Section 8: Metallization Jaeger Chapter 7 EE143 – Ali Javey Multilevel Metallization EE143 – Ali Javey Multilevel Metallization Components FOX Si substrate ( InteMetal Oxide e.g. BPSG. Low-K dieletric) (e.g. PECVD Si Nitride) EE143 – Ali Javey Interconnect RC Time Delay Interconnect Resistance RI =R/L = ρ / (WAlT Al) Interconnect-Substrate Capacitance CV ≡C/L = WAl εox / Tox Interconnect-Interconnect Capacitance CL ≡ C/L = TAl εox / SAl * Values per unit length L EE143 – Ali Javey Interconnect Requirements • low ohmic resistance – interconnects material has low resistivity • low contact resistance to semiconductor device • reliable long-term operation EE143 – Ali Javey Resistivity of Metals Commonly Used Metals Aluminum Titanium Tungsten Copper Less Frequently Utilized Nickel Platinum Paladium EE143 – Ali Javey Ohmic 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 contact EE143 – Ali Javey Contact Resistance Rc Heavily doped Semiconductor surface Metal Contact Area For a uniform current density flowing across the contact area Rc = ρc / (contact area ) 2 ρc of Metal-Si contacts ~ 1E-5 to 1E-7 Ω-cm 2 ρc of Metal-Metal contacts < 1E-8 Ω-cm EE143 – Ali Javey Contact Resistivity −1 ⎛ ∂J ⎞ Specific contact resistivity ρ ≡ c ⎜ ⎟ ⎝ ∂V ⎠ at V~0 ⎡2 m*ε ⎛ φ ⎞⎤ ρ = exp ⎢ s ⎜ B ⎟⎥ c ⎜ ⎟ ⎣⎢ h ⎝ N ⎠⎦⎥ φB is the Schottky barrier height m = electron mass N = surface doping concentration h =Planck’s constant 2 ρc = specfic contact resistivity in ohm-cm 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 EE143 – Ali Javey Aluminum 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% Si EE143 – Ali Javey Alloying of Contacts Alloy toObtainVery ContactLow Resistivit y Specific ResistivitContact y −6 2 =ρc 1x . 2 Ω 10 cm − ContactResistance RC ρ RA= =c contact area C A li JaveyAEE143 – Electromigration High current density causes voids to form in interconnections “Electron wind” causes movement of metal atoms EE143 – Ali Javey Electromigration • Copper added to aluminum to improve lifetime (Al, 4% Cu, 1% Si) 1 ⎛ E ⎞ MTF ∝ exp⎜ A ⎟ J 2 ⎝ kT ⎠ J = current density E A = activation energy MTF = mean time to failure • Heavier metals (e. g. Cu) have lower activation energy EE143 – Ali Javey 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 EE143 – Ali Javey Metal CVD Processes TiN – used as barrier-metal layer – deposition processes: 6 TiCl 8+ NH4 3 → 6 TiN + + 242 HCl N 2NHTiCl4 + 2 H3+ 2→2 TiN+ 8 HCl TiCl2 N4+ 24+ H2 → 2 TiN+ 8 HCl li JaveyAEE143 – Electroplating EE143 – Ali Javey Dual Damascene Process EE143 – Ali Javey Salicides • Self-Aligned Silicide on silicon and polysilicon • Often termed “Salicide” EE143 – Ali Javey Low-K Dielectrics EE143 – Ali Javey Porous low-k dielectric examples EE143 – Ali Javey.