Understanding Atomic Layer Deposition:
Successes & Limitations and Recipe Development
J Provine & Michelle Rincon November 1, 2012 Stanford University
NNIN ALD Roadshow (special thanks to Ganesh Sundaram, Eric Deguns,
Prof. H. Brongersma, and Prof. Fritz Prinz)
What is ALD? aka How does it work?
• First let’s look atomistically. – A rose by any other name…
• Then we’ll look at what that means in ways we can measure. – Microscopically? Nanoscopically? Angstroscopically?
• Then briefly back at the atomic level to see behind the curtain.
In air H2O vapor is adsorbed on most surfaces, forming a hydroxyl group. With silicon this forms: Si-O-H (s)
After placing the substrate in the reactor, Trimethyl Aluminum (TMA) is pulsed into the reaction chamber.
Tri-methyl aluminum Al(CH3)3(g)
C H
H H
H
Al
O
Methyl group (CH3)
Substrate surface (e.g. Si)
ALD Example Cycle for Al2O
3 Deposition
Al(CH3)3 (g) + : Si-O-H (s) :Si-O-Al(CH3)2 (s) + CH4
Trimethylaluminum (TMA) reacts with the adsorbed hydroxyl groups, producing methane as the reaction product
C
H
H
H
H
Al
O
Reaction of TMA with OH
Methane reaction product CH4
H
H H
H H C
C
Substrate surface (e.g. Si)
ALD Cycle for Al2O3
C H H
Al
O
Excess TMA Methane reaction product CH4
H H C
Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups, until the surface is passivated. TMA does not react with itself, terminating the
reaction to one layer. This causes the perfect uniformity of ALD. The excess TMA is pumped away with the methane reaction product.
Substrate surface (e.g. Si)
ALD Cycle for Al2O
3
C H H
Al
O
H2O
H H C
O H H
After the TMA and methane reaction product is pumped away, water vapor (H2O) is pulsed into the reaction chamber.
ALD Cycle for Al2O
3
2 H2O (g) + :Si-O-Al(CH3)2 (s) :Si-O-Al(OH)2 (s) + 2 CH4
H
Al
O
O
H2O reacts with the dangling methyl groups on the new surface forming aluminum-oxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse.
Again metane is the reaction product.
O Al Al
New hydroxyl group
Oxygen bridges
Methane reaction product
Methane reaction product
ALD Cycle for Al2O
3
H
Al
O
O
The reaction product methane is pumped away. Excess H2O vapor does not react with the hydroxyl surface groups, again causing perfect passivation to one atomic layer.
O O Al Al
ALD Cycle for Al2O
3
One TMA and one H2O vapor pulse form one cycle. Here three cycles are shown, with approximately 1 Angstrom per cycle. Each cycle including pulsing and pumping takes e.g. 3 sec.
O
H
Al Al Al
H H
O O
O O O O O
Al Al Al O O
O O O
Al Al Al O O
O O O
Al(CH3)3 (g) + :Al-O-H (s) :Al-O-Al(CH3)2 (s) + CH4
2 H2O (g) + :O-Al(CH3)2 (s) :Al-O-Al(OH)2 (s) + 2 CH4
Two reaction steps in each cycle:
ALD Cycle for Al2O
3
Measurement Technique: Ellipsometry
• Measure change in reflected polarization
• Fast and extremely accurate (<1Å) if material model is known
• Stacks of varying films and absorptive films can cause issues
• Average over a large area
Microscopically (Nanoscopically?) What makes a deposition ALD?
• Step One: Linear growth rate
• Is anything wrong here?
y = 1.259x
0
200
400
600
800
1000
1200
1400
0 200 400 600 800 1000 1200
Th
ick
ne
ss (
Å)
Number of Cycles
• Step Two: Self-limiting deposition/cycle
Microscopically (Nanoscopically?) What makes a deposition ALD?
Saturation Curve at 250°C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0 0.5 1.0 1.5 2.0 2.5
Gro
wth
Pe
r C
ycle
Precursor Dose (seconds)
ALD “Window”
ALD
Window
Temperature
Desorption limited
Condensation limited
Activation energy limited
Decomposition limited
Growth Rate
Å/cycle
Saturation
Level
Each ALD process has an ideal process “window” in which growth is saturated at a monolayer of film.
Outside the ALD Window
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
1.3
125 175 225 275
Growth per cycle (A)
Temperature (qC)
Thermal Alumina Plasma Hafnia
Outside the ALD Window
• If not all of the reactants are removed from the chamber during the purge…
• An easy indication of this is a loss in film uniformity.
