Signal detection in electron microscopy Marek Unčovský Vybrané partie z elektronové mikroskopie 2 •Introduction • What is detector •Detected signals in electron microscopes •Technologies used for detection in electron microscopy •Detector design considerations •Summary Contents 3 • Detector is part of imaging system of microscope • It collects signal radiation in EM, amplifies and converts into form which can be digitized and visualized • Detectors can be divided into different categories based on: •Type of signal they collect •Technology they use Detector basics – what is detector? 4 Electron beam interaction with sample •Scattering processes: - Elastic - Rutherford or Mott scattering (1) -> Low Loss BSE - Compton scattering (3) - Inelastic - Inner/interband transitions (5) - Inner shell ionization (6) -> SE, AE - X-ray emission (4) - phonon and plasmon emission (2) -> BSE 5 • Electron or ion beam interaction with sample produce various types of particles/radiation = signal • Electrons – Secondary – Backscattered – Auger – Transmitted – Absorbed (specimen current) • Photons – Cathodoluminescence – X-ray • Ions – Secondary Types of signals detected in electron microscopes 6 •Electron signals • Secondary electrons – (SE), E<50eV, small escape depth (~10nm) → best resolution • Backscattered electrons – (BSE), 50eV50eV, characteristic peaks, surface material composition information • Transmitted electrons (sample must be thin enough) • Absorbed electrons/current Electrons http://www4.nau.edu/microanalysis/Microprobe- SEM/Signals.html   5.1 1.0 )( E mx  = 7 • Electrons emitted by the sample under electron beam (inner shell ionization effects) • Small escape depth → high resolution • Yield depends on local sample tilt → Topography contrast • Yield depends on local magnetic or electrostatic fields • Signal is polluted by SE created by BSE in sample – SE2, or on some other surface in specimen chamber (usually final lens) – SE3 → noise (information from different part of with different contrast) Secondary electrons 8 • Different yield for different materials → material contrast • Yield changes with primary beam energy → for most materials there is equilibrium point where secondary emission balances primary beam current, i.e. no charging occurs even in case that sample is insulator. Secondary electrons 9 • Primary beam electrons reflected by the sample (elastically or inelastically) • Yield depends on atomic number of sample material → low loss BSEs reflected close to beam axis – high take off angle • Yield depends on local tilt of sample surface → BSEs reflected far from beam axis – low take off angles • Yield depends on crystal orientation → channeling contrast & EBSD(P) = Electron Back Scattered Diffraction (Pattern) Backscattered electrons 10 • SE image BSE image Examples of SE and BSE images 11 Z- contrast Topography Material & topography contrast in BSE signal 12 •Transition of electron in atom filling inner shell vacancy results in release of energy •Energy may be transferred to another electron which is ejected from the atom •Characteristic peaks for elements – analytical method AES- Auger Electron Spectroscopy •Low energies (50eV-3keV)-> small escape depth = surface sensitive method •Extreme surface sensitivity and weakness of signal require usually UHV setup Auger electrons 13 •Electrons transmitted through sample without scattering or scattered to space below sample •Only possible for samples with thickness smaller than interaction volume •Signal depends on: • Sample thickness • Sample material • Crystal orientation •Standard imaging - TEM •Scanning transmission electron microscopy – STEM •Electron energy loss spectroscopy - EELS Transmitted electron - TEM 14 • Electrons transmitted through sample without scattering or scattered to space below sample • Only possible for samples with thickness smaller than interaction volume • Signal depends on: • Sample thickness • Sample material • Crystal orientation • Signals: • BF – Bright Field – electrons scattered close to beam axis or transmitted without scattering • DF – Dark Field – electrons scattered in higher angle from beam axis • HADF – High Angle Dark Field – Higher angle than DF Transmitted electrons – STEM (Scanning Transmission Electron Microscopy) High Angle Dark Field Annular Dark Field Bright Field 15 •Cross section of multilayer (Pt, Mo, Si) Bright Field Dark Field STEM - examples 16 • UV to IR light (160nm-2000nm) emitted by the sample under electron irradiation • Effect occurs only in certain materials (semiconductors, minerals, organic molecules) • Direct detection of light emitted by sample, or more complex instruments with monochromator to obtain spectra of emitted light Cathodoluminescence 17 • Electron beam induced emission of X-ray has two components • Continuous (“brehmstrahlung”) • Characteristic X-ray – dependent on atomic structure of sample • Peaks of characteristic X-ray corresponds to energy emitted by electron when changing energy levels in atom, thus they enable to determine atomic compound of sample (not chemical structure) Characteristic X-Ray X-ray K L M N O K K K L L 18 •EDS or WDS ( also EDX, WDX) • Energy (Wave) Dispersive Spectroscopy (X-ray) • EDS – faster x WDS - more accurate (better energy resolution) • X-ray spectra • X-ray mapping Characteristic X-ray 19 •Secondary Ions • Secondary ions are sputtered from sample by ion beam – only in FIB or SDB • Yield depends on material and crystal orientation • Can be used for: • Imaging (channeling contrast) • SIMS – Secondary Ion Mass Spectroscopy – enable to measure atomic compound of sputtered sample •Specimen current • Signal of absorbed current is complementary to SE+BSE signal • Usually large currents and long dwell times are needed Other 20 •Topography, shape – Secondary electrons •Material (qualitatively) – Backscattered electrons, Cathodoluminescence STEM •Material (quantitatively) - Characteristic X-ray, EELS Signals in EM - summary 21 • Each detector consist of: • collection part with primary amplification mechanism depending on technology • Preamplifier with additional electronic gain, offset and filtering capabilities which prepares signal for frame grabber Basic Detector Scheme 22 •All detectors used in electron microscopy use several detection principles • Scintillation detectors • Use materials which emit light when irradiated by electrons • Semiconductor detectors • Use photodiodes or avalanche diodes with thin entrance window • Gaseous ionization detectors • Use amplification in gas environment • Continuous multipliers • Amplification in continuous dynode structure in chamber environment • Light detectors • For CL detection, uses mirrors and lightguides to get emitted light to PMT • Conversion detectors • Not special category - detectors which use some structure to convert real signal into different type of signal which can be then detected by some of the above mentioned detector schemes Technologies used for signal detection & amplification 23 • Signal electrons are converted into light in scintillator • Light is transported through lightguide to photomultiplier which amplifies signal and converts it to current • SE detection - scintillator has to be biased so SE’s has sufficient energy to scintillate • BSE detection – No bias needed • SI (secondary ion) detection – conversion electrode needed to convert ions to SE’s, these can be amplified • TEM phosphor screens/screens of TEM cameras Scintillation detectors 24 •Scintillator • Single crystal (YAG, YAP..), Powder (P47 = Y2SiO5..) or organic (PBO..) • Fluorescence caused by • Transition between vibrational states of organic molecules • De-excitation of electrons excited in states in forbidden band (inorganic scintillators) • Basic properties • Decay time • Light output • Emission spectrum • Electron energy needs to be in range of keV to produce light in scintillator • Lightguide • Has to be matched to scintillator (similar refraction index) and in good optical contact with scintillator and PMT Components of scintillation detectors 25 •Photomultiplier (PMT) • Electron tube with photosensitive cathode -external photoelectric effect • Photocathode –> external photoelectric effect (Alkali – Sb, Cs, Te, Rb, or III-V semiconductors GaAs…) • Dynodes (BeO,MgO,GaP…) • Basic parameters • Spectral sensitivity of photocathode • Gain • Linearity • Dark current Components of scintillation detectors kn VA= 26 •ETD – Everhardt Thornley detector – standard detector of SEM systems • Detector uses biased grid to attract low energy SEs • Scintillator is biased to 10kV to provide enough light output • In-chamber or in-column versions •Robinson type BSE detector – Scintillation BSE detector • Annular scintillator with central hole for electron beam • HAADF STEM detector for TEM Examples of scintillation detectors 27 • Charged particle passing through semiconductor causes production of electron hole pairs • Ratio for all materials and types of radiation • Ionization energy ~3eV for both Ge and Si, Fano factor ~0.1 ->good energy resolution • Several detector types • P/N, PIN diodes • Avalanche diodes – APDs • CCD cameras • Direct detectors – CMOS • Hybrid pixel detectors • X-ray detectors for spectorscopy Semiconductor detectors .const Eg i   28 • Equilibrium of drift and diffusion current at p-n junction forms depletion region •Reverse bias often used to enlarge depletion region (fully depleted detector) • Increase active volume • Decrease capacitance of the detector •Shallow implant on the surface needed to not block low energy electrons Semiconductor detector - diode 29 •Bulk material • Semiconductor wafer - Si, Ge, GaAs.. •Dopants to create p and n regions • Introduced from the surfaces using diffusion or ion implantation •Metallization to make