SYNTHESIS OF COPPER NANOWIRES WITH NANO-TWIN SUBSTRUCTURES

11Joon-Bok LeeJoon-Bok Lee22Dr. Bongyoung I. YooDr. Bongyoung I. Yoo22Dr. Nosang V. MyungDr. Nosang V. Myung

1Department of Chemical Engineering, A-217 Engineering Quadrangle, Princeton University, Princeton, NJ 08544-5263, USA2Department of Chemical Engineering, University of California, Riverside, CA 92521, USA

OutlineOutline

Purpose of ResearchPurpose of Research– Usage of copper nanowires in VLSI (very-Usage of copper nanowires in VLSI (very-

large scale integration)large scale integration)

Objective of ResearchObjective of Research

Experimental ProceduresExperimental Procedures– Copper thin film electrodepositionCopper thin film electrodeposition– Template-based copper nanowire fabricationTemplate-based copper nanowire fabrication

Results and DiscussionsResults and Discussions

IntroductionIntroduction

Integrated circuitIntegrated circuit– Discovery in 1970-driving force in advent of computer Discovery in 1970-driving force in advent of computer

systemssystems– Contain transistors and other semiconducting devicesContain transistors and other semiconducting devices– metal interconnections that serve as interconnections metal interconnections that serve as interconnections

for each componentfor each component– 1997: Electrodeposited Copper replaces sputtered 1997: Electrodeposited Copper replaces sputtered

Aluminum as interconnecting materialAluminum as interconnecting materialMuch higher conductivity and lower electromigrationMuch higher conductivity and lower electromigration

IntroductionIntroduction

Advancements in technologyAdvancements in technology increasing increasing interconnections in smaller areasinterconnections in smaller areas– International Technology Roadmap for International Technology Roadmap for

Semiconductors (ITRS) 2005: 100 million Semiconductors (ITRS) 2005: 100 million transistors, 100,000 I/O in 30nmtransistors, 100,000 I/O in 30nm22 chips by chips by 20152015

Copper wires must also be reduced to the Copper wires must also be reduced to the nanometer scalenanometer scale– Need high electrical conductivity and tensile Need high electrical conductivity and tensile

strengthstrength

A cross section of a microchip showing copper interconnections.

IntroductionIntroduction

strength of materials increase with decreasing strength of materials increase with decreasing grain sizes that form the materialsgrain sizes that form the materials– Smaller grain sizes give greater grain boundaries Smaller grain sizes give greater grain boundaries

(GB)(GB)– GBs resist propagation of dislocationsGBs resist propagation of dislocations– But GBs also scatter electrons-higher resistanceBut GBs also scatter electrons-higher resistance

Twin Boundary (TB) blocks dislocation but Twin Boundary (TB) blocks dislocation but maintains conductivitymaintains conductivityOptimum: find methods to make nanowires with Optimum: find methods to make nanowires with TBsTBs– No known attempts in literature or otherwiseNo known attempts in literature or otherwise

An example of twinAn example of twin

boundaries found boundaries found

within specially within specially

prepared copper prepared copper

thin film samplesthin film samples

Grain Boundary

ObjectiveObjective

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

Determination of electrodeposition Determination of electrodeposition conditionsconditions– Form contiguous copper thin films without Form contiguous copper thin films without

powdery depositspowdery depositsPlated on Brass substrates with 99.9% copper as Plated on Brass substrates with 99.9% copper as anodeanodeAcid copper electrolyteAcid copper electrolyteDirect Current and Pulse-reverse current testedDirect Current and Pulse-reverse current tested

Selective chemical etching for grain size Selective chemical etching for grain size observationobservation

ProcedureProcedure

Electrodeposition of Copper nanowiresElectrodeposition of Copper nanowiresProcedureProcedure

Anodization of Al to form alumina Anodization of Al to form alumina templatestemplates– A) clean and cut Al to appropriate sizeA) clean and cut Al to appropriate size– B) Anodization of AlB) Anodization of Al

20V Al anode Platinum coated Titanium 20V Al anode Platinum coated Titanium cathodecathode

– C) formation of hexagonally close C) formation of hexagonally close packed Aluminapacked Alumina

