Proceedings Volume 1324

Modeling of Optical Thin Films II

Michael Ray Jacobson
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Proceedings Volume 1324

Modeling of Optical Thin Films II

Michael Ray Jacobson
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 1 December 1990
Contents: 6 Sessions, 24 Papers, 0 Presentations
Conference: 34th Annual International Technical Symposium on Optical and Optoelectronic Applied Science and Engineering 1990
Volume Number: 1324

Table of Contents

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Table of Contents

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  • Invited Papers
  • Process Parameter Models
  • Film Growth Models
  • Film Microstructure
  • Film Property Models I
  • Film Property Models II
Invited Papers
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Revisiting structure zone models for thin film growth
Karl H. Guenther
More than 20 years ago Movchan and Demchishin published their structure zone model (SZM) of thin film morphology as a function of a single macroscopic parameter, the normalized substrate temperature. In all its simplicity, this model already reflects many experimental observations of thin film growth with surprising accuracy. Later modifications of the model which included the influence ofresidual gas pressure and electrical bias potential in sputtering extended its validity to this family of deposition processes. Evolutionary features of thin film growth and the fractal nature of thin film structures were also discussed. None of these previous models, however, providedfor a vitreous, fully dense structure as observed with some ion and plasma assisted deposition processes. In 1988, the author proposed an extension of the original Movchan-Demchishin SZM with afourth zone reflecting the vitreousphase. In thispaper, we will discuss the validity ofthis extension and generalize the 4-zone model.
Effects of surface diffusion on thin-film morphology: a computer study
Robert B. Sargent
A two-dimensional hard-disk model of thin-film deposition is described; the model is of the type originally introduced by Henderson et al.1 We have implemented a simple (and necessarily approximate) way to incorporate the effects of surface diffusion in our model, and a means to connect the input parameters of the computer algorithm to the evaporation parameters of substrate temperature and evaporation rate. The effects of surface diffusion and vapor-incidence angle on film structure are explored; the effects of surface diffusion predicted by the model correlate with the transition in film structure (Zone 1 to Zone 2) first described by Movchan and Demchishin.2
Unusual fluorination effects on superconducting films
Stanford R. Ovshinsky, Rosa T. Young
In this paper we extend our previous fluorination work presenting an unusual fluorination effect in growing device quality superconducting film directly on sapphire. The superiority of the film quality is attributed to the fact that fluorine plays a significant role in the control of nucleation and in the enhancement of the growth rate of the superconducting film in the basal plane, therefore, an "epitaxial" film is obtained. Furthermore, the high quality fluorinated film can be grown at a lower temperature. As a result, the grain boundary weak link effect and the interface diffusion between the superconducting film and the substrate are minimized. The superconducting film with a high critical current density and a very smooth surface is achieved. With our technique, we believe that device quality superconducting film could be grown not only on sapphire but on other flexible inexpensive continuous substrates for high field applications which could lead to a major technological advancement.
Process Parameter Models
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Source emission pattern polynomial representation
Ricardo Flores-Hernandez, Francisco De Villa
A method to obtain accurate thickness data to characterize the emission patterns of evaporation sources is described. Thickness data is obtained through digital image processing algorithms applied to the monochromatic transmission bands digitized from a set of multilayer Fabry-Perot filters deposited on large flat circular substrates. These computer image-processed taper-thickness patterns are reduced to orthonormal polynomial series expansions in two steps, using Tschebyshev and associated Legendre polynomials. The circular glass substrates employed to characterize each type of evaporation source are kept stationary during the evaporation process of evaporation of each layer to obtain the specific thickness distribution for each type of source.
Knowledge-based optical coatings design and manufacturing
Karl H. Guenther, Avelino J. Gonzalez, Hoi J. Yoo
The theory of thin film optics is well developed for the spectral analysis of a given optical coating. The inverse synthesis - designing an optical coating for a certain spectral performance - is more complicated. Usually a multitude of theoretical designs is feasible because most design problems are over-determined with the number of layers possible with three variables each (n, k, t). The expertise of a good thin film designer comes in at this point with a mostly intuitive selection of certain designs based on previous experience and current manufacturing capabilities. Manufacturing a designed coating poses yet another subset of multiple solutions, as thin if in deposition technology has evolved over the years with a vast variety of different processes. The abundance of published literature may often be more confusing than helpful to the practicing thin film engineer, even if he has time and opportunity to read it. The choice of the right process is also severely limited by the given manufacturing hardware and cost considerations which may not easily allow for the adaption of a new manufacturing approach, even if it promises to be better technically (it ought to be also cheaper). On the user end of the thin film coating business, the typical optical designer or engineer who needs an optical coating may have limited or no knowledge at all about the theoretical and manufacturing criteria for the optimum selection of what he needs. This can be sensed frequently by overly tight tolerances and requirements for optical performance which sometimes stretch the limits of mother nature. We introduce here a know1edge-based system (KBS) intended to assist expert designers and manufacturers in their task of maximizing results and minimizing errors, trial runs, and unproductive time. It will help the experts to manipulate parameters which are largely determined through heuristic reasoning by employing artificial intelligence techniques. In a later state, the KBS will include a module allowing the layman user of coatings to make the right choice.
Computer-optimized optical monitoring wavelengths
Ricardo Flores-Hernandez
Experimental and computer procedures are described to calculate in advance the optimal monitoring wave lengths for each layer for any all-dielectric multilayer stack on any substrate holder configuration. Thickness ratios [tooling factors] between monitoring and working substrates are obtained experimentally to compute the spectral behavior of the irregular stack that has to be deposited on the monitoring substrate to obtain the desired, correct all-dielectric thin film stack on the working substrates. The computational algorithm takes into account the overall monitoring beam's spectral response. The results obtained are a set of possible turning-point monitoring wave lengths for each layer to be deposited and plots the transmittance or reflectance behavior of these wave lengths as a function of layer thickness growth, to allow the user to select the most convenient among them.
Improved sensitivity in ellipsometry of thin biochemical films by employing sublayers
Jinyu Wang, Joseph D. Andrade, Jinn-Nan Lin, et al.
Ellipsometry is widely used for investigating the optical properties of thin films on planar substrates, including films of adsorbed proteins or polymers. The average thickness and effective refractive index of the adsorbed layer are calculated by measuring the A and 'P ellipsometry parameters. Unfortunately the thickness of the adsorbed protein layers is often too thin to significantly affect the and 'I' parameters. However, using a substructure consisting of an additional sublayer placed between the substrate and the adsorbed layer, we can improve the sensitivities of both and 'P to changes in the adsorbed layer, provided that the thickness of the sublayer is optimized. We show that for a Si02 layer on a Si wafer, the optimum Si02 thickness is about 1350 A when the incident angle is 70 degrees and the wavelength is 6328 A. The materials of the sublayer can be metal, semiconductor and/or dielectric.
Interaction between dispersive and inhomogeneous models for interpreting spectral ellipsometric data of thin films
Charles K. Carniglia, Karl N. Schrader
Spectral ellipsometric data taken on thin-film samples are often used to obtain optical properties of the thin-film materials. A homogeneous model is usually used for the films and a dispersive refractive index is varied to fit the data. Often the data do not fit a homogeneous model. In this case the film is modeled by one or more layers to approximate an inhomogeneous film. A common procedure is to fix the dispersion curve for the film material and to model the indices of the various layers with density or porosity as the adjustable parameter. This paper demonstrates that dispersion and film inhomogeneity can affect ellipsometric data in a similar fashion in certain spectral regions. Thus, the dispersion curve derived using a homogeneous film model may be in error due to inhomogeneities in the film. An example is presented for a silica (SiO2) film on a silicon (Si) substrate. Ellipsometric data are calculated using handbook values for the refractive indices of the materials, but for an inhomogeneous silica film. These data are analyzed using a homogeneous film with a variable Cauchy dispersion equation. The best-fit dispersion curve is found to deviate significantly from the handbook data for silica.
Film Growth Models
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Nucleation and growth of thin films
Brian L. Evans, Shi Xu
A study has been made of the nucleation and growth of thin microcrystalline films of Cu,Au,Pt,Ni prepared by ion-beam sputtering. In-situ measurements of the film resistance during deposition have been analysed in terms of percolation theory for the early, discontinuous, phase and thin film, grain boundary, scattering for the quasi-continuous phase. This analysis yields values of the percolation threshold, fractional coverage and lateral grain dimensions, for different deposition rates, which are compared with the corresponding values obtained from direct observations and soft X-ray multilayers reflection spectra. The minimum thickness of the metallic nuclei is shown to be four atom layers.
Simulation of the early stages of thin film formation and columnar growth
Mihai A. Popescu
Computer simulations of the structure of 13-atom metallic clusters formed by deposition on crystalline and non-crystalline substrates were carried out. The formation of discontinuous thin films during vapor deposition and the growth of bidimensional columns were simulated.
Ballistic simulation of optical coatings deposited over topography
R. N. Tait, S. K. Dew, Tom J. Smy, et al.
The use of SIMBAD, a two dimensional ballistic deposition simulation of the growth of thin films, is suggested for investigation of refractive index inhomogeneities in integrated optics devices. Refractive index variation as a function of packing fraction is obtained experimentally for evaporated MgF and sputtered SiO2 by depositing films at angles. These relationships are used to translate SIMBAD density predictions to refractive index predictions for films deposited on integrated optics topographies.
