Shallow Junctions: Contacts - Stanford University · 2005-04-26 · Page ‹#› Stanford University 11 Saraswat / EE311 / Contacts Fermi Level Pinning Energy band structure of the

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Stanford University Saraswat / EE311 / Contacts1

Prof. Krishna Saraswat

Department of Electrical EngineeringStanford UniversityStanford, CA 94305

[email protected]

Shallow Junctions: Contacts


Outline

•Junction/contact scaling issues

•Shallow junction technology

•Ohmic contacts

Need to understand the physics of contacts

resistance and develop technology to minmize it

•Technology to form contacts

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Conduction Mechanisms forMetal/Semiconductor Contacts

Contact resistance strongly depends on barrier height (φB) and doping density

EfV

I

Ohmic

Schottky

(c) Field emission.!

(a) Thermionic emission

(b) Thermionic-field emission

Low doping

Medium doping

Heavy doping

φB


Specific Contact Resistivity (ρc)

!V

!V

n+

V = Vbulk + 2Vcontact = I (Rbulk + 2Rcontact)

For a uniform current density

Rcontact =dVcontact

dI=!cA

Rbulk =dVbulk

dI=!l

A

•Specific contact resistivity and not contact resistance is the fundamentalparameter characterizing a contact

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Tunneling - Ohmic ContactsFs

Fm Jsm Xd =2 K !o " iq Nd

When Xd ≤ 2.5 – 5 nm, electrons can “tunnel” throughthe barrier. Required doping is:

!

Ndmin"

2 K #o $ i

q Xd2

" 6.2 %1019cm

&3for Xd = 2.5 nm

Jsm

=A*T

kFs! P(E)(1" F

m)dE

P(E) ~ exp -2!

B

h

"sm*

N

#

$ %

&

' (

!

Jsm" exp #2xd 2m*q$B # qV( ) /h2[ ]

!

"c =" co exp2#Bh

$sm*

N

%

&

' '

(

)

* * ohm + cm 2

Net semiconductor to metal current is

P(E) is the tunneling probability given by

Current can be shown to be

Specific contact resistivity is of the form

ρc primarily depends upon • the metal-semiconductor work function, φΒ, • doping density, N, in the semiconductor and • the effective mass of the carrier, m*.


Specific contact resistivity

Specific Contact Resistivity to P-type Si

(S. Swirhun, PhD Thesis, Stanford Univ . 1987)

!c = !co exp2"Bqh

#sm*

N

$

% & &

'

( ) ) ohm * cm2

P-type Si

Specific contact resistivity, ρc ↓•As doping density N↑•Barrier height φB ↓

Spec

ific c

onta

ct re

sistiv

ity (Ω

cm2 )

NA (cm-3)

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Specific Contact Resistivity to N-type Dopants

(S. Swirhun, PhD Thesis, Stanford Univ . 1987)

• Similar trends for N-type Si

• For a given doping density contactresistance is higher for n-type Si than p-type.

• This can be attributed to the barrier height• φBn > φBp

Spec

ific c

onta

ct re

sistiv

ity (Ω

cm2 )

ND (cm-3)


Solid Solubility of Dopants in Silicon

•Problem is worse for p-type dopants (B), solid solubility is lower•Maximum concentration of dopants is limited by solid solubility

PROBLEM: Solid solubility of dopants does not scale !

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Barrier Height of Metals and Silicides to SiIdeal Schottky model

Practical barrier withFermi level pinning

φBN + φBP = Eg

Φm < χ Φm > χ

. (Ref: S. Swirhun, PhD Thesis, Stanford Univ. 1987)

Barrier height to n- and p-type Si(φBN hollow symbols and φBP solid symbols)

φBN ⇒ 2Eg/3φBP ⇒ Eg/3


Strategy for Series Resistance Scaling

0306090120150180210240270300

Rcsd

Rdp

Rext

Rov

Source/Drain Engineering

Box ProfileLow-BarrierSilicide(ΦB = 0.2 eV)

Box ProfileMidgap Silicide

Graded JunctionMidgap Silicide

LG = 53 nm

S/D

Ser

ies

Res

ista

nce

[Ωµm

]

Source: Jason Woo, UCLA

But ΦB is controlled by Fermi level pinningHow can we reduce ΦB ? More on this in the metal gate discussion.

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Fermi Level Pinning

Energy band structure of the Schottky contact and the electron energy dependenceof the charging character of the metal semiconductor interface states.

The metal work function is pinned near the charge neutrality level. The charge neutrality level is defined as the energy level at which the character of

the interface states changes from donor-like to acceptor-like. The charge neutrality level is situated at around one-third of the band gap in the

case of silicon ⇒ φbn = 2Eg/3 and φbp = Eg/3 Can we alter the charge neutrality level? It may be possible to do so by modifying

the interface. An issue of current research.


