Se ha denunciado esta presentación.
Se está descargando tu SlideShare. ×

Scanning Electron Microscope.pptx

Cargando en…3

Eche un vistazo a continuación

1 de 72 Anuncio

Más Contenido Relacionado

Similares a Scanning Electron Microscope.pptx (20)

Más reciente (20)


Scanning Electron Microscope.pptx

  1. 1. Introduction to Electron Microscopy 1 R. Basori
  2. 2. How to Study Nanomaterial ? Processing Synthesis Properties Applications Micro/Nano Structures Characteriza- tion Materials (Device Fabrication) 2
  3. 3. Sample Characterization Techniques Microscopic Techniques Spectroscopic Techniques (Interaction of matter with Electron) (Interaction of matter with Photon/light) Scanning Electron Microscope (SEM) Transmission Electron Microscope (TEM) X-ray photoelectron spectroscopy (XPS)  UV-Visible FTIR, Raman etc…. 3
  4. 4. Why electron microscopy? 4
  5. 5. The overall design of an electron microscope is similar to that of a light microscope 5
  6. 6. 6
  8. 8. 8 Fundamentals of Electron Optics Refraction and reflection of Electron beam when encountering the region of potential difference. ( ) 1 ( ) 2 Sin r V Sin i V  An electron passing from a lower potential V1 to higher potential V2 undergoes refraction as defined by Snell’s law. i r V1 V1 V2 V2 i V1 V1 V2 V2 V1<V2 V1>V2 r’ r
  9. 9. 9 ( ) 1 ( ) 2 Sin r V Sin i V  1 2 V i Sin V   1 2 V i Sin V   Refraction, Reflection, r’ = i Fundamentals of Electron Optics Electron beam on passing through a region of potential difference with V1<V2, experience a retardation making an angle of refraction greater than incident angle.
  10. 10. 10
  11. 11. Why Electron Microscope ? Characteristic of a Microscope  Magnification  Resolution  Depth of Focus • Higher magnification 11
  12. 12. Magnification (M): Size of image Actual size of the object M=25/f 12
  13. 13. Resolution: Power of resolving or identifying two closely spaced particles separately Limit of Resolution Where, NA numerical aperture of the microscope sin NA n   nrefractive index of the media Resolving Power of a Microscope R∞ 1 D R high , D low  good microscope D  0.61 D NA   Optical microscope of λ=400 -700 nm, D>200 13
  14. 14. 6.63 x10-34 Limit of resolution, ( ) D D D Resolving Power 1 D  14
  15. 15. 15
  16. 16. Depth of Field/Focus: Another important aspect to resolution is the axial (or longitudinal) resolving power of an objective, which is measured parallel to the optical axis and is most often referred to as depth of field/focus. When considering resolution in optical microscopy, a majority of the emphasis is placed on point-to-point lateral resolution in the plane perpendicular to the optical axis. It decreases with increased magnification. Sharpness of an image depends upon the numerical aperture ‘NA’ and the refractive index n of the lens material. 16
  17. 17. Optical Microscope Electron Microscope Electron Microscope Electron Microscope 17
  18. 18. K L M Elastically scattered electron Inelastically scattered electron Unscattered electron Nucleus Electron shells Ejected electrons Electron beam ΔE E-ΔE E E E High-energy electron going through the sample Interaction of unscattered and elastically scattered electrons will give image. 18
  19. 19. (Topography) 19
  20. 20. Scanning Electron Microscope 20
  21. 21. A Scanning Electron Microscope (SEM) is an instrument for observing and analyzing the surface microstructure of a bulk sample using a finely focused beam of electrons. SEM can achieve 1 – 4 nm imaging resolution depending on the instruments. Primary Applications: •Surface topography / morphology •Composition analysis •Contrast •And more …… What is Scanning Electron Microscope ? 21
  22. 22. Basic components of electron microscopes 22
  23. 23. Rotor Rotary Pump Air is drawn into the pump through an inlet and it is compressed. Consists of a stationary part, Stator and a moving part, Rotor, assembled inside a casing. This rotor is driven by an electric motor at a constant speed. Vacuum System : 23
  24. 24. Diffusion Pump Vacuum System : It consists of a chamber housing a oil vessel with a heater, a chimney and a nozzle. On chamber's outer surface, cooling coils carrying water are wound. The heater vaporizes the oil and these hot vapors rise into the vapor chimney. The hot vapors are deflected downwards by an annular nozzle or a jet assembly mounted at the top of the chimney. This jet, moving downwards at supersonic speeds, imparts momentum to randomly moving gas molecules in the chamber. This momentum deflects the molecules towards the pump outlet. A backing pump is constantly used to remove the gas molecules. The hot oil condenses on cold walls and returns to the vessel at bottom. 24
  25. 25. Turbo Molecular Pump Vacuum System : It consists of alternate layers of stator and rotor discs. The rotor rotates at a very high RPM. The blades are mounted at an optimum angle, on both stator and rotor. This high speed rotation imparts momentum to the gas particles upon collision with the rotor discs. The high speed molecules are directed towards the exit using the stator discs. 25
