Presentation is loading. Please wait.

Presentation is loading. Please wait.

Scanning Electron Microscopy (SEM)

Similar presentations


Presentation on theme: "Scanning Electron Microscopy (SEM)"— Presentation transcript:

1 Scanning Electron Microscopy (SEM)
Short description Beam parameters influence on image e- gun components filaments lenses Beam-sample interaction electron scattering Image formation Goldstein, Scanning Electron Microscopy and X-ray Microanalysis

2 Scanning Electron Microscope (SEM)
Field of view: 5x 5 mm2 – 500 x 500 nm2 V-shaped Filament Resolution: down to 1 nm Extractor Beam accelerator Deflecting Plates Electron Column Image Display Primary e- Beam Scan quadrupole e- Detector Backscattered Electrons Sample

3 How to sweep an electron beam
Optical axis First coil deviate beam from optical axis Second coil brings beam back at optical axis on the pivot point Image formation point by point collecting signal at each raster point S L L = raster length on sample W = working distance S = raster length on screen Magnification = S/L L = 10 m, S = 10 cm M depends on working distance

4 Effect of beam parameters on image
V0 = beam voltage ip = beam current p = beam convergence angle dp = beam diameter at sample

5 Effect of beam parameters on image
Resolution High resolution mode Noise on signal ip = beam current dp = beam diameter Good compromise ip = 1 pA, dp = 15 nm ip = 320 pA, dp = 130 nm ip = 5 pA, dp = 20 nm High current mode Resolution too low

6 Effect of beam parameters on image
Depth of focus If p is small, dp changes little with depth, so features at different heights can be in focus p = 15 mrad p = 1 mrad

7 Effect of beam parameters on image
Electron energy V0 < 5 kV, beam interaction limited to region close to surface, info on surface details V kV, beam penetrates into sample, info on interior of sample V0 = beam voltage

8 Electron column e- are produced and accelerated
Beam is reduced to increase resolution Beam is focused on sample

9 Filament e- are accelerated to anode and the hole allows
Wehnelt: focuses e- inside the gun Controls intensity of emitted e- Grid connected to filament with variable resistor e- exit filament following + lines The equipontential line shape has focussing effect and determines 0 and d0 e- are accelerated to anode and the hole allows a fraction of this e- to reach the lenses

10 Filament Electron column Equipotential lines Filament head
Electron beam The equipontential line shape has focussing effect and determines 0 and d0 Electron column

11 Operating principle: thermionic electron emission
Filament types Lanthanum hexaboride (LaB6) Tungsten hairpin (most common) 0.120 mm Tungsten wire Operating principle: thermionic electron emission LaB6 crystal 0.20 mm

12 Filament types Tungsten hairpin Lanthanum hexaboride (LaB6)
thermionic electron emission Ac = 120 A/cm2K2 Ew = work function To reduce filament evaporation  operate the electron gun at the lowest possible temperature Materials of low work function are desired. Tungsten hairpin Lanthanum hexaboride (LaB6) Ew = 4.5 eV Jc = 3.4 A/cm2 at 2700 K Lifetime hours Energy width  0.7 eV Operating pressure 10-5 mbar Ew = 2.5 eV Jc = 40 A/cm2 at 1800 °K Lifetime hours Energy width  0.3 eV Operating pressure 10-6 mbar

13 Thermal Field Emission
Filament types Thermal Field Emission Operating principle: thermionic electron emission + Tunnelling W-Zr crystal 0.20 mm I = A/cm2 at 1800 °C Lifetime > 1000 hours Energy width  0.1 eV Small source dimension (few nm) Operating pressure 10-9 mbar

14 Lanthanum hexaboride (LaB6) Thermal Field Emission
E gun brightness Beam current changes throughout the column Brightness is conserved throughout the column R p dp Tungsten hairpin Lanthanum hexaboride (LaB6) Thermal Field Emission dp: 30 – 100 m dp: 5 – 50 m dp: 5 nm  = 106 A/sr cm2  = 108 A/sr cm2  = 105 A/sr cm2

