Chapter 12 Chemical Mechanical Polishing - …

1 11 Chapter 12 Chemical MechanicalPolishing2 Objectives List applications of CMP Describe basic structure of a CMP system Describe slurries for oxide and metal CMP Describe oxide CMP process. Describe metal Polishing process. Explain the post-CMP clean23 MaterialsDesignMasksIC FabTestPackagingFinal TestThermalProcessesPhoto-lithographyEtc hPR stripImplantPR stripMetalizationCMPD ielectricdepositionWafersWafer Process Flow4 Overview Multi layer metal interconnection is required Planarization of dielectric layers is important Depth of focus (DOF) requires a flat surface toachieve high resolution The rough dielectric surface can also causeproblems in later metallization35 Tungsten CMP Tungsten has been used to form metal plugs CVD tungsten fills contact/via holes andcovers the whole wafer. Need to remove the bulk tungsten film fromthe surface Fluorine based plasma etchback processes Tungsten CMP replaced etchback6P-EpiP-WaferMetal 3Al Cu AlloyIMD 3 USGM etal 4Al CuUSGS ilicon NitrideAl Cu AlloyN-WellP-WellBPSGn+n+p+p+STIUSGWAl Cu AlloyUSGM1M2Al CuUSGWIMD 1 IMD 2 TiSi2 TiTiN ARCWTi/TiNTi/TiNSidewallSpacer, USGPMD BarrierNitrideIMD 3 Passivation 1 Passivation 2 PMDCMP PSG, WCMP USG, WCMP USG, WCMP USGWCMP USGCMOS IC47 Definition of Planarization Planarization is a process that removes thesurface topologies, smoothes and flattensthe surface The degree of planarization indicates theflatness and the smoothness of the surface8 Definition of PlanarizationCompletely Conformal Film, No PlanarizationConformal and Smooth, No Planarization59 Definition of PlanarizationPartial PlanarizationGlobal Planarization10 Definition of PlanarityR PlanarityR( m)

2 Surface to > 30 Local to 10030to Planarization> 100< Surface smoothing and local planarizationcan be achieved by thermal flow oretchback Global planarization is required for thefeature size smaller than m, whichcan only be achieved by CMP12 Traditional Planarization Methods Thermal flow Sputtering etchback Photoresist etchback, Spin-on glass (SOG) etchback713 Thermal Flow A common method for dielectricplanarization, , pre-metal dielectric High temperature operation, ~1000 C PSG or BPSG reflow, become soft and startto flow due to the surface tension Limitations: High temp causes excessivedopantdiffusion Higherdopantconcentration can low downtemp, however, also cause metal corrosion ifphosphorusdopantis too high (> 7 wt%)14As DepositedBPSGP-type substratep+p+N-wellP-type substrateSiO2n+n+p+p+815 After Thermal FlowBPSGP-type substratep+p+N-wellP-type substrateSiO2n+n+p+p+16 Etch Back Reflow temperature is obviously too highfor IMD can melt aluminum metal Other planarization method is needed forIMD Sputteringetchback(ion mill)

3 And reactiveetchbackare employed917 Etch Back Argon sputtering etchback chip off dielectricat corner of the gap and taper the openings Subsequent CVD process easily fills the gapwith a reasonable planarized surface Reactive ion etchback process with CF4/O2chemistry further planarizes the surface18 CVD USGUSGAl Cu SiBPSGP-type substratep+p+N-wellP-type substraten+n+p+p+SiO21019 Sputtering Etch Back of USGUSGAl Cu SiBPSGP-type substratep+p+N-wellP-type substraten+n+p+p+SiO220 CVD USGUSGAl Cu SiBPSGP-type substratep+p+N-wellP-type substraten+n+p+p+SiO21121 Reactive Etch Back of USGUSGAl Cu SiBPSGP-type substratep+p+N-wellP-type substraten+n+p+p+SiO222 Photoresist Etchback PR spin-coats can baking Planarized solid thin film on wafer surface Plasma etch process with CF4/O2chemistry Oxide etched by F and PR by O Adjusting CF4/O2flow ratio allows 1:1 ofoxide to PR selectivity.

4 Oxide could be planarized after etchback1223 After Oxide DepositedOxideMetalMetal24 Photoresist Coating and BakingOxidePhotoresistMetalMetal1325 Photoresist EtchbackOxideMetalMetal26 Photoresist EtchbackOxideMetalMetal1427 Photoresist EtchbackOxideMetalMetal28 Photoresist Etchback When F etch oxide, O will be released Higher PR etch rate due to extra oxygen PR etchback can t planarize very well After the PR etchback, dielectric filmsurface is flatter than it is just deposited. In some cases, more than one PR etchbackis needed to achieve required flatness1529 SOG Etchback SOG replaces PR Advantage: some SOG can stay on the wafersurface to fill the narrow gaps PECVD USG liner and cap layer USG/SOG/USG gap fill and surfaceplanarization Sometimes, two SOG coating-curing-etchbackprocesses are used30 SOG Etchback1631 Necessity of CMP Photolithography resolutionR = K1 /NA To improve resolution,NA or DOF = K2 /2(NA)2, both approaches toimprove resolution reduceDOF DOF is about 2,083 for m and1,500 for m resolution.

