Oct. 30th- Nov. 1st, 2017

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Thomas LaGrange, Ph.D. Faculty Lecturer and Senior Staff Scientist Electron Sources, Optics and Detectors SEM Doctoral Course MS-636 Oct. 30th- Nov.

The vacuum level varies between regions of the electron microscope; the highest vacuum level (<10-7 Pa or 10-9 mbar) being in the gun where electrons are emitted and accelerated and is required to

The specimen area also requires a high vacuum level to reduce contamination, which is especially important for chemical analysis that requires the electron beam to rest in the same area for a long

Turbo-molecular and oil-diffusion pumps for high vacuum cannot work against atmospheric pressure and they require a mechanical prevacuum pump to back them for proper and efficient operation.

1 Thomas LaGrange, Ph.D. Faculty Lecturer and Senior Staff Scientist Electron Sources, Optics and Detectors SEM Doctoral Course MS-636 Oct. 30th- Nov. 1st, 2017 Summary Electron propagation is only possible through vacuum. The vacuum level varies between regions of the electron microscope; the highest vacuum level (<10-7 Pa or 10-9 mbar) being in the gun where electrons are emitted and accelerated and is required to keep source and accelerator clean. The specimen area also requires a high vacuum level to reduce contamination, which is especially important for chemical analysis that requires the electron beam to rest in the same area for a long time. Hydrocarbon build up (contamination) on the observed area often results from a low system vacuum level in combination with existing surface contaminants. Turbo-molecular and oil-diffusion pumps for high vacuum cannot work against atmospheric pressure and they require a mechanical prevacuum pump to back them for proper and efficient operation. Electron beams are generated either by thermal emission or field emission. Field emission sources provide lower energy spread and high coherence required high resolution spatial resolution imaging and electron spectroscopy. Electrons are focused by simple round magnetic lenses which properties resemble the optical properties of a wine glass. Unlike in light optics the wavelength (2pm for 300kV) is not the resolution limiting factor. Lens aberrations however and instabilities of the electronics (lens currents etc.) limit the resolution of even the best and most expensive transmission electron microscopes to about 50pm. Recording an image means detecting electrons. Depending on their energy electrons can be detected by different detectors. A high detector efficiency and a high signal to noise ratio allows faster recording and reduces the exposure (beam damage) of the sample to the electron beam. A high linearity and high dynamic range permits to quantify images and to record high and low intensities in one image (important for diffraction experiments). 2 1

Components of a scanning electron microscope Scan coils 1. Electron propagation is only possible through vacuum! 2. Need a good vacuum system to reduce contamination!

2 Components of a scanning electron microscope Scan coils 1. Electron propagation is only possible through vacuum! 2. Need a good vacuum system to reduce contamination! 3 Outline: This section of the course describes main components of the scanning electron microscope: pumping systems, electron sources, electron optics and detectors. 1) Pumping Systems 2) Electron sources 3) Electron optics A. Basics B. Magnetic Lens C. Aberrations and Spatial Resolution Limits in a SEM D. Modern SEMs 4) Detectors A. Secondary electrons B. Back scattered electrons 4 2

1) Pumping system: block diagram Gun Ion Pump Column Ion Pump Buffer/ Reservoir Tank SEM Column and Sample Chamber Electron Gun Gun Valve Secondary pump valve Diffusion Pump Primary pump

1 Pa) Mechanical pump Secondary vacuum (<10-4 Pa) Oil diffusion pump Turbomolecular pump High and ultra-high vacuum Gun & specimen area (<10-6 Pa) Ion getter pump Cold trap 5 Mechanical Rough

3 1) Pumping system: block diagram Gun Ion Pump Column Ion Pump Buffer/ Reservoir Tank SEM Column and Sample Chamber Electron Gun Gun Valve Secondary pump valve Diffusion Pump Primary pump Valve Air Primary vacuum (>0.1 Pa) Mechanical pump Secondary vacuum (<10-4 Pa) Oil diffusion pump Turbomolecular pump High and ultra-high vacuum Gun & specimen area (<10-6 Pa) Ion getter pump Cold trap 5 Mechanical Rough Pump Vent Valve 1) Pumping system: Primary vacuum 6 Rotary vane pump Uses oil (wet) which can be a source of contamination Noisy but not active all of the time (buffer tank backing) Cheap and low maintenance Scroll pumps No oils or sources of contamination, Dry Expensive Lower ultimate vacuum pressure (10-2 torr) High maintenance, scrolls need to be yearly 3

cold trap (limits oil vapor migration) 7 1 ) Pumping system: Secondary vacuum Turbomolecular pumps

