Optical Constants Of Films Research Articles

Characterization of TFT-LCD and OLEDs Devices by Phase Modulated Spectroscopic Ellipsometry for Display Applications

ABSTRACTSpectroscopic ellipsometry is one of the most accurate and reliable optical technique to characterize polymers, liquid crystals (LCs) and organic light emitting diodes (OLEDs). Because these devices are formed by complex structure including optical anisotropy, absorbing and graded materials, the correct use of spectroscopic ellipsometer required a combination of the proper choice of hardware and the appropriate ellipsometric model. In this work, we presents ellipsometric results obtained by a commercially available phase modulated spectroscopic ellipsometer (PMSE) on a full TFT-LCD structure characterized from UV to NIR. As expected, strong anisotropy and inhomogeneous optical properties were found respectively on LCs and ITO materials. We also presents measurements of film thickness and optical constants of each layer constituing an OLED structure

Dynamic In Situ Spectroscopic Ellipsometry of the Reaction of Aqueous Iron(II) with 2,2‘-Bipyridine in a Thin Nafion Film

Dynamic in situ spectroscopic ellipsometry studies of the chemical reaction between ferrous ion and 2,2'-bipyridine (bpy) in a thin Nafion film are presented. A simple prototype system composed of a thin Nafion film on a glass substrate was used throughout the work. The reaction was detected by optically monitoring the formation of the strongly absorbing complex ion, Fe(bpy)3(2+) (epsilon 520 = 7.70 x 10(3) M(-1) cm(-1) in 0.1 M NaCl). The changes in film optical constants, n and k, and the thickness upon exposure of it to various solutions were monitored in a flow cell with the film on the backside of the substrate relative to the interrogation by light. A "step-by-step" approach was used to isolate the component parts of the system in which the film was consecutively exposed to solutions in the following order: supporting electrolyte, bpy, and, last, ferrous iron solution. The optical properties of the materials were quantitatively described before and during mass transport within the film by modeling using the appropriate multilayer optical models, i.e., the Cauchy equation for nonabsorbing media and the Urbach and Tauc-Lorentz (oscillator) functions for a film that absorbed. The experiments done allowed study of the diffusion in the film and the chemical reactions that are important in the sensing scheme for ferrous iron. Ligand (bpy) diffusion followed a two-stage diffusion mechanism described by a Berens-Hopfenberg model for incremental sorption (D25 = 7.04 x 10(-13) cm2 s(-1)). The stabilities of the appropriate systems, i.e., Nafion film with bpy, iron, and iron complex, were studied by exposing equilibrated films to circulating supporting electrolyte solutions. The measurements gave important insights into a set of film chemical reactions and, in turn, selective film dynamics. This work exemplifies the usefulness of spectroscopic ellipsometry in monitoring the kinetics of a chemical reaction in situ, as well as the changes in the film physical properties under dynamic conditions.

A computer program for determination of thin films thickness and optical constants

A computer simulation program for processing transmission spectra of amorphous optical thin films deposited on weakly absorbing substrates and evaluation of the refractive index n, extinction coefficient k and thickness d was developed. The computer code is the implementation of an optical characterisation algorithm based on the determination of the upper and lower envelopes of the transmission spectrum interference fringes. Inhomogeneities in the thickness of the analysed films, which are responsible of a shrinking in the fringes amplitude, can be considered in the program. Relative errors in the calculated values of n, k and d have been determined using simulated transmission spectra in both cases of homogeneous and inhomogeneous films. The thickness and the refractive index of uniform films are calculated with an accuracy ≤0.5%, while the accuracy in the case including inhomogeneities is ≤2%. Simulation results for chalcogenide thin films deposited by pulsed laser deposition (PLD) on microscope slabs and glass slides are reported.

Percolation threshold in ultrathin titanium films determined byin situspectroscopic ellipsometry

We report on the use of in situ spectroscopic ellipsometry to determine the film thickness, optical constants, and growth rate for titanium films grown on silicon oxide by pulsed filtered cathodic vacuum arc. The growth rate and film thickness are shown to be highly controllable. We also report the results of simultaneous in situ spectroscopic ellipsometry and in situ conductivity measurements to independently verify the ability of both techniques to determine the percolation threshold. Percolation is shown to occur at a film thickness in the range 2.7 to $3.1(+∕\ensuremath{-}0.1)\mathrm{nm}$. The ability of spectroscopic ellipsometry to accurately determine the void fraction in a thin film enables us to apply the percolation formalism based on a critical exponent to this system.

