transmission electron microscopy skills:vacuum science in em lecture 6

8/8/2019 Transmission Electron Microscopy Skills:Vacuum Science in EM Lecture 6

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Vacuum Science in EMVacuum Science in EMLecture 6Lecture 6


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OutlineOutlineReview of general vacuum conceptsReview of general vacuum concepts

Very brief!Very brief!

Types of vacuum gauges used inTypes of vacuum gauges used in TEMsTEMsPiraniPirani

PenningPenning

Types of pumps used inTypes of pumps used in TEMsTEMsMechanical / Roughing pumpMechanical / Roughing pump

Diffusion PumpDiffusion PumpTurbomolecularTurbomolecular pumpspumps -- Turbo pumpsTurbo pumps

Ion pumpsIon pumps

ColdCold--trapstraps

A typical vacuum systemA typical vacuum system


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Very brief vacuum overviewVery brief vacuum overviewWhy do we care in EM?Why do we care in EM?

Electrons scatter off of gasesElectrons scatter off of gases

Gases contaminate samplesGases contaminate samples

Pressure (P) units are a messPressure (P) units are a mess1 Pascal (Pa) = 7.51 Pascal (Pa) = 7.5

1010

--33

Torr (Torr (

1010--22

Torr) = 10Torr) = 1022

mbarmbarRanges (approx.)Ranges (approx.)Low vacuum: 760 Torr > P > 1 TorrLow vacuum: 760 Torr > P > 1 Torr

Medium vacuum: 1 Torr > P > 10Medium vacuum: 1 Torr > P > 10--33 TorrTorr

High vacuum: 10High vacuum: 10--33 Torr > P > 10Torr > P > 10--88 TorrTorr

Ultrahigh vacuum: P < 10Ultrahigh vacuum: P < 10--99

Surface science people will say itSurface science people will say its not UHV until 10s not UHV until 10--1010 TorrTorr


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Very brief vacuum overviewVery brief vacuum overviewKinetic theory assumptions:Kinetic theory assumptions:

Large # of moleculesLarge # of molecules

Adjacent molecules separated by distances that areAdjacent molecules separated by distances that are

large compared to their diameterlarge compared to their diameter

Molecules in constant motionMolecules in constant motion

Molecules exert no force on each other except duringMolecules exert no force on each other except during

collisioncollision

PressurePressureRate at which momentum is transferred to a surfaceRate at which momentum is transferred to a surface

P =1

3

nmvrms2


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Very brief vacuum overviewVery brief vacuum overviewVelocities follow a MaxwellVelocities follow a Maxwell--BoltzmanBoltzman distributiondistribution

Mean free path (Mean free path ())

Particle flux:Particle flux:

= nkT

2m

12

vmean = 8kTm

1 2

vrms = 3kTm

1 2

=1

21 2 do2n

1 2

=0.67

P (Pa)=

0.005

P (Torr)


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Very brief vacuum overviewVery brief vacuum overviewMonolayer formation time:Monolayer formation time:

tml =

1

do2 =

4

nvdo2

AtmosphericAtmospheric 11 nanosecnanosec1010--33 TorrTorr 11 millisecmillisec1010--66 TorrTorr 1 sec1 sec1010--88 TorrTorr 100 sec100 sec1010--1010 TorrTorr 3 hrs3 hrs


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Vacuum gaugesVacuum gaugesPiraniPirani

Use a heated wire to formUse a heated wire to formpart of a Wheatstonepart of a Wheatstonebridge circuitbridge circuitPP TT RRRange: 1000 to 10Range: 1000 to 10--44 TorrTorrUncertainty in accuracyUncertainty in accuracyisis 5%5%


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Vacuum gaugesVacuum gaugesIon gauges (hot cathode)Ion gauges (hot cathode)

Force exerted by particles isForce exerted by particles istoo small to measure in hightoo small to measure in high--vacvac & UHV conditions& UHV conditionsIndirect readings neededIndirect readings neededOperationOperation

Filament heated to produceFilament heated to produce ee--ss

Accelerated to gridAccelerated to grid

Ionize particles, which areIonize particles, which are

subsequently collectedsubsequently collected

Range: 10Range: 10--33 to 10to 10--88Not particularly accurateNot particularly accurate

