SEM part 1.pdf

Introduction to scanning electron microscopy (SEM) B Basic concepts and operating principle Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition “Scanning Electron Microscopy and Associated...

Introduction to scanning electron microscopy (SEM) B Basic concepts and operating principle Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition “Scanning Electron Microscopy and Associated Techniques: Overview” (pages VII-XIV) MOL-32228 Electron Microscopy Tampere University History of scanning electron microscopes Louis de Broglie 1925: Dual nature of matter Hans Busch 1927: First electromagnetic lens Knoll 1935: Concept demonstration of scanning electron microscope Manfred von Ardenne 1938: First scanning electron microscope Cambridge Scientific Instrument Company 1965: First commercial scanning electron microscope  Optical microscopy vs. scanning electron microscopy Optical microscope Scanning electron microscope  Operating principle of SEM Sample Image Electron beam stays at the first stop of the sample for the dwell time Detector detects signal during this time The signal level defines the grayscale of the corresponding pixel in the image Point-by-point correspondence  Operating principle of SEM Sample Image Electron beam moves to the next stop of the sample Detector detects the signal The signal level defines the grayscale of the corresponding pixel in the image  Operating principle of SEM Sample Image The first row is scanned through The greyscale of first row pixels is defined The electron beam moves to the next row Point-by-point correspondence [Goldstein et al. Scanning electron microscopy and X-ray microanalysis, 3rd Edition] Magnification and resolution Magnification: Resolution: The ability to resolve two lines which have a = (minimum) distance of Limage Lspecimen Depth-of-field Depth of field is the length around the plane of optimum focus at which the sample is still in focus For SEM the depth of field can be up to 600 μm, whereas for optical microscope only 20 μm can be achieved http://www.emal.engin.umich.edu  The structure of SEM 1. Formation and control of energetic electron beam Electron gun Lens system Associated electronics Vacuum unit 2. Scanning (position and movement) of the beam Scanning coil 3. Electron-specimen interactions 4. Detection of selected signal type Detectors 5. Image formation by point-by-point correspondence Electronics + Vacuum unit SEM operation parameters Beam electron energy Adjusted with acceleration voltage Typically 0.1 - 30 keV Beam size and focus Adjusted with lens current and apertures Associated electronics Diameter typically 0.5 nm – 1 μm Beam current Adjusted with lens current and apertures Typically 1 pA – 1 μA Beam convergence angle Adjusted with apertures and working distance Typically 0.001 - 0.05 rad Affects the depth-of-field Magnification Adjusted by controlling the scanned area Signal type Adjusted by selecting the detector to be used (+ detector parameters) + Vacuum unit Examples of the effects of operation parameters The SEM operator optimizes the operation parameters to achieve the best possible outcome for each case Simplified examples: Aim is good resolu on → operator selects small beam diameter = low signal level = poor contrast/visibility Aim is good depth of field → operator selects small aperture size = small beam divergence angle = good depth-of-field BUT low beam current = low signal level = poor contrast/visibility Selection of signal type: dominating topographic contrast OR compositional contrast (qualitative/quantitative) OR combination  20-60% of the power of SEM signals the electron beam converts to heat Introduction to scanning electron microscopy (SEM) B Basic concepts and operating principle Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition “Scanning Electron Microscopy and Associated Techniques: Overview” (pages VII-XIV) MOL-32228 Electron Microscopy Tampere University Electron beam – sample interactions text Concepts of elastic and inelastic scattering and interaction volume Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition Chapter 1 MOL-32228 Electron Microscopy Tampere University Reminder: Dual nature of matter Huygens 1678: Light is a longitudinal wave Newton 1690: Light is a stream of particles Röntgen 1895: Detection of X-rays Thompson 1897: Detection of electrons De Broglie 1924: Dual nature of matter = ℎ = ℎ Beam electron’s energy originates from the potential difference of the electron gun, i.e. the acceleration voltage ( = ) and is kinetic energy ( = ½ ) Beam and sample scales Electron beam The sample is typically 0.5-5 cm in diameter, but the studied area smaller In a high-quality SEM the beam is about 1 nm e- e- e- in diameter There are less than hundred atoms in the direct view within the area of the beam at the surface Au atoms Ø 0.288 nm Beam Ø 1 nm Electron beam – specimen interactions Inelastic Interactions take place through a variety of physical scattering Signal processes collectively referred to as scattering e- E i events The scattering events transfer energy from the beam e- electrons to the specimen atoms and/or alter the Ei Elastic direction of travel of the beam electrons scattering The beam electron–specimen interactions produce the signals that convey information about the E < Ei specimen and are converted into images and/or spectra The scattering events are classified into inelastic and E ≈ Ei elastic scattering =‘Signal’ Electron beam – specimen interactions The probability of scattering events is described with the scattering cross-section Q The average distance between the scattering events is called mean free path  Ei Elastic scattering In an elastic scattering event, the direction of the beam electron path is deviated as it interacts with the electric field of an atom E ≈ Ei Deviations are typically 5°-10°, but can be up to =‘Signal’ 180° The energy of the electron remains approximately the same The cross-section for an elastic scattering event Z=atomic number E=Beam