Gold-alumina cermet photothermal films

Cermet films of gold and aluminum oxide were sputter deposited onto silicon substrates. These films consisted of nanometer-sized gold grains embedded in a matrix of aluminum oxide and were designed to absorb visible radiation and minimally emit infrared radiation. Ex situ and in situ spectroscopic ellipsometry were used to characterize film optical constants from deep ultraviolet to middle infrared. The optical constants, for cermet films with gold volume fractions ranging from 0 to 1, were used in predictive optical models. Thermal performance was optimized using a new algorithm based on energy balances. This algorithm weights the parameter fit in predictive optical models to an ideal reflectance spectrum. This weighting factor emphasizes fitting in more significant spectral ranges and de-emphasizes fitting in less significant wavelength ranges. Absorptivity, emissivity, and their ratio were calculated for optimized film structures.

Optical modeling of iridium thin film erosion under oxygen plasma exposure

This work is motivated by interest in use of iridium metal films in the space environment. Optical metrology is a sensitive tool for study of film roughness, optical constants, and microstructural properties. In the present paper, in situ spectroscopic ellipsometry is applied to monitor oxygen plasma etching of films of iridium∕chromium on fused silica substrates, using an electron cyclotron resonance plasma source. Optical modeling allows for a fundamental understanding of the plasma etching mechanism. Analysis of spectroscopic psi and delta data at selected time slices suggests the iridium film is etched away gradually upon plasma irradiation, while showing no signs of oxidation or other chemical changes. Nanoscale atomic mixing and index of refraction grading are observed.

On the feasibility of colored glazed thermal solar collectors based on thin film interference filters

Glazed thermal solar collectors, typically equipped with black, optical selective absorber sheets, exhibit good energy conversion efficiency. However, the black color, and sometimes the visibility of tubes and corrugations of the metal sheets, limit the architectural integration into buildings. In order to overcome this drawback, interference filters are considered as a promising approach. Multilayered thin film stacks deposited on the cover glass can produce a colored reflection hiding the black absorber without a great loss of energy. These interference filters are designed and optimized by numerical simulation. Such coatings are deposited by vacuum processes (e.g. magnetron sputtering) and also via the SolGel method. Optical measurements, such as real-time laser-reflectometry and spectrophotometry, are suitable to determine film thicknesses and optical constants of individual layers, and to measure color coordinates and solar transmittance for the multilayer stacks. Advantages and disadvantages of the different coating processes are discussed.

Film structure and optical constants of magnetron-sputtered fluoride films for deep ultraviolet lithography

Stoichiometric fluoride films have been fabricated by magnetron sputtering of fluoride targets with SF 6 at around room temperature. Refractive index ( n ) and extinction coefficient ( k ) of the optimal AlF 3 film are 1.37 and 6×10 −4 at 193 nm, and in case of the optimal LaF 3 film, n and k are 1.75 and 9×10 −4 at 193 nm. The two-step process, which consists of target pre-fluorination and film deposition, enables LaF 3 films to grow with dense and bulky structure. An anti-reflective coating at 193 nm was fabricated, and its reflectance and transmittance are 0.03% and 99.2%, respectively.

Film structure and optical constants of magnetron-sputtered fluoride films for deep ultraviolet lithography

Stoichiometric fluoride films have been fabricated by magnetron sputtering of fluoride targets with SF 6 at around room temperature. Refractive index ( n) and extinction coefficient ( k) of the optimal AlF 3 film are 1.37 and 6×10 −4 at 193 nm, and in case of the optimal LaF 3 film, n and k are 1.75 and 9×10 −4 at 193 nm. The two-step process, which consists of target pre-fluorination and film deposition, enables LaF 3 films to grow with dense and bulky structure. An anti-reflective coating at 193 nm was fabricated, and its reflectance and transmittance are 0.03% and 99.2%, respectively.

Optical properties of aluminium oxide thin films prepared at room temperature by off-plane filtered cathodic vacuum arc system

An off-plane filtered cathodic vacuum arc (FCVA) system has been developed to deposit large area (up to 8 inch) optical coatings with good uniformity controlled by scanning magnetic field system. With this FCVA system, aluminium oxide thin films were deposited with high deposition rate (1.5-0.5 nm/s) on Si (100) and quartz substrates at room temperature under different oxygen pressures (1×10 −2–3.73×10 −2 Pa). The evolution of Al 2p core level and oxygen concentration in the films was studied by XPS spectra. In addition to the good uniformity (±2%) in terms of thickness and smooth surface (RMS: approx. 0.1 nm), the as-grown films were amorphous with low stress (less than 0.5 GPa). The variation of deposition rate with oxygen pressure was investigated. Optical properties, such as film transmittance and optical constants of aluminium oxide thin films formed under different oxygen pressures were determined. Film properties demonstrate the potential applications for aluminium oxide thin films in optical coatings and for FCVA technology in deposition of metal oxide optical coatings.

