Gate contacts Al cannot withstand the temperatures needed for MOSFET fabrication. Instead, gate contacts are made of Polysilicon, doped heavily,.or Silicides: For lower resistivity than poly Si Make by sputtering CVD, or by depositing metal on Si then reacting Silicide over poly-Si(polycide)on the gate takes advantage of the reliable Si/oxide inter face but with lower resistance Silicides can be made on gate, source and drain by self-aligned process C"salicide): the silicide forms only at the places where the metal contacts the silicon then the unreacted metal is removed Many possibilities WSi,, TaSi,, MoSi,, TiSi,, CoSi, All have different microstructures. thermal stabilities and reactivities ll/703 3.155J6.l52J 19 Mechanical stability As wafer is thermally cycled during processing packaging, mechanical stresses can build up due to thermal mismatch Deposited films, oxides etc also grow with internal stress. These stresses can cause delamination and void or hillock formation Mechanical properties of interconnect muterial Thermal modulus Hardness Melting material coefficient(C) Y/(1-1)(MPa) (kg mm) point (C) Al(111)23.1×10-° 1.143×10° 8.41×10-6 1.699×1081-14 TAI 123×10-6 660-750 Si(10026×10- 1.805×10° 1417 Si(1) 2.290×10° 0.55×10-6 083×10° 170011/17/03 3.155J/6.152J 20 11/17/03 3.155J/6.152J Gate contacts Al cannot withstand the temperatures needed for MOSFET fabrication. Instead, gate contacts are made of: Silicides Many possibilities WSi2, TaSi2, MoSi2, TiSi2 2 Several complex crystal structures may be possible in each system. 19 Polysilicon, doped heavily, …or ... : For lower resistivity than polySi. Make by sputtering, CVD, or by depositing metal on Si then reacting. Silicide over poly-Si (“polycide”) on the gate takes advantage of the reliable Si/oxide interface but with lower resistance Silicides can be made on gate, source and drain by self-aligned process (“salicide”): the silicide forms only at the places where the metal contacts the silicon, then the unreacted metal is removed. , CoSi All have different microstructures, thermal stabilities and reactivities. Mechanical stability As wafer is thermally cycled during processing & packaging, mechanical stresses can build up due to thermal mismatch. Deposited films, oxides etc. also grow with internal stress. These stresses can cause delamination and void or hillock formation. 10