Interconnects - Stanford University · 2006-05-08 · 2 tanford University 3 EE311 / Al...
Transcript of Interconnects - Stanford University · 2006-05-08 · 2 tanford University 3 EE311 / Al...
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EE311 / Al Interconnects1 tanford Universityaraswat
Outline
•Interconnect scaling issues
•Aluminum technology
•Copper technology
Interconnects
EE311 / Al Interconnects2 tanford Universityaraswat
Properties of Interconnect Materials
Metals
Silicides
Barriers
Material Thin film
resistivity
(µ! " cm)
Melting
point (˚C)
Cu 1.7-2.0 1084
Al 2.7-3.0 660
W 8-15 3410
PtSi 28-35 1229
TiSi2 13-16 1540
WSi2 30-70 2165
CoSi2 15-20 1326
NiSi 14-20 992
TiN 50-150 ~2950
Ti3 0W7 0 75-200 ~2200
N+ polysilicon 500-1000 1410
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EE311 / Al Interconnects3 tanford Universityaraswat
SILICIDES - Short local interconnections which have to be exposed to hightemperatures and oxidizing ambients, e.g., polycide and salicide structures.
REFRACTORY METALS – Via plugs, future gate electrodes, localinterconnections which need very high electromigration resistance.
TiN, TiW – Barriers, glue layers, anti reflection coatings and short localinterconnections.
Al, Cu - for majority of the interconnects
Interconnect Architecture
EE311 / Al Interconnects4 tanford Universityaraswat
Al has been used widely in the past and is still used Low resistivityEase of deposition, Dry etching Does not contaminate SiOhmic contacts to Si but problem with shallow junctionsExcellent adhesion to dielectricsProblems with Al
• Electromigration ⇒ lower life time• Hillocks ⇒ shorts between levels• Higher resistivity
Why Aluminum?
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Electromigration due to electron wind induced diffusion of Al through grain boundaries
SEM of hillock and voids formation due to electromigration in an Al line
Electromigration induced hillocks and voids
voidHillock Void
Metal
MetalDielectric
Electromigration
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FscatFfield +
E field FTOTAL = FSCAT + FFIELD
!
" qz#
r E
!
"D
= µr #
q! *
kT"D "
r E =
!
J = "r E =
r E
#
!
D = Doe"kT
Ea
!
"D = J #qZ *
kTD0e$Ea
kT
!
MTF " e EakT
!
MTF =A
"mJneEa
kT
Using Einstein’s relation
current density is related to E field by
Diffusivity is given by
Therefore
Meantime to failure is thus of the form of
A phenomenological model for MTF is γ is the duty factorM, n are constants
Electromigration Theory
Where qz* is effective ion charge
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• Energy dissipated is increasing as performance improves• Average chip temperature is rising
Thermal Behavior in ICs
4
5
6
7
8
9
50 70 90 110 130 150 170 190
Technology Node [nm]
Chi
p Ar
ea [c
m2 ]
75
100
125
150
175
Max
imum
Pow
er D
issi
patio
n [W
]
Source: ITRS 1999
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Temperature Current Density
MTF =A
rmJnexp
Ea
kT
! " # $
% &
r is duty cycleJ is current densityEa is activation energyT is temperatureA, m, and n are materials related constants
Mean time to failure is
Parametric Dependencies of Electromigration
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Ref: Yi, et al., IEEE Trans. Electron Dev., April 1995
MTF =A
rmJnexp
Ea
kT
! " # $
% & r = duty cycle
J =current density
EM Limited Device Integration DensityEM Limited Interconnect Width
Electromigration-Induced Integration Limits
Better packing techniques will be needed in the future tominimize the chip temperature rise
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CompositionMaterials
Electromigration: Material and Composition
Adding Cu to Al decreases its selfdiffusivity and thus increasesresistance to electromigration
Materials with higher activationenergy have higher resistance toelectromigration
E a (e
V)
Temperature (K) Cumulative failure (%)
Failu
re T
ime
(hou
rs)
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EE311 / Al Interconnects11 tanford Universityaraswat
(a)
(b)
(c)
Electromigration: Grain StructureTop view
• For Al grain boundary diffusion DGB >> Ds or DL(Ds = surface diffusion, DL = lattice diffusion)
• In a bamboo structure grain boundary diffusion isminimized
DGB
DS
DL
Near bamboo structure
Self Diffusion in Polycrystalline Materials
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Bamboo Structure
With sufficient grain boundary migration a "bamboostructure" may develop
No grain boundary diffusion in the bamboo structure
Effect of Post Patterning Annealing on the GrainStructure of the Film.