15 Electromagnetic Lenses
Demagnification of beam crossover image (d0) to get high resolution (small dp) d0: 5 – 100 m for filaments d0: 5 nm for TFE Beam focussing High demag needed Low demag needed Fringe field coils radial parallel

16 Electromagnetic Lenses
Focusing process e- interacts with Br and Bz separately -e (vz x Br) produces a force into screen Fqin giving e- rotational velocity vqin vqin interacts with Bz produces a force toward optical axis Fr = -e (vqin x Bz) f = focal length the distance from the point where an electron first begins to change direction to the point where it crosses the axis. The actual trajectory of the electron will be a spiral The final image shows this spiraling action as a rotation of the image as the objective lens strength is changed.

17 Electromagnetic Lenses
Lens coil current and focal length I = lens coil current N = number of coils V0 = accelerating voltage Increasing the strength (current) of the lens reduces the focal distance the focal length will become longer at higher accelerating voltages for the same lens current

18 Demagnification of beam crossover image (d0) = object
Comparison to optical lenses Demagnification of beam crossover image (d0) = object Beam crossover d0 = tungsten diameter = 50 m Scaling from the figure, the demag factor is 3.4 so d1 = d0/m = 14.7 m CONDENSER LENSES: the aim is to reduce the beam diameter

19 Objective Lenses Scope: focus beam on sample They should contain:
Scanning coil Stigmator Beam limiting aperture Scope: focus beam on sample They also provide further demagnification Pinhole No B outside Large samples Long working distances (40 mm) High aberrations Snorkel B outside lens Large samples Separation of secondary from backscattered e- Long working distances Low aberrations Immersion Sample in B field Small samples Short working distances (3 mm) Highest resolution Low aberrations Separation of secondary from backscattered e-

20 Effect of aperture size
Aperture size: 50 – 500 m Decrease 1 for e- entering OL to a a determines the depth of focus Determines the beam current Reduces aberrations

21 Effect of working distance
Increase in WD  increase in q  m smaller  larger d  lower resolution but longer depth of focus

22 Effect of condenser lens strenght
Strong Weak Decrease q1 and increase p2  larger m Higher Ibeam Lower Ibeam Lower dp Higher dp Increase in condenser strenght (current)  shorter q  larger m and smaller d Also it brings a beam current reduction, so a compromise between current and resolution is needed

23 Gaussian probe diameter
Understand how probe size varies with probe current Calculate the minimum probe size and the maximum probe current Distribution of emission intensity from filament = gaussian with size dG dG = FWHM Knowing emitter source size, dG may be calculated from the total demagnification With no aberrations, keeping dG constant would allow to increase ip by only increasing p

24 Spherical aberrations
Origin: e- far from optical axis are deflected more strongly e- along PA gives rise to gaussian image plane No aberration e- along PB cross the optical axis in ds So at the focal plane there is a disk and not a point Spherical aberration disk of least confusion Cs = Spherical aberration coefficient  f Immersion and snorkel Cs ~ 5 mm Pinholes Cs ~ mm So one need to put a physical aperture to limit aberrations

25 Aperture diffraction To estimate the contribution to beam
diameter one takes half the diameter of the diffraction disk nm rad eV

26 Chromatic aberrations
Origin: initial energy difference of accelerated electrons Chromatic aberration disk of least confusion For tungsten filament E = 3 eV At 30 KeV E/E0 = 10-4 At 3 KeV E/E0 = 10-3 Cs = Chromatic aberration coefficient  f

27 Astigmatism Origin: machining errors, asymmetry in coils, dirt
Result: formation ow two differecnt focal points Effect on image: Stretching of points into lines Can be compensated with octupole stigmator

28 Astigmatism

29 Beam-sample interaction
Simulation of e- trajectories Backscattered e- Silicon V0 = 20 KV TFE,  = A/sr cm2 dp = 1 nm Ib = 60 pA Main reason of large interaction volume: Elastic Scattering Inelastic scattering

