Halo Doping: Physical Halo Doping: Physical Effects and Compact Effects and Compact

ModelingModeling

S. Mudanai, W. Shih, R. Rios, S. Mudanai, W. Shih, R. Rios,

P. Packan and S.W. LeeP. Packan and S.W. Lee

OverviewOverview

IntroductionIntroduction

Compact modeling of halo Compact modeling of halo doping for threshold voltagedoping for threshold voltage––Mobility artifactMobility artifact

Mechanism of output resistance Mechanism of output resistance degradationdegradation

Compact modeling requirementsCompact modeling requirements

IntroductionIntroductionScaling requires the use of Scaling requires the use of Halo implants for control of Halo implants for control of short channel devicesshort channel devices

–– Halo implant profile Halo implant profile optimized for best optimized for best performance for the digital performance for the digital devicesdevices

Non-uniform doping along the channel leads to

– Reverse short channel effect

– Long channel DIBL

– Degradation of output resistance at longer lengths

Gate

Source Drain

Halo Doped region

Low doped centre

Netdoping

Halo doping effectsHalo doping effects

L

LHalo LHalo

Dop

ing

con

cent

ratio

n

NHalo

Nbulk

L

2LHalo

Dop

ing

con

cent

ratio

n

NHalo

Dop

ing

con

cent

ratio

n

L

2LHaloNHalo

In compact models RSCE is modeled by using In compact models RSCE is modeled by using a length dependent doping concentration.a length dependent doping concentration.

–– Match the Match the subthresholdsubthreshold current across the current across the 3 transistors to extract an equivalent 3 transistors to extract an equivalent doping.doping.

The core quantities in a compact model are The core quantities in a compact model are calculated assuming a uniform doping.calculated assuming a uniform doping.

Bulk

Halo

Halo

Halo implant effects: linear Halo implant effects: linear regionregion

IV characteristics are not IV characteristics are not the same.the same.On region current for halo On region current for halo device is higher than the device is higher than the uniformly doped deviceuniformly doped device

0.1um0.1um

Dop

ing

con

cent

ratio

n

2e18

3e17

1.0um0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4Vgs(V)

Ids(

mA

/m

)

1.4e+18cm-33e+17cm-3

2e+18cm-3

3-Transistor modelHalo transistors=0.1umHalo doping = 2e+18cm-3Bulk doping = 3e+17cm-3

Bulk

Halo

HaloEquivalentDopinguniformSame vt

Mobility artifactMobility artifact

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

0.1 1 10 100

Channel length (micron)

Idlin

(hal

o)/Id

lin(u

nifo

rm)

Nhalo=2e18;Nbulk=3e17

Nhalo=1e18;Nbulk = 3e17

Modeling of the nonModeling of the non--uniform doping along uniform doping along the channel with the channel with ““averagedaveraged”” doping doping requires an increase in requires an increase in ““mobility parametermobility parameter””used.used.

This required increase is This required increase is –– a function of channel a function of channel

length.length.

–– a function of difference a function of difference in the bulk and halo in the bulk and halo doping.doping.

Bulkthgs

BulkHalo

thgs

Halo

dsoxds

VVL

VVL

WVCI

−+

−

≈ 2µ

Long channel DIBLLong channel DIBL

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

distance along the channel (micron)

Chan

nel p

oten

tial (

V)

Vds = 1.2V

Vds = 0.05V

2-D device simulation

Numerical device simulation

Occurs due to barrier reduction on the drain side.Occurs due to barrier reduction on the drain side.Most compact models use a term proportional to Most compact models use a term proportional to vdsvds in in their DIBL formulations to describe this effect.their DIBL formulations to describe this effect.

Halo Implant effects: Halo Implant effects: saturation regionsaturation region

Output resistance Output resistance

degradation in saturation.degradation in saturation.

Drain saturation voltage Drain saturation voltage

((VdsatVdsat) for the uniformly ) for the uniformly

doped device is slightly doped device is slightly

higher than the higher than the VdsatVdsat for for

the halo doped device.the halo doped device.

