high mobility materials and novel device structures for high...

High Mobility Materials andNovel Device Structures for

High Performance Nanoscale MOSFETsProf. (Dr.) Tejas Krishnamohan

Department of Electrical EngineeringStanford University, CA

&Intel CorporationSanta Clara, CA

2Prof. (Dr). Tejas Krishnamohan, ICSI-5, Marseille, 2007

OutlineOutline

••Need for high mobility channelNeed for high mobility channel

••Bulk Ge PMOSBulk Ge PMOS

••StrainStrain

••QuantizationQuantization

••HeterostructureHeterostructure

••SchottkySchottky SS--DD

••NMOSNMOS

••SummarySummary


High Mobility Channel Impact OnHigh Mobility Channel Impact OnDevice PerformanceDevice Performance

Increasing brings us closer to the ballistic limit

Low m*transport High injLow r

I sat qNSourcevinj 1r1r

Source

NsourceInjected (vinj)Back scattered (r)

Drain inj low field mobility


•But, carrier velocity increase has saturatedwith scaling…

•MOSFET delay has continued to decrease by use of Sistrain to boost velocity…

•and, velocity boosting will also saturate withstrain-based Si band engineering…

High-µ channel: Getting there (LG~10nm) and proceeding beyond

Carrier velocity increase isparamount for performance scaling

Historical CMOS Performance vs. Scaling: The 1/LG “law”

Motivating Focus for HighMotivating Focus for High--µµ ChannelChannel

10 1000

5

10

15

20x106

ElectronsHoles

Strained Si

Channel Length (nm)In

ject

ion

velo

city

(cm

/s)

Courtesy: D. Antoniadis (MIT)

Si

High µ


Picking the Right HighPicking the Right High--µµ MaterialMaterial

Why Ge?

•More symmetric and higher carrier mobilitiesHighest hole mobility

•Easier integration on Si

•Lower temperature processing

17.717.714.814.812.412.4161611.811.8DielectricDielectricconstantconstant

0.170.170.360.361.4241.4240.660.661.121.12BandgapBandgap ((eVeV))

85085050050040040019001900430430Hole mobilityHole mobility

77000770004000040000920092003900390016001600Electron mobilityElectron mobility

InSbInSbInAsInAsGaAsGaAsGeGeSiSiMaterialMaterial PropertyProperty


•Problem #1: Surface Passivation

Solutions: GeONGeON,, high-dielectrics

•Problem #2: Low bandgap higher leakage

Solutions: Innovative channel and device structures, e.g.,Si/Ge Heterostructures, strain

•Problem #3: Parasitic resistance

Solutions: Schottky (Metal) S-D

Problems With Ge and SolutionsProblems With Ge and Solutions


GeGe PassivationPassivation byby GeOGeOxxNNyy

GeON

Ge

SiO2

Al

GeON growth by RTP in NH3

LPCVD SiO2 deposition

Minimal hysterisis < 30 mV

Midgap Dit of 4-5 1011 /cm2-eV

Excellent for CMOS isolation

Need reduction for gatePethe et al, IEEE SISC, Dec 2006

P-Ge

N-Ge


Electrical characteristicsElectrical characteristics -- GeOGeOxxNNyy

Good PMOS and NMOS characteristics

2.2X mobility enhancement in p-type

High electron mobility

PMOS

NMOS


High Mobility MaterialsHigh Mobility Materials

Effective mass vs. BandgapEffective mass vs. Bandgap

( Fischetti et al, JAP 1996 )

Strain vs. BandgapStrain vs. Bandgap

Smaller Effective Mass and SmallerSmaller Effective Mass and Smaller BandgapBandgap Larger BTBT and Larger Off State Current.Larger BTBT and Larger Off State Current.

Bandgap

strain

Small m*,Small

Eg

Krishnamohan et al., IEEE TED May 2006 (Invited)


I sat qNSourcevinj 1r1r

High Mobility Channel Impact OnHigh Mobility Channel Impact OnDevice PerformanceDevice Performance

Increasing brings us closer tothe ballistic limit

Disadvantages

Low Eg High Leakage CurrentsLow m* High Tunneling Leakage

Low Density of StatesHigh High subthreshold slope

Leakage currents may hinderscalability

Low m*transport High injLow r

Conduction band

Source

NsourceInjected (vinj)Back scattered (r)

Drain

Advantages

Valance band

0.010.110.001

0.01

0.1

1

10

100

1000

Gate Length (µm)

ActivePower

Passive Power

1994 2004

Po

wer

Den

sity

(W/c

m2 )

Passi

vePower

Active Power

Courtesy: Ed Nowak (IBM)


High ION+

Low IOFF

Double Gate- Better electrostatics

- Minimize OFF-state drain-source leakage

High Mobility Channel- High drive current and low intrinsic delay

High-K dielectrics- Reduced gate leakage

LogLog(I(IDSDS))

VgVg00

IIoff,minoff,min

BTBTBTBT

IIonon

New Structures and Materials for NanoscaleNew Structures and Materials for NanoscaleMOSFETsMOSFETs

Gate

Gate

High µchannel

High-K dielectric

S D


BTBT ModelingBTBT Modeling

Our model captures… Band structure information All Possible Transitions between

bands (Full Band) Energy Quantization Quantized Density of States

Kim, Krishnamohan, and Saraswat, IEEE DRC, 2007


BTBT ModelingBTBT Modeling

Can simulate the BTBT current for different materials.Matched with available experimental data.


