Ion Effects in Optical Films
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ION EFFECTS IN OPTICAL FILMS U. J. Gibson Thayer School of Engineering, Dartmouth College, Hanover NH 03755 ABSTRACT Ion bombardment during growth of thin films has been shown to be a powerful technique for alteration of a wide variety of film properties from index of refraction and stoichiometry to density and abrasion resistance. A brief review of the deposition processes and ion effects of relevance to the production of optical films is presented. Application of the technique to some particular problems in films with both optical and protective roles, and the use of ion beams to alter the chemical composition and hence index of films will be discussed. Both homogeneous and spatially non-uniform coatings will be discussed, including generation of multilayer filters and gradient index structures in waveguiding films. INTRODUCTION The optical properties of films are of paramount importance for optical applications, but coatings are generally required to fulfill a number of other requirements in terms of environmental stability, mechanical strength, adhesion, etc. The use of ions during production of these films has been shown to have dramatic effects on a wide range of properties. Dc and rf sputtering, remote plasma discharges, deposition of films by direct deposition from ions or ionized clusters, ion beam sputtering (IBS), and ion-beam assisted deposition (IBAD, or lAD) have all been used for modification and improvement of optical thin films. The use of broad beam ion beam sources has been particularly emphasized in research, since the energy and flux of the beam can be varied over wide ranges, and is decoupled from the actual vaporization process. A number of comprehensive reviews on the use of ion beams for deposition have been published in recent years [1-8]. Early work on the use of hollow cathode ion sources [9,10] indicated the value of low energy (20-100eV) ion processing for optical thin films. Mechanical properties could be improved, and optical absorption reduced, by the addition of oxygen ions during deposition of materials such as SiO 2 and A12 0 3 . With the advent of the Kaufman-type broad-beam hot cathode ion guns (originally developed as space thrusters) the effects of energy vs. ion current, and more materials systems were investigated. Independent control over the voltage and current permits optimization of the bombardment parameters for each particular application. In general, high energy bombardment results in both chemical alteration and physical rearrangement of the structure, while at low energies, the effects tend to be chemical. For most optical applications, the bombarding energy should be kept below about 1 keV (per ion) in order to avoid generating optically absorbing defects. The low end of the energy range is usually limited by the capabilities of the sources, which do not generate much current at low energies. Most work has been done above 50eV for this reason. For each material system, optimum values of the bombardment energy and flux must be determined, since there is often a tra
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