Electron Microscopy 3. SEM Image formation, detection, resolution, signal to noise ratio, interaction volume, contrasts SEM is easy! Just focus and shoot "Photo"!!! Please comment this picture... Any idea what it might be???
SEM is easy! Just focus and shoot "Photo"!!! Does this one sound more obvious? SEM is easy! Just focus and shoot "Photo"!!! At least this one??
Image formation SEM, signals Backscattered electrons photons IR,UV,vis. inelastic elastic e-beam Secondary electrons X-rays Absorbed current, EBIC Auger electrons Secondary electrons (~0-30eV), SE Backscattered electrons (~evo), BSE Auger electrons Photons: visible, UV, IR, X-rays Phonons, Heating Absorbtion of incident electrons (EBIC-Current)
SEM imaging with electrons Energy spectrum of electrons leaving the sample SE BSE Auger electrons SE: secondary electrons 0-50eV BSE: backscattered electrons E>50eV Electron detectors Photomultiplier Everhart-Thornley detector Collects and detects lower energy (<100eV) electrons: SEM backscattered electrons
Electron detectors semiconductor BSE semiconductor detector: a silicon diode with a p-n junction close to its surface collects the BSE (3.8eV/ehole 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 SEM, secondary electrons Electrons with low energy (0-50eV) leaving the sample surface Intensity depends on inclination of the surface topography
SEM, backscattered electrons Electrons with high energy (~Eo) backscattered from below the surface Intensity depends on density (atomic weight) ~composition Nb 3 Sn in Cu matrix Depth of field: principle Airy: d A =0.61 /nsin the image is considered as unsharp when it reach the double of the Airy disc: h=d A /tg =0.61 cos /nsin 2 if small aperture h~1/ 2 h ~ 0.1µm pour =70 h ~ 10 µm pour =10 the depth of field of the optical microscope is very low! defocused h h defocused 2 2d A d A
Depth of field: photons and SEM SEM ~10-3 rad resolution Résolution 10µm 1µm 100nm 10nm 10mm 1mm 0.1mrad 1nm LM ~10-20 Profondeur depth of de field champ h h 100µm 1mrad 10mrad 10µm LM SEM =0.5 LM =500 mnm 1µm 0.1µm 10 10 2 10 3 10 4 10 5 Grandissement magnification (grossissement) M G Changing depth of focus with the diameter of the objective aperture 100 µm 50 µm 30 µm
Comparison of different microscopes Some typical SEM Zeiss Ultra HR-SEM At CIME: Zeiss NVision40 (FIB) Resolution 1.0 nm @ 15 kv, 1.7 nm @ 1 kv, 4.0 nm @ 0.1 kv Magnification12-900,000x in SE mode Acceleration Voltage0.02-30 kv Probe Current4 pa - 10 na Standard Detectors: EsB Detector with filtering grid High efficiency In-lens SE Detector Everhart-Thornley Secondary Electron Detector
Resolution Gun and lens design define the smallest probe size = resolution..? Other parameters: Rayleigh criterium Screen resolution, pixel size (at low mag) Interaction volume (delocalization) Signal to noise ratio Smallest probe size Rayleigh criterion How to measure resolution? Two point images (airy discs) are resolved when the intensity between two intensity maxima drops down to 81%
probe size and resolution (no noise) model 100nm diam. particles particles 25nm diam., probe dia 2nm particles 100nm diam., probe dia 2nm particles 50nm diam., probe dia 2nm probe size and resolution (with noise) model 100nm diam. particles particles 25nm diam., probe diam 2nm particles100nm diam., probe diam 2nm particles 50nm diam., probe diam 2nm
SEM: Effet of beam current, probe diameter and exposure time 500nm 10 pa/10 s 10 pa/160 s 100 pa/160 s 1 na/160 s Good resolution, but noisy Good resolution, Less statistical noise Slight loss in resolution, Less noisy Very little noise, but strong decrease in resolution SEM: Limiting parameters on resolving power 1. High magnification The probe size (generation of SE1) r d probe 2. The volume of interaction (generation of SE2+SE3 from BSE) 3. Low magnification The screen (or recording media) pixel size d screen r d screen /magnification
