2.Components of an electron microscope. a) vacuum systems, b) electron guns, c) electron optics, d) detectors. Marco Cantoni, 021/

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1 2.Components of an electron microscope a) vacuum systems, b) electron guns, c) electron optics, d) detectors Marco Cantoni, 021/ Centre Interdisciplinaire de Microscopie Electronique CIME MSE-603 Electron guns, lenses, detectors Marco Cantoni 1 Summary Electron propagation is only possible through vacuum. The vacuum level varies in the different areas of an electron microscope. The highest vacuum level (<10-7 Pa or 10-9 mbar) is required in the gun where electrons are emitted through field emission. Also the specimen area requires a high vacuum level especially for chemical analysis when the electron beam is resting for a longer time in the same area. Hydrocarbon build up (contamination) on the observed area is often the result of a low system vacuum level. Turbomolecular and oil-diffusion pumps for high vaccum cannot work against atmospheric pressure and need a mechanical prevaccum pump in order to function. Electron beams can either be generated by thermal emission (thermionic sources, cheap) or field emission. Only field emission sources can provide the necessary low energy spread and coherence for modern high resolution electron microscopy 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. However lens aberrations 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). MSE-603 Electron guns, lenses, detectors Marco Cantoni 2

2 Components of an electron microscope Vacuum system! Source: electron gun Lenses and apertures Sample holder (stage) Detector(s) common SEM and TEM Specific for each technique MSE-603 Electron guns, lenses, detectors Marco Cantoni 3 Pumping system 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 Vaccum level in space: 1 Pa at 100km above earth surface MSE-603 Electron guns, lenses, detectors Marco Cantoni 4

3 Primary vacuum Rotary vane pump Uses oil noisy MSE-603 Electron guns, lenses, detectors Marco Cantoni 5 Secondary vacuum Oil diffusion pump Vibration free Contamination possible oil vapor High pumping capacity (>500 l/s) Best with cold trap MSE-603 Electron guns, lenses, detectors Marco Cantoni 6

4 Secondary vacuum Turbomolecular pump Rotation speed rpm Magnetic bearings Pumping volumes l/s MSE-603 Electron guns, lenses, detectors Marco Cantoni 7 High / Ultra-high vacuum Ion getter pump no vibrations No exit: improves vacuum! MSE-603 Electron guns, lenses, detectors Marco Cantoni 8

5 Contamination Oil vapors from oil diffusion pump Heat the sample up to 100 C Cool the sample down to -200 C (danger ice!) Clean the sample holders regularly Don t touch samples and sample holders (even with gloves) Use a plasma cleaner before observation MSE-603 Electron guns, lenses, detectors Marco Cantoni 9 SOURCES (gun) LaB 6 Cathode MSE-603 Electron guns, lenses, detectors Marco Cantoni 10

6 Emission of electrons metal vacuum (with electrical field) Electric field Thermionic emission Shottky emission field-enhanced thermionic emission (10 8 V/m) Extended Shottky emission thermally assisted field emission Cold field emission tunnel effect (quantum tunnelling) temperature MSE-603 Electron guns, lenses, detectors Marco Cantoni 11 Emission of electrons metal vacuum (with electrical field) Electric field Thermionic emission Shottky emission field-enhanced thermionic emission (10 8 V/m) Extended Shottky emission thermally assisted field emission Cold field emission tunnel effect (quantum tunnelling) temperature MSE-603 Electron guns, lenses, detectors Marco Cantoni 12

7 Electron gun Important parameters Emitted current, energy Energy dispersion Brightness current per surface unit and solid angle Coupling to the column the gun incorporates often a first lens (Wehnelt, gun lens) MSE-603 Electron guns, lenses, detectors Marco Cantoni 13 Thermionic gun Tungsten wire heated up to 2800K LaB 6 crystal heated to 1900K Advantage simple, cheap no high vacuum required maintenance friendly Disadvantages low brightness high energy dispersion large source size (30um) MSE-603 Electron guns, lenses, detectors Marco Cantoni 14

