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 WAlTAl Interconnect Substrate Capacitance CV C L WAl ox Tox Interconnect Interconnect Capacitance CL C L TAl ox 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 EE143 Ali Javey Aluminum to p type silicon forms an ohmic contact Remember Al is ptype dopant Aluminum to n type silicon can form a rectifying contact Schottky barrier diode Aluminum to n silicon yields a tunneling contact 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 EE143 Ali Javey cm2 Contact Resistivity 1 Specific contact resistivity J 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 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 to Obtain Very Low Contact Resistivity Specific Contact Resistivity c 1 2 x10 6 cm 2 Contact Resistance RC RC EE143 Ali Javey c A A contact area 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 EA 1 exp kT J2 J current density E A activation energy MTF mean time to failure MTF 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 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 EE143 Ali Javey 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
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