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Steven M. George
Dept. of Chemistry, Univ. of Colorado
Boulder, Colorado 80309
Steven.George@Colorado.edu
Thermal Atomic Layer Etching (ALE)
of Silicon Using an Oxidation and
“Conversion-Etch” Mechanism
Thermal Atomic Layer Etching (ALE)
Using Sequential Surface Reactions
2
Thermal Si ALE Using an Oxidation and
“Conversion-Etch” Mechanism
Oxidation
Ligand-Exchange & Conversion
FluorinationRepeat
(C)
Si
AlF(CH3)2
Si(CH3)4
SiO2
Al(CH3)3
H2O
O2
Etched
Thickness
Si
(A)
(B)HF
Al2O3 Al2O3
Si
SiO2
AlF3
Si
SiO2
Repeat
Al2O3
SiO2
3
Outline
1. Thermal Al2O3 ALE Using Fluorination &
Ligand-Exchange
2. Thermal SiO2 ALE Using “Conversion Etch”
3. Thermal Si ALE Using Oxidation &
“Conversion Etch”
4
100 ALE Cycles
Mass change per cycle
= -15.9 ng/cm2
Etch rate = 0.51
Å/cycle
Al2O3 ALE Using Al(CH3)3 and HF
Y. Lee, J.M. DuMont & S.M. George, Chem. Mater. 28, 2994 (2016). 5
Mass Loss During Al(CH3)3 and HF
Exposures for 3 Cycles of Al2O3 ALE
Constant mass changes
with each reactant
exposure
Y. Lee, J.M. DuMont & S.M. George, Chem. Mater. 28, 2994 (2016). 6
Al2O3 ALE via Fluorination &
Ligand Exchange
7
Outline
1. Thermal Al2O3 ALE Using Fluorination &
Ligand-Exchange
2. Thermal SiO2 ALE Using “Conversion Etch”
3. Thermal Si ALE Using Oxidation &
“Conversion Etch”
8
SiO2 ALE at 300°C Using Al(CH3)3 & HF
0.31 Å/cycle
0.20 Å/cycle
0.15 Å/cycle
0.027 Å/cycle
Pre
ssu
re
J.W. DuMont et al., ACS Appl. Mater. Interfaces 9, 10296 (2017). 9
Reactions during “Conversion-Etch”
Conversion of SiO2 to Al2O3:
1.5SiO2 + 2Al(CH3)3 Al2O3 + 1.5Si(CH3)4
Fluorination of Al2O3:
Al2O3 + 6HF 2AlF3 + 3H2O
Etch of AlF3:
AlF3 + 2Al(CH3)3 3AlF(CH3)2
10
Pressure-Dependence of TMA Reaction
During “Conversion-Etch”
TMA converts Si-O species
to Al-O species.
1.5SiO2 + 2Al(CH3)3
Al2O3 + 1.5Si(CH3)4
TMA etches AlF3 layer.
AlF3 + 2Al(CH3)3
2AlF(CH3)2
Difference Spectra
J.W. DuMont et al., ACS Appl. Mater. Interfaces 9, 10296 (2017). 11
Pressure-Dependence of HF Reaction
During “Conversion-Etch”
HF Fluorinates Al-O
Species
Al2O3 + 6HF
2AlF3 + 3H2O
Difference Spectra
J.W. DuMont et al., ACS Appl. Mater. Interfaces 9, 10296 (2017). 12
Mechanism of Thermal SiO2 ALE by
“Conversion-Etch”
TMA removes AlF3 &
converts SiO2 to Al2O3
HF converts Al2O3 to AlF3
13
Outline
1. Thermal Al2O3 ALE Using Fluorination &
Ligand-Exchange
2. Thermal SiO2 ALE Using “Conversion Etch”
3. Thermal Si ALE Using Oxidation &
“Conversion Etch”
14
Spectroscopic Ellipsometry
Measurement of Both Top SiO2 & Si
15
Si ALE Using O2-HF-TMA Sequence
-30
-20
-10
0
10
-10
-5
0
5
10
15
0 20 40 60 80
Si T
hic
kn
es
s C
ha
ng
e (
Å)
SiO
2 Th
ickn
es
s (Å
)
Si ALE Cycles
290oC
O2, 250 Torr
HF, 1 Torr
TMA, 1 Torr
O2-HF-TMA
0.4 Å/cycle
Si ALE Linear decrease in Si
thickness
Si etch rate: 0.4
Å/cycle
Surface oxide
remains constant
Surface oxide
thickness: ~11 Å
A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018). 16
-35
-30
-25
-20
-15
-10
-5
0
0 20 40 60 80
30 Torr O2
90 Torr O2
170 Torr O2
250 Torr O2
Si T
hic
kn
es
s C
ha
ng
e (
Å)
Si ALE Cycles
0.2 Å/cycle
0.4 Å/cycle
290oC
HF, 1 Torr
TMA, 1 Torr
O2-HF-TMA
Si ALE vs. O2 Pressure
Linear decrease in Si
thickness at all O2
pressures
Etch rate increases with
O2 pressure
Higher oxidation at
higher O2 pressures
17A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018).
Si ALE vs. TMA and HF Pressure
-25
-20
-15
-10
-5
0
0 10 20 30 40 50 60
0.5 Torr HF
1 Torr HF
1.5 Torr HF
Si T
hic
kn
es
s C
ha
ng
e (
Å)
Si ALE Cycles
0.4 Å/cycle
290oC
O2, 250 Torr
TMA, 1.5 Torr
O2-HF-TMA
TMA pressure dependence
consistent with conversion process HF pressure independence suggests that
HF fluorinates all converted Al2O3
-25
-20
-15
-10
-5
0
0 10 20 30 40 50 60
0.5 Torr TMA
1.5 Torr TMA
2 Torr TMA
0.8 Torr TMA
Si T
hic
kn
es
s C
ha
ng
e (
Å)
Si ALE Cycles
0.2 Å/cycle
0.4 Å/cycle
290oC
O2, 250 Torr
HF, 1 Torr
O2-HF-TMA
TMA Pressure Dependence HF Pressure Dependence
18A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018).
Si ALE of Ultrathin Si Films
No difficulty etching
ultrathin Si films
Linear decrease in film
thickness
No effect from silicon
quantum confinement
0
20
40
60
80
100
0 50 100 150 200
Si T
hic
kn
es
s (
Å)
Si ALE Cycles
290oC
O2, 250 Torr
TMA, HF, 1 Torr
O2-HF-TMA
0.5 Å/cycle
19A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018).
Applications for Thermal Si ALE
20
1. Isotropic etching of three-dimensional
structures such as silicon nanowires
2. Surface smoothening to reduce silicon line edge
roughness
3. Surface polishing, surface cleaning and
removing surface damage
Conclusions – Thermal Si ALE
1. O2 Oxidizes Si to SiO2
2. TMA Converts SiO2 to Al2O3
3. HF Fluorinates Al2O3 to AlF3
4. TMA Removes AlF3 by Ligand-Exchange
21
Extra Slides
QCM Studies in Hot Wall, Viscous
Flow Reactor
Pump
Heated Substrates
Gas Switching
Valves
Throttle
Valve
Quartz Microbalance
HeatersFlow Tube
N2
H2O
TMA
J.W. Elam et al., Rev. Sci. Instrum. 73, 2981 (2002).
FTIR Spectroscopy Studies of Surface
Species & Etched Material
Film Thickness Measurement with
Spectroscopic Ellipsometry
LightSource
Detector
Sample Insulation
Manifold
ALEChamber
Pump
Gas In
Gas out
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