Atomic Layer Etching of SiO2:
Challenges And Opportunities*
Gottlieb S. Oehrlein,
Department of Material Science and Engineering and Institute for
Research in Electronics and Applied Physics
University of Maryland, College Park
Atomic Layer Etch and Atomic Layer Clean Technology Workshop
San Francisco – April 21st, 2014
Acknowledgements
D. Metzler – PhD graduate student at University of Maryland
Collaborations
Sebastian Engelmann, Robert Bruce, Eric Joseph - IBM Research
Valery Godyak, and Mark Kushner, University of Michigan, Ann Arbor –
collaboration and discussions
Dr. N. Fox-Lyon, E. Bartis, A. Knoll, M. Vollmer, P. Tang (all at University
of Maryland), Benjamin Alexandrovich - collaboration in parts of the
project
Funding
We gratefully acknowledge financial support of this work by the
National Science Foundation under award No. CBET-1134273 and US
Department of Energy (DE-SC0001939).
2
Motivation
Atomic-scale controllability for fabrication technologies
ALE faces issue of self-limitation:
Requires:
Insignificant physical sputtering
Negligible spontaneous chemical etching
Rauf et al. 1 and Agarwal and Kushner2 established computational
simulations :
Showed possibility of ALE for the fluorocarbon/SiO2 and Si system
Based on sub-nanometer-scale FC deposition
1) Rauf et al., J. Appl. Phys. 101(3) (2007)
2) Agarwal et al., J. Vac. Sci. Technol. A 27(1) (2009)
3
A Molecular Dynamics Investigation of Fluorocarbon Based
Layer-by-Layer Etching of Silicon and SiO2
• Crystalline SiO2
Laboratory for Plasma Processing of Materials
4
A Molecular Dynamics Investigation of Fluorocarbon Based
Layer-by-Layer Etching of Silicon and SiO2
≈5Å
• Crystalline SiO2
Laboratory for Plasma Processing of Materials
SiO2 sample after CF3+ ion
bombardment
5
A Molecular Dynamics Investigation of Fluorocarbon Based
Layer-by-Layer Etching of Silicon and SiO2
For low ion energies:
Self-limited Si removal
For 50 eV:
Sputter rates decrease
Laboratory for Plasma Processing of Materials
6
Etch profiles for Ar/c-C4F8 and Ar ALE
etching of a SiO2-over-Si self-aligned
contact.
The etch begins with 20 ML of Si
aligned with 20 ML of SiO2. A highly
selective etch of the contact is
achieved in 20 cycles of PALE.
7
Outline
Introduction
Brief Review of Computational Modeling
Experimental Approaches
Time-Dependent Etch Rates & Surface
Chemistry
Electrical Characterization
Conclusion and Summary
Laboratory for Plasma Processing of Materials
8
Process Description
– Continuous inductively coupled plasma (Ar), periodic precursor injection,
bias
– A full cycle consists of:
A. Deposition Step Short precursor pulse
B. Etch Step
Removal of modified surface layer
– In-situ ellipsometry allows real-time monitoring of thickness changes
1.5 s
C4F8
Bias Power
Ar
10 V
35 s
10 s
10 mTorr pressure, 50 sccm, 200 W source power
A
B
Laboratory for Plasma Processing of Materials
A
9
B
A
B
