Vacuum-Science-and-Technology

TechnologyofThinFilmVacuumScience&TechnologyXuhaiZhangSchoolofMaterialScienceandEngineeringSoutheastUniversityOutline1.Conceptofvacuum2.KineticTheoryofGases3.GasTransportandPumping4.VacuumPumps5.VacuumSystems6.MonitoringtheVacuumEnvironment7.Conclusion2Whatisthe“Vacuum”3ConceptofvacuumInvacuumscientificfield,vacuumisaregionwithagaseouspressuremuchlessthanatmosphericpressure.FeaturesofvacuumThepressureinvacuumislowerthan1atm,sotheevacuatedchambersareunderpressurefromatmosphere.FeaturesofvacuumTherearelessgasinthestateofvacuum,thefrequencyofcollisionbetweengasmolecularswouldbedecrease.Wherecanwefindthevacuum?1.2Unitofvacuum•InternationalSystemofUnits：Pascals1atm=101.325kPa•traditionalunitofpressure:Torr1Torr＝133.3224Pa1.3VacuumqualityVacuumqualityTorrPaAtmosphereAtmosphericpressure7601.013×10+51Lowvacuum760to251×10+5to3×10+31to0.03Mediumvacuum25to1×10−33×10+3to1×10−1Highvacuum1×10−3to1×10−91×10−1to1×10−7Ultrahighvacuum1×10−9to1×10−121×10−7to1×10−10Extremelyhighvacuum1×10−121×10−10Outerspace1×10−6to3×10−171×10−4to3×10-15Perfectvacuum0001.3VacuumqualityVacuumevaprationsputteringLowpressureCVDSEM,TEMLowvacuum102PaMediumvacuum102~10-1Pa10Pa100Pa10PaHighvacuum10-1~10-5Pa10-3Pa10-2Pa10-3PaUltrahighvacuum10-5PaOutline1.Conceptofvacuum2.KineticTheoryofGases3.GasTransportandPumping4.VacuumPumps5.VacuumSystems6.Conclusion17Outline1.Conceptofvacuum2.KineticTheoryofGases3.GasTransportandPumping4.VacuumPumps5.VacuumSystems6.MonitoringtheVacuumEnvironment7.Conclusion182KineticTheoryofGasesIdealgass–Gasmoleculeslookasindependentelasticspheres•noattractiveorrepulsiveforcesbetweenmolecules–separatedfromeachotherbydistancesthatarelargecomparedtotheirsize2.KineticTheoryofGases•Thelargenumberofatomsormoleculesmakingupthegasareinacontinuousstateofrandommotionthatisintimatelydependentontheirtemperature.•Thegasparticlescollidewitheachotheraswellaswiththewallsoftheconfiningvessel.•Asteady-statedistributionofmolecularvelocitiesisgivenbytheMaxwell-Boltzmannformula2.KineticTheoryofGasesRTMvevRTMvf22232)2(4)(Maxwell-BoltzmanndistributionM--molecularweightT--absolutetemperatureR--gasconstantVelocitydistributionsforAlvaporandH2gas.2.KineticTheoryofGasesMRTva8Forair,T=300K,va≈460m/s,1030milesperhourAveragevelocities2.KineticTheoryofGasesMomentumtransferfromthegasmoleculestothecontainerwallsgivesrisetotheforcesthatsustainthepressureinthesystem.n--thenumberperunitvolumeNA--Avogadro'snumbern/NA—thenumberofmolesperunitvolumeAAaNnRTNMvnp82Thegaspressureisrelatedtototheirkineticenergyortemperature.Pressure2.KineticTheoryofGasesThemeandistancetraveledbymoleculesbetweensuccessivecollisions21dnd--collisiondiameterofgasmoleculen--thenumberperunitvolumeAveragecollisionfrequency：va/λThemean-freepathExample•Forairatroomtemperatureandatmosphericpressure,λmfp~50nm,assumingd≈0.5nm,•airmoleculesmakeabout1010collisionspersecond•Sogasesmixtogetherratherslowlyeventhoughtheindividualmoleculesaremovingatgreatspeeds.P=0.1Pa，10-3Pa?Themean-freepathKineticTheoryofGases•GasimpingementFlux:Thenumberofmoleculesthatstrikeanelementofsurface,perpendiculartoacoordinatedirection,perunittimeandareaFactor¼isthecoefficientobtainedfromtheaveragetreatmentofmovingdirectionandvelocitydistributation.4anvVa--Averagevelocitiesn---thenumberperunitvolume•Knudsenequation–ImpingementFluxisproportiontoP–ininverseproportiontosquarerootofTandMMRTpNA24anvMRTva8AAaNnRTNMvnp82GasimpingementFluxApplication:ContaminationofsubstrateTimeforcompletemonolayercoverageofasurface•Inairatatmosphericpressureandambienttemperatureasurfacewillacquireamonolayerofgasin3.5x10-9s•Highvaccum10-8Pa，10h•10-4Pa？pNMRTNNA2GasimpingementFluxMoleculardensity,incidencerate,meanfreepath,andmonolayerformationtimeasafunctionofpressureOutline1.Conceptofvacuum2.KineticTheoryofGases3.GasTransportandPumping4.VacuumPumps5.VacuumSystems6.MonitoringtheVacuumEnvironment7.Conclusion303.1GasflowregimeLaminarflowTurbulentflowViscousflowMolecularflowKnudsennumberReynoldsnumberGasflowregime3.1GasflowregimeMolecularflow•Atlowpressure,themeandistancebetweenmolecularcollisionsislargecomparedtothedimensionsofthesystem,thereareonlymolecule-chamberwallcollisions.Viscousflow（laminarflow/turbuletflow）•Athigherpressure,collisionbetweenmolecularpredominaterelativetomolecule-chamberwallcollisions.3.1.1CriteriafordistinguishingbetweentheflowregimesKnudsennumber•MolecularflowKn1•ViscousflowKn110•IntermediaterflowKn=1~110D—systemdimensionsλ—meanfreedistanceDKn3.1.2Laminarflow•Atlowgasvelocitiestheflowislaminarwherelayered,parallelgasflowlinesmaybeimagined.Undertheseconditionsthelaminarflowvelocityiszeroatthewallsofatube,butitincreasestoamaximumatthetubeaxis.3.1.3Turbunetflow•Forhigherflowvelocitiesthegaslayersarenolongerparallelbutswirlandareinfluencedbyanyobstaclesintheway.Inthisturbulentflowrange,cavitiesoflowerpressuredevelopbetweenlayers.3.1.4CriteriafordistinguishingbetweentheflowregimesReynoldsnumber–LaminarflowRe1200–TurbulentflowRe2200–Intermediateflow2200Re1200d--characteristicdimensionofthesyste