Basics of Scanning Electron Microscopy

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1 Thomas LaGrange, Ph.D. Faculty and Staff Scientist Basics of Scanning Electron Microscopy SEM Doctoral Course MS-636 April 11-13, 2016

2 Outline This chapter describes the principles of a scanning electron microscope (SEM), the signals generated and associated contrast mechanisms. It starts with a description of the SEM and detectors used, the formation of an electron probe and how SEM parameter s influence on the spatial resolution. In order to understand the image formation and the contrast mechanisms involved, an explanation of the electron-matter interaction volume is given followed by a discussion on the origin of the secondary and back-scattered electrons (SE and BSE) will be given. This forms the theoretical basis for how we interpret the different possible contrasts mechanisms that form a SEM images, which will be discussed within this presentation, including artifacts. The presented material concludes with application examples and charging effects that can be reduced with lower accelerating voltages and charge compensation devices.

3 Outline 1) SEM operation 2) Detectors 3) Electron probe and resolution 4) Depth of field 5) Electron-matter interaction volume 6) Secondary and back-scattered electrons 7) Contrast mechanisms 8) Examples 9) Charging effects

4 1) SEM operation Electrons are accelerated to high energies (high spatial resolution!) Condenser lens system defines probe size and control probe current In some SEMs, there is a variable objective aperture between condenser lens 1 and 2 that used to control convergence angle (depth of field) Scanning coils above Objective lens raster beam on sample Objective lens focus probe on sample Various detectors surrounding sample collected radiated signals Scan coils 4

1) SEM operation Image formed step by step by the sequential scanning of the sample with the electron probe Image acquisition as numerical data Bulk sample Imaging the sample surface (from 1 nm to 1

5 1) SEM operation Image formed step by step by the sequential scanning of the sample with the electron probe Image acquisition as numerical data Bulk sample Imaging the sample surface (from 1 nm to 1 µm depth depending on the analyzed signal Contrast is due to secondary electrons (SE) emission or back scattered electrons (or sometimes to photons, X-rays, absorbed current) Resolution: 1 nm to 10 nm

1) SEM operation Incident electrons interaction with the sample produces: Secondary electrons SE topography, low energy 0-50 ev Backscattered electrons BSE atomic number Z, energy ev 0 Auger

6 1) SEM operation Incident electrons interaction with the sample produces: Secondary electrons SE topography, low energy 0-50 ev Backscattered electrons BSE atomic number Z, energy ev 0 Auger Electrons : not detected in conventional SEM, surface analysis Cathodoluminescence: photons UV, IR, vis Absorbed current, electron-holes pairs creation, EBIC plasmons Sample heating (phonons) Radiation damages: chemical bounding break, atomic displacement out of site (knock-on)

7 1) SEM operation Energy spectrum of electrons leaving the sample

8 2) Detectors: Quick Recap A "light" source a detector (eye, photographic plate, video camera... a magnification section (lenses, apertures...) a sample (+ a "goniometer") an illumination section (lenses, apertures...)

2) Detectors In-lens detectors On axis In-Lens SE detector Small collection angle Topographical information High resolution (low working distances, less spherical aberration and

9 2) Detectors In-lens detectors On axis In-Lens SE detector Small collection angle Topographical information High resolution (low working distances, less spherical aberration and collection geometry limits SE2 and SE3 electrons) On axis In-Lens EsB detector Small collection angle Loss BSE more Z- contrast information Bias control for improved selectivity and contrast 11

10 3) Electron probe and resolution A "light" source a detector (eye, photographic plate, video camera... a magnification section (lenses, apertures...) a sample (+ a "goniometer") an illumination section (lenses, apertures...)

11 3) Electron probe and 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 13

12 3) Electron probe and resolution Resolving power ("resolution"): Rayleigh criterion

$3) Electron probe and resolution Raleigh s Criterion: Diffraction Limited Resolution d = 0.61 n sin θ = 0.$

13 3) Electron probe and resolution Raleigh s Criterion: Diffraction Limited Resolution d = 0.61 n sin θ = 0.61 λ β d Airy Diffraction Disks Green light : 532 nm, (objective collection angle)~1 rad 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 Lens aberrations limit spatial resolution to ~1 nm 15

14 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 16

$3 = C a s LaB6 source Chromatic aberration æ DE d ch = C ch E + 2 DI ö ç a è I ø Diffraction (Airy, Rayleigh) l$ d d = 0.

