Cambridge NanoTech ALD Tutorial

Cambridge NanoTech ALD Tutorial
July 2011

ALD Applications
Other applications
Roll to rollInternal tube linersNano-glueBiocompatibleMagnetic
Chemical
CatalysisFuel cells
Semi / Nanoelectronics
Flexible electronics
Gate dielectricsGate electrodesMetal InterconnectsDiffusion barriersDRAMMultilayer-capacitorsRead heads
MEMS
Etch resistanceHydrophobic / antistiction
Optical
AntireflectionOptical filtersOLED layersPhotonic crystalsTransparent conductorsElectroluminescenceSolar cellsLasersIntegrated opticsUV blockingColored coatings
Nanostructures
Inside poresNanotubesAround particlesAFM tips
Wear resistant
Blade edgesMolds and diesSolid lubricantsAnti corrosion

ALD Films
- ALD films deposited with digital control of thickness; “built layer-by layer”
- Each film has a characteristic growth rate for a particular temperature
Common ALD Materials
ALD Deposition Rates at 250°C
Oxides
Al2O3, HfO2, La2O3, SiO2, TiO2, ZnO, ZrO2, Ta2O5, In2O3, SnO2, ITO, FeOx, NiO2, MnOx, Nb2O5, MgO, NiO, Er2O3
Nitrides
WN, Hf3N4, Zr3N4, AIN, TiN, TaN, NbNx
Metals
Ru, Pt, W, Ni, Co
Sulphides
ZnS
1.26 Å
1.08 Å
0.38 Å

Benefits of ALD
Perfect films
Digital control of film thickness
Excellent repeatability
100% film density
Amorphous or crystalline films
Conformal Coating
Excellent 3D conformality
Ultra high aspect ratio (>2,000:1)
Large area thickness uniformity
Atomically flat and smooth coating
Challenging Substrates
Gentle deposition process for sensitive substrates
Low temperature and low stress
Excellent adhesion
Coats challenging substrates – even teflon

ALD Reaction Sequence
ALD is based on the spatial separation of precursors
A single ALD cycle consists of the following steps:
1) Exposure of the first precursor
2) Purge or evacuation of the reaction chamber to remove the non-reacted precursors and the gaseous reaction by-products
3) Exposure of the second precursor – or another treatment to activate the surface again for the reaction of the first precursor
4) Purge or evacuation of the reaction chamber
Single Cycle
Precursor A
Purge
Precursor B
Purge
Time

ALD Example Cycle for Al2O3 Deposition
Tri-methyl
aluminum
Al(CH3)3(g)
Methyl group
(CH3)
Al
H
C
H
H
H
O
Substrate surface (e.g. Si)
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.

Methane reaction
product CH4
H
Reaction of
TMA with OH
H
C
H
H
H
H
C
C
H
H
H
Al
O
Al(CH3)3 (g) + : Si-O-H (s) :Si-O-Al(CH3)2(s) + CH4
ALD Cycle for Al2O3
Trimethylaluminum(TMA) reacts with the adsorbed hydroxyl groups,
producing methane as the reaction product

ALD Cycle for Al2O3
Methane reaction
product CH4
Excess TMA
H
H
C
C
H
H
Al
O
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.

ALD Cycle for Al2O3
H2O
O
H
H
H
H
C
C
H
H
Al
O
After the TMA and methane reaction product is pumped away, water vapor (H2O) is pulsed into the reaction chamber.

2 H2O (g) + :Si-O-Al(CH3)2(s) :Si-O-Al(OH)2(s) + 2 CH4
ALD Cycle for Al2O3
Methane reaction product
New hydroxyl group
Methane reaction
product
Oxygen bridges
H
O
O
Al
Al
Al
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.

ALD Cycle for Al2O3
H
O
O
O
Al
Al
Al
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.