Controlled FC Deposition
Deposited Thickness/Pulse [Å]
100
7.44 * NC4F8 - 0.71
10
C4F8 pulsed
1
Ar continuous
10 mTorr
400 W
0.1
0.0
0.2 0.4 0.6 0.8
19
NC4F8/Pulse [10 ]
1.0
Controlled deposition of fluorocarbon films on the order of Ångstrom
by varying precursor flow and/or pulse time
Laboratory for Plasma Processing of Materials
10
Time-Dependent Etch Rates
Thickness Change [Å]
5
0
-5
C4F8 pulsed
-10
-15
Ar continuous
10 mTorr
25 eV EIon
200 W
1.5 s
C4F8
Pulse
10 s
No Bias
35 s
Bias
-20
0
50
100
150
200
Time [s]
250
• Good reproducibility
• Short pulses allow to control thickness of deposition
Laboratory for Plasma Processing of Materials
11
300
350
Time-Dependent Etch Rates
Thickness Change [Å]
5
0
-5
C4F8 pulsed
-10
-15
Ar continuous
10 mTorr
25 eV EIon
200 W
1.5 s
C4F8
Pulse
10 s
No Bias
35 s
Bias
-20
0
50
100
150
200
Time [s]
250
• Good reproducibility
• Short pulses allow to control thickness of deposition
Laboratory for Plasma Processing of Materials
12
300
350
Time-Dependent Etch Rates
Thickness Change [Å]
5
0
-5
C4F8 pulsed
-10
-15
Ar continuous
10 mTorr
25 eV EIon
200 W
1.5 s
C4F8
Pulse
10 s
No Bias
35 s
Bias
-20
0
50
100
150
200
Time [s]
250
• Good reproducibility
• Short pulses allow to control thickness of deposition
Laboratory for Plasma Processing of Materials
13
300
350
Time-Dependent Etch Rates
15
C4F8 pulsed
FC Etching
1.5 s C4F8
Ar continuous
Pulse
25 eV EIon
10 s No Bias
Thickness Change [Å]
10
SiO2
Etching
35 s Bias
FC Etching
5
SiO2
Etching
0
-5
(a)
t0
0
10
3 s C4F8
Pulse
tf
(b)
20
30
40
50 0
Time [s]
10
• Strong impact of FC layer
• Time dependent etch rates due to FC depletion
Laboratory for Plasma Processing of Materials
14
20
30
40
50
Time-Dependent Etch Rates
15
C4F8 pulsed
FC Etching
1.5 s C4F8
Ar continuous
Pulse
25 eV EIon
10 s No Bias
Thickness Change [Å]
10
SiO2
Etching
35 s Bias
FC Etching
5
SiO2
Etching
0
-5
(a)
t0
0
10
3 s C4F8
Pulse
tf
t0
(b)
20
30
40
50 0
Time [s]
10
• Strong impact of FC layer
• Time dependent etch rates due to FC depletion
Laboratory for Plasma Processing of Materials
15
20
tf
30
40
50
FC And Ion Energy Impact
• Etch rates depend on:
o FC film thickness
o Ion energy
Chemically enhanced
etching
• Saturation effect when
reaching critical FC film
thickness
No additional mixing
into SiO2
Laboratory for Plasma Processing of Materials
16
Intensity [a.u.]
15 Å
102
O1s
CF
SiFx
CF2
C1s
Si2p
5Å
SiO2
SiOF
SiO2
SiOF
C-C
C-CFx
CF
CF2
CF3
Surface Chemistry
F1s
After Deposition
During Etch
After Etch
107
284
Laboratory for Plasma Processing of Materials
294
531
Binding Energy [eV]
17
536
686
691
Intensity [a.u.]
15 Å
102
O1s
CF
SiFx
CF2
C1s
Si2p
5Å
SiO2
SiOF
SiO2
SiOF
C-C
C-CFx
CF
CF2
CF3
Surface Chemistry
F1s
After Deposition
During Etch
After Etch
107
284
• Mixing of F into SiO2
Laboratory for Plasma Processing of Materials
294
531
Binding Energy [eV]
18
536
686
691
Intensity [a.u.]