15 Diameter (nm) 3) Electron probe and resolution Resolution in probe mode (SEM, STEM) Spherical aberration d sph 3 = C a s LaB6 source Chromatic aberration æ DE d ch = C ch E + 2 DI ö ç a è I ø Diffraction (Airy, Rayleigh) l d d = 0.61 n sin a Brightness conservation 4I d g = p 2 b 1 a Combined d ech = 2 d g + 2 dsph 2 + d ch + 2 dd Convergence angle(mrad) Probe with coherent source: see Mory C, Cowley J M, Ultramicroscopy A/cm 2 sr

16 3) Electron probe and resolution SEM: Limiting parameters on resolving power with SE 1. High magnification The probe size (generation of SE1) r~d probe 2. The size of the interaction volume (generation of SE2+SE3 from BSE): energy and atomic number influence 3. Low magnification The screen (or recording media) pixel size d screen r~d screen /magnification

17 3) Electron probe and resolution How to increase resolving power? Reduce the probe current at constant dose Reduce probe size Reduce interaction volume Reduce C sph Increase gun brightness Increase exposure time t Decrease spot size Increase accelerating voltage Reduce accelerating voltage Short focus lenses: in-lens, semi in-lens, Snorkel Field emission gun: Cold emission, thermal assisted, Schottky effect Reduce C sph and increase Dedicated columns: Gemini transmitted brightness (Merlin), XL30,

3) Electron probe and resolution SEM: Effect

loss of resolution, still less statistical

18 3) Electron probe and resolution SEM: Effect of current, probe size and image acquisition time good resolution, but statistical noise Good resolution, less statistical noise smal loss of resolution, still less statistical noise very few statistical noise, but high resolution loss!

19 3) Electron probe and resolution probe size and resolution (no noise) particles 25 nm diam., probe dia 2 nm model 100 nm diam. particles particles 50 nm diam., probe dia 2 nm particles 100 nm diam., probe dia 2 nm

20 3) Electron probe and resolution Probe size and resolution (with noise) model 100 nm diam. particles particles 50 nm diam., probe diam 2 nm particles 25 nm diam., probe diam 2 nm Particles100 nm diam., probe diam 2 nm

21 3) Electron probe and resolution Current/probe diameter Thermionic source: spherical aberration is the most important I max = 3p 2 16 bc sph Field emission source: gun aberrations and chromatic aberration are more important I max = cd d 8 3 Tiré de L.Reimer, SEM

22 3) Electron probe and resolution How to increase resolving power? Reduce the probe current at constant dose Reduce probe size Reduce interaction volume Increase exposure time t Decrease spot size Increase accelerating voltage Reduce accelerating voltage Reduce C sph Increase gun brightness Short focus lenses: in-lens, semi in-lens, Snorkel Field emission gun: Cold emission, thermal assisted, Schottky effect Reduce C sph and increase transmitted brightness Dedicated columns: Gemini, XL30,

Diameter (nm) Diameter (nm) Diameter (nm)

angle(mrad) Convergence angle(mrad) Modern

23 Diameter (nm) Diameter (nm) Diameter (nm) 3) Electron probe and resolution Thermionic SEM : low voltage? Convergence angle(mrad) Convergence angle(mrad) Convergence angle(mrad) Modern SEM short focus length: C sph =17 mm, C ch =9 mm, E=1.5 ev, = A/cm 2 sr

24 c. Electron probe and resolution How to increase resolving power? Reduce the probe current at constant dose Reduce probe size Reduce interaction volume size Reduce C sph Increase gun brightness Reduce C sph and increase transmitted brightness Increase exposure time t Decrease spot size Increase accelerating voltage Reduce accelerating voltage Short focus lenses: in-lens, semi in-lens, Snorkel Field emission gun: Cold emission, thermal assisted, Schottky effect Dedicated columns: Gemini, XL30,

25 3) Electron probe and resolution Interaction volume versus E 0 1m 1m Cu 20keV Cu 5keV Penetration depth in Cu as a function of incident energy E 0 and proportion of BSE (Monte-Carlo simulation) Z = cte 1m Cu 1keV Cu 1keV

26 3) Electron probe and resolution How to increase resolving power? Reduce the probe current at constant dose Increase exposure time t Reduce probe size Reduce interaction volume size Reduce C sph Increase gun brightness Reduce C sph and increase transmitted brightness Decrease spot size Increase accelerating voltage Reduce accelerating voltage Short focus lenses: in-lens, semi in-lens, Snorkel Field emission gun: Cold emission, thermal assisted, Schottky effect Dedicated columns: Gemini, XL30,