Average pore size 30nmAverage pore size 30nm

– D), E) selective chemical etchingD), E) selective chemical etchingD) Aluminum backingD) Aluminum backing

E) Barrier layer etching to open poresE) Barrier layer etching to open pores

Electrodeposition of Copper nanowiresElectrodeposition of Copper nanowires

ProcedureProcedure

– F) Sputter Au seed layerF) Sputter Au seed layerTo form working conductive electrodeTo form working conductive electrode

– G) Place templates on glass slide to form G) Place templates on glass slide to form workable electrodeworkable electrode

Copper tape and silver paint used to form Copper tape and silver paint used to form electrical connectionelectrical connection

– H), I) electrodeposition of nanowiresH), I) electrodeposition of nanowiresSame electrolyte solutionSame electrolyte solution

– J), K) Isolation of alumina template with J), K) Isolation of alumina template with enclosed nanowiresenclosed nanowires

J) removal from glass slide through acetoneJ) removal from glass slide through acetone

E) mechanical removal of gold seed layerE) mechanical removal of gold seed layer

– L) Chemical dissolution of alumina L) Chemical dissolution of alumina templatetemplate

Grain Size versus Current DensityGrain Size versus Current Density

Grain size decreases as direct current is increased. Agitation increases grain size.

Increasing current

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Current Density (mA/cm^2)

Gra

in S

ize

(nm

^2)

Without Agitation

With Agitation

Prelim. Grain Size TestsPrelim. 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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Direct Current Forward-Reverse Current

Gra

in S

ize

(n

m^

2)

Average current:24mA/cm 2̂

Prelim. Grain Size TestsPrelim. Grain Size Tests

Figure 4. The efficacy was nearly 100% for most of current density conditions.

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0 20 40 60 80 100 120

Current Density (mA/cm^2)

Cu

rren

t E

ffic

ien

cy (

per

cen

t)

Prelim. Dep. Rate TestsPrelim. Dep. Rate Tests

Figure 5. The deposition rate seems to linearly increase as a function of current density.

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0 20 40 60 80 100 120

Current Density (mA/cm^2)

Dep

osit

ion

Rate

(n

m/s

eco

nd

)

DC

Forward -ReverseLinear (DC)

Alumina TemplatesAlumina Templates

Figure 8. alumina template cross sections, taken after 2hours, 3hours, and 4 hours of oxidation. (19, 44, 65 micrometers, respectively)

2 hours 3 hours 4 hours

Alumina TemplatesAlumina Templates

Figure 9. The thickness seems to linearly increase as a function of time in oxidation.

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0. 00 0. 50 1. 00 1. 50 2. 00 2. 50 3. 00 3. 50 4. 00 4. 50

Current Densi t y (mA/ cm̂2)

Depo

siti

on r

ate

(nm/

seco

nd)

Templates with enclosed nanowiresTemplates with enclosed nanowires

Nanowire deposition in custom aluminatemplates.

Processed nanowires from the same template.

Results-300nm thick nanowiresResults-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.

Nanowire LengthsNanowire Lengths

Copper nanowire, diameter 30 nanometers. Grown under 20mA/cm2 forward 60mA/cm reverse conditions

Copper nanowire, diameter, 30 nanometers. Higher resolution.

Future PlansFuture Plans

Further nanowires have been made with Further nanowires have been made with custom anodized alumina templates custom anodized alumina templates – sent to TEM for imaging and confirmation of sent to TEM for imaging and confirmation of

nanotwin structure growthnanotwin structure growth

If nanotwin structures within the nanowires If nanotwin structures within the nanowires are confirmedare confirmed– further testing to find out the optimum current further testing to find out the optimum current

condition and other aspects will be donecondition and other aspects will be done

AcknowledgementAcknowledgement

I would like to thank:I would like to thank:

Dr. B.Y. YooDr. B.Y. Yoo

Dr. Nosang MyungDr. Nosang Myung

UCR NSF REU BRITE programUCR NSF REU BRITE program

UCR Nano Electrochemical System UCR Nano Electrochemical System Laboratory (NESL) Laboratory (NESL)