contacts • Evaporation or sputterring •Passivation to protect semiconductor surfaces •Guard ring – additional junction to isolate main junction from detector edge •Each production step only on selected parts of surface – masks, photoresists Semiconductor detector – diode Fabrication 30 •Energy needed to create one electron – hole pair in silicon is 3.6eV → 30keV electron creates ideally > 8000 electron hole pairs, 1keV electron only ~270 •Thickness of front implant – “dead layer” – low energy electron efficiency •Capacitance and serial resistance t = RC (C = 10pF, R=100W -> t = 1 ns) •Leakage current – Crystal imperfections, temperature, (mechanical damage, surface contamination) Semiconductor detector – diode Parameters R eV E II beam beam -= )1( 62.3 det  31 •Avalanche photodiodes (APDs) • Contrary to PIN diodes charge carriers are accelerated by high bias (up to 2500V) • Avalanche electron multiplication in the depletion region similar to process in PMT • Higher gain x Higher dark current = noise • Recently arrays of APDs are produced which may replace PMT in future (MPPC-multipixel photon counter, SiPM – Silicon photomultiplier) Semiconductor (solid state) detectors 32 •Advantages • Segmentation of semiconductor detector is very easy, fabrication of multichannel detectors is easier than with scintillation detectors • Makes use of progress in silicon industry, not special production like scintillators or PMTs • Signal is led via cabling in chamber – it does not block excessive spatial angle around the sample •Disadvantages • Low keV sensitivity (technology improves) • Sensitivity to mechanical damage of surface layer • Relatively higher input cost (masks, production preparation etc.) • Signal routing in chamber is vulnerable to EMC interference Semiconductor (solid state) detectors 33 • Several types of BSE detectors placed below pole piece, different segmentation • STEM detectors placed below the sample • In column detectors mounted inside or above objective lens Semiconductor detectors – examples 34 •High resolution CCD cameras •Fiber optic or lens coupling to scintillator •Cooling to suppress thermal noise (peltier cooler) •Variety of different position possibilities Pixelated detectors CCD camera - TEM 35 •SEM/SDB uses CCD mostly for EBSD • Electron Back Scattered Diffraction Pattern • Information about crystal lattice orientation • Specimen tilted to 70 degrees • EBSD pattern (Kikuchi bands) images taken while scanning • Crystal orientation determined for each part of sample (Hough transform) • Scintillator coupled to CCD camera Pixelated detectors CCD camera - EBSD 36 Pixelated detectors CMOS camera – direct detection - TEM •High resolution / low dose •Radiation hardness is important •Pixel sizes <5um – comparable to interaction volume •Interaction volume reduction by backthinning •Resolution >20Mpix •Speeds >1000fps Vybrané kapitoly z elektronové mikroskopie 37 Pixelated detectors Hybrid detectors •Combination of readout chip (common) and sensor •Sensors tailor made for application – Si, GaAs, CdTe… •Higher radiation hardness •Frame based or data driven mode •Pixel sizes ~10s of um Vybrané kapitoly z elektronové mikroskopie 38 •X-ray detectors measure energy of incoming photon by the amount of ionizations it produces in detector material •Si(Li) and SDD detectors available •Si(Li) – lithium drifted silicon • PIN diode (3-5mm thickness) with ~1000V bias • For noise reduction it requires liquid nitrogen cooling • External FET transistor to convert current to voltage • Resolution ~ 130eV FWHM @ Mn Ka line X-ray detectors 39 •SDD detector • Transversal field generated by serie of ring electrodes causes charge carrier to drift to small collection electrode • This concept allows significantly higher countrate than Si(Li) • FET transistor integrated in the chip improves energy resolution and throughput • High purity allows to use Peltier cooling instead of liquid nitrogen • Resolution ~ 130eV FWHM @ Mn Ka line • Countrate up to 1,000,000 cps, better energy resolution at given countrate X-ray detectors 40 •SDD chip •Si(Li) detector •SDD detector X-ray detectors - examples 41 •Low Vacuum or ESEM (Environmental Scanning Electron Microscopy) mode is used for imaging of non conductive samples •Chamber pressure vary between 10-4000 Pa -> biased scintillation detectors cannot be used • Amplification in chamber gas environment – biased electrode (~0-800V) •Maximum bias given by Paschen law •Gain is determined by gap distance, gas type, pressure and anode bias •Optimum exists in gain dependence on Pd/V0 Inelastic mean free path ~ length needed to achieve ionization energy Gaseous ionization detectors bdp dpa V +  = )ln( 0VBPd d APe eg - = =   0 0.2 0.4 0.6 0.8 1 1.2 0 0.05 0.1 0.15 Pd/V (torr-mm-V-1 ) NormalisedSEGain 1 10 100 1000 10000 100000 0 100 200 300 400 500 600 Anode Bias (V) Gain 42 •Additional effects