uurida kile struktuuri lihtsamaid vahendeid kasutades. Reaalsete kilede kirjeldamine murdumisnäitaja mittehomogeensust arvestavas lähenduses lubab ka paremini aru saada mitmekihiliste katete käitumisest ja loodetavasti parandada nende omadusi.
A work deals with the optical thin films of low absorption in a case where the film refractive index is a function of film thickness. It is shown that a film on a transparent substrate may be modelled based only on its transmission spectrum if Lorentz dispersion model is applied for its dispersion and a structure consisting of two sublayers is used. It is shown a multiparameter optimization yields reproducible results for the films of different thickness. A smooth transition between the two sublayers still improves a merit function for fitting, but it is not clear whether an obtained improvement demonstrates a real structure or simply better representation of measured spectrum including a noise. For vacuum evaporated films (MgF2, Y2O3 etc.) the fitting yields a negative inhomogeneity of a refractive index (higher refractive index in contact with a substrate and lower in contact with ambient) in which one of the sublayers must have an optical thickness less than quarter of the wavelength for near UV region. While a fit merit function may be the same for the two-layer inhomogeneity and a linear inhomogeneity model, measurement of a physical thickness of a film may help to make a choice. It is shown for electron beam evaporated Y2O3 film a thin sublayer is placed in contact with a fused silica substrate. For atomic layer deposited films the both types of two-layer inhomogeneity (negative and positive) are possible. The analysis of the TiO2 films shows film inhomogeneity type may change depending on process carrier gas flow rate or on temperature. Using a multiparameter fitting makes it possible to obtain the results also for the films too thin for using the usual interference analysis methods. For a lowest carrier gas rate a thin sublayer of less index of refraction is shown to have a place in contact with substrate. The results of this work should help to understand a structure of the thin films based on the transmission measurements, which are less demanding in instrumentation. Taking into account film inhomogeneity should also help in constructing of the thin film optical coatings.
A work deals with the optical thin films of low absorption in a case where the film refractive index is a function of film thickness. It is shown that a film on a transparent substrate may be modelled based only on its transmission spectrum if Lorentz dispersion model is applied for its dispersion and a structure consisting of two sublayers is used. It is shown a multiparameter optimization yields reproducible results for the films of different thickness. A smooth transition between the two sublayers still improves a merit function for fitting, but it is not clear whether an obtained improvement demonstrates a real structure or simply better representation of measured spectrum including a noise. For vacuum evaporated films (MgF2, Y2O3 etc.) the fitting yields a negative inhomogeneity of a refractive index (higher refractive index in contact with a substrate and lower in contact with ambient) in which one of the sublayers must have an optical thickness less than quarter of the wavelength for near UV region. While a fit merit function may be the same for the two-layer inhomogeneity and a linear inhomogeneity model, measurement of a physical thickness of a film may help to make a choice. It is shown for electron beam evaporated Y2O3 film a thin sublayer is placed in contact with a fused silica substrate. For atomic layer deposited films the both types of two-layer inhomogeneity (negative and positive) are possible. The analysis of the TiO2 films shows film inhomogeneity type may change depending on process carrier gas flow rate or on temperature. Using a multiparameter fitting makes it possible to obtain the results also for the films too thin for using the usual interference analysis methods. For a lowest carrier gas rate a thin sublayer of less index of refraction is shown to have a place in contact with substrate. The results of this work should help to understand a structure of the thin films based on the transmission measurements, which are less demanding in instrumentation. Taking into account film inhomogeneity should also help in constructing of the thin film optical coatings.
Description
Väitekirja elektroonilisest versioonist puuduvad publikatsioonid.
Keywords
doktoritööd, füüsika, kiled