High Frequency Properties and Applications of High Frequency Properties and Applications of Carbon Nanotube (CNT) Carbon Nanotube (CNT)
HaoHao XinXin
ECE Dept. & Physics Dept.ECE Dept. & Physics Dept.
University of ArizonaUniversity of Arizona
OutlineOutline
• Introduction and motivation
• Microwave VNA characterization
• THz time domain free space measurement
• Conclusion
http://ece.arizona.edu/~mwca/http://ece.arizona.edu/~mwca/Research Areas
• Novel Electronically Scanned Antennas• Meta-material for mmW applications• Nano Devices and Antennas• Novel THz Sources and Components• Monolithic Microwave IC (MMIC)• 3-D Integrated mmW Circuits and
Antennas• Power Harvesting Applications• Biological Inspired Direction Finding
Our research focus on microwave / mmW / THz antennas, circuits and applicationsCapabilities including: Complete Design / Simulation / Fabrication / Characterization
Prof. Hao [email protected]
60 GHz Packaged Antenna Fully Compatible with Si RFIC
3890 µm
Novel Electronic-Scanned Array
High Frequency Carbon Nanotube Research
Power Amplifier MMIC
Antenna feed
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Metamaterial Based Antennas
3-D THz Rapid Component Fabrication
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Frequency (GHz)
MeasurementSimulation
ARO US Air Force
DoE
ARO US Air Force
DoE
AROARO US Air ForceUS Air Force
DoEDoE
• Complete microwave to THz characterization facilities– Network analyzers (up to 110 GHz), THz equipments– Semiconductor parametric analyzer– Spectrum analyzer, signal generators, power meters,
noise figure meter, etc.– Wafer probe station, antenna anechoic chamber
Agilent E8361A Network Analyzer 10 MHz to 67 GHz, 1.85mm test ports various coaxial and waveguide cal kits
Probe Station with various probe Positioners DC to millimeter probes and calibration substrates
Time Domain THz Spectrometer(TDS) 50 GHz – 2 THz
Testing FacilitiesTesting Facilities
• Full capability clean rooms at engineering and optical science (a brand new SEM & NPGS)
• In-house CVD system for nanotube growth• In-house PCB fabrication equipment
Chemical Vapor Deposition SystemFor Carbon Nanotube Growth
• Access of semiconductor foundries– Raytheon RRFC / Teledyne Scientific Company / MIT
Lincoln Laboratory (GaAs, InP, and CMOS)
EvaporatorSputter E-Beam LithographyMask Aligner
Dry Film PCB Fabrication Equipment
Fabrication FacilitiesFabrication Facilities
• Advanced electromagnetic and circuit simulation tools– Linear / Non-linear circuit simulators– Finite-element frequency domain and time domain
simulators– Agilent Advanced Design Systems, Momentum, HFSS,
CST, SEMCAD++, Cadence, Sonnet …– Work Stations with 12 GB RAM– Evaluating parallel EM simulation tools such as GEMS
Modeling / Simulation CapabilitiesModeling / Simulation Capabilities
3-D structure of MWNT
Images are taken from www.nanotech-now.com
Background and MotivationBackground and Motivation
Carbon Nanotube (CNT) (Diameter ~ nm)
• Discovered by S. Lijima in 1991
• Rolled-up sheet(s) of graphene
• Metallic CNT can have current density 1000 times
greater than metals (> 109 A/cm2 vs. < 107 A/cm2 for Cu)
• One of strongest and stiffest materials
SWNT (Single-Walled)
• Chirality => Semconducting or metallic
• Bandgap can be tuned by the chirality of SWNT
MWNT (Multi-Walled)
• Multi-layers in parallel => Tend to be metallic
Main technology issue: material control / fabrication
Lieber et.al. J. Phys. Chem. B 104, 2794-2809 (2000)
SWNT Young’s modulus: ~ 1TPatensile strength: ~ 30GPa
Structure and properties of SWNTStructure and properties of SWNT
Semiconducting SWNT – active devices
Metallic SWNT – passive interconnects
Possibly: all carbon IC
Metallic
Semiconducting
Field Effect Transistors
- Significant better device performance metric1
- Up to THz intrinsic cutoff frequency predicted2
Passive Applications: Interconnect, Antenna
• Transmission line model of CNTs
• Quantum / Kinetic inductance (~ 10,000 higher than magnetic inductance)
