Electro Scan Electro Scan Electro Scan Electro Scan Environmental Scanning Electron Environmental Scanning Electron Microscopes Microscopes 1 1
Seeing Things You’ve Never Seeing Things You’ve Never Seen Before Seen Before Electro Scan Electro Scan Uncoated Silicon Nitride Dissolving Table Salt Living Aphid Uncoated Silicon Nitride Dissolving Table Salt Living Aphid Oxidizing Iron 800º C Crystallizing KCL 600º C Oil and Water Droplets Oxidizing Iron 800º C Crystallizing KCL 600º C Oil and Water Droplets 2 2
ESEM - Environmental SEM ESEM - Environmental SEM Electro Scan Electro Scan I Investigate samples in a variety of Investigate samples in a variety of I environments manipulating pressure, environments manipulating pressure, temperature, humidity, and composition of temperature, humidity, and composition of ambient gas or liquid. ambient gas or liquid. I Observe non-conductive, wet, dirty, Observe non-conductive, wet, dirty, I outgassing, dynamic samples without outgassing, dynamic samples without cleaning or coating. cleaning or coating. 3 3
Scanning Electron Microscope Scanning Electron Microscope (SEM) (SEM) Electro Scan Electro Scan Gun Gun Gun Electron Source Electron Source Electron Source Chamber Chamber Chamber Wehnelt Wehnelt Wehnelt Anode Anode Anode Condenser Lenses Condenser Lenses Condenser Lenses Objective Aperture Objective Aperture Magnification Objective Aperture Magnification Magnification Control Control Control Scan Coils Scan Coils Scan Coils Scan Signals Scan Signals Scan Signals Objective Lens Objective Lens Objective Lens Detector Detector Detector Sample Sample Sample Image Signal Image Signal Image Signal High Vacuum High Vacuum Sample Chamber High Vacuum Sample Chamber Sample Chamber Display CRT Display CRT Display CRT Pump Pump Pump Mechanical Mechanical Mechanical Pump Pump Pump An SEM forms an image by scanning a finely An SEM forms an image by scanning a finely focused beam of electrons over the sample surface. focused beam of electrons over the sample surface. 4 4
SEM Signal Generation SEM Signal Generation Electro Scan Electro Scan Primary beam Primary beam Primary beam Cathodoluminescence Cathodoluminescence Cathodoluminescence Characteristic X-rays Characteristic X-rays Characteristic X-rays (light) (light) (light) Bremsstrahlung Bremsstrahlung Bremsstrahlung Backscattered Backscattered Backscattered X-rays X-rays X-rays electrons electrons electrons Auger electrons Auger electrons Auger electrons Secondary Secondary Secondary electrons electrons electrons Heat Heat Heat Transmitted electrons Transmitted electrons Transmitted electrons Specimen Specimen Specimen Elastically scattered electrons Elastically scattered electrons Elastically scattered electrons current current current The beam electrons generate a variety of signals as they interact with sample atoms. The beam electrons generate a variety of signals as they interact with sample atoms. 5 5
SEM Electron Optics SEM Electron Optics Electro Scan Electro Scan Electron Source Electron Source Electron Source (Crossover) (Crossover) (Crossover) Electrons at Electrons at Electrons at wider angles wider angles wider angles focus closer focus closer focus closer to lens to lens to lens Divergence Divergence Divergence Slower electrons Slower electrons Slower electrons focus closer focus closer focus closer to lens to lens to lens Condenser Lens Condenser Lens Condenser Lens Image of Source Image of Source Image of Source (demagnified) (demagnified) (demagnified) Minimum Spot Size Minimum Spot Size Minimum Spot Size Excluded by Aperture Excluded by Aperture Excluded by Aperture Increased Increased Increased Divergence Divergence Divergence Image of Source Image of Source Objective Lens Aperture Image of Source Spherical Aberration Objective Lens Aperture Chromatic Aberration Spherical Aberration Objective Lens Aperture Chromatic Aberration Chromatic Aberration Spherical Aberration (further demagnified) (further demagnified) (further demagnified) Sample Sample Sample Column Optics Lens Aberrations Column Optics Lens Aberrations Column Optics Lens Aberrations The electron optics of the column are designed to demagnify the image of the electron The electron optics of the column are designed to demagnify the image of the electron source, forming the smallest possible spot on the sample surface. source, forming the smallest possible spot on the sample surface. Lens aberrations limit the demagnification. Lens aberrations limit the demagnification. 6 6
