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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
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
0
100
200
300
400
500
600
700
800
900
1000
0 5 10 15 20 25 30
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
0
50
100
150
200
250
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.
95
96
97
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100
101
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.
0
5
10
15
20
25
30
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40
45
50
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.
0
10
20
30
40
50
60
70
80
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