4 1) Pumping system: Secondary vacuum Oil diffusion pump Vibration free cheap, reliable and low maintenance Possible Contamination sources: oil vapor High pumping capacity (>500 l/s) Best with cold trap (limits oil vapor migration) 7 1 ) Pumping system: Secondary vacuum Turbomolecular pumps Rotation speed 20-50,000 rpm Magnetic bearings Pumping volumes l/s No oils or sources of contamination 8 4

1 ) Pumping system: High / Ultra-high vacuum Ion getter

vacuum by trapping gas molecules in sputtered Ti layers

and for proper operation 9 2) Electron Sources LaB 6

5 1 ) Pumping system: High / Ultra-high vacuum Ion getter pump (IGP) no vibrations No exit ; no pumping improves vacuum by trapping gas molecules in sputtered Ti layers Requires High Vacuum via secondary pumping before using and for proper operation 9 2) Electron Sources LaB 6 Cathode Tungsten Field Emission Tip

heated to overcome the work function to push electrons in vacuum level Tungsten wire heated to ~2800K

6 2) Electron Sources: Comparison of Sources 11 2) Electron Sources: thermionic gun Electron boil from surface.heated to overcome the work function to push electrons in vacuum level Tungsten wire heated to ~2800K LaB 6 crystal heated to 1900K Main advantages: simple, cheap, no high vacuum required and maintenance friendly Disadvantages: low brightness, high energy spread and large source size ( m) 12 6

Increasing Temperature 300 K 1700 K 10/30/2017 2) Electron Sources: Emission of electrons metal vacuum (with electrical field) Increasing electric field Thermionic emission Schottky emission

Cold FE 13 2) Electron Sources: Field emission guns (emitter) Cathodes (tips) Cold field emission (E 10 9 V/m) W single crystal with a sharp tip (radius ~25nm) Advantages Small energy dispersion (<0.

7 Increasing Temperature 300 K 1700 K 10/30/2017 2) Electron Sources: Emission of electrons metal vacuum (with electrical field) Increasing electric field Thermionic emission Schottky emission field-enhanced thermionic emission (10 8 V/m) Extended Schottky emission thermally assisted field emission Cold field emission tunnel effect (quantum tunnelling) Hot Schottky Cold Schottky Hot FE Cold FE 13 2) Electron Sources: Field emission guns (emitter) Cathodes (tips) Cold field emission (E 10 9 V/m) W single crystal with a sharp tip (radius ~25nm) Advantages Small energy dispersion (<0.1eV) high coherence, high brightness -> higher resolution at lower energies Disadvantages: expensive, high vacuum necessary cold emission needs flushing (cleaning) after 8 hrs Thermally assisted emission: Schottky effect Large tip size W/Zr tip at K ZrO complexes lower work function Continuous operation self-cleaning ZrO 2 reservoir 14 7

2) Electron Sources: Field emission guns (extraction) First anode (extractor) Some kv 5.

8 2) Electron Sources: Field emission guns (extraction) First anode (extractor) Some kv V/m Second anode Final acceleration Grounded- to slight voltage for focusing and increasing electron current Characteristics Tip and anodes form an electrostatic condensor Cross-over (virtual source) size is Ø~5-30nm 15 3) Electron Optics: Basics Object plane Focal plane Image plane Carter/Williams 16 8

9 3) Electron Optics: Basics Parallel beams are focused to one point in the focal plane Object plane Parallel beams are focused to one point in the focal plane Focal plane Image plane tiré de Carter/Williams 17 3) Electron Optics: basics Image plane 1. Over-focus 2. Focus 3. Under-focus tiré de Carter/Williams 18 9

3) Electron Optics: Basics Apertures Angle limiting apertures Defines convergence angle Used to change depth of field, current, and probe size in combination with condenser lens system tiré de

10 3) Electron Optics: Basics Apertures Angle limiting apertures Defines convergence angle Used to change depth of field, current, and probe size in combination with condenser lens system tiré de Carter/Williams 19 3) Electron Optics: Condenser lens system, SEM First Condenser Lens (C1) Defines probe size Small spot <-> small current Large spot <-> high current Used to change Probe Current 20 10

$lenses for electrons Light: glass lenses deflection of light through changing refraction index Electrons: (charged$

11 3) Electron Optics: Condenser lens system, SEM Double Condenser Lens systems in modern microscopes allow increased flexibility in controlling probe current and convergence angle on the sample 21 3) Magnetic Lenses: Lenses for light, lenses for electrons Light: glass lenses deflection of light through changing refraction index Electrons: (charged particles) Electrostatic lenses Magnetic lenses: Lorentz Force! **Variable focus (no moving parts) Tunable correctors (astigmatism) 22 11