• Resonance frequency 100 times lower (~ ): much smaller antenna
Potential High Frequency Applications: GHz Potential High Frequency Applications: GHz -- THzTHz
1. J. Guo et al, IEDM, pp. 711 (2002) 2. P. J. Burke, Solid-StateElectron., 40, 1981, (2004)
LC/1
Challenge of Single-tube measurement
• Significant mismatch between single CNT’s high intrinsic impedance and 50Ω microwave testing system
• Parasitics often dominate the intrinsic CNT properties
• Difficulty of test fixture fabrication
• Kinetic inductance at microwave frequency has not been experimentally verified
HighHigh--frequency measurement of CNTfrequency measurement of CNT
CNT
50 Ω50 Ω
RCNT~10kΩmismatchmismatch
A CNT across electrodes using Electrophoresis A CNT across electrodes using Electrophoresis
Alternative approach: measure an ensemble of CNTs (CNT paper, array etc.) and extract intrinsic properties
May be necessary for practical applications (bundles, thin film devices, etc. are currently more promising)
Alternative Measurement ApproachAlternative Measurement Approach
CNT ensembleExtraction
Incident & ScatteredEM waves
Intrinsic CNTProperties
Limitation of reported CNT ensemble measurements
• Measured only transmission or reflection
• The assumption of µ=1 had to be made
• Lack of error analysis
Goal: Study high frequency properties of CNTs
• Vector Network Analyzer (10 MHz – 110 GHz)
• THz Time Domain Spectrometer (50 GHz – 3 THz)
• Characterization of both SWNT and MWNT
Characterization of CNT EnsemblesCharacterization of CNT Ensembles
MWNT Paper Sample MWNT Paper Sample
• Thickness: 3.5 mil (~90 μm)
• MWNTs are randomly oriented
Suspend MWNTsin a fluid
Filter the fluid onto a membrane
Support
Dry the membraneand remove the
MWNT paper
PreparationPreparation
MWNT paper photoMWNT paper photo SEM imageSEM image
VNA Experimental SetupVNA Experimental Setup
• Characterized with state-of-art Agilent E8361A PNA network analyzer (10MHz-67GHz)
• Calibrated with waveguide cal kit before measurement
• MWNT paper sandwiched between two rectangular waveguides
• Both of magnitudes and phases of reflection (S11) and transmission (S21) are measured
Port 1 Port 2
• Nicolson-Ross-Weir (NRW)
approach is implemented
• m=0 is selected and verified
• ε = ε’ - j ε” and μ = μ’ - j μ” are complex permittivity and permeability normalized to free space.
Extracting Intrinsic Material PropertiesExtracting Intrinsic Material Properties
Incident
ReflectedS11
TransmittedS21
t
1 21 11V S S= +
2 21 11V S S= −
1 2
1 2
1 VVXV V+
=+
2 1X Xξ = ± −
2
21VV
ξξ−
Γ =−
0
[ ln | | ( 2 )]rpvZ
j mk t
ημ ξ θ π
′= − +
2 2rpvZ
μεη
=′
Extracted Permittivity and PermeabilityExtracted Permittivity and Permeability
• Resulting conductivity σ=ε”ωε0 ~ 1500 S/m, close to DC conductivity 1000 S/m
• No significant magnetic response
• Negative µ” are due to numerical spill over from ε”
-500
0
500
1000
1500
2000
0 10 20 30 40 50
Frequency (GHz)
ε’
0
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2000
3000
4000
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Frequency (GHz)
ε”-2
-1
0
1
2
0 10 20 30 40 50
Frequency (GHz)
μ’
-2
-1
0
1
2
0 10 20 30 40 50
Frequency (GHz)
μ”
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0
500
1000
1500
2000
0 10 20 30 40 50
Frequency (GHz)
ε’
0
1000
2000
3000
4000
0 10 20 30 40 50
Frequency (GHz)
ε”-2
-1
0
1
2
0 10 20 30 40 50
Frequency (GHz)
μ’
-2
-1
0
1
2
0 10 20 30 40 50
Frequency (GHz)
μ”
1. L. Wang, R. Zhou, and H. Xin, IEEE Trans. Microwave Theory and Tech., Vo. 56, No. 2, pp. 499-506, February, 2008
Verification of the AlgorithmVerification of the Algorithm
assigned with computed ε and µ
Excitation
Excitation
HFSS simulation ModelHFSS simulation Model
• A 3.5-mil thin slab is assigned with extracted ε and µ and sandwiched between two waveguides
• The simulated S-parameters are consistent with measured S-parameters
Ansoft’s HFSS (High Frequency Structure Simulator) is employed for simulation.
