Scanning Electron Microscopy

Transcription

1 Scanning Electron Microscopy Instrument Imaging Chemical Analysis (EDX) Structural and Chemical Analysis of Materials J.P. Eberhart John Wiley & Sons, Chichester, England, Scanning Electron Microscopy and X-Ray Microanalysis J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer, J. Michael Kluwer Academic/Plenum Publishers, New York, 2003.

2 1. A column which generates a beam of electrons. 2. A specimen chamber where the electron beam interacts with the sample. 3. Detectors to monitor the different signals that result from the electron beam/sample interaction A viewing system that builds an image from the detector signal.

3 Reduced image of crossover Project the crossover image onto the specimen X-Y translation + rotation

4 SEM Image not formed by focusing of lenses X-ray maps can be displayed. Resolution not limited by lens aberrations (in the usual sense of image forming lenses is limited by the objective lens aberrations which determines the minimum probe size). Imaging involves digital processing online image enhancement and offline image processing. Resolution limited by probe size and beam spreading on interaction with specimen. Hence, resolution depends on the signal being used for the formation of the image.

5 A fine electron probe is scanned over the specimen. Various detectors (Secondary Electron (SE), Back Scattered Electron (BSE), X- Ray, Auger Electron (AE) etc.) pick up the signals. The amplified output of a detector controls the intensity of the electron beam of a CRT (synchronized scanning) of the pixel of display Scanning Electron Beam Various Detectors (SE, BSE, EDX, AE) Display on CRT Parameter Resolution Magnification Values ~ 40 Å (SE); ~ ( ) Å (BSE) Note that the resolution depends on the type of signal being used Depth of field High (~ m) Importance of SEM Size of specimen 1 5 cm (usual range)

6 Many signals are generated by the interaction of the electron beam with the specimen. Each of these signals is sensitive to a different aspect of the specimen and give a variety of information about the specimen. Auger Electrons Backscattered Electrons (BSE) Absorbed Electrons Incident High-kV Beam SPECIMEN Secondary Electrons (SE) Characteristic X-rays Bremsstrahlung X- rays Visible Light Electron-Hole Pairs Elastically Scattered Electrons In a SEM these signals are absent Direct Beam Inelastically Scattered Electrons

7 Signals An important point to note is the fact that the different signals are generated essentially* from different regions in the specimen. This determines: as to what the signal is sensitive to the intensity of the signal. Not to scale The X-rays generated by the electrons are the Primary X-rays The primary X-rays can further lead to electronic transitions which give rise to the Secondary X-rays (Fluorescent X-rays) Interaction volume volume which the electrons interact with Sampling volume volume from which a particular signal (e.g. X-rays) originates * Monte Carlo simulations are used to find the trajectory of electrons in the specimen and determine the probability of various processes

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9 X-ray fluorescence and Auger electrons kev e Electron from beam knocks out a core electron Photoelectrons 2 Transition from higher energy level to fill core level

10 3 Generation of x-rays accompanying the transition 4 Further the x-ray could knock out an electron from an outer level this electron is called the Auger electron

11 Electron-hole pairs and cathodoluminescence Photoluminescence Photon induced light emission e Incident electron excites an electron from the valence band to the conduction band creating an electron hole pair Cathodoluminescence Electron induced light emission Conduction band Band gap e Semiconductors Valence band hole Cathodoluminescence (CL) Spectroscopy h OR Bias Electron beam induced current ( EBIC) Charge collection microscopy

12 Secondary Electrons (SE) Produced by inelastic interactions of high energy electrons with valence electrons of atoms in the specimen which cause the ejection of the electrons from the atoms. After undergoing additional scattering events while traveling through the specimen, some of these ejected electrons emerge from the surface of the specimen. Arbitrarily, such emergent electrons with energies less than 50 ev are called secondary electrons; 90% of secondary electrons have energies less than 10 ev; most, from 2 to 5 ev. Being low in energy they can be bent by the bias from the detector and hence even those secondary electrons which are not in the line of sight of the detector can be captured.

