Applications of electron-beam generated plasmas to materials processing

Nonequilibrium (T_e≫T_ion,gas) plasma processing often allows for greater process control with reduced environmental impact when compared to other materials processing methods, and therefore presents tremendous opportunities in the areas of thin film development and surface modification. The U.S. Naval Research Laboratory\´s "Large Area Plasma Processing System" (LAPPS) has been developed based on the high-energy (2 keV) electron-beam ionization process, with the goal of maximizing the benefits of plasma processing over large areas (∼1 m²). This system has been shown to be: 1) efficient at producing plasma in any gas composition; 2) capable of producing low-temperature plasma electrons (<0.5 eV) in high densities (10⁹-10¹² cm^-3); and 3) scalable to large area (square meters). In this paper, the progress of a number of applications using LAPPS is discussed. Nitride growth in stainless steel was investigated, which demonstrated high rates (over 20 μm/h¹2/) at low temperatures (≤462°C). Complementary mass spectrometry showed that the nitriding results correlated to the flux of atomic ions delivered to the substrate. LAPPS was also combined with magnetron sputtering sources to form hybrid systems for thin film deposition of nitrides. The simultaneous operation of LAPPS with a titanium magnetron sputter source increased the growth of the <200> orientation in TiN films, due to the increased atomic nitrogen ion flux. Polymer pretreatment studies were also initiated in these systems; polytetrafluoroethylene substrates pretreated with an oxygen LAPPS exposure demonstrated a significant increase in copper and aluminum film adhesion compared to untreated substrates, with the dominant factor believed to be the changed surface morphology. Similarly dramatic fluorination of polyethylene was demonstrated with plasmas generated in Ar/SF₆ mixtures.