ee143 f2010 lecture 16 metallizationee143/fa10/lectures/lec_16.pdfee143 f2010 lecture 16 7...
TRANSCRIPT
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Metallization• Interconnects
– Typical current density ~105 A/cm2
– Wires introduce parasitic resistance and capacitance• RC time delay
• Inter-Metal Dielectric-Prefer low dielectric constant to reduce capacitance
• Multilevel Metallization (2-10 levels of metalwiring)– Reduction in die size– Higher circuit speed ( shorter interconnect distance)– Flexibility in layout design
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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FOX
Si substrate
( InteMetal Oxide e.g. BPSG. Low-K dieletric)
(e.g. PECVD Si Nitride)
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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• Interconnections and Contacts– RC Time Delay– Interconnect resistance– Contact resistance– Dielectric Capacitance– Reliability - Electromigration
• Multilevel Metallization• Surface Planarization Techniques
Outline
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Interconnect ResistanceRI =R/L = / (WAlTAl)
Interconnect-SubstrateCapacitanceCV C/L = WAl ox / Tox
Interconnect-InterconnectCapacitanceCL C/L = TAl ox / SAl
* Values per unit length L
Interconnect RC Time Delay
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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InterconnectionsInterconnect Requirements
• low ohmic resistance– interconnects material has low resistivity
• low contact resistance to semiconductordevice
• reliable long-term operation
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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• Metal (Low resistivity)
– for long interconnects
• Highly doped Poly-Si (medium resistivity)
–for short interconnects
• Highly doped diffused regions in Si substrate(medium resistivity)
–for short interconnects
Possible interconnect materials
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Resistivity of Poly-Si
Resistivity of Pure Metals
Poly-Si (min) =1E-3 -cm
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Metal Contact to SiTunneling “ohmic” contacts
SiO2
Al
n+
n-Sie 1019 - 1021/cm3
SiO2
Al
p+
p-Si
Xd
Ec
Ev
I
Vh
MSi
> 1019/cm3
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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SchottkyRectifyingContact
SchottkyTunnelingOhmic Contact
Tunneling
Thermionicemission
Small depletion width(highly doped Si)
Large depletion width(lightly doped Si)
metal semiconductor
Depletion region
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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For a uniform current densityflowing across the contact areaRc = c / (contact area )
c of Metal-Si contacts ~ 1E-5 to 1E-7 -cm2
c of Metal-Metal contacts < 1E-8 -cm2
MetalContactArea
Heavily dopedSemiconductor surface
Contact Resistance Rc
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Bs
c Nh
m
*2exp
Approaches to lowering of contact resistance:1) Use highly doped Si as contact semicodnuctor2) Choose metal with lower Schottky barrier height
B is the Schottky barrier heightN = surface doping concentrationc = specfic contact resistivity in ohm-cm2
c V
J
1
at V~0
m = electron massh =Planck’s constant= Si dielectric constant
Specific contact resistivity
Contact Resistivity c
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Schottky Barrier heightsof common metals contactsto n-type semiconductors
Note:B (n-type) + B (p-type)= Eg of semiconductor
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Al Spiking Problem
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Al-Si Eutectic Behavior
At thesinteringtemperatureof about450C aftermetalization,Si is solublein Al up to ~1% but Al isnot soluble inSi.
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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1. Add ~2% Si to Al to prevent Si outdiffusion
2. Use diffusion barrier layer to block Al/substrate interaction
Al Spiking Problem: Solutions
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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electron flow
moment transferredfrom electrons
Thermally excitedmetal atom out of lattice site
Metal Latticeatomic potential
position x
Electromigration Motion
Metal atom at lattice site(equilibrium)
•Electromigrated metal atoms move along the same direction of electrons
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Hillock formation (can short neighboring metal lines)
Void formation(metal line becomes open)
2 flux in 1 flux out
MassAccumulation
1 flux in 2 flux out
MassDepletion
Grainboundaries
Electromigrated atoms dominated by grain boundary diffusion
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Electron flow
HillockFormation
VoidFormation
LargeHillock(dendrite)
SEM micrographs ofAluminumelectromigrationfailure
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Dendrite Fomation Void Formation
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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* MTF defined as time for 50% of test samples to fail
MTF J –2 exp [ EA/ kT]
J = current density in Amp/cm2EA = activation energy ( ~ 0.5-0.8 eV for metals)
Median Time to Failure (MTF) ofElectromigration
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
22* Tradeoffs are etching difficulty and increased electrical resistivity
Shown are Acceleratedtest data:Testing temp ~225CJ ~ 1E6 Amp/cm2
Alloying Al with other elements will retardgrain boundary diffusion
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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How alloy precipitates can block grain boundary diffusion
Example: Al-Cu (1-4%) alloy. After sintering, AlCu3 compound willprecipitate along the grain boundaries.
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Suggested Metallization for 0.25m linewidths
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
Metal Silicides
Metal silicides can be used as :
1) Low-resistivity metal for source/drain contacts2) Shunting metal with poly-Si as interconnects
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
Metal Silicides can be prepared by:
1) Sputtering Deposition2) CVD3) Metal-Si Reactions
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
Properties of Metal Silicides
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Metal Deposition Techniques
• Sputtering has been the technique of choice– high deposition rate– capability to deposit complex alloy compositions– capability to deposit refractory metals– uniform deposition on large wafers– capability to clean contact before depositing metal
• CVD processes have recently been developed(e.g. for W, TiN, Cu)– better step coverage– selective deposition is possible– plasma enhanced deposition is possible for lower deposition
temperature
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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1) Tungsten (W)– used as contact plug, also as first-level metal
– blanket (non-selective) deposition processes:
HFWHWF 63 26
2446 2 HHFSiFWSiHWF
Example Metal CVD Processes
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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2) TiN– used as barrier-metal layer
– deposition processes:
HClTiNHNHTiCl 8222 234
234 NHCl24TiN6NH8TiCl6
HClTiNHNTiCl 8242 224
Metal CVD Processes (cont.)
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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3) CVD Copper
Metal-OrganicCu compound (gas)
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Trench filling with CVD Cu
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Cu Plating
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Electroless Cu Plating
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Low-K Dielectrics
Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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Professor N Cheung, U.C. Berkeley
Lecture 16EE143 F2010
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• Copper interconnects
• Low-K dielectrics(Fluorinated SiO2,polymers, xerogels...)
To further reduce RCtime delay:
Advanced Metalization Materials
*This photo is an idealizationonly, with an airgap betweenthe Cu layers.