Quantization EffectsQuantization Effects

ΔETbody

S

D

G

BTBT

Small Tunneling RateSmall Tunneling Rate

ΨeΨh

Eg

Large Tunneling BarrierLarge Tunneling BarrierStrong QuantizationStrong Quantization

TunnelBarrier > Eg

S

D

GBTBT

Large Tunneling RateLarge Tunneling Rate

ΨeΨh

Eg

Eg

Thick Body DGFET

Thin Body DGFET

Thin Body Increases Tunneling Barrier Height.Thin Body Increases Tunneling Barrier Height. Lower BTBT.Lower BTBT.

Oxide

Oxide

Tbody

ΔE

OxideChannel

Krishnamohan et al., VLSI Symposium 2006

High mobility - Small Eg

Quantization - Large Eg


Effect of Quantization on Valleys inEffect of Quantization on Valleys in GeGe

Body ThicknessBody Thickness vsvs QuantizationQuantizationGeGe PMOSPMOS

GeGe Band StructureBand Structure

E

k

<111><100>

Conduction Band

Heavy holesLight

Split off

ValenceBand

ГValleyL ValleyΔΔEEГГ ΔΔEELL

Energy Quantization ofEnergy Quantization of ГГ> Energy Quantization of L> Energy Quantization of LInIn GeGe --valley leakage is strongly suppressed with ultravalley leakage is strongly suppressed with ultra--thin bodythin body

3 5 100

0.1

0.3

0.5

Tbody (nm)

En

erg

yL

evel

(eV

)

E

ELΔΔEEГГ

ΔΔEELL

En

erg

yL

evel

(eV

)

Tbody (nm)

Kim, Krishnamohan, Nishi and Saraswat, IEEE SISPAD, 2006


Biaxial strained Si andBiaxial strained Si and GeGe PMOSPMOS

Krishnamohan et al., SSDM 2007

Strain modifies the band structure and directly affects theStrain modifies the band structure and directly affects theleakage properties of the device.leakage properties of the device.

Lowest leakage obtained for ~50% strainedLowest leakage obtained for ~50% strained--GeGe


Biaxial strained Si andBiaxial strained Si and GeGe PMOSPMOS

TensileCompressive

Optimal Performance Tradeoff:- Biaxial Compressively Strained (2-3%) Germanium

TensileCompressive

(%)

ION IOFF

LG=16nm, Tbody = 5nm, Tox = 0.9nm, Vdd=0.7V

Krishnamohan et al., IEEE IEDM 07


UniaxialUniaxial strained Si andstrained Si and GeGe PMOSPMOS

TensileCompressive

TensileCompressive

Optimal Performance Tradeoff:- Uniaxial Compressively Strained Ge (<3GPa) <100>- Uniaxial Compressively Strained (>3GPa) Si <110>

ION IOFF

Krishnamohan et al., IEEE IEDM 07


TEMTransport in high µ Ge

High Ion

High E-field in wide bandgap SiLow E-field in Ge

Low leakage

Eg due to quantization in Ge thin filmLow leakage

StrainedStrained--Ge Heterostructure SOI PMOSGe Heterostructure SOI PMOS

Krishnamohan, Krivokapic, Uchida, Nishi and Saraswat, IEEE TED, May 2006 (Invited)


Low BTBT leakage:Low BTBT leakage: Eg due to quantization in Ge

thin film Reduced E-field in Ge

Mobility Id-Vg

4X improvement over Si4X improvement over Si due to:due to: Strain in GeStrain in Ge Reduced scattering due toReduced scattering due to

––Reduced EReduced E--field in Gefield in Ge––Channel away from the interfaceChannel away from the interface

Si

HFET on SOISi

HFET on Bulk

HFET on SOI

HFET on Bulk

Krishnamohan, Krivokapic, Uchida, Nishi and Saraswat, IEEE TED, May 2006

StrainedStrained--Ge Heterostructure SOI PMOSGe Heterostructure SOI PMOS


Materials:Relexed Si (r-Si), Strained Si (s-Si),Relaxed-Ge (r-Ge), Strained-Ge (s-Ge),Strained-SiGe (s-SiGe)

Terminology (x,y) for channel material

x = Ge content in the channel material and

y = Ge content in an imaginary relaxed (r)substrate to which the channel is strained (s)

Monomaterial Heterostructure-FET

Structures

Krishnamohan, Jungemann, Kim, Nishi and Saraswat, VLSI Symp. June 2006

LG=16nm, TS = 5nm, Vdd=0.7V

Power-Performance

Ge/Si PMOS Ultimate PerformanceGe/Si PMOS Ultimate PerformanceComparisonComparison


Source: Prashant MajhiSEMATECH/Intel

Si

Ge

Si-Ge QW

SEMATECH Results on Strained QuantumSEMATECH Results on Strained QuantumWellsWells vsvs Relaxed Ge Channel pRelaxed Ge Channel p--MOSMOS


SchottkySchottky SS--DD heterostructureheterostructure

2nm Ge

n-Sin-Si

Ni-SNi-S Ni-DNi-D

p+ SiGep+ SiGeLTOLTOGeGe

Good ohmic S-D contacts and low parasitic resistance.Advantages of heterostructure (low leakage –high mobility).