3) Electron Optics: Lenses Electrostatic lenses Applied potenial and electric field lines define deflection force and

Variable focus (no moving parts) Tunable correctors (astigmatism) Pole piece metal cone that confines the magnetic field

25 23 3) Electron Optics: Lorenz force F = Force on electron V = electron velocity B = Magnetic field strength

moving electron: Lorenz force F = -e(vxb) Component of v B: v r << v Magnitude of Force F=-eVBsin Focal length depends

12 3) Electron Optics: Lenses Electrostatic lenses Applied potenial and electric field lines define deflection force and focusing Magnetic lenses: Lorentz Force! Variable focus (no moving parts) Tunable correctors (astigmatism) Pole piece metal cone that confines the magnetic field No (simple) divergent lens! multi-poles lenses Image rotation! ) Electron Optics: Lorenz force F = Force on electron V = electron velocity B = Magnetic field strength (rotationally symmetric) e = charge of electron = angle to optic axis Interaction of an electromagnetic field on a moving electron: Lorenz force F = -e(vxb) Component of v B: v r << v Magnitude of Force F=-eVBsin Focal length depends on B increasing B lowers f To obtain the same focal at higher electron voltages, B must increase Spiral with radius r = mv r /eb All electrons crossing the axis in one point are focused into the same point,, at frequency (cyclotron frequency) =eb/m Image rotation!

SHORTER FOCAL LENGTH Upper polepiece Lower polepiece High lens strength =>short focal length B = vector parallel to field B r = radial component of field (vector perpendicular to axis) B z = axial

13 Out of focus astigmatic Ok! Clichés: P.-A. Buffat 10/30/2017 3) Electron optics: Magnetic lens focal length Focal Length (f):point on Z axis where initially parallel rays cross the axis after passing through the lens HIGHER LENS STRENGTH = SHORTER FOCAL LENGTH Upper polepiece Lower polepiece High lens strength =>short focal length B = vector parallel to field B r = radial component of field (vector perpendicular to axis) B z = axial component of field (vector parallel to axis) B varies depending on position in lens Force strongest at center of the gap Low lens strength = long focal length 27 3) Electron Optics: aberrations: Lens aberrations Focus Astigmatism sperical and chromatical aberrations Can be corrected or minimized Physical limits Diffraction limited resolution 26 13

3) Electron Optics: Chromatic Aberration Focal length varies with energy critical for nonmonochromatic beams (advantage for FE guns) 27 3) Electron Optics: Spherical Aberration Focal length varies

14 3) Electron Optics: Chromatic Aberration Focal length varies with energy critical for nonmonochromatic beams (advantage for FE guns) 27 3) Electron Optics: Spherical Aberration Focal length varies with distance from optical axis, i.e., rays from the center to edge of the lens have different focal points Image of the object is dispersed (or blurred) along the optical axis Circle of least confusion d s = ½ C s

This cause the image to be direction 29 3) Electron probe and resolution Lens aberrations

15 3) Electron Optics: Aberrations-astigmatism Astigmatism: focal length varies in different planes. This cause the image to be direction 29 3) Electron probe and resolution Lens aberrations are one of the main limitations to obtaining high spatial resolution BUT astigmatism (second order aberrations of the lens systems) can be easily corrected in mordern microscopes 30 15

3) Electron Optics: Aberration Correctors Astigmatism: Light optics: Correction with cylindrical lenses Electron optics: Correction with quadrupole lenses, 2 quadrupole lenses under 45 degree allow

$quadrupole and octopole lenses 31 3) Electron Optics: Resolution Raleigh s Criterion: Diffraction Limited Resolution d Green light : 532 nm, (objective collection angle)~1 rad Airy Diffraction Disks$

16 3) Electron Optics: Aberration Correctors Astigmatism: Light optics: Correction with cylindrical lenses Electron optics: Correction with quadrupole lenses, 2 quadrupole lenses under 45 degree allow to control strength and direction of correction Spherical Aberration: Light optics: correction with combination of convergent and divergent lenses Electron optics: Correction with hexapole or quadrupole and octopole lenses 31 3) Electron Optics: Resolution Raleigh s Criterion: Diffraction Limited Resolution d Green light : 532 nm, (objective collection angle)~1 rad Airy Diffraction Disks d = 0.61 n sin θ = 0.61 λ β n= 1.7 for oil immersion lens ** d=190nm Electrons 10 kev : nm n=1 for vacuum =0.1 rad given SEM geometry **d=0.075 nm Thermionic Lens aberrations SEM limited limit by spatial source brightness resolution to ~1 nm FEG SEM limited by lens aberrations 32 16

