Ion implantation and optical devices

Abstract

Ion beams can be used to change many surface properties, such as chemical stability, stress, electrical conductivity, energy gap, defect energy states or refractive index. The depth scale accessible with small ion implanters (i.e. 0–400 kV) is a few microns and therefore modification of thin films or controlled etching is possible. For optical components in integrated optic circuits ion beams can be used to define mask patterns, etch surface features, develop planar lenses and write optical waveguides. Many of the techniques are identical to those applied to semiconductors. However, properties of specifically optical interest are the refractive index, electro-optic coefficient and optical absorption. Localized index changes can be introduced by the addition of specific impurity ions of selected polarizability or by changes in lattice density from energy deposited from the ion beam. In glass systems the relaxation of the network may enhance or reduce the index, depending on the details of the structure and the implanted impurity. For crystalline solids radiation damage dominates and the trend is to form lower index amorphous systems. The optical absorption associated with the radiation damage may be annealed without loss of control of the index. Alternatively the colour centres may be utilized for luminescence or photoconductive properties. Electro-optic properties are generally degraded by the original implantation and optically active elements are best defined by implantation in the regions adjoining the waveguides. Several simple electro-optic components have been formed by ion implantation and these will be discussed. The development of complex devices and the full exploitation of ion beams with thin film targets for optical systems is still in its infancy, but the great potential of these combined techniques is already apparent.