MT-0.6026 Electron microscopy Scanning electron microscopy and electron probe microanalysis Eero Haimi Research Manager Outline 1. Introduction Basics of scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) Introduction to sample preparation 2. Background of measurement principles and methods Electron beam specimen interaction Equipment technology 3. Measurement techniques Secondary (SE) and backscattering (BSE) electron imaging with SEM X-ray spectrometry using EDS and WDS Electron backscattering diffraction (EBSD) 4. Applications examples 1
2. Background of measurement principles and methods Electron beam specimen interactions SEM+EDS+WDS+EBSD equipment technology 3 Electron beam specimen interactions Electron scattering Backscattered electrons Secondary electrons X-ray emission X-ray continuum Characteristic x-rays Interaction volumes and emission regions of different signals 4 2
Electron beam specimen interactions Electron beam: acceleration voltage 0,2 30 kv Electrons Secondary electrons (SE) Auger electrons Characteristic x-rays Continuous x-rays Photons Back-scattered electrons (BSE) Cathode luminescence Heat Electric current Sample material Electron scattering Electron beam (Auger Electrons) Secondary electrons (SE) Backscattered electrons (BSE) Electron current Sample 6 3
Electron scattering Elastic Inelastic Scattering from electron cloud Scattering from atomic nucleus Coherent Incoherent 7 Elastic electron scattering Scattering with almost no energy loss is called elastic. In this case, less than 1 ev energy is transferred from the scattering electron to the sample. Interaction takes place primarily between scattering electron and atom nucleus. Scattering is for the most part coherent. However, as a result of scattering direction of electron movement can change. Generally, the direction changes only few degrees, but there is a small probability that the degree of angular change can be anything between 0-180. On average, change in the angle increases as a function of atomic number Z. If multiple elastic changes in direction of electron movements takes place frequently times enough, some of the primary beam electrons can escape the sample without practical loss of energy. These electrons are called backscattered electrons (BSE). 4
Elastic electron scattering C Fe Monte Carlo simulation 25 electron trajectories (25 kv) 9 Backscattered electron yield as a function of atomic number h = backscattering coefficient 5
Backscattered electron yeild as a function of sample tilt angle Fe, 25 kv 0, h = 0,28 70, h = 0,54 11 Backscattered electron diffraction Coherently backscattered electrons are capable for diffractive interaction. Intensity variation of backscattered electron diffraction pattern is characteristic to crystal structure and orientation of the sample. 12 6
Inelastic electron scattering In inelastic scattering electron loses kinetic energy. Several scattering mechanisms exists. Secondary electron emission Auger-electron emission Characteristic x-ray emission Emission of x-ray continuum Cathode luminescence (with certain materials) Fonon scattering Plasmon scattering 13 Secondary electron emission When primary beam electrons are interacting with conduction band and valence electrons, secondary electrons (SE) can be ejected from the sample. Kinetic energies of these electrons are less than 50eV. Therefore, only secondary electrons that has been generated near the sample surface are capable of escaping the material. Secondary electrons are also generated by backscattering electrons. l = free mean path 14 7
Effect of topography on secondary electron emission Energy distribution of emitted electrons 8
Charge ballance I beam = I scattered + I specimen I beam = (h+d)i beam + I specimen Electron beam specimen interactions Secondary electrons (SE) Back-scattered electrons (BSE) Auger electrons Characteristic x-rays Continuous x-rays Cathode luminescence Heat Electric current 9
X-ray emission Continuous radiation (Bremsstrahlnung, white radiation) Characteristic radiation 19 Continuous x-rays Continuous x-rays (bremsstrahlung ) are generated, when primary beam electrons are decelerated by interaction with Coulombic field of atoms. Energy distribution of this radiation is continuous. The most energetic radiation reaches so called Duane- Hunt (short wave limit) limit. If the sample is not charged, the limit has same value as the whole kinetic energy of primary electrons. 20 10
