Generation
and applications of a broad atomic oxygen beam with a high flux-density
via collision-induced dissociation of O2
Zhiqiang Hana,b,
Liying Songa,b,
Po-Wan
Shumb,d*, Woon-Ming Lau c,a,b,*
a Beijing Advanced Innovation Center for Materials
Genome Engineering, Center for Green Innovation, School of Mathematics
and Physics, University of Science and Technology Beijing, Beijing
100083, Chinab Shunde Innovation School, University of Science and
Technology Beijing, Foshan, Guangdong 528399, China
c School of Chemistry and Chemical Engineering,
Linyi University, Linyi, Shandong,
276000, China
d The Sun Age (Guangdong) New Energy Ltd, China
Corresponding Author
*Woon-Ming Lau. Email:leolau@lyu.edu.cn
*Po-Wan Shum. Email:samshum@thesunage.com
Abstract
We detail the generation of a pulsed
atomic oxygen (AO) broad beam with a high flux-density via
collision-induced dissociation of O2 to support
practical industrial exploitation of AOs, particularly for facilitating
2-dimenstional oxidation/etching at a fast rate of one-monolayer per
second in an area ≥1000cm2. This innovation fuses the
following interdisciplinary concepts: (a) a high density of
O+ can be produced in an electron-cyclotron-resonance
(ECR) O2 plasma; (b) O+ can be
extracted and accelerated with an aperture-electrode in the plasma; (c)
O+ with adequate kinetic energy can initiate a cascade
of gas-phase collisions in the presence of O2; (d)
collision-induced dissociation of O2 yields AOs with
adequate kinetic energy which can cause additional collision-induced
dissociation of O2. Computational simulations of such
collisions, with both ab initio molecular dynamics and direct simulation
Monte Carlo methods, are used to guide the experimental generation of
the proposed AO-beam. We experimentally demonstrate the highest known AO
mean flux-density of about 1.5×1016 atoms
cm-2 s-1 in a broad-beam, and use it
to oxidatively modify a self-assembled molecular layer of siloxane on a
silicon wafer. In addition, we also demonstrate the growth of
Al2O3 through an AO-assisted atomic
layer deposition process at a room temperature.
KEYWORDS. Collision, Molecular dynamics, Monte Carlo, Oxidization,
Atomic layer deposition
Introduction
Atomic oxygen (AO), generated by the
dissociation of molecular oxygen, exhibits high chemical reactivity and
has been extensively employed in investigating material oxidation and
corrosion mechanisms, as well as in developing novel oxidation processes
in the chemical industry.[1-6] Particularly, the generation of a
broad beam of AOs for the controlled growth of a metal-oxide monolayer
and the controlled oxidative-etch of an organic/carbonaceous
device-constituent is highly desirable to fuel the emergency of
2-dimensional structures with a monolayer accuracy in advanced
electronics, optoelectronics and other functional devices.[7-9] For
example, atomic layer deposition (ALD) of metal-oxide has indeed become
a crucial technique in many device-manufacturing processes but its
practical exploitation has been stalled by the difficulty in quickly
purging the water vapor which is used to oxidize the metal-oxide
precursor.[10-13] In principle, AO is more active than water vapor
and AO can be purged quickly. In the engineering design of an AO-beam
generator for practical manufacturing, the flux-density of AOs should be
considerably higher than 1015 atoms
cm-2 s-1 in order to support the
growth/etch of a monolayer-substance per second, with a broad beam
covering an area of not less than 300cm2. An option of
pulsing the AO-beam on and off every second further enables the precise
control of monolayer-wised oxide-growth and etching per second. Such an
AO-beam generator is ideal but has been generally conceived to be
technically infeasible.
Historically, the development of an AO broad-beam was first quested for
the facilitation of space-exploration. In brief, when space-crafts and
artificial satellites operate within low Earth orbit (LEO), chemical
reactions causing detrimental damage of such space-facilities occur
because the presence of O2 in LEO dissociates into AO by
solar ultraviolet irradiation and AO colliding with space-facilities
traveling at a sonic speed is violent and reactive. More specially, the
kinetic energy of such AO (about 5eV) indeed matches the molecular
chemical bond strength (3-5eV) of typical materials and an AO flux of
1015 atoms
cm-2 s-1 is sufficient to cause
collision-induced chemical-modification of one monolayer per
second.[14] As such, the research in this specialized field has
fostered the development of AO-beam equipment with controllable kinetic
energy up to 5eV and high throughput (1015 atoms
cm-2 s-1). This, in turn, can
facilitate the investigation of novel oxidation mechanisms and
applications by regulating the kinetic energy and throughput of AOs.