Optical properties of aluminium oxide thin films prepared at room temperature by off-plane filtered cathodic vacuum arc system

An off-plane filtered cathodic vacuum arc (FCVA) system has been developed to deposit large area (up to 8 inch) optical coatings with good uniformity controlled by scanning magnetic field system. With this FCVA system, aluminium oxide thin films were deposited with high deposition rate (1.5-0.5 nm/s) on Si (100) and quartz substrates at room temperature under different oxygen pressures (1×10−2–3.73×10−2 Pa). The evolution of Al 2p core level and oxygen concentration in the films was studied by XPS spectra. In addition to the good uniformity (±2%) in terms of thickness and smooth surface (RMS: approx. 0.1 nm), the as-grown films were amorphous with low stress (less than 0.5 GPa). The variation of deposition rate with oxygen pressure was investigated. Optical properties, such as film transmittance and optical constants of aluminium oxide thin films formed under different oxygen pressures were determined. Film properties demonstrate the potential applications for aluminium oxide thin films in optical coatings and for FCVA technology in deposition of metal oxide optical coatings.

Methods for the determination of the optical constants of thin films from single transmission measurements: a critical review

Optical transmission measurements are commonly used for the routine determination of thin film optical constants. This paper presents an overview of the different methods of evaluating these transmission data, leading to values for the complex refractive index. Three different groups of methods are distinguished using: (1) at least two different optical measurements; (2) dispersion relations or general physical constraints to approximate the behaviour of the wavelength-dependent refractive index; and (3) a `virtual' measurement as a second variable. The methods from groups (2) and (3) (requiring only a single transmission measurement) are treated in more detail and are evaluated in terms of their accuracy.

Optical characteristics of very thin films

Very thin films with physical thickness of 5–150 nm are of theoretical and practical interest. We derive general expressions for the transmittance and reflectance by development of the Abelès characteristic matrix elements to second order in ( d/ λ), where d is the thickness and λ is the wavelength. Then the film optical constants are evaluated from simple algebraic set of equations. A computer simulation of spectrophotometric experiment in VIS-NIR and evaluation of the complex refractive index are presented for films with thickness < λ/30 and dispersion of the complex refractive index. As an illustration, optical constants are analyzed for a 15 nm vacuum deposited As 2S 3 film.

Optical constants and roughness study of dc magnetron sputtered iridium films

Extremely smooth thin films of iridium have been deposited onto superpolished fused silica substrates using dc magnetron sputtering in an argon plasma. The influence of deposition process parameters on film microroughness has been investigated. In addition, film optical constants have been determined using variable angle spectroscopic ellipsometry, over the spectral range from vacuum ultraviolet to middle infrared (140 nm–35 μm). Because the Ir films were optically thick and the surface roughnesses were measured by atomic force microscopy then accounted for in the optical model, the as-determined film optical constants are expected to be the best available for Ir bulk metals, minimally affected by surface overlayers or microstructure.

Computations of the optical properties of metal/insulator-composites for solar selective absorbers

Solar selective absorbers are very useful for photo thermal energy conversion. The absorbers normally consist of thin films (mostly composite), sandwiched between the antireflection layer and (base layer on) a metallic substrate, selectively absorbing in the solar spectrum and reflecting in the thermal spectrum. The optical performance of the absorbers depends on the thin film design, thickness, surface roughness and optical constants of the constituents. The reflectivity of the underlying metal and porosity of the antireflection coating plays important roles in the selectivity behavior of the coatings. Computer simulations, applying effective medium theories, have been used to investigate the simplest possible design for composite solar selective coatings. A very high solar absorption is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the air interface. The destructive interference created in the visible spectrum has increased the solar absorption to 98%. This paper also addresses the optical performance of several metals/dielectric composites like Sm, Ru, Tm, Ti, Re, W, V, Tb, Er in alumina or quartz on the basis of their refractive indices. The antireflection coating porosity and surface roughness has been analyzed to achieve maximum solar absorption without increasing the thermal emittance. Antireflection layer porosity is a function of dielectric refractive index and has nominal effect on the performance of the coating. While, up to the roughness of 1×10 −7 m RMS, the solar absorption increases and for higher roughness, the thermal emittance increases only.

