H. A. Macleod

Multiple determination of the optical constants of thin-film coating materials

The seven participating laboratories received films of two different thicknesses of Sc2O3 and Rh. All samples of each material were prepared in a single deposition run. Brief descriptions are given of the various methods used for determination of the optical constants of these coating materials. The measurement data are presented, and the results are compared. The mean of the variances of the Sc2O3 refractive-index determinations in the 0.40–0.75-nm spectral region was 0.03. The corresponding variances for the refractive index and absorption coefficient of Rh were 0.35 and 0.26, respectively.

Optical and microstructural properties of hafnium dioxide thin films

Abstract We have applied a variety of analytical tools to educe the composition and morphology of thin films of hafnium dioxide deposited under a variety of conditions. Surface analytical techniques include scanning electron microscopy, transmission electron microscopy, electron diffraction, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. The results from these measurements are correlated with the observed optical properties. Electron diffraction reveals that films deposited below 300 °C are amorphous and spectrophotometry uncovers a small optical inhomogeneity. The refractive index and inhomogeneity are strongly influenced by the oxygen backfill present during film condensation. Ion-assisted deposition (IAD) removes the optical inhomogeneity, increases the refractive index, and eliminates the air-to-vacuum spectral shift observed in films deposited without IAD. In addition, low energy IAD is found to increase the refractive indices of the films without affecting the optical inhomogeneity. This is explained by the preferential sputtering of hydroxide from the growing film surface by the low energy ions.

Monitoring of optical coatings

Monitoring implies the measurement and control of the parameters of optical thin films during deposition with the object of ensuring the production of an acceptable coating. Monitoring techniques usually concentrate on film thickness as the most important parameter. The principal monitoring arrangements in current use are surveyed with a brief account of their relative merits. It is shown that they are less able to cope with refractive-index errors than with simple thickness errors and that tight control of material parameters is required to take advantage of recent advances in monitoring techniques.

Refractive index and inhomogeneity of thin films

The fact that the optical characteristics of thin-film materials are generally different from those of the same materials in bulk form is well known. The differences depend very much on the conditions in which the deposition has been carried out. A good understanding of these differences, their causes, and the influence of deposition parameters is vital if we are to be able to improve coating quality. We have developed two complementary methods with the objective of deriving information on the index of refraction and its variation throughout the thickness of the film. Perceptible optical inhomogeneity is normally present and appreciable inhomogeneity is frequently present in thin films. Such inhomogeneity is usually associated with layer microstructure. The first is a postdeposition technique that makes use of measurements in air of the transmittance and reflectance of the layer under study over a wide wavelength region. The second, in contrast, makes use of in situ measurements, that is measurements made under vacuum and during the actual deposition of the layer. We shall show with the help of several examples that the two methods lead to results that are consistent and demonstrate the existence in deposited materials of an inherent variation of the index of refraction normal to the surface. The thermal sensitivity of the layer properties and their tendency to adsorb atmospheric moisture must be taken into account before the residual differences between the two techniques can be explained.

Ion assisted deposition of thermally evaporated Ag and Al films

Optical, electrical, and microstructural effects of Ar-ion bombardment and Ar incorporation on thermally evaporated Ag and Al thin films are investigated. The results show that as the momentum supplied to the growing films by the bombarding ions per arriving metal atom increases, the refractive index at 632.8 nm increases and the extinction coefficient decreases, lattice spacing expands, grain size decreases, electrical resistivity increases, and trapped Ar increases slightly. In Ag films, stress reverses from tensile to compressive and in Al films compressive stress increases. In the Al films the change in optical constants can be explained by the variation in void volume. The reversal of stress from tensile to compressive in Ag films requires a threshold level of momentum. The increase in electrical resistivity is related to the decrease in grain size and increase in trapped Ar in both types of film. Many of these properties correlate well with the momentum transferred, suggesting that the momentum is an important physical parameter in describing the influence of ion beam on growing thin films and determining the characteristics of thin metal films prepared by ion assisted deposition.

OPTICAL-CONSTANT CALCULATION OVER AN EXTENDED SPECTRAL REGION : APPLICATION TO TITANIUM DIOXIDE FILM

An iterative algorithm has been developed that takes starting values derived by an envelope method but then minimizes the influence of the envelopes and emphasizes the actual measured data. This combination avoids the difficulties inherent in the accurate drawing of the envelopes and makes it possible to extract the thickness and the optical constants of semiconducting and dielectric films over a wide spectral region, including regions of high absorption.

Ion-assisted deposition of lanthanum fluoride thin films

Ion-assisted deposition has been used to deposit lanthanum fluoride thin films with near-unity film packing densities and no significant increase in absorption. Rutherford backscattering analysis has determined the effect of ion bombardment on the film stoichiometries including the degree of fluorine deficiency. Oxygen atoms or compounds appear to occupy most of the available anion vacancies if sufficient oxygen is available in the ion beam or the residual atmosphere.

Reactive ion assisted deposition of aluminum oxynitride thin films

Optical properties, stoichiometry, chemical bonding states, and crystal structure of aluminum oxynitride (AlO(x)N(y)) thin films prepared by reactive ion assisted deposition were investigated. The results show that by controlling the amount of reactive gases the refractive index of aluminum oxynitride films at 550 nm is able to be varied from 1.65 to 1.83 with a very small extinction coefficient. Variations of optical constants and chemical bonding states of aluminum oxynitride films are related to the stoichiometry. From an x-ray photoelectron spectroscopy analysis it is observed that our aluminum oxynitride film is not simply a mixture of aluminum oxide and aluminum nitride but a continuously variable compound. The aluminum oxynitride films are amorphous from an x-ray diffraction analysis. A rugate filter using a step index profile of aluminum oxynitride films was fabricated by nitrogen ion beam bombardment of a growing Al film with backfill oxygen pressure as the sole variation. This filter shows a high resistivity to atmospheric moisture adsorption, suggesting that the packing density of aluminum oxynitride films is close to unity and the energetic ion bombardment densifies the film as well as forming the compound.

Computer simulation of thin film growth: applying the results to optical coatings

Computer simulation of thin film growth has been used extensively to gain insight into the origin and nature of the microstructure of vapor-deposited thin films. Usually, however, no attempts are made to predict film properties other than column angle and film density from such simulations. The aim of our work is to derive quantitative data from computer simulations in order to be able to predict relevant properties of optical coatings. The deposition of 2500 to 25,000 particles has been simulated on different computers by random deposition of two-dimensional hard disks, using a simple relaxation scheme. Statistical analysis of the results yields quantitative data for the density, column angle, and column period. On the basis of these results, a simple model has been developed for the microstructure of a three-dimensional film. The birefringence and the shape of water-penetration fronts in evaporated optical coatings, predicted from this model, are confirmed by experiment.

New Christiansen filters

A number of new optical filters that are based on the well-known Christiansen effect of scattering light in a heterogeneous medium have been fabricated. In addition to the conventional solid-in-liquid systems, we fabricated some solid-in-solid filters, which we refer to as solid Christiansen filters. Here we report on their fabrication, performance, and applications. From our experiments we have also derived the dispersion curve for a liquid styrene monomer in the visible region.