SCANNING ELECTRON MICROSCOPE

BYGAJENDRA KUMARID- 2011uit1721

WHAT IS SEM