H
H
H
O
O
O
O
O
Al
Al
Al
O
O
O
O
O
Al
Al
Al
O
O
O
O
O
Al
Al
Al
O
O
O
Al(CH3)3 (g) + :Al-O-H (s) :Al-O-Al(CH3)2(s) + CH4
ALD Cycle for Al2O3
One TMA and one H2O vapor pulse form one cycle. Here three cycles are shown, with approximately 1 Angstrom per cycle.
Two reaction steps in each cycle:
2 H2O (g) + :O-Al(CH3)2(s) :Al-O-Al(OH)2(s) + 2 CH4

ALD Deposition Characteristics
ALD is insensitive to dose after saturation is achieved
Deposition rate remains unchanged with increasing dose
MgO Saturation Curve at 250°C
Linear MgO Deposition

ALD “Window”
Each ALD process has an ideal process “window” in which growth is saturated
Process parameters inside the ALD window allowfor reliable and repeatable results
The ALD window is defined by the precursor volatility / stability
Decomposition limited
Condensation limited
Growth Rate
Å/cycle
ALD
Window
Saturation
Level
Temperature
Desorption limited
Activation energy limited

ALD Reaction Temperatures
ALD is a chemistry driven process
Based on precursor volatility/reactivity
Most ALD Processes
Reactor Temp
>400°C
250°C
300°C
150°C
150°C
Room T
200°C
100°C
350°C
High precursor volatility, lower thermal stability of precursors
Lower precursor volatility, Slow desorption of precursors

High Aspect Ratio Coatings
ALD is uniquely suited to coat ultrahigh aspect ratio structures enabling precise control of the coatings thickness and composition.
Cambridge NanoTech’s research systems offer deposition modes for ultra high aspect ratio (>2,000:1)
“Capillary tube”
Cross Sectional SEM
AAO template*
*Image courtesy of the University of Maryland

Compositional Uniformity
Refractive Index – Ellipsometry
Cross sectional EDX
2.104
2.103
2.101
2.101
2.105
2.102
2.104
2.104
2.101
2.103
2.103
2.103
2.099
2.101
2.104
Al2O3 Silica aerogel foam
Ta2O5 - 500Å film

ALD Precursors
Good ALD precursors need to have the following characteristics:
Volatility
Vapor pressure (> 0.1Torr at T < 200°C) without decomposition
Stability
No thermal decomposition in the reactor or on the substrate
Reactivity
Able to quickly react with substrate in a self-limiting fashion (most precursors are air-sensitive)
Byproducts
Should not etch growing film and/or compete for surface sites
Availability
Precursor cylinders

Plasma Enhanced (PE)ALD
Remote Plasma as a reactant
Expands ALD window for materials by decreasing activation energy
Lower temperature possible: avoids precursor decomposition
Faster deposition cycle times
Fewer contaminates in films
Single Cycle
Fiji PE-ALD chamber
Precursor A
Purge
Plasma On
Plasma Purge
Time

Plasma Enhanced (PE)ALD
Plasma ALD processes are used for a variety of oxides, nitrides, and metals, including titanium nitride, platinum, and other materials, allowing for low resistivity of titanium nitride, and significantly lower temperatures for depositing platinum.
Cambridge NanoTech Fiji Manifold
Cambridge NanoTech Fiji Chamber

Variety of Material Types Possible
ALD allows for the fabrication of different types of materials, all in the same deposition chamber, without the need for different hardware configurations.
Doped films: single “layers” of dopant film in between bulk
Doped films do not require “activation” by annealing
(B) Nanolaminate Films: stacks of alternating layers
(C) Graded films: composition slowly changes from material A to material B
M1

Low Temperature ALD
Some ALD processes can deposit films < 150°C: Al2O3, HfO2, SiO2, TiO2, ZnO, ZrO2, Ta2O5, SnO2, Nb2O5, MgO
Ideal for merging organics with inorganics
Compatible with photoresist, plastics, biomaterials

Product Portfolio
Cambridge NanoTech ALD systems are engineered for a wide variety of applications from research to high-volume manufacturing. These systems deposit precise, conformal and ultra-thin films on multiple substrates. Their simplified system designs yield low startup and operating costs.
Savannah
Fiji
Phoenix
Tahiti
Compact, cost-effective system for research
Plasma system for research
Batch manufacturing system
Large area manufacturing system
Production
Research

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Cambridge NanoTech ALD Tutorial

by CambridgeNano on Aug 10, 2011

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