15 Å
102
O1s
CF
SiFx
CF2
C1s
Si2p
5Å
SiO2
SiOF
SiO2
SiOF
C-C
C-CFx
CF
CF2
CF3
Surface Chemistry
F1s
After Deposition
During Etch
After Etch
107
284
• Change in FC composition
Laboratory for Plasma Processing of Materials
294
531
Binding Energy [eV]
19
536
686
691
Continuous C4F8 Addition and FC Wall Coverage
10
10
C4F8 Admixture
During After
The C4F8 flow is cycled as shown
9
10
3/2
-3
EEPF [eV cm ]
schematically below
EEPFs are measured:
A. During the C4F8 injection to
study continuous precursor
admission
B. After the C4F8 injection to
study FC wall coverage
effects
0%
2%
4%
6%
8%
8
10
7
10
2 min 2 min
C4F8
Ar
2%
4%
6%
0
5
10
15
Electron Energy [eV]
20
8%
10 mTorr pressure, 50 sccm total, 200 W source power, -10 V
A
B
A
Laboratory for Plasma Processing of Materials
B
A
B
20
A
B
Continuous C4F8 Addition and FC Wall Coverage
C4F8 admission shows:
Vp initially slightly decreasing for small amounts of C4F8 before rising
A clear drop in Ne and rise in Te
Te saturates above 4 % admixture
FC wall coverage after C4F8 admission shows:
A clear drop in Vp and Ne
No significant change in Te
16
6
5
3.4
10
Plasma Potential Vp [V]
12
3.6
Electron Temperature Te [eV]
-3
Electron Density Ne [10 cm ]
14
10
8
6
4
During C4F8 Admixture
2
Pure Ar after Admixture
0
0
2
4
6
8
C4F8 Admixture [%]
10
4
3.2
3
3.0
2
2.8
1
0.2
During C4F8 Admixture
Pure Ar after Admixture
Pure Ar after Admixture
0
0
Laboratory for Plasma Processing of Materials
2
4
6
8
C4F8 Admixture [%]
21
During C4F8 Admixture
0.0
10
0
2
4
6
8
C4F8 Admixture [%]
10
Plasma Properties During a Single Cycle
10
10
• EEPFs show a small impact and fast
I
recovery after short precursor pulses
During Deposition
During Etch
After Etch
1.5 s Pulse, Cycle 4
II III
-3
EEPF [eV cm ]
9
3/2
• Typical values during the etch step are:
o Plasma Potential, Vp: 13.8 V
o Electron Density, Ne: 6.3 x 1010 cm-3
o Electron Temperature, Te: 3.13 eV
10
8
10
7
10
1.5 s
I
II
0
III
5
10
15
Electron Energy [eV]
C4F8
Bias Power
Ar
10 V
80 s
10 s
10 mTorr pressure, 50 sccm, 200 W source power
A
B
Laboratory for Plasma Processing of Materials
A
22
B
A
B
20
Plasma Properties During a Single Cycle
Vp and Te spike during the pulse, while Ne drops
The change in Te is very small
A recovery to initial values occurs within 20 s
Laboratory for Plasma Processing of Materials
23
Impact of Longer Gas Pulses
10
10
II
III
I
During Deposition
During Etch
After Etch
5.0 s Pulse, Cycle 4
10
3/2
-3
EEPF [eV cm ]
9
8
10
7
10
0
5
10
15
Electron Energy [eV]
5s
I
II
20
III
C4F8
Bias Power
10 V
80 s
15 s
Ar
10 mTorr pressure, 50 sccm, 200 W source power
A
B
A
B
Laboratory for Plasma Processing of Materials
24
A
B
Impact of Longer Gas Pulses
Longer pulses lead to:
A slower recovery of Vp
A larger drop and slower recovery in Ne
A larger spike in Te
Te recovers more quickly than Ne and Vp after precursor injection
Laboratory for Plasma Processing of Materials
25
Conclusions
• Controlled removal of 1 Å to 5 Å SiO2 per etching cycle is
possible using:
o Deposition of thin FC films (to 1 ~Å) to enable chemically
enhanced sputter etching;
o Vanishing etch rate when chemical etchant supply is
exhausted
• Time resolved SiO2 etch rates strongly reflect their dependence
on chemical reactant supply and ion energy
• Our results1 are consistent with computational simulations by
Rauf et al. 2 and Agarwal and Kushner3
1. Metzler et al., J. Vac. Sci. Technol. A 32(2) (2014)
2. Rauf et al., J. Appl. Phys. 101(3) (2007)
3. Agarwal et al., J. Vac. Sci. Technol. A 27(1) (2009)
Laboratory for Plasma Processing of Materials
26
Challenges
• Control of chamber chemistry is essential
• C4F8 pulses have a
• significant impact on plasma properties within each cycle
• reduced but cumulative impact from cycle to cycle due to wall
effects
• Selectivity issues at low ion energies and for periodic precursor
injection
• Performance for 3-dimensional structures needs to be
established
Laboratory for Plasma Processing of Materials
27