Potential Solutions for S/D Engineering• Rdp & Rcsd Scaling (ρc ↓)

⇒ Maximize Nif ( Rsh,dp ↓):

- Laser annealing

- Elevated S/D⇒ Minimize ΦB:

- Dual low-barrier silicide

(ErSi (PtSi2) for N(P)MOS)

• Rov & Rext Scaling

⇒ Dopant Profile Control:

ultra-shallow highly-doped box-shaped SDE profile

(e.g., laser annealing, PAI + Laser Annealing)

Rcsd Rdp RextRov

x

y = 0

GateSidewall

Silicide

Next(x)

Nov(y)

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Bandgap Engineering

• Si1-xGex S/D & germanosilicide contact− Smaller bandgap for Si1-xGex

− Reduction of Rcsd with single contact metal

From M. C. Ozturk et al. (NCSU), IEDM2002


Contact Resistance: 3D Model

• Current flow in a contact is highlynon-uniform

• Contact resistance does not scalewith area

Silicon

Contact

Metal

I

Current I

I

I

Silicide

!" J =#Jx

#x+#J y

#y+#Jz

#z= 0

J = !"E = "#v

!" #!V = 0

Majority carrier continuity equationoutside the contact is

Current density in the semiconductor is

Combining these two equations we obtain

!

Itot

= " J # dA$

Total current over the contact area is

Solution of the above equations givesinformation about contact resistance.However, calculations are very involved.

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Transmission Line Contact Model

I(x) = I1 exp !x

"c Rs

#

$ %

&

' ( = I1 exp ! x lt( )

lt = !c Rs

lt is the characteristic length of thetransmission line - the distance at which63% of the current has transferred into themetal.

A simplified 1D solution of the contacts is


Measurement of Contact Resistanceand Specific Contact Resistivity (ρc)

Rf !"cwd

For a very large value of lt or for d << lt

• Rf gives reasonable assessment of the source/drain contact resistanceincluding the resistance of the semiconductor under the contact

• Specific contact resistivity, ρc, can be calculated by measuring I, Vf or Ve• Measurement of Rf or Re is not straightforward and needs specialized

test structures

!

Re

=Ve/I =

Rs"c

w sinh d / lt( )

!

Rf =Vf /I =Rs"c

wcoth d / lt( )

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Test Structure to Measure Contact Resistance:Transmission Line Tap Resistor

V24 = Vf + IRSi + Vf

Rt =V24

I= 2Rf + Rsls w

Rf = Vf / I1 =Rs!c

wcoth d / lt( ) is a very small number

Rf


Test Structure to Measure Contact Resistance:Cross-bridge Kelvin Structure

N+ Diffusion

VkRk =Vk

I=

V14

I23

=!c

l2

l

l

.

.

I

Metal

.l

l

N+ Diffusion

Metal

Contact

1 2

3 4

Cross-bridge Kelvin structure used to measure an averagecontact resistance, called RK in the figure

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• Specific contact resistivity (ρc) is a fundamental property of theinterface and should be independent of contact area

• 1-D models overestimate the contact resistance (Rc)• 2-D models give more accurate results and should be used

Error in Specific Contact Resistivity due to 1-D Modeling1-D model 2-D model

Specific contact resistivity (ρc) Contact resistance

Loh, et al., IEEE TED, March 1987.


Outline



•Ohmic contacts

•Technology to form contacts

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Aluminum Contacts to Si

Oxide

Silicon

Aluminum

N+

Oxide

•Silicon has high solubility in Al ~ 0.5% at 450ºC•Silicon has high diffusivity in Al•Si diffuses into Al. Voids form in Si which fill with Al: “Spiking” occurs.


Al/Si Alloy Contacts to SiAl-Si phase diagram

By adding 1-2% Si in Al to satisfy solubilityrequirement junction spiking is minimmized

But Si precipitation can occur whencool down to room temperature

⇒ bad contacts to N+ Si

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Silicide Contacts

•Silicides like PtSi, TiSi2 make excellent contacts to Si

•However, they react with Al

•A barrier like TiN or TiW prevents this reaction

Oxide

Silicon

Aluminum

N+

Oxide

TiN

TiSi2

PtSi

TiWBarrier

Contact


Silicide Contacts

Similar methods are used for other silicides

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Interfacial reactions

Integrity of ohmic contacts due to aphysical barrier between Al and silicide

ΦB (eV)

T (°C)

Schottky barrier reductiondue to Al reaction with PtSi


Barriers

•Silicides react with Al at T < 400°C •A barrier like TiN or TiW prevents this reaction upto T > 500°C

Structure Failure

Temperature(˚C)

Failure Mechanism

(Reaction products)

Al/PtSi/Si 350 Compound formation

(Al2Pt, Si)

Al/TiSi2/Si 400 Diffusion

(Al5Ti7Si12, Si at 550˚C)

Al/NiSi/Si 400 Compound formation

(Al3Ni, Si)

Al/CoSi2/Si 400 Compound formation

Al9Co2, Si)

Al/Ti/PtSi/Si 450 Compound formation

(Al3Ti)

Al/Ti30W70/PtSi/Si 500 Diffusion

(Al2Pt, Al12W at 500˚C)

Al/TiN/TiSi2/Si 550 Compound formation

(AlN, Al3Ti)

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Outline



•Ohmic contacts

•Silicided junctions

Documents

Shallow Junctions: Contacts - Stanford University · 2005-04-26 · Page ‹#› Stanford University 11 Saraswat / EE311 / Contacts Fermi Level Pinning Energy band structure of the