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A High Density Carbon Nanotube Capacitor for
Decoupling Applications

• Mark M. Budnik, Arijit Raychowdhury,
• Aditya Bansal, Kaushik Roy
• July 27, 2006

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A High Density Carbon Nanotube Capacitor

• Introduction to Decoupling Capacitors
• Carbon Nanotube Capacitor Physical Structure
• Carbon Nanotube Electrical Model
• Carbon Nanotube vs. Conventional Capacitors
• Capacitance per Unit Area
• Leakage per Unit Area
• Conclusions

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Introduction to Decoupling Capacitors

• Decoupling capacitors are used to reduce supply
  voltage variations in advanced processors

Input Voltage
i (t)
-
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Integrated Decoupling Capacitor Structure
A
t
A
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Traditional Decoupling Capacitors

• Problems
• Parallel plate topology - low capacitance / unit
  area
• Expensive die area
• High leakage current
• Algorithm placement
• Improvements?
• Improve dielectric material - limited
• Increase area - more expensive, more leakage
• Decrease dielectric thickness - more leakage
• Increase number of layers - unproven

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Carbon Nanotube Capacitor Alternative

• Metallic, single wall carbon nanotubes
• Offer large surface area to volume ratio

1nm
1nm
? 1?m
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Carbon Nanotube Capacitor (CNCAP)
C
C
A
A
C Cathode A Anode
C
C
A
A
C
C
A
A
C
C
A
A
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CNCAP Electrical Model
Parallel CNTs
CNCAP Model
R/2
L/2
L/2
R/2
Cathode
Front
End
CQ
CQ
L
CG
CC
CQ
CT
R/2
L/2
L/2
R/2
Front
End
R
CQ
CG
Anode
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Capacitance Per Unit Area
Separation 2 nm 3 nm 4 nm
CC 22.8 aF / µm 18.1 aF / µm 15.6 aF / µm
CT 20.4 aF / µm 16.6 aF / µm 14.4 aF / µm
4xCT 81.6 aF / µm 66.4 aF / µm 57.6 aF / µm
Capacitor Technology 2018 MOS CNCAP, s2nm CNCAP,
s3nm CNCAP, s4nm
ITRS Capacitance ( fF / µm2 ) 11 - - - - - - - - -
200 CNT Layers Capacitance ( fF / µm2 ) - -
- 2,710 1,660 1,160
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Capacitance Leakage Per Unit Area
ILEAK
Capacitor Technology 2018 MOS CNCAP, s2nm CNCAP,
s3nm CNCAP, s4nm
Capacitance ( fF / µm2 ) 11 2,710 1,660 1,160
Leakage Current ( / µm2 ) lt 20 fA 1.83 µA 27.5
pA 0.586 fA
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Conclusions

• Traditional MOS parallel plate capacitors
• Limited in ability to serve as decoupling
  capacitors
• Limited improvements for the forseeable future
• Metallic, single wall carbon nanotubes
• High surface area to volume ratio
• Small inter-tube spacing can result in
  appreciable capacitance per unit length
• May be placed in multiple layer bundles
• 3-D carbon nanotube capacitor structure
• High capacitance per unit area ( gtgt 11fF /
  µm2 as a function of the number of layers)
• Low leakage current per unit area ( lt 1fA /
  µm2 for inter-tube spacing of 4nm)