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Plasma EtchingROCHESTER INSTITUTE OF TECHNOLOGYMICROELECTRONIC ENGINEERINGPlasma EtchingDr. Lynn Fullerhttp://people.rit.edu/lffeeeMicroelectronic EngineeringRochester Institute of Technology82 Lomb Memorial DriveRochester, NY 14623-5604Tel (585) 475-2035Fax (585) duRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn Fuller4-17-13 Plasma Etch.pptPage 1

Plasma EtchingOUTLINE Introduction Plasma Etching Metrics – Isotropic, Anisotropic,Selectivity, Aspect Ratio, Etch Bias Plasma and Wet Etch Summary The Plasma State - Plasma composition, DC & RF Plasma Plasma Etching Processes - The principle of plasmaetching, Etching Si and SiO2 with CF4 Other Plasma Processes - Sputtering, resist stripping, CVD Equipment Advanced Plasma Systems Trends, Recent Advances SummaryRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 2

Plasma EtchingINTRODUCTION - IDEAL ETCHING PROCESSPrior to etchresistfilm to be etchedsubstrateIdeal etching is the accurate transfer of the pattern to the underlying filmAfter etchresistsubstrateNo process is ideal, some anisotropic plasma etches are closeRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 3

Plasma EtchingWET ETCHNGThermometerProbe withglass coverTeflon CoverHolesPlastic Screwfor HandleWafer BoatWafersHot Plate50 CControllerRochester Institute of TechnologyMicroelectronic EngineeringTeflon Stirrer& Guide PlateGenerally Wet Etching is Isotropic April 17, 2013 Dr. Lynn FullerPage 4

Plasma EtchingWET ETCHINGresistBefore EtchsubstrateBlockingRochester Institute of TechnologyMicroelectronic r Isotropic Etch April 17, 2013 Dr. Lynn FullerPage 5

Plasma EtchingDIRECTIONALITY OF THE ETCHxDegree of AnisotropyA (z-x)/zA 1A 0zresistresistzzsubstrateAnisotropic EtchA 0.9xUndercutresistzsubstrateIsotropic Etch No r Institute of TechnologyMicroelectronic EngineeringOveretch Isotropic EtchAnisotropic Etch April 17, 2013 Dr. Lynn FullerA NegPage 6

Plasma EtchingSELECTIVITYRprRfPhotoresistNitrideRoxPad OxideRf etch rate for nitride filmRpr etch rate for photoresistRox etch rate for pad oxideSiliconWe want Rf high and Rpr, Rox lowSelectivity of film to Photoresist Rf/RphSelectivity of film to pad oxide Rf/RoxRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 7

Plasma EtchingUNIFORMITYEtch rate non-uniformity (%) (Maximum etch rate-Minimum etch rate)(Maximum etch rate Minimum etch rate)x100%Example: Calculate the average etch rate, etch rate uniformitygiven the etch rates at center, top left, top right, bottom right,bottom left are 750, 812, 765, 743, 798 nmRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerAnswer: 773.6 nmand 4.4%Page 8

Plasma EtchingWET ETCHING CHARACTERISTICSAdvantages:Simple equipmentHigh throughput (batch process)High selectivityDisadvantages:Isotropic etching leads to undercuttingUses relatively large quantities of etch chemicals, must immerse waferboats, must discard partially used etch to maintain etch rateHot chemicals create photoresist adhesion problemsSmall geometries difficult, etch block caused by surface tensionCritical Etch time, dimensions change with etch time, bias developsChemical costs are highDisposal costs are highRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 9

Plasma EtchingTHE NEED FOR PLASMA ETCHINGAdvanced IC Fabrication with small geometries requires precisepattern transferSub Micrometer Geometry is commonLine widths is often comparable to film thicknessSome applications require high aspect ratioSome materials wet etch with difficultyHigh aspect ratio, anisotropicetch; only possible throughplasma etchRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 10

Plasma EtchingPLASMA ETCHING CHARACTERISTICSAdvantages:No photoresist adhesion problemsAnisotropic etch profile is possibleChemical consumption is smallDisposal of reaction products less costlySuitable for automation, single wafer, cassette to cassetteDisadvantages:Complex equipment, RF, gas metering, vacuum, instrumentationSelectivity can be poorResidues left on wafer, polymers, heavy metalsParticulate formationStringers, profile effectsRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 11

Plasma EtchingGEC CELLSF6CF4CHF3O2H2ArHeRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 12