influencing/limiting signal of gaseous detectors • BSE signal amplification • Primary beam amplification • Skirt effect – elastic scattering of primary beam Gaseous ionization detectors       ++= d S d S eII BSEPEd     0 0 2 4 6 8 10 0 5 10 15 20 25 30 Electron Energy (keV) mean-free-path(mm) 10 torr 5 torr1 torr 23 21 364 L T p E Z rs       = 43 •Advantages • Cheap production, no special know how needed (PCB, metal electrodes) • Can be tailor made for given application • Robust and user exchangeable •Disadvantages • ESEM versions of detectors (with PLA) limit field of view significantly • Slower response time due to current induced by slow ions produced in amplification cascade • Needs biased amplifier which complicates multichannel detector schemes Gaseous ionization detectors 44 •Structures created from material with high secondary emission yield (or covered with layer with this feature) •Electric field is distributed along the channel of multiplier placed directly in SEM/SDB chamber •Signal particle creates SE in input part of detector and this is further amplified similarly to PMT •More channels can be combined in microchannel plate → position sensitive detectors Continuous multipliers 45 •Advantages • Small size w.r.t. PMT detectors x similar gain • Can be used to detect both electrons and ions •Disadvantages • Sensitive layer providing amplification degrades with time and use, detectors need to be exchanged after some time, when used heavily it could be a few months • Biased preamplifier needed for this type of detectors Continuous multipliers 46 •Uses mirrors to transport light generated by electron beam on the sample to light sensor •Examples • Centaurus CL – version of BSE detector with exchangeable tip – instead of scintillator only mirror placed above sample the rest of detector remains the same – simple CL detecor • Gatan Chroma CL – Detector with monochromator enabling to determine also spectrum of emitted light Cathodoluminescence detectors 47 •BSE mode of TLD • BSEs converted on mirror electrode to SEs, then detected •SI mode of ICE • Secondary ions converted on conversion ring to SE’s, then detected •BSE mode of GBSD • BSE’s converted on inner electrode of detector to SE’s , then detected Conversion Detectors 48 •Scintillation • Coupling to PMT or other light sensitive devices • SE, BSE detection • Fast, low noise •Solid state – semiconductor • Variety of use cases, BSE, EBSD, EDS… • Direct detection, multichannel detection • Benefits from progress in semiconductor industry •Gas ionization • Low Vac, ESEM, charge neutralization •Other technologies • Continuous multipliers • Cathodoluminescence Technologies used for signal detection Summary 49 Detector design considerations •Collection efficiency • Simulation tools enable modeling of collection efficiency of different detector configurations including beam sample interaction •Narrow point spread function • Scintillator decay time, semiconductor sensor capacitance •Low additional noise • Low losses in signal transport (scintillator photocathode matching, lightguide transport efficiency, signal wiring capacitance) • Noise free amplification, reduced electronic noise Vybrané partie z elektronové mikroskopie NSNR ~ 2 2 in out SNR SNR DQE = 50 Detector design considerations •Possibility of signal filtering • Spatial (angular distribution) • Energy •Simultaneous acquisition • Variety of signals at the same time • Multichannel detection, acquisition and processing •Spatial requirements • Space around the sample is limited • Number of SEM/SDB/TEM accessories is increasing Vybrané partie z elektronové mikroskopie 51 Detector design example •ETD • Collection efficency ~30% • P47 scintillator 1signal electron ~100photons – decay time ~80ns • Losses in scintillator, lightguide and interfaces ~20-30 photons reaching photocathode • According to cathode DQE (30- 50%) ~10 photoelectrons produced • Amplification in PMT => 106 electrons at PMT anode Vybrané partie z elektronové mikroskopie 52 • Detectors are key part of the electron microscope – eyes of the system • Detect signals produced by interaction of electron or ion beam with sample • Electrons (secondary, backscattered, Auger…) • Photons (X-Ray, Cathodoluminescence) • Ions • Use different technologies to convert and amplify particle signal into current/voltage which can be digitized and visualized • Scintillators • Semiconductors • Gas amplification Summary 53 •Knoll, G. F.: Radiation detection and measurement, ISBN 0-471- 07338-5 •Spieler, H.: Introduction to radiation detectors, http://www- physics.lbl.gov/~spieler/physics_198_notes_1999/ •Reimer L.: Scanning Electron Microscopy – Physics of Image Formation and Microanalysis, ISBN: 3-540-63976-4 •Hamamatsu photonics K.K.: Photomultiplier Tubes – Basics and Applications http://sales.hamamatsu.com/assets/applications/ETD/pmt_handboo k_complete.pdf Literature