Nano-Scale Structures Fabricated using Anodic
Aluminum Oxide Templates
Outline
I
II
III
IV
V
Introduction and Motivation
Porous Alumina Masks
Results
Conclusions
NanoLab Experiments
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Objective:
Introduction
Fabricate ordered arrays of structures on the nanometer scale using porous alumina
templates.
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Integrated Circuits
Moores Law:
 Dr. Gordon E. Moore, founder of Intel, predicted in 1965 that the
number of transistors per IC doubles every 18 months.
http://www.intel.com/research/silicon/mooreslaw.htm
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Semiconductor Roadmap
Year
1999 2002
2005 2008
2011
2014
Technology
180
130
100
70
50
35
Generation (nm)
DRAM Half Pitch 180
130
100
70
50
35
(nm)
MPU Gate Length 140
85-90 65
45
30-32 20-22
(nm)
Gate Oxide
1.9-2.5 1.5-1.9 1-1.5 0.8-1.2 0.6-0.8 0.5-0.6
Thickness (nm)
MPU transistor
6.6 M 18 M
44 M 109 M 269 M 664 M
-2
density (cm )
MPU Speed
1250 2100
3500 6000
10000 16903
(MHz)
Cost/transistor
1735 580
255
110
49
22
(microcents)
Important characteristics of “The 1999 National Technology Roadmap
for Semiconductors” published by the SIA.
 Current technology hits a roadblock in about 2012 in terms of fabrication and
device operation.
 Alternative patterning techniques and computing schemes are needed (e.g.
Quantum, Molecular, Optical Computers, Carbon Nanotubes based devices, etc.).
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Motivation: General
What is Anodic Porous Alumina?
 Aluminum oxide grown on an Al substrate in an electrolytic cell. The
resulting structure consists of an array of tunable nanometer-sized pores
surrounded by an alumina backbone.
Purpose:
 To understand the mechanisms involved in the growth and ordering of
anodic porous alumina.
Motivation:
 Interest in using anodic porous alumina as a nano- template to
fabricate nanometer-sized structures (e.g. nanofabrication of quantum
dots).
Why do we want to fabricate nanostructures?
1. Fundamental physical interest in the nanometer size regime. Properties of nanosized structures are different from their bulk and molecular counterparts.
2. Technological applications as electronic and optical devices.
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Motivation: Applications
Commercially available
Anopore filter.
1. Physics:
 Explore optical, electrical, and
magnetic quantum confinement.
http://www.2spi.com/catalog/s
pec_prep/filter2.html
2. Engineering:
 Microfiltration.
 Optical waveguides and
photonic crystals for
optical circuits.
 Template for carbon
nanotube growth for
electronic, mechanical
applications.
 Ordered arrays of
quantum dots for lasers,
photodetectors.
 ULSI memory devices
and ICs.
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Porous Alumina used as optical
waveguide.
H. Masuda, et. al., Jpn. J. Appl. Phys.
38, L1403 (1999).
Ordered arrays of carbon
nanotubes fabricated using a
porous alumina template.
J. Li, et al., Appl. Phys. Lett.
75(3), 367 (1999).
Overview of Anodic Oxide Films
Fabrication
 Anodize aluminum in electrolyte
(e.g. Oxalic Acid)
Two main types of anodic oxide films can be grown
depending on the nature of the electrolyte:
1. Barrier-Type Films:
 Grown Oxide Insoluble in Electrolyte
 Nearly Neutral Electrolytes (pH 5-7)
2. Porous-Type Films:
 Grown Oxide Slightly Soluble in Electrolyte
 Aqueous Sulfuric, Oxalic, and Phosphoric Acid Electrolytes
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Historical Timeline
 1920’s Porous alumina starts to be used commercially to protect and finish
bulk Al surfaces.
 1940’s-1960’s With advent of electron microscopes, first characterization
of structure of porous alumina, but growth theories are experimentally
unsubstantiated.
 1970 Manchester group does first real experimental work showing pore
radius dependence on applied voltage,etc.
 1992 First “quantitative” theoretical attempt to explain pore growth from
first principles by Belorus group.
 1995 Japanese group discovers pores will self-order into close packed array
under the right anodization conditions.
 1996-Present Use of porous alumina for nano-applications abound.
