Chapter 11 PVD and Metallization - ntut.edu.tw

1 12006/5/231 Chapter 11 PVD and Metallization2006/5/232 Metallization Processes that depositmetal thin filmonwafer Definition Applications PVD vs. CVD Methods Vacuum Metals Processes Future Trends2006/5/234 MaterialsDesignMasksIC FabTestPackagingFinal TestThermalProcessesPhoto-lithographyEtc hPR stripImplantPR stripMetalizationCMPD ielectricdepositionWafersWafer Process Flow32006/5/235 Physical Vapor Deposition2006/5/236 PVD Vaporizing solid materials Heating or sputtering Condensing vapor on the substrate surface Very important part of Metallization Methods Evaporation Sputtering42006/5/237 PVD Methods.

2 Evaporation Filaments Flash hot plate Electron beam2006/5/238 Thermal EvaporatorWafersAluminumChargeAluminum VaporHigh Current SourceTo Pump10-6 Torr52006/5/239 Electron Beam EvaporatorWafersAluminumChargeAluminum VaporPower SupplyTo Pump10-6 TorrElectronBeam2006/5/2310 PVD Methods: Sputtering DC Diode (simplest form) RF Diode DC Magnetron (most popular)62006/5/2311 SputteringMomentum transfer will dislodge surfaceatoms offAr+2006/5/2312DC Diode SputteringTargetArgon PlasmaWafer Chuck-VWaferMetal film72006/5/2313 Schematic of Magnetron SputteringMagnetsErosiongroveTargetHighe r plasmadensityMagnetic fieldline2006/5/2314 Magnetron Sputtering Most widely used PVD system More sputter from grove Better WTW uniformity cross wafer High deposition rate Good step coverage Good process (thickness)

3 Control82006/5/2315 PVD Chamber with ShieldTargetShield,LinerWafer ChuckWafer2006/5/2316 Sputtering Purer film Better uniformity Single wafer,better processcontrol Larger size waferEvaporator More impurities Batch process Cheaper toolSputtering vs. Evaporator92006/5/2317 PVD Vacuum Requirement Residue gases on the vacuum chamber wall H2O formation ,.. Water can react with Al to form Al2O3 Affects conductivity of interconnections Only way to get rid of H2O: reachultra highvacuum, 10-9 Torr2006/5/2318 PVD Vacuum Requirement Cluster tool Staged vacuum Loading station: 10 6 Torr Transfer chamber: 10 7to 10 8 Torr Deposition chamber: 10 9 Torr102006/5/2319 PVD Vacuum: Pumps Wet pump (oil diffusion pump): atm to 10-3 Torr, phasing out from fabs.

4 Rough (mechanical) pump: atm to 10-5 Torr Turbo pump: 10-2to 10-7 Torr Cryo pump: to 10-10 Torr Ion pump: to 10-11 Torr2006/5/2320 Endura PVD SystemPVDT argetPVDC hamberCVDC hamber112006/5/2321 PVD vs. CVD CVD: Chemical reaction on the surface PVD: No chemical reaction on the surface CVD: Better step coverage (50% to ~100%)and gap fill capability PVD: Poor step coverage (~ 15%) and gapfill capability2006/5/2322 PVD vs. CVD PVD: higher quality, purer deposited film,higher conductivity, easy to deposit alloys CVD: always has impurity in the film,lower conductivity, hard to deposit alloys122006/5/2323 Contact/Via Process Degas Pre-clean Ti PVD TiN PVD TiN CVD N2-H2plasma treatment W CVD2006/5/2324 Aluminum Interconnection Process Degas Pre-clean Ti PVD Al-Cu PVD TiN PVD132006/5/2325 Degas Heat wafer to drive away gases andmoisture on wafer surface Outgassing can cause contamination andhigh resistivity of deposited metal film2006/5/2326 Pre-clean Remove the native oxide Reduce the contact resistance Sputtering with argon ions RF plasma142006/5/2327 Pre-clean ProcessMetalNative OxideAr+Argon Plasma2006/5/2328 Titanium PVD Reduce

5 Contact resistance Larger grain size with low resistivity Wafer normally is heated to about 350 Cduring the deposition process Improve the surface mobility Improve step coverage152006/5/2329 Collimated Sputtering Used for Ti and TiN deposition Collimator allows metal atoms or molecules tomove mainly in vertical direction Reach the bottom of narrow contact/via holes Improves bottom step coverage2006/5/2330 Collimated SputteringPlasmaCollimatorMagnetsTargetF ilmVia holes162006/5/2331 Metal Plasma System Ti, TiN, Ta, and TaN deposition Ionize metal atoms through inductivecoupling of RF power in the RF coil Positive metal ions impact with thenegatively charged wafer surface vertically Improving bottom step coverage Reduce contact resistance2006/5/2332 Ionized Metal PlasmaTargetPlasmaVia Hole VRFI nductiveCoils172006/5/2333 Titanium nitride PVD Reactive sputtering process Ar and N2 N2molecules dissociate in plasma Free nitrogen radicals react with Ti to forma thin layer of TiN on target surface.

