PNNL-13501 - Pacific Northwest National Laboratory

Hybrid Plasma Process for Vacuum Deposition of Transparent Conductive Oxide Films
Study Control Number: PN00052/1459
John D. Affinito
Transparent conductive oxide films are needed for solar cell electrodes, flat panel display electrodes, solar-thermal control
window films for buildings and greenhouses, and various military stealth applications. A new hybrid plasma process is
being developed for vacuum deposition of transparent conductive oxide thin film coatins on low temperature substrates
(below 70°C).
Project Description
A new plasma processing capability has been studied at
Pacific Northwest National Laboratory. It involves a
focused, two-phase project to develop and reduce to
practice a hybrid plasma deposition process for the
vacuum fabrication of high quality transparent conductive
oxide coatings on low temperature substrates. Plasma
diagnostic and process monitoring instrumentation may
be integrated with the vacuum deposition chamber,
process hardware, and a process monitoring and control
strategy. For each phase, film quality may be assessed by
a wide variety of ex situ optical, electrical, and surface
science techniques. Hall and four-point probe
measurements may be used to determine conductivity,
carrier concentration, and mobility. Crystallinity may be
determined by x-ray diffraction, scanning electron
microscopy, atomic force microscopy, and step-height
profilometry. The plasma frequency, free carrier collision
time, and effective mass may be determined by optical
transmission and reflectance measurements in the near
ultraviolet, visible, near infrared, and infrared portions of
the spectrum. Chemical composition of the coatings may
be assessed by spectroscopy.
Introduction
Plasmas have a wide variety of materials processing and
manufacturing applications. Some of the most attractive
plasma processing technology applications include
production of nano-particle powders, surface treatments,
plasma-assisted vacuum thin film deposition, and plasma
only reduced pressure-to-vacuum thin film deposition.
Nano-particles have application in drug delivery,
improved starting materials for sintering processes and
high specific surface area materials for catalysis,
wastewater treatment, and isotope separations. Plasma
surface treatments may be used to improve wetting and
adhesion of subsequent coatings or to provide controlled
functionalization of surfaces for purposes of
biocompatibility, sensing, and a variety of separation and
320 FY 2000 Laboratory Directed Research and Development Annual Report
treatment processes. Plasma, and plasma assisted, thin
film deposition processes are increasingly being proven to
provide higher density films with improved crystallinity
at higher deposition rates than is attainable with
conventional evaporation and sputtering techniques. One
major advantage in the use of plasmas in surface
treatments and coatings is the ability of the plasma to
provide a great deal of surface energy with relatively
modest bulk heating of the substrate being treated or
coated. As well, in contrast to the use of ion beams,
surface damage due to kinetic bombardment can be
significantly reduced with plasmas since plasma sources
do not rely on high voltage extraction of the ions to
present the plasma to the substrate.
The hybrid plasma process for this study involves a closecouple
output flux of a high density, forced flow, oxygen
plasma source with the metal atom output flux from one
or more radiatively heated precision thermal evaporation
sources. The maximum deposition rate achievable, while
maintaining high quality highly transparent and highly
conductive transparent conductive oxide films, is
dependent on both the degree and the intimacy of the flux
mixing, as well as the degree of ionization of the mixed
oxygen-metal atom flux. Metal atom source development
is based on recent specialized, and proprietary sources for
thermal vacuum evaporation deposition of lithium metal.
The metal atom sources are designed and built as part of
this work.
Several existing plasma sources must be tested using
modified versions of hollow cathode, helical resonator,
in-line hollow cathode plasma electron emitter sources,
and a customized helical resonator with a pre-ignition
stage. The plasma sources are evaluated with respect to
film quality, deposition rate, and ease of integration with
the metal atom sources.
In addition to design, fabrication, and integration of
specialized plasma and metal atom sources, the
achievement of the project goals requires development of