Plasma EtchingTHE PLASMA STATEPlasma - A partially ionized gas with equal numbers of positive andnegative particles. Overall the plasma remains electrically neutral.Glow Discharge - A non-ideal plasma. Some regions are positivelycharged, others are negative. A wide variety of particles exist in thedischarge in addition to ions and electrons, including for example,radicals, excited species, and various fractured gas moleculescreated by collisions between electronics and gas molecules oratoms. Overall, the discharge system must remain electricallyneutral even though some portions of it are not. (Glow Dischargeand Plasma are terms that are used interchangeably in dry etching)Collisions – Ions and electrons are accelerated by the electric field,and collide with other gas particles and bombard all surfaces.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 13

Plasma EtchingTHE PLASMA STATEIon, (Positive) - A positively charged particle - a gas molecule oratom with and electron removed.Radical - A neutral gas particle (atom or molecule) that exists ina state of incomplete chemical bonding and is thereforechemically reactive. It is formed by the fracturing of a gasmolecule by a high energy electron collision.Example: O2 e- -- 2ORochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 14

Plasma EtchingPLASMA COMPOSITIONA typical plasma contains:Neutral Molecules at a density ofRadicalsElectronsPositive ions10e16/cm310e14/cm310e8/cm310e8/cm3There are a million times more radicals than ions or electrons.Radicals form more easily and their lifetime is much longer.Ions don’t etch, radicals do. Ions affect the process by energetic(physical) bombarding of the surface, influencing the chemicalprocesses of etching.Radicals are responsible for the dry etching process. They arechemically active and react with the surfaces to produce volatileRochester Institute of Technologyproducts.Microelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 15

Plasma EtchingBASICS OF PLASMA ETCHINGCF4 is inert gasadd electron impact to produce fluorine radicals:CF4 e CF3 F 2 e (Dissociative Ionization)CF4 e CF3 F e(impact dissociation)Then chemically form volatile SiF4:Si 4F SiF4 (gas)DischargeSiF4CF4FRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullersiliconPage 16

Plasma EtchingADDED GASESHydrogen - reduces fluorine concentration by combination to form HFOxygen - Increases fluorine concentration by combining with carbonwhich would otherwise require fluorine or reacting with CF3 toliberate FArgon – Increases plasma density increasing fluorine radical conc.Helium – Carries heat away and helps photoresist survivalGasCF4C2F6C3F8CHF3C:F Ratio1:41:31:2.71:3SiO2:Si Selectivity1:13:15:110:1Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 17

Plasma EtchingETCH RATERelative Etch Rate1.00.8Etch Rate nm/min100Poly800.6600.4400.22010 20 30 40Percent O250%Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPressure 25 mTorrFlow Rate 40 sccmResistSiO2Poly/Si10 20 30 40 50%Percent H2 in CF4 H2MixturesPage 18

Plasma EtchingSILICON ETCHING MECHANISMCF4 is Freon 14F/C ratio is 4CF4 e- -- CF3 F eF radicals adsorb on silicon surface; SiF4 desorbsCF3 radicals also adsorbCF3 F -- C4 desorbsThe presence of carbon on the surface reduces the amount offluorine available to etch silicon. Carbon will leave the surface bycombining with F reducing fluorine, carbon can remain on thesurface forming C-F polymers which in turn inhibits etching. HighF/C ratio favors etching. Adding O2 can increase etch rate andincreases selectivity over oxide.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 19

Plasma EtchingSILICON DIOXIDE ETCHING MECHANISMC3 and F radicals adsorb. C bonds with oxygen at the surface F bondswith Si. By-products are CO, CO2, COF2, SiF4. The addition of H2removes F from the system by forming stable HF gas. Addition of H2therefore decreases the effective F/C ratio and increases selectivity ofSiO2 with respect to silicon. As H2 is increased, it begins to consumefluorine H F HF This slows the formation of SiF4 and slows theremoval of Silicon. Polymerization will be promoted on all surfaces,which tends to inhibit etching. On horizontal surfaces however, ionicbombardment provides enough energy to cause the carbon/hydrogen tocombine with surface oxygen. Released CO and H2O expose thesurface silicon which is removed by combining with released fluorineradicals. Silicon will not be etched because of the absence of oxygenat the surface.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 20

Plasma EtchingDRYTEK QUAD ETCH RECIPE FOR CC AND VIARecipe Name:ChamberPowerPressureGas 1Gas 2Gas 3Gas 4FACCUT3200W100 mTorrCHF3 50 sccmCF410 sccmAr100 sccmO20 sccm(could be changed to N2)TEOS Etch RateAnnealed TEOSPhotoresist Etch Rate:Thermal Oxide Etch Rate:Silicon Etch RateTiSi2 Etch Rate494450117441821US Patent 5935877 - Etchprocess for forming contactsover titanium ter Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerDrytek QuadPage 21

