Plasma enhanced chemical vapor deposition(PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin filmsystems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasmasystems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others.

Plasma deposition of optical films and coatings: A review

The WIND imaging interferometer (WINDII) was launched on the Upper Atmosphere Research Satellite (UARS) on September 12, 1991. This joint project, sponsored by the Canadian Space Agency and the French Centre National d'Etudes Spatiales, in collaboration with NASA, has the responsibility of measuring the global wind pattern at the top of the altitude range covered by UARS. WINDII measures wind, temperature, and emission rate over the altitude range 80 to 300 km by using the visible region airglow emission from these altitudes as a target and employing optical Doppler interferometry to measure the small wavelength shifts of the narrow atomic and molecular airglow emission lines induced by the bulk velocity of the atmosphere carrying the emitting species. The instrument used is an all-glass field-widened achromatically and thermally compensated phase-stepping Michelson interferometer, along with a bare CCD detector that images the airglow limb through the interferometer. A sequence of phase-stepped images is processed to derive the wind velocity for two orthogonal view directions, yielding the vector horizontal wind. The process of data analysis, including the inversion of apparent quantities to vertical profiles, is described.

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WINDII, the wind imaging interferometer on the Upper Atmosphere Research Satellite

Ion beam assisted deposition, the bombardment of a thin film with a beam of energetic particles during deposition, provides a powerful technique for modifying the microstructure and properties of thin films and coatings. Various experimental approaches used for ion beam assisted deposition are described and the physical basis for the effects is examined. Observations on modification of nucleation and growth behaviour, microstructure development, compound synthesis, and applications to the modification of properties such as intrinsic stress, adhesion, surface mechanical properties, corrosion and oxidation resistance, optical properties, and electrical properties, are reviewed.

Use of ion beam assisted deposition to modify the microstructure and properties of thin films

Kinetics of bone cell organization and mineralization on materials with patterned surface chemistry.

Pulsed laser deposition (PLD) technique is used to deposit pure and Al-doped ZnO thin films at different temperatures on glass substrates. From the transmission data from optical spectroscopy the band gap energy Eg of the films is derived. The dependences of Eg on the deposition temperatures of the pure and Al-doped ZnO films are different. The band gap energy of the pure ZnO increases and saturates with temperature. However, Eg of Al-doped ZnO shows an exponential decrease. Refractive indices of 1.9–2.1 in the VIS are determined by the spectroscopic ellipsometry (SE). Photoluminescence (PL) data reveal the strong near band emission by increasing the deposition temperature. # 2003 Elsevier Ltd. All rights reserved.

Band gap energy of pure and Al-doped ZnO thin films

A versatile method for determination of the optical constants is described that can be applied to a variety of coating materials. It is based on the use of an optical thin film synthesis program to adjust the constants of dispersion equations until a good fit is obtained between measured and calculated spectral transmittance and/or reflectance curves. The sensitivity of the determination can be increased by a suitable combination of measurement quantities. Because more than the minimum amount of data can be used, sensitivity to measurement errors and the chances of obtaining multiple solutions can both be reduced. To illustrate the method optical constants are determined of MgF2, ZnS, MgO, Inconel, and Si films in the visible part of the spectrum and of ITO films in the 0.4–12.0-μm range.

Determination of optical constants of thin film coating materials based on inverse synthesis

After a brief description of the threats which face the issuers of documents and valuable papers, the concept of thin film anticounterfeiting coatings proposed by the NRCC is explained. The colorimetric considerations which govern the design of such devices are reviewed. Typical coating designs suitable for use in transmission and reflection are discussed. Reflection coatings can be constructed with and without an integral black absorber coating. The need for geometric patterns in the anticounterfeiting coating is explained. Various methods for producing such logos are proposed. Mention is made of the various numerical tools developed for the design and investigation of single films and multilayers, their desensitization to construction errors, and for the investigation of different process control strategies. During the duration of the project, experiments were performed on the optical constants and nonoptical properties of a number of metal and dielectric films considered for use in the anticounterfeiting coatings. These and some of the processes and equipment investigated during the development of a prototype production facility are reviewed. Finally, some security and economic aspects of the devices are considered.

Research on thin film anticounterfeiting coatings at the National Research Council of Canada

A hard to simulate, readily distinguishable from counterfeits, optical interference authenticating device comprises a substrate, and two optical interference coatings in interfacial contact on one side of the substrate, one of the optical interference coatings being a contrast coating, the other of the optical interference coatings being a form depicting coating for depicting a form relative to the contrast coating. The optical interference coatings each comprising at least one optical interference layer the material and thickness of which has been selected for the coatings to have different, particular, known spectral reflectance and spectral transmittance characteristics from one another when viewed at a particular angle, such that the form depicting coating is visible by reflected or transmitted light of particular coloration, when viewed at the particular angle, at least in part by optical interference of light partically reflected or transmitted at the interfacial contact. A further optical interference contrast coating may be provided on the substrate and the form depicting coating may be sandwiched between the contrast coatings. The substrate may be transparent, semi-transparent or opaque, depending how the spectral characteristics are to be observed. A marginal edge portion of the form depicting coating may taper in thickness to provide a multicolored auriole around the form. A plurality form depicting coatings may be provided which overlie one another to provide a more complex pattern.

Form depicting, optical interference authenticating device

An ion-assisted evaporation process is described for the formation of ITO films on substrates that cannot be heated beyond 150° C. Refractive indices and absorption coefficients were measured in the visible part of the spectrum for some of the films. The resistivity of the films varied between 0.7 and 8 × 10−3 Ω cm, depending on the process parameters. The properties were uniform over 25-cm diam areas.

Transparent, conducting indium tin oxide films formed on low or medium temperature substrates by ion-assisted deposition.

This paper reviews the different types of merit functions that have been used in the past in optical thin film calculations. Attention is drawn to the power of merit functions which operate on complicated quantities that require one or more integral expressions for their definition. To prove this point, several thin film problems are solved in which the CIE coordinates, luminous transmittances or reflectances, solar absorptance, and blackbody emittances of a multilayer are specified.

F. C. Ho

Papers

Determination of optical constants of thin film coating materials based on inverse synthesis

Research on thin film anticounterfeiting coatings at the National Research Council of Canada

Form depicting, optical interference authenticating device

Transparent, conducting indium tin oxide films formed on low or medium temperature substrates by ion-assisted deposition.

Merit functions for more effective thin film calculations.