22T3
Plasma-Surface Interactions for Materials Nanofabrication and Space
Propulsion: A Unified Study for Technology Developments and Future Progress
プラズマ・表面相互作用の研究展開：ナノ加工プロセスと宇宙航行推進
Kouichi Ono
斧 高一
Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyoto University
Unit of Synergetic Studies for Space, Kyoto University
Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
京都大学工学研究科・宇宙総合学研究ユニット，〒606-8501 京都市左京区吉田本町
A better understanding of plasma-surface interactions is indispensable for utilizing plasmas in hand. This
paper presents an overview of the evolution of our understanding of several phenomena associated withplasma-surface interactions in the fields of materials nanofabrication and space propulsion. A numericalapproach to reproduce plasma-surface interactions and profile evolution relies on a continuum model, discrete or Monte Carlo-based model, and also classical molecular dynamics simulation, to promote a unifiedstudy of the interactions for future technological developments.

1. Introduction
Plasma-surface interactions can occur in a
number of research and application fields ofplasma: materials processing, lighting, display, actuator, welding, controlled nuclear fusion, andspace development. The interactions involve a
number of physical and chemical issues of enormous complexity that occur on surfaces, such as
surface chemistry of neutrals, ion-solid and radiation-solid interaction physics, materials science, andplasma physics of the sheath. It is often assumed
in low-pressure plasmas that the positive ions enter
the surface after being accelerated through thesheath thereon, while neutrals tend to enter the surface isotropicallly or thermally without being influenced by the sheath. Moreover, the short-wavelength radiation incident on the surface often causes
some modification of surfaces as well as photoelectron emission therefrom. The phenomena of interest have evolved from being macroscopic orphenomenological to microscopic or atomistic.
This paper presents an overview of the evolution of our understanding of several phenomena
associated with plasma-surface interactions in thefields of materials nanofabrication and space electric propulsion, with emphasis being placed on aunified study for technological developments andfuture prospects. In materials processing [1], theplasma-surface interactions on substrates are indispensable to achieve the processing (etching, deposition, and modification), while the interactions onchamber walls and other nearby surfaces are required to be understood, to control the concentration of ions and neutrals in the plasma. In electricor plasma propulsion system [2], the interactions ofpropellant ions with chamber walls and electrodes
erode them over time, which tends to limit the lifetime of thrusters in operation.
2. Materials Nanofabrication
Dry or plasma etching is an indispensable
processing technique in the fabrication of modern
microelectronic devices. Increasingly strict requirements are now being imposed on plasma etching technology, as ultra-large-scale integrated(ULSI) circuit device dimensions continue to bescaled down to << 100 nm [3−5]. The requirements include the precise or nanometer-scale control of etched profile, critical dimension (CD),roughness, and their microscopic uniformity (oraspect-ratio dependence), together with that of etchrate, selectivity, and damage.
In front-end-of-line (FEOL) processes, thenanometer-scale control of Si etching in chlorineand bromine-based plasmas is indispensable for thefabrication of gate electrodes and shallow trench
isolation, through suppressing profile anomalies ofsidewalls and bottom surfaces. In practice, sidewall anomalies, such as tapering, bowing, footing(or corner rounding), and notching, largely affect
the CD and its difference between isolated anddense patterns and also between p- and n-type gates.
The roughness or ripple on sidewalls is also a critical issue to be resolved, because the resulting lineedge roughness (LER) and line width roughness(LWR) on feature sidewalls are responsible for varied gate widths and thus the variability of transistor
performance. Anomalies of bottom surfaces, suchas microtrenching and roughness (or residue), affectthe uniformity of bottom surfaces and lead to the10 nm

100 nm

1 nm

Bottom-up approachTop-down approach
VLSI

0.1 nm (1 A)

ULSI

Nanoelectronics

Conventional model
MD Simulation(String, Level-set,Atomic-scale cellular model (ASCeM)
Cellular)
UV,VUVBr containing plasma

PlasmaNeutralElectron Ion (Radical)+Plasma
Fluid
PIC/MCDSMC

SheathPlasmaReactor
Wafer stage

Cl containing plasma
Cl+ O SiClxOyClSiClx

Substratesurface,interface
Substrate

SiCl4Cl+Clenergetic BrBr

Vacuum

Mask
SiClxOy
SiClxSiSolid (Si)SiSiO2 film

2.719 Å32.6 Å

32.6

ÅFig.1. Modeling and simulation of plasma-surface interactions in materials nanofabrication.