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Frequency (GHz)
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Frequency (GHz)
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Frequency (GHz)
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Frequency (GHz)
Measured
Simulated
Measured
Simulated
Measured
Simulated
Measured
Simulated
dB (S
11)
dB (S
21)
Pha
se (S
11) (
deg)
Pha
se (S
21) (
deg)
0 20 40 60-0.5
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-0.1
Frequency (GHz)
0 20 40 60-180
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Frequency (GHz)
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Frequency (GHz)
0 20 40 60-40
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0
Frequency (GHz)
Measured
Simulated
Measured
Simulated
Measured
Simulated
Measured
Simulated
dB (S
11)
dB (S
21)
Pha
se (S
11) (
deg)
Pha
se (S
21) (
deg)
Error AnalysisError Analysis
2 2 2 2( | 11|) ( 11) ( | 21|) ( 21)| 11| 11 | 21| 21
S S S SS S S Sε ε ε εε′ ′ ′ ′∂ ∂ ∂ ∂′Δ = Δ + Δ∠ + Δ + Δ∠
∂ ∂∠ ∂ ∂∠
2 2 2 2( | 11|) ( 11) ( | 21|) ( 21)| 11| 11 | 21| 21
S S S SS S S Sε ε ε εε′′ ′′ ′′ ′′∂ ∂ ∂ ∂′′Δ = Δ + Δ∠ + Δ + Δ∠
∂ ∂∠ ∂ ∂∠
2 2 2 2( | 11|) ( 11) ( | 21|) ( 21)| 11| 11 | 21| 21
S S S SS S S Sμ μ μ μμ′ ′ ′ ′∂ ∂ ∂ ∂′Δ = Δ + Δ∠ + Δ + Δ∠
∂ ∂∠ ∂ ∂∠
2 2 2 2( | 11|) ( 11) ( | 21|) ( 21)| 11| 11 | 21| 21
S S S SS S S Sμ μ μ μμ′′ ′′ ′′ ′′∂ ∂ ∂ ∂′′Δ = Δ + Δ∠ + Δ + Δ∠
∂ ∂∠ ∂ ∂∠
• The partial derivatives are computed numerically. For example, to calculate ∂ε’/∂|S11|, we evaluate ε’ with |S11|+δ while all other parameters remaining the same (δ: small number, selected to be 1E-4), then . The S-parameters uncertainties are provided by Agilent.
/ | 11| [ (| 11| ) (| 11|)] /S S Sε ε δ ε δ′ ′ ′∂ ∂ = + −
Error Analysis (contError Analysis (cont’’))
The systematic errors of ε” (less than +/- 15%) are much smaller than ε’ , μ’ and μ”.
ε” extracted from different data sets are fairly close (within +/-6.5%), but ε’ , μ’ and μ” change dramatically. Consistent with the above theoretical prediction.
Overestimation of errors ( 6.5%<15% ) is due to
• Worst-case S-parameter uncertainties
• Numerical partial derivative evaluations
Systematic errors are more sensitive on S11 uncertainties than S21uncertainties. Consistent with expectation.
• Most of energy is reflected. Change of material properties leads to larger difference on S21 than S11, in other words, little variation in S11 will cause dramatic change on ε and μ.
