Karl H. Guenther

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.

Microstructure of vapor-deposited optical coatings

The microstructure of thin films applied by conventional physical vapor deposition for use as optical coatings is columnar for most of the materials commonly used. This has been established for about a decade through numerous experimental observations employing microfractographical replication for use with high resolution transmission electron microscopes. Scanning electron microscopes are more useful investigating coating defects, the most remarkable of these defects being known as nodules. From fundamental considerations of nucleation and growth of thin films, the origin of both columns and nodules and the dependence of their appearance on the deposition conditions are discussed in some detail. A simple 2-D simulation model assuming limited surface mobility of adatoms or admolecules shows striking similarities to peculiar properties of both columnar and nodular growth seen in actual investigations. Conclusions are drawn as to how the two types of microstructure influence general film properties and, in particular, how they influence possible laser damage mechanisms.

Surface roughness measurements of low-scatter mirrors and roughness standards

The surface roughness of low-scatter mirrors and roughness standards can be measured in various ways: mechanically, optically, and electronoptically. In this paper several useful methods and instruments—total integrated scattering, stylus profilometer, optical heterodyne profilometer, variable angle scatterometer, and electron mirror interference microscope—are presented. Qualitative surface roughness assessment is provided by Nomarski microscopy and transmission electron microscopy of surface replicas. Samples were prepared at Balzers, and their surface roughness was measured at several laboratories using the methods given above. The results of these measurements are compared, and reasons for the differences are discussed.

Physical and chemical aspects in the application of thin films on optical elements

First, an introductory review of deposition-dependent microstructural peculiarities of real optical thin films is presented. Then, some physical and chemical properties of particular interest for the user of coated optical elements are surveyed. Of the optical properties, the refractive index is discussed in detail, and reflectivity, light scattering, and surface plasmon excitation are also included. Of the mechanical properties, adhesion, hardness, abrasion as well as some aspects of intrinsic stress are illustrated with practical examples. The importance of the thermal stability of optical coatings is emphasized, and some examples of severe alterations of coating properties after thermal treatment are given. The degradation of coatings as a result of radiation impact is also treated briefly. Finally, the discussion of diffusion processes within some coatings and in the surface region of substrates leads to a survey of selected chemical reactions between the substrate and the coating, within the coating itself, and between the coating and the environment.

Absorption and thermal conductivity of oxide thin films measured by photothermal displacement and reflectance methods

Photothermal reflectance and photothermal displacement measurements of optical absorption and thermal conductivity are reported for electron-beam-(EB) deposited and ion-plated (IP) thin films of TiO(2), Ta(2)O(5), and ZrO(2). Of the particular set of samples investigated, the EB films have higher absorption than the IP films. The absorption of the EB samples decreases over a period of ~ 90 min on irradiations with an Ar-ion laser of 488-nm wavelength. By contrast, the absorption of the IP samples changes insignificantly or not at all. Photothermal displacement area scans of coating surfaces yield lower defect densities for the IP samples compared with the EB samples for all three oxide materials. The feasibility and limitations of photothermal measurements for thin-film optical and thermal characterizations are discussed.

Comparative study of titanium dioxide thin films produced by electron-beam evaporation and by reactive low-voltage ion plating

Titanium dioxide (TiO(2)) is often used as a high refractive-index material for multilayer optical coatings. However, the optical properties of TiO(2) films depend strongly on the deposition process and its parameters. A comparative study of TiO(2) films fabricated by conventional electron-beam evaporation and by reactive low-voltage ion plating that uses different phases of Ti-O as starting materials is reported. Results on the variability of TiO(2) thin films are analyzed in relation to process parameters. The potential of fabricating high and low refractive-index multilayer stacks with TiO(2) only, by employing two different deposition processes, is presented with a practical example.

Microstructure analysis of thin films deposited by reactive evaporation and by reactive ion plating

Electron microscopy, stylus‐type surface profilometry, and Raman microprobe characterization show distinct differences between thin films deposited by reactive evaporation and by reactive ion plating. Reactive evaporation yields thin films with the well‐known columnar microstructure with appreciable surface roughness and other deficiencies. Low‐voltage, high‐current reactive ion plating deposition produces thin films which are smooth and dense. Cross‐section electron micrographs of ion plated coatings reveal a densely packed polycrystalline structure for ZrO2 , while TiO2 appears to form vitreous films. Molecular dynamics computer simulation of the film formation process is in good qualitative agreement with the experiments. The results suggest the expansion of the Movchan–Demchishin structure zone model with an additional zone 4 for the vitreous phase, resulting from superthermal film formation conditions (thermal spiking).

Electrooptic beam deflection using the leaky mode of a planar waveguide

Electrooptic beam deflection using the leaky mode of a TiO/sub 2/ thin-film waveguide in optical contact with a LiNbO/sub 3/ crystal is discussed. Initial measurements yielded a deflection efficiency of 0.34 mrad/kV and suggest potential for more than an order of magnitude improvement. By depositing the waveguide directly onto the electrooptical crystal, a rugged miniature device that has the potential for high-speed operation and an angular deflection efficiency equal to or higher than that of currently available bulk devices can be fabricated. In addition to the strict tolerances on the coating thickness, the deposited films must have extremely low losses, a high refractive index, and be resistant to environmental effects.<<ETX>>

Revisiting structure-zone models for thin-film growth

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.

Design Of Nonpolarizing Achromatic Beamsplitters With Dielectric Multilayer Coatings

Optical beamsplitters often consist of repeated pairs of high and low index quarter-wave layers. At oblique angles of incidence, such coatings typically have a fairly high polarization ratio. Reflectance, transmittance, and phase for the two orthogonal planes of polarization, s and p, are different in general. Here, we present the results of the design of all-dielectric beamsplitter coatings with very low polarization ratios. An initial sinusoidal refractive index profile, optimized with a refining computer program, yields a 50±1% beamsplitter in the 450 to 650 nm wavelength range, with less than 0.5% (abs.) difference between the s and p reflectance in most of this interval. Matching the elements of the characteristic matrix of this design with those of a generic homogeneous multilayer stack yields the starting design A(HL)7HS for a reflectance to transmittance ratio of R:T = 50:50% and 30:70% beamsplitters, which are optimized for the 500 to 600 nm wavelength range and angles of incidence of 40°, 50°, and 60° using a computer program based on a damped least squares refining technique. The average deviation from the nominal beamsplitting ratio is less than 0.5% for all given design examples. The maximum deviations are about 2% in this wavelength range.