recess and damage in gate fabrication.
Moreover, new materials are also being required to be precisely etched in integrating theminto device fabrication [3−5]. In FEOL, recent
efforts have been made to replace gate silicon-oxides and -oxynitrides by high dielectric constant (k>20) materials such as HfO2, ZrO2, and theirsilicates and aluminates, where highly selective
etching of high-k over underlying Si is indispensable for their removal prior to forming source anddrain contacts. For the fabrication of high-k gate
stacks, gate electrodes of conventional poly-Si arebeing required to be replaced by metal gates such as
TiN, TaN, Ru, Pt, and Ir, to suppress some problemof the depletion layer present in doped poly-Si.
Chlorine- and bromine-based plasmas are also often
employed in etching of these high-k and metal electrode materials.
In these situations, the modeling and simulation of plasma-surface interactions is required for abetter understanding of the physics and chemistry
underlying the phenomena observed, as well as fora theoretical design of plasma reactors and processes with less experimental efforts [5,6]. The
model is divided in four areas or categories: aplasma reactor or equipment model (including thesheath region on reactor chamber walls and substrates), model for plasma-surface interactions forblanket substrate surfaces and chamber walls,
model for plasma-surface interactions for smallfeature surfaces (model for profile or topologicalevolutions), and model for electrical characteristics
and damages of substrate materials and interfaces.
A Monte Carlo-based atomic-scale cellular model(ASCeM) is available to simulate the feature profile
evolution on nanometer scale during processing,and a classical molecular dynamics (MD) simulation is also available to clarify the profile evolution
as well as surface reaction kinetics on atomic scale,as shown in Fig, 1.

3. Space Propulsion
Small spacecraft has recently attracted increasing attention in space technology to reduce theoverall mission costs and increase the launch rates,
which in turn requires miniature or micro thrusters
including micro electric propulsion systems or micro plasma/ion thrusters [7,8]: DC current microarcjet thruster, DC microplasma thruster, microwave-excited microplasma thruster as shown in Fig.
2, micro Hall thruster, micro ion thruster, and dielectric capillary discharge acceleration using gas
fuels; field emission electric propulsion and colloid
thruster using liquid fuels; vacuum arc microthruster, micro laser-ablation plasma thruster,
micro pulsed plasma thruster, and ferroelectric
plasma thruster using solid fuels. In these systems,
the surface-to-volume ratio of microplasma sources
is relatively high, and so the particle and energy
loss to chamber walls is severe, thus resulting insignificant plasma-surface interactions to erode thechamber walls. The erosion is most significant ataround the end of the antenna in Fig. 2, where theplasma density is calculated to be high. The numerical model consists of an electromagnetic module for microwave propagation in interacting withplasmas and a fluid module for plasma flows withtwo (electron and heavy particle) temperatures.
MetalGas

DielectricInsulator

L = 10 mm

MicrowaveAntenna
PlasmaGas
Coaxial cable

PlasmaD = 0.2 mm(1 μm) 1000 nm

D = 1.5 mm

Plasma source

NozzleFig.2. Microwave-excited microplasma thruster ofelectrothermal type.
References[1] M. A. Lieberman and A. J. Lichtenberg: Principles of
Plasma Discharges and Materials Processing, 2nd ed.
(Wiley, New York, 2005).
[2] G. P. Sutton and O. Biblarz: Rocket Propulsion Elements, 7th ed. (Wiley, New York, 2001),[3] H. Abe et al.: Jpn. J. Appl. Phys. 47 (2008) 1435.
[4] International Technology Roadmaps for Semicon
ductors (ITRS) 2007 Edition, 2008 Update[http://www.itrs.net][5] K. Ono et al.: Thin Solid Films 518 (2010) 3461.
[6] H. Tsuda et al.: Jpn. J. Appl. Phys. 50 (2011)
08JE06; 50 (2011) 08KB02.
[7] T. Takahashi et al.: Phys. Plasmas 16 (2009)083505 ; 18 (2011) 063505; J. Appl. Phys. (submitted).
[8] Y. Takao et al.: J. Appl. Phys. 108 (2010) 093309.