Optical Coatings

Thin-Film Production

Vacuum Deposition

Melles Griot manufactures thin films by a process known as vacuum deposition. Uncoated substrates are placed in a large vacuum chamber capable of achieving a vacuum of at least 10-6 torr. At the bottom of the chamber is a source of the film material to be vaporized, as shown in the figure below. The substrates are mounted on a series of rotating carousels, arranged so that each substrate sweeps in planetary style through the same time-averaged volume in the chamber.

Schematic view of typical vacuum deposition chamber

Thermal Evaporation

The source of vaporized material is usually one of two types. The simpler, older type relies on resistive heating of a thin folded strip (boat) of tungsten, tantalum, or molybdenum by a high direct current. Small amounts of the coating material are loaded into the boat. A high current (10-100 A) is passed through the boat, which undergoes resistive heating. The coating material is then vaporized thermally. Because the chamber is at a greatly reduced pressure, there is a very long mean free path for the free atoms or molecules, and the heavy vapor is able to reach the moving substrates at the top of the chamber. Here it condenses back to the solid state, forming a thin, uniform film. Several problems are associated with thermal evaporation. Some useful substances can react with the hot boat, which can cause impurities to be deposited with the layers, changing optical properties. In addition, many materials, particularly metal oxides, cannot be vaporized this way, because the material of the boat (tungsten, tantalum, or molybdenum) melts at a lower temperature. Instead of a layer of zirconium oxide, a layer of tungsten would be deposited on the substrate.

Soft Films

Until the advent of electron bombardment as a superior alternative, only materials that melted at moderate temperatures (2000ºC) could be incorporated into thin-film coatings. Unfortunately, the more volatile materials also happen to be the softer materials, which produce less resilient films. Consequently, early multilayer coatings deteriorated fairly quickly and required undue amounts of care during cleaning. More important, sophisticated designs with performance specifications at several wavelengths could not be easily produced since these designs required many individual layers, and the softness of the layers made some of these films impractical.

Electron Bombardment

Electron bombardment has become the accepted method of choice for optical thin-film fabrication. This method is capable of vaporizing even highly involatile materials, such as titanium oxide and zirconium oxide. Using large cooled crucibles precludes reaction of the heated material with the metal of the boat or crucible. A high-flux electron gun (1 A at 10 kV) is aimed at the film material contained in a large, water-cooled, copper crucible. Intense local heating melts and vaporizes some of the coating material in the center of the crucible without causing undue heating of the crucible itself. For particularly involatile materials, the electron gun can be focused to intensify its effects. Careful control of temperature and vacuum conditions ensures that most of the vapor is in the form of atoms or molecules, as opposed to clusters. This produces a more even coating with better optical characteristics and improved longevity.

Ion-Assisted Bombardment

Ion-assisted bombardment is a coating technique that can offer unique benefits under certain circumstances. Ion assist during coating leads to a higher atomic or molecular packing density in the thin-film layers. This results in a higher refractive index and, most important, superior mechanical characteristics. Specifically, the lack of voids in the more efficiently packed film means that it is far less susceptible to water-vapor absorption. Water absorption by an optical coating can change the index of refraction of layers and, hence, the optical properties. Water absorption can also cause mechanical changes that can ultimately lead to failure. Ion-assisted coating can also be used for cold processing. Eliminating the need to heat parts allows cemented parts, such as achromats, to be safely coated.

Monitoring And Controlling Layer Thickness

A chamber set up for multilayer deposition has several sources that are preloaded with various coating materials. The entire multilayer coating is deposited without opening the chamber. A source is heated, or the electron gun is turned on, until the source is stable. The shutter above the source is opened to expose the chamber to the vaporized material. When a particular layer is deposited to the correct thickness, the shutter is closed and the source is turned off. This process is repeated for the other sources. The most common method of monitoring the deposition process is optical monitoring. A monitor beam of light passes through the chamber and is incident on a blank monitor substrate. Reflected light is detected using photomultiplier and phase-sensitive detection. As each layer is deposited onto the reference blank, the intensity of reflected light from it oscillates in a pseudo sine wave (rectified). The turning points represent quarter- and half-wave thicknesses at the monitor wavelength, with intermediate thicknesses between. Deposition is automatically stopped as the reflectance of the reference surface passes through the appropriate point.

