EE143 F2010 Lecture 16 Metallizationee143/fa10/lectures/Lec_16.pdfEE143 F2010 Lecture 16 7 "Metal (Low resistivity) ... Low-resistivity metal for source/drain contacts 2) Shunting

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



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FOX

Si substrate

( InteMetal Oxide e.g. BPSG. Low-K dieletric)

(e.g. PECVD Si Nitride)



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• Interconnections and Contacts– RC Time Delay– Interconnect resistance– Contact resistance– Dielectric Capacitance– Reliability - Electromigration

• Multilevel Metallization• Surface Planarization Techniques

Outline



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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



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InterconnectionsInterconnect Requirements

• low ohmic resistance– interconnects material has low resistivity

• low contact resistance to semiconductordevice

• reliable long-term operation



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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



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Resistivity of Poly-Si

Resistivity of Pure Metals

Poly-Si (min) =1E-3 -cm



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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



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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



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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



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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



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Schottky Barrier heightsof common metals contactsto n-type semiconductors

Note:B (n-type) + B (p-type)= Eg of semiconductor



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Al Spiking Problem



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Al-Si Eutectic Behavior

At thesinteringtemperatureof about450C aftermetalization,Si is solublein Al up to ~1% but Al isnot soluble inSi.



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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



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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



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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



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Electron flow

HillockFormation

VoidFormation

LargeHillock(dendrite)

SEM micrographs ofAluminumelectromigrationfailure



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Dendrite Fomation Void Formation



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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



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



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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.



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Suggested Metallization for 0.25m linewidths



Metal Silicides

Metal silicides can be used as :

1) Low-resistivity metal for source/drain contacts2) Shunting metal with poly-Si as interconnects



Metal Silicides can be prepared by:

1) Sputtering Deposition2) CVD3) Metal-Si Reactions



Properties of Metal Silicides



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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



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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



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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.)



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3) CVD Copper

Metal-OrganicCu compound (gas)



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Trench filling with CVD Cu



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Cu Plating



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Electroless Cu Plating



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Low-K Dielectrics



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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.

Documents

EE143 F2010 Lecture 16 Metallizationee143/fa10/lectures/Lec_16.pdfEE143 F2010 Lecture 16 7 "Metal (Low resistivity) ... Low-resistivity metal for source/drain contacts 2) Shunting