Characteristics of Hydrogen Negative Ion Production depending on Electron Energy Distributions in Inductively Coupled Plasmas

Abstract

The goal of the RF driven hydrogen negative ion (H- ion) source driver developments is to promote the generation of H- ions and their precursors for a large extracted H- ion beam current with a high RF efficiency. Without full understanding the underlying physics of the hydrogen plasma chemistry and the inductively coupled plasma (ICP), it cannot comprehend phenomena in RF H- ion source drivers, then cannot also achieve this goal. To understand the underlying physics, the electron energy distribution function (EEDF) and H- ion generation in low-pressure inductively coupled hydrogen plasmas is investigated using both theoretical and empirical approaches. A global model was developed to investigate the densities of H- ions and other species in a low-pressure inductively coupled hydrogen plasma with a bi-Maxwellian EEDF. Compared to a Maxwellian plasma, bi-Maxwellian plasmas have higher populations of low-energy electrons and highly vibrationally excited hydrogen molecules that are generated efficiently by the high-energy electrons. This leads to higher reaction rates of the dissociative electron attachments responsible for H- ion production. The model indicated that the bi-Maxwellian EEDF at low pressures is favorable for the creation of H- ions. The dual frequency antenna ICP was developed to bi-Maxwellize the EEDF by controlling the driving frequency-dependent collisionless heating. The dual frequency antenna ICP consists of a 2 MHz-driven solenoidal antenna wound around a cylindrical chamber and a 13.56 MHz-driven planar antenna placed on the top of it. Compared to the conventional single frequency antenna ICPs, the dual frequency antenna ICP reveals two distinctive characteristics, i.e., an increase in the power transfer efficiency and the bi-Maxwellization of the EEDF due to the collisionless heating. These characteristics allow the dual frequency antenna ICP to accomplish the enhanced generation of H- ions and their precursors with a high RF efficiency. In addition, the source pulsing for the enhancement of the volume H- ion production was investigated by introducing the newly devised time derivative of EEDF – electron energy characteristic. The experimental result shows that H- ion density in the after-glow is about 17 times of that in the active-glow. It was found that this is due to the electron cooling in the after-glow and the long lifetime of highly vibrationally excited molecules.