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 metal wiring) Reduction in die size Higher circuit speed ( shorter interconnect distance) Flexibility in layout design 1
FOX Si substrate ( InteMetal Oxide e.g. BPSG. Low-K dieletric) (e.g. PECVD Si Nitride) 2
Outline Interconnections and Contacts RC Time Delay Interconnect resistance Contact resistance Dielectric Capacitance Reliability - Electromigration Multilevel Metallization Surface Planarization Techniques 3
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Interconnect RC Time Delay Interconnect Resistance RI =R/L = / (WAlTAl) Interconnect-Substrate Capacitance CV C/L = WAl ox / Tox Interconnect-Interconnect Capacitance CL C/L = TAl ox / SAl * Values per unit length L 5
Interconnect Interconnections Requirements low ohmic resistance interconnects material has low resistivity low contact resistance to semiconductor device reliable long-term operation 6
Possible interconnect materials 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 7
Resistivity of Poly-Si Resistivity of Pure Metals Poly-Si (min) =1E-3 -cm 8
Metal Contact to Si Tunneling ohmic contacts Al e n+ SiO2 M Ec Ev I Al h Si 1019-1021/cm3 n-si p+ Xd SiO2 > 1019/cm3 V p-si 9
metal semiconductor Depletion region Schottky Rectifying Contact Schottky Tunneling Ohmic Contact Large depletion width (lightly doped Si) Thermionic emission Tunneling Small depletion width (highly doped Si) 10
Contact Resistance Rc Heavily doped Semiconductor surface Metal Contact Area For a uniform current density flowing across the contact area Rc = c / (contact area ) c c of Metal-Si contacts ~ 1E-5 to 1E-7 -cm2 of Metal-Metal contacts < 1E-8 -cm2 11
Contact Resistivity c 1 J Specific contact resistivity c V at V~0 2 m * s c exp h B N B is the Schottky barrier height N = surface doping concentration c = specfic contact resistivity in ohm-cm2 m = electron mass h =Planck s constant = Si dielectric constant Approaches to lowering of contact resistance: 1) Use highly doped Si as contact semicodnuctor 2) Choose metal with lower Schottky barrier height 12
Schottky Barrier heights of common metals contacts to n-type semiconductors Note: B (n-type) + B (p-type) = Eg of semiconductor 13
Al Spiking Problem 14
Al-Si Eutectic Behavior At the sintering temperature of about 450C after metalization, Si is soluble in Al up to ~1 % but Al is not soluble in Si. 15
Al Spiking Problem: Solutions 1. Add ~2% Si to Al to prevent Si outdiffusion 2. Use diffusion barrier layer to block Al/substrate interaction 16
Electromigration Motion Metal Lattice atomic potential electron flow moment transferred from electrons Thermally excited metal atom out of lattice site position x Metal atom at lattice site (equilibrium) Electromigrated metal atoms move along the same direction of electrons 17
Electromigrated atoms dominated by grain boundary diffusion Hillock formation (can short neighboring metal lines) Grain boundaries Void formation (metal line becomes open) 2 flux in 1 flux out Mass Accumulation 1 flux in 2 flux out Mass Depletion 18
Void Formation Electron flow Hillock Formation Large Hillock (dendrite) SEM micrographs of Aluminum electromigration failure 19
Dendrite Fomation Void Formation 20
Median Time to Failure (MTF) of Electromigration * MTF defined as time for 50% of test samples to fail MTF J 2 exp [ EA/ kt] J = current density in Amp/cm2 EA = activation energy ( ~ 0.5-0.8 ev for metals) 21
Alloying Al with other elements will retard grain boundary diffusion Shown are Accelerated test data: Testing temp ~225C J ~ 1E6 Amp/cm2 * Tradeoffs are etching difficulty and increased electrical resistivity 22
How alloy precipitates can block grain boundary diffusion Example: Al-Cu (1-4%) alloy. After sintering, AlCu3 compound will precipitate along the grain boundaries. 23
Suggested Metallization for 0.25 m linewidths 24
Metal Silicides Metal silicides can be used as : 1) Low-resistivity metal for source/drain contacts 2) Shunting metal with poly-si as interconnects
Metal Silicides can be prepared by: 1) Sputtering Deposition 2) CVD 3) Metal-Si Reactions
Properties of Metal Silicides
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 28
Example Metal CVD Processes 1) Tungsten (W) used as contact plug, also as first-level metal blanket (non-selective) deposition processes: WF6 SiH 4 W SiF4 2 HF H 2 WF6 3 H 2 W 6 HF 29
Metal CVD Processes (cont.) 2) TiN used as barrier-metal layer deposition processes: 6TiCl 4 8NH 3 6TiN 24HCl N 2 2TiCl 4 2 NH 3 H 2 2TiN 8 HCl 2TiCl 4 N 2 4 H 2 2TiN 8 HCl 30
3) CVD Copper Metal-Organic Cu compound (gas) 31
Trench filling with CVD Cu 32
Cu Plating 33
Electroless Cu Plating 34
Low-K Dielectrics 35
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Advanced Metalization Materials To further reduce RC time delay: Copper interconnects Low-K dielectrics (Fluorinated SiO2, polymers, xerogels...) *This photo is an idealization only, with an airgap between the Cu layers. 38