Transparent thin-film characterization by using differential optical sectioning interference microscopy

We propose an optical thin-film characterization technique, differential optical sectioning interference microscopy (DOSIM), for simultaneously measuring the refractive indices and thicknesses of transparent thin films with submicrometer lateral resolution. DOSIM obtains the depth and optical phase information of a thin film by using a dual-scan concept in differential optical sectioning microscopy combined with the Fabry-Perot interferometric effect and allows the solution of refractive index and thickness without the 2pi phase-wrapping ambiguity. Because DOSIM uses a microscope objective as the probe, its lateral resolution achieves the diffraction limit. As a demonstration, we measure the refractive indices and thicknesses of SiO2 thin films grown on Si substrate and indium-tin-oxide thin films grown on a glass substrate. We also compare the measurement results of DOSIM with those of a conventional ellipsometer and an atomic force microscope.     

General approach to reliable characterization of thin metal films

Optical constants of thin metal films are strongly dependent on deposition conditions, growth mode, and thickness. We propose a universal characterization approach that allows reliable determination of thin metal film optical constants as functions of wavelength and thickness. We apply this approach to determination of refractive index dispersion of silver island films embedded between silica layers.     

Estimation of the thickness and the optical parameters of several stacked thin films using optimization

The reverse engineering problem addressed in the present research consists of estimating the thicknesses and the optical constants of two thin films deposited on a transparent substrate using only transmittance data through the whole stack. No functional dispersion relation assumptions are made on the complex refractive index. Instead, minimal physical constraints are employed, as in previous works of some of the authors where only one film was considered in the retrieval algorithm. To our knowledge this is the first report on the retrieval of the optical constants and the thickness of multiple film structures using only transmittance data that does not make use of dispersion relations. The same methodology may be used if the available data correspond to normal reflectance. The software used in this work is freely available through the PUMA Project web page (http://www.ime.usp.br/~egbirgin/puma/).     

Development of thickness measurement program for transparent conducting oxide thin films

The gallium doped zinc oxide has been one of the candidates for the transparent conducting oxide thin film electrode. It is not suitable to use a conventional light interference method to measure the thickness of the gallium doped zinc oxide thin film because the refractive index and extinction coefficient of the thin film is unknown during the optimization of the deposition conditions. In this paper, we report on the details of the film thickness program which uses the measured optical and electric properties and relationship between the plasma frequency and the optical constant of the film. The obtained film thickness of the prepared gallium doped zinc oxide thin film using the program was comparable with thicknesses measured by a cross-sectional analysis of the atomic force microscopy and the surface profiler. Moreover, the optical constant of refractive index and extinction coefficient of the film could also be estimated.     

Growth and characterization of indium oxide thin films prepared by spray pyrolysis

Highly transparent and conducting indium oxide thin films are prepared on glass substrates from precursor solution of indium chloride. These films are characterized by X-ray diffraction, scanning electron microscopy and optical transmission. The preferential orientation of these films is found to be sensitive to deposition parameters. A comparative study has been made on the dependence on the thickness of the film on substrate temperatures with aqueous solution and 1:1 C₂H₅OH and H₂O as precursors. Films deposited at optimum conditions have 167 nm thickness and exhibited a resistivity of 2.94 × 10⁻⁴ Ω m along with transmittance better than 82% at 550 nm. The analytical expressions enabling the derivation of the optical constants of these films from their transmission spectrum only have successfully been applied. Finally, the refractive index dispersion is discussed in terms of the single-oscillator Wemple and Didomenico model.     

Simultaneous measurement of optical constants of dispersive material at visible and infrared wavelengths

A systematic method has been proposed to evaluate the complex refractive indices and thicknesses of dispersive polymer thin films in both visible and infrared regions. The curve-fitting method has been applied to the measurement of the film thicknesses and complex refractive indices in the visible and near-infrared regions. The accuracy of the evaluated film thickness is of the order of +/-0.5%. The presented method is useful for characterizing the optical properties of polymers whose refractive indices are near to those of substrates.     

Variable refractive index optical coatings

The discrete and restricted values of refractive index of the bulk optical materials at present available are a serious limitation on the usefulness of these materials for optical coatings. This limitation can be overcome by utilizing the atom-by-atom condensation feature of the growth of vapour-deposited thin films, which allows the homogeneous mixing of different materials irrespective of their solubility restrictions. We have used this feature of co-deposition of different materials to form mixed optical materials of variable refractive index, the variation being determined by the composition of the source material. Measurements of the optical constants of these films, prepared by co-evaporation of mixtures of ZnS and MgF₂ of various compositions, have been made. The refractive index of the mixed films is found to be in good agreement with the values predicted on the basis of the Lorentz-Lorentz theory. In addition, the optical equivalence of alternate layers of ZnS and MgF₂ with step thicknesses ranging from 5 to 250 Å has been studied. For step thicknesses less than 100 Å, the optical properties of the composite films are equivalent to those for the homogeneously mixed films. For larger step thicknesses, considerable and complicated interference effects are observed. Thus, variable refractive index composite films can be prepared by (a) co-deposition and (b) deposition of alternate discrete layers of different materials as long as the step thickness does not exceed about 100 Å. Furthermore, these techniques of obtaining optical films of mixed materials can be extended to any combination of materials and therefore they open up a new field in materials technology.     

