SYNTHESIS OF COPPER NANOWIRES WITH NANO-TWIN SUBSTRUCTURES

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SYNTHESIS OF COPPER NANOWIRES WITH NANO-TWIN
SUBSTRUCTURES

• 1Joon-Bok Lee
• 2Dr. Bongyoung I. Yoo
• 2Dr. Nosang V. Myung
• 1Department of Chemical Engineering, A-217
  Engineering Quadrangle, Princeton University,
  Princeton, NJ 08544-5263, USA
• 2Department of Chemical Engineering, University
  of California, Riverside, CA 92521, USA

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Outline

• Purpose of Research
• Usage of copper nanowires in VLSI (very-large
  scale integration)
• Objective of Research
• Experimental Procedures
• Copper thin film electrodeposition
• Template-based copper nanowire fabrication
• Results and Discussions

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Introduction

• Integrated circuit
• Discovery in 1970-driving force in advent of
  computer systems
• Contain transistors and other semiconducting
  devices
• metal interconnections that serve as
  interconnections for each component
• 1997 Electrodeposited Copper replaces sputtered
  Aluminum as interconnecting material
• Much higher conductivity and lower
  electromigration

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Introduction

• Advancements in technology? increasing
  interconnections in smaller areas
• International Technology Roadmap for
  Semiconductors (ITRS) 2005 100 million
  transistors, 100,000 I/O in 30nm2 chips by 2015
• Copper wires must also be reduced to the
  nanometer scale
• Need high electrical conductivity and tensile
  strength

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A cross section of a microchip showing copper
interconnections.
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Introduction

• strength of materials increase with decreasing
  grain sizes that form the materials
• Smaller grain sizes give greater grain boundaries
  (GB)
• GBs resist propagation of dislocations
• But GBs also scatter electrons-higher resistance
• Twin Boundary (TB) blocks dislocation but
  maintains conductivity
• Optimum find methods to make nanowires with TBs
• No known attempts in literature or otherwise

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• An example of twin
• boundaries found
• within specially
• prepared copper
• thin film samples

Grain Boundary
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Objective

• Understand effect of electrodeposition conditions
  for synthesizing copper nano-twinned nanowires
• Investigate meterials properties, including
  morphology and microstructures, of copper
  nanowires
• Investigate electrical properties of copper
  nanowires by measuring temperature dependent
  electrical resistivity

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Procedure

• Determination of electrodeposition conditions
• Form contiguous copper thin films without powdery
  deposits
• Plated on Brass substrates with 99.9 copper as
  anode
• Acid copper electrolyte
• Direct Current and Pulse-reverse current tested
• Selective chemical etching for grain size
  observation

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Procedure

• Electrodeposition of Copper nanowires

• Anodization of Al to form alumina templates
• A) clean and cut Al to appropriate size
• B) Anodization of Al
• 20V Al anode Platinum coated Titanium cathode
• C) formation of hexagonally close packed Alumina
• Average pore size 30nm
• D), E) selective chemical etching
• D) Aluminum backing
• E) Barrier layer etching to open pores

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Procedure

• Electrodeposition of Copper nanowires

• F) Sputter Au seed layer
• To form working conductive electrode
• G) Place templates on glass slide to form
  workable electrode
• Copper tape and silver paint used to form
  electrical connection
• H), I) electrodeposition of nanowires
• Same electrolyte solution
• J), K) Isolation of alumina template with
  enclosed nanowires
• J) removal from glass slide through acetone
• E) mechanical removal of gold seed layer
• L) Chemical dissolution of alumina template

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Grain Size versus Current Density
Increasing current
Grain size decreases as direct current is
increased. Agitation increases grain size.
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Prelim. Grain Size Tests
Figure 3. Grain size was similar or slightly
decreased in reverse-forward plating as compared
to direct current plating
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Prelim. Grain Size Tests
Figure 4. The efficacy was nearly 100 for most
of current density conditions.
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Prelim. Dep. Rate Tests
Figure 5. The deposition rate seems to linearly
increase as a function of current density.
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Alumina Templates
2 hours
3 hours
4 hours
Figure 8. alumina template cross sections, taken
after 2hours, 3hours, and 4 hours of oxidation.
(19, 44, 65 micrometers, respectively)
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Alumina Templates
Figure 9. The thickness seems to linearly
increase as a function of time in oxidation.
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Templates with enclosed nanowires
Nanowire deposition in custom alumina templates.
Processed nanowires from the same template.
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Results-300nm thick nanowires
Copper nanowire, length 8.4 micrometers. Grown
under 20mA/cm2 forward 60mA/cm reverse
conditions
Copper nanowire, length 12.7 micrometers. Grown
under16mA/cm2 conditions.
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Nanowire Lengths
Copper nanowire, diameter 30 nanometers. Grown
under 20mA/cm2 forward 60mA/cm reverse
conditions
Copper nanowire, diameter, 30 nanometers. Higher
resolution.
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Future Plans

• Further nanowires have been made with custom
  anodized alumina templates
• sent to TEM for imaging and confirmation of
  nanotwin structure growth
• If nanotwin structures within the nanowires are
  confirmed
• further testing to find out the optimum current
  condition and other aspects will be done

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Acknowledgement

• I would like to thank
• Dr. B.Y. Yoo
• Dr. Nosang Myung
• UCR NSF REU BRITE program
• UCR Nano Electrochemical System Laboratory (NESL)