EE143 – Ali JaveySection 8: MetallizationJaeger Chapter 7EE143 – Ali JaveyMultilevel MetallizationEE143 – Ali JaveyFOXSi substrate( InteMetal Oxide e.g. BPSG. Low-K dieletric)(e.g. PECVD Si Nitride)Multilevel Metallization ComponentsEE143 – Ali JaveyInterconnect ResistanceRI=R/L = ρ / (WAlTAl)Interconnect-Substrate CapacitanceCV≡C/L = WAlεox/ ToxInterconnect-Interconnect CapacitanceCL≡ C/L = TAlεox/ SAl* Values per unit length LInterconnect RC Time DelayEE143 – Ali JaveyInterconnect Requirements• low ohmic resistance– interconnects material has low resistivity• low contact resistance to semiconductor device• reliable long-term operationEE143 – Ali JaveyResistivity of MetalsCommonly Used MetalsAluminumTitaniumTungstenCopperLess Frequently UtilizedNickelPlatinumPaladiumEE143 – Ali JaveyOhmic Contact Formation• Aluminum to p-type silicon forms an ohmic contact [Remember Al is p-type dopant]• Aluminum to n-type silicon can form a rectifying contact (Schottky barrier diode)• Aluminum to n+ silicon yields a tunneling contactEE143 – Ali JaveyFor a uniform current density flowing across the contact areaRc= ρc/ (contact area )ρc of Metal-Si contacts ~ 1E-5 to 1E-7 Ω-cm2ρc of Metal-Metal contacts < 1E-8Ω-cm2MetalContact AreaHeavily dopedSemiconductor surfaceContact Resistance RcEE143 – Ali Javey⎥⎥⎦⎤⎢⎢⎣⎡⎟⎟⎠⎞⎜⎜⎝⎛=BscNhmφερ*2expApproaches to lowering of contact resistance:1) Use highly doped Si as contact semicodnuctor2) Choose metal with lower Schottky barrier heightφBis the Schottky barrier heightN = surface doping concentrationρc= specfic contact resistivity in ohm-cm2⎟⎠⎞⎜⎝⎛∂∂≡−cVJρ1at V~0m = electron massh =Planck’s constantε= Si dielectric constantSpecific contact resistivityContact ResistivityEE143 – Ali JaveyAluminum Spiking and Junction Penetration• Silicon absorption into the aluminum results in aluminum spikes• Spikes can short junctions or cause excess leakage• Barrier metal deposited prior to metallization• Sputter deposition of Al - 1% SiEE143 – Ali JaveyAlloying of Contactsareacontact =A ResistanceContact 102.1 y ResistivitContact SpecificyResistivitContact LowVery Obtain Alloy to26ARRcmxcCCcρρ=−Ω=−EE143 – Ali JaveyElectromigrationHigh current density causes voids to form in interconnections“Electron wind” causes movement of metal atomsEE143 – Ali JaveyElectromigration• Copper added to aluminum to improve lifetime (Al, 4% Cu, 1% Si)• Heavier metals (e. g. Cu) have lower activation energyMTF ∝1J2expEAkT⎛ ⎝ ⎜ ⎞ ⎠ ⎟ J = current densityEA= activation energyMTF = mean time to failureEE143 – Ali JaveyMetal 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 temperatureEE143 – Ali JaveyTiN– used as barrier-metal layer– deposition processes:HClTiNHNHTiCl 8222234+→++234NHCl24TiN6NH8TiCl6++→+HClTiNHNTiCl 8242224+→++Metal CVD ProcessesEE143 – Ali JaveyElectroplatingEE143 – Ali JaveyDual Damascene ProcessEE143 – Ali JaveySalicides• Self-Aligned Silicide on silicon and polysilicon• Often termed “Salicide”EE143 – Ali JaveyLow-K DielectricsEE143 – Ali JaveyPorous low-k dielectric
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