Charles K. Carniglia

Comparison of the properties of titanium dioxide films prepared by various techniques.

Fourteen university, government, and industrial laboratories prepared a total of twenty pairs of single-layer titanium dioxide films. Several laboratories analyzed the coatings to determine their optical properties, thickness, surface roughness, absorption, wetting contact angle, and crystalline structure. Wide variations were found in the optical and physical properties of the films, even among films produced by nominally the same deposition techniques.

Materials for optical coatings in the ultraviolet.

In this work, we used 248-nm 20-nsec laser pulses to measure laser-damage thresholds for halfwave-thick single layers of fifteen potential UV coating materials, for highly reflective coatings made of thirteen combinations of these materials, and for antireflective coatings made using five combinations of the materials. Refractive index, absorption, position of the UV absorption edge, stress, and environmental stability were measured for the halfwave-thick single layers. The first three of these parameters are closely related and were generally correlated with damage thresholds of the single-layer coatings. Thresholds of HR coatings were correlated with absorption in the high-index materials used in the coatings and with refractive index of the low-index materials. Thresholds of AR films were not well correlated with properties of single-layer coatings.

Optical and crystalline inhomogeneity in evaporated zirconia films

Electron-beam-deposited zirconia (zirconium dioxide) films are known to be inhomogeneous. They have a higher refractive index near the substrate and a lower index at the outer surface. Zirconia films deposited on high temperature (300°C) substrates are also known to be crystalline, exhibiting both a cubic and a monoclinic phase. X-ray diffraction studies of zirconia films of various thicknesses show that films which have an optical thickness of less than a quarter of a wavelength at 600 nm are cubic, while films that are thicker consist of both cubic and monoclinic zirconia. As the film thickness increases beyond a quarterwave optical thickness of 600 nm, the amount of the cubic phase remains constant and the amount of the monoclinic phase increases linearly with thickness. This implies that the film nucleates with a cubic structure but that after the film reaches a critical thickness, the surface conditions become more favorable for the growth of the monoclinic phase. This also suggests several possible models for the inhomogeneity in index. Spectral analysis indicates that the cubic portion of the film is optically inhomogeneous, while the monoclinic phase is homogeneous and has a lower refractive index.

Stress reduction in ion beam sputtered mixed oxide films

Thin films deposited by ion beam sputtering typically have a high compressive stress. This paper demonstrates that this stress can be reduced by cosputtering two materials. Thin film mixtures of zirconia (ZrO(2)) and silica (SiO(2)) were prepared with a range of compositions using ion beam sputtering. The refractive index was found to vary almost linearly with composition. The large stress observed in zirconia films was found to be reduced significantly by the addition of silica.

Refractive index determination using an orthogonalized dispersion equation.

An orthogonal form of a general three-term dispersion equation is presented which is useful for determination of the refractive index of transparent optical materials from measured spectral data. The orthogonal basis functions are dependent on the spectral range of the data and whether the spacing of the data is uniform with respect to wavelength or wavenumber. The application of the technique to the reduction of ellipsometric measurements of a single layer thin film of zirconia on a fused silica substrate is presented. The measured data were fit to a Conrady dispersion equation and to the corresponding orthogonal basis set. The convergence to the solution was an order of magnitude faster for the orthogonal form of the dispersion equation.

Comparison of several shortwave pass filter designs

The simplest shortwave pass filter design consists of repetitions of the basic period of the form (0.5L H 0.5L), where L and H refer to quarterwaves of low and high index materials at a particular wavelength. Such a filter has a passband limited by the first- and third-order rejection bands. The width of the passband can be extended to shorter wavelengths by further subdividing the basic period using materials of intermediate indices. Alternate shortwave pass filter designs can be obtained by approximating a continuous index profile in which the natural log of the refractive index follows a sinusoidal profile. Films with such a continuous index profile are commonly called rugate filters. Four discrete index approximations to the rugate filter are compared. They are found to differ with respect to the bandwidth of the shortwave passband.

