Abstract

We describe the current approaches for depositing high performance multilayer (ML) coatings on large area figured optics by using dc magnetron sputtering. The highest normal incidence reflectivity of 66% at 130 Å has been achieved by using ML composed of alternating layers of Mo and Si. The optimum structure, corresponding to smooth pure layers separated by narrow interlayers of mixed composition, is obtained by sputtering at low Ar pressure of 1.75 mTorr. In magnetron sputtering at low Ar pressure the energy flux is provided by the reflected Ar neutrals. However, we have observed that high quality ML can also be grown at higher Ar pressure (10 mTorr) by applying a dc bias to the substrate, thereby extracting Ar ions from the plasma that bombard the growth surface. Excellent coating uniformity has been achieved by implementing a combination of large area sources, small source-to-substrate separation, and compound planetary motion of the substrates. This approach requires no masking to generate a uniform deposition profile, which significantly simplifies the accommodation of different targets, substrates, and deposition conditions. Furthermore, since the intrinsic deposition profile is uniform, simple masks can be used to produce axisymmetric variations in the ML period when laterally graded coatings are required.

© 1992 Optical Society of America