5 Here we assumedK1=K2, =248 nm(DUV), andNA= of CMP m pattern require roughness < 2000 Only CMP can achieve this planarization When feature size > m, other methodscan be used1733 Advantages of CMP Planarized surface allows higher resolution ofphotolithography process The planarized surface eliminates sidewallthinning because of poor PVD step coverage34 Metal Line Thinning Due to theDielectric StepMetal 1 Metal 2 IMD 1 PMDS idewall Thinning1835 Planarized Dielectric Surface, noMetal Line Thinning EffectMetal 1 IMD 1 PMDM etal 236 Advantages of CMP Eliminate the requirement of excessiveexposure and development to clear the thickerphotoresist regions due to the dielectric steps This improves the resolution of via hole andmetal line pattering processes Uniform thin film deposition Reduce required over etch time Reduce chance of undercut or substrate loss1937 PROver Exposure and OverDevelopmentMetal 2 Metal 2 IMD 1 PRPRN eeds more exposureand developmentPossible CD loss due to moreexposure and development38 Rough Surface, Long Over EtchMetal 2 Metal 2 IMD 1 PRPRNeed a long overetch to remove2039 Flat Surface, Short Over EtchMetal 2 Metal 2 PRPRVery litter overetch is requiredIMD 140 Advantages of CMP CMP reduce defect density,improve yield Reducing the process problems in thin filmdeposition, photolithography, and etch.

6 CMP also widens IC chip design parameters CMP can introduce defects of its own Need appropriate post-CMP cleaning2141 Applications of CMP STI formation Dielectric layer planarization PMD and IMD Tungsten plug formation Deep trench capacitor42 Applications of CMPSTICMP USGCMP PSG, WCMP PSG, WCMP USGCMP USGCMP USGCMP WCMP W2243 Applications of CMP Copper interconnection. Copper is very difficult to dry etch, Dual damascene: process of choice Tungsten plug is a damascene process44 Applications of CMP It uses two dielectric etch processes, one via etch and one trench etch Metal layers are deposition into via holesand trenches. A metal CMP process removes copper andtantalum barrier layer Leave copper lines and plugs imbeddedinside the dielectric layer2345 PECVD NitrideP-EpiP-WaferN-WellP-Welln+STIp+p+ USGWPSGN itriden+46 PECVD USGP-EpiP-WaferN-WellP-Welln+STIp+p+USGW PSGUSGN itriden+2447 PECVD Etch Stop NitrideP-EpiP-WaferN-WellP-Welln+STIp+p+ USGWPSGUSGN itriden+48 PECVD USGP-EpiP-WaferN-WellP-Welln+STIp+p+USGW PSGUSGn+USG2549 Photoresist CoatingP-EpiP-WaferN-WellP-Welln+STIp+p+ USGWPSGUSGn+USGP hotoresist50 Via 1 Mask2651 Via 1 Mask Exposure and DevelopmentP-EpiP-WaferN-WellP-Welln+STI p+p+USGWPSGUSGn+USGP hotoresist52 Etch USG.

7 Stop on NitrideP-EpiP-WaferN-WellP-Welln+STIp+p+ USGWPSGUSGn+PhotoresistUSG2753 Strip PhotoresistP-EpiP-WaferN-WellP-Welln+STI p+p+USGWPSGUSGn+USG54 Photoresist CoatingP-EpiP-WaferN-WellP-Welln+STIp+p+ USGWPSGUSGn+USGP hotoresist2855 Metal 1 Mask56 Metal 1 Mask Exposure andDevelopmentP-EpiP-WaferN-WellP-Welln+ STIp+p+USGWPSGUSGn+USGP hotoresist2957 Etch USG and NitrideP-EpiP-WaferN-WellP-Welln+STIp+p+ USGWPSGn+PhotoresistUSGUSG58 Strip PhotoresistP-EpiP-WaferN-WellP-Welln+STI p+p+USGWPSGn+USGUSG3059 Deposit Tantalum Barrier LayerP-EpiP-WaferN-WellP-Welln+STIp+p+US GWPSGn+USGUSG60 Deposit CopperP-EpiP-WaferN-WellP-Welln+STIp+p+U SGWPSGn+USGUSGC opper3161 CMP Copper and TantalumP-EpiP-WaferN-WellP-Welln+STIp+p +USGWPSGn+USGUSGCuM162 PECVD Seal NitrideP-EpiP-WaferN-WellP-Welln+STIp+p+ USGWPSGn+USGUSGCuM13263 CMP Hardware Polishing pad Wafer carrier Slurry dispenser64 Chemical Mechanical PolishingSlurryPolishing PadPressureWafer HolderWaferMembranePlatenSlurry DispenserRetaining Ring3365 Linear Polishing SystemSlurryPressureWafer CarrierWaferMembraneSlurry DispenserRetaining RingPadConditionerBelt and Polishing PadSupport Fluid Bearing66 Orbital PolishingDown ForceSlurryWaferCarrierFilm cOrbital Motion, pPolishPad3467 Polishing Pad Porous, flexible polymer material cast, sliced polyurethane or urethane coatedpolyester felt Pad directly affects quality of CMP process Pad materials: durable, reproducible,compressible at process temperature Process requirement: high topographyselectivity to achieve surface planarization68 Polishing PadHardness Harder Polishing pad: higher removal rateand better within die (WID) uniformity Softer pad: better within wafer (WIW)uniformity.

