Electron Microscopy 3. SEM Image formation, detection, resolution, signal to noise ratio, interaction volume, contrasts 3-1 SEM is easy! Just focus and shoot "Photo"!!! Please comment this picture... Any idea what it might be??? 3-2
SEM is easy! Just focus and shoot "Photo"!!! Does this one sound more obvious? 3-3 SEM is easy! Just focus and shoot "Photo"!!! At least this one?? 3-4
Image formation 3-5 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) 3-6
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 3-7 Electron detectors Photomultiplier Everhart-Thornley detector Collects and detects lower energy (<100eV) electrons: SEM backscattered electrons 3-8
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 3-9 SEM, secondary electrons Electrons with low energy (0-50eV) leaving the sample surface Intensity depends on inclination of the surface topography 3-10
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 3-11 Some typical SEM Zeiss Ultra HR-SEM Resolution 1.0 nm @ 15 kv, 1.7 nm @ 1 kv, 4.0 nm @ 0.1 kv At CIME: Zeiss NVision40 (FIB) 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 3-12
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 3-13 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 3-14
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 3-15 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 3-16
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 3-17 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 3-18
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% 10nm BSE=44% Cu 1keV 10nm U 1keV 3-19 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% 3-20
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! 3-21 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) 3-22
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...) 3-23 Dust on WC (different Z materials flat material rough material low Z material low Z material thin low Z material SE 25 kv BSE 3-24
This image cannot currently be displayed. 3-25 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) 2009 Marco Cantoni 3-26
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) 3-27 (From L. Reimer, Image Formation in Low-Voltage Scanning Electron Microscopy, (1993)) 3-28
Tin balls 3-29 3-30
Change in secondary electron contrast with accelerating voltage (from L.Reimer, Image formation in the low-voltage SEM) 3-31 Contraste enhancement at low voltage: less delocalization by SE2. An example: a fracture in Ni-Cr alloy SE, 5 kv SE, 30 kv 3-32
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 (blue 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 3-33 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: 3-34
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: 3-35 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: 3-36
SEM low kv imaging No specimen preparation needed: Low kv imaging of non-conducting samples Liquid filled organic membranes Zeiss Nvision 40 3-37 SEM low kv imaging No specimen preparation needed: Low kv imaging of non-conducting samples Liquid filled organic membranes Zeiss Nvision 40 3-38
SEM low kv imaging Purely organic specimen: non-conductive, low density: Metal coating 15nm Ag/Pd coating 3nm Os coating HeLa Cells, Graham Knott, Nvision 40 3-39 SEM low kv imaging Easy samples: SC wire Nb 3 Sn in Cu matrix 3-40
SEM low kv imaging classical conditions: 20kV Everhard-Thornley detector (SE) Solid state BSE detector 3-41 SEM low kv imaging SE and BSE imaging 3-42
SEM low kv imaging Contamination 3-43