Resolution (nm) 10/30/2017 3) Electron Optics: Resolution Limits for a modern SEM d ech d d 2 g 2 sph d 2 ch d 2 d 33 3) Electron Optics: Resolution at Low kv Résolution (nm) 100 50 10 5 Low voltage,

17 Resolution (nm) 10/30/2017 3) Electron Optics: Resolution Limits for a modern SEM d ech d d 2 g 2 sph d 2 ch d 2 d 33 3) Electron Optics: Resolution at Low kv Résolution (nm) Low voltage, high resolution Basse tension/haute résolution: Observation - observation of the de la real surface surface réelle Uncoated - échantillons samples non-métallisés - faible endommagement dû au Very faisceau little beam damage 1985 FE LaB 6 W High Haute voltage, tension/haute high resolution résolution: Edge - effets effects, de bord fine details not resolved - détails fins non-résolus - fort endommagement dû au Beam faisceau damage Tension High Tension d'accélération Voltage, (kv) kv 17

size Different particles have different interaction volumes sizes and thus

18 3) Electron Optics: Resolution Spatial resolution depends on the size of the interaction volume Interaction volume differs material, accelerating voltage, spot size Different particles have different interaction volumes sizes and thus different resolution for the same microscope settings 35 3) Electron Optics: Modern Microscopes 36 18

19 3) Electron Optics: Modern Microscopes 37 3) Electron Optics: Modern Microscopes -Conventional Mode magnetic field inside pole-piece 38 19

Secondary electrons spiral upward in the strong magnetic field of the lens and are collected by the detector positioned above

20 3) Electron Optics: Immersion lens Modern Low kv SEMs specimen inside lens A small specimen sits inside the lens gap Has very short focal distance, range of 2 to 5 mm Short focal distance means low aberrations, small probe sizes, and high resolution Secondary electrons spiral upward in the strong magnetic field of the lens and are collected by the detector positioned above the lens (**In-Lens Detectors) 39 3) Electron Optics: Modern Microscopes BSE at 2kV: ~1nm resolution BSE at 500V: ~5 nm resolution 40 20

3) Electron Optics: Modern Microscopes MERLIN Analytical power for the sub-nanometer world 41 3) Electron Optics: Modern Microscopes MERLIN Analytical power for the sub-nanometer world High stability

energies Energy selective Backscatter detector (EsB) In-lens Secondary Electron detector GEMINI II final lens GEMINI II design Complete detection system Proven GEMINI final lens design New double

21 3) Electron Optics: Modern Microscopes MERLIN Analytical power for the sub-nanometer world 41 3) Electron Optics: Modern Microscopes MERLIN Analytical power for the sub-nanometer world High stability field emitter cathode Maximum probe current 300 na Beam Double Booster condenser lens Brightness Aperture independent of the electron probe probe current adjustment maintained for low landing energies Energy selective Backscatter detector (EsB) In-lens Secondary Electron detector GEMINI II final lens GEMINI II design Complete detection system Proven GEMINI final lens design New double condenser lens for highest probe current possibilities (40 na) Beam booster technology maintains brightness of all electron probes including low landing energies True on-axis in-lens SE and BSE detectors 42 21

22 Combined Deceleration of high voltage electrons and the filtering of aberrated electrons provides improved resolution at Low kv imaging 43 3) Electron Optics: Modern Microscopes 44 22

23 3) Electron Optics: Modern Microscopes 45 3) Electron Optics: Modern Microscopes 46 23

3) Electron Optics: Modern Microscopes 3)

10mm Stage Range XYZ/RT 110 x 110 x 65mm /

24 3) Electron Optics: Modern Microscopes 3) Electron Optics: Modern Microscopes HR-SEM located CIME-SION 15kV Resolution 1.0 nm 1kV Resolution 1.6 nm 1.4 nm (with BD) In-lens Detectors T1 (segmented A +B), T2, T3* Analytical WD 10mm Stage Range XYZ/RT 110 x 110 x 65mm / Beam Current range 1pA- 400nA Low Vacuum Optional up to 50 pa 48 24

3) Electron Optics: FEG Microscopes Specifications of Zeiss Gemini 300 Resolution @ 15 kv- 1.2 nm Resolution @ 1 kv -2.2 nm Source Schottky Thermal Field Accelerating Voltage 0.