Increasing timeand/or temperature
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e-
Spanning grain Spanning grain
Polygranular cluster
A B
atomic flux
clustercluster
electric current, J
back flux from stress
Tensile stressmetal atom depletion
Compressive stressmetal atom accumulation
spanning grain
polygranular cluster
On an average the flux is uniform ina uniform grain size textured film.
If the grain size is non uniformthe flux will vary accordingly.
e-
! (stress)to
t1t"
!ss
!o
!(t1)
L
comp.
tens i le
build up of stress within polygranularcluster with time due to electromigration
Microstructural Inhomogeneities and StressInduced Electromigration
EE311 / Al Interconnects14 tanford Universityaraswat
e-
! (stress)to
t1t"
!ss
!o
!(t1)
L
comp.
tens i le
•Eventually, a steady-state is approached where the forward flux isequal to the backward flux due to stress.
•A criterion for failure of an interconnect under electromigrationconditions could be if the steady-state stress is equal to or greater thansome critical stress for plastic deformation, encapsulent rupture, largevoid formation, etc.
•The maximum steady state stress would occur at x=0 or x=L
Build up of stress within polygranular clusterwith time due to electromigration.
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• For very large line width/grain size (w/d) values, no spanning grains are present. Nopoints of flux divergence are present and the time to failure is relatively high.
• As w/d decreases, some spanning grains occur. This results in divergences in the flux,causing stress to develop that can be greater than the critical stress for failure (with L >Lcrit for most or all of the polygranular clusters). The lines would fail and the median timeto failure would decrease.
• As w/d decreases more, more spanning grains are present, but the lengths of thepolygranular cluster regions between them are shorter, many of them shorter than Lcrit.The stresses that develop, which create the back fluxes and which in turn eventually stopthe electromigration flux, are in many cases smaller than the critical stress.
• For small enough values of w/d bamboo or near-bamboo structures are present. Thepolygranular clusters are few, and for some lines, all the clusters are shorter than Lcrit andthe lines will not fail by this mechanism. The MTTF thus increases dramatically.
e-
!
!ss
L1
!crit
!
!ss
L2
!crit
!ss
!crit !ss!crit
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Electromigration: Grain Texture• Empirical relationship (for Al & Al alloys)
MTF !eµ
"2 log[
I(111)
I(200)
]3 S. Vaidya et al., Thin Solid Films, Vol. 75, 253, 1981
103
104
105
106
107
1.8 1.9 2.0 2.1 2.2 2.3
Tim
e-t
o-F
ailure
(sec)
1/T (10-3
/K)
(111) CVD Cu
Ea = 0.86 eV
(200) CVD Cu
Ea = 0.81 eV
213238263 188°C
Ref: Ryu, Loke, Nogami and Wong, IEEE IRPS 1997.
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Structure
Layered Structures: ElectromigrationLayering with Ti reduces Electromigration
Time (arbitrary units)C
umul
ativ
e fa
ilure
Al-Si
Layered Al-Si/Ti
Improvements in layered films due to redundancy
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Mechanical properties ofinterconnect materials
Material Thermal expansion coefficient
(1/˚C)
Elastic modulus,
Y/(1- ) (MPa)
Hardness (kg/mm2)
Melting point (˚C)
Al (111) 23.1 x 10-6 1.143 x 105 19-22 660
Ti 8.41 x 10-6 1.699 x 105 81-143 1660
TiAl3 12.3 x 10-6 - 660-750 1340
Si (100) 2.6 x 10-6 1.805 x 105 - 1412
Si (111) 2.6 x 10-6 2.290 x 105 - 1412
SiO2 0.55 x 10-6 0.83 x 105 - ~1700
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Layering with Ti reduces hillock formation
Layered Al/TiHomogeneous Al/TiPure Al
Surface profile
Layered Structures: Hillocks
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Difference in the expansioncoefficients of the film and substratecauses stress in the thin film.
(a) Film as-deposited (stress free).
