Determination of film thickness and refractive index in one measurement of phase-modulated ellipsometry

Ellipsometry is often used to determine the refractive index and/or the thickness of a polymer layer on a substrate. However, simultaneous determination of these parameters from a single-wavelength single-angle measurement is not always possible. The present study determines the sensitivity of the method to errors of measurement for the case of phase modulated ellipsometry and identifies conditions for decoupling film thickness and refractive index. For a specific range of film thickness, both the thickness and the refractive index can be determined from a single measurement with high precision. This optimal range of the film thickness is determined for organic thin films, and the analysis is tested on hydrogel-like polymer films in air and in water.     

Treasure of the Past VII: Measurement of the Thickness and Refractive Index of Very Thin Films and the Optical Properties of Surfaces by Ellipsometry

The use of the ellipsometer for the measurement of the thickness and refractive index of very thin films is reviewed. The Poincaré sphere representation of the state of polarization of light is developed and used to describe the reflection process. Details of the operation of the ellipsometer are examined critically. A computational method is presented by which the thickness of a film of known refractive index on a reflecting substrate of known optical constants may be calculated directly from the ellipsometer readings. A method for computing both the refractive index and thickness of an unknown film is also developed. These methods have been applied to the determination of the thickness of an adsorbed water layer on chromium ferrotype plates and on gold surfaces. In the former case the thickness was 23 to 27 Å, and in the latter was 2 to 5 Å. The measurement of the thickness and refractive index of barium fluoride films evaporated on chromium ferrotype surfaces is used as an illustration of the simultaneous determination of these two quantities.     

Measurement of the Thickness and Refractive Index of Very Thin Films and the Optical Properties of Surfaces by Ellipsometry

The use of the ellipsometer for the measurement of the thickness and refractive index of very thin films is reviewed. The Poincaré sphere representation of the state of polarization of light is developed and used to describe the reflection process. Details of the operation of the ellipsometer are examined critically. A computational method is presented by which the thickness of a film of known refractive index on a reflecting substrate of known optical constants may be calculated directly from the ellipsometer readings. A method for computing both the refractive index and thickness of an unknown film is also developed. These methods have been applied to the determination of the thickness of an adsorbed water layer on chromium ferrotype plates and on gold surfaces. In the former case the thickness was 23 to 27 Å, and in the latter was 2 to 5 Å. The measurement of the thickness and refractive index of barium fluoride films evaporated on chromium ferrotype surfaces is used as an illustration of the simultaneous determination of these two quantities.     

Calculation of film thickness for dip coated antireflective films

Antireflective films require an accurate film thickness to be able to perform optimally. The ideal thickness on most films is a quarter wavelength optical thickness (QWOT). The physical thickness of the QWOT film depends on the refractive index of the material that is being used. Each layer of the antireflective coating will have different optimal conditions for applying the film. When using the dip coating method, these conditions are withdrawal speed and concentration of solution.When using the currently accepted equations derived by Yang et al. to calculate the film thickness an error of 31.7% was noted when compared with the experimentally measured film thickness. Realising that the refractive index of the film plays a role in determining the thickness of the film, the equations were modified to take refractive index into account. Once this was taken into account, the calculated film thickness deviated from the measured film thickness by 8.7%. This error can be attributed to experimental errors which involve temperature and concentration fluctuations.     

Measurement of the physical thickness and refractive index of thin epitaxial garnet films

A technique for the measurement of refractive index and physical film thickness of epitaxial garnet films is described which utilizes variable wavelength measurements. Experimental results are presented for gadolinium gallium garnet substrates and two different bubble domain film compositions. From these results, it is concluded that the technique is not applicable to wafers with films on both sides due to the differences in film thickness of the two films. For single sided wafers, the refractive index can be determined with an accuracy of ±0.3 percent.     

Refractive index of polycrystalline submicrometer polymer thin films: Thickness dependence

Dielectric films used in insulating applications are becoming consistently thinner, hence the thickness of thin and ultathin films is an important design parameter. There exists a need for characterizing and understanding the thickness dependence of properties of films. The refractive index for low dielectric polytetrafluoroethylene crystalline submicrometer thin films is investigated by using an optical spectrometer coupled with a hot stage to monitor their thickness-dependent behavior. It is demonstrated that the refractive index has a strong dependence on film thickness, which can be related to the microstructure and morphology of the film as characterized by Fourier transform infrared spectroscopy and scanning electron microscopy.     

