Slide 1Slide 2Slide 3Slide 4Where the credit belongsImaging modesParameters Determining ResolutionCurrents in an SEM (W-filament)Electron sources/guns: optionsWhen do we need which kind of SEM?Tungsten Hairpin FilamentsCold Field Emitters (FEG)Flashing: required of cold-FEGs, not Schottky thermal field emittersCold FEG Gun behaviorTypical characteristicsSchottky EmittersThe Schottky EmitterSchottky PerformanceNano tips - atomic sized FEGRegular and Nano Tips(1) Source SizeHow to choose?Brightness(2) Source BrightnessConservation of brightnessEmitter brightness(3) Energy SpreadSlide 28Slide 29SummaryLensesHitachi’s view of Practical electron lenses…Another view of lensesThe ideal lensSpherical AberrationStigmation: correction for spherical aberrationsChromatic AberrationDiffractionEffect of aberrationsContributions to actual beam diameterPerformance vs Beam EnergyHow the SEM operates 1:Getting the beam to raster There are two major challenges with operating an SEMCreating an image requires correctly establishing about a dozen parametersInterpreting the resulting image also requires a lot of skill and experienceOther than that, it’s really easy!Imaging Inputs (operator controls)Everhart-ThornleyThrough the Lens (TTL)LowVacvCD/4QBSHelixkV probe currentbrightnessworking distancemagnification dwell timedetector choice3010110100200500100050101.6020406080100624 4590180510152025303540515202535contrast0204060801001001,000,0001,000 10,000100,000Slide stolen from Charles Lyman (with many changes)Schematic drawing of scanning electron microscopeC1 lens currentC3 lenscurrent Raster beamDeterminemagnificationEverhart-Thornley detector 15Intro to Hi Performance SEMWhere the credit belongsAll slides with the yellow graphic are courtesy of David Joy, U of TennesseeDavid Joy probably knows more about electron microscopy than anyone else alive6Intro to Hi Performance SEMImaging modesResolution: gives maximum resolution!High current: for optimum contrast, EDX and EBSDDepth of focus: large depth of field is a great attribute of the SEM. Use long working distanceLow voltage modeBetter topographic informationAbility to overcome charging7Intro to Hi Performance SEMParameters Determining ResolutionAccelerating potential: VProbe current: IpBeam diameter: dpConvergence angle: αpCurrents in an SEM (W-filament)Filament current: Current that heats a tungsten filament, typically 2.-2.8 A. Strongly affects filament lifetime. Similar for Schottky FEG, but only heated to 17 KEmission current: total current leaving the filament, typically about 4 μA for W-filament, 4 μA for FEG.Beam current: Portion of emission current that transits the anode aperture; decreases going down the column.Probe current: a calculated number related to the current on the sample, typically 1 pA – 1 nA.Specimen current: the current leaving the sample through the stage, typically about 1% of the probe current. Remember that one electron incident on the sample can generate many in the sample…a 2 keV electron can generate hundreds at 5 eV.FEI also defines a parameter called “spot size” which is proportional to the log2(probe current); proportionality constant depends on aperture size.9Intro to Hi Performance SEMElectron sources/guns: optionsThe requirements for modern SEMs call for nanometer resolution, high current into small probe sizes, and effective low voltage operationSuch needs make the venerable thermionic gun obsolete for top of the line SEMsSo all high performance SEMs now use some more advanced form of electron sourceW-filament machines are still much less expensive and adequate for many applications10Intro to Hi Performance SEMWhen do we need which kind of SEM?The FEG SEM offers high performance not just high resolutionThis means large probe currents (up to a few nanoamps, [Ip in Leo goes to 5 μA] important for EDS and EBSD), and small diameter electron probes (from 1 to 3nm), over a wide energy range (from 0.5 -30keV). The FEG SEM performance package involves both the gun and the probe forming lensesHuge difference in resolution between FEG and W-filament at very low voltageA FEG SEM will cost about twice as much as a W-filament machine!11Intro to Hi Performance SEMTungsten Hairpin FilamentsThe electron source is the key to overall performanceThe long time source of choice has been the W hairpin sourceBoils electrons over the top of the energy barrier - the current density Jc depends on the temperature and the cathode work function f- Richardson’s equation….. Jc=AT2exp(-e/kT)Cheap to make and use ($12.58 ea) and only a modest vacuum is required. No vac-ion pump. Last tens of hours.workfunction eV conduction bandvacuum levelthermionicelectronicThermionic electronsSchematic Model of Thermionic Emission12Intro to Hi Performance SEMCold Field Emitters (FEG)Electrons ‘tunnel out’ from a tungsten wire because of the high field obtained by using a sharp tip (100nm) and a high voltage (3-4kV) Jc=AF2/.exp(-B1.5/ F)The Fowler-Nordheim equation shows that the output is temperature independent – hence the name ‘cold’Needs UHV but gives long life and high performanceworkfunction eV conduction bandvacuum levelpotentialdistancebarrierFieldF V/cmFlashing: required of cold-FEGs, not Schottky thermal field emittersEach tip should show a consistent emission current when it is flashedCompare the tip current with its own usual value not with that from other tips If the value is low, flash several times until the current recoversExcessive flashing may blunt the tip14Intro to Hi Performance SEMCold FEG Gun behavior(Hitachi and JEOL make cold-FEG microscopes)The tip must be atomically clean to perform properly as a field emitterEven at 10-6 Torr a monolayer (“one Langmuir”) of gas is deposited in just 1 sec so the tip must be cleaned every time before it is used; tip needs 10-10 TorrCleaning is performed by ‘flashing’ - heating the tip to white heat for a few seconds. This burns off (desorbs) the gas15Intro to Hi Performance SEMTypical characteristicsThe tip is usually covered with a mono- layer of gas after 5-10 minutesThe emission then stabilizes for a period of from 2 hours (new machine) to 8 hours (mature machine).On the Hitachi S4700, S4800, and S5500 the tip must be