8 Hard pads easier to cause scratches. The hardness is controlled by pad chemicalcompositions or by cellular Pad Cells absorb Polishing slurry Filler improve Mechanical properties Polishing pad surface roughness determinesthe conformality range. Smoother pad has poorer topographicalselectivity less planarization effect. Rougher pad has longer conformality range andbetter planarization Polishing result70 Hard Rough PadPolishing PadFilmWaferPad Movement3671 Soft Smooth PadPolishing PadFilmWaferPad Movement72 Pad Conditioning Pad becomes smoother due to the Polishing Need to recreate rough pad surface In-situ pad conditioner for each pad The conditioner resurfaces the pad Removes the used slurry Supplies the surface with fresh slurry3773 Polishing Pad and Pad ConditionerWaferCarrierSlurryDispenserPa dConditionerPolishingPad74 Polishing Head Polishing head is also called wafer carrier It consists of a Polishing head body Retaining ring Carrier membrane Down force driving system3875 Polishing HeadPolishing Head BodyRetaining RingCarrierMembrane76 Schematic of Polishing HeadVacuum ChuckDownforcePressureRestraining RingPositioningWaferRestrainingRingRestr ainingRingMembraneCarrierChamber3977

9 Pad Conditioner Sweeps across the pad to increase surfaceroughness required by planarization andremoves the used slurry Conditioner is a stainless steel plate coatedwith nickel-plated diamond grits Diabond CMP conditioner: stainless steelplate coated with CVD diamond film plateddiamond grids78 Surface of CMP ConditionersStainless Steel PlateNickel FilmDiamond GritsConventionalStainless Steel PlateDiamond FilmDiamond Grits (~ 20 m)DiabondSilicon Substrate4079 CMP Slurries Chemicals in the slurry react with surfacematerials, form Chemical compounds thatcan be removed by abrasive particles Particulate in slurry mechanically abradethe wafer surface and remove materials Additives in CMP slurries help to achievedesired Polishing results80 CMP Slurries CMP slurries work just like toothpaste Chemicals kill gems, remove tartar, andform protection layer on the teeth Particles abrade away unwanted coatingfrom tooth surface during tooth brushing4181 CMP Slurries Water-based chemicals with abrasive particles andchemical additives Different Polishing processes require differentslurries Slurry can impact removal rate, selectivity,planarity and uniformity Slurries always are engineered and formulated fora specific Slurries Oxide slurry: alkaline solution with silica Metal slurry.

10 Acidic solution with alumina Additives control the pH value of slurries oxide, pH at 10 to 12 metal, pH at 6 to 24283pH ValuespH02468101214 More AcidicMore Basic7 Neutral1359111384 Slurry Delivery Slurry components are stored separately DI water with particulate additives for pH control oxidants for metal oxidation Flow to a mixer to mix at required ratio4385 Slurry FlowDI + SuspensionspH AdjusterOxidantLFCLFCLFCM ixerCMPToolLFC: liquid flow controller86 Oxide Slurry Based on experience of optical industry,which polish silicate glass to make lensesand mirrors for a long time Oxide slurry is a colloidal suspension offine fumed silica (SiO2) particles in water KOH is used to adjust the pH at 10 to 12 NH4OH can also be used4487 Oxide Slurry Abrasives: fumed silica particles Normally contain ~ 10% solids Shelf lifetime of up to 1 year with propertemperature control88 Fumed Silica Fumed silica particles are formed in a vaporphase hydrolysis of SiCl4in a hydrogen-oxygen flame2 H2+ O2 2 H2 OSiCl4+ 2 H2O SiO2+ 4 HCl Overall reactionSiCl4+ 2 H2+ O2 SiO2+ 4 HCl 4589 Fumed Silica Particle FormationH2>1800 C<1710 CAggregatesAgglomeratesO2 SiCl4 CollectionSystemCooling90 Fumed Silica ParticlesCourtesy of Fujimi Corporation4691 Metal Polishing Slurry Metal CMP process is similar to the metalwet etch process Oxidant reacts with metal to form oxide Metal oxide is removed Repeat metal oxidation and oxide removal92 Metal Polishing Slurry The metal CMP slurries usually are pH-adjusted suspensions of alumina (Al2O3)