Characteristic x-rays When acceleration voltage of primary beam electrons is increased adequately, specific intensity peaks are formed on top of continuous x-ray spectrum at wavelengths (energies) that are characteristic to each element. After appearance, these characteristic wavelengths are independent of acceleration voltage. Mo 21 Emission of characteristic x-rays Characteristic x-ray are emitted by the following process: a) The interaction of a high energy electrons with an atom result in ejection of an electron from inner atomic shell. (Also x-ray photons are capable for the same process that result fluorescence radiation). The beam energy must be bigger than critical excitation (ionization) energy E c : Ue > E c Ionization leaves an atom in an excited state that has higher energy than the ground state. The critical excitation energy is larger than the energy of corresponding x-ray photon. (Fluorescence radiation do not excite same type of atoms again.) 22 11
Characteristic x-ray emission b) De-excitation (relaxation) takes place, when an electron from an outer shell fills the empty state (Texcitation < 10-8 s). The difference between the two shell energies equals the energy of the characteristic x-ray: hn = E f -E i Consequently, if: L peaks b a b g K peaks f = K --> K-series i = L -> Ka-lines a i = M -> Kb-lines i = N -> Kg-lines f = L --> L-series i = M -> La-lines i = N -> Lb-lines f = M --> M-series i = N -> Ma-lines M peaks a 23 Quantum mechanics of electron transitions Electron transitions that take place as a result of relaxation of excited state have not equal probabilities. Some of the transitions are even quantum mechanically forbidden. Calculation of quantum mechanical transition probabilities shows that transitions with: Only with accurate spectrometers fine structures of atomic energy levels are detected in charecteristic x-ray measurements. - Dl = ±1 - Dj = 0 tai ±1 are allowed. 24 12
Characteristic x-rays Auger-electron emission Atomic excitation state can be relaxed instead of x-ray photon emission by emission of Auger-electron that has also characteristic energy. Auger and x-ray yields per excitation state equals one. The relative proportions depend on atomic number. Auger-emission is more probable in the case of light elements. As a consequence, characteristic x-ray emission of light elements is not as intensive as it is in the case of heavier elements. Auger electrons are measured in surface analytics but not in SEM, because Augerelectrons generated deeper in the sample loses their characteristic energy quicly in inelastic scattering processes. 26 13
Physical background of x-ray spectroscopy Inelastic electron scattering is capable of producing characteristic x-rays, when electron energies exceed the critical energy of exitation. Characteristic x-ray wavelengths are specific to elements; they depend on electron shell structure. The wavelengths obey Moseley's law: or l -1/2 = C(Z-s) E = D(Z-F) 2 where C,s,D ja F are electron shell dependent constants and E is energy of the radiation. Relation between wavelengths and energy: E = hc/l wavelength dispersive spectrometry (WDS) energy dispersive spectrometry (EDS) Interaction volumes Electron range Fluorescence Absorption Emission zones of different signals 28 14
Primary electron range Monte Carlo electron trajectory simulation Acceleration voltage = 5kV Acceleration voltage = 25kV Carbon sample 29 Kanaya & Okayaman formula R = (0,0276*M*E 0 1,67 )/(Z 0,89 r) R = electron range (μm), M = atomic weight (g/mol), Z= atomic number, ρ = sample density (g/cm 3 ), E o = incident beam energy (kev) 30 15
Electron ranges in different materials C Fe 5 kv 25 kv 31 Fluorescence Cr-Ka Fe-Ka 32 16
Absorption Part of electrons and photons are absorbed in the sample. Electron absorption is stronger than photon absorption Absorption is dependent on electron or photon energy, material thickness (length of scattering path), density and mass absorption coefficients 33 Schematic illustration of interaction volumes for various signals 34 17
Characteristic x-ray range Empirical formula: ρr = 0.064(Eo 1.68 -Ec 1.68 ) R = x-ray range (depth of x-ray production) (mm) Eo = beam energy (kev) Ec = critical excitation energy (kev), ρ = density (g/cm3) (Anderson-Hasler) 35 Influence of acceleration voltage (beam energy) and atomic number on interaction volume Fe 18
Optimum resolution optimal empty resolution poor S/N ration Effect of magnification on optimum pixel size Magnification Scan area on sample Pixel size (for 10 x 10 cm display) (1000 x 1000 pixel scan) 10 1 cm 2 10 mm 100 1 mm 2 1 mm 1 000 100 mm 2 100 nm 10 000 10 mm 2 10 nm 100 000 1 mm 2 1 nm 38 19
Resolution and interaction volume Electron beam specimen interactions Secondary electrons (SE) Back-scattered electrons (BSE) Auger electrons Characteristic x-rays Continuous x-rays Cathode luminescence Heat Electric current 20
Overview of instrument capabilities High magnification Large depth of field Chemical information in micrometer scale (BSE, EDS, WDS) Crystallographic information (EBSD) Special techniques (EBIC, CL, voltage contrast) In-situ experiments (temperature, strain, etc.) More that just a microscope More that just composition and structure 21