Practical skills in scanning electron microscope

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