The interaction volume: Monte-Carlo simulations Electron Flight Simulator ($$$ Small World / D. Joy) old DOS!!!! http://www.small-world.net Single Scattering Monte Carlo Simulation (Freeware) "Monte Carlo Simulation" Mc_w95.zip by Kimio KANDA http://www.nsknet.or.jp/~kana/soft/sfmenu.html CASINO (Freeware) " monte CArlo SImulation of electron trajectory in solids " by P. Hovongton and D. Drouin http://www.gel.usherbrooke.ca/casino/what.html Number/Energy of backscattered electrons by Monte-Carlo simulations W BSE 41% BSE 43% BSE 52% energy energy energy 1 kv 3 kv 30 kv C BSE 10% BSE 8% BSE 5% energy energy energy
BSE=6% Penetration and backscattering vs elements (Z) 1 C 20keV V acc = 20kV = cte Depth of electron penetration vs Z and yield of electron backscattering BSE (Monte- Carlo simulation): BSE=33% 1 BSE=50% Cu 20keV 1 U 20keV 1 BSE=32% Z = cte Depth of electron penetration in Cu vs energy E 0 and yield of electron backscattering BSE (Monte-Carlo simulation): Cu 20keV 1 Cu 5keV BSE=33% 1 BSE=35% Cu 1keV Cu 1keV
10nm BSE=14% Penetration and backscattering V acc = 1kV = cte C 1keV Depth of electron penetration vs Z and yield of electron backscattering BSE (Monte- Carlo simulation): BSE=34% Cu 1keV 10nm 10nm U 1keV BSE=44% 200nm BSE=8% Penetration and backscattering V acc = 5kV = cte C 5keV Depth of electron penetration vs Z and yield of electron backscattering BSE (Monte- Carlo simulation): 200nm BSE=33% Cu 5keV 200nm U 5keV BSE=47%
BSE Line scan Monte-Carlo Simulations (Casino 2.42) e-beam: 1.5kV 10nm 50nm Electron trajectories in C back-scattered trapped C C+Os C C+Os C SEM: "true" secondary electrons SE1 and "converted BSE" secondaries SE2+SE3 Different types of SE 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 (from L. Reimer, Scanning Electron Microscopy) x 0,y 0 intensity and delocalisation of SE stemming from the probe at x 0,y 0 (leading to the X 0,Y 0 pixel intensity on the image) The SE signal always contain a high resolution part (SE1 from the probe) and an average (low resolution) part from SE2+SE3!
Relative contribution of SE1 and SE2 (+SE3) vs primary energy total total total SE2 SE1 The total intensity (green+brown) is attributed to the (x,y) pixel, here at 0 nm on this 1-D model (adapted from D.C. Joy Hitachi News 16 1989) 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 Z=0.5) A fast way to phase mapping I BSE =I pe with I pe the intensity of the primary beam, the BSE yield 0.28 0.11 (SE1) SE: low or no chemical contrast but for light elements the topographical contrast will dominate on rough surfaces I SE =I pe +I SE3 I pe ( pe + pe sur ) Al Ni SE1 SE2 SE3 with the total SE yield, pe the yield for SE1 and sur the SE3 yield for materials surrounding the sample (pole-pieces...)
Yield for SE and BSE emission per incident electron vs tilt SE ~1/cos SE (40 )/ SE (0 ) 1.30 Monte-Carlo: for Al: BSE (0 ) 0.15 BSE (40 ) 0.24 BSE (40 )/ BSE (0 ) 1.60 for Cu: BSE (0 ) 0.31 BSE (40 ) 0.40 BSE (40 )/ BSE (0 ) 1.30 Al Cu (adapted from L. Reimer, Scanning electron microscopy) SE yield ~1/cos BSE yield Dust on WC (different Z materials flat material rough material low Z material low Z material thin low Z material SE 25 kv BSE
Toner particle (penetration in light material) SE 28 kv BSE
Topographic contrast in SE mode penetration depth ("range") >>SE escape length Relative yield of SE vs angle of incidence on the sample surface 1-10nm I 0 I( ) I I pe ( ) I (0) cos (adapted from D.C. Joy Hitachi News 16 1989) Size and edge effects intensity profile on image Do not forget, in SEM: The signal is displayed at the probe position, not at the actual SE production position!!! (adapted from L.Reimer, Scanning Electron Microscopy)
(From L. Reimer, Image Formation in Low-Voltage Scanning Electron Microscopy, (1993)) Tin balls
Change in secondary electron contrast with accelerating voltage (from L.Reimer, Image formation in the low-voltage SEM)
Contraste enhancement at low voltage: less delocalization by SE2. An example: a fracture in Ni-Cr alloy SE, 5 kv SE, 30 kv SEM: Effect of the accelerating voltage on penetration and SE signal (from D.C. Joy Hitachi News 16 1989) 20 kv: strong penetration, SE3/SE4 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/SE4 are produced in the C film together with SE1/SE2. The C film and its defects become visible