Plasma EtchingCONTACT CUT ETCH RECIPETheory: The CHF3 and CF4 provide the F radicals that do the etching of thesilicon dioxide, SiO2. The high voltage RF power creates a plasma and the gassesin the chamber are broken into radicals and ions. The F radical combines with Sito make SiF4 which is volatile and is removed by pumping. The O2 in the oxide isreleased and also removed by pumping. The C and H can be removed as CO,CO2, H2 or other volatile combinations. The C and H can also form hydrocarbonpolymers that can coat the chamber and wafer surfaces. The Ar can be ionized inthe plasma and at low pressures can be accelerated toward the wafer surfacewithout many collisions giving some vertical ion bombardment on the horizontalsurfaces. If everything is correct (wafer temperature, pressure, amounts of polymerformed, energy of Ar bombardment, etc.) the SiO2 should be etched, polymershould be formed on the horizontal and vertical surfaces but the Ar bombardmenton the horizontal surfaces should remove the polymer there. The O2 (O radicals)released also help remove polymer. Once the SiO2 is etched and the underlying Siis reached there is less O2 around and the removal of polymer on the horizontalsurfaces is not adequate thus the removal rate of the Si is reduced. The etch rate ofSiO2 should be 4 or 5 times the etch rate of the underlying Si. The chamber shouldbe cleaned in an O2 plasma after each wafer is etched.US Patent 5935877 - Etch process forRochester Institute of TechnologyMicroelectronic Engineeringforming contacts over Titanium Silicide April 17, 2013 Dr. Lynn FullerPage 22

Plasma EtchingPICTURES OF M1-M2 VIA CHAINRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 23

Plasma EtchingSEM OF 6µm LINES / 2X2µm VIASRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 24

Plasma EtchingPOLYMER FORMATIONO2 additionsCF4 plasmaIncreasing anisotropicIncreasing SiO2:SiSelectivityPolymerFormationIsotropicEtchIon Bombardment EnergyIncreasing anisotropicIncreasing PressureH2 additionsIncreasing IsotropicRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 25300 eV

Plasma EtchingDRY ETCHING SPECTRUMPressurePhysical (Sputtering)Momentum TransferDirectional Etch PossiblePoor SelectivityRadiation Damage PossibleLow 100 mTorr100 mTorrReactive Ion EtchingPhysical and ChemicalVariable AnisotropyVariable Selectivity400 mTorrChemical Plasma EtchingFastIsotropicHigh SelectivityLow radiation DamageEnergyHigh EnergyLow EnergyRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 26

Plasma EtchingPLASMA ETCHING OF VARIOUS 2O3AlAl2O3GaAsKind of Gas PlasmaCF4, CF4 O2, CCl2F2, SF6CF4, CF4 O2, CF4 N2, SF6CF4, CF4 O2, HF* ,SF6CCl2F2, C3F8**, C2F6 H2**CF4, CF4 O2, SF6CF4, CF4 O2CF4, CF4 O2C2Cl2F4CF4 O2, C2Cl2F4 O2, C2Cl3F3 O2CF4CF4Cl2, CCl4, CCl4 AirCl2 Ar, CCl4 ArCCl4, CCl4 Ar, BCl3CCl4, CCl4 Ar, BCl3CCl2F2Remarkdoped or undoped*selectiveevaporate or sputteroxidation methodRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 27

Plasma EtchingION ASSISTED ANISOTROPIC ETCHINGTwo mechanisms are proposed to explain the phenomenon of ion assisted anisotropy.Anisotropic etching is believed to result from a combination of physical and chemicalremoval processes. The ratio of vertical etch rate to horizontal etch rate may be increasedeither by reducing the horizontal rate or by increasing the vertical rate.ION INDUCED DAMAGE MECHANISM:In this model, bombarding ions have sufficient energy to break crystal bonds, making thefilm more accessible and the surface more reactive to the active chemical etchants. At thesidewalls, where there is essentially no ion bombardment, the etching process proceeds atthe nominal chemical etch rates.SURFACE INHIBITOR MECHANISM:In some etch chemistries, the surface exposed to the plasma is likely to become coated witha chemisorbed film of etchant radicals and unsaturated species, which polymerize andadhere tenaciously to the material being etched. The resulting polymer coating inhibits thechemical reactions necessary to etch. Ion bombardment can cause the polymers to desorb,exposing horizontal surfaces to the etching gas. Vertical surfaces experience little or nobombardment, therefore etching in the horizontal direction can be completely blocked.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 28

Plasma EtchingION ASSISTED ANISOTROPIC ETCHINGSURFACE DAMAGESURFACE INHIBITORPhotoresistPhotoresistPolymerDamageSpeeds chemical reactionon horizontal surfacesSlows chemical reactionon vertical surfacesRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 29

