Achieving Improved Optical Thin Film Control and Uniformity of Silicon Dioxide by Using Ion Assisted Deposition

We have found that the deposition of critical optical thin film stacks with silicon dioxide from an electron beam gun is severely limited by the stabilty of the evaporation pattern of the material. We had not obtained satisfactory results in some of the more demanding applications with either solid or granular starting material. The amount of material deposited on a central monitor chip or control crystal in a box coater did not have a reproducible ratio to that received at other positions in the chamber. We solved the problem by evaporating silicon monoxide from a resistance source with simultaneous oxygen Ion Assisted Deposition (IAD) using a Denton CC102R Cold Cathode Ion Source to produce silicon dioxide of high density and low absorption. The films are sufficiently uniform and repeatable over a full diameter calotte in a 760mm box coater. We describe here our observations of the operating characteristics of the ion source and the deposition scheme which we have found most satisfactory. INTRODUCTION, THE PROBLEM Figure 1 illustrates the variable distribution of silicon dioxide evaporant from an electron beam gun as is commonly experienced in physical vapor deposition. The “cloud” may be broad or narrow and not necessarily normal to the general surface of the material if there is “tunnelling”. This is explained by the erratic melting/sublimation of silicon dioxide surfaces in both granular and solid disk forms. In our box coaters, we have experienced significant difficulty in achieving satisfactory repeatablility of more complex coatings and sometimes even four layer antireflection coatings due to this problem. The properties of silicon dioxide such as its low index, clarity, weathering and abrasion resistance can otherwise be very desirable. We have therefore put forth extensive effort to retain these desirable properties while overcoming the limitations.