30 Beam-sample interaction
Elastic Scattering 0 Elastic scattering cross section Z = atomic number; E = e- energy (keV); A = atomic number N0 = Avogadro’s number;  = atomic density Elastic mean free path = distance between scattering events Silicon  = 2.33 g/cm3 Z = 14 A = 28 N0 =

31 Beam-sample interaction
Inelastic Scattering Inelastic scattering energy loss rate Z = atomic number A= atomic number N0 = Avogadro’s number  = atomic density Ei = e- energy in any point inside sample J = average energy loss per event Eb = 20 KeV The path of a 20 KeV e- is of the order of microns, so the interaction volume is about few microns cube

32 Beam-sample interaction
20 KeV beam incident on PMMA with different time periods Interaction volume Simulation Energy transferred to sample

33 Influence of beam parameters on beam-sample interaction
Beam energy 10 KeV 20 KeV Fe 30 KeV Elastic scattering cross section Inelastic scattering energy loss rate Longer  Lower loss rate

34 Influence of beam parameters on beam-sample interaction
Smaller and asymmetric interaction volume Incidence angle 45° Fe 60° surface surface Scattering of e- out of the sample Reduced depth Same lateral dimensions

35 Influence of sample on beam-sample interaction
Atomic number V0 = 20 keV Fe (Z=26) C (Z=6) Fe, k shell C, k shell Elastic scattering cross section 10% to 50% of the beam electrons are backscattered They retain 60% to 80% of the initial energy of the beam Reduced linear dimensions of interaction volume Inelastic scattering energy loss rate

36 Influence of sample on beam-sample interaction
Atomic number V0 = 20 keV Ag (Z=47) U (Z=92) U, k shell Ag, k shell More spherical shape of interaction volume

37 Signal from interaction volume (what do we see?)
Backscattered electrons Secondary electrons Backscattered e-

38 BSE dependence Backscattered electron coefficient 60°
Monotonic increase Relationship between  and a sample property (Z) This gives atomic number contrast 60° If different atomic species are present in the sample Ci = weight concentration

39 BSE dependence Incidence angle 60° n = intensity at normal
Line length: relative intensity of BSE Strong influence on BSE detector position 60°

40 BSE dependence Energy distribution Lateral spatial distribution
The energy of each BSE depends on the trajectory inside sample, hence different energy losses Region good for high resolution Region I: E up to 50 % Becomes peaked with increasing Z Gives rise to loss in lateral resolution At low Z the external region increases

41 BSE dependence Sampling depth Sampling depth is typically 100 -300 nm
Percent of  Fraction of maximum e- penetration (microns) RKO defines a circle on the surface (center in the beam) spanning the interaction volume Sampling depth is typically nm for beam energies above 10 keV

42 Signal from interaction volume (what do we see?)
Energy distribution of electrons emitted by a solid Secondary electrons Energy: 5 – 50 eV Probability of e- escape from solid  = e- mean free path

43 Secondary electrons Origin: electron elastic and inelastic scattering
SURFACE SENSITIVE SE1 = secondary due directly to incident beam Beam resolution SE2 = secondary generated by backscattered electrons BSE resolution Carbon: SE2 /SE1 = 0.18 Low backscattering cross section Aluminum: SE2 /SE1 = 0.48 Copper: SE2 /SE1 = 0.9 High backscattering cross section Gold: SE2 /SE1 = 1.5 SE Intensity angular distribution: cos

44 Image formation Backscattered e- Secondary e- Volume sensitive
Surface sensitive Sampling depth ~ nm

45 Image formation Many different signals can be extracted from beam-sample interaction So the information depends on the signal acquired, is not only topography

46 Image formation The beam is scanned along a single vector (line) and the same scan generator is used to drive the horizontal scan on a screen Signals to be recorded For each point the detector collects a current and the intensity is plotted or the intensity is associated with a grey scale at a single point A one to one correspondence is established between a single beam location and a single point of the display Magnification M = LCRT/Lsample But the best way is to calibrate the instrument