6 Argon ions sputter the TiN from the targetsurface and deposit it on the wafer surface2006/5/2334 Three Applications of TiNPSGTiSi2n+TiN, PVDTiN glue layer,PVD & CVDWAl-CuTiN ARC, PVD182006/5/2335Al-Cu PVD Ultra high vacuum to remove moisture andachieve low film resistivity. Cluster tool with staged vacuum dry pumps, turbo pumps and cryopump A cryopump can help a PVD chamber toreach up to 10-10 Torr base pressure byfreezing the residue gases in a frozen trap2006/5/2336Al-Cu PVD Standard processandhot aluminum process Standard process: Al-Cu deposition overtungsten plug after Ti and TiN deposition Normally deposit at ~ 200 C Smaller grain size, easier to etch Metal annealing to form larger grain size lower resistivity highelectromigrationresistance (EMR)192006/5/2337Al-Cu PVD Hot aluminum process To fill contact and via holes with Al insteadof W plug, thus reducing contact resistance Several process steps.

7 Ti deposition Al-Cu seed layer is deposited at low <200 C Bulk Al-Cu layer is deposited at highertemperatures (450 C to 500 C)2006/5/2338 Applications Interconnection Gate and electrodes Micro-mirror Fuse202006/5/2339 CMOS: Standard MetallizationP-waferN-WellP-WellSTIn+n+U SGp+p+Metal 1, Al CuBPSGWP-epiTiSi2 TiN, ARCTi/TiN2006/5/2340 Applications: Interconnection212006/5/2341 Applications: Interconnection Dominate the Metallization processes Al-Cu alloy is most commonly used W plug, technology of 80s and 90s Ti as welding layer TiN: barrier, adhesion and ARC layers The future is---Cu!

8 2006/5/2342 Applications: Gate and Electrode Al gate and electrode Polysilicon replace Al as gate material Silicide WSi2 TiSi2 CoSi2, MoSi2, TaSi2,.. Pt, Au,..as electrode for DRAM capacitors222006/5/2343Q & A Can we reduce all dimensions of metalinterconnection line at the same ratio? R= l/wh. When we shrink all dimensions(lengthl, widthw, and heighth) accordinglyto the shrinking of the device feature size,resistanceRincreases, Slower circuit and more power consumption2006/5/2344 Conducting Thin Films232006/5/2345 Conducting Thin Films Polysilicon Silicides Aluminum alloy Titanium Titanium nitride Tungsten Copper Tantalum2006/5/2346 Polysilicon Gates and local interconnections Replaced aluminum since mid-1970s High temperature stability Required for post implantation anneal process Al gate can not use form self-aligned source/drain Heavily doped LPCVD in furnace242006/5/2347 Silicide Much lower resistivity than polysilicon TiSi2, WSi2.

9 And CoSi2are commonly used2006/5/2348 Silicide TiSi2and CoSi2 Argon sputtering removes the native oxide Ti or Co deposition Annealing process forms silicide Ti or Co don t react with SiO2, silicide is formedat where silicon contacts with Ti or Co Wet strips unreacted Ti or Co Optional second anneal to increase conductivity252006/5/2349 Self-aligned Titanium SilicideFormationPolysilicon gateGate oxiden-n-n+n+TiPolysilicon gateGate oxiden-n-n+n+TiSi2 TiSi2 TiPolysilicon gateGate oxiden-n-n+n+TiSi2 TiSi2 TitaniumdepositionSilicide annealingTitanium wetstriping2006/5/2350 Tungsten Silicide Thermal CVD process WF6as the tungsten precursor SiH4as the silicon precursor.

10 Polycide stack is etched Fluorine chemistry etches WSix Chlorine chemistry etches polysilicon Photoresist stripping RTA increases grain size and conductivity262006/5/2351 Aluminum Most commonly used metal The fourth best conducting metal cm cm Gold cm cm It was used for gate before mid-19702006/5/2352 Aluminum-Silicon Alloy Al make direct contact with Si at source/drain Si dissolves in Al and Al diffuses into Si Junction spike Aluminum spikes punctuate doped junction Short source/drain with the substrate ~1% of Si alloyed in Al can stop this ! Thermal anneal at 400 C to form Si-Al alloyat the silicon-aluminum interface272006/5/2353p+p+Al-SiJunction Spiken-type SiliconAlAlAlSiO22006/5/2354 Electromigration Aluminum is a polycrystalline material Many mono-crystalline grains Current flows through an aluminum line Electrons constantly bombards the grains Smaller grains will start to move This effect is calledelectromigration282006/5/2355 Electromigration Electromigration tear the metal lineapart Higher current density in the remaining line Aggravates the electron bombardment Causes further aluminum grain migration