David JoyA Clean MachineTerminologyCommon Vacuum UnitsSlide 5Slide 6Slide 7Qualitative Vacuum RangesVacuum pumpsScroll PumpsAlternatively….Rotary Vane Mechanical PumpSlide 13Turbomolecular pumpTurbo pump performanceIon PumpsThe triode pumpIon pump performanceCryogenic pumpVacuum GaugesPirani gaugePirani calibrationPenning (Cold cathode) GaugeCapacitance ManometerSlide 25Ion gaugesO-ring sealsUHV metal to metal sealsVacuum HygieneCleaning samplesClean is not for ever ...Plasma cleaningThe Dark Side of SEMUnwanted Beam InteractionsUnwanted beam interactionsRadiolysis is….Radiolysis damage in PolymersDose does matterIs a high beam energy bad?‘Mythbuster’ factDamage in semiconductorsThermal damage?Other beam induced damageContamination and EtchingModern SEMs are very cleanbut samples are not..Contamination and EtchingLow magnificationBlack squares...High magnificationVirtue of necessity..Temperature effectsTemperature Effects IIThe Cold FingerWithout a cold fingerWith a cold finger in use...Controlling contaminationSlide 58David JoyDr. David C. JoyDistinguished ProfessorMaterials Science and EngineeringUniversity of Tennessee Knoxville, TN D.Phil., University of Oxford (UK): A Study of Magnetic Domains in the SEMHitachi payrollA Clean MachineThe FEGSEM cannot operate except in a clean, ultra-high vacuum. (Numerous caveats for low vacuum and ESEM operation.) This requires items such as:Scroll PumpsTurbo Molecular PumpsIon Pumpsas well as associated gaugesand even with the cleanest of vacuums there is still The Dark Side of SEM that must be faced….Terminology“Low vacuum” = High pressure“High Vacuum” = Low pressureCommon Vacuum UnitsThere are many varied units that are used to specify pressuresThe Torr, the Bar and the Pascal are in common use..... but the Pascal is the SI recommended unit for pressure and so is the best choice for documentation1 Atmospheric pressure is 760 mm Hg = 1 Bar = 105 Pa1 Torr = 1 mm Hg1 Torr = 1/760 of an atmosphere = 132 Pa1 milliTorr = 0.13Pa = 1 μmHg 1mbar = 1/1000 Atm = 0.76 Torr = 100Pa1 Pa = 7.6 milliTorr = 7.6 μmHgPressure: Units of MeasurePressure exerted by a column of fluid:P ≡ F/A = mg/A = ghA/A = gh  h 1 Atm (mean sea level) = 760 Torr = 1013 mBar = 1.01x105 Pa = 101.3 kPa = 14.7 psi = 34 ft. waterAverage atmospheric pressure in SLC is about 635 Torr, 12.3 psi, 28.4 ft water…“Kinds of Pressure”Gauge Pressure: measured with respect to ambient.Absolute pressure: measured with respect to vacuumCar tires, basketballs, boilers, LN2 tanks, JFB/MEB compressed air supply…Vacuum systems, cathode ray tubes, light bulbs, barometersMean Free Path in GasesWith sufficient accuracy for approximate calculations we may take:λ = 7 x 10-3/p mbar-cmλ = 5 x 10-3/p Torr-cmλ = 5/p μmHg-cmQualitative Vacuum RangesLow vacuum (SEC) 760 to 1 TorrMedium vacuum (SEC) 1 to 10-3 TorrHigh vacuum (Chamber) 10-3 to 10-6 TorrVery high (Column) 10-6 to 10-9 TorrUltra-high (Gun) 10-9 and lower FEGSEMs contain examples of each vacuum levellaminarmolecularVacuum pumpsFor each of the vacuum ranges identified earlier there is one or more type of pump that is best Pumps are always used in combination - one pump is used to start the nextThe sequencing of the pump down is crucial and so this is done under computer controlScroll PumpsScroll pumps are the foundation of clean vacuum systemsThey consist of two Archimedes’ screws machined into aluminum plates mounted so that the spirals interleaveOne plate is held fixed while the other oscillates. Gas is trapped between the spirals and forced out to the exit portPumping speed is constant from Atmospheric pressure down to about 1000Pa and the ultimate pressure is about 10PaScroll pumps are oil-free and require neither inlet nor outlet valvesThe world’s oldest pump technology – Archimedes’ screwAlternatively….Roughing can also be carried out using a diaphragm pumpOscillation of the diaphragm alternately pulls gas in one port and then expels it through the other.Oil free pumps are clean but typically a factor of 3x slower, and 3x more expensive, than pumps containing oil – but worth the wait and expenseRotary Vane Mechanical PumpRobustInexpensiveOperates to ambient pressureSingle stage and two stageTurbomolecular pumpArchimedian screw - runs at 20000+ rpmNeeds electronic protection / control for the bearings in case of loss of powerProduces a clean, oil-free, high vacuum down to 10-6 Pa (10-8 Torr)Must be backed: scroll pump, diaphragm pump or rotary oil pump.Turbo pump performanceTurbo pumps can start even at atmospheric pressure (although they labor) and they can go down to 10-8 TIt is best to pre-pump the system with a clean backing systemTMP do not pump all gases with same efficiency - large molecules are pumped faster than smaller molecules1 milliT = 0.13Pa = 1 μmHg Turbo pump performanceIon PumpsIonized molecules spiral in magnetic field and get buried in Ti wall coatingA large number of these structures are run in parallel to improve the pumping speedDiode pumps only handle gases that are easily ionized (no noble gases)The triode pumpIf noble or unusual gases are expected to be found in the SEM (nitrogen, helium, counter gases from a WDS system etc.) then a triode pump must be used.The additional electrode then makes it possible to ionize these gasesIon pump performance“The” UHV pump - goes to 10-9 Pa (10-11 Torr) and below in a properly designed vacuum systemRequires no backing…more than a little misleading… in fact it works best in a sealed system. Entrainment pump!The IP requires a periodic bake-out into rough pumped system to clean the buried gas from the pump. This is done during the gun bake procedureCheck for electrical instability by slapping the pump with an open hand. Instability indicates need for a bakeIon Pump PerformanceCryogenic pumpCryo-pumps use liquid helium and activated charcoal absorbers to pump to 10-12 TVery high pumping speedsNo vibration or magnetic fieldsBut they need periodic bake-outs into a rough pump to clean the absorbersThey are expensive to run unless used with a closed-circuit (Stirling engine) liquid He pumpVacuum GaugesVacuum systems must be monitored constantly to