3.2 Estimations of the AO Flux-Density generated by the AO
Generator
In the present work, the well-known Kapton mass loss method for
measuring AO flux densities is adopted. In addition, a
self-assembled monolayer (SAM) of
ethyl trimethyl siloxane (ETS) formed on an oxidized silicon wafer is
exposed to the AO beam of the AO generator for testing the AO oxidative
behavior. Both the changes in hydrophilicity by water contact angle
measurements (WCA) and surface composition by X-ray photoelectron
spectroscopy (XPS) are characterized as a function of AO exposures. The
experimental details are included in SI.
Results and Discussion
The experimental results from the Kapton mass loss measurements and the
WCA measurements are included in Figure S5 . In brief, the
Kapton mass loss measurements confirm that for the injection of 300eV
O+ at a flux density of 1×1015 ions
cm-2 s-1 into the tube chamber of
the AO generator, a broad AO beam with a flux density of 1.5
×1016 atoms cm-2s-1 is detected at a distance of 6cm from the exit of
the tube chamber. The Kapton mass loss measurements also confirm that
the AO fluxes at the specimen-placement plane of 6cm from the exit plane
are uniform with a change of less than 10% within the central area of
30cm in diameter. Since the tube chamber has a diameter of 30cm, the
diameter of the broad AO beam at 6cm outside the tube exit is expected
to be about 36cm, due to random scattering of AOs. Thus, the AO flux
density right at the tube exit is expected to be 2.2
×1016 atoms cm-2s-1 and the AO flux is 1.53×1019atoms s-1. Since the ion flux density of
1×1015 ions cm-2s-1 is injected into the tube, with an injection
diameter of 20cm, the ion flux is 3.14 ×1017 ions
s-1. Therefore, on the average, 48.7 AOs are generated
by one 300eV O+ and roughly about 24 collision-induced
dissociations of O2 are driven by each ion injection.
As for the exploitation of AOs, a drop in WCA of 20ois evidence with one second of the AO beam-pulse onto the
C2H5-SAM. The surface oxidation is also
revealed with the C1s XPS spectral analysis shown in Figure 5(a). In
this set of data, the sample prior to any AO exposure should have a
surface composition of SiO2 terminated by
-C2H5 and should thus show a C 1s
spectrum of those carbon atoms in -C2H5.
The trace amounts of spectral signals of C-O, C=O and O-C-O chemical
species are attributed to the practically inevitable adsorption of
residual carbonaceous contaminants such as lubricating oils in the
non-perfect ultrahigh vacuum environment during XPS analyses. A
comparison of the spectral data with and without an AO dosage of
1.5×1016 atoms cm-2 clearly shows a
significant increase of the amounts of C-O, C=O and O-C-O chemical
species on the AO-exposed sample. Thus, the carbonaceous contamination
merely compromises but does not mask out the detection of AO-oxidation
of the -C2H5 chemical group.
Nevertheless, a quantitative analysis of the degree of oxidation of the
-C2H5 chemical group is impossible due
to this carbonaceous contamination. The drop in WCA of
20o, however, firms up the effectiveness of AO surface
oxidation of a hydrocarbon surface/film.
Atomic layer deposition (ALD) of aluminum oxide is widely used as a
protective encapsulant. Typically, one cycle of such an ALD process
comprises a half cycle of TMA adsorption and a subsequent half cycle of
TMA oxidation. Commonly, a flux of water molecules is used to complete
the half cycle of TMA oxidation within a fraction of a second, but the
complete purge of the residual water molecules is slow and typically
requires a duration of 10 seconds or more due to the high surface
sticking efficiency of water. In the present work, the flux density of
1.5×1016 AOs
cm-2s-1 is expected to be sufficient
for the completion of TMA oxidation in less than one second and the
purge of AOs is virtually instantaneous. A trial run of ALD, with one
second of TMA exposure followed by one second pulse of AOs, for 50
cycles indeed yields the deposition of about 7nm of aluminum oxide on a
copper disc. The XPS data summarized in Figure 5(b) confirms
the fast deposition of aluminum oxide at room temperature with
AO-assisted ALD. The effect on the passivation of a copper disc against
thermal oxidation is demonstrated by the experimental evidence
summarized in Figure S5 .
Conclusion
In the present work, AIMD simulations offer clear atomistic snapshots of
collision cascades induced by hiring O+ with a kinetic
energy of 0-300eV to O2. Dissociative collisions of
O2 and generation of AOs are evident, and the
dissociation threshold requirement of the O+ kinetic
energy can be as low as 14eV which critically depends on the exact
collision configuration. Through the examination of some 200 collision
configurations, a comprehensive picture of collision-induced
O2 by hyperthermal O+ is obtained. The
database is then used to facilitate DSMC simulations of AO production in
the practical experimental design of a high-flux AO generator equipped
with an ECR oxygen ion source powered to 800W for the extraction of an
ion flux density of 1×1015 ions cm-2s-1 with an extraction area of about
300cm2, and with the ions fired into a drift tube
(30cm in diameter) containing O2 at 0.14Pa. In essence,
DSMC simulations confirm each 300eV O+ bullet
initiates cascades of collisions with some of them having collision
energy higher than the dissociation thresholds estimated by AIMD
simulations. As such, a microscopic mechanistic portrait of AO
generation is yielded. Subsequent experimental AO flux density
measurements by the well-known Kapton erosion method gives a
quantitative estimate of 49 AOs generated by one 300eV
O+ injection. Since the dissociation energy of
O2 is about 5.1eV, on the average, about 122eV of the
kinetic energy carried by each O+ bullet is used to
drive collision-induced dissociations of 24 O2, with the
rest of input energy raising the kinetic energy and
vibrational/rotational energy of each collision partners to an average
of roughly 0.18eV which is significantly higher than their average
energy in thermal equilibrium at room temperature. As such, the
experimental AO broad-beam with a beam-size of 1000cm2and a record-high flux density of 1.5×1016 atoms
cm-2 s-1 bears an average energy of
0.18eV for each AO or an AO temperature of slightly higher than
1100oC. When a self-assembled molecular layer of ethyl
trimethyl siloxane on a silicon wafer is exposed to this AO beam for one
second, the water contact angle drops by 20o and XPS
surface analysis also confirms the surface oxidation of the hydrocarbon.
In addition, the same AO-beam has also been demonstrated to facilitate
the atomic layer deposition of Al2O3with a record-fast growth rate of 7nm per 100 seconds. In comparison,
the typical growth with water vapor to replace AO as the oxidizing agent
has a typical growth rate of 7nm per 500-1000 seconds. The present work
thus articulates the science and technology of practically generating a
high-flux broad-beam of AOs and demonstrates some applications of such a
technology innovation.