25 3) Electron Optics: FEG Microscopes Specifications of Zeiss Gemini kv- 1.2 nm 1 kv -2.2 nm Source Schottky Thermal Field Accelerating Voltage kv Probe Current 4 pa 40 na Variable Pressure Pa Detectors: EHT, VPSE, BSD, VP-BSE, Inlens SE and EsB Backscatter Detector (BSD) HD Maximum Scan Speed 100 ns/pixel Magnification 10 1,000,000 Image Framestore 3 k 2 k pixels Stage Type 5 axis compucentric stage Just Installed and operating since September 49 4) Detectors: SEM, signals Backscattered electrons photons IR,UV,vis. inelastic elastic e-beam Secondary electrons Absorbed current, EBIC X-rays Auger electrons Secondary electrons (~0-30eV), SE Backscattered electrons (~evo), BSE Photons: visible, UV, IR, X-rays Auger electrons Phonons, Heating Absorbtion of incident electrons (EBIC-Current) 50 25

26 4) Detectors: SEM imaging with electrons Energy spectrum of electrons leaving the sample SE: secondary electrons 0-50eV BSE: backscattered electrons E>50eV 51 4) Electron detectors: Secondary Electrons Electrons with low energy (0-50eV) leaving the sample surface Intensity depends on inclination of the surface topography 52 26

4) Electron detectors: Secondary Electrons Photomultiplier Everhart-Thornley detector Collects and detects lower energy (<100eV) electrons: The positive collector voltage ( +200 à +400V) attracts the

27 4) Electron detectors: Secondary Electrons Photomultiplier Everhart-Thornley detector Collects and detects lower energy (<100eV) electrons: The positive collector voltage ( +200 à +400V) attracts the SE toward the detector, the 10kV post acceleration give them enough energy to create a bunch of photons for each SE 53 4) Electron detectors: In-lens SE In-lens SE (Secondary Electron detector) Topographical information with on-axis in-lens SE detector On axis In-Lens SE detector 54 Complete detection system: Small collection angle Topographical information Unique double in-lens detection Acquisition of pure secondary and backscatter electron signals Separation of compositional, topographical and crystalline surface information High resolution (low working distances, less spherical aberration and collection geometry limits SE2 and SE3 electrons) 27

to surface charging 55 4) Electron detectors: backscattered electrons Electrons with high energy (~Eo)

28 4) Electron detectors: In-lens SE In-lens SE detector Everhart Thornley detector Charging! In-lens detectors collect SE1 electrons, those that emanate from the near surface, which are more sensitive to surface charging 55 4) Electron detectors: backscattered electrons Electrons with high energy (~Eo) backscattered from below the surface Intensity depends on density (atomic weight) ~composition contrast (Z) Nb 3 Sn in Cu matrix 56 28

5) Electron detectors: Semiconductor BSE BSE semiconductor detector: a silicon diode with a p-n junction close to its surface collects the BSE (3.

distribution info Detects higher energy (>5kV) electrons: SEM backscattered electrons 57 4) Electron detectors: In-lens BSE (EsB) Energy filtering grid

detection system Unique double in-lens detection Acquisition of pure secondary and backscatter electron signals Separation of compositional,

29 5) Electron detectors: Semiconductor BSE BSE semiconductor detector: a silicon diode with a p-n junction close to its surface collects the BSE (3.8eV per electron hole pair) large collection angle slow (poor at TV frequency) some diodes are split in 2 or 4 quadrants to bring spatial BSE distribution info Detects higher energy (>5kV) electrons: SEM backscattered electrons 57 4) Electron detectors: In-lens BSE (EsB) Energy filtering grid EsB (Energy selective Backscatter detector) Compositional contrast with on-axis in column EsB detector Si 3 N 4 TiN Si Ti 59 GEMINI II design Complete detection system Unique double in-lens detection Acquisition of pure secondary and backscatter electron signals Separation of compositional, topographical and crystalline surface information On axis In-Lens EsB detector Small collection angle Loss BSE more Z-contrast information Bias control for improved selectivity and contrast 29

Electrons T1- Red (BSEs) T2- Blue (BSEs)

30 4) Electron Detectors: TENEO has 3 in-lens detectors T1 collects BSEs with large angles giving a mix of compositional and topography contrast T2 collects BSE with low angles (high Z contrast) T3 collects SE1 giving near surface topography imaging T1 Backscattered Electrons T3 Secondary Electrons T1- Red (BSEs) T2- Blue (BSEs) T3-Green (SE1) 60 Perovsikite solar cell material Images from Emad Oveisi QUESTIONS? 61 30

Oct. 30th- Nov. 1st, 2017

Thomas LaGrange, Ph.D. Faculty Lecturer and Senior Staff Scientist Electron Sources, Optics and Detectors SEM Doctoral Course MS-636 Oct. 30th- Nov. 1st, 2017 Summary Electron propagation is only possible