5 10 15 20 25 30 35 40 45 50500
1000
1500
Frequency (GHz)
ε′
5 10 15 20 25 30 35 40 45 500
2000
4000
6000
8000
Frequency (GHz)
ε′′
Intrinsic Permittivity of MWNTIntrinsic Permittivity of MWNT
• Bruggeman Effective Medium Theory (EMT) applied to remove the effect of air in MWNT paper and extract the intrinsic permittivity of MWNT
• ε’ and ε” are roughly doubled
0a eff b eff
a b
a eff b eff
f fK K
ε ε ε ε
ε ε ε ε
− −+ =
+ +
Composite with a & b
1 qK
q
−=
2
1 2
1/
1/ 2 /
aq
a a=
+
f’s are volume fractions
a’s are diameter & lengthof individual MWNT
THz Time Domain Spectrometer (TDS) SetupTHz Time Domain Spectrometer (TDS) Setup
• Femto-second laser excites a low-temp grown GaAs substrate
• THz pulse generated and transmitted to sample
• Same laser pulse optically delayed to detector: coherent measurement
• Transmission and reflection magnitudes & phases can be measured1. Z. Wu, L. Wang, A. Young, S. Seraphin, and H. Xin, J. App. Phys., Vol. 103, No. 9, May, 2008 (This article was also published in the June 2, 2008 issue of the Virtual Journal of Nanoscale Science & Technology (www.vjnano.org)
• Photoconductive THz-TDS system (50GHz-1.5THz)• Transmission Setup
– Sample-out as reference spectrum– Sample-in as sample spectrum
• Reflection Setup– Metal plate as reference object (reflection -1)– CNT sample and metal plate surfaces placed at the same
position (misalignment <12.7 μm)
– Incidence angle 26°
7
Detailed THz Experimental SetupDetailed THz Experimental Setup
• Large reflection from MWNT sample• Small transmitted signal through the sample• Higher order multiple reflections ignored
8
Metal PlateMWNT
Free SpaceMWNT
ReferenceMWNT Refl.MWNT Trans.
Frequency DomainTransmissionReflection
THzTHz--TDS ResultsTDS Results
' "z z jz= −
01( , 0)1i
zR zz
θ −≈ =
+
))1(2(exp2)1(
4)( 00 d/λnπjz
zn,zT −+
=2
2
0)/(sin1cos
)/(sin1cos),(
nz
nzznR
ii
ii
θθ
θθ
−+
−−=
' "n n jn= −Complex refractive index: Wave impedance:
First order complex transmission and reflection coefficients:
n and z numerically solved from T0 and R0
No need to assume n = 1 / z (or µ = 1)
If 0°, z and thus n can be analytically solved from T0 and R0iθ ≈
(Normal-incidence approximation)
/n zε = n zμ = ⋅Permeability:Complex permittivity:
9
Material Parameters Extraction AlgorithmMaterial Parameters Extraction Algorithm
Multiple branches of n’ solution due to the phase ambiguity in measured T0
10
• Good consistency between VNA and TDS measured n’ solution branches
• Analytically solving n’solutions with approximation gets close results
0iθ ≈ o
0 020
1 ( 2 )1 2
Tn mR d
λππ
′= − ∠ +−50 100 150 200 250 300 350
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0
100
200
300
Frequency (GHz)
Mul
tiple
bra
nche
s of
n′ s
olut
ions
50 100 150 200 250 300 350-500
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0
100
200
300
Frequency (GHz)
Mul
tiple
bra
nche
s of
n′ s
olut
ions
MultipleMultiple--Branch Branch SoultionsSoultions of Index of Refractionof Index of Refraction
• m = 0 branch in TDS results is the physically correct branch
• The continuity of n’ at the boundary frequency is kept
• Good consistency of n’ and n”between VNA and TDS results
• Statistical error bars show good repeatability
11
Extracted nExtracted n’’ and nand n”” of MWNT Samplesof MWNT Samples
0
1000
2000
3000
4000
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d ε"
0
200
400
600
800
1000
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d ε'
-2
0
2
4
6
8
10
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d μ"
-4
-2
0
2
4
6
8
10
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d μ'
(a) (b)
(c) (d)
0
1000
2000
3000
4000
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d ε"
0
200
400
600
800
1000
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d ε'
-2
0
2
