A High Density Carbon Nanotube Capacitor for

Decoupling Applications

A High Density Carbon Nanotube Capacitor for

Decoupling Applications

Mark M. Budnik, Arijit Raychowdhury,

Aditya Bansal, Kaushik Roy

July 27, 2006

Mark M. Budnik, Arijit Raychowdhury,

Aditya Bansal, Kaushik Roy

July 27, 2006

A High Density Carbon Nanotube Capacitor

Introduction to Decoupling Capacitors

Carbon Nanotube Capacitor Physical Structure

Carbon Nanotube Electrical Model

Carbon Nanotube vs. Conventional CapacitorsCapacitance per Unit AreaLeakage per Unit Area

Conclusions

Introduction to Decoupling Capacitors

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

i (t)

Inpu

t Vol

tage

+

-

Integrated Decoupling Capacitor Structure

t

A

AA

Ct

Traditional Decoupling Capacitors

ProblemsParallel plate topology - low capacitance / unit areaExpensive die areaHigh leakage currentAlgorithm placement

Improvements?Improve dielectric material - limitedIncrease area - more expensive, more leakageDecrease dielectric thickness - more leakageIncrease number of layers - unproven

Carbon Nanotube Capacitor Alternative

Metallic, single wall carbon nanotubes

Offer large surface area to volume ratio

~ 1nm

~ 1nm

1m

Carbon Nanotube Capacitor (CNCAP)

C C

C C

C C

C C

A A

A A

AA

AA

C = Cathode

A = Anode

CNCAP Electrical Model

Parallel CNTs CNCAP ModelF

ront

End

R/2 L/2 L/2 R/2

CQ

CG

Fro

nt

End

R/2 L/2 L/2 R/2

CQ

CG

CQ

CQ

CC

Cathode

Anode

L

R

CT

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, s=2nm

CNCAP, s=3nm

CNCAP, s=4nm

ITRS Capacitance

( fF / µm2 )

11

- - -

- - -

- - -

200 CNT Layers

Capacitance ( fF / µm2 )

- - -

2,710

1,660

1,160

Capacitance Leakage Per Unit Area

Capacitor

Technology

2018 MOS

CNCAP, s=2nm

CNCAP, s=3nm

CNCAP, s=4nm

Capacitance

( fF / µm2 )

11

2,710

1,660

1,160

Leakage Current

( / µm2 )

< 20 fA

1.83 µA

27.5 pA

0.586 fA

ILEAK

Conclusions

Traditional MOS parallel plate capacitorsLimited in ability to serve as decoupling capacitors

Limited improvements for the forseeable future

Metallic, single wall carbon nanotubesHigh 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 structureHigh capacitance per unit area ( >> 11fF / µm2 as a function of the number of layers)

Low leakage current per unit area ( < 1fA / µm2 for inter-tube spacing of 4nm)