SEM Resolution SEM Resolution Electro Scan Electro Scan Convergence Convergence Convergence Angle Angle Angle Spot Spot Spot Diameter Diameter Diameter SEM resolution is ultimately limited by the diameter of the spot SEM resolution is ultimately limited by the diameter of the spot formed by the beam on the sample surface. formed by the beam on the sample surface. 7 7
Volume of Interaction Volume of Interaction Electro Scan Electro Scan Primary electron beam Primary electron beam Primary electron beam Source of Source of Source of Sample secondary electrons Sample secondary electrons Sample secondary electrons Source of Source of Source of backscattered electrons backscattered electrons backscattered electrons Source of Source of Source of electron-excited electron-excited electron-excited characteristic X-rays characteristic X-rays characteristic X-rays Beam electrons generate signals throughout a region Beam electrons generate signals throughout a region known as the Volume of Interaction known as the Volume of Interaction 8 8
Resolution and Contrast Resolution and Contrast Electro Scan Electro Scan S S E E Gold on Carbon Toner Tungsten Carbide Gold on Carbon Toner Tungsten Carbide B B S S E E Resolution is dependent on sample type as well as signal type. Resolution is dependent on sample type as well as signal type. 9 9
Depth of Field Depth of Field Electro Scan Electro Scan Electron Beam Electron Beam Electron Beam Sample surface Sample surface Sample surface Depth of Depth of Depth of Field Field Field Plane of Best Plane of Best Plane of Best Focus Focus Focus Region in Effective Focus Region in Effective Focus Region in Effective Focus The small convergence angle of the beam in an SEM yields excellent depth of field. The small convergence angle of the beam in an SEM yields excellent depth of field. 10 10
Characteristic X-rays Characteristic X-rays Electro Scan Electro Scan X-ray X-ray X-ray K K K Photon Photon Photon Lines Lines Lines L L L β γ β β γ γ Lines Lines α Lines α α β β β α α α Outer Outer Outer Shell Shell Shell Electron Electron Electron M M M α α α Line Line Line Inner Shell Inner Shell Inner Shell Primary Primary Primary Electron Electron Electron Electron Electron Electron The energy of a characteristic X-ray is determined by The energy of a characteristic X-ray is determined by the atomic structure of the emitting element. the atomic structure of the emitting element. 11 11
X-ray Spectrum X-ray Spectrum Electro Scan Electro Scan An X-ray spectrum shows the intensity of X-ray emissions An X-ray spectrum shows the intensity of X-ray emissions from various elements present in the sample. from various elements present in the sample. 12 12
X-ray Maps X-ray Maps Electro Scan Electro Scan X-ray maps can show the spatial distribution of elements in the sample. X-ray maps can show the spatial distribution of elements in the sample. 13 13
Electron Gun Electron Gun Electro Scan Electro Scan Filament Filament Filament Cathode voltage Cathode voltage Cathode voltage (e.g. -30 kV) (e.g. -30 kV) (e.g. -30 kV) Wehnelt voltage Wehnelt voltage Wehnelt voltage (e.g -30.5 kV) (e.g -30.5 kV) (e.g -30.5 kV) Wehnelt Wehnelt Wehnelt Electron "crossover" Electron "crossover" Electron "crossover" Anode (0 V) Anode (0 V) Anode (0 V) electrons electrons electrons The high voltages used in an electron gun require a high vacuum. The high voltages used in an electron gun require a high vacuum. 14 14
Everhart-Thornley Detector Everhart-Thornley Detector Electro Scan Electro Scan Electron Beam Electron Beam Electron Beam Photo-multiplier Photo-multiplier Photo-multiplier Scintillator Scintillator Scintillator (+12 kV) (+12 kV) (+12 kV) Secondary Secondary Secondary Electron Electron Electron Light guide Light guide Signal Out Light guide Signal Out Signal Out Collector grid Collector grid Collector grid (+300 V) (+300 V) (+300 V) Secondary electrons Secondary electrons Secondary electrons The high voltages used in a conventional secondary electron detector The high voltages used in a conventional secondary electron detector require a high vacuum in the sample chamber. require a high vacuum in the sample chamber. 15 15
Charging Artifacts Charging Artifacts Electro Scan Electro Scan Nonuniform Charge Balance Typical Charging Artifacts Nonuniform Charge Balance Typical Charging Artifacts at 1.7 kV at 20 kV at 1.7 kV at 20 kV 16 16
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