Transcript Slide 1

Ultra-Low Coverage Spontaneous Etching and Hyperthermal
Desorption of Aluminum Chlorides from Cl2/Al(111)
Tyler J. Grassman, Gary C. Poon, and Andrew C. Kummel
University of California, San Diego
Gordon Research Conferences: Dynamics at Surfaces – August 10-15, 2003
Abstract
Results
Non-resonant multiphoton ionization (MPI) and time-of-flight mass spectrometry (TOF-MS) have been
used to monitor the desorption of aluminum chloride (AlxCly) etch products from the Al(111) surface at
100 K and 500 K during low-coverage (< 5% monolayer) monoenergetic Cl2 (0.11 eV to 0.65 eV) dosing.
The desorption products in this low-coverage range show predominantly hyperthermal exit velocities
under all dosing conditions. For example, with 0.27 eV incident Cl2, the etch product was found to have a
most-probable velocity of 517 ± 22 m/s at an Al(111) surface temperature of 100 K. This corresponds to
22 times the expected thermal desorption translational energy for AlCl3. Sticking probability
measurements and AlxCly etch rate measurements show etching even at Cl2 coverages of less than 5%
monolayer at surface temperatures between 100 K and 500 K. These experimental results are consistent
with a combination of fast-time-scale and low Cl surface coverage formation of aluminum chlorides and
the presence of activated aluminum chloride chemisorption states having potential energies above the
vacuum level, and are contrary to the standard high-coverage picture of aluminum chloride etch requiring
high surface coverage (>1 monolayer) and subsurface Cl diffusion. Density functional theory calculations
yield results that are consistent with both our experimental findings and mechanistic descriptions.
Measured velocities, translational energies, and peak-to-width ratios for desorbing aluminum chlorides at
all incident Cl2 translational energies and Al(111) surface temperatures studied.
Dose Species Einc (eV)
DFT: Bonding to Al Adatoms
Ts (K)
vp (m/s)
EAlCl (eV)a
2
6
Cl2/He: lowc
0.65
100
612.3 ± 21.2
―
0.522 ± 0.037
0.65 ± 0.06
highd
0.65
100
255.5 ± 9.2
―
0.092 ± 0.007
0.84 ± 0.08
Cl2/He
0.65
500
653.1 ± 15.5
0.296 ± 0.014
―
0.56 ± 0.02
Cl2/Ne
0.27
100
517.7 ± 22.0
―
0.378 ± 0.032
0.82 ± 0.03
Cl2/Ne
0.27
500
552.9 ± 14.2
0.213 ± 0.011
―
0.76 ± 0.03
pure Cl2
0.11
100
460.3 ± 22.3
―
0.303 ± 0.029
0.69 ± 0.06
pure Cl2
0.11
500
533.3 ± 9.8
0.197 ± 0.007
―
0.71 ± 0.05
Cl2/He:
Bonded
vp:w
E Al Cl (eV)b
3
Non-bonded
DE = -2 eV
Cl
Al
Al(ad.)
DFT slab calculations of Cl adsorbates on the Al(111) surface near an Al adatom indicate a strong
energetic preference for Cl atoms to bond to Al adatoms rather than remain dispersed on the surface
(out of bonding range). This is likely to be the case for regrowth islands and step edges, as well. The
surface reaction, 3Cl(ad.) + Al(ad.) → AlCl3(ad.), is computationally found to be about 2 eV
thermodynamically favorable. These AlCl3 adsorbates are also likely be highly mobile on the surface, as
the surface binding potential for the molecules (with respect to the different possible adsorption sites)
was found to have a maximum corrugation of only 0.1 eV.
a: Translational energy calculated from mass of AlCl3 etch product and measured vp
b: Translational energy calculated from mass of Al2Cl6 etch product and measured vp
c: Total dosing time ≤ 10 sec
d: Total dosing time > 10 sec
Experimental
Methods
QMS
Al(111)
Experimental
800
AlCl3 (thermal)
700
Al2Cl6 (thermal)
(Ts = 100 K)
600
500
400
300
200
100
850
Experimental
800
AlCl (thermal)
700
AlCl3 (thermal)
600
500
300
200
100
0
1327
535
TOF-MS
LEED
(b)
(d)
0.7
Thermal
h
Al x Cl y n

Al  Cl  AlCl 
nh




 Al  Cl  AlCl

Exit Energy, Eexit (eV)
0.6

0.5
Thermal
(Ts = 100 K)
AlCl3 (exp)
0.6
Al2Cl6 (exp)
0.4
0.3
0.2
0.1
0.0
AlCl (exp)
0.5
AlCl3 (exp)
0.4
0.3
0.2
0.1
0.27
0.65
0.11
0.6
Observed Al2Cl6 TOF
0.5
Expected 100 K Al2Cl6 TOF
0.4
0.3
0.0
0
30
60
90 120 150 180 210 240 270 300
Time (msec)
(b)
1.0
(Inc. Cl2/Ne
Ts = 500 K)
0.9
Normalized Al+ Intensity (arb.)

 md 1 
1
S (t )
 
