EE143 S06 Lecture 18 Nonplanar Metallization Planar Metallization Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Caps and Plugs oxide oxide The plug material can be same as interconnect material e g Cu or different material e g W Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Non Optimized Planarization example Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Good Planarization Example Five Level Metallization for Company C W plugs for contacts and vias W for metal 1 Al Ti metal 2 3 4 Al metal 3 CMP for all dielectrics Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Surface Planarization Benefits for Lithography Processes Lower Depth of Focus requirement Reduced optical reflection effects on resist profiles Reduced resist thickness variation over steps topview Benefit for Etching Processes Reduced over etch time required due to steps Benefit for Deposition Processes Improved step coverage for subsequent layer deposition Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Planarization Factor 1 final step height initial step height Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Planarization A bad example Topography management must start at lower layers Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Planarization Techniques 1 Spin on glass or polyimide 2 Deposit and Etchback 3 Chemical Mechanical Polishing CMP Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Spin On Glass SOG Professor N Cheung U C Berkeley EE143 S06 Lecture 18 SOG Annealing Cure 400 500oC inorganic backbone polymer exact composition depends on SOG type 800 1100oC densified glass inorganic SOGs can be performed in N2 O2 or steam steam allows densification to occur at lower temperatures Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Polyimides Polymers excellent thermal stability up to 450oC good dielectric properties r 3 3 1016 cm superior chemical resistance Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Deposit and Etchback 1 Deposit thick oxide layer 600 1000nm 2 Spin on resist or polymer to planarize surface 3 Etch back with a process that has equal oxide and resist polymer etch rates e g CF4 O2 dry etch 4 Deposit second oxide layer Simple process requiring equipment and materials already available in the lab Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Chemical Mechanical Polishing CMP Wafer is polished using a slurry containing silica abrasives 10 90 nm particle size etching agents e g dilute HF Backing film provides elasticity between carrier and wafer Polishing pad made of polyurethane with 1 mm perforations rough surface to hold slurry Professor N Cheung U C Berkeley EE143 S06 Lecture 18 CMP Configurations Rotating wafer Rotating pad Professor N Cheung U C Berkeley Rotating Wafer Linear track pad EE143 S06 Lecture 18 CMP Process Control For reference only Professor N Cheung U C Berkeley EE143 S06 Lecture 18 CMP Rate Preston Model Local Removal rate R Kp P v where P local applied pressure v relative pad wafer velocity Kp Preston coefficient unit in pressure 1 function of film hardness Young s modulus slurry pad composition and structure Professor N Cheung U C Berkeley EE143 S06 Lecture 18 CMP Selectivity CMP of Professor N Cheung U C Berkeley Selectivity to Si3N4 SiO2 6 1 poly Si 280 1 W 75 1 Al 40 1 EE143 S06 Lecture 18 Problems encountered in CMP Pattern dependence Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Pattern Dependence of CMP Note Y axis highly magnified High isolated features polish fast Increased pressure at corners of features creates rounding Professor N Cheung U C Berkeley EE143 S06 Lecture 18 CMP Endpoint Detection 1 Polisher Motor Current Professor N Cheung U C Berkeley EE143 S06 Lecture 18 CMP Endpoint Detection 2 Optical Interference Professor N Cheung U C Berkeley EE143 S06 Lecture 18 CMP Application Example RIE of Cu difficult due to low vapor pressure of by products Cu lines formed by CMP Cu has to be encapsulated by a liner e g TiN to prevent out diffusion into SiO2 and Si Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Single Damascene Process ILD Inter Level Dielectric 1 2 5 3 6 4 Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Dual Damascene Cu Metallization with Diffusion Barrier Cu Professor N Cheung U C Berkeley EE143 S06 Lecture 18 Dual Damascene Process Sequence Professor N Cheung U C Berkeley EE143 S06 Professor N Cheung U C Berkeley Lecture 18
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