 1998 Although mechanism for ordering still not clear, German group
proposes one possible mechanism.
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Porous Alumina
 Anodize aluminum in electrolyte (e.g. Oxalic Acid).
 Oxide grows at the metal/oxide and oxide/electrolyte
interfaces, pores initiate at random positions by field-assisted
dissolution at the oxide/electrolyte interface.
Apparatus
 Ordering requires appropriate potentials and long anodization
times.
 Ordering results from repulsion between neighboring pores
due to mechanical stress at the metal/oxide interface.
Resulting Structure
H. Masuda and K. Fukuda, Science 268, 1466 (1995).
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Barrier-Type Anodic Oxide Films
Growth Mechanism
 Oxide growth proceeds at the Aluminum
anode (+).
 Hydrogen gas is evolved at the Platinum
cathode (-).
 The current between the cathode and
anode is carried by the electrolyte.
 Oxidation reactions at theAl anode
2 Al  3OH
 Al2O3  3H

 6e 
{ 2 Al  3O 2   Al2O3  6e  }
 Electrolysis of water at aluminum oxide/ electrolyte interface
H 2O  OH

 H  (aq)
{ 2 H 2O  2O 2  4 H  (aq) }
 Reduction reaction at the cathode:
2 H   2e   H 2 
 The overall electrochemical reaction occurring is:
2 Al  3H 2 O  Al2 O3  3H 2
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Barrier-Type Anodic Oxide Films
Growth Mechanism
 Oxide growth proceeds at the metal/oxide and the oxide/electrolyte
interface.
 Growth proceeds due to the motion of ions under the applied field.
 Growth at the metal/oxide interface is due to oxygen containing anions
(mainly OH- and O2-) moving through interstitial/vacancy sites.
 Growth at the oxide/electrolyte interface is due to Al3+ cations moving
through interstitial/place exchange mechanisms.
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Overview of Film Anodization
 Oxide growth proceeds via ionic
conduction and reaction of Al
cations and oxygen containing
anions under the influence of an
applied field.
(e.g. 2Al+ + 3OH-  Al2O3+3H++6e-)
 Pores initiate at random positions
through field-assisted dissolution of
the oxide at the oxide/electrolyte
interface.
 Initially oxide growth
dominates. (I)
 Dissolution becomes
competitive, barrier layer
thins, and pores initiate. (II)
 Approaches steady state
where both mechanisms occur
at roughly the same rate. (III
and IV)
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V.P. Parkhutik, and V.I. Shershulsky, J. Phys. D:Appl. Phys. 25, 1258 (1992).
Porous-Type Anodic Oxide Films
Field-Assisted Dissolution
 Application of a field across the oxide polarizes the oxide bonds.
 This polarization effectively lowers the activation energy for
dissolution of the oxide.
 This promotes solvation of Al3+ ions by water molecules and the
removal of O2- ions by H+ ions.
 This NUE/Bumm
processes&isJohnson
strongly dependent on the E-field strength.
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Ordered Growth of Porous Alumina
 In 1995, Japanese group found that
pores will self-order under the right
anodization conditions.
 The two most important conditions
are narrow voltage ranges and long
anodization times.
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Ordered Nano-Templates
 Tunable diameters and spacings from
20 nm to 500 nm.
 Polycrystalline structure: ordered
micron-sized domains, defects at grain
boundaries.
 Low temperature growth produces
unordered 4-10 nm arrays.
Ordered Oxalic
Near-Ordered Sulfuric
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Ordered Growth of Porous Alumina
 Ordered pore arrays obtained in three
different electrolytes for long anodization
times and appropriate voltages (specific for
each electrolyte).
 Polycrystalline structure with perfectly
ordered domains a few microns in size.
Defects occur at grain boundaries.
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To create an ordered
through-hole mask:
Mask Processing
1. Anodize for a long time
allowing pores to order.
1.
2. Chemically remove the
alumina in a mixture of
phosphoric and chromic acid.
2.
3. Anodize for a short time
(now pores are ordered).
3.
4. Coat top surface of alumina
with a polymer (collodion)
to protect it from further
processing.
4.
5. Remove Al Substrate in a
saturated HgCl2 solution.
5.
6. Remove the barrier layer in
5 wt.% Phosphoric Acid.
6.
