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The Chemical Products
of Ionic, Molecular, and Cluster Beam Deposition
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Motivations
- Better understanding of the chemical processes
that occur during molecular cluster-surface collisions
- Predictions about growth of strongly adhering
thin films
- Improved understanding of how thin-film
nucleation and growth from beam deposition is affected by reactions
conditions such as impact velocity and beam configuration (as molecules
or molecular clusters).
- Improved understanding of effect of reaction
conditions on thin-film nucleation and growth
- Determine mechanisms involved in film growth via
cluster beam deposition
(a) Modification of Polystyrene
Substrate: Hydrocarbon vs. Hydrocarbon Ions
i) C3H5+
at 50 eV vs. CH3+ at 20 eV
- C3H5+
dissociates into fragments that are embedded within the polystyrene
matrix 65% of the time.
- CH3+ scatters from the
surface 20 % of the time (sometimes taking a surface H atom with it).
The rest of the time it embeds within the surface without dissociating.
- Despite the fact that the energy/atom is nearly
the same in the last two cases the processes that occur on impact are
very different and depend heavily on the composition and unique
chemistry of the incident ion.
ii) C3H5+
vs. CH3+ at same energies
- At the same incident energy of 20 eV CH3+
and C3H5+ show similar distributions
of scattering and penetration into the surface
- At the same incident energy of 50 eV both ions
dissociate most of the time. However, C3H5+
dissociates into a much wider variety of species than CH3+
iii) C3H5+
Isomeric Ions at 50 eV
- Differences in dissociation products that grow
the thin film as a result of changes in the structure of the incident
ion:
- when the incident structure contains CH3
pieces, CH3 are predicted make up 11-13 % of the products;
otherwise, CH3 makes up only ~ 1 % of the products
- CH3-CH=CH+ is easiest to
dissociate while the CH2-CH+-CH2 is
the most difficult to dissociate
iv) Energy Transfers
- Scattering from the surface
- CH3+: 84% incident energy
transferred to polystyrene, 11% remains ion kinetic energy, 5%
transformed into ion internal energy
- C3H5+: 95%
incident energy transferred to surface, 2% remains ion kinetic energy,
3% transformed to ion internal energy
- Embedding in the surface
- CH3+ and C3H5+:
97-98% incident energy transferred to the polystyrene and 2-3%
transformed to ion internal energy
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(b) Modification of Polystyrene
Substrate: Hydrocarbon vs. Fluorocarbon Ions
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1) System Setup
The system
contains about 9000 atoms with 52 Å × 30 Å × 56
Å in dimension. In order to control the temperature, thermal
state atoms is set around the interested deposition region. During the
deposition the system is controlled at 300 K. Four kinds of ions are
used in the deposition, C3H5+, CH3+
and C3F5+, CF3+.
The fluence is comparable to the experimental conditions. The
deposition energy is 50 eV/ion.
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2) The depth profile of
incident ions
Different
incident ions and conditions produce different depth profiles. Ions
that easily react with the substrate will not penetrate the substrate
to as great an extent. In addition, ions with smaller incident
velocities and larger sizees will similarly penetrate only a short
distance insto the substrate. Thus, C3H5+
ions tend to aggregate together and mostly stay near the surface. In
contrast, CH3+ ions penetrate more deeply.
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3) Density of species
Hydrocarbon ions
are more reactive then fluorocarbon ions, since most of these ions and
their fragments bond with the polystyrene substrate. For C3H5+
ions, the majority species is CnHm, where n >
3 and m > 5. This tells us that C3H5+
ions tend to aggregate together.
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4) Percentage of intact
polystyrene chains
The ions can be ranked as
follows from those that produce the larged modification of the surface
to the least amount of modification: CH3+, CF3+,C3F5+
and C3H5+.
| Depth (Å) |
C3F5+ |
CF3+ |
C3H5+ |
CH3+ |
| 0 - 7 |
0 |
25 |
0 |
0 |
| 7 - 14 |
25 |
25 |
25 |
0 |
| 14 - 21 |
75 |
25 |
100 |
25 |
| 21 - 28 |
100 |
100 |
100 |
0 |
| 28 -35 |
100 |
100 |
100 |
100 |
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5) Percentage of intact phenyl
rings
CH3+
ions modify the phenyl rings to the greated extent.
| Depth (Å) |
C3F5+ |
CF3+ |
C3H5+ |
CH3+ |
| 0 - 7 |
22.7 |
27.3 |
9 |
4.5 |
| 7 - 14 |
86.4 |
36.4 |
40.9 |
4.5 |
| 14 - 21 |
100 |
60 |
90 |
20 |
| 21 - 28 |
100 |
100 |
100 |
31.8 |
| 28 -35 |
100 |
100 |
100 |
50 |
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(c) Modification of
Diamond (111) Substrate by Organic Molecular Cluster Deposition
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Growth of thin films under beam
impact
- When system energy is less than 3 eV/molecule of
molecular bond energy thin film nucleation and growth occurs.
- Effect of cluster size, incident velocity and surface
reactivity on film nucleation explored.
- Processes predicted to occur during experimental film
growth include surface cratering, adhesion of chemical products to
surface, partial sputtering of film, and continued growth.
Beam Impacts
- The simulations provide details of how changes in
reaction conditions influence thin film growth.
- Mechanisms of cluster-beam deposition include
polymerization, adhesion, surface penetration and sputtering.
- Heavier clusters cause more surface damage.
Cluster vs. Molecular Beam
- Effect of cluster size, molecular reactivity, surface
temperature and surface reactivity on thin-film nucleation through
molecular cluster deposition explored. Mechanisms of cluster-beam
deposition include polymerization, adhesion, surface penetration and
sputtering.
- No significant differences predicted for molecular beam
and molecular cluster beam deposition.
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