Convergence angle(mrad) Convergence angle(mrad) Convergence angle(mrad) Snorkel Regular

27 Diameter (nm) Diameter (nm) Diameter (nm) 3) Electron probe and resolution Short focus length or FEG? Convergence angle(mrad) Convergence angle(mrad) Convergence angle(mrad) Snorkel Regular focus length FEG C sph =1.7 mm, C ch =1.9 mm C sph =17 mm, C ch =9 mm C sph =17 mm, C ch =9 mm = A/cm 2 sr, E=1.5 ev = A/cm 2 sr, E=1.5 ev = A/cm 2 sr, E=0.4 ev

28 Resolution (nm) 3) Electron probe and resolution Resolution loss at low voltage Résolution (nm) Low Basse voltage, ten sio high n/h resolution au te réso lu tio n : Observation - ob servatio of n the d e real la su surface rface réelle - échan tillon s n on -métallisés Uncoated samples - faible en do mmag ement dû au Very little beam damage faisceau 1985 FE LaB 6 W High Hau te voltage, ten sio n/h high au te resolution réso lu tio n: - effets de bo rd Edge effects, fine details not - détails fins no n-réso lu s resolved - fo rt end o mmag emen t dû au More faisceau beam damage Shorter objective lens focal length and C s Tension High Tension d'accélération Voltage, (kv) 30

29 3) Electron probe and resolution Influence of working distance and objective aperture on resolution Increasing working distance will: increase depth of focus increase probe size and thus decrease resolution. increase the effects of stray magnetic fields and thus decrease resolution increase aberrations due to the need for a weaker lens to focus Increasing aperture size will: increase current and produced signals better signal to noise increase aberration and thus decrease resolution. decrease depth of field 31

30 4) Depth of field Light bulb filament

31 4) Depth of field Depth of field as a function of d probe h h 2a 2d A d A The depth of field is the depth for which the image can be focused. The depth of field increases when the convergence angle of the probe (a) decreases. Increase the working distance Reduce objective aperture size ì ï h prof.champ = max í î ï 2d sonde 1 a 2 pixel "image " G 1 a

32 4) Depth of field Effect of working distance (WD) and aperture on the depth of field

33 4) Depth of field Examples

34 4) Depth of field 100 µm Another example of the effect of the objective aperture diameter 50 µm 30 µm

35 4) Depth of field Measuring depth of field: stereoscopy

36 5) Electron-matter interaction volume Elastic interaction Total kinetic energy and momentum are constant The light electron interacts with the electrical field in the heavy atom: Rutherford scattering. Only little energy is transferred, the electron speed does not change significantly in amplitude but only in direction (elastic scattering). Back scattered electron Incident electron Scattering angle 100 kv 1000 kv C Au C Au mev 0.03 mev 9 mev 0.5 mev ev 9 mev 2.7 ev 0.17 ev ev 0.6 ev 179 ev 11 ev ev 1.2 ev 359 ev 22 ev

37 5) Electron-matter interaction volume Inelastic interaction part of the total kinetic energy is dissipated (energy loss) vibration in molecules or atomic lattice in crystals (phonons mev-100mev) collective oscillations of electrons (plasmons 10 ev) intra- and interband transitions ( mev- 1 ev) inner shell atom ionization ( 50 ev to150 kev -> ev 0!) bond breaking ev, atom displacement ev (requires V acc 100kV...1MV, no longer SEM!)

38 5) Electron-matter interaction volume Mean free path (penetration depth) Elastic cross-sections s el and mean free path L el, total (elastic+inelastic) mean free path L t and electron range R The mean free path is the average path that an electron progagates before it scatters off an atom

39 5) Electron-matter interaction volume Monte-Carlo simulations 1. Electron Flight Simulator ($$$ Small World / D. Joy) old DOS!!!! 2. Single Scattering Monte Carlo Simulation (Freeware) "Monte Carlo Simulation" Mc_w95.zip by Kimio KANDA 3. CASINO (Freeware) " monte CArlo SImulation of electron trajectory in solids " by P. Hovongton and D. Drouin

40 5) Electron-matter interaction volume Number/Energy of backscattered electrons by Monte-Carlo simulations W BSE 41% BSE 43% BSE 52% 1 kv 3 kv 30 kv C BSE 10% BSE 8% BSE 5%

electron penetration vs Z and yield of backscatter electrons BSE

41 5) Electron-matter interaction volume Penetration and back- scattering vs elements (Z) BSE=6% V acc = 20kV = cte Depth of electron penetration vs Z and yield of backscatter electrons BSE (Monte-Carlo simulation): 1m C 20 kev 1m U 20 kev BSE=50% 1m Cu 20 kev BSE=33%

