SEMScanning Electron Microscope

• Type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons.

• electron beam is generally scanned in a raster scan pattern.

• electrons interact with electrons in the sample, producing various signals that can be detected and that contain information about the sample's surface topography(secondary electrons) and composition(Back scattered electrons) and crystal orientation(X-rays).

• An electron beam is thermionically emitted from an electron gun fitted with a tungsten filament cathode.

• The electron beam typically has an energy ranging from 0.2 keV to 40 keV.

• Electron beam is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter.

• primary electron beam interacts with the sample within an interaction volume(100 nm to 5 micron into the surface).

• Sample preparation:1. Sample must be electrically conductive.2. Nonconductive specimens are coated with

ultrathin coating of electrically conducting material by vacuum sputtering such as tungsten, chromium, graphite, gold, platinum and indium.

3. Biological samples require chemical fixation to preserve and stabilize their structure and they are deprived of moisture by air drying.

• Energy exchange results in:1. Reflection of high energy electrons(Elastic

scattering).2. Emission of secondary electrons(Inelastic

scattering).3. Emission of electromagnetic radiation

(characteristic X-rays).

Detectors Detection of Secondary Electrons• Secondary electrons are detected by Everhart- Thornley

detector which is a type of scintillator-photomultiplier system.

• Low energy Electrons are first attracted to a low positive voltage on Faraday cage.

• Accelerated by higher voltage in scintillator to convert into light photons.

• Photons are then multiplied in number by Photomultiplier.

• Amplified electrical signal output by the photomultiplier is displayed as a two-dimensional intensity distribution

Detection of Backscattered electrons• Backscattered electrons (BSE) consist of high-

energy electrons that are reflected or back-scattered out of the specimen interaction volume.

• Everhart- Thornley detector can be used with negative charge on the faraday cage.

• Best detected by dedicated BSE detectors.• Can be used to contrast between areas of different

chemical composition.• Heavy elements (high atomic number) backscatter

electrons more strongly than light elements (low atomic number) thus appear brighter in the image.

• It has the ability to image bulk materials (not just thin films or foils).

• Resolution can range from 1nm-20nm. • SEM micrographs have a large depth of

field yielding a characteristic three-dimensional appearance.

• 3d view of pollen grain