However, the development of high-throughput AO-beam equipment is stalled
by numerous technical limitations and difficulties.[14, 15] First,
although AOs can be readily produced by pyrolysis, photodissociation and
plasma-reactions, the kinetic energy of AOs thus produced is
uncontrollable and typically in a tame region of a few meV. [16-20]
Technically, hyperthermal AOs with kinetic energy up to 10eV can be
generated by the well-established molecular-beam technology via
high-pressure-jet acceleration of O2, with
laser-dissociation of the accelerated O2.[20, 21]
Practically, this costly method can typically give an AO-beam with a
cross-sectional diameter not more than 1mm, and is thus suitable for
fundamental research but not for engineering of industrial
exploitations. In this context, industrial engineering has turned to the
adoption of the electron-cyclotron-resonance (ECR) plasma technology
which is known to facilitate the most intensive ionization-induced
dissociation of molecules in gas phase, including the dissociation of
O2 into AOs and oxygen ions.[22] Oxygen ions thus
generated are extracted from the plasma and then accelerated by an
electric field to regulate their kinetic energy. They are finally
converted to AOs with the desirable kinetic-energy by collision and
charge-transfer with a metal-plate mounted near the exit of the oxygen
ion beam extracted from the ECR plasma. This technology has indeed been
used to produce an AO-beam of about 5eV in kinetic energy, with a
cross-sectional beam diameter of about 20cm. The AO-beam size is limited
by the practical size of an ECR plasma source. This known demonstration
of a practical broad AO-beam with a controllable kinetic energy has,
nevertheless, the following deficiencies: (a) the AO flux density is
limited by the ion flux density and even an ECR plasma can only support
a maximum ion flux density of about 1x1015 ions
cm-2 s-1 and a beam of about
300cm2; (b) ion neutralization by charge-transfer with
a metal-plate inevitably leads to ion-sputtering and metal-contamination
of the AO-beam; (c) the AO-beam must be used close to the ion-exit from
the plasma in order to prevent a severe flux-density drop caused by
broadening of the ion beam and AO-beam. The placement of a
workpiece-specimen close to a plasma which gives strong ultraviolet
radiation and excited atoms/molecules, however, risks detrimental
damages of the specimen.
Here, an innovative and practical
method is reported to overcome the deficiencies of this AO-beam
technology. The present innovation articulates the science and
technology of converting a broad
ECR-based oxygen-ion beam with a flux of 1×1015 ions
cm-2 s-1 to a broad AO beam with a
flux of 1.5×1016 atoms cm-2s-1 via collision-induced dissociation of
O2 into AOs, as shown in Figure 1 . In this AO
generator, one ion injection with a kinetic energy of 300eV initiates
many collision cascades in which on the average about 130eV of the 300eV
injection-energy is used to drive about 24 O2dissociations each costing a dissociation energy of about 5.1eV. Since
ion extraction can be easily pulsed, the resultant AO flux can also be
engineered with a pulsing frequency not less than one on/off cycle per
second. The objective is the facilitation of industrial exploitation of
AO surface-modification and AO-assisted metal-oxide deposition, to
realize the rising demands of layer-by-layer
surface-modification/deposition with a controlled operation of one
atom/molecular monolayer per second, with a scalable
modification/deposition area of ≥1000cm2. In the
following sections, the
fundamental concepts of AO generation by gas-phase dissociative
collisions initiated by oxygen ions extracted and accelerated from an
oxygen plasma are explained
by ab initio molecular dynamics
(AIMD) of the kinematics in dissociative
O+-O2, O-O2 and
O2-O2 paired-collisions, particularly to
pin-point the threshold kinetic energy required for O2dissociation in different collision-impact trajectories.
With these critical attributes,
direct simulation Monte Carlo (DSMC) method of the generation of a
high-flux AO-beam by extracting O+ from a plasma into
a low-pressure O2 atmosphere are detailed.
The simulations are then
translated into the engineering designs of a practical AO-beam
generator, as shown in Figure 1 , with an AO flux density more
than 1016 atoms cm-2s-1 and a beam ≥1000cm2 at the
proximity of the generator exit, in a pulse-operation mode with a
pulsing frequency as high as one on/off cycle per second.
The generator comprises an array
of 2-by-2 ECR plasma sources, with a total power of 800W. These
functional features are demonstrated and validated.
More specifically, the oxidative
modification of a self-assembled molecular layer of alkyl silane on the
surface of an oxidized silicon wafer is also explained. Further, the ALD
of alumina with a fast rate of one-deposition cycle per 2 seconds is
articulated. The supplementary
information (SI) provides further elaboration on the details.
Simulations