8 MSE-603 Electron guns, lenses, detectors Marco Cantoni 15 Field emission guns Cathods Cold field emission (E 10 9 V/m) W monocristal with sharp tip tip radius ~100nm Thermally assisted emission: Shottky effect W/Zr tip at K Advantages Small energy dispersion (<0.4eV) high coherence, high brightness -> higher resolution at lower energies Disadvantages expensive high vacuum necessary cold emission needs flushing (cleaning) after 8 hrs MSE-603 Electron guns, lenses, detectors Marco Cantoni 16

9 Field emission guns First anode (extractor) Some kv V/m Second anode Final acceleration Grounded Characteristics Tip and anodes form an electrostatic condensor Cross-over (source) is virtual Ø~5nm MSE-603 Electron guns, lenses, detectors Marco Cantoni 17 Most common: LIMS Liquid Metal Ion Source Ion gun (FIB) W tip Liquid metal wets the tip through surface tension and electrostatic force Ionization and emission by field effect (~10 10 V/m) High brightness Small emitting surface (Taylor cone) Small Ion probes (~5nm) possible: FIB Focused Ion Beam LMIS Source: FEI Beam Technology Division MSE-603 Electron guns, lenses, detectors Marco Cantoni 18

10 MSE-603 Electron guns, lenses, detectors Marco Cantoni 19 Optics, basics tiré de Carter/Williams MSE-603 Electron guns, lenses, detectors Marco Cantoni 20

11 Optics, basics Object plane Focal plane Image plane Carter/Williams MSE-603 Electron guns, lenses, detectors Marco Cantoni 21 Optics, basics tiré de Carter/Williams TEM: transmitted beam, diffracted beams MSE-603 Electron guns, lenses, detectors Marco Cantoni 22

12 Optics, basics Focus Over-focus Under-focus tiré de Carter/Williams MSE-603 Electron guns, lenses, detectors Marco Cantoni 23 Optics, basics Angle limiting apertures Collection angle Use: condensor lens system Diffraction contrast tiré de Carter/Williams MSE-603 Electron guns, lenses, detectors Marco Cantoni 24

13 Condensor lens system Convergent illumination <-> parallel illumination probe mode (SEM ou STEM) «projection» mode TEM MSE-603 Electron guns, lenses, detectors Marco Cantoni 25 Condensor lens system, SEM Condensor aperture Convergence angle intensity (current) MSE-603 Electron guns, lenses, detectors Marco Cantoni 26

14 Condensor lens system, SEM Condensor I (C1) Defines probe size Total Current MSE-603 Electron guns, lenses, detectors Marco Cantoni 27 Projector lens system, TEM TEM: Intermediate and projector lenses Projection of the back focal plane to the screen diffraction mode Projection of the image plane to the screen image mode (haute resolution) mode DIFFRACTION mode IMAGE MSE-603 Electron guns, lenses, detectors Marco Cantoni 28

15 Projector lens system, TEM TEM: Intermediate and projector lenses Projection of the back focal plane to the screen diffraction mode Projection of the image plane to the screen image mode (haute resolution) mode DIFFRACTION mode IMAGE MSE-603 Electron guns, lenses, detectors Marco Cantoni 29 Lenses for electrons Light: glass lenses deflection of light through changing refraction index Charged particles Lorentz Force! Electrostatic lenses Magnetic lenses Particularity: Variable focus Tunable correctors (astigmatisme) MSE-603 Electron guns, lenses, detectors Marco Cantoni 30

16 Electrons in a magnetic field Optical axis α Homogeneus field, α small Component of v // B almost unchanged Component of v B: v r << v Spiral with radius r = m vr/eb All electrons crossing the axis in one point are focused into the same point, α, v r Focal length depends on B increasing B lowers f MSE-603 Electron guns, lenses, detectors Marco Cantoni 31 Magnetic lens Field with rotational symmetry Lorenz Force : F = -e v ^ B e on optical axis: F = 0 e not on optical axis : deviated optical axis: symmetry axis Scherzer 1936: Magnetic lens with rotational symmetry: Aberration coefficients: C s : spherical C c : chromatical Always positive!! Resolution limit: D res = 3/ 4 1/ λ Cs Example: λ= nm, C s = 1 mm D res = = 1.8Å MSE-603 Electron guns, lenses, detectors Marco Cantoni 32