47 Image formation Digital image: numerical array (x,y,Signal)
8 bits = 28 = 256 gray levels Signal: output of ADC Resolution = 2n 16 bits = 216 = gray levels Pixel = picture element Pixel is the size of the area on the sample from which information is collected Actually is a circle Considering the matrix defining the Dimension of Pixel Element Length of the scan on sample number of steps along the scan line The image is focused when the signal come only from a the location where the beam is addressed At high magnification there will be overlap between two pixel

48 Image formation There is overlapping of pixel signal intensity
For a given experiment (sample type) and experimental conditions (beam size, energy) the limiting magnification should obtained by calculating the area generating signal taking into account beam-sample interactions and compare to pixel size beam Area producing BSe- V0 = 10 keV, dB = 50 nm on Al, dBSE = 1.3 m  deff = 1.3 m on Au dBSE = 0.13 m  deff = 0.14 m 10x 10 cm display There is overlapping of pixel signal intensity Different operation settings for low and high magnification

49 Depth of field Depth of field D = distance along the lens axis (z) in the object plane in which an image can be focused without a loss of clarity. To calculate D, we need to know where from the focal plane the beam is broadened Broadening means adjacent pixel overlapping The vertical distance required to broaden a beam r0 to a radius r (causing defocusing) is For small angles

50 Depth of field How much is r?
On a CRT defocusing is visible when two pixels are overlapped  r = 1 pixel (on screen 0.1 mm) But 1 pixel size referred to sample depends on magnification To increase D, we can either reduce M or reduce beam divergence Beam divergence is defined by the beam defining aperture

51 Depth of field Optical SEM

52 Detector Everhart-Thornley Secondary + BSE Grid Positive: BSE+SE
Grid negative: only BSE The bias attracts most of SE solid angle acceptance: 0.05 sr Geometric efficiency: 0.8 %

53 Topographic contrast Intensity of SE and BSE depends on
beam/sample incidence angle () and on detector/sample angle () BSE coefficient increase with  BSE emission distribution ~ cos  SE emission distribution ~ sec  Detector position and electron energy window are important

54 Topographic contrast Negative bias cage to exclude secondary e-
- Detector is on one side of sample  anysotropic view - Small solid angle of acceptance  small signal - High tilt angle Analogy to eye view Dierctional view High contrast due to orientation of sample surfaces

55 Topographic contrast Positive bias cage to accept secondary e-
Contributions: Direct BSE+SE SE distribution intensity I ~ sec  Variation in SE signal between two surfaces with different  dI = sec  tan  d So the contrast is given by dI/I = tan  d The SE are collected from most emitting surfaces since the positive bias allows SE to reach the detector Analogy to eye view

56

57 High resolution imaging
High resolution signal if selected in energy SE1 : e- directly generated by beam BSE1 : low energy loss (<2%) e- from beam SE2 : e- generated by BSE into sample BSE2 : higher energy loss e- from beam High resolution signal generated by BSE1, SE1 Separation of signal is necessary to obtain high resolution

58 Low mag High mag Silicon V0 = 30 KV TFE,  = 1 108 A/sr cm2 dp = 1 nm
Ib = 60 pA Low mag Scan width at X = 10x10 m2 image 1024x1024, pixel width 10 nm SE1 - BSE1 width = about 2 nm Scanning at low M means field of view larger than SE2 emission area So there is large overlap between pixel And the changes are due only to SE2 variations Beam penetration depth = 9.5 m Emission area = 9.5 m FWHM = 2 nm High mag Scan width at X = 1x1 m2 image 1024x1024, pixel width 1 nm Scanning at high M means field of view smaller than SE2 emission area So as the beam is scanned, no changes in SE2 but changes are due to SE1 SE2 gives large random noise

59 Ag NP on glass Carbon nanotubes TiO2 on silicon

60 SEM in FOOD Schematic representation of gaseous SED
the role of imaging gas in VP-SEM B. James / Trends in Food Science & Technology 20 (2009) 114