42 5) Electron-matter interaction volume Penetration and backscattering vs elements (Z) V acc = 5 kv = cte Depth of electron penetration vs Z and yield of backscattered electrons BSE (Monte-Carlo simulation): 200 nm BSE=8% C 5 kev 200nm BSE=33% 200 nm BSE=47% Cu 5 kev U 5 kev

43 5) Electron-matter interaction volume Penetration and backscattering vs elements (Z) V acc = 1 kv = cte Depth of electron penetration vs Z and yield of backscattered electrons BSE (Monte-Carlo simulation): 10 nm BSE=14% C 1keV 10 nm BSE=34% 10 nm U 1keV BSE=44% Cu 1keV

44 5) Electron-matter interaction volume Penetration and backscattering vs energy(e) Z = cte Depth of electron penetration in Cu vs energy E 0 and yield of backscattered electrons BSE (Monte-Carlo simulation): 1µm Cu 20 kev 1µm Cu 5 kev Cu 1 kev 1µm Cu 1keV

6) Secondary Electrons and Backscattered Electrons "true" secondary electrons SE1 and "converted BSE" secondaries SE2+SE3 Various SE types from SE1: incident probe SE2: BSE leaving the

45 6) Secondary Electrons and Backscattered Electrons "true" secondary electrons SE1 and "converted BSE" secondaries SE2+SE3 Various SE types from SE1: incident probe SE2: BSE leaving the sample SE3: BSE hitting the surroundings although this signal is gathered around the probe, its intensity is only attributed to the pixel corresponding to the actual probe position x 0,y 0

46 6) Secondary Electrons and Backscattered Electrons "true" secondary electrons SE1 and "converted BSE" secondaries SE2+SE3 The SE signal always contain a high resolution part (SE1 from the probe) and a combined average (low resolution) part from SE2+SE3!

47 6) Secondary Electrons and Backscattered Electrons Relative contribution of SE1 and SE2 (+SE3) vs primary energy total total total SE1 SE2 The total intensity (green and brown) is attributed to the (x,y) pixel, here at 0 nm on this 1-D model (adapted from D.C. Joy Hitachi News )

48 6) Secondary Electrons and Backscattered Electrons Yield for SE and BSE emission per incident electron vs atomic number Z sample surface polished (no topography) and perpendicular to the incident beam direction (intermediate energy E 0 15 kev) BSE: chemical contrast for all the elements (sensitivity DZ=0.5) A fast way to phase mapping I BSE =I pe h with I pe the intensity of the primary beam, the BSE yield d: (SE1) SE: low or no chemical contrast but for light elements the topographical contrast will dominate on rough surfaces I SE =I pe d +I SE3 =I pe (d pe +d pe h+d sur h) SE1 SE2 SE3 with d the total SE yield, d pe the yield for SE1 and d sur the SE3 yield for materials surrounding the sample (pole-pieces...) Al Ni

49 6) Secondary Electrons and Backscattered Electrons Dust on WC (different Z materials) flat material rough material low Z material low Z material thin low Z material SE 25 kv BSE

50 6) Secondary Electrons and Backscattered Electrons Contaminated area around a soldering spot

51 6) Secondary Electrons and Backscattered Electrons Toner particle (penetration in light material) SE 28 kv BSE

52 7) Contrast mechanisms Size and edge effects

53 7) Contrast mechanisms Topographical contrast in SE mode: Effect of sloped surfaces penetration depth ("range") >>SE escape length 1-10nm I 0 I( ) I( q) = I 0 d( I ( 0 ) cosq Relative yield of SE vs angle of incidence on the sample surface (adapted from D.C. Joy Hitachi News )

54 7) Contrast mechanisms Size and edge effects (From L. Reimer, Image Formation in Low-Voltage Scanning Electron Microscopy, (1993))

55 7) Contrast mechanisms Size and edge effects Do not forget, in SEM: The signal is displayed at the probe position, not at the actual SE production position!!! intensity profile on image

7) Contrast mechanisms SE and BSE topography contrast For one position (x,y) of the electron probe: BSE escape from a "pear" volume around the probe position SE1 escape from a