7. Remove collodion and place
alumina on desired
substrate.
7.
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AFM of Unopened
Barrier Layer
(1 mm x 1 mm)
H. Masuda et al. , Jpn. J. Appl. Phys. 35, L126 (1996).
Pattern Transfer Techniques: Results
1. Etching Processes
Fluorine Beam
Ion Beam
Transfer mask pattern via etching into
substrate for ordered arrays of
trenches.
Transfer mask pattern via ion etching
into substrate for ordered arrays of
trenches or pillars.
2. Growth Processes
Sputtering and Thermal
Deposition
Transfer mask pattern via deposition
onto substrate for ordered arrays of
dots.
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F-Etched Array of Si(001) Nano-Holes
SAMPLE:
F-ETCH:
PORES:
~500nm thick Free-Standing AAO/Si(001)
1 min. 20 sec.
TSUB = 250oC
Width 70 nm, Depth 100-120 nm
TOP DOWN VIEW
X-SECT. VIEW
50 nm
200 nm
 Walls are ~30 nm thick (near top).
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Ion Etched Array of GaAs Nano-Holes
SAMPLE:
ION BEAM:
PORES:
OBLIQUE VIEW
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~500nm thick Free-Standing
AAO/GaAs(100)
500 eV Ar+, 0.05 mA/cm2
Time = 2hrs. 12min.
Width 50 nm, Depth 50-60 nm
~TOP DOWN VIEW
X-SECT. VIEW
Thermally Evaporated Nano-Dots: MgF2
SEM Top Views
MgF2 dots/Si
Au dots/SiO2
200 nm
AFM Views
Line Scan
18
Height (nm)
16
14
12
~60 nm
10
8
6
4
3-D
Rendered
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2
Height:
12 nm  11% 0
Diameter: 60 nm  9% 0
Spacing: 110 nm  5%
100
200
300
400
Distance (nm)
500
Thermally Evaporated Nano-Dots: Gold
 Porous alumina used as an evaporation
mask to grow quantum dots.
H. Masuda et al. , Jpn. J. Appl. Phys. 35,
L126 (1996).
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Ion Etched Array of GaAs Nano-Pillars
SAMPLE:
ION BEAM:
PILLARS:
~20nm thick Fe dots on GaAs(100).
500 eV Ar+, 0.05 mA/cm2
Time = 17 min.
Width 50 nm, Height 50 nm
OBLIQUE VIEW
TOP DOWN VIEW
X-SECT. VIEW
Note: No Fe
remaining.
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Evaporated Catalyst Dots For Carbon
Nanotube Growth
SAMPLE:
~20nm thick Fe catalyst dots on 100nm Ti/Si
GROWTH:
CVD using Methane gas at 500 Torr, 800oC
NANOTUBES: Multi-walled tubes, ~10s of microns long
TOP DOWN VIEW
 Collaboration with
Dr. Shen Zhu of
Marshall Space
Flight Center.
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Conclusions
Fabricated ordered, arrays of nanostructures using porous alumina templates as
masks:
 Arrays of 50 nm wide trenches in Si and GaAs by atom-beam and sputter
etching.
 Arrays of 50 nm dots of various materials onto substrates by evaporation
and sputtering.
 Arrays of nano-pillars in Si and GaAs by etching nano-dot arrays.
Future
 Make pores smaller (to 5 nm) using sulfuric acid electrolytes and low temp.
anodization.
 Seed for carbon nanotube growth.
 Explore optical, electrical, and magnetic properties of nanostructures.
 Explore ways to transfer single or arbitrary dot/trench patterns.
 Fabricate such nanostructures in situ in multichamber MBE system.
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NanoLab Class: AAO Templated Structures
• Fabricate AAO Masks:
– Ordered and Disordered Oxalic Masks (50 nm/100 nm).
– Ordered film: 15 hr first anodization.
– Disordered film: 1 hr first anodization.
• Lift-Off onto Silicon and Quartz substrates.
– Silicon substrates for SEM characterization.
– Quartz substrates for UV-Vis characterization.
• Thermally Evaporate Gold onto all Samples
– Must be done one sample at a time, because alignment is critical.
• Characterize Samples
– AFM -both samples
– SEM of Au dots on Silicon.
– UV-Vis of Au dots on Quartz.
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