electron energy φ0=Angle of view Inelastic scattering Inelastic scattering involves loss of beam electron Signal energy e- E i Subsequent inelastic scattering events set a limit on how far the beam electron can travel in the specimen After an inelastic scattering event, the beam electron has lower energy and has deviated slightly (< 1°) from its original path E < Ei Inelastic scattering events produce a variety of secondary signals that can be used in electron microscopy Inelastic scattering The average energy loss of beam electron in an inelastic scattering event corresponds the ionisation potential of the sample material The energy loss (dE) within a distance travelled (ds) i.e. the Bethe expression The total path of electrons in the sample is called Bethe range Beam energy ↑ = Rate of energy loss ↓ = Bethe range ↑ Z=atomic number Atomic number ↑ = Rate of energy loss ↑ ρ=density = Bethe range ↓ A=atomic weight (g/mol) Electron interaction volume Interaction volume Both elastic and inelastic scattering events spread the beam electrons into a 3D volume within the sample Especially the elastic scattering events widen the volume in horizontal direction The exact path of the electrons is complex and random Stepwise Monte Carlo simulations are used to simulate the electron paths Equations for scattering cross-section used Monte Carlo simulation with 500 trajectories Interaction volume - size The size of the interaction volume depends on the beam energy and sample composition Note the scale: The AU interaction volume contains ~1010 Au atoms Interaction volume - size Interaction volume - size Kanaya-Okayama range is an estimate of the interaction volume radius that contains >95% of the trajectories Bethe range vs. Kanaya-Okayama range  Interaction volume - shape Acceleration voltage and sample composition affect the shape of the interaction volume Hemispherical to pear-like P. Dumcumb and P.K. Shields, Brit. J. Appl. Phys. 14 (1965) 779. Cross section for elastic scattering: how soon do the beam electrons start to spread? Energy loss during inelastic scattering: how soon do the beam electrons stop? Interaction volume - symmetry Tilt angle affects the symmetry of the interaction volume At 0° tilt the shape is rotationally symmetric Interaction volume – energy deposition The electrons’ energy decreases towards the periphery of the volume The effect of interaction volume on resolution Sample Image https://www.gel.usherbrooke.ca/casino Electron beam – sample interactions text Concepts of elastic and inelastic scattering and interaction volume Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition Chapter 1 MOL-32228 Electron Microscopy Tampere University  Electron signals text Generation of secondary and backscattered electrons Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition Chapters 2 and 3 MOL-32228 Electron Microscopy Tampere University Image contrast Contrast Ctr is the difference in signal level S between two adjacent areas Signal level (intensity) is dependent on the signal yield arriving the detector Signal yield from the sample depends on electron beam, sample and detector If the signal yield follows a predictable response to a specimen property, “contrast mechanism” can be established Contrast ≠ resolution Bad contrast Good contrast Signals As the result of the scattering events, different signals are generated that convey data for for imaging and/or elemental analysis Basic SEM signals are Secondary and backscattered electrons for imaging X-rays for elemental analysis Bulk sample Backscattered electrons (BSE) Backscattered electrons (BSE) The origin of BSEs are beam electrons which have undergone several elastic scattering events and eventually escape from the entrance surface High energy electrons, E≈Ei The relative number of emitted BSEs is called backscattered electron coefficient: The number of detected BSEs define the BSE signal intensity Backscattered electrons Absorbed electrons Backscattered electron yield The backscattered electron coefficient is strongly proportional to the atomic number of the sample, especially at low Z values = −0.0254 + 0.016 − 1.86 10 + 8.3 10 As a result, areas with higher average Ag Cu atomic weight exist brighter in a BSE image: compositional/atomic number/Z contrast W For homogenous materials where Ci is the weight fraction of material i Different metals Metal coated fibre reinforced polymer composite Backscattered electron yield The backscattered electron coefficient increases with increasing sample tilt 1 9 = = (1 + ) As a result, vertical surfaces exist brighter in a BSE image: topographic contrast The effect of beam energy on the backscattered electron coefficient is low = 0.9211 = 0.1382 − = 0.1904 − 0.2235 ln + 0.1292 ln − 0.01491 ln BSE yield vs. tilt angle Escape depth of backscattered electrons Escape depth describes the depth below surface from which the BSEs can escape the specimen Escape depth ≠ interaction volume The escape depth of BSEs is large due to their high energy ~50-300 nm Escape depth of BSEs The escape depth depends on the acceleration voltage, sample composition and tilt angle BSE images at various incident beam energies of a semiconductor device consisting of silicon and various metallization layers at different depths  Radial distribution of backscattered electrons The BSEs are emitted mostly nearby the incident beam position, but also around it Tilt angle θ=0° Tilt angle θ≠0° As a result of deep BSE escape depth and wide radial distribution, the emitted BSE information comes from a 3D volume which size is significantly higher when compared with the beam diameter Angular distribution of backscattered electrons In 0° tilt geometry, the BSE yield isrotationally symmetrical around the incident beam and follows a cosine rule In tilted geometries, the BSE yield becomes asymmetric The angular distribution of BSEs have a strong effect on the most preferable BSE detector position and the detected BSE signal level Secondary electrons (SE)  e- (E