Nearly Monoenergetic Positive Ion Beam with Self-Neutralized Space Charge Extracted from a Pulsed Plasma and Its Application
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Space charge neutralization of an ion beam extracted from a plasma is crucial for advanced plasma processes which require precise control of the ion flux and the width of the ion energy distribution (IED). In previous studies, filaments thermionically emitting electrons were used for neutralizing the space charge, which would otherwise cause the ion beam to diverge owing to Coulomb explosion. However, the performance of the neutralizing filaments is restricted by their limited lifetime and required extra power supplies. This study reports that a positive ion beam with a self-neutralized space charge can be extracted from a pulsed plasma. In particular, a nearly monoenergetic ion beam was realized by applying a synchronous DC bias to a “boundary electrode” in the afterglow (power-off) of the plasma. A retarding field energy analyzer (RFEA) and a movable Faraday cup were used to measure the IED and the time resolved ion and electron current densities, as a function of position along the beam axis, to unravel the mechanism of self-neutralization of the space charge of the ion beam. When a +100 V DC bias was applied to the boundary electrode during the afterglow, a ~100 eV beam (ion density ~10^(9) cm^(-3)) emerged that was space-charge neutralized by periodic injection of electrons, mainly during the early afterglow, into the region downstream of the extraction grid. A background plasma (electron density ~10^(10) cm^(-3)) was formed by these electrons, in conjunction with relatively low-energy (1-10 eV) ions that exited the inductively coupled plasma (ICP) source when the DC bias was off, as well as formed by resonant charge exchange of the 100 eV beam ions with the background Ar atoms, in the downstream region near the extraction grid. The behavior of the beam IED as a function of pressure and power of the ICP as well as the timing of application of the DC bias voltage was also studied. Highly anisotropic reactive ion beam etching of polysilicon masked with SiO2, has been demonstrated using a monoenergetic Ar+ ion beam (generated by a pulsed ICP) in the presence of Cl2 gas. Vertical sidewall profiles and smooth flat bottom surfaces were achieved. Specifically, the sample was placed 2 cm downstream of the ion extraction grid and electrons effusing from the pulsed plasma (mainly during the early afterglow) neutralized not only the positive ion space charge, but also the surface of the silicon dioxide mask. Thus, etching artifacts that have been reported in the literature due to insulator surface charging did not appear in the present system.