(40 ) = I PE d 1/cos40 =I PE 13% SE1 contrast =26% I BSE (0 )=I PE =I PE 31% I BSE (40 )=I PE 37% BSE contrast = 18% I SE2+3 = I BSE d= I PE h d I SE2+3 (0 ) = I PE 37% 10%= I PE

56 7) Contrast mechanisms SE and BSE topography contrast For one position (x,y) of the electron probe: BSE escape from a "pear" volume around the probe position SE1 escape from a thin layer under the entrance surface of the probe SE2 escape from a thin layer under the escape surface of BSE contrast = 2(I 1 -I 2 )/(I 1 +I 2 ) I SE (0 )=I PE d=i PE 10% I SE (40 ) = I PE d 1/cos40 =I PE 13% SE1 contrast =26% I BSE (0 )=I PE =I PE 31% I BSE (40 )=I PE 37% BSE contrast = 18% I SE2+3 = I BSE d= I PE h d I SE2+3 (0 ) = I PE 37% 10%= I PE 3.1% out of 10% I SE2+3 (40 ) = I PE 37% 10%=I PE 3.7% out of 13% BSE topographical contrast is not negligible! Chemical contrast is better observed only on flat, polished surfaces

absorbed (0V) The trajectories of BSE are not strongly affected by the electrical field, most BSE miss

57 7) Contrast mechanisms Comparison of SE and BSE contrast modes SE BSE +200V ET detector backscattered and transmitted e - create SE, some of them are driven to the ET detector by the electric field BSE are absorbed (0V) The trajectories of BSE are not strongly affected by the electrical field, most BSE miss the detector If you look down to the column, the "light" seems to come from the Everhardt-Thornley detector.

58 7) Contrast mechanisms Topographical contrast at low energy Effect of the incidence angle (adapted from D.C. Joy Hitachi News )

59 7) Contrast mechanisms Contrast in image suggest the object in the center has a stepped pyramid morphology

60 7) Contrast mechanisms Rotated by 180 degrees, and now it looks like an etched pit on the surface

61 7) Contrast mechanisms Detector position? Image rotated 180 degrees Detector position? pyramid?.. OR.. etch-pit?

62 7) Contrast mechanisms Influence of voltages on the resolution and contrast mechanisms 64

63 7) Contrast mechanisms Low voltage better resolution and imaging of fine surface features low Z materials (smaller interaction volume and less SE2 signal) High voltage better spatial resolution for isolated particles 65

64 7) Contrast mechanisms Change in SE contrast with the voltage (from L.Reimer, Image formation in the low-voltage SEM)

$example: a fracture$ Enhancement

65 7) Contrast mechanisms An example: a fracture in Ni-Cr alloy Enhancement topograpical contrast at low voltage: less delocalization by SE2. SE, 5 kv SE, 30 kv

7) Contrast mechanisms Effect of the accelerating voltage on penetration and SE signal (from D.C. Joy Hitachi News 16 1989) 20 kv: Strong penetration, SE3 is a much larger signal than SE1/SE2.

66 7) Contrast mechanisms Effect of the accelerating voltage on penetration and SE signal (from D.C. Joy Hitachi News ) 20 kv: Strong penetration, SE3 is a much larger signal than SE1/SE2. It reveals the copper grid under the C film via the electron backscattering, but the structure of the film itself is hidden 2 kv: Low penetration, only a few electrons reach the copper grid and most of the SE3 are produced in the C film together with SE1/SE2. The C film and its defects become visible

8) Examples Physical limit to the imaging in secondary electron mode Tin grains on a thin carbon film (TEM supporting grid) HRSEM 25 kv 1 nm nominal resolution left: SE

67 8) Examples Physical limit to the imaging in secondary electron mode Tin grains on a thin carbon film (TEM supporting grid) HRSEM 25 kv 1 nm nominal resolution left: SE right: scanning transmitted electrons (STEM bright-field) SE: e - /e - coulombic STEM: Rutherford (e-/electric field in atom) (from B. Ocker, Scanning Microscopy 9 (1995) 63 )

68 8) Examples Physical limit of the imaging with secondary electrons (from B. Ocker, Scanning Microscopy 9 (1995) 63 ) The average grain size looks larger in SE (12.3 nm) than in STEM (9.1 nm) "Delocalization": the elastic scattering in STEM (Rutherford) occurs at a much closer distance from the atom nucleus than the inelastic coulombic e - /e - interactions required to eject a SE