Effects of undercoats and overcoats on damage thresholds of 248 nm coatings

Previous experiments have demonstrated that 1064 nm high reflectors benefit from the addition of halfwave silica overcoats, and that 1064 nm antireflection coatings can be improved by adding halfwave silica undercoats or barrier layers. In each case, a statistical improvement of about 50% has been observed. This paper reports similar results for coatings designed for 248 nm. The high reflectors were scandia/magnesium fluoride quaterwave stacks. Three design variations were tested: with no overcoat, with a halfwave silica overcoat, and with a halfwave magnesium fluoride overcoat. The presence of the overcoat more than doubled the threshold of the reflectors. The highest threshold, 8.5 joules/sq. cm, was measured on a reflector with a magnesium fluoride overcoat. Two material combinations were used for the four-layer antireflection coatings: scandia/silica and scandia/magnesium fluoride. Each of these combinations was coated without a barrier layer, with a silica barrier layer, and with a magnesium fluoride barrier layer. The barrier layer was an undercoat with a halfwave optical thickness. Varying degrees of improvement in thresholds, ranging up to 50%, were found in all cases with barrier layers. The highest thresholds exceeded 6 joules/sq. cm for scandia/silica coatings with silica barrier layers.

Comparison of the Optical Properties of some High-Index Oxide Films prepared by Ion Beam Sputter Deposition with those of Electron Beam Evaporated Films

A typical thin film of a high-index oxide coating material deposited by electron-beam (E-beam) evaporation has a refractive index lower than the bulk value for the material. This indicates that the film is porous, having a density less than the bulk density. In addition, such films are often inhomogeneous, having a higher refractive index near the substrate surface. Thin films of the same materials deposited by ion beam sputter deposition (IBD) techniques tend to have higher and more uniform refractive indices, indicating that IBD films are denser and more homogeneous. We have measured the index of refraction n, the extinction coefficient k and the degree of inhomogeneity of several high-index oxide materials deposited by IBD and by standard E-beam evaporation. The materials investigated were the oxides of zirconium, tantalum, hafnium, niobium and aluminum. In all cases, the IBD films were found to have higher refractive indices than their evaporated counterparts. The IBD films, in general, exhibited an inhomogeneity which was the reverse of the inhomogeneity found in the E-beam films. This indicates a difference in the nucleation and growth of the films. Tantala films show an absorption band in the visible spectral region resulting from electron trapping sites. Thus, this material is probably not suitable for coatings for visible and UV laser systems.

Feature issue on optical interference coatings

The feature issue on Optical Interference Coatings, stimulated by the June 1992 Topical Meeting, covers the wider field of optical surface treatments after polishing. It is the latest in a series that has been running every four years since 1976.

A 1.06µm Laser Absorption Calorimeter for Optical Coatings

We describe a laser calorimeter for measuring the absorption of an optical coating deposited on a thin disc. The sample with diameter 2.54cm and thickness between 0.02 to 0.13cm is irradiated with 6 watt CW Nd:YAG laser. The temperature of the coated disc is compared to that of a reference disc using thermistors in mechanical contact with the samples. Measurements are made under vacuum (100 torr) to reduce temperature variations caused by convection. The calorimeter was evaluated by measuring coated and uncoated discs of various thicknesses with absorptances from 9 x 10 - 6 to 5 × 10 - 3 . These measurements demonstrated a repeatability of ′ 2% at an absorptance level of 1 × 10 4 and a sensitivity for a 0.04cm thick substrate of 3.8 × 10 5 μV/watt. The lower absorption limit of the calorimeter is 2 x 10 - 6 . The results of measurements on polished, uncoated fused silica discs with thicknesses from 0.04 to 0.13cm show a surface absorptance of 6 × 10 - 6 and a bulk value of 5 x 10 5 /cm.