Sputtering Promises Lower Temperatures, High Speed

Dr. Tuckerman Moss

If you are truly excited by optical production equipment, the biannual Optatec trade shows are the place for you. Nestled among this year's surface generators, planetary polishers and OEM optical components was a new sputtering system from Satis Vacuum Industries Vertrieb AG of Baar, Switzerland.

Sputtering is an attractive alternative to the more conventional thermal physical vapor deposition of optical coatings because it can be accomplished at much lower substrate temperatures -- typically no higher than 50 °C -- and the resulting layers exhibit higher packing density and greater film hardness. And sputtering is fast: The Satis system has a mean sputter rate of about 3 nm/s and a full evacuation time of less than six minutes, resulting in a cycle time for a conventional antireflection coating of about 15 minutes.

The Satis system uses one doped silicon target, which is usually DC-pulse-discharged in argon. Many thin-film systems can be created with this 150-mm-diameter target by injecting selected reactive gases into the plasma during coating. For example, adding oxygen produces a high-purity SiO₂coating with an index of 1.47; adding nitrogen would raise the index value to 2.05. As a result, users can control the refractive index of a film by varying the reactive gas rather than by using different targets for each layer. The growth rates are so constant that the film thickness can be determined as a function of time.

Controlling the reactive gas mixture enables the user to realize thin-film systems that cannot be produced in any other way. Moreover, variable index gradients can be produced by injecting different reactive gases in a timed manner. This eliminates the need to build a multilayer optical filter system by depositing a series of homogeneous layers. Rather, a user lays down a monolithic layer of continuously varying refractive index, a method known as Rugate design.

There are several advantages to producing filters in this manner:

* Because there are no discrete boundaries between high- and low-index layers, internal layer
tension is reduced, and there is less cracking and layer detachment.
* Moisture diffusion at boundary layers is eliminated because there are no boundaries.
* Thresholds for high-power laser damage are higher.
* Index gradients enable the design of broader-bandwidth filters.