Recent Advances in Hollow Cathode Technology for Plasma-Enhanced ALD—Plasma Surface Modifications for Aluminum and Stainless-Steel Cathodes

Abstract

Recent designs have allowed hollow cathode gas plasma sources to be adopted for use in plasma-enhanced atomic layer deposition with the benefit of lower oxygen contamination for non-oxide films (a brief review of this is provided). From a design perspective, the cathode metal is of particular interest since—for a given set of conditions—the metal work function should determine the density of electron emission that drives the hollow cathode effect. However, we found that relatively rapid surface modification of the metal cathodes in the first hour or more of operation has a stronger influence. Langmuir probe measurements and hollow cathode electrical characteristics were used to study nitrogen and oxygen plasma surface modification of aluminum and stainless-steel hollow cathodes. It was found that the nitridation and oxidation of these metal cathodes resulted in higher plasma densities, in some cases by more than an order of magnitude, and a wider range of pressure operation. Moreover, it was initially thought that the use of aluminum cathodes would not be practical for gas plasma applications, as aluminum is extremely soft and susceptible to sputtering; however, it was found that oxide and nitride modification of the surface could protect the cathodes from such problems, possibly making them viable.

Downstream Electric Field Effects during Film Deposition with a Radio Frequency Plasma and Observations of Carbon Reduction

“…A brief description of the MEAglow growth system used is warranted. The most prominent feature of the system is that it has a stainless-steel hollow cathode plasma source, of the type recently used to greatly reduce oxygen contamination in ALD-grown films [22,23]. This was used to provide active nitrogen species in an ultra-high vacuum (UHV) growth chamber with load lock.…”

“…This was used to provide active nitrogen species in an ultra-high vacuum (UHV) growth chamber with load lock. There have been a number of papers [5,18,23] that have characterized this specific plasma source, which is actually the earliest prototype for some 80 other hollow cathode plasma source units that now operate around the world. However, as explained by Butcher, Georgiev and Georgieva [23], that specific design was retired from production early on in favor of designs that covered greater pressure ranges and larger deposition areas.…”

“…There have been a number of papers [5,18,23] that have characterized this specific plasma source, which is actually the earliest prototype for some 80 other hollow cathode plasma source units that now operate around the world. However, as explained by Butcher, Georgiev and Georgieva [23], that specific design was retired from production early on in favor of designs that covered greater pressure ranges and larger deposition areas. The plasma works as a remote radio-frequency (RF) source (13.56 MHz), with the substrates positioned in the afterglow of a nitrogen plasma.…”

“…The plasma works as a remote radio-frequency (RF) source (13.56 MHz), with the substrates positioned in the afterglow of a nitrogen plasma. The pressure range for hollow cathodes is determined by the hole dimensions, where electrons are confined to generate the hollow cathode effect, as well as by the material of which the cathode is made (see reference [23]). Smaller holes are used for higher pressures.…”

Reducing the β-Ga2O3 Epitaxy Temperature to 240 °C via Atomic Layer Plasma Processing

“…Atomic layer deposition (ALD), a technique based on self-limiting surface ligand exchange reactions, has been gaining great interest in the last decades, in large part because it enables low-temperature thin-film growth with atomic-scale precision, ultimate 3D conformality, and wafer-scale uniformity. − In particular, plasma-enhanced atomic layer deposition (PEALD) has played a critical role in advancing the low-temperature growth of a wide range of crystalline semiconductor films. − Ga 2 O 3 films have been successfully grown at low substrate temperatures (typically less than 300 °C) via both thermal and plasma-enhanced ALD. − Although most of the ALD studies in literature report as-grown amorphous Ga 2 O 3 films, − recently, there have been a few publications on producing as-grown crystalline films at substrate temperatures ranging 190–295 °C on sapphire substrates via PEALD. − However, these films still lacked higher crystal quality due to either having partial amorphous phases or a mixture of varying Ga 2 O 3 polymorphs (α and β mixture). − Thus, Rafie Borujeny et al needed to employ a short post-deposition thermal annealing of their 190 °C as-grown film at 550 °C to obtain a pure β-Ga 2 O 3 phase on sapphire . More recently, we have reported on the first-time achievement of as-grown crystalline single-phase β-Ga 2 O 3 films on Si and glass substrates at a low temperature of 200 °C via PEALD .…”