(b) Expansion of the film andsubstrate with increasingtemperature.
(c) Biaxial forces compress the Alfilm to the substrate dimension
Stress due to Thermal Cycling
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Process Induced Stress
0 100 200 300 400 500
300
200
100
0
-100
Temperature (˚C)
Str
ess
(MP
a)
Pure Al0.64 micron thick
Heating
Cooling
Plastic deformation
Elastic deform
ation
Silicon
Aluminum
! (in Si)
Si pulls inward on Al at interface,inducing compressive stress on Al.
! (in Al)
Expansion of Al withrespect to Si
EE311 / Al Interconnects22 tanford Universityaraswat
Stress Measurement
• Es is Young’s modulus of the substrate,• vs is Poisson’s ratio for the substrate,• (Es/1-vs) is the biaxial modulus of thesubstrate,
• ts is the substrate thickness,• tf is the film thickness, and• Rf is the radius of curvature induced bythe film
!
" =Es
1#$ s
%
& '
(
) *
ts2
6t f Rf
Biaxial stress in the film
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Hillocks
Hillock formation due to compressive stress and diffusion alonggrain boundaries in an Al film.
Al
Al
Al
ILD
ILD
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Current Aluminum InterconnectTechnology
oxide
1. oxide deposition
2. via etch
resist
barrier W
4. W & barrier polish
3. barrier & W fill
Fabrication of Vias
W Plug
0.5 micron
Oxide
Glue
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Current Aluminum InterconnectTechnologyFabrication of Lines
Ti / TiNAl(Cu)Ti
5. metal stack deposition
6. metal stack etch
8. oxide polish
7. oxide gapfill
oxide
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Current Aluminum InterconnectTechnology
(Curtsey of Motorola)
Al(Cu) - Metal 1
TiTiN
TiN
TiNTi
TiNAl(Cu) - Metal 2
Silicon TiSi2TiN
W
W
N+
Oxide
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Low Dielectric Constant (Low-k) MaterialsOxide DerivativesF-doped oxides (CVD) k = 3.3-3.9C-doped oxides (SOG, CVD) k = 2.8-3.5H-doped oxides (SOG) k = 2.5-3.3
OrganicsPolyimides (spin-on) k = 3.0-4.0Aromatic polymers (spin-on) k = 2.6-3.2Vapor-deposited parylene; parylene-F k ~ 2.7; k ~ 2.3F-doped amorphous carbon k = 2.3-2.8Teflon/PTFE (spin-on) k = 1.9-2.1
Highly Porous OxidesXerogels k = 1.8-2.5
Air k = 1
EE311 / Al Interconnects28 tanford Universityaraswat
Thermal Conductivity forCommonly Used Dielectrics
Dielectric Film Thermal Conductivity
(mW/ cm°C)HDP CVD Oxide 12.0
PETEOS 11.5
HSQ 3.7
SOP 2.4
Polyimide 2.4 3550 70 130 180100
0
1
2
3
4
0
0.2
0.4
0.6
0.8
1
1.2
Technology Node [nm]
Die
lect
ric C
onst
ant
Thermal C
onductivity[ W
/ mK
]
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Embedded Low-k Dielectric ApproachWhy embed Low-k dielectriconly between metal lines?
Mechanical strength
Moisture absorption in low-k
Via poisoning/compatibilitywith conventional dry etchand clean-up processes
CMP compatibility
Ease of integration and cost
Thermal conductivity
VIA
VIA
METMET LOWk
METMET LOWk
SiO2(k~4.2)
SiO2(k~4.2)
SiO2(k~4.2)
Source: Y. Nishi, TI
EE311 / Al Interconnects30 tanford Universityaraswat
Air-Gap Dielectrics
2.0
2.5
3.0
3.5
4.0
4.5
0.2 0.4 0.6 0.8 1Line/Space (um)
K eff Experimental
Simulated
Keff vs. Feature Size
Capacitance
Total Capacitance (pF)6.0 9.0 12.0 15.0 18.0 21.0 24.0
1
10
305070
90
99Air-GapStructure
HDP OxideGapfill
Cum
ulat
ive
Pro
babi
lity
Al
SiO2
Al
Air
• Reduced capacitance between wires• Heat carried mostly by the vias• Electromogration reliability is better because of stress relexation
allowed by free space
Electromigration