17 Magnetic lens Electron optics: no sharp interface at lens «surface» iron e-beam No divergent lens! Electron beam diverges by itself Electrostatic repulsion Pole piece coil multi-poles lenses Correction of aberrations Pole piece metal cone that confines the magnetic field Image rotation! MSE-603 Electron guns, lenses, detectors Marco Cantoni 33 Lens aberrations sperical and chromatical aberrations Astigmatism Can be corrected or minimised Physical limits Diffraction effect Aberrations: Clichés: P.-A. Buffat MSE-603 Electron guns, lenses, detectors Marco Cantoni 34

18 chromatical aberration Focal length varies with energy critical for non-monochromatic beams (advantage for FE guns) MSE-603 Electron guns, lenses, detectors Marco Cantoni 35 Spherical aberration Focal length depends on the distance from optical axis Image of the object is dispersed along the optical axis Circle of least confusion d s = ½ C s α 3 MSE-603 Electron guns, lenses, detectors Marco Cantoni 36

19 Aberrations: astigmatism Astigmatism: focal length varies in different planes. MSE-603 Electron guns, lenses, detectors Marco Cantoni 37 correctors Astigmatism: Light optics: correction with cylindrical lenses Electron optics: Correction with quadrupole lenses: 2 quadrupole lenses under 45 degree allow to control strenght 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 Cs-corrector MSE-603 Electron guns, lenses, detectors Marco Cantoni 38

20 Aberrations: diffraction MSE-603 Electron guns, lenses, detectors Marco Cantoni 39 Résolution SEM Limite SEM modèrne MSE-603 Electron guns, lenses, detectors Marco Cantoni 40

21 Resolution: SEM vs TEM MSE-603 Electron guns, lenses, detectors Marco Cantoni 41 Resolution SEM vs TEM MSE-603 Electron guns, lenses, detectors Marco Cantoni 42

22 Resolution: SEM Résolution (nm) Low voltage, high resolution Basse tension/haute résolution: - Observation observation of de the la surface real surface réelle - Uncoated échantillons samples non-métallisés - faible endommagement dû au Very faisceau little beam damage FE 1985 LaB 6 W Haute High voltage, tension/haute high resolution résolution: - effets de bord Edge effects, fine details not - resolved détails fins non-résolus - fort endommagement dû au Beam faisceau damage Tension d'accélération (kv) MSE-603 Electron guns, lenses, detectors Marco Cantoni 43 Resolution of a TEM Resolution depends on the aberration of the objetive lens: Chromatic: depends on ΔE/E; 300 kev; Cc ~ 1 mm, not critical Diffraction: wave lenght λ = kev Spherical Aberration: limiting!!! In 2000: a standard noncorrected TEM 300 kev provides a resolution of ~2Å S. Pennycook et al., MRS Bull. 31, 36 (06) MSE-603 Electron guns, lenses, detectors Marco Cantoni 44

23 semiconductor Electron detectors BSE semiconductor detector: a silicon diode with a p-n junction close to its surface collects the BSE (3.8eV/e - -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 MSE-603 Electron guns, lenses, detectors Marco Cantoni 45 Electron detectors Photomultiplier Everhart-Thornley detector Collects and detects lower energy (<100eV) electrons: : SEM backscattered electrons MSE-603 Electron guns, lenses, detectors Marco Cantoni 46

24 Electron detectors From Caméra CCD (charge coupled device) 1kx1k, 2kx2k, 4kx4k Pixel + high dynamic range, sensitive -Slow (no TV rate) -expensive MSE-603 Electron guns, lenses, detectors Marco Cantoni 47 Electron detectors Imaging plates High dynamic range grey levels Absolutely linear senisitvity big surface Numerical Image MSE-603 Electron guns, lenses, detectors Marco Cantoni 48

25 Electron detectors Imaging plates High dynamic range grey levels Absolutely linear senisitvity big surface Numerical Image MSE-603 Electron guns, lenses, detectors Marco Cantoni 49 Electron detectors Application: electron diffraction MSE-603 Electron guns, lenses, detectors Marco Cantoni 50