shallow junctions: contacts - stanford...
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Stanford University Saraswat / EE311 / Contacts1
Prof. Krishna Saraswat
Department of Electrical EngineeringStanford UniversityStanford, CA 94305
Shallow Junctions: Contacts
Stanford University Saraswat / EE311 / Contacts2
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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Stanford University Saraswat / EE311 / Contacts3
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
Stanford University Saraswat / EE311 / Contacts4
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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Stanford University Saraswat / EE311 / Contacts5
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*.
Stanford University Saraswat / EE311 / Contacts6
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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Stanford University Saraswat / EE311 / Contacts7
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)
Stanford University Saraswat / EE311 / Contacts8
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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Stanford University Saraswat / EE311 / Contacts9
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
Stanford University Saraswat / EE311 / Contacts10
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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Stanford University Saraswat / EE311 / Contacts11
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.
Stanford University Saraswat / EE311 / Contacts12
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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Stanford University Saraswat / EE311 / Contacts13
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
Stanford University Saraswat / EE311 / Contacts14
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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Stanford University Saraswat / EE311 / Contacts15
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
Stanford University Saraswat / EE311 / Contacts16
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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Stanford University Saraswat / EE311 / Contacts17
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
Stanford University Saraswat / EE311 / Contacts18
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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Stanford University Saraswat / EE311 / Contacts19
• 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.
Stanford University Saraswat / EE311 / Contacts20
Outline
•Junction/contact scaling issues
•Shallow junction technology
•Ohmic contacts
•Technology to form contacts
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Stanford University Saraswat / EE311 / Contacts21
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.
Stanford University Saraswat / EE311 / Contacts22
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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Stanford University Saraswat / EE311 / Contacts23
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
Stanford University Saraswat / EE311 / Contacts24
Silicide Contacts
Similar methods are used for other silicides
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Stanford University Saraswat / EE311 / Contacts25
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
Stanford University Saraswat / EE311 / Contacts26
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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Stanford University Saraswat / EE311 / Contacts27
Outline
•Junction/contact scaling issues
•Shallow junction technology
•Ohmic contacts
•Silicided junctions