Plasmas have been used for some time to deposit coatings on optical elements such as lenses and filters, as well as for more mundane applications such as reflectors on automobile bumpers or highway signs. A newer use under development is the manufacture of multilayer optical fibers. Furthermore, a more glamorous and potentially more important application looming on the horizon is the fabrication of integrated circuits containing photonic elements—a necessary step in the move toward optical computing. To handle photons on a chip, a polymer such as polymethylmethacrylate (PMAA) can be spin-coated onto a patterned silicon wafer, and plasma deposition can be used to create a graded-index structure that can then be patterned and etched. This is obviously a new research direction with many problems to be overcome but with a large payoff.
One of the most commonly used plastics is methyl methacrylate (MMA), commonly known as Plexiglas or Lucite. In this material, the long polymer chains are arranged in a linear (not cross-linked) manner, resulting in a comparatively weak material. On the other hand, plasma polymerized methyl methacrylate (PPMMA) has a dense, highly cross-linked structure and can be used to strengthen the surfaces of plastic containers, textiles, or even metal automobile parts. This can be done by exposing the surface to an MMA or HMDSO plasma. Another advantage of the plasma polymerization process is the retention of the original monomer structure, whereas other processes tend to change it. It is not understood how the cross-linked structure is formed, what the precursors are, and what plasma parameters will optimize the process. Though there is not yet any commercial application of this process, it is clear that further research may lead to the improvement of many manufactured products.
Deposition of new materials is a suitable use for the excellent laboratory ECR equipment in NRL's ion/plasma processing group. As mentioned above, the development of a low-permittivity dielectric for integrated circuits is a challenge—such a development would be a significant contribution to ULSI technology. Formation of III-V compounds such as GaN, using ECR as a deposition source, may well be a rich field of investigation. Materials developed here could then be incorporated into unique circuits made in the fabrication line of the Surface and Interface Sciences Branch in the Electronics Division. Production of such one-of-a-kind devices could justify the maintenance, though not the upgrading, of a fabrication facility at NRL.
The extensive diagnostics capabilities at NRL in the Chemical Vapor Processing Group and the Surface and Interface Sciences Branch laboratories could be applied to the problem of understanding the precursor radicals in the deposition and polymerization processes. Though it has great potential, plasma polymerization is not well understood, and NRL 's personnel are capable of making significant progress in this direction also.