Oxygen plasma effects on optical properties of ZnSe films

Zinc selenide is an infrared transparent semiconductor material being considered for use in space as an infrared optical coating. In this work, zinc selenide thin films of different thicknesses were exposed to an electron cyclotron resonance generated oxygen plasma, often used to “simulate” the low earth orbital environment. The maximum fluence used in our experiments was equivalent to ∼16 years in the low earth orbital environment. ZnSe thin film optical constants (both before and after oxygen plasma exposure) were determined using variable angle spectroscopic ellipsometry from the vacuum ultraviolet at 146 nm through the middle infrared to 40 μm. A parametric dispersion model (Herzinger–Johs) was successfully used to fit the optical data over the entire range from ultraviolet to infrared. Comparing the pre- and post-oxygen plasma exposure data, few changes were observed in the middle infrared region, while drastic changes were seen in the vacuum ultraviolet through visible to near infrared (0.73–8.5 eV). This suggests that chemical changes upon plasma exposure, including oxidation, are found mainly in a thin layer near the surface. As the proposed application is for infrared coatings, and few infrared changes were seen under conditions roughly equivalent to 16 years in low earth orbit, ZnSe may indeed be useful for space infrared applications. Performance simulations of ZnSe coated infrared-operating electrochromic thermal-control surfaces confirm this conclusion.

FTIR Ellipsometry as a Tool for Studying Organic Layers: From Langmuir-Blodgett Films to Can Coatings

The capabilities of infrared ellipsometry to examine structurally various thin organic layers deposited on different substrates are demonstrated in this contribution. The presented examples involve: Langmuir-Blodgett film on gold substrate, anisotropic film of polystyrene on a silicon wafer, implanted fluorinated surface layer into a bulk polyethylene fuel tank and can coating deposited on electrolytically-plated steel (tinplate). Different calculational procedures were used to determine the film optical constants and thickness.

In situ optical characterization during MPACVD diamond filmgrowth on silicon substrates using a bichromatic infrared pyrometer underoblique incidence

In this paper, we carry out in situ optical characterization ofdiamond films on silicon substrates using an infrared bichromaticpyrometer under oblique incidence. We develop an optical model taking intoaccount the effective emissivity of the diamond film/silicon substratesystem. This model allows the determination of the diamond and siliconoptical constants, the system real temperature, the incubation period andthe growth rate, from the apparent temperature measured during the diamondgrowth. These parameters are found from the experimental data and fromminimizing the mean square error using the Levenberg-Marquardt algorithm.The model can be extended to other partially transparent deposits onabsorbent substrates at any observation angle and the first-hand values ofthe film optical constants can be found without the knowledge of thesubstrate optical properties.The pyrometric results thus obtained on the diamond films deposited bymicrowave plasma-assisted chemical vapour deposition in a CH4-H2gas mixture are discussed and compared to those deduced from Ramanspectroscopy in order to investigate the diamond purity. As the CH4concentration is augmented, the decrease in the diamond purity thus foundis in agreement with the increase in the film extinction coefficientevaluated from pyrometry. Moreover, the behaviour of the apparenttemperature pseudo-period provides information on the growth process withrespect to the growth rate and the film real index variations.

Optical constants of vacuum-evaporated cadmium sulphide thin films measured by spectroscopic ellipsometry

Optical constants of vacuum-evaporated CdS thin film have been determined by spectroscopic ellipsometry in the photon energy range from 1.2 to 5.5 eV at room temperature. The dielectric constant values were found to be substantially lower than those for the bulk CdS. The optical constants of the film were also determined using the optical transmittance measurements and the results were discussed.

Variable angle spectroscopic ellipsometry of fluorocarbon films from hot filament chemical vapor deposition

Hot filament chemical vapor deposition using hexafluoropropylene oxide as the precursor gas yielded two sets of fluorocarbon films, one with varying OH/COOH content and the other with varying grain aspect ratio, as revealed by Fourier transform infrared spectroscopy and atomic force microscopy, respectively. Variable angle spectroscopic ellipsometry was performed to derive film thickness and film optical constants. A uniaxial Cauchy–Urbach dispersion layer, with separate in-plane and out-of-plane dispersion parameters, was found to realistically describe the films. Derived film thickness agreed well with profilometry measurements. Anisotropy in index of refraction n and extinction coefficient k was on the order of 10−2 and 10−5 to 10−3, respectively. The relationship between the complex index of refraction and the dielectric function allowed the optical dielectric constant ε1 to be calculated. The presence of OH did not affect the film optical dielectric constant significantly. Even though OH/COOH groups are considered to contribute adversely by increasing the dielectric constant, their effect may have been masked by the dominance of F atoms. The optical dielectric constant decreased linearly with increasing grain aspect ratio. The presence of voids was thought to lower the effective index of refraction and optical dielectric constant, and the increase in grain asymmetry created more of these voids due to less efficient packing. By assuming a linear contribution from bulk grains and voids to the effective optical dielectric constant, film porosity was derived. A maximum film porosity of 33%, corresponding to an optical dielectric constant of 1.74, was observed.