S t   4 exp 
0
2 
t
t
 2kT t 
0.1
0.8
m
kT
d
kT
v p  sl  2
tp
m
0.6
0.5
0.4
Observed AlCl3 TOF
1
E p  mv 2p  2kT
2
“most-probable translational
energy”
Expected 500 K AlCl3 TOF
0.2
0.1
0.0
0
10 20
30
40
50
60
Time (msec)
70
80
90 100
(a)
Maxwell-Boltzmann-like time-of-flight distribution curves for the etch products from the 0.27 eV incident Cl2 beam on
the (a) 100 K and (b) 500 K Al(111) surface. The solid curves show the experimentally observed desorption distribution,
and the dashed curves show the expected thermal desorption time-of-flight distribution for the etch product mass and
surface temperature of interest, as indicated in the figures. The most probable time-of-flights are indicated by the
vertical, single-headed arrows. The full-width half-max of the experimental distributions are indicated by the horizontal,
double-headed arrows.
1.0
500 m/s Al2Cl6 component
(Ts = 100 K)
0.9
Conclusion
(a)
(b)
0.8
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.7
0.6
0.5
0.4
0.3
Begin surface
Cl2 exposure
0.2
0
1
2
3
4
5
0
10
20
0.7
0.5
550 m/s AlCl3 component
(Ts = 500 K)
0.4
0.3
Begin surface
Cl2 exposure
0.1
30
40
Time (sec)
50
60
70
80
(b)
0.6
0.2
0.1
0.0
1.0
0.9
0.8
“most-probable velocity”
Thermal distribution should exhibit most-probably velocity
to width ratio (vp:w) of about 1. An experimental ratio
smaller than unity indicates a wider distribution than would
be expected for a purely thermal desorption mechanism.
0.7
0.3
d sl
2
“most-probable time of
flight”
2
sl
0.2
tp 
Etch Rate
Profiles
Al
Cl
DFT slab calculations of the Cl/Al(111) adsorbate/terrace system meant to examine the possibility of
adsorbate clustering do not show a thermodynamic preference for clustering geometries. However, the
calculations do indicate a strong preference for ontop adsorption sites, with differences in total energies
of -0.4 to -0.9 eV compared to other sites. Ontop-site Cl adsorbates are also found to pull the Al terrace
atom to which they were bonded out of the surface plane by 0.4 Å, thereby likely making them more
vulnerable to attack by other atomic or molecular adsorbates, and helping to replenish nucleation sites.
Energy (arb.)
0.7
Etch Rate (arb.)
0.8
Values extracted from experimental time-of-flight spectra
compared against values taken from expected thermal
distributions (flux-weighted Maxwell-Boltzmann for density
detector):
Etch Rate (arb.)
Normalized Al+ Intensity (arb.)
0.9
0.65
Etch product exit velocities and energies plotted against incident velocities and energies, respectively, for
surface temperatures of 100 K and 500 K. The open symbols (squares, circles, triangles, diamonds) represent
experimental data, while the filled symbols represent the expected values from a purely thermal desorption
mechanism. The data clearly shows that the etch products are exiting the surface at hyperthermal velocities.
Time-of-Flight
Distributions
(Inc. Cl2/Ne
Ts = 100 K)
0.27
Cl2 Incident Energy, Einc (eV)
Cl2 Incident Energy, Einc (eV)
1.0
(Ts = 500 K)
0.0
0.11
(a)
1327
0.7
Exit Energy, Eexit (eV)
Collimator
850
Cl2 Incident Velocity, vinc (m/s)
Cl2 Incident Velocity, vinc (m/s)
(Cl2/He, Cl2/Ne
pure Cl2)
Clustered
Dispersed
400
0
535
DFT: Bonding on Terraces
(Ts = 500 K)
900
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
10
20
30
40
exit kinetic energy
chemisorption well
0
1
2
3
4
5
Distance From Surface (arb.)
0.0
0
Standard Activated
Chemisorption Model
50
60
70
80
Time (sec)
Etch rate profiles of the 0.27 eV incident energy Cl2 on (a) the 100 K and (b) the 500 K Al(111) surface.
The insets are blow-ups of the shaded regions and show the first 5 seconds of data. As seen in the figures,
etching begins immediately upon exposure of the Al(111) surface to the low-flux Cl2 molecular beam, at
surface coverages of < 5% monolayer. Such results indicate fast time-scale surface agglomeration of
adsorbed Cl atoms/molecules and submolecular aluminum chlorides.
Energy (arb.)
Ionizer (210.2 nm)
900
1000
vacuum
level
AES
(c)
1000
vacuum
level
Pulsed Nozzle
(10Hz)
(a)
Exit Velocity, vexit (m/s)
Skimmer
Mechanical
Chopper
(7 ms)
Exit vs. Incident Velocities/Energies
Exit Velocity, vexit (m/s)
• Very low Cl2 flux (~ 2x1014 cm-2 sec-1) pulsed molecular beam
• MPI (210.2 nm) and TOF-MS → photodissociate AlxCly, detect Al+
• King & Wells type sticking measurements with QMS
• DFT-GGA with Vanderbuilt ultrasoft pseudopotentials and plane-wave basis, 7×6×1 21 k-point
Monkhorst-Pack, 250 eV plane-wave cut-off for Cl/Al(111) adsorbate system (Vienna Ab-initio
Simulation Package, VASP)
Activated Chemisorption
State Model
activated chemisorption state
exit kinetic energy
chemisorption well
Distance From Surface (arb.)
The unusual desorption phenomena observed in this work is consistent with a model consisting of a
combination of fast surface agglomeration of Cl and AlxCly adsorbates (as seen in the etch rate profiles,
as well as the computational data) and the existence of activated aluminum chloride chemisorption
states, with potential wells above the vacuum level. The activated chemisorption state model is
diagrammed in the figure above (b), and is compared with the standard activated chemisorption model
(a) in which the chemisorption well is below the vacuum level and desorbing species must surmount an
activation barrier.