15.12.2015

SEM 2 N D CE MM WORKS H OP : S C ANNING ELEC T RON MI C ROSCOP E MA JA KOBLAR, S C. E NG. PHYSICS

Scanning electron microscope General considerations of SEM:  magnification and resolution  depth of focus (field,depth) - DOF  preparation  conductive  vacuum compatible

SEM components we will discuss:  vacuum system  microscope column  sample chamber  graphic user interface (JSM-7600F) (Images: ammrf) CENTER FOR ELECTRON MICROSCOPY AND MICROANALYSIS

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15.12.2015

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Vacuum system rough, high, ultra high and

extremely high

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Vacuum  to produce a mean free path for electrons greater than the lenght of the electron column  to avoid collisions between electrons of the beam and molecules  to prevent beam induced chemical reactions  required for stable emission (arcing and damaging the filament) and for some detectors and electrostatic lenses

R = 8,3145 J/mol K … universal gas co T … temperature d … diameter of molecules NA = 6,0221 1023 /mol… Avogadro´s n P … pressure

The division of areas of low pressure: rough

abbreviation

Vacuum area

Pressure [mbar]

numerical density [particles/m3]

LV (sl: GV)

low vacuum

1000 - 1

1019 - 1016

MV (sl. SV)

medium vacuum

1 - 10-3

1016 - 1013

HV (sl. VV)

high vacuum

10-3 - 10-7

1013 – 109

UHV (sl. UVV)

ultra high vacuum

10-7 - 10-12

109 - 104

EXV (sl. EVV)

extremely high vacuum

under 10-12

under 104

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Atmospheric pressure: 760 Torr = 101.3 kPa = 1013 mbar Pressure: Pa (SI unit), mbar, bar, Torr (USA)

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760 Torr  1 bar → Torr ~ mbar 1 mbar = 100 Pa

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Types of vacuum pumps in SEM Gas transfer vacuum pumps: ◦ Rotary pump ◦ Diffusion pump ◦ Turbomolecular pump

Entrapment vacuum pumps: ◦ Ion pump ◦ Cold trap

(Image: electronica.ugr.es)

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Rotary Pump Pumps by rotation of an off-center cylinder. Initial pump (to pump the chamber after sample change, in airlock exchange). Good efficiency for high pressure – 100 L/min. (Video: YouTube)

Disadvantages: ◦ vibrations ◦ oil vapour ◦ maintenance

(

JXA-840A

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JSM-7600F

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Diffusion Pump Molecules diffuse into the active part of the pump where they are trapped and removed. Very high pumping speed, pumps also light gases. Tolerant with particles.

(Im (Video: YouTube)

Disadvantages: ◦ needs warm up and special oil, which is evaporated ◦ needs cooling too ◦ can only work vertically (Image: pchemlabs.com)

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Turbomolecular Pump A jet turbine and works by momentum transfer (multiple stages of rotating blades (rotor) spaced between fixed blades (stator). High pumping speed but not for light gases. Clean, no warm up, oil free. (Video: YouTube)

Disadvantages: ◦ ◦ ◦ ◦

relatively expensive not tolerant to particles can fail catastrophically high vacuum is pure hydrogen (Image: wikipedia)

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Sputter Ion Pump Air molecules are removed from the chamber by gettering them onto a surface by ionizing gas within a magnetically confined cold cathode discharge Can achieve ultrahigh vacuum. No moving parts. (Video: YouTube)

Ions sputter Ti, which reacts with residual gases and buries these under a film.

(Image: nau.edu

Disadvantages: ◦ not very efficient for water ◦ gasses not permanently removed JSM-7600F

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Liquid nitrogen trap The gettering is accomplished by making the pump wall very cold (cryopump). Molecules are trapped on the surface (sorption). No moving parts. Good for water (water vapor desorbs slowly from the internal surfaces of the chamber).

(Video: YouTube)

Disadvantages: ◦ frequently degassed ◦ gasses not permanently removed JSM-7600F

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15.12.2015

(Image: JEOL)

JSM-7600F, JSM-5800 and JXA-840A CENTER FOR ELECTRON MICROSCOPY AND MICROANALYSIS

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Basic principle of SEM operation

(Image: ammrf)

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electron gun

Microscope column

Gun aperture and alignment coils Gun isolation valve Faraday cup Aperture angle control lens

In column energy filter (R-filter)

condenser to shape the beam apertures to limit the beam scan coils to deflect the beam objective lenses to focus the beam

(Image: JEOL)

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Electron gun CENTER FOR ELECTRON MICROSCOPY AND MICROANALYSIS

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Thermionic emission To produce an electron beam:

(Image: ammrf)