Higher drive in saturation Higher drive in saturation

region for the halo device.0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

5.E-05

6.E-05

7.E-05

0 0.5 1 1.5Vds(V)

Ids(

A)

Halo doped device

Uniformly doped deviceVdsatHalo Vdsat uniform

Numerical device simulationSame linear Vth

region for the halo device.

The halo device pinch –off region occursin the halo implant region, since that regionis closest to the drain and has a thresholdvoltage higher than the uniformly doped region α

VthVgsVdsat −=

22--d device simulation: d device simulation: potential profile potential profile

0.20.30.40.50.60.70.80.9

11.11.2

0 0.2 0.4 0.6 0.8 1

Distance along the channel(um)

Cha

nnel

pot

entia

l (V) Vds =1.2

Vds=0.5Vds=0.2

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Distance along the channel (um)

Cha

nnel

pot

entia

l(V)

Vds = 1.2Vds=0.5Vd=0.2

UNIFORM Doping Halo Doping

0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

5.E-05

6.E-05

7.E-05

0 0.5 1 1.5Vds(V)

Ids(

A)

Halo doped device

Uniformly doped device

Uniform doping: In saturation additional drain bias is dropped in the pinch off regionnear the drainHalo doping: In saturation channel potential continues to increase in the bulk doped part,as though it was still in linear region.

22--T equivalent circuitT equivalent circuit22--d device simulations have shown that the d device simulations have shown that the Rout degradation occurs because of drain Rout degradation occurs because of drain side barrier modulation and the source side side barrier modulation and the source side barrier does not play a role.barrier does not play a role.

Vg

Vs

Vd

Vs

Vg

Vd

Lh

L-Lh

Vd’

Halo

Bulk

22--T equivalent circuitT equivalent circuit

Halo Dopedregion

Low doped region

SourceDrain

I II III

Vd’

Vdsathalo Vds2

Potentialdrop

Vds1

Vds3

Vds1: Linear region Vds1: Linear region VdVd’’ resistive potential divider resistive potential divider Halo/Bulk.Halo/Bulk.

Vds2/3: Saturation region Vds2/3: Saturation region VdVd’’ potential divider of potential divider of VdsatVdsathalohalobetween Region II and region Ibetween Region II and region I

Increasing the drain bias increases the length of Region III, Increasing the drain bias increases the length of Region III, reducing the length of region II reducing the length of region II increasing increasing VdVd’’

1. Pinch-off points moves closer to the source of the Halo

2. Barrier reduction already accounted through the long channel DIBL modelPinch

off

22--Tequivalent circuit Tequivalent circuit --simulationsimulation

0

0.005

0.01

0.0 0.5 1.0VDS(V)

IDS(

mA

/um

)Uniform doping 1.3e18

composite circuit with CLM

composite circuit without CLM

The halo doped transistor of length Lhalo is simulated with and without channel length modulation.

Vs

Vg

Vd

Lh

L-Lh

Vd’

Halo

Bulk

Vg

Vs

Vd

Equivalentdoping

22--T equivalent circuit T equivalent circuit -- simulationsimulation

GmHalo

Gmeq

GmBu

lk

GdsHal

o

GdsBul

k

Routeq

Vd

Vd’

Vd+

-Vgd’

+

-

Vg

+

-

Vg

The intermediate node The intermediate node voltage continues to voltage continues to increase in saturation. increase in saturation. This results in increasing This results in increasing current in saturation.current in saturation.

Vs

Vg

Vd

Lh

L-Lh

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Vds(V)

Vd's

(V)

Composite circuit with CLM

Composite circuit without CLM

Vd’

So why does adding Halo So why does adding Halo implant make Rout worse ?implant make Rout worse ?