GeGe NMOSNMOS

•Interface state density looks asymmetric from initialmeasurements.•Skewed to conduction band•Can severely degrade the electron mobility

A new full conductance method measured at low T to measure Dit

Measurement of Dit in Ge:•Weak inversion response•Smaller interface trap timeconstant

In collaboration with IMEC


High Mobility IIIHigh Mobility III--V Channel NMOS?V Channel NMOS?

••Main advantage of a semiconductor with a smallMain advantage of a semiconductor with a smalltransport mass is its high injection velocity.transport mass is its high injection velocity.

••BUTBUT……

–– Low DOSLow DOS ––> C> Charge transfer toharge transfer to valleys in L and Xvalleys in L and X

–– SmallSmall EgEg ––> High BTBT leakage> High BTBT leakage

–– HigherHigher--kk ––> Worse Short Channel Effects> Worse Short Channel Effects

••We have investigated and benchmarked DoubleWe have investigated and benchmarked Double--GateGatenn--MOSFETsMOSFETs with different channel materials (GaAs,with different channel materials (GaAs,InAsInAs,, InSbInSb, Ge, Si) taking into account band structure,, Ge, Si) taking into account band structure,quantum effects, BTBT and shortquantum effects, BTBT and short--channel effects.channel effects.

Charge Quantization

Weaklyquantized

Stronglyquantized

ΓΓLL XX

quantizationquantization

ΔEg

Tbody Tbody

ΔEg


Off Current: Band EngineeringOff Current: Band Engineering

Thin bodyThin body :••Small BTBSmall BTBT due to enT due to energy quantizationergy quantization••Small bandgap materialsSmall bandgap materials : large BTBTlarge BTBT••Quantization depends on massQuantization depends on mass

VVDDDD DependenceDependence ((TTbodybody=5nm)=5nm) TTbodybody DependenceDependence (V(VDDDD=0.9V)=0.9V)

Small VSmall VDDDD :••Over 100x Reduction in BTBTOver 100x Reduction in BTBT..••Large bandgap : large reductionLarge bandgap : large reduction••Small bandgap : small reductionSmall bandgap : small reduction

InAssSi

sGeGe

GaAsSi

InAs

sSi

sGe

Ge

GaAsSi

Kim, Krishnamohan, Nishi and Saraswat, IEEE SISPAD, 2006


NMOS Drive Currents (Ballistic)NMOS Drive Currents (Ballistic)Body Thickness Effect

IIONON forfor IIIIII--V materials is similar toV materials is similar to GeGeFor lowFor low TTbodybody ccharge spills intoharge spills into L and XL and X . Low I. Low IONONInnovative device structures needed to improveInnovative device structures needed to improve IIONON

VDD Effect

Tbody=5nm VDD=1V

TTbodybody (nm)(nm)

TTOXOX = 1nm= 1nm, L, LGG = 15nm, I= 15nm, IOFFOFF = 0.1= 0.1μμA/A/μμmm

Pethe, Krishnamohan, Kim, Wong, Nishi and Saraswat, IEEE IEDM, 2005


SummarySummary Bulk GermaniumBulk Germanium

SurfaceSurface passivationpassivation ofof GeGe demonstrated

High mobility bulk Ge PMOS demonstrated

NMOS results are very encouraging but still needs improvement

IIoffoff may limit scalability of very µ materials like Ge, and III-V

Innovative device structures will be needed to exploit excellent transportproperties of High-µ and Small-EG materials.

Strained-Germanium Heterostructures

Ultra-thin strained-Ge quantum well devices fabricated on UT-SOI

High IHigh Ionon and lowand low IIoffoff PMOS demonstrated in Si/PMOS demonstrated in Si/GeGe heterostructureheterostructure

Demonstrated metal SDemonstrated metal S--DD heterostructureheterostructure ss--GeGe FETsFETs with low parasitic resistancewith low parasitic resistance

BTBT tunneling model developed and caliberated with experimental data.

IIIIII--VV MOSFETsMOSFETs

IIoffoff may limit scalability of very µ materials like Ge, InAs and InSb.

ION in most III-V materials dominated by transport in L-valley under quantization –advantages of low transport mass diminished.

Innovative device structures will be needed to exploit excellent transportproperties of High-µ and Small-EG materials.


AcknowledgementsAcknowledgementsFunding:MARCO, DARPA, NSF, Intel, Stanford INMP

Collaborations:Prof. Krishna SaraswatProf. Yoshio NishiProf. Paul McIntyreProf. Philip WongProf. Christoph Jungemann

Students:Abhijit PetheDuygu KuzumDonghyun KimKoen Martens (IMEC)

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