◦ an emitter (electrode), ◦ a surrounding cathode (Wehnelt cylinder/ grid cap) and ◦ an anode with a central hole. (Image: nau.edu)

𝐸 = 𝐸𝑤 + 𝐸𝐹

Richardson- Dushman equation: 𝐽𝑡ℎ = −𝐴𝑇 2 𝑒𝑥𝑝

−𝑒 𝐸𝑤

Tungsten: T = 2700 K, Ew= 4,5 eV → Jth =3,4 A/cm2 LaB6: T = 1800 K, Ew= 2,5 eV → Jth =40 A/cm2

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𝑘𝑇 Jth ... Thermionic current density (A/m2) A … constant for thermionic emitters (120 A/cm2K2) T… metal temperature (K) e … electron charge (C) Ew … metal work function (eV) k … Boltzmann's constant (J/kg)

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W gun (JSM-5800, JXA-840A)

400 µm

JSM-5800

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Field emission Fowler – Nordheim tunneling

The cathode is a tungsten rod, very sharp point (107 V/cm), by applying potential to the first anode (3-5kV). Electrons can escape cathode without applying any thermal energy.

Vacuum

Thermionic Ew(SE)

Ew

Very high vacuum ( 1000

> 5000

> 2000

Cathode regeneration

not required

not required

not required

every 6 to 8 hours

Sensitivity to external influence

minimal

minimal

low

high

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(Table: tedpella)

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* virtual source; 1hPa= hectopascals = 100 Pa

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Condenser to shape the beam CENTER FOR ELECTRON MICROSCOPY AND MICROANALYSIS

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Condenser lens

Vo

Similar to glass lenses in optical microscopes. Main role of EM lenses is to demagnify the source of electrons to form a much smaller diameter probe. Two main lenses used in SEM:

The greater the len resulting beam diam convergence angle

◦ Condenser lenses ◦ Objective lenses (Images: ammrf)

The main role of the condenser lens is to control the size of the beam and determines the number of electrons in the beam which hit the sample.

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Four par Vo… the αp… the ip… the e dp… the

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Apertures to limit the beam CENTER FOR ELECTRON MICROSCOPY AND MICROANALYSIS

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Apertures Three (JSM-5800: 100 µm, 30 µm and 20 µm) Four (JSM-7600F: 110 µm, 70 µm, 50 µm and 30 µm) Reduces the beam current. Lowers the angular spread and spherical aberration.

30 µm

50 µm

70 µm

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110 µm

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Electromagnetic lenses to focus the beam CENTER FOR ELECTRON MICROSCOPY AND MICROANALYSIS

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Objective lens Focuses electrons on the sample at the working distance.

Properties: ◦ large demagnification ◦ two sets of deflection coils are included – scan coils ◦ stigmator is built in it ◦ includes the beam shift coils ◦ the visible light microscope optics (EMPA)

(Images: ammrf)

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Sample chamber motorized stage (x,y,z,t,r) detectors

EDS RIBE

RBEI

SEI GB

LEI LN2

(Image: JEOL)

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Pole piece

EDS LEI

Cold trap

EDS BEI

SEI

RBEI

stage

stage Turbo pump

JSM-7600F

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JSM-5800

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Signals and interactions

(Image: ammrf)

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Signals and interactions

(Image: ualberta)

(Image: Low Voltage Electron Microscopy: Principles and Applications)

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Figure (b) displays inelastic processe emissions, with re levels.

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SE vs BSE Secondary electrons ◦ ◦ ◦ ◦

Backscattered electrons

High resolution Strongly topography sensitive Little element sensitive Sensitive to charging

◦ Lower resolution ◦ Atomic number contrast in particular strong signal to heavy atoms ◦ Less sensitive to charging

(Video: YouTube)

(Video: YouTube)

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(Images: ammrf)

R filter – electrostatic lens fixed

fixed

fixed variable

SE mode

SB mode CENTER FOR ELECTRON MICROSCOPY AND MICROANALYSIS

BE mode [email protected]

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Electrostatic lense

Objective Lens SEI

LEI

Magnetic filed Sample (Image: JEOL)

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GENTLE BEAM Obtain higher image resolution by decelerating primary electron beam just before landing at the specimen

Gentle Beam

Normal

Acc. Voltage

Landing Voltage Stage Bias Voltage Landing Voltage = Accelerating Voltage

Landing Voltage =Acc. Voltage - Bias Voltage (Images: JEOL)

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SEM vs GB mode (Images: JEOL)

Improves image resolution Reduces charging and beam damage Shows pure surface topographic information

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Improves image resolution Reduces charging & beam damage Shows pure surface topographic information

SEM vs GB mode SEM

GB mode

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