Fabrication of a novel organic polymer thin film

Fabrication of organic polymer thin films and organic semiconductors are critical for the development of sophisticated organic thin film based devices. Radio Frequency plasma polymerisation is a well developed and widely used fabrication technique for polymer thin films. This paper describes the fabrication of an organic polymer thin film from a monomer based on Lavandula angustifolia. Several polymer thin films were manufactured with thicknesses ranging from 200 nm to 2400 nm. The energy gap of the polymer thin film was measured to be 2.93 eV. The refractive index and extinction coefficient was determined to be 1.565 (at 500 nm) and 0.01 (at 500 nm) respectively. The organic polymer thin film demonstrates the possibility of an environmentally friendly, cost effective organic semiconductor.     

Grating-coupler assisted infrared reflection absorption spectroscopy for the characterization of organic thin films

We demonstrate how grating-coupler assisted infrared reflection absorption spectroscopy can be used to simultaneously determine the chemical identity and relative thickness of organic thin films. With a grating substrate, a threshold anomaly associated with passing off of the -1 diffracted order occurs at grazing angles of incidence, resulting in a sharp absorbance in the infrared. The position of this peak is sensitive to the grating geometry as well as the dielectric environment near its surface. Thus, shifts in the peak position can be used to determine the relative thickness of adsorbed films or quantify molecular adsorption events. To illustrate the characteristics and sensitivity of this phenomenon, several samples were prepared and tested, including self-assembled alkanethiolate monolayers with 11-mercaptoundecanoic acid, 11-mercapto-1-undecanol, decanethiol, and a covalently linked layer of bovine serum albumin on a commercial, gold-coated grating. For these samples, the position of the threshold absorbance peak shifted to lower wavenumbers as film thickness increased, which is consistent with calculated shifts based upon an increasing refractive index at the interface. The sensitivity of this shift was measured to be 3.7 cm(-1) nm(-1). These results illustrate how a grating substrate can be exploited in a standard infrared reflectance measurement to provide additional information about the relative thickness of adsorbed surface films.     

Monitoring the deposition of an interference film by differential reflection of light

The change in the reflection (differential reflection) of light from an interference film as a result of the deposition of an ultrathin layer on it is investigated. Formulas describing the differential reflection around the reflectivity minima and maxima of the film are obtained by a perturbation method. It is shown that these formulas and the corresponding differential measurements can be used for an easy and unambiguous determination of the thickness and refractive index not only of ultrathin surface layers but also of the interference films themselves. The proposed method is especially convenient for monitoring the deposition of thin-film structures. Pis’ma Zh. Tekh. Fiz. 24, 78–86 (October 26, 1998)     

Optical properties of an octadecylphosphonic acid self-assembled monolayer on a silicon wafer

We report a study of a full-coverage octadecylphosphonic acid (OPA or ODPA) self-assembled monolayer (SAM) spin-coated on the native oxide layer (SiO₂) of a single crystalline silicon (c-Si) wafer using spectroscopic ellipsometry (SE) and reflectometry (SR). The OPA SAM showed characteristics of being a dielectric film in visible range and becoming absorbing in deep-UV range. By assuming an optical stack model of OPA/SiO₂/c-Si for the OPA monolayer system and adopting the parameterized Tauc-Lorentz dispersion model, we obtained an excellent fit of the model to the SE and SR data, from which dispersion of optical functions as well as thickness of the OPA film were deduced. The OPA film thickness measured by atomic force microscopy (AFM) on partial coverage OPA samples was used as the initial trial film thickness in the fitting processes. The deduced OPA film thickness from SE and SR data fitting was in good agreement with that obtained by AFM.     

Use of artificial neural networks to predict thickness and optical constants of thin films from reflectance data

Artificial neural networks and the Levenberg–Marquardt algorithm are combined to calculate the thickness and refractive index of thin films from spectroscopic reflectometry data. Two examples will be discussed, the first is a measurement of thickness and index of transparent films on silicon, and the second is a measurement of three thicknesses and index of poly-silicon in a rough poly-silicon on oxide stack. A neural network is a set of simple, highly interconnected processing elements imitating the activity of the brain, which are capable of learning information presented to them. Reflectometry has been used by the semiconductor industry to measure thin film thickness for decades. Modeling the optical constants of a film in the visible region with a Cauchy dispersion model allows the determination of both thickness and refractive index of most transparent thin films from reflectance data. The use of an alloy interpolation model for the optical constants of poly-silicon allows the determination of thicknesses and poly optical constants. In this work artificial neural networks are used to obtain good initial estimates for thickness and dispersion model parameters, these estimates are then used as the starting point for the Levenberg–Marquardt algorithm which converges to the final solution in a few iterations. These measurement programs were implemented on a Nanometrics NanoSpec 8000XSE.     