Molecular-dynamics simulation of thin-film growth and relaxation
Pierre A. Deymier, Robert B. Sargent
We simulate the growth of a thin film in two dimensions with a computer implementation of the molecular dynamics (MD) method. The system consists of a krypton substrate maintained at a temperature of about 10 degrees Kelvin, toward which argon atoms are periodically directed (with a velocity corresponding to 120 degrees Kelvin). The resulting argon film follows the (horizontal) spacing of the krypton lattice until the thickness of the film approaches an average thickness of about 10 monolayers. As deposition proceeds, the configuration of the film changes to incorporate an edge misfit dislocation at the film-substrate interface; this relieves the interfacial stress. We also apply the MD method to study the relaxation of thin-film structures predicted by a hard-disk growth model. We consider two variations of the growth model; the first is similar to that described by Henderson et al.,6 the second is a variation which incorporates the effect of surface diffusion. The voids in the relatively open microstructure predicted by the Henderson model are very effective in relieving interfacial stress. The numerous lattice defects (grain boundaries, dislocations, and vacancies) in the denser microstructure predicted by the second type of hard-disk model result in a film with high stress.
Simulations of glass surfaces-structure, water adsorption, and bond rupture
Stephen H. Garofalini
Molecular dynamics simulations of the structure of silica glass surfaces formed in a perfect vacuum as well as in the presence of a water vapor show the type, location, and concentration of specific features formed in the surface. A bond rupture mechanism which causes silanol formation far removed from the original reaction site is observed. The 3-membered ring is proposed as a site for H adsorption in the glass.
Film Microstructure
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Theoretical distribution of the structural uniformity of the vapor-deposited thin films
Wei Zhang, G. Q. Chang
According to the tangent rule, which relates the deposition angle to the growth direction of the columns, which comprise many vapor-deposited thin films, and according to the relative geometric and trigonometric relations between the evaporation source and the substrate, the theoretical distribution formulas describing the uniformity of the colunmar structure are given for four kinds of general fixtures. These distributions have been computed for various conditions. We also analyze and discuss a structural thin-film model, and have concluded that the structure obtained with the rotary spherical fixture is the best of several fixture alternatives.
Inhomogeneity in layers of dielectric high index materials: a simplified study for an antireflection coating
A simple procedure to evidence inhomogeneity in the refractive index of a single dielectric thin film included in a AR coating is developed. It requires only the use of standard measurement equipment: a spectrophotometer and a three wavelength e 1 lipsometer. The results show a good agreement between different characterization methods: reflectance, ellipsometry and single layer spectrophotometric analysis.
Structure-related anisotropic index of the optical thin films
Wei Zhang, Jian-Ying Fan, Y. Mu
The cylindrical model of optical thin films microstructure is assumed for the derivation of the equations expressing the structure-dependent anisotropic optical parameters in this paper. The equations are given for various values of cylinder obliquity and for several incident plane orientations. The equations are based on a capacitive model in which two index components are given and on the dielectric ellipsoid theory. They are applied to modeling ZnS thin films and the computed results have been given. We discuss the results, especially noting consistency of our model-based computations with measurements.
Method to destroy the columnar structure of optical thin films
Wei Zhang, Jian-Ying Fan, Heng-qian Zhao, et al.
A method based on continuously modulating the vapor deposition angle, δ, in the range from 0° < δ < 180° at an appropriate rate is proposed, by which the typical colunmar structure of vapordeposited thin films can be prevented in favor of a more compact structure, similar to a bulk material. The growth rates of such films are quantifed, and the their microstructure is simulated by computer. At the same time, an experimental study has been performed successfully. Lastly, the mechanism underlying these improved properties is analyzed and discussed.
Film Property Models I
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Study of stresses in optical thin films by optical fiber technology
Shouyao Sun, Jiu Lin Zhou, Xuequan Zan
In-situ measuring the intensity and characteristics stresses in optical coatings during deposition can get a lot of important information for improving the properties of optical thin films, which is more important for some devices uesd in modern science and technology. This paper gives a new method which uses a kind of most in-fashion sensing technology, a single-mode optical fiber sensing system developed in the middle of 1900s to test the stresses in optical coatings during deposition with the aid of a single-chip micro-computer and a micro-printer. The experimental results show that this method takes advantages of high sensitivity wide dynamic range, small volume, available in-situ measurement and easily interfaced to any vacuum system.
Stress-related phenomena in reactively dc magnetron sputtered aluminum nitride thin films
Roland Zarwasch, H. Oefner, Eduard P. Rille, et al.
