Roger P. Netterfield

Ion-beam-assisted deposition of thin films.

Some effects on the properties of electron-beam evaporated thin films produced by ion bombardment of the growing film are reported. Substantial increases in the packing densities of SiO2 , TiO2 , and ZrO2 films have been produced as measured by the reduction in the adsorption of moisture when the films are exposed to a humid atmosphere. In a ZrO2-SiO2 multilayer interference filter, changes in the wavelength of the peak transmittance on exposure to the atmosphere have been reduced from 8 nm for films deposited without ion bombardment to <1 nm for ion-beam-assisted films.

Properties of CeO2 thin films prepared by oxygen-ion-assisted deposition.

Thin films have been prepared by electron-beam evaporation of CeO2, where the growing film has been bombarded with oxygen ions. The packing density of the films has been increased from ∼0.55 without ion bombardment to unity with bombardment as determined by moisture adsorption measurements. The refractive index, extinction coefficient, and scattering loss are reported for a range of ion energies from 50 to 1200 eV. The ratio of ion-current density to film growth rate required to produce films that did not adsorb moisture was found to be a minimum for ion energies in the 300–600-eV range. Absorption and scatter losses are smallest for the lower ion energies and the crystal structure of CeO2 films is relatively stable under ion bombardment although ion-assisted films tend to be less crystalline than evaporated layers.

Characteristics of titanium arc evaporation processes

The characteristic photon and ion emissions from a titanium cathodic arc evaporator have been studied. The composition of the background gas in the vacuum vessel was found to influence the emissions strongly. High resolution optical spectroscopy revealed that the neutral titanium atoms had a lower velocity distribution than had the ionized particles. Ion energy analysis indicated that charge exchange processes dominate at high background pressures. External magnetic fields applied to the plasma stream were found to excite the vapour and to accelerate the ions. Internal fields applied to the cathode surface were found to increase the cathode spot velocity to a maximum of around 25 m s-1. Application of an external field resulted in deposited titanium films with a reduced microdroplet component whereas an internal field had no effect. The film quality further improved with the introduction of reactive gases such as oxygen and nitrogen. The properties of the deposited titanium, TiO2 and TiN films were examined as a function of the external field strength.

Ion‐assisted deposition of mixed TiO2‐SiO2 films

Mixed thin films of TiO2 and SiO2 were produced by coevaporation from separate electron‐beam sources and simultaneous bombardment of the growing film with oxygen ions. The optical properties of the films were determined during growth by in situ ellipsometry and the surface composition of the deposited films studied by in situ ion scattering spectroscopy, ex situ x‐ray photoelectron spectroscopy, and energy filtered electron diffraction. The correlation between the optical and surface characterization is presented. There is evidence of local variations in the relative concentrations of TiO2 and SiO2. The position of the Si 2p binding energy depends on the TiO2 content in the film, indicating the possible formation of an intimate mixture.

Unambiguous determination of optical constants of absorbing films by reflectance and transmittance measurements

We describe a three-stage process for the determination of the optical constants n and k of thin absorbing films on the basis of reflectance, transmittance, and thickness measurements. The first stage uses a bivariate optimization based on the functions (1 ± R)/T, R denoting a reflectance and T the transmittance. The second stage uses a phase-variate approach based on an algorithm for locating the complex zeros of analytic functions. The third stage uses an evolution curve, giving the single-wavelength reflectance as a function of film thickness. We illustrate the process using practical examples drawn from recent studies of films of amorphous silicon, hydrogenated carbon, and hydrogenated silicon.

Low sideband guided-mode resonant filter

A guided-mode resonant filter with low sideband reflections is proposed. It is shown that, for serious reduction of out-of band reflectance, the combination of waveguide-grating filter design with conventional antireflective stack design methods is not adequate. To achieve symmetrical low sideband reflectances, independent control of various layer thicknesses is necessary. At a given illumination angle with appropriate control of the waveguide thickness, a specific resonant grating filter is designed whose out-of-band reflectance on both sides of the resonant peak is well below 10(-4). Even 50 nm away from the peak, on both sides, the out-of-band reflectance remains below 10(-3). Analysis of the variation in the main manufacturing parameters indicates that such filters can be reliably produced with present-day technologies.

Optical properties and stress of ion-assisted aluminum nitride thin films.

The optical properties and stress in thin films of aluminum nitride have been studied as a function of ion energy and the ion-to-vapor arrival ratio (J(i)/J(v)) for the N(2)(+) ion-assisted deposition of aluminum. The ion energy was varied between 75 and 1000 eV. The refractive index was found to depend on the ion energy and flux. The highest index was 2.10 at 633 nm. The films were found to be highly transparent over the wavelength region 275-800 nm. The films were found to be free of major oxide contamination, and x-ray photoelectron spectroscopy studies revealed the presence of excess nitrogen in films prepared at high J(i)/J(v) values. The film stress was also found to be related to the ion energy, and an anomalously high compressive stress of -5.0 GPa was found for 100-eV N(2)(+)-assisted depositions. By comparison, films prepared by magnetron deposition were found to have lower refractive indices (1.97-1 99, n633) and higher stress (-8 to -12 GPa) when deposited in pure nitrogen.

Influence of ion assistance on the optical properties of MgF(2).

The optical properties of MgF(2) films prepared by evaporation and ion-assisted deposition have been determined from transmittance and near-normal incidence reflectance measurements and also from electron-energy loss spectroscopy (EELS). The results show that oxygen-ion assistance leads to higher extinction coefficients for wavelengths <180 nm. Transmission electron microscopy studies show that the crystal grain size of MgF(2) films is not strongly affected by oxygen or argon-ion bombardment. The presence of MgO in the films is inferred from RBS measurements and proposed to be the major factor influencing VUV losses. EELS is also demonstrated to be a valuable technique for determination of optical properties from the near-infrared to x-ray regions of the spectrum.

Ion‐assisted deposition of bulklike ZrO2 films

Stable thin films of zirconium dioxide are produced by ion‐assisted electron beam deposition at room temperature and 300 °C. The method yields films with substantially increased packing density resulting in refractive indices close to the bulk value. The improvement in optical properties is accompanied by an amorphous to cubic crystalline transition of the film structure which is mixed with a monoclinic phase for heated films.

Mechanical characteristics of optical coatings prepared by various techniques: a comparative study.

Good performance of optical coatings depends on the appropriate combination of optical and mechanical properties. Therefore, successful applications require good understanding of the relationship between optical microstructural and mechanical characteristics and film stability. In addition, there is a lack of standard mechanical tests that allow one to compare film properties measured in different laboratories. We give an overview of the methodology of mechanical measurements suitable for optical coatings; this includes depth-sensing indentation, scratch resistance, friction, abrasion and wear testing, and stress and adhesion evaluation. We used the techniques mentioned above in the same laboratory to systematically compare the mechanical behavior of frequently used high- and low-index materials, namely, TiO2, Ta2O5, and SiO2, prepared by different complementary techniques. They include ion-beam-assisted deposition by electron-beam evaporation, magnetron sputtering, dual-ion-beam sputtering, plasma-enhanced chemical-vapor deposition, and filtered cathodic arc deposition. The mechanical properties are correlated with the film microstructure that is inherently related to energetic conditions during film growth.