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.     

The substrate effect on Ge doped GaN thin films coated by thermionic vacuum arc

This study focuses on characterization and understanding of the substrate effect on Ge doped GaN thin films coated onto transparent substrates. The produced films were deposited onto unheated glass and unheated polyethylene terephthalate by using thermionic vacuum arc technique. Gallium nitride and germanium pellets were used in the thin film production. Reflectance, refractive index and thicknesses of Ge doped GaN thin films were measured by optical interferometer using Cauchy model for fitting. The transmittances were determined in the wavelength range between 200 and 1000 nm by using UV–Vis double beam spectrophotometer. The optical Tauc method was used to determine the band gap energies of produced thin films. Surface morphologies of produced thin films were characterized by atomic force microscopy and also field emission scanning electron microscopy. In conclusion, the substrate effect on the optical and morphological properties of the produced thin films was observed.     

Influence of substrate absorption on the optical and geometrical characterization of thin dielectric films

The role played by a glass substrate on the accurate determination of the optical constants and the thickness of a thin dielectric film deposited on it, when well-known envelope methods are used, is discussed. Analytical expressions for the two envelopes of the optical transmission spectra corresponding to film. with both uniform and nonuniform thicknesses are derived, assuming the substrate to be a weakly absorbing layer. It is shown that accurate determination of the refractive index and the film thickness is notably improved when the absorption of the substrate is considered. The analytical expressions for the upper and lower envelope, are used to characterize optically and geometrically both uniform and nonuniform amorphous chalcogenide films. The results obtained are compared with those derived by use of expressions for the envelopes that neglect the substrate absorption. The comparison shows that overestimated refractive indexes and underestimated thicknesses are obtained when the conventional approach, in which the substrate absorption is neglected, is used.     

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.     

Structural and optical properties of Ag2SeTe nano thin films prepared by thermal evaporation

Semiconducting Ag₂SeTe thin films were prepared with different thicknesses onto glass substrates at room temperature using thermal evaporation technique. The structural properties were determined as a function of thickness by XRD exhibiting no preferential orientation along any plane, however the films are found to have peaks corresponding to mixed phase. The XRD studies were used to calculate the crystallite size and microstrain of the Ag₂SeTe films. The calculated microstructure parameters reveal that the crystallite size increases and micro strain decreases with increasing film thickness. The refractive index, dielectric constants and thereby the optical bandgap of the films were calculated from transmittance spectral data recorded in the range 400?C1200 nm by UV?CVIS-Spectrometer. The direct optical bandgap of the Ag₂SeTe thin films deposited on glass substrates with different thicknesses 50?C230 nm were found to be in the range 1.48?C1.59 eV. The carrier density value is estimated to be around 9.8 × 10²¹ cm^?1 for the film thickness of 150 nm. The compositions estimated from the optical band gap studies reveal a value of 0.75 for Tellurium concentration. These structural and optical parameters are found to be very sensitive to the thin film thickness.     

Diagnostic in TCOs CVD processes by IR pyrometry

Infra red pyrometry is a sensitive, simple and low-cost technique commonly used for the measurement of the deposition temperature in CVD processes. We demonstrate in this work that this optical technique can be used as diagnostic tool to provide fruitful informations during the growth under atmospheric pressure of TiO₂ films on various substrates chosen as an example of transparent oxide. Significant variations of the pyrometric signal were observed during the deposition of TiO₂ thin films due to interferences in the growing film resulting from multi-reflections at the interfaces and scattering induced by the surface roughness. Modeling of the time dependence of the IR pyrometric signal allows simultaneously the determination of the layer thickness, the growth rate, surface roughness and refractive index of the thin films under the growth conditions. This diagnostic technique can be used for various transparent thin films grown on opaque substrates and is well adapted to control CVD processes operating either under atmospheric or low pressure and more generally any thermal treatment processes.     

Optical and dielectric properties of double helix DNA thin films

In this work, the thin film of wheat DNA was deposited by spin-coating technique onto glass substrate, and the optical and dielectric properties of the double helix DNA thin film were investigated. The optical constants such as refractive index, extinction coefficient, dielectric constant, dissipation factor, relaxation time, and optical conductivity were determined from the measured transmittance spectra in the wavelength range 190–1100 nm. Meanwhile, the dispersion behavior of the refractive index was studied in terms of the single oscillator Wemple–DiDomenico (W–D) model, and the physical parameters of the average oscillator strength, average oscillator wavelength, average oscillator energy, the refractive index dispersion parameter and the dispersion energy were achieved. Furthermore, the optical band gap values were calculated by W–D model and Tauc model, respectively, and the values obtained from W–D model are in agreement with those determined from the Tauc model. The analysis of the optical absorption data indicates that the optical band gap E_g was indirect transitions. These results provide some useful references for the potential application of the DNA thin films in fiber optic, solar cell and optoelectronic devices.     

