Production of Reactive Species in Low-Temperature Hydrogen Plasmas

The use of hydrogen plasmas in the microelectronics industry and nuclear fusion makes their understanding of significant interest. Two reactive species of particular importance are negative ions and atomic hydrogen (H atom).
Negative-ion sources are applicable to neutral beam injection for magnetic confinement fusion. Dielectric materials, such as diamond, are being investigated as a potential alternative to low-work function metals for the surface production of negative ions. In this work, we employ photoemission yield spectroscopy (PYS) to determine the electronic structure and ionisation threshold of doped diamond, including phosphorus doped diamond, a novel material for negative-ion production, and graphite after deuterium plasma exposure (2 Pa, 150 W) at varying sample temperatures (30-700◦C). The ionisation threshold of conductive graphite increases with sample temperature while the negative ion production decreases, as measured by a magnetised retarding field energy analyser and mass spectrometry. In contrast, while the ionisation threshold of diamond is observed to change with sample temperature and the dopant used, this appears to have negligible effect on negative-ion production. This suggests that hydrogen retention and the bandgap are more influential.
We used two-photon absorption laser induced fluorescence (TALIF) to measure H-atom densities in two pulsed, radio-frequency (rf) plasma sources in pure H2. In an inductively coupled plasma (ICP), the temporal variation of the H-atom density in the afterglow demonstrates the significance of gas heating in the high-power-density systems. In a capacitively coupled plasma (CCP) configuration, spatially resolved H-atom densities near the powered electrode surface are linked with rf-phase-resolved optical emission structures of Hα (30-125 Pa, 10-125 W, 720-1025 Vpp). Differences in the spatial profiles of Hα emission and H-atom densities are explained by the diffusion of H-atoms on the time scale of their lifetime.
The increased knowledge and understanding of the production of negative ions and atomic hydrogen in low-pressure hydrogen plasmas is of interest for neutral beam injection systems and materials processing.