Aluminum nitride films were deposited on fused silica by reactive dc magnetron sputtering from an Al-target in an Ar/N2 atmosphere. In-situ measurements during the deposition provide data concerning mechanical stresses inherent to the growing thin films. By variation of both the composition of the sputtering gas (Ar,N2) and the total gas flow in the vacuum chamber, the occuring intrinsic stresses could be shifted in magnitude and direction. Stress values of the thin films ranged from -1.2GPa (compressive) to +1.2GPa (tensile) when the Ar/N2 ratio was varied between 3:1 and 1:3 for the different total gas flows of 5Osccm, lOOsccm, and 200sccm (corresponding to total gas pressures of approximately 2x101Pa, 4x101Pa, and 8x101Pa respectively). Investigation of optical film properties, such as refractive index, as well as of structural properties were carried out and the results were related to the state of stress the films were in. The optical characterization (n,k) was achieved by photospectrometry. Structure and chemical composition were analysed by electron diffraction,transmission electron microscopy (ThM) and Auger electron spectroscopy (AES) respectively.
Hole burning in multilayer structures: effect of the third order susceptibility
The refractive index dependence on temperature and electric field can cause drastic modifications on the optical properties of optical filters. A simulation has been developed which handles electric field and temperature dependent refractive indices in multilayer stacks. In this paper we have restricted ourselves to an electric field dependence and results obtained for single line rugate filters designed by the modified Fourier transform technique, are presented.
Anisotropic dispersion and inhomogeneous dispersion in the evaporated thin films
Wei Zhang, Heng-qian Zhao, Mao-hua Yang
The anisotropic and inhomogeneous dispersion ofevaporated thin films are considered in this paper. We present possible models of both based on the basic dispersion equation, and discuss methods to measure dispersion of both anisotropic and inhomogeneous indices, n(, 0) and n(X, t), respectively . These methods are applied to a series of ZnS thin films deposited with a range of incident angles [0] and thicknesses [1]. The coefficients in the proposed dispersion models have been computed according to the experimental results for 0 nm < t < 150 nm for n(A, t) and over 0° < 0 < 500 for n(0). We conclude with an analysis and discussion of both indices.
Film Property Models II
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Modeling the properties of unbacked thin films
John O. Stoner Jr.
Thin films are used as coatings and filters on solid substrates in optics, condensed-matter physics, and many other fields. Ballistic-aggregation simulations have served well to explain the properties of such films and to suggest methods for their improvement. Thin films are also widely used without backings, as self-supporting structures. We have used simulations to advantage in the modeling of such components. To explain some of their properties, 3-D modeling is essential and a simple modeling program is adequate. Our studies using both 2-D and 3-D methods explain qualitatively the tendency of such films to curl, the directional character of that curling, the differing x-ray reflectance from the two sides of such a film, and the variation of mechanical and other properties upon position in a large film.
Structural refinement of superlattices from x-ray diffraction
Ivan K. Schuller, E. Fullerton, H. Vanderstraeten, et al.
We have performed extensive model calculations in order to understand the effect that roughness has on the X-ray diffraction from multilayers. We have developed models to calculate the low and the high angle X-ray diffraction spectra, including kinematic and dynamical simulations. These model calculations were used to understand a variety of systems including crystalline/crystalline, crystalline/amorphous and amorphous/amorphous multilayers. These model calculations were compared with the diffraction spectra of multilayered systems prepared using sputtering and Molecular Beam Epitaxy (MBE) techniques. Using the experience acquired from these model calculations we have recently developed a comprehensive nonlinear optimization program to refine the structure of multilayers from X-ray diffraction spectra. Kinematic formulation is used to refine the high angle data and dynamical (Fresnel) formalism was used to fit the low angle spectra. A comparison of the results obtained from the structural refinements with EXAFS and artificially prepared rough multilayers indicates that this type of approach give a reliable and speedy determination of the roughness, interdiffusion and lattice parameter variations in multilayers. This work has been described in a number of papers in the last few years.
Optical modeling of mixed dielectric/metal media multilayers
Mariasha Gorlin, Rafi Gatt, Sasha Yoffe, et al.
There are already a large and varied number of existing selective coatings which are used for photothermal energy conversion, but lately coatings based on thin films produced by sputtering have begun to be used in industry. A coating of this type is based on a multilayer stack consisting of both dielectric and absorbing layers. Included in the absorbing layers are layers which have a dispersion of metal particles in a ceramic matrix, and in general the metal concentration in these layers is not uniform but graded. Programs for the calculation of optical parameters of such a multilayer cannot currently be purchased commercially. Therefore it was decided to develop a program which would calculate the reflectance, solar absorbtivity, thermal emissivity, and photothermal efficiency of a selective coating. Throughout the use of this program, it is possible to calculate the highest photothermal efficiency for any temperature, to evaluate the influence of each individual layer on the resultant behaviour of the entire stack, and to calculate the angular dependence of the solar absorbtivity.