In this review paper, we provide a survey of nucleation and growth models for thin films. Centered on a simplistic computer simulation of random ballistic agglomeration of molecules represented as two dimensional hard-disks, which is valid for the limit case of very low surface mobility or of no surface mobility at all, we explain various properties of thin film optical coatings, notably their sometimes unstable behaviour.
Optical coatings are increasingly gaining importance in volume applications like displays, optical data storage, architectural coatings, lighting, etc. This has brought movement into the classical methods of preparing optical coatings. The more important trends are:
The microstructure, crystallinity, and stoichiometry of vacuum deposited thin films are modified substantially if the layer is bombarded by ions during growth. The properties of optical films prepared by ion-assistance are often superior to those produced in vapor deposition. These ion-induced modifications will be explained in terms of recently developed theories and computer simulations.
The plasma deposition process can produce coatings with highly desirable properties, attributable to the ion and neutral chemistry at the surface of the growing film. The ultimate technical success of this technique depends upon the ability to control the reactive species in the plasma, their fluxes, and their energies. Recently, progress has been made in identifying deposition mechanisms and understanding how they can be controlled via the available parameters such as plasma excitation power and frequency, reactor geometry, gas pressure, and flow rates. The need to understand and control the mass transport mechanisms at the growing surface has become very clear, and convincing evidence has been demonstrated that back-etching or "surface scrubbing" can be important. This effect can be used to selectively scavenge undesirable phases in the growing film. The influence of these mechanisms on film properties will be discussed. A brief description will also be given of the properties of different types of films available by this process, and a number of examples will be discussed that show how plasmas have been controlled to achieve desired properties for these films.
There is considerable evidence that thin film morphology, which includes both the film top surface and internal void network structure, is a key link between film preparation processes and resulting film characteristics and properties. Quantitative preparation-property relations can only be obtained after a realistic morphology model is developed in sufficient detail. In this study we consider morphology in the context of our evolutionary structure zone model and introduce several methods for quantifying recorded images of the morphology.
Problems of light scattering in multilayer dielectric thin films arise from non-uniform variations in optical indices. The purpose of this work was to evaluate the surface roughness and interfaces in zirconia/silica multilayers with respect to scattering. Scatter and surface roughness were evaluated with angular resolved scatterometry (at 633 nm) and stylus profilometry. The multilayer thin film interfaces and growth morphologies were characterized with transmission electron microscopy using thin cross sections prepared by ultra-microtomy. Multilayer designs made of thick zirconia layers showed both a higher scattered intensity and scattering at higher angles than those designs composed of thinner zirconia layers. Stylus profilometry data agreed with this trend, indicating that surface roughness is determined more by the zirconia layer thickness than by the number of layers. Transmission electron microscopy (TEM) provided an explanation for this behavior. Zirconia was observed to nucleate and form a vertical columnar microstructure on silica from which evolved a coarse, dendritic growth morphology. The silica, on the other hand, seemed to moderate the contribution of the zirconia microstructure to the multilayer surface texture.
Submicrometer dielectric films have been analyzed by Raman spectroscopic techniques with respect to phase composition and homogeneity. Residual film stress has also been evaluated from measured vibrational band shifts in sol-gel as well as sputter-deposited films using available data from bulk studies of the pressure dependence on Raman frequencies. Dynamic studies of temperature and laser-induced phase transformations have been performed in situ, suggesting the applicability of this technique for film analysis during deposition. The potential advantages and limitations associated with Raman characterization of thin dielectric films will be addressed in this review using examples from current research in this area.
We measure the thermal impedance of an interface in a thin metallic film using picosec-ond transient thermoreflectance. Our measurements on sputtered nickel-copper (Ni-Cu), nickel-molybdenum (Ni-Mo), nickel-titanium (Ni-Ti), and nickel-zirconium (Ni-Zr) indicate that the thermal impedance of the interface is directly related to the atomic size mismatch of the constituent elements. The interface decreases the thermal diffusion in the film compared to that measured for the constituent single element films.