Adding halo implant Adding halo implant on the drain side on the drain side forces the forces the transistor to transistor to saturate at a lower saturate at a lower VdsatVdsat

Beyond saturation Beyond saturation the high doped the high doped region on the drain region on the drain side acts like a side acts like a scalerscaler of drain bias of drain bias to the bulk doped to the bulk doped transistor

0.0E+00

1.0E-05

2.0E-05

3.0E-05

4.0E-05

5.0E-05

6.0E-05

0 0.2 0.4 0.6 0.8 1 1.2 1.4Vds(V)

Id(A

)

Halo peak = 3e18Halo peak = 1.5e18No halo

Numerical device simulation

Bulk doping = 3e17

Vds

Vdreducing

block

Vgs Bulk transistortransistor

Halo doping variationHalo doping variationAt the same gate bias At the same overdrive

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 0.5 1 1.5VDS(V)

GD

S(A

/V)

nominal device

halo dopingincreased

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 0.5 1 1.5

VDS(V)

GD

S(A

/V)

nominal device

increased halodoping

Bulk Halo

Typically compact models do not take into account the barrier between the bulk and halo doped region. Hence, they cannot predict the increased output resistance due to reduced halo implant.

Bulk doping variationBulk doping variationAt the same gate bias, numerical device simulations

2D device simulations: Increasing the bulk doping results in higher threshold voltage resulting in higheroutput resistance.Increasing the bulk doping also results in a reduced barrier between the halo andbulk regions resulting in higher output resistance.

Bulk

Halo

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

0 0.2 0.4 0.6 0.8 1 1.2VDS

GD

Snominal device

vt reduced by 100mV

vt increased by 100mV

Gds Gds vsvs LengthLength

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 0.2 0.4 0.6 0.8 1 1.2

Length(um)

Gds

(A/V

)

Non-uniformlydopeduniformly doped

VDS =0.5 V; VGS =0.6V

22--D device simulations D device simulations were performed with were performed with halo doping and halo doping and uniform doping such uniform doping such that effective that effective VthVth is the is the same.same.

The longer channel The longer channel lengths > 0.1um show lengths > 0.1um show Rout degradationRout degradation

The degradation The degradation reduces with reducing reduces with reducing channel length.channel length.

Rout degradation Rout degradation modelingmodeling

RequirementsRequirements––Function of difference in threshold Function of difference in threshold voltage for the halo and bulk doping.voltage for the halo and bulk doping.

––Capture the results of varying the Capture the results of varying the bulk/halo dopingbulk/halo doping

––Ineffective for very short channel Ineffective for very short channel lengthslengths

––Reflect the Reflect the vdsatvdsat changes with changes with different halo dopingdifferent halo doping

Capacitance modeling in Capacitance modeling in Halo devicesHalo devices

At low frequencies At low frequencies the CV curve of a the CV curve of a halo doped devices halo doped devices exhibits a lower exhibits a lower threshold voltagethreshold voltage–– The centre lower The centre lower

doped region inverts doped region inverts before the source and before the source and drain ends of the drain ends of the channel.channel.

At high frequencies At high frequencies the halo doped the halo doped transistor exhibits a transistor exhibits a pronounced NQS pronounced NQS effect.effect.

0.02.04.06.08.0

10.012.014.016.018.0

0.0 0.5 1.0 1.5Vgs

Cgg

(fF/

m)

Unifrom dopingHalo Doping

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1.0 10.0 100.0Freq(GHz)

CG

G n

orm

aliz

ed to

CG

G @

low

freq

uenc

yhalo dopinguniform doping

ConclusionsConclusions

Compact modeling of halo devices using Compact modeling of halo devices using uniform doping results in a mobility uniform doping results in a mobility artifact.artifact.

22--D numerical device simulations used to D numerical device simulations used to explain output resistance degradation in explain output resistance degradation in long channel deviceslong channel devices

22--T equivalent transistor approach shows T equivalent transistor approach shows that CLM in halo transistor causes Rout that CLM in halo transistor causes Rout degradation.degradation.

Analytical model requirements for compact Analytical model requirements for compact modeling of Rout degradation identified.modeling of Rout degradation identified.