ZrO2 Uniformity Optimization Normal Chamber Thickness
Map
Average Thickness vs. Pulse Time
16
0102030405060708090
100
0 0.1 0.2 0.3 0.4
Th
ick
ne
ss (
Å)
Precursor Pulse Time (s)
Non-Uniformity vs. Pulse Time
Silicon Wafers
0
20
40
60
80
100
120
140
0 0.1 0.2 0.3 0.4
No
n-u
nif
orm
ity
(% o
f a
ve
ra
ge
th
ick
ne
ss)
Precursor Pulse Time (s)
Measurements in Angstroms
ZrO2 Uniformity Optimization Under-Dosed
Chamber Thickness Map
Average Thickness vs. Pulse Time
17
0102030405060708090
100
0 0.1 0.2 0.3 0.4
Th
ick
ne
ss (
Å)
Precursor Pulse Time (s)
Non-Uniformity vs. Pulse Time
0
20
40
60
80
100
120
140
0 0.1 0.2 0.3 0.4
No
n-u
nif
orm
ity
(% o
f a
ve
ra
ge
th
ick
ne
ss)
Precursor Pulse Time (s)
Silicon Wafers
Thickness drop-off
High non-uniformity
Measurements in Angstroms
ZrO2 Uniformity Optimization Chemical Vapor Deposition
(CVD) Chamber Thickness Map
Average Thickness vs. Purge
Time
18
0
20
40
60
80
100
0 10 20 30 40
Th
ick
ne
ss (
Å)
Purge time after
precursor pulse (s)
Non-Uniformity vs. Purge Time
0
0.5
1
1.5
2
2.5
3
3.5
0 10 20 30 40
No
n-u
nif
orm
ity
(% o
f a
ve
ra
ge
th
ick
ne
ss)
Purge time after
precursor pulse (s)
Silicon Wafers
Measurements in Angstroms
HfO2 Uniformity Optimization
Normal ALD Process Non-Uniformity vs. Pulse Time
19
Substrate
Precursor-saturated surface
00.20.40.60.8
11.21.41.6
0 10 20 30 40
No
n-u
nif
orm
ity
(% o
f a
vg
th
ick
ne
ss)
Purge time after
precursor pulse (s)
Film
Purge time is too long
Precursor-saturated surface Precursor-saturated surface Precursor-saturated surface Precursor-saturated surface
Desorption! Average Thickness vs. Purge Time
0
20
40
60
80
100
120
0 10 20 30 40T
hic
kn
ess (
Å)
Purge time after
precursor pulse (s)
Lower thickness
Higher Non-uniformity
What is going on atomistically?
• Compositional analysis – Very complete material map – Accuracy .1-10% depending
on material • Weighted average over a
depth of ~10nm • Lateral spot size is typically
>10µm (system dependent)
• Secondary ions emitted from the surface (and sub-surface)
• Extremely accurate trace contamination
• Depth profiling • Significant damage to surface • Interface mixing
What is going on atomistically? We need a technique to look 1 atom deep.
LEIS and The Hypocratic Oath
LEIS on an ALD film of ZrO2
LEIS on an ALD film of ZrO2
The little secret of ALD…if you know this, you know more than 90% of the people I’ve talked to about ALD.
Platinum Nucleation
0
50
100
150
200
250
0 200 400 600 800 1000
Film
Thi
ckne
ss (Å
)
# of Cycles
Thermal 270°C
Thermal 100°C
Pt from MeCpPtMe3 and O
2
Plasma Enabled (PE)ALD
ͻ Remote Plasma as a reactant – Widens ALD window for materials by decreasing
activation energy – Avoids precursor decomposition or damaging
substrates with limited thermal budget – Remote ICP source prevents substrate damage from
ions – Faster deposition cycle times – Fewer contaminates in films – Smaller nucleation delay
ͻ Film Examples
– Low temperature oxides – Metal nitrides – Metallic films
• High-Aspect Ratio Structures
– Radical recombination prevents greater than ~20:1
Fiji PE-ALD chamber
Benefits of Plasma
• Precursor temp 90°C
• Nucleation is eliminated when using O2 plasma as reactant
• Constant growth rate per cycle even at low process temp
Decreased nucleation for metallic films
0
50
100
150
200
250
300
350
400
0 200 400 600 800 1000
Film
Thi
ckne
ss (Å
)
# of Cycles
Thermal 270°C
Plasma 270°C
Thermal 100°C
Plasma 100°C
Pt from MeCpPtMe3 and O2
Pt Nucleation Pt from MeCpPtMe
3 and O
2
Thermal vs Plasma ALD
Plasma ALD Pt on Al2O
3 Plasma ALD Pt on thermal SiO
2
Nanolaminate Formation / Doping
� “Doping” in ALD � Doping is carried out by substituting a pulse of
precursor M1 with dopant precursor M2
� This allows exploration of a wide concentration window without having to prepare new targets for each concentration (as in sputtered depositions)
� Grated films with uniform properties possible
� Annealing films to “activate” dopants is not required
Substrate
M1
M2
M1
Dopant cycle M2
Summary • The Canonical view of ALD is valuable for
understanding, but it does not provide full understanding
• A more complete understanding is useful – Selective deposition – Surface preparation – Troubleshooting
• For analysis there is no magic bullet – Best to heavily armed – The right analysis for your interest
Questions?
On to Process Development
High quality recipe example: MgO
High quality recipe example: MgO
High quality recipe example: MgO
Guard against “unstable” precursors
Sufficient dose is needed
System Purge
• Reactant X must be fully cleared before reactant Y is introduced to the system – Otherwise CVD-like growth
• Purge time depends on temperature (and chemical)
Exposure Mode • For high aspect ratios or to improve deposition
uniformity/consistency • Close stop valve before pulse and allow
saturation time before purge
Boost for Low Volatility Precursors
Summary • ALD can be an extremely stable, repeatable,
controllable process • However, good recipe characterization and
precursor selection is essential • Resources
– Academic literature – Cambridge Nanotech (many well established recipes) – ALD work at stanford:
snf.stanford.edu/SNF/equipment/chemical-vapor-deposition/ald/
Finally: Some Things Learned During Roadshow
• Equipment – University of Maryland:
Beneq – UC Berkeley: Fiji and
Picosun – UT Austin: Savannah and
Fiji (w/ turbo pump) – Arizona State: Savannah – Cornell: Oxford – Washington (Seattle):
Oxford – GA Tech: Fiji – Harvard: Fiji and Savannah – Utah: Fiji
• Processes – Maryland: thermal
vanadium oxide – UC Berkeley: thermal
Ru – UT Austin: thermal
BeO2