Plasma EtchingPOLYMER FORMATIONIt is known that flourocarbon gases such as CHF3, CF4, C3F8 etc. product unsaturatedcompounds in the plasma, leading to polymer formation and deposition on the wafer surface andelectrodes. Polymer formation and the boundary between polymerization and etchingconditions depend upon the fluorine to carbon (F/C) ratio. Addition of oxygen to the plasmachemistry increases F/C ratio and reduces polymer formation. The addition of oxygen,unfortunately also increases the removal rate of photoresists. Energetic ion bombardment willshift the polymerization-etching boundary to lower F/C ratios.PROBLEMS WITH POLYMERS:Deposits cam form on all surfaces of the chamber, affecting reproducibility of the etch process.Polymers are a source of particulate contamination.Cleaning of chambers must be performed regularly in order to prevent build up. This representsreduced up-time.ADVANTAGES OF POLYMERS:Properly controlled polymer deposition can allow anisotropic etching with otherwise purelychemical isotropic etch chemistries.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 30

Plasma EtchingBOSCH ICP (PLASMA THERM)The Bosch process uses two chemistries, one to generate polymersand the other to etch silicon. The etch machine switches betweenthe two every few seconds to ensure that the sidewalls are coveredwith polymer allowing fast, deep trenchetching. (the substrate is on a chuck thatis cooled by liquid nitrogen. 5µm spaces 200µm etch depth 40:1 aspect ratio 2µm/min Si etch rate 75:1 selectivity tophotoresistRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 31

Plasma EtchingSTS ETCH SYSTEM AT RITSF6 and C4F81 to 10 um/min,Oxide, Nitride or Photoresist masks.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 32

Plasma EtchingSTS ETCH SYSTEM AT RIT13 sec etch in SF6 at 130 sccm plus O2 at 13 sccm7 sec polymer deposition in C4F8 at 80 sccm600 watts RF power45 mTorr Pressure during etch100 V wafer bias during etch3 um/min etch rateRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 33

Plasma EtchingALUMINUM ETCHINGPRECONDITIONING OF ETCH CHAMBER BREAKTHROUGH - This is to remove native aluminum oxide (Al2O3) from the surface ofthe wafer by reduction in Hydrogen or by Sputtering by bombardment with Argon at highenergies or both. Water vapor will scavenge Hydrogen and grow more Al2O3 causing nonreproducible initiation times.ALUMINUM ETCHING – because AlF3 is not volatile, a Chlorine based etch is needed toetch aluminum. BCl3, CCl4, SiCl4 and Cl2 are all either carcinogenic or highly toxic. As aresult the pump oils, machine surfaces and any vapors must be treated carefully. AlCl3 willdeposit on chamber walls. AlCl3 is hygroscopic and absorbs moisture that desorbed once aplasma is created causing Al2O3 breakthrough problems.ALLOYS - Aluminum often has a few percent of Silicon or Copper. Silicon is removed bythe Chlorine, Copper is not and requires a special process.PUMPS - BCl3 form nonvolatile residue upon contact with oxygen or water and causesfilters and exhaust ducts to clog readily.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 34

Plasma EtchingLAM 4600 ALUMINUM ETCHERPlasma ChemistryCl2 – Reduces Pure AluminumBCl3 – Etches native Aluminum Oxide-Increases Physical SputteringN2 – Dilute and Carrier for the chemistryChloroform – Helps Anisotropy andreduces Photoresist damageRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 35

Plasma EtchingLAM4600 ANISOTROPIC ALUMINUM ETCHStepPressureRF Top (W)RF BottomGap (cm)N2BClCl2ArCFORMCompleteTime 510300088Time Endpoint818041000125325252308Oetch10%Fuller, December 2009Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn 10 sNorm Val5670Trigger105%Slope Page 36

Plasma EtchingRESULTS FROM NEW ALUMINUM PLASMA ETCHPhotoresist on Metal TwoPhotoresist RemovedRochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 37

Plasma EtchingCOPPER ETCHING1. Copper does not form any volatile compounds with known plasmaetch gases, and therefore cannot be RIE etched.2. Copper can be sputter etched, but this technique has no selectivity.3. Contamination of the fab with copper is a serious concern.The Damascene process has become an attractive enabling methodfor patterning copper by CMP.Rochester Institute of TechnologyMicroelectronic Engineering April 17, 2013 Dr. Lynn FullerPage 38

Plasma EtchingPLASMA STRIPPING

Plasma Etching Page 2 OUTLINE Introduction Plasma Etching Metrics – Isotropic, Anisotropic, Selectivity, Aspect Ratio, Etch Bias Plasma and Wet Etch Summary The Plasma State - Plasma composition, DC & RF Plasma Plasma Etching Processes - The principle of plasma etching, Etching Si and SiO2 with CF4

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