Scattering

Reflectance and transmittance are usually the most important optical properties specified for a thin film, closely followed by absorption. However, the degree of scattering caused by a coating is often the limiting factor in the ability of coated optics to perform in certain applications. Scattering is quite complex. The overall degree of scattering is determined by imperfections in layer interfaces and interference between photons of light scattered by these imperfections, as shown in the illustration below.

Interface imperfectoins scattering light in a multilayer coating

It is also a function of the granularity of the layers. This is difficult to control as it is an inherent characteristic of the materials used. Careful modification of deposition conditions can make a considerable difference to this effect. The most notable examples of applications where scattering is critical are intracavity mirrors for low-gain lasers, such as certain helium neon laser lines and continuous-wave dye lasers.

Temperature and Stress

A major problem with thin films is caused by inherent mechanical stresses. Even with careful control of the vacuum, source temperature, and optimized positioning of the optics being coated, many thin-film materials do not deposit well on cold substrates. This is particularly true of involatile materials. Raising the substrate temperature a few hundred degrees improves the quality of these films, often making the difference between usable and useless film. The elevated temperature seems to allow freshly condensed atoms (or molecules) to undergo limited surface diffusion. Optics that have been given a multilayer thin-film coating at an elevated temperature require very slow cooling to room temperature. Thermal expansion coefficients of substrate and film materials are likely to be somewhat different. As cooling occurs, the coating contracts and produces stress in the layers. Many pairs of coating materials do not adhere particularly well to each other owing to different chemical properties and bulk packing characteristics. Temperature-induced stress and poor interlayer adhesion are the most common thickness limitations for optical thin films. Until new technologies, such as ion-assisted deposition, are developed into true production tools, stress must be reduced by minimizing overall coating thickness and by carefully controlling the production process.

Intrinsic Stress

Even in the absence of thermal-contraction-induced stress, the layers often are not mechanically stable because of intrinsic stress from interatomic forces. The homogeneous thin film is not the preferred phase for most coating materials. In the lowest energy, natural form of the material, molecules are aligned in a crystalline symmetric fashion. This is the form in which intermolecular forces are more nearly in equilibrium. In addition to intrinsic molecular forces, intrinsic stress results from poor packing. If packing density is considerably less than 100%, the intermolecular binding may be sufficiently weakened to make the layer totally unstable.

Production Control

Two major factors are involved in producing a coating to perform to a particular set of specifications. First, sound design techniques must be used. If design procedures cannot accurately predict the behavior of a coating, there is little chance that satisfactory coatings will be produced. Second, if the manufacturing phase is not carefully controlled, the thin-film coatings produced may perform quite differently from the computer simulation. Melles Griot uses the latest computer design programs with exhaustive iterations to ensure that the final design is optimized. Manufacturing high-quality thin films is not trivial. At Melles Griot, more effort is expended on monitoring thin-film manufacture than on any other single manufacturing procedure. Without such careful monitoring, the tedious design and optimization phase would be wasted. Great care is taken in coating production at every level. Not only are all obvious precautions taken, such as thorough precleaning and controlled cool down, but even the smallest details of the manufacturing process are carefully controlled. Our thoroughness and attention to detail ensures that the customer will always be supplied with the best design, manufactured to the highest standards.

Quality Control

All batches of Melles Griot coatings are rigorously and thoroughly tested for quality. Even with the most careful production control, this is necessary to ensure that only the highest quality parts are shipped. Our inspection system meets the stringent demands of MIL-I-45208A and our spectrophotometers are calibrated to standards traceable to the National Institute of Standards and Technology (NIST). Upon request, we can provide complete environmental and photometric testing to MIL-C-675 and MIL-M-13508. All are firm assurances of dependability and accuracy.

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Optics Guide © 2004 Melles Griot Inc.