69 8) Examples Al x Ga 1-x As/GaAs "quantum wire" (2-D quantum well) x=0.20 GaAs x=0.55 SE mode image on a cleaved surface. The SE 2 (BSE chemical) contrast dominates this image in absence of topographical contrast (SE 1 =cte) (by courtesy of Dr. K. Leifer, IPEQ/EPFL) QW

70 8) Examples Contrast reversal in BSE mode at low accelerating voltage Si Cu Ag Au from L.Reimer, Image formation in low-voltage SEM

71 9) Charging effects Fiberglass on epoxy 1 kv 5 kv, high current 5 kv, low current (by courtesy of B. Senior CIME/EPFL)

72 9) Charging effects Charging-up on a mask for microelectronic (SiO 2 substrate, photoresist, SE mode) V acc >> E 2 V acc E 2

73 9) Charging effects Charging-up on spherical silica particles TV scan slow scan Charging of particle surface lead to anomalous contrast given a flying saucer like morphology 5 kv at 1.5 kv, close to the neutrality point, particles recover their spherical type contrast 1.5 kv

an electric field that repells the SE toward the substrate

74 obj pole-piece 9) Charging effects Charging-up of an insulating particle of dust 0V Negative charges left on the particle create an electric field that repells the SE toward the substrate around the dust (adapted from L. Reimer Scanning Electron Microscopy)

75 j. Charging effects Extreme charging-up: electrons are reflected by the sample and hit the microscope sample chamber!!! ET (adapted from Philips Bulletin)

76 9) Charging effects SE contrast is improved and distortions are minimized at low voltages fiberglass on epoxy Which polarity?????? by courtesy B. Senior/CIME

9) Charging effects Total yield for electron emission (SE + BSE) on insulators E 1 and E 2 are critical energies where 1 electron leaves the surface for each incident electron: neutrality when ev acc

77 9) Charging effects Total yield for electron emission (SE + BSE) on insulators E 1 and E 2 are critical energies where 1 electron leaves the surface for each incident electron: neutrality when ev acc = E 2 charging-up disappears! ev acc = E 1 is unstable, ev acc = E 2 is stable Caution: E 1 and E 2 are specific to the material, but also change with the incidence angle q! Caution: this simple (simplest!) model is not quantitative for insulators because charge implantation and removal depends of the scanning speed and precise sample geometry

9) Charging effects Observation of insulating samples Charging-up is reduced or even cancelled when working at E 2 Charging-up may be cancelled under partial atmosphere in a "low vacuum" or "low

78 9) Charging effects Observation of insulating samples Charging-up is reduced or even cancelled when working at E 2 Charging-up may be cancelled under partial atmosphere in a "low vacuum" or "low pressure" SEM, ESEM Caution the "skirt" (incident electrons from the probe are scattered out of it by the atmosphere reduced resolution and contrast delocalized microanalysis (may attain mm!) Using a charge compensator device Cliché Kontron (Kuschek)pour CIME

79 9) Charging effects Contrast reversal in SE mode close to the neutrality point SiO 2 -Cr mask for TEG-FET transistors production SiO 2 (E 2 ~3.0keV) Cr (E 2 ~1.8keV) 3.0 kv 1.8 kv Cliché Kontron (Kuschek)pour CIME

80 9) Charging effects Some values of the neutrality E 2 energy E 2 : upper neutrality energy E m : maximum emission energy d m : maximum yield at E m adapted from: E. Plies, Advances in Optical and Electron Microscopy,13 (1994) p 226

9) Charging effects Charge compensator device Gas introduced near the sample Gas is ionized by electron beam Surface charge is reduced due to ion

81 9) Charging effects Charge compensator device Gas introduced near the sample Gas is ionized by electron beam Surface charge is reduced due to ion bombardment SEM is operated at high voltages instead low voltages, i.e., ionization of the gas and thus surface charge compensation is better at higher voltages

82 9) Charging effects Surface potential (voltage) contrast (from Golstein et al, Practical SEM (1975))

83 Other effects 1) Leakage: Magnet field from distribution board Stray fields from power lines in the surrounding walls High-tension line located too close to the instrument 2) Low floor strength (acoustic vibrations) 3) Improper grounding 4) Detector discharging and/or contaminated

84 QUESTIONS? 86

SEM Doctoral Course MS-636. April 11-13, 2016

SEM Doctoral Course MS-636. April 11-13, 2016 Thomas LaGrange, Ph.D. Faculty Lecturer and Senior Staff Scientist Electron Sources, Optics and Detectors SEM Doctoral Course MS-636 April 11-13, 2016 Summary Electron propagation is only possible through