Hot cathodeHot cathode

ee-- ee-- ii++

GridGrid

CollectorCollectorFilamentFilament


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Vacuum gaugesVacuum gaugesIon gauges (cold cathodeIon gauges (cold cathode -- Penning)Penning)

Similar idea, but onceSimilar idea, but once ee--ssemitted, they are spunemitted, they are spunaround in a magnetic fieldaround in a magnetic fieldLeads to increasedLeads to increasedsensitivity, longer pathssensitivity, longer pathsRange: 10Range: 10--22 to 10to 10--1111 TorrTorr

Cold cathodeCold cathode


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MechanicalMechanical pumpspumpsRotary pumpRotary pump

From ambient to 10From ambient to 10--33 TorrTorr

RoughRough oror roughingroughingpumppump

Oil basedOil based

Dirty, loud, lots of vibrationDirty, loud, lots of vibration

Scroll pumpScroll pumpOil freeOil free

Like aLike a turbo pumpturbo pump (more(more

later) but designed to coverlater) but designed to coverthe low vacuum rangethe low vacuum range

Cleaner, still has someCleaner, still has some

vibrationvibration

Becoming the standardBecoming the standard


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Diffusion pumpsDiffusion pumpsHot oil used to trap gasesHot oil used to trap gasesVery efficientVery efficientVibration freeVibration freeRange: capable of 10Range: capable of 10--33 toto1010--1111 TorrTorr

Remember, that is only ifRemember, that is only if

system can handle 10system can handle 10--1111

Torr!Torr!Not used in UHV, though, forNot used in UHV, though, for

fear of backfear of back--streamingstreaming


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TurbomolecularTurbomolecular pumpspumpsRotary blade captures molecules, forces themRotary blade captures molecules, forces themout other sideout other sideCan operate from ambient to 10Can operate from ambient to 10--99 TorrTorr

In about an hourIn about an hour

Generally, you use a rough pump inGenerally, you use a rough pump in

conjunction with a turboconjunction with a turbo

Can introduce vibrationCan introduce vibration


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Ion pumpIon pumpElectrons emitted fromElectrons emitted fromcathodecathodeSpun around by a magneticSpun around by a magneticfieldfieldIncident gas is ionizedIncident gas is ionizedIonized gas accelerated intoIonized gas accelerated intopump walls, where it stickspump walls, where it sticksCan coat walls with Ti, toCan coat walls with Ti, tohelp that processhelp that process

Ti sublimation pumpTi sublimation pump

Ti coating renewed every dayTi coating renewed every day

Range: 10Range: 10--55 to 10to 10--1111 TorrTorrNo vibrationsNo vibrations


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CryoCryo Pumps / cold fingersPumps / cold fingersCold surfaces attract & retain residual molecularCold surfaces attract & retain residual molecularspeciesspecies

CrCryoyo--condensationcondensation Molecules attrMolecules attracted to condensations sites on cold surface, as Tacted to condensations sites on cold surface, as T

decreases, their residence time there increasesdecreases, their residence time there increases

CryoCryo--ssorptionorption

WeWeak vanak van derder Waals attraction of molecules to surfaceWaals attraction of molecules to surface

CryoCryo--trappingtrapping

Trapping on one gas within the frozen porous condensate ofTrapping on one gas within the frozen porous condensate ofanotheranother

Used in EM in two major waysUsed in EM in two major waysCold fingersCold fingers oror antianti--contamination devicescontamination devices

To help pumps (older microscopes)To help pumps (older microscopes)

Residual contaminants lost when surface warms upResidual contaminants lost when surface warms up


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Vacuum systemVacuum system


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Actual vacuum systemActual vacuum system2000FX2000FX


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ContaminationContaminationMain worry isMain worry isresidualresidualhydrocarbonshydrocarbons

Pump oilPump oil

Specimen &Specimen &

specimen holderspecimen holder

You can helpYou can helpUse gloves, useUse gloves, use

care (doncare (dont toucht touch

past the opast the o--ring)ring)

Residual waterResidual waterFilmFilm

transmission electron microscopy skills:vacuum science in em lecture 6

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