  26. 26. 26
  27. 27. Mean Free Path and Pressure (λ): 1 – 2 m 27
  28. 28. •“Gun Valve” separating the upper column from the rest of the micro sample exchange; appropriate vacuum •Use gloves when mounting samples and transferring them to the column •Samples are dry and free of excessive outgassing. Precaution: 28
  29. 29. • There are three main types of gun – Thermionic Gun - electrons are emitted from a filament when heated sufficiently by passing current through it. Field Emission Gun (FEG) - a very strong electric field is used to extract electron from a material filament. Thermal Field (Schottky) Emission Gun – electrons are ejected for combined action of both heating and electric field. Electron Source/Gun 29 Purpose: Produce electrons Roughly shape the beam Set the electron energy (accelerating voltag
  30. 30. Thermionic Gun •Filament is heated and begins to produce electrons. •Electrons leave the filament tip with a negative potential so accelerate towards the earthed anode and into the microscope column. •A small negative bias on the Wehnelt then focuses the beam to a crossover which acts as the electron source. Example: Tungsten (W), Lanthanum hexaboride (LaB6) 30
  31. 31. Tungsten filament assembly Tungsten filament LaB6 filament tip • Can also use LaB6 crystal grown to a tip – gives a brighter beam than W for same kV. Thermionic Sources Tungsten: Low work function High Melting temperature ~ 3700 K Emit electron at 2700K 31
  32. 32. •An extraction voltage of around 2kV is applied to the first anode to create an intense electric field to allow electrons to escape from the tip. •The second anode is then used to accelerate the electrons into the microscope at the required energy. •Combination of the two anodes focuses the beam into a crossover creating a fine beam source. • A very strong electric field (~109Vm-1) draws electrons from a very fine metallic tip (usually W). FEG source – ZrO/W tip Field Emission Gun (FEG) 32
  33. 33. Field Emission Gun (FEG) Major Advantages: •High resolution •Very long potential source lifetime • Low operating temperature (~300K) • High brightness • High current density (~1010 A/m2) • Small source size < 0.01 um • Highly spatially coherent, small energy spread Major Disadvantages: •Small source size •Not good for large area specimen, easy lose current density •Lowest maximum probe current. •Poor current stability. •Require ultra high vacuum in gun area. In field emission, electrons tunnel through a potential barrier, rather than escaping over it as in thermionic emission. 33
  34. 34. Thermal Field (Schottky) Emission Gun Combined heating and electric field (hybrid source) 34
  35. 35. Comparison of Sources 35
  36. 36. 36 Thermionic Sources Major Advantages: •High resolution •Very long potential source lifetime • Low operating temperature (~300K) • High brightness • High current density (~1010 A/m2) • Small source size < 0.01 um • Highly spatially coherent, small energy spread Major Disadvantages: •Small source size •Not good for large area specimen, easy lose current density •Lowest maximum probe current. •Poor current stability. •Require ultra high vacuum in gun area. Field Emission Gun (FEG) Thermal Field (Schottky) Emission Gun
  37. 37. 37 EM field exerts a force on a moving electron in a direction normal to both the field and the propagation direction of the electrons.
  38. 38. Condenser Lens Objective Lens Two types of electromagnetic lenses lenses in EM V V 38
  39. 39. Function of Condenser Lenses: Spot size 39
  40. 40. Function of Objective Lenses: Focus 40
  41. 41. Ideal and Actual lenses: Rays ideally focused to same plane. But, in reality single focal plane does not occur. 41
  42. 42. Since the focal length ‘f’ of a lens is dependent on the strength of the lens, it follows that different wavelengths will be focused to different positions. Chromatic aberration of a lens is seen as fringes around the image due to a “zone” of focus. Lens Defects (Chromatic Aberration) E +ΔE 42
  43. 43. The simplest way to correct for chromatic aberration is to use illumination of a single wavelength! Correction of Chromatic Aberration In light optics chromatic aberration can be corrected by combining a converging lens with a diverging lens. This is known as a “doublet” lens Increase accelerating voltage which will decrease spreading of electron energy and make almost monochromatic High accelerating voltage 43
  44. 44. Lens Defects (Spherical Aberration) The fact that wavelengths enter and leave the lens field at different angles results in a defect known as spherical aberration. Peripheral Beam Paraxial Beam Aperture Solution: Add a limiting aperture 44
  45. 45. Lens Defects (Aperture Diffraction) 45
  46. 46. Lens Defects (Astigmatism) Reasons: •Slight imperfection in manufacturing pole pieces and copper winding 46
  47. 47. Astigmatism Correction: Objective lend adjustment 47