Physical limit to the imaging in secondary electron mode 1/2 Tin grains on a thin carbon film (TEM supporting grid) HRSEM 25 kv 1nm nominal resolution left: SE right: scanning transmitted electrons (STEM) (from B. Ocker, Scanning Microscopy 9 (1995) 63 ) SE: e - /e - coulombian STEM: Rutherford (e-/electric field in atom) Physical limit to the imaging in secondary electron mode 1/2 (from B. Ocker, Scanning Microscopy 9 (1995) 63 ) The average grain size looks larger in SE (12.3nm) than in STEM (9.1nm) "Delocalisation": the elastic scattering in STEM (Rutherford) occurs at a much closer distance from the atom nucleus than the inelastic coulombian e - /e - interaction required to eject a SE
Mo buried and etched fibers: SE1 and SE2 5 kv SE 10 µm 15 kv SE 10 µm SE1 low low bright/dark (edges) low SE2 (BSE) high low medium med -ium Mo 30 kv SE 10 µm Ni3Al etching Contrast reversal in BSE mode at low accelerating voltage At low voltage, the yield for BSE is no longer a monotone fuction of the atomic number Si Cu Ag Au from L.Reimer, Image formation in low-voltage SE
5kV, high current Charging effects Fiberglass on epoxy 1kV 5kV, low current (by courtesy of B. Senior CIME/EPFL) Improving SE contrast at low voltage Which polarity? fiberglass on epoxy Autumn by 2010 courtesy B. Senior/CIME Experimental Methods in Physics Marco Cantoni
Total yield for electron emission (SE + BSE) on insulators E1 and E2 are critical energies where 1 electron leaves the surface for each incident electron: neutrality 1 taux -0 >>0 >0 when ev acc = E2 charging-up disappears! ev acc = E1 is unstable, ev acc = E2 is stable Caution: E1 and E2 are specific of the material, but also change with the incidence angle! E 1 E 2 0 1000 2000 3000 énergie kev Caution: this simple (simplist!) model is not quantitative for insulators because charge implantation and removal depends of the scanning speed and precise sample geometry TV scan Charging-up on silica particles/al slow scan 5kV charges at the particle surface lead to anomalous contrast as a flying saucer 1.5kV at 1.5 kv, close to the neutrality point, particles recover their sphere contrast
TV scan Charging-up on spherical silica particles slow scan 5kV 1.5kV Charging-upof an insulating particle of dust Negative charges left on the particle create an electric field that repells the SE toward the substrate around the dust 0V obj pole-piece (adapted from L. Reimer Scanning Electron Microscopy)
Backscattered and secondary electrons BSE CeO 2 particles in a Na(Ce)La(MoO 4 ) 2 matrix are revealed by BSE even from slightly below the surface (brown circles). They also appear in the SE image thanks to the SE2 (produced by BSE). Small SiO 2, remaining from polishing, are light and nearly transparent to primary e -. They weakly backscatter but emit SE on all their surface and give the main contribution to SE signal. SE SEM low kv imaging No specimen preparation needed: Low kv imaging of non-conducting, low density samples Al2O3 Nano-crystals FEI Magellan Operator: Ingo Gestmann Samples: Marco Cantoni
SEM low kv imaging No specimen preparation needed: Low kv imaging of non-conducting, low density samples Al2O3 Nano-crystals FEI Magellan Operator: Ingo Gestmann Samples: Marco Cantoni SEM low kv imaging No specimen preparation needed: Low kv imaging of non-conducting, low density samples Carbon nano-tubes (MWCT) FEI Magellan Operator: Ingo Gestmann Samples: Marco Cantoni
SEM low kv imaging No specimen preparation needed: Low kv imaging of non-conducting samples Liquid filled organic membranes Zeiss Nvision 40 Marco Cantoni SEM low kv imaging No specimen preparation needed: Low kv imaging of non-conducting samples Liquid filled organic membranes Zeiss Nvision 40 Marco Cantoni
SEM low kv imaging Purely organic specimen: non-conductive, low density: Metal coating 15nm Ag/Pd coating 3nm Os coating HeLa Cells, Graham Knott Marco Cantoni, Nvision 40 SEM low kv imaging Easy samples: SC wire Nb 3 Sn in Cu matrix
SEM low kv imaging Easy samples: Everhard-Thornley detector (SE) Solid state BSE detector SEM low kv imaging Easy samples:
SEM low kv imaging Easy samples: