Scanning transmission electron microscope (2)

Outline

• History(TEM)• Background• Components• Specimen Preparation• Imaging method• Contrast formation• Modifications• STEM• References

Scanning Transmission Electron Microscope

Specimen Preparation

• Prepare a specimen with at least part of its thickness at about 100 nm, depending on the atomic weight of specimen materials.

• Pre-Thinning:1st specimen less than 1mm thick is prepared Reducing the thickness to about 0.1mm before

final thinning to 100 nm thickness• Final Thinning: involveIon MillingElectrolytic ThinningUltramicrotomy

Ion Milling

• Uses a beam of energetic ions to bombard specimen surfaces to reduce the thickness by knocking atoms out of a specimen

• General procedurea) Dimple grindingb) ion milling ion beam of 1–10 keV

bombarded specimen is placed in the

center at an angle of 5-30◦

Electrolytic Thinning

• Reducing specimen thickness to 100nm

• General procedureA specimen placed in

an electrochemical cell as anode

A suitable reduce specimen thickness

Common technique is jet polishing

Electrolytic thinning completed in 3–15 minutes.

Ultramicrotomy

• Reducing specimen thickness to 100nm

• General procedureA specimen is mounted

in a holder against the cutting tool

The specimen should be trimmed to have a tip held against the knife

The holder gradually moves toward the knife while it repeatedly moves up and down

STEM

• The basic principle of image formation fundamentally different from static beam TEM

• small spot size is formed on the sample surface with the condenser lenses

• This probe is scanned on the sample surface

• the signal is detected by an electron detector, amplified and synchronously displayed on CRT with the scan coils

Cont…

• DETECTOR1. Small disk on the column axis which

detects the transmitted beam (BF STEM image) or diffracted beam (DF STEM image)

2. Annular detector (a plate with a hole) which detects all the diffracted beams except the transmitted one (ADF STEM)

• Resolution limited by the spot sizehave poorer resolution but better contrast

TEM =

STEM

MATERIALS CHARACTERIZATION Introduction to Microscopic and Spectroscopic Methods by Yang Leng -ISBN 978-0-470-82298-2