13 Secondary Electrons Some Z contrast! SE are generated by 3 different mechanisms: SE(I) are produced by interactions of electrons from the incident beam with specimen atoms SE(II) are produced by interactions of high energy BSE with specimen atoms SE(III) are produced by high energy BSE which strike pole pieces and other solid objects near the specimen.

14 Back Scattered Electrons (BSE) Produced by elastic interactions of beam electrons with nuclei of atoms in the specimen Energy loss less than 1 ev Scattering angles range up to 180, but average about 5 Many incident electrons undergo a series of such elastic event that cause them to be scattered back out of the specimen The fraction of beam electrons backscattered in this way varies strongly with the atomic number Z of the scattering atoms, but does not change much with changes in E 0. n n BSE IE Dependence on atomic number BSE images show atomic number contrast (features of high average Z appear brighter than those of low average Z)

15 Detectors Note that SE not in traveling in the line of sight can also be captured by the detector Secondary Electrons Backscattered Electrons

16 Magnification The magnification in an SEM is of Geometrical origin (this is unlike a TEM or a optical microscope) Probe scans a small region of the sample, which is projected to a large area (giving rise to the magnification). Area scanned on specimen Area projected onto display

17 Depth of field Dependent on the angle of convergence of the beam Depth of field is the same order of magnitude as the scan length Magnification 10,000 Scan length 10 m Depth of Field 8 m

18 What determines the resolution in an SEM? Probe size (probe size is dependent on many factors) Signal being used for imaging This is because the actual interaction volume/cross section is different from the probe diameter. Additionally, each signal is sensitive to a different aspect of the specimen. In terms of parameters: Accelerating voltage Beam current Beam diameter Convergence angle of beam

19 Topographic Contrast in SEM Inclination Effect Shadowing Contrast Line of sight with the detector Edge/Spike Contrast

20 Operating parameters affecting signal quality Accelerating Voltage Probe Current Working Distance Specimen Tilt Aperture Size Edge effect Contamination Charging

21 Operating Parameter Gun voltage Working distance Values ~20 kev ~26 mm Probe size W filament ~30 Å LaB 6 Field Emission Vacuum W filament 10 5 Torr LaB 6 Field Emission 10 8 Torr Torr Probe current Probe diameter Resolution This leads to decrease in image intensity we have to use a brighter source (W filament < LaB 6 < Field Emission gun)

22 Comparison of Electron Sources at 20kV Work Function Operating Temperature Units Tungsten LaB 6 FEG (cold) FEG (thermal) FEG (Schottky) ev K Current Density Crossover Size A/m 2 5* μ m <0.005 < Brightness A/cm 2 sr Energy Speed ev Stability %/hr <1 <1 5 5 ~1 Vacuum PA Lifetime hr >1000 >1000 >1000

23 Increasing Resolution Probe size Probe Current strength of condenser lens Working Distance Leads to Beam convergence angle spherical aberration Working Distance Specimen Tilt Aperture Size Edge effect Contamination Charging

24 Image Processing Any signal picked up by a detector can be converted to an electrical signal and be used of imaging Contrast processing +ve to ve contrast, gamma control etc. Contrast quantification contour mapping, colour mapping Image integration signal integration over a number of scans ( SNR) Usual image analysis phase fractions etc.

25 Backscattered Electron Images Emission of Backscattered electrons = f(composition, surface topography, crystallinity, magnetism of the specimen) Composition Z number Topography and composition information is separated using detector Crystallinity channeling contrast (& EBSD) (the BSE intensity changes drastically on or around Bragg s condition) Poorer spatial resolution

26 Backscattered Electron Signals Detectors Signal A Signal B A + B A B Composition (COMPO) Topography (TOPO)

27 COMPO: A + B TOPO: A B Specimen: Metallic 20 kv, 1100 Ref: SEM Manual, JEOL

28 Secondary Electron Image (SEI) Backscattered Electron Image (BEI) X-ray image (Si) X-ray image (Al) Specimen: Metallic 20 kv, 1100 Composition via: BEI EDX Ref: SEM Manual, JEOL

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30 High Resolution Unclear surface structures More edge effect More charge-up More damage Accelerating Voltage Clear surface structures Less damage Less charge-up Less edge effect Low resolution