61 SEM in FOOD ‘‘bloomed’’ chocolate. 50 μm
Blades of cocoa butter present on the surface Image taken with sample at 5 °C using nitrous oxide at ~ 100 Pa (0.8 torr) as imaging gas

62 SEM in FOOD 20 μm VP-SEM image of commercially produced mayonnaise.
Image taken with sample at 5.0 °C using water vapor at around 670 Pa (5.0 torr) as imaging gas. Light continuous phase is water mid grey discrete phase is oil. Darkest grey areas are air bubbles Disadvantages of conventional SEM techniques insulating specimens impossibility of examining hydrated samples without altering their state (drying or freezing) Sample preparation treatments introduce artifacts No studies of dynamic processes for such samples

63 Scanning Auger Microscopy (SAM) e- Detector V-shaped Filament
Extractor Deflecting Plates Backscattered Electrons e- Detector Primary e- Beam Sample Chemical Map Electron Energy Analyzer Scanning Auger Microscopy (SAM) Auger Spectrum

64 Auger Spectroscopy Ekin Evac EF e- e- EK(XYZ)= EB(X)-EB(Y)-EB(Z)- VB
M2,3 3p M1 3s e- e- L2,3 2p L1 2s K 1s One-Hole Initial State De-Excitation Auger Process Two-Hole Final State Ground State XYZ Auger Process One-Particle Scheme Energy Conservation EK(XYZ)= EB(X)-EB(Y)-EB(Z)- EK(XYZ) = KE of Auger electron EB(X) = BE of X level EB(Y) = BE of Y level EB(Z) = BE of Z level

65 EK(XYZ)= EB(X)-EB(Y)-EB(Z)-F+R-
Usually additional terms must be included accounting for the two-hole final state correlation interaction and the relaxation effects EK(XYZ)= EB(X)-EB(Y)-EB(Z)-F+R- F Two-Hole Final State Correlation Energy R Two-Hole Relaxation Energy Eb One electron binding energy K L1 M1 VB Evac EF Ekin L2,3 M2,3

66 Auger Process Nomenclature KL1M2 Auger Process
L1L2M1 Coster-Kronig Process (the initial hole is filled by an electron of the same shell) CCC Core-Core-Core Transition CCV Core-Core-Valence Transition CVV Core-Valence-Valence Transition KL1M2 L1L2M1 Ekin Ekin Evac Evac EF EF VB VB M2,3 M2,3 M1 M1 L2,3 L2,3 L1 L1 K K

67 Competitive processes
Auger X-Ray Fluorescence Electron Relative Probabilities of Relaxation by Auger Emission and by X-Ray Fluorescence Emission EF 3d M4,5 3p M2,3 3s M1 Photon 2p L3 2p L2 2s L1 1s K For lines originating from shell L and M the Auger yield remains much higher than X-ray emission

68 Principal Auger Lines while Spanning the Periodic Table of the Elements
CHEMICAL SENSITIVITY

69 Electron distribution spectrum
Pulse Counting Mode Derivative Mode Since Auger emission lines are often very broad and weak, their detectability is enhanced by differentiating of the spectrum

70 Chemical environment sensitivity
Gas Solid

71 Auger Electron Spectroscopy Quantitative Analysis
In analogy to what developed for XPS, one can determine the atomic concentration (Ci) of the atomic species present in the near-surface region of a solid sample Ci Atomic Concentration of the i-th species si Orbital Sensitivity Factor of the i-th species Ii Spectral Intensity Related to the i-th species

72 Au N6,7VV Si L2,3VV Auger Spectra as Measured at Selected Points of the Self-organized Agglomerated Au/Si(111) Interface Island Flat region

73 Si L2,3VV Auger Line Shape as Measured at Selected Points of the Self-organized Agglomerated Au/Si(111) Interface Island Flat region


Download ppt "Scanning Electron Microscopy (SEM)"

Similar presentations


Ads by Google