Adhesion and strain are two important physical properties which determine the success or failure of thin-film coatings in optical applications. In this paper we describe the design and operation of a dynamically loaded scratch tester for making measurements of relative adhesive strength, and a modulated transmission ellipsometer for measuring total and internal strain. Numerous examples for coating/substrate systems of interest to optical applications are given.
ZnS-Ge and ZnSe-Ge thin film mixtures formed by codeposition (analog) and by sequential layering of alternate materials (digital) were characterized for stress and crystalline structure. The stress in sequentially deposited films varied linearly with composition between the stress values of the pure materials (compressive for ZnS and ZnSe and tensile for Ge). The stress in codeposited ZnS-Ge films deviated from linearity near the tensile-compressive crossover composition and at the ZnS-rich end. Films with 5 - 25% by weight (w/o) Ge composition had lower stress and much higher crystallinity than pure ZnS films. The crystallinity of ZnSe-Ge films on the other hand decreased upon addition of the Ge while the stress exhibited a slight increase at - 10 w/o Ge. These results have significant implications for the design of gradient-index thin film structures.
Designers of optical multilayer coatings commonly assume that the individual layers will be ideally homogeneous and isotropic. In practice, it is very difficult to control the conditions involved in the complex evaporation process sufficiently to produce such ideal films. Clearly, changes in process parameters, such as evaporation rate, chamber pressure, and substrate temperature, affect the microstructure of the growing film, frequently producing inhomogeneity in structure or composition. In many cases, these effects are interdependent, further complicating the situation. However, this process can be simulated on powerful, interactive, and accessible microcomputers. In this work, we present such a model and apply it to estimate the influence of an inhomogeneous layer on multilayer performance. Presently, the program simulates film growth, thermal expansion and contraction, and thickness monitoring procedures, and includes the effects of uncertainty in these parameters or noise. Although the model is being developed to cover very general cases, we restrict the present discussion to isotropic and nondispersive quarterwave layers to understand the particular effects of inhomogeneity. We studied several coating designs and related results and tolerances to variations in evaporation conditions. The model is composed of several modular subprograms, is written in Fortran, and is executed on an IBM-PC with 640 K of memory. The results can be presented in graphic form on a monochrome monitor. We are currently installing and implementing color capability to improve the clarity of the multidimensional output.
Several independent techniques have been explored for the measurement of optical constants of a single film on a glass substrate. One technique, inspired by Abeles, is based on comparing the reflectance of coated and uncoated sections of a substrate under "s" and "p" polarized light until a balance is found. This method is effective over a wide range of film thicknesses. Another technique is an envelope method based on an inhomogeneous model. To begin, an assumption is made of reflectance and transmittance on a spectrophotometer. The index is determined from measurements of the apparent refractive index . If the index and the thickness of the film are large enough to allow waveguide transmission, then the most direct means of measuring film index or thickness of the film is by determining the effective index "N." With a prism coupler, "N" can be found by measuring the coupling angle.
Oxide films on glass substrates, both single and multilayers, were produced by a special reactive ion plating process in the new automatic plating system BALZERS BAP 800. Structure, optical and mechanical film properties have been examined as a function of the deposition parameters. Multilayer systems consisting of TiO2 and Si02 films were deposited, and tests were made concerning optical characteristics, reproducibility and stability. Because of the unique characteristics of the films ion plating promises to become the new technology for the production of optical coatings.
Ion-assisted deposition (IAD) has been used to deposit magnesium fluoride, cryolite (Na3A1F6), and lanthanum fluoride films on ambient temperature substrates. IAD magnesium fluoride coatings show increased density and moisture resistance, while IAD lanthanum fluoride coatings exhibit increased refractive indices. Cryolite films, however, do not significantly benefit from IAD. Oxygen, rather than argon, as the bombarding species reduces bombardment-induced absorption in the visible, but may produce an additional oxide absorption edge in the ultraviolet.