31 Low atomic number High atomic number Low accelerating voltage High accelerating voltage

32 kv 30 kv Specimen: Toner Accelerating voltage Increased contribution of BSE Low surface contrast Charging Ref: SEM Manual, JEOL

33 kv 25 kv Specimen: Sintered powder Accelerating voltage Better surface contrast Not sharp at high magnifications WD or probe diameter Ref: SEM Manual, JEOL

34 kv 5 kv Specimen: Evaporated Au particles Accelerating voltage Better image sharpness Improved resolution Ref: SEM Manual, JEOL

35 kv 5 kv Specimen: Paint coat Accelerating voltage Low surface contrast More BSE contributions from within the specimen Ref: SEM Manual, JEOL

36 Specimen tilt 5kV, Specimen: IC chip TILT Improve quality of SE images complete survey of topography Stereo images images at 2 angles Ref: SEM Manual, JEOL

37 Smooth image Deteriorated resolution More damage Probe current High-resolution obtainable Grainy image

38 10 kv, na 0.1 na 10 pa Specimen: Ceramic Probe current image sharpness surface smoothness Ref: SEM Manual, JEOL

39 Greater depth of field Low resolution Working Distance working distance spherical aberration (spot size resolution improves) High resolution Low depth of field working distance Depth of field (wide cone of electrons) The working distance is the distance between the final condenser lens and the specimen

40 Large current e.g. Better for EDX Low resolution Smaller depth of field Aperture size (objective lens) High resolution Greater depth of field Grainy image

41 Edge Effect SE emission from protrusions and circumferences appear bright Accelerating voltage Greater the edge effect (edges become brighter) 5 kv 25 kv Tilt: 50, 720 Specimen: IC chip Ref: SEM Manual, JEOL

42 Charging Due to low conductivity of sample Coating with a conducting material to avoid charging To charging Voltage, probe current, tilt specimen 4 kv Specimen: Foreleg of vinegar fly 10 kv Accelerating voltage Charging Ref: SEM Manual, JEOL

43 Contamination Due to residual gas in the vicinity of the electron probe Leads to reduced contrast and loss in image sharpness Usually caused by scanning a small region for long time Contamination 5 kv Specimen: ITO Ref: SEM Manual, JEOL

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45 Backscattered Electron Diffraction Diffraction of Backscattered electrons: 1) Channeling contrast, 2) Diffraction patterns (EBSD) Weaker than atomic number contrast required good BSE detector The BSE intensity changes drastically on or around Bragg s condition Poorer spatial resolution

46 EBSD A stationary electron beam strikes a tilted crystalline sample and the diffracted electrons form a pattern on a fluorescent screen. Pattern is characteristic of the crystal structure and orientation of the sample region from which it was generated. Used to measure the crystal orientation, measure grain boundary misorientations, discriminate between different materials, and provide information about local crystalline perfection.

47 A diffraction pattern from nickel collected at 20 kv accelerating voltage

48 Indexing: Kikuchi bands are labelled with the Miller indices of the crystal planes that generated them (red). The planes project onto the screen at the centre of the bands. Kikuchi band intersections are labelled with crystal direction that meets the screen at this point (white). This direction is the zone axis of the planes corresponding to the intersecting Kikuchi bands. The nickel crystal unit cell superimposed on the diffraction pattern in the orientation which generates this pattern. The crystal planes are labelled which correspond to the (2-20) and (020) Kikuchi bands in the diffraction pattern. The Kikuchi band width depends on the d-spacing of the corresponding plane. The (200) plane d-spacing is wider than the (2-20) plane so the Kikuchi bands from (200) planes are narrower than those from (2-20) planes.

49 The symmetry of the crystal is shown in the diffraction pattern. For example, four fold symmetry is shown around the [001] direction by four symmetrically equivalent <013> zone axes. Changes in the crystal orientation result in movement of the diffraction pattern. The simulated diffraction pattern is from a sample tilted 70 to the horizontal and the crystal orientation is viewed along the direction perpendicular to the sample

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