Metallization Interconnects Typical current density ~10 5 A/cm 2 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 R I =R/L = ρ / (W Al T Al ) Interconnect-Substrate Capacitance C V C/L = W Al ε ox / T ox Interconnect-Interconnect Capacitance C L C/L = T Al ε ox / S Al * Values per unit length L 5
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 Metal Silicides 15-80 µω-cm Poly-Si (min) =1E-3 Ω-cm 8
Metal Contact to Si Tunneling ohmic contacts X d Al M Si E c n + n-si SiO 2 e 10 19-10 21 /cm 3 E v Al I h p-si p + SiO 2 > 10 19 /cm 3 V 9
metal semiconductor Depletion region Schottky Rectifying Contact Large depletion width (lightly doped Si) Thermionic emission Schottky Tunneling Ohmic Contact Tunneling Small depletion width (highly doped Si) 10
Contact Resistance Rc Contact Area Metal Heavily doped Semiconductor surface For a uniform current density flowing across the contact area R c = ρ c / (contact area ) ρ c of Metal-Si contacts ~ 1E-5 to 1E-7 Ω-cm 2 ρ c of Metal-Metal contacts < 1E-8 Ω-cm 2 11
Contact Resistivity ρ c Specific contact resistivity ρ c J V 1 at V~0 ρ c 2 exp m s B = h * ε φ N φ B is the Schottky barrier height N = surface doping concentration ρ c = specfic contact resistivity in ohm-cm 2 m = electron mass h =Planck s constant ε= 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: Φ B (n-type) + Φ B (p-type) = E g 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 position x Thermally excited metal atom out of lattice site 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 1 flux in 2 flux out Mass Accumulation 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 [ E A / kt] J = current density in Amp/cm2 E A = 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/cm 2 * 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 0.25µm linewidths 24
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 25
Example Metal CVD Processes 1) Tungsten (W) used as contact plug, also as first-level metal blanket (non-selective) deposition processes: WF + + + + SiH W SiF 2HF H 6 4 4 2 WF + 3H W + 6HF 6 2 26
Metal CVD Processes (cont.) 2) TiN used as barrier-metal layer deposition processes: 6 TiCl + + + 8NH 6TiN 24HCl N 4 3 2 2 2 TiCl + 2NH + H 2TiN + 8HCl 4 3 2 2 TiCl + N + 4H 2TiN + 8HCl 4 2 27
3) CVD Copper Metal-Organic Cu compound (gas) 28
Trench filling with CVD Cu 29
Cu Plating 30
Electroless Cu Plating 31
Low-K Dielectrics 32
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Advanced Metalization Materials To further reduce RC time delay: Copper interconnects Low-K dielectrics (Fluorinated SiO 2, polymers, xerogels...) *This photo is an idealization only, with an airgap between the Cu layers. 35