Optical coatings are used in lasers systems for fusion research to control beam propagation and reduce surface reflection losses. The performance of coatings is important in the design, reliability, energy output, and cost of the laser systems. Significant developments in coating technology are required for future lasers for fusion research and eventual power reactors.

I have spoken several times before on the topic of the Air Force's (AF) (and DoD's) needs for improved thin film coatings. My viewpoints are still changing after more than 5 years of significant involvement in DoD's thin film programs. I have chosen to address the overall thin film program rather than restrict myself to the originally proposed title "Coatings for Laser Weapons" for two reasons: First, because I cannot satisfactorilly discuss the important problems in the application programs to this international audience, and second, because I feel it's time to identify and catalyse a transformation I see beginning to take place in the thin film community. To accomplish this, I will indulge in a fair amont of speculation about future developments, research results, and approaches, so that the overall picture of the desired (envisioned) state of coating science stands out clearly. Our task as researchers is to bring about this desired state over the next several years.

The microstructure of ortical thin films is strongly columnar with a large internal surface area and void volume. This structure has great influence on the optical and mechanical properties of the films and is responsible for many of the difficulties experienced with the fabrication and subsequent performance of optical coatings. The microstructure, some of its effects on film properties, and the latest ideas on its origins are surveyed.

High quality optical coatings of metal oxides were prepared by Activated Reactive Evaporation (ARE). A cold cathode discharge is used for generation of low-energy oxygen ions and excited molecules. A survey of the operating parameters for the preparation of TiO2, BeO, In2O3, SnO2 and SiO2 coatings is given. Some results and problems are discussed in connection with the production of low-loss laser coatings, high power UV mirrors and conductive antireflection coatings.

The design is described of a prototype semi-continuous roll coater for the manufacture of optical multilayer coatings. It consists of an unload-, a process-, and a rewind module. The process module has been designed to accommodate both resistance and electron beam gun sources. Although the prototype can deposit only one layer at a time, an expanded production machine should be capable of depositing several layers onto the moving web on each pass.

Methods of deposition have been developed which are adapted for the vacuum roll-to-roll coating of plastic with optical thin films. Planar magnetron sputtering and low pressure plasma assisted CVD are shown to be peculiarly fitted for this application. The construction of such an apparatus is described and examples of indium and indium-tin oxide films made by these techniques are given.

Vacuum roll coating equipment is built for an increasingly large number of different products for various applications. A variety of technical solutions with regard to the main functional construction groups allow the coating equipment to be taylored according to the given requirements of special products. With a system of interchangeable groups it is possible to compose specialised vacuum roll coating equipment.

A process is described for the continuous coating of rolls of flexible material up to 36" wide with a transparent electrically conductive layer of indium tin oxide. These coatings are produced by either direct sputtering or by reactive magnetron sputtering from a metal alloy target. The latter process is preferred from a cost standpoint. Coatings are available with visible light transmittances and electrical resistances ranging from greater than 0.8 and less than 100Ω/□ to 0.9 and greater than 1MΩ/□ respectively. A variety of substrates, such as polyester, Teflon, Kapton, polycarbonate can be coated. Uses of these coatings range from use in transparent touch panels to static drains and heat mirrors. Several of these applications are discussed briefly.

Architectural glass coating has gained relevance in the entire energy consumption discussion worldwide. This affects solar control films for the reduction of climatisation costs in hot climate regions and heat mirrot coatings to reduce energy costs for room heating in regions of moderate climate conditions. Both types of coatings are deposited in a high rate sputtering process. The specific requirements to the magnetron cathode to reach special film properties are described. For the solar control films a low magnetic field strength version of the magnetron cathode is used to improve film hardness. For the heat mirror coatings a high magnetic field strength version of the magnetron cathode is described, especially to prevent the oxidation of IR-reflecting intermediate metal film. Optical values of solar control films on the base of stainless steel and titanium are given. Special colour effects, especially in reflection are described. Heat mirrors are exhibited as three films systems with silver or copper as the intermediate metal films. Transmission and reflectivity values are reported.

Production techniques have been developed and proven in a production facility owned and operated by Airco Temescal in Carleton, Michigan. This plant deposits single-and multilayer films by sputtering on glass for architectural applications. Other plants have been built for handling glass substrates up to 100 inches wide to 144 inches long with average line speeds up to 12 feet per minute. This proven technology is applicable to the manufacture of other products, such as mirrors and glass with conductive coatings in large quantitites.