A Technique For Monitoring Optical Thin Film Thickness By Direct Evaporation Onto The End Of An Optical Fiber Is Described. The Optical Fiber Is Suspended Inside A Coating Chamber So That The Fiber Distal End Faces The Evaporation Source And Is Illuminated With A Chopped Light Source. The Light Which Is Reflected From The Fiber End Is Returned Through The Fiber And Is Collectyed By A 45° Mirror With A Hole In It. The Signal Is Detected And Displayed On Led Displays And A Chart Recorder. Multiple Fibers Are Used To Monitor Individual Layers In A Stack, Or A Single Fiber Is Used After Cleaving The End With A Tool To Produce A Clean, Flat, Uncoated Surface Without Breaking The Vacuum.
We have investigated the properties of magnesium fluoride films made using ion assisted deposition, with argon or Freon as the source gas. The effect of the ion species and residual gases on the chemical, physical, and optical properties have been determined. For all films, the optical performance is strongly depen-dent on the presence of residual oxygen-bearing gases. For the Freon bombarded films the incorporation of carbon is not seen to degrade their transparency, and a more stoichiometric Mg:F ratio is acheived.
A conventional box coating chamber has been equipped with three dc magnetron sputtering cathodes, reactive gas (02, N2, H2) pressure and flow control, a planetary substrate holder, and optical monitoring to produce a low-temperature batch coating system for multilayer optical coating fabrication using oxides, nitrides, semi-conductors, and semitransparent metals. The system realizes the benefits traditionally derived from this geometry: maximum area of uniform coating thickness, spatially averaged coating flux impingement angles, and precise thickness monitoring. The use of sputtering cathodes adds the benefits of increased flexibility in choice of materials deposited, increased adatom energy at the substrate, and increased chemical reactivity in a gas discharge. Dense, durable, fully reacted dielectric coatings are achieved at low processing temperatures. Metal targets (sources) are used for ease of fabricatton, and dc power avoids radio frequency (rf) impedance matching requirements. In this paper, the chamber and components are described; operation of the cathodes is explained; and thickness uniformity profiles, deposition rates, and substrate temperature data are related. A list of materials investigated to date is presented with pertinent optical properties and intrinsic mechanical stress values. Illustrative performance examples for multilayer coatings on glasses and plastics are included.
The effects of ultrasound-assisted deposition (UAD) of ZrOx , Ta0x , and MgFx films on their optical properties have been investigated. The direction of vibration is transverse to the direction of growing films on substrates that are glued to tubular piezoelectric transducers driven by a power amplifier. Results indicate that ultrasonic powers above 20 W/cm2 are required to induce detectable changes in UV absorption and vacuum-to-air shifts of spectral profiles. UAD is likely to induce oxygen and fluoride deficiencies in oxide and fluoride films and increase the packing density of films. No significant changes between UAD and conventional films were observed in x-ray diffraction analysis, humidity testing, and Nomarski microscopy.
Properties of metal oxide thin films deposited using ion assisted deposition were studied. Previously, we reported increased values of film refractive index, reduction in iqical scatter and changes in film crystalline phase for coatings deposited using ion assisted deposition.1,2 In this report we extend the previous work, and present results illustrating the effects of energetic bombardment of the growing film during deposition of Ta205, Al203 and Si02. The optical constants, optical scatter characteristics, environmental stability and stress measurements of these films are reported. In addition, coatings were deposited onto heavy metal fluoride substrates at low (150°C) temperature using ion assisted deposition. Results illustrating improvement in substrate environmental durability and abrasion resistance are reported.
The free electron laser generates harmonic radiation at high intensity in a photon energy region which has been shown to produce absorption in high performance mirror coatings. I review the degradation measurements, and discuss the hypotheses concerning the nature of the mechanism. The available experimental options for discovering the mechanism and measuring the scaling relations are considered.
The use of narrow band filters (NBF) to demonstrate various architectures for optical computing is currently of interest. We are performing a systematic search of standard narrow band filter/substrate designs for arrangements optimized with respect to several criteria; we are concerned with the variation of switching power and time with spacer layer thickness, overall reflectivity, and substrate thermal conductivity. Our coatings are evaporated in a dedicated box coater and are optically monitored with the help of a remote laser. Work on design and uniformity is in progress.