A method for the determination of the complex refractive index of non-metallic thin films using photometric measurements at normal incidence

A method is proposed for the determination of the complex refractive index of non-metallic thin films using photometric measurements at normal incidence over an extended wavelength interval. A necessary condition for the applicability of the method is the existence of maxima and minima in the reflectance due to interference effects. The problem of multiple solutions is analysed and the optical thicknesses at the extrema are used for choosing the correct solutions. For absorbing films two alternative procedures are described. One procedure requires an approximate value of the film thickness to start with and refines it during the calculation. The other procedure does not depend on a previous knowledge of the thickness but gives it as a result together with n(λ) and k(λ). A separate procedure is proposed for transparent and almost transparent films. An application of the method is carried out in which excellent agreement is obtained between calculated and experimental results.     

Determination of the optical constants (n and k) of inhomogeneous thin films with linear index profiles

A new method for the determination of optical constants of absorbing inhomogeneous thin films is proposed. It requires measurements at normal incidence of the reflectance and transmittance of the film. In an inhomogeneous thin film, the optical constants vary along the thickness of the film. It has been reported in the literature that only the spatial integral value of the absorption index needs to be considered if its value is small. Therefore, in the proposed method, the mean value of the absorption index was used. The validity of this assumption was tested. On the other hand, the variation in the refractive index along the thickness of the film was taken into account. The method is discussed along with the nature of the solutions obtained and the effects of various parameters and assumptions. The method is applied successfully to inhomogeneous thin films of zirconium oxide.     

Optical-constant calculation of non-uniform thickness thin films of the Ge10As15Se75 chalcogenide glassy alloy in the sub-band-gap region (0.1–1.8 eV)

Optical-transmission spectra are very sensitive to inhomogeneities in thin films. In particular, a non-uniform thickness produces a clear shrinking in the transmission spectrum at normal incidence. If this deformation is not taken into account, it may lead to serious errors in the calculated values of the refractive index and film thickness. In this paper, a method first applied by Swanepoel for enabling the transformation of an optical-transmission spectrum of a thin film of wedge-shaped thickness into the spectrum of a uniform film, whose thickness is equal to the average thickness of the non-uniform layer, has been employed. This leads subsequently to the accurate derivation of the refractive index in the subgap region (0.1–1.8 eV), the average thickness, as well as a parameter indicating the degree of film-thickness uniformity. This optical procedure is applied to the particular case of freshly-prepared films of the Ge₁₀As₁₅Se₇₅ ternary chalcogenide glassy alloy. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-oscillator model. The optical-absorption edge is described using the ‘non-direct transition' model proposed by Tauc, and the optical energy gap is calculated by Tauc's extrapolation. Finally, the photo-induced and thermally induced changes in the optical properties of the a-Ge₁₀As₁₅Se₇₅ layers are also studied.     

光反射干涉谱新方法测试薄膜厚度和光学常数

依据测量薄膜和光之间相互作用可确定薄膜特性的原理,并基于光反射干涉谱与德国最新研发薄膜分析软件SCOUT的新方法可测量已知或未知材料的多层薄膜厚度及其折射率n、消光系数k。通过实际测试证明：该方法可测试单晶硅、玻璃、ITO玻璃基底上沉积薄膜的厚度,样品基本不需要特别准备,对样品无破坏性,测试精准。理论上可以测量所有透光或半透光薄膜的厚度和光学常数,操作非常简便,适合于镀膜行业的在线检测和实时监控,且SCOUT软件在多层膜及多种材料的研发、制备等方面具有应用潜力。     

Extraction of optical constants of zinc oxide thin films by ellipsometry with various models

Spectroscopic ellipsometry was used to extract the optical constants of zinc oxide (ZnO) thin films deposited on (100) silicon substrate by filtered cathodic vacuum arc technique. Three dispersion models, namely, Sellmeier dispersion model, Cauchy model and Forouhi–Bloomer model, were evaluated for determining the optical constants of ZnO thin films below the energy band gap. The study shows that the Cauchy model provides the best spectral fittings among these three models. Above the energy band gap, two ellipsometric models, namely, two-phase model and three-phase point-by-point fit, were used. This study reveals that the initial values used in the point-by-point fitting play a critical role. It also shows that the refractive index and the extinction coefficient calculated with the two-phase model can be used as the initial values for the point-by-point fitting. The spectral dependence of the refractive index and extinction coefficient obtained in this work is comparable with the data reported in the literature. In sum, a reliable methodology for determining the optical constants of ZnO thin films in the ultraviolet-visible-near infrared range (2501100 nm) has been developed.     

Simultaneous determination of thickness and optical constants of polymer thin film by analyzing transmittance

We present a novel method for determining both the thickness and the optical constants of a weakly absorbing thin film upon a nearly transparent substrate through analysis of transmittance measured at various incident angles with coherent light. We demonstrate this method for a polymer thin film. The refractive indices and extinction coefficients of poly(DRI-anthranilic acid) at wavelengths of 1,064, 632.8, and 532 nm were determined for the first time to our knowledge. We also confirmed the validity of our method with a polystyrene thin film whose optical constant was known. It was found that a thickness of a few hundred nanometers can easily be measured and that this method offers simplicity as well as the capability of in situ measurement.