Results of measurements on mechanical properties of metal films (Ag, Al, Cr) and of some dielectric films (mainly MgF2, CaF2, BaF2) used in optics are presented and discussed. The obtained values showed a strong dependence on preparation conditions (residual atmosphere, deposition rate). Structural and microstructural investigations provided the data required to calculate the intrinsic tensile stress of polycrystalline MgF2 and Cr films according to a grainboundary model.

The reactive deposition of both silica and titania films was investigated using an oxygen ion source similar to that developed by Ebert. The ion source used a DC gas discharge to generate oxygen ions and a graphite exit orifice to direct the ion beam to the substrate. It was found that this source could produce ion currents of 1.26 mA at a discharge voltage of 400 volts. The source materials were SiO and TiO evaporated from resistively heated boats. The measured optical constants of the titania films showed that increasing ion currents decreased the absorption constant and increased the refractive index. Infrared attenuated total reflectance (ATR) measurements were used to study the stoichiometry, moisture content, and dangling bonds of the silica films. These measurements showed that the stoichiometry of the silica films approached that of SiO2 as the ion current was increased. Laser calorimetry was used to measure the infrared absorptance at 1.06 μm to determine if the titania films were oxygen deficient. X-ray diffraction measurements on the titania films indicated that they were essentially amorphous.

Ion beam techniques can be applied advantageously during different phases of optical coatings. Prior to deposition, substrates can be ion beam cleaned with rare gas or reactive gas ions, depending upon the substrate materials involved. During deposition, ion beam sputtering can be employed to produce films of superior optical and mechanical qualities. A hybrid technique has been demonstrated in which coating material initially is sputtered by the ion beam, and subsequently it is generated thermally due to target heating by the ion beam. In this way advantages of sputter deposition are realized during initial stages of film growth, and faster deposition rates can then be achieved using thermal generation. Another hybrid deposition technique involves ion bombardment of the optical surface while simultaneous condensation of thermally generated dielectric material occurs. Potential advantages of this technique include controlled changes of physical and chemical characteristics of the film.

Highlights of a multiyear effort to develop new or improved thin-film, optical-coating materials through the use of reactive sputtering techniques are presented. Reactive sputtering is shown to be an extremely versatile technique capable of straightforward synthesis of broad classes of materials. The exceptional utility of sputtering for preparation of hard coatings such as oxides, nitrides and novel materials based on Si and Ge is described. Some of these coating materials cannot be made by conventional evaporative techniques. Reactive sputtering allows precise control of coating composition, microstructure and the resulting optical properties. Supporting data are presented for TiO2, for which record high damage thresholds were obtained, and for Si-based coatings, for which record low infrared absorptance was achieved. Transparent conductive indium tin oxide (ITO) coatings with low sheet resistance and high visible and near infrared transmission can also be made. These coatings have many electro-optic contact and electromagnetic shielding applications. Examples of multilayer coatings such as all-dielectric and dielectric-enhanced mirrors made from reactively sputtered materials are included, and simple yet elegant fabrication techniques are introduced. The reactive sputtering technique and equipment used specifically for optical coatings are briefly described, and comparison is made with the conventional evaporative approach.

Diamondlike carbon thin films can be made by several different processes. We discuss two methods we have used to produce these films: deposition by low energy carbon ion beam and rf decomposition of hydrocarbon gases. In many ways, the films made by the two methods are similar, but there are some slight differences. The films have been characterized by electron spectroscopy, optical spectroscopy and transmission electron microscopy, and these measurements will be discussed. The films are mechanically hard, resist abrasion, transparent in the infrared and less so in the visible with a refractive index that can be varied between 1.8 and 2.3. Very efficient single layer quarterwave AR coatings have been produced on silicon solar cells. Other applications will be discussed.

