practical skills in scanning electron microscope
TRANSCRIPT
Dr.Nawfal H.AldujailiElectron microscope UnitUniversity of KufaFaculty of Science 03-07
04.2016
Practical Skills in Scanning Electron Microscope
Content• Definiton of scanning electron
microscope• Optical Vs scanning electron
microscopy• Working principal• Construction of SEM-
Instrumentation• To familliaze with the types of
signals obtained from electron matter interaction
• Low vacuum imaging• Liquid specimen imaging
Characteristic information:SEMTopography
The surface features of an object and its texture
Morphology The shape and size of the particles making up the object
Composition The elements and compounds that the object is composed of and the relative amounts of them
Optical microscopes VsElectron microscopes
Resolution Depth of field Magnification
Resolution
Blood corpuscles
SEM
Optical microscope
Depth of fieldDepth of field: The range of heights (distances) on the specimen surface for which the image is in focus
Optical microscope SEM
MagnificationMagnification : defined by the ratio of the length of the scan on the CRT (LCRT) and length of the scan on the specimen (Lspec)
SEM magnification can be changed by adjusting the length of the scan on the specimen corresponding to a constant length of scan on the CRT
Electrons need a vacuum
SEM Components
JSM 6701F
COMPONENTS • Electron gun• Electromagnetic
lenses• Scanning coils• Detectors• Sample stage• Vacuum system
Electron sourcesThermionic Emission: Source is heated until electrons overcome work function.
Tungsten LaB6
Field emission: Source is a sharp tungsten tip. Electron are extracted by a strong electric field.
Electron sources
Comparison of electron sources at 20kV
• Condenser lens reduce the diameter of the electron beam
• Objective lens – final probe forming Final focusing of the beam on to specimen Resolution depends on the quality of the objective lens The focus knob controls this lens
Magnetic lenses
Electron beam-sample interaction
Primary
electron
Secondary Electron
Characteristic
X-rayAuger electron
Photon
Backscatter electron
EDS
Primary Backscattered Electrons
Compositional Information
Secondary ElectronsTopographical &
morphological Information
Sample
Secondary electron imaging• Low energy electrons arising from inelastic
scattering• Emitted from the top 1-50nm zone of the per-
shaped excitation area• Detected by secondary electron detector• Imaging specimen topography and morphology
SEM Images
Hela cells Skin collagen
Human blood cells MRSA bacteria
High/Low Vacuum Mode Imaging
e-e-e-
e-e-e-
xSamples
e-e-e
-e-e-
e-Samples
eM+
eM
++
M Residual gas
High Vacuum Low Vacuum
Why low pressure? • Reduce the charging of electron• Specimens with a lot of outgassing, unstable specimen in high vacuum, frozen water containing sample, etc
Who are potential users ?Research tool in fields such as polymer, life science, geological, medical and forensic science
Low vacuum= more versatile From non conductive to wet samples
Low Vacuum Mode Imaging
SEM offer higher magnification, resolution and depth of
field SEM provides details information of topography,
morphology, composition Secondary electrons are useful for imaging
morphology features Backscattered electrons are used for
compositional imaging as a contrast is directly proportional to atomic number
Wet samples can be observed with low vacuum mode or quantomix holder
Conclusion