  48. 48. Apertures Advantages Increase contrast by blocking scattered Electrons Decrease effects of chromatic and spherical aberration by cutting off edges of a lens Disadvantages Decrease resolution due to effects of diffraction Decrease resolution by reducing half angle of illumination Decrease illumination by blocking scattered electrons 48
  49. 49. Scanning coils 49
  50. 50. Back scattered electron detector Secondary electron detector Source Electromagnetic lenses, Apertures etc. Specimen Detectors 50
  51. 51. 51
  52. 52. (Topography) 52
  53. 53. 53
  54. 54. to maintain the conservation of energy and momentum 54
  55. 55. Incident beam Auger Electron: 55
  56. 56. 56
  57. 57. Interaction Volume The ‘interaction volume’ is the area of the sample excited by the electron beam to produce a signal. The penetration of the electron beam into the sample is affected by the accelerating voltage used, the higher the kV the greater the penetration. The effective interaction volume can be calculated using the electron range, R:- 1.67 0 0.89 0.0276 ( ) AE R m Z    Where A atomic weight (g/mole), Z atomic number, ρ density (in g/cm3) Eo energy of the primary electron beam (in kV). Accelerating voltage (kV) Primary Electron Range (µm) 50 3.1 30 0.99 5 0.16 1 0.01 (10 nm) Fe  A=55.85, Z=26, ρ=7.78 g/cm3 57
  58. 58. Interaction Volume (R) 58
  59. 59. 1 kV 5 kV 15 kV Accelerating Voltage The dimensions of the interaction volume increases with accelerating voltage The probability of elastic scattering is inversely proportional to beam energy 59
  60. 60. Depth based information Surface based information Accelerating voltage reduction  reduced interaction volume  surface information 60
  61. 61. 61
  62. 62. C, 6 Fe, 26 Au, 79 Atomic Number The dimensions of the interaction volume will decrease with higher atomic number elements. The rate of energy loss of the electron beam increases with atomic number and thus electrons do not penetrate as deeply into the sample The probability of elastic scattering increases with A – causing the interaction volume to widen. Accelerating voltage, 5kV 62
  63. 63. The angle at which the beam strikes the specimen and the distance from the surface are important factors in how much of signal escapes from the specimen. 63
  64. 64. Elements of different atomic number SE or BSE emitted from different element are different , create different contrast in image 64
  65. 65. 65
  66. 66. Primary Beam 66
  67. 67. + + + + + Faraday Cage +300 V 67
  68. 68. Image Formation in SEM 68
  69. 69. 69
  70. 70. ‘Light’ region is made up predominantly of Fe. (i.e. the heaviest element) ‘Grey’ region is made up predominantly of Ca. ‘Dark’ region is made up predominantly of Si and Al. (i.e. the lightest elements) Energy Dispersive Spectroscopy on the SEM • There were 3 distinct regions in the Backscattered Image Back scattered electrons are deflected more by heavier atoms leading to a brighter contrast in BEI images – the lighter the region the heavier the element present. The Y-axis shows the counts (number of X-rays received and processed by the detector) and the X-axis shows the energy level of those counts. 70
  71. 71. An electron microscope is operating with an accelerating voltage ~100KV under vacuum of ~10-8 mbar. If half angle of incident beam cone is 0.01 radian, what will be the resolution of the electron microscope? [sin(0.01rad)=0.00999, h=6.62x10-34m2 kg / s, e=1.6x10-19C , m=9.1x10-31kg] For electron energy of 10kV, calculate the interaction volume of Fe having A=55.85, Z=26, ρ=7.78 g/cm3 Where A atomic weight (g/mole), Z atomic number, ρ density (in g/cm3) Eo energy of the primary electron beam (in kV). 71
  72. 72. Thank You !! 72

Notas del editor

  • As the term says we are utilizing transmission mode of the electrons to find out the features inside the materials. So in this particular case, we wants the material to be transparent to the electron so that electrons get transmitted through the material and provide information about the materials.
  • Wehnelt electrode - an electrode in the electron gun assembly of some thermionic devices, used for focusing and control of the electron beam. A Wehnelt acts as a control grid and it also serves as a convergent electrostatic lens. An electron emitter is positioned directly above the Wehnelt aperture, and an anode is located below the Wehnelt. The anode is biased to a high positive voltage (typically +1 to +30 kV) relative to the emitter so as to accelerate electrons from the emitter towards the anode, thus creating an electron beam that passes through the Wehnelt aperture.
  • After coating tungsten tip with zirconium oxide operating temperature of tungsten reduces from 2700 K to 1800 K.
  • To get high field strength with low applied bias, field emitting tips are made sharp.
  • Changing converging point of the beam, some part of the beam is going to be cut and we will get small spot size i. e. low current and high resolution.
  • The second lens that we have is called objective lens. Based on changing the lens strength we can change the poit at which the system focuses.
  • Energy dispersive x-ray spectroscopy is techniques used for elemental analysis or chemical composition of the elements