Achieving Stable Results with Titanium Dioxide

Titanium dioxide has been a favored high index material by some, but it appears to be less popular with others because of difficulties in obtaining a stable result. We discuss what we think is the root of the problem and how it may be rectified. The need to have negligible absorption places a lower limit on the background oxygen pressure and some constraints on temperature and deposition rate. Our interpretation of reported results and our own experience is that the material being evaporated reaches an equilibrium with the background oxygen in the chamber after some period of being melted. The oxygen rich material is releasing excess oxygen until it comes to equilibrium, while the oxygen poor material must take up oxygen from the background to reach equilibrium. As a result, our philosophy and practice has been to achieve the most stable process practical by the techniques and material choices which we use in “melting-in” starting material. An equation derived from our experience and others’ is given for the index of refraction of titania as a function of background oxygen pressure, temperature, and deposition rate without ion-assisted deposition (IAD). The effects of IAD are also discussed.