Since 1880, when first applied in the incandescent lamp industry, chemical vapor deposition (CVD) has been employed in a diverse group of technologies. At present, CVD plays vital roles in microelectronic, wear- and radiation-resistant coatings, fiberoptics, and the purification and fabrication of exotic materials, from ultra-low expansion glasses to high-purity refractory metals. In the CVD process, a precisely mixed stream of gases is injected into a cavity where it encounters a heated substrate, which provides the thermal activation energy that initiates a chemical reaction in the vapor. This reaction leaves behind a solid film on the substrate surface, while the other reaction products are entrained in the gas stream and exhausted from the cavity. The reaction normally takes place below or at atmospheric pressure, and the substrate can be heated by optical, infrared, or radio frequency radiation. CVD has four major advantages over most other thin film deposition techniques. First, the process allows tight control over gas stream flowrate and composition, which leads to predictable and repeatable film composition and to graded structures, if desired. Second, the thermal activation of the reaction establishes thermal equilibrium at the site of film deposition, producing tight, highly coordinated structures. Third, the throwing power of CVD is excellent. Last, the technique is especially well suited to the deposition of refractory materials difficult by other processes. Therefore, the application of this technology to optical thin films promises to be fruitful; the important steps have been taken in a number of areas.

A novel plasma-activated CVD process has been employed to deposit SiO2 and mixed oxide (SiO2+ Al2O3) covers directly on silicon solar cells. SiO2 covers as thick as 130μm were deposited near 20μm per hour at substrate temperatures below 150°C. Deposition stresses and cell fracture become serious problems above 60μm thickness.

Large wavefront phase errors introduced by small thickness changes in multilayer high reflectance coatings are discussed. The importance of measuring these wavefront phase errors at the wavelength the coating is to be used is illustrated. Plans for an interferometer working at a wavelength of 3.8 microns for measuring wavefront phase errors introduced by coating nonuniformity are described.

Expanding interest in infrared (IR) thin-film coatings is stimulating development of IR ellipsometers which can provide the optical constant measurements necessary to correctly design these coatings. Infrared ellipsometers are patterned after their visible counterparts; however, they suffer from a component limitation that is generally not a problem for visible instruments. Polarizers for use in the IR generally have low-contrast, limited-wavelength coverage, and beam distortion problems which limit the accuracy of IR ellipsometers. However, corrections can be made if the instrument is properly modeled. The complexity of these corrections and of ellipsometric calculations requires the use of a computer and associated codes. These instrument limitations and computer codes are discussed, and a state-of-the-art automated ellipsometer for use from 1 to 12 pm is described. Examples of measurements with this instrument show the precision attainable.

Laser calorimetry has been widely used to measure thin film optical absorption in filmed substrates with total film-substrate system absorptance of 1 x 10-2 and below. This paper will discuss the advantages and disadvantages of the various type calorimeters presently used to measure witness-size samples in the absorptance range of 1 x 10-2 to 1 x 10-5. Special emphasis will be given to single-layer films. Considered will be the implications of front versus back surface films, film thickness, film-substrate interface absorption, and laser coherence on the interpretation of the measured total absorptance. Throughout the entire presentation, the main concern will be the errors present in calorimetric measurements.

Infrared spectral emittance measurements can be used to determine the optical characteristics of thin films. The method is discussed and some examples are presented.

Optical reflectance and transmittance are frequently employed in the evaluation of thin film materials and components. Accurate measurement of these quantities is an experimental problem with numerous subtle sources of systematic error. For precise analysis, it is essential that these measurements be performed at the actual temperatures of use - in many modern applications, this implies temperatures substantially different from room temperature. This paper describes methods generally employed for both absolute and relative measurements, including optical and electronic signal processing implications. Although detector spatial nonuniformity is widely recognized as a serious concern, equally so and often totally neglected are variations in transmission of windows employed in elevated or cryogenic temperature measurements. Another serious problem, often not adequately recognized, concerns the degradation of the sample from condensed contaminants at cryogenic temperatures or oxides at high temperatures. An instrument will be described which successfully addresses these problems and is capable of providing measurements of sample reflectance or transmittance while protecting the sample under ultrahigh vacuum (< 10-9 torr). Relative reflectance data taken with this instrument are presented and analyzed which have an accuracy of ±3 x 10 -5.

The difficulties in making precision high reflection measurements of thin film optical coatings is described. The need for detector assemblies with very uniform response over a given area is explained. Examples of detector assemblies using shaped diffusor elements are given.

Measurement is reported at 4 deg K (and blocked transmittance below 10-5) of PbTe/ZnS thin-film filters deposited on Ge substrates. The reduced carrier-absorption which is obtained by cooling these PbTe films is found to accord with simple theory. Advantage for various high-performance multilayers by cooling is significant at the longer wavelengths, and has been verified.