4
6
8
10
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d μ"
-4
-2
0
2
4
6
8
10
0 100 200 300 400
VNATHz-TDS
Frequency (GHz)
Extr
acte
d μ'
(a) (b)
(c) (d)
12
Extracted MWNT Permittivity and PermeabilityExtracted MWNT Permittivity and Permeability
2 21
2 21 1( )
p pc i i
ω ωε ε
ω ω ω ω ω= − +
− Γ − + Γ +
ε’ and ε” simultaneously fittedCovering microwave to THz
13
DrudeDrude--Lorentz Model FittingLorentz Model Fitting
0 50 100 150 200 250 300 350-25
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-5
0
5
10
15
Frequency (GHz)
Ext
ract
ed n′
Transmission and ReflectionTransmission onlyVNA measurement
0 50 100 150 200 250 300 3500
5
10
15
20
25
30
35
40
Frequency (GHz)
Ext
ract
ed n′′
Transmission and ReflectionTransmission onlyVNA measurement
0 50 100 150 200 250 300 350-25
-20
-15
-10
-5
0
5
10
15
Frequency (GHz)
Ext
ract
ed n′
Transmission and ReflectionTransmission onlyVNA measurement
0 50 100 150 200 250 300 3500
5
10
15
20
25
30
35
40
Frequency (GHz)
Ext
ract
ed n′′
Transmission and ReflectionTransmission onlyVNA measurement
))1(2(exp2)1(
4)( 00 d/λnπjn
nn,zT −+
=
With n = 1 / z (or µ = 1) assumption:
Solve complex n from T0
14
Transmission Measurement Only EvaluationTransmission Measurement Only Evaluation
21 exp( 4 " / )1
zLossFactor mag n fd cz
π⎡ ⎤−⎛ ⎞= −⎢ ⎥⎜ ⎟+⎝ ⎠⎢ ⎥⎣ ⎦
Due to one round trip ofinternal reflections in sample(Transmission/ Reflection)
Loss factor calculated with extracted n” and z
Higher order signal < 10% oflast order signal magnitude for~ 60GHz and up
Verified extraction algorithms
15
Multiple Reflections Effect in Sample Multiple Reflections Effect in Sample
Characterization of SWNT Thin Film on Substrates Characterization of SWNT Thin Film on Substrates
Consider CNT layer as a boundary providing surface current
Air(Medium 1)
SubMedium 2
Reflection coeff:
Transmission coeff:
s
s
YY
σσ
−
+
−Γ =
+
12
s
YTY σ+
=+
1 20
1, nY Y Y YZ Z± = ± = =With
SurfaceConductivity
SWNT Thin Film: ~ 300 nm thickness
3.2mm Pyrex glass
5mm thick window glass3.2mm thin window glass
Thick substrate necessary to avoid multiple reflections
Characterization of SWNT Thin Film on Substrates Characterization of SWNT Thin Film on Substrates
100 200 300 400 500 600 700-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
Freq, GHz
surfa
ce c
ondu
ctiv
ity, r
eal p
art
05200611
100 200 300 400 500 600 700-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
Freq, GHz
surf
ace
cond
uctiv
ity, i
mag
inar
y pa
rt
05200611
• More accurate real conductivity• Measured surface conductivity
– σ*t = 0.015 S (THz) and 0.0023 S (DC 4-point measurement)– σ ~ 105 Simens
• Peaks measured in conductivity– Systematic error / plasmonic resonances?– More study is under way
Under Study Under Study –– Single Tube MeasurementSingle Tube Measurement
Fabricate Grids on Insulating SubstrateFabricate Grids on Insulating Substrate Disperse CNT and Locate Using AFMDisperse CNT and Locate Using AFMEBL Fabrication of Designed Test FixtureEBL Fabrication of Designed Test Fixture
1 um1 um
Microwave Property Probing and ExtractionMicrowave Property Probing and Extraction
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0 20 40 60 80 100
without Tubewith Tube
Frequency (GHz)
S21
(dB
)
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-50
0
50
100
0 20 40 60 80 100
with Tubewithout Tube
S21
Phas
e (D
eg)
ConclusionsConclusions
• Carbon nanotube maybe useful for high frequency applications
• Bulk (paper / thin film) materials characterized (8 – 400 GHz)
• Intrinsic material properties (permittivity & permeability) extracted
• Microwave & THz data are consistent
• Drude-Lorentz model fits measured permittivity
• Correlation of single tube and ensemble results under way
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