Optical and microstructural characterization of sol/gel derived cerium-doped PZT thin films

Optical properties of cerium-doped PZT thin films on sapphire prepared by a sol-gel technique are investigated using both transmission and reflection spectra in the wavelength range 200 to 900 nm. The refractive index, extinction coefficient and thickness of the film are determined from the measured transmission spectra. The packing density of the film is calculated from its refractive index using the effective medium approximation (EMA), and average oscillator strength and wavelength are estimated using a Sellmeir-type dispersion equation. Absorption coefficient (α) and the band gap energy (E_g) of each film composition are also calculated. Possible correlations of microstructure and phase formation behaviour with changes in band gap energy and other optical properties are discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Determination of properties of wedged, nonuniformly thick, and absorbing thin films by using a new numerical method

Nonuniformity in the thickness of thin films can severely distort their transmission spectra as compared with those of flat, smooth films. Methods that extract properties such as refractive index, thickness, and extinction coefficient of such films can suffer inaccuracies when they are applied to wedged or nonuniformly thick films. To accurately extract optical properties of nonuniform films, we have developed a novel numerical method and efficient constitutive relations that can determine film properties from just the transmission spectrum for films that are locally smooth with negligible scattering loss. This optimum parameter extraction (OPE) method can accommodate films with two-dimensional thickness variation that would result in significant errors in the values of refractive index and film thickness if not considered. We show that for carefully chosen test cases and for actual pulsed-laser-deposition AlN thin films, properties such as refractive index, extinction coefficient, and film thickness were very accurately determined by using our OPE method. These results are compared with previous techniques to determine the properties of thin films, and the accuracy of and applicable conditions for all these methods are discussed.     

Optical anisotropy,molecular orientations,and internal stresses in thin sulfonated poly(ether ether ketone) films

The thickness, the refractive index, and the optical anisotropy of thin sulfonated poly(ether ether ketone) films, prepared by spin-coating or solvent deposition, have been investigated with spectroscopic ellipsometry. For not too high polymer concentrations (≤5 wt%) and not too low spin speeds (≥2000 rpm), the thicknesses of the films agree well with the scaling predicted by the model of Meyerhofer, when methanol or ethanol are used as solvent. The films exhibit uniaxial optical anisotropy with a higher in-plane refractive index, indicating a preferred orientation of the polymer chains in this in-plane direction. The radial shear forces that occur during the spin-coating process do not affect the refractive index and the extent of anisotropy. The anisotropy is due to internal stresses within the thin confined polymer film that are associated with the preferred orientations of the polymer chains. The internal stresses are reduced in the presence of a plasticizer, such as water or an organic solvent, and increase to their original value upon removal of such a plasticizer.     

Nanoparticle processing for optical applications – A review

This article provides an overview of current research on nanoparticle processing for optical applications. We elaborate on four nanoparticle processing methods: (i) an aerosol spray method, (ii) nanoparticle dispersion, (iii) a nanoparticle coating method for making films, and (iv) an electrospinning method for making fibers from a nanoparticle dispersion. The use of aerosol spray methods for the preparation of nanoparticles and nanostructured particles for application in optical materials is reviewed. Nanoparticle dispersion techniques for the synthesis of unique composite organic/inorganic materials with unique optical properties are discussed. Preparation of self-assembled monolayer particle films, layer-by-layer films, and fibers consisting of nanoparticles was also introduced. We also highlight the range of unique optical properties associated with optical materials that result from nanoparticle processing, such as a controllable refractive index, transparency, photoluminescence, photonic crystal, and plasmon resonance.     

The properties of NMOB/cadmium arachidate alternated multilayers

100 bilayers 2-nitro-5-(N-methyl-N-octadecyl)aminobenzoic acid (NMOB) and cadmium arachidate (CdA) alternated Langmuir–Blodgett (LB) film was fabricated. The thickness of NMOB monolayer is 2.85 nm measured by small angle X-ray diffraction. It shows that NMOB molecule stands vertically in the alternated film. A DFT method, B3LYP, was used to calculate the energies of various conformations of NMOB with 4-31G* basis set. The computational results show that the tilted conformation is the most stable and the energy difference between this conformation and the vertical one is 31 kJ/mol. The refractive indices of NMOB/CdA alternated films were also determined at different wavelengths by waveguide measurement.     