Optical communication systems need non-polarizing edge filters for the construction of wavelength division multiplexer (WDM). We report about our investigations to produce such filters by using the material combination Si02/Si. The advantage of this combination is the high refractive index difference allowing the production of steep edges. On the other hand,the silicon layers could only be made fairly non-absorbing by depositing them onto heated substrates (300°C) and subjecting them afterwards to an additional heat treatment. So these layer systems can up to now not be deposited onto cemented optical fiber components.
Numerical optimization studies have led to a new multilayer design for a narrow bandpass filter. Its unique feature is the absence of traditional spacer layers. Electric field intensity profiles indicate that it is similar to a double cavity but the spacer layers are defined by pairs of low- and high-index layers forming partitioned cavities. Other partitionings have been studied. It is shown that the partitioning configuration can control somewhat the electric field intensity profile, which should lead to higher transmission. Another advantage of partitioned cavity filters is that they are less sensitive to random thickness errors in fabrication and are amenable to an error compensation scheme. Some filters were fabricated using ellipsometric monitoring with error compensation including one which achieved 83% peak transmission and about 1% bandwidth.
The efficacy of high efficiency lamps can be improved by recycling the unused infrared energy. This is accomplished by reflecting the infrared portion of the emitted energy back onto the emitter with a hot mirror coating on the lamp envelope. The envelopes of these lamps operate at high temperatures so that special coating designs are used which function well at elevated temperatures. The coatings can also be used with other lamp applications, such as flashlamps and color correcting or selective lamps.
Calculations of the reflectance, transmittance, Kerr magneto-optical polarization rotation, and related constants are important in the design of multilayer thin-film media for magneto-optical disks for data storage. Different computation methods have been suggested in the literature for specific structures.1-3 We have established a powerful technique based on matrix methods and have written a program to calculate all the optical responses of multilayer structures. Calculations for a few specific models have been made, and these predict Kerr rotation angles > 2° for normal incidence. These calculations can be readily extended to different media structures. Oblique incidence calculations are also being incorporated. We feel that disk media designs can be analyzed very effectively with this technique.
Electrically conductive (EC) and Infrared (IR) transparent thin metal film coatings for the 3 - 5 um and 8 - 12 um wavebands have been developed. Surface resistivity from 5 to 200 ohms/square have been studied. A transmittance value as high as 90% at 10.6 um has been achieved. The performance of these developed coatings are compared with the characteristics of the few IR transparent EC coatings reported in the literature. The fundamental difficulties in producing a highly transparent, wide optical bandwidth coating with low resistivity are discussed. The choice of a semiconductor or metal type EC coating are compared. The optical performance of these two types of IR EC coatings are calculated for equal electrical properties (surface resistivities). Induced transmission optical designs for both EC coating types are presented and the peak transmittance, and bandwidth achieved are compared. Measured results are compared with the calculated values. To date, the developed IR EC coatings have been vacuum deposited on ZnS, ZnSe, Ge, MgF2, Al203 and IR transmitting glass substrates materials. These coatings have also been deposited on a variety of flat optics and on a near hemispherical dome (9" diameter). Some present appli-cations and potential applications of these IR EC coatings are discussed. For example, a 3 - 5 um/millimeter wave aspheric beamsplitter has been produced. Electromagnetic Inter-ference (EMI) shielding of a MgF2 dome with a 3 - 5 um IR EC coating has been accomplished and verified by measurements.
In the Kretschmann arrangement of coupling light into surface plasmon resonances, the base of a prism is coated with a metal film and the light is incident on that film internally. If the thickness of the film is correctly chosen, then light incident at particular angle will be coupled completely into the plasmons. Thus coupling leads to a narrow, deep dip in the internal reflectance of the prism base as a function of incidence angle. Any change in the character of the metal surface results in measurable changes in the incidence angles; thus, the phenomenon is an extremely sensitive indicator of surface conditions. In particular, the effect can detect and monitor very small amounts of surface contamination. The metal layer can also be overcoated with a thin dielectric layer, so that changes induced in contamination of the latter can be observed. The surface plasmon phenomenon and the experimental arrangement employed for contamination detection will be described; experimental results on residues left by strippable coatings will also be presented.