Electron Beam Generated Plasmas for Ultra Low T_eProcessing

Abstract

The Naval Research Laboratory (NRL) has developed a processing system based on an electron beam-generated plasma. Unlike conventional discharges produced by electric fields (DC, RF, microwave, etc.), ionization is driven by a high-energy (∼ few keV) electron beam, an approach that can be attractive to atomic layer processing applications. In particular, high electron densities (10 10 -10 11 cm −3 ) can be produced in electron beam generated plasmas, where the electron temperature remains between 0.3 and 1.0 eV. Accordingly, a large flux of ions can be delivered to substrate surfaces with kinetic energies in the range of 1 to 5 eV. This provides the potential for controllably etching and/or engineering both the surface morphology and chemistry with monolayer precision. This work describes the electron beam driven plasma processing system, with particular attention paid to system characteristics and the ability to control the generation and delivery of ions to the surface and their energies. Electron beam generated plasmas are produced by injecting a highenergy electron beam into a gas background, which will ionize, dissociate, and excite atoms or molecules as it traverses the gas volume. While the basic inelastic processes that lead to species production are the same as those in discharge plasmas, the use of energetic electron beams to drive production results in plasmas that have very different properties than conventional discharges. Some of these properties are attractive for plasma-based atomic layer processing applications where, whether etching, depositing, or chemically modifying materials, fine control over the flux and energy of ions is needed. In the case of atomic layer etching, perhaps the single most important need is to tightly control the kinetic energy of ions incident to the processing surface so as to avoid damage while maintaining a reasonable etch rate. [1][2][3] In this regard, a significant advantage of beam-driven plasmas is the inherently low electron temperature T e , which is typically a fraction of an eV in most gas mixtures of technological interest. Importantly, this is true regardless of the plasma density. Thus, one can produce a large fluence of reactive ion and neutral species where the kinetic energy of the ions is as low as a few eV.The interest in electron beam generated plasmas for materials processing can be traced back at least 4 decades. In the early 1970s Bunshah 4 described an electron beam vapor deposition system that utilizes an electron beam to both vaporize the metal and "activate" the background gas. It was also noted by Dugdale 5 that high-energy electron beam systems developed for welding could be employed for "soft vacuum vapor deposition" in a manner similar to that of Bunshah. In the 1980s, Collins and co-workers at Colorado State University, developed electron beam produced plasmas for plasma enhanced chemical vapor deposition of SiO 2 . 6,7 This system used a sheet-like beam of multi-keV electrons injected parallel to the growth substrate. A similar configurat...

Plasma-based chemical functionalization of graphene to control the thermal transport at graphene-metal interfaces

“…However, the resulting species densities and energies can be significantly different [21] due to the large difference in energies of the electrons that drive production. In typical discharges, species are produced by a low energy distribution of plasma electrons (T e  few eV) that have been heated via an applied electric field to overcome the elastic and inelastic electron collisions that reduce their energy, such that some fraction of the resulting electron population is energetic enough to sustain the plasma through ionization.…”

“…The production of species via energetic electron beams can be written as, (2) where J b is the beam current density, N i is the number density of the parent molecule or atom, σ i is the cross section for species production (for a given beam energy), and k is a proportionality coefficient. [21] There are several important implications derived from Eq.…”

“…Plasma-based processes are readily scalable to meet the demands of wafer-scale fabrication and thus, when paired with large-area graphene growth techniques, [17,18] provide a path toward industrial-level production of graphene-based electronics. While many different plasma sources have been used to functionalize or process graphene [19,20], electron beam generated plasmas are wellsuited for graphene processing as they are capable of providing large fluxes of very low energy ions, [21] which is an important factor in avoiding the damage to delicate structures often associated with conventional discharge plasmas [22]. Thus, with the appropriate choice of operating conditions, the type and amount of functional groups can be introduced at the metal-graphene interface and used to regulate the cross-plane thermal conductance.…”

Surface treatment by the ion flow from electron beam generated plasma in the forevacuum pressure range

“…Variation of electron beam current and energy, as well as composition of gaseous atmosphere, enables variation of plasma density and temperature. Due to low temperature of electron component and relatively high concentration beam plasma is used for etching of thin (monoatomic) surface layer [6][7].…”

Correlating charge fluence with nanoparticle formation during in situ plasma synthesis of nanocomposite films

“…In addition, the use of few keV electron beams favor the production of ions over metastable and other excited species, and leads to a very low electron temperature plasma. This yields a processing environment dominated by large fluxes of very low energy ions and electrons . Keeping the Ar + ion kinetic energies below 5 eV eliminates the etching and damage sometimes associated with conventional discharge plasmas .…”