Transmission interference spectrometric determination of the thickness and refractive index of barrier films formed anodically on aluminum

Aluminum oxide films were formed anodically under carefully controlled conditions and prepared for transmission by dissolving part of the base metal in a solution containing iodine.A spectrophotometric technique to determine the refractive index of the anodic oxide is presented. This technique, based on the transmission interference spectra of two films with slightly different thicknesses, yielded precise results and enabled problems derived from the relatively low refractive index of alumina to be overcome. The film thickness was also calculated from the data, and the results indicated that the refractive index did not depend on the film thickness, at least for the range under study.The anodic alumina showed normal dispersion over the UV, visible and near- IR regions and its refractive index varied from 1.88 at 200 nm to 1.65 at 1300 nm.     

Electrical and optical properties of InSbSe<Subscript>3</Subscript> amorphous thin films

Electrical conductivity, I–V characteristics and optical properties are investigated for InSbSe₃ amorphous thin films of different thicknesses prepared by thermal evaporation at room temperature. The composition of both the synthesized material and thin films were checked by energy dispersive X-ray spectroscopy (EDX). X-ray analysis indicated that all samples under investigation have amorphous structure. The dc electrical conductivity was measured in the temperature range (303–393 K) and thickness range (149–691 nm). The activation energy ΔE _σ was found to be independent of film thickness in the investigated range. The obtained I–V characteristic curves for the investigated samples are typical for memory switches. The switching voltage increases linearly with film thickness in the range (113–750 nm), while it decreases exponentially with temperature in the range (303–393 K). The switching process can be explained according to an electrothermal process initiated by Joule-heating of the current channel. Measurements of transmittance and reflectance in the spectral range (400–2,500 nm) are used to calculate optical constants (refractive index n and absorption index k). Both n and k are practically independent of film thickness in the investigated range (149–691 nm). By analysis of the refractive index n the high frequency dielectric constant ε_∞ was determined via two procedures and was found to have the values of 9.3 and 9.15. Beyond the absorption edge, the absorption is due to allowed indirect transitions with energy gap of 1.46 eV independent on film thickness in the investigated range.     

Effects of composition and thickness on the electrical properties of indium oxide/tin oxide multilayered films

Indium oxide/tin oxide multilayered films with a 2 nm pair thickness were deposited on glass substrates at temperatures lower than 100° C by an ion-beam sputtering method. The structure, electrical properties and visible transmissivity were investigated as a function of composition and the total thickness on as-deposited and annealed films. X-ray diffraction analysis showed that the as-deposited 200 nm thick film (with 0.15 nm tin oxide layers) was partially crystalline and films thinner than 100 nm were amorphous or microcrystalline. The roomtemperature resistivity of as-prepared films increased with the increase of an average tin oxide layer thickness from ∼0.05 to ∼0.3 nm (ideal monolayer thickness) under a constant total thickness ∼100 nm. We observed a decrease of the Hall mobility with the increase of the total film thickness from 10 to 200 nm in as-deposited samples containing 0.15 nm tin oxide layers.     

Thickness dependence of structural and optical properties of indium tin oxide nanofiber thin films prepared by electron beam evaporation onto quartz substrates

Indium tin oxide (ITO) thin films, produced by electron beam evaporation technique onto quartz substrates maintained at room temperature, are grown as nanofibers. The dependence of structural and optical properties of ITO thin films on the film thickness (99-662 nm) has been reported. The crystal structure and morphology of the films are investigated by X-ray diffraction and scanning electron microscope techniques, respectively. The particle size is found to increase with increasing film thickness without changing the preferred orientation along (2 2 2) direction. The optical properties of the films are investigated in terms of the measurements of the transmittance and reflectance determined at the normal incidence of the light in the wavelength range (250-2500 nm). The absorption coefficient and refractive index are calculated and the related optical parameters are evaluated. The optical band gap is found to decrease with the increase of the film thickness, whereas the refractive index is found to increase. The optical dielectric constant and the ratio of the free carrier concentration to its effective mass are estimated for the films.