This document provides an overview of specimen preparation and imaging methods for scanning transmission electron microscopy (STEM). It discusses how specimens are thinned to around 100nm thickness through techniques like ion milling, electrolytic thinning, and ultramicrotomy. It also explains how STEM works by scanning a small probe across the sample to form images based on transmitted and diffracted beams detected by different types of detectors, providing better contrast than conventional TEM though with lower resolution due to the larger probe size.

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

2. Scanning Transmission Electron
Microscope

13. 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: involve
Ion Milling
Electrolytic Thinning
Ultramicrotomy

14. Ion Milling
• Uses a beam of energetic
ions to bombard
specimen surfaces to
reduce the thickness by
knocking atoms out of a
specimen
• General procedure
a) Dimple grinding
b) ion milling
 ion beam of 1–10 keV
bombarded
 specimen is placed in
the center at an angle of

15. Electrolytic Thinning
• Reducing specimen
thickness to 100nm
• General procedure
A 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.

16. Ultramicrotomy
• Reducing specimen
thickness to 100nm
• General procedure
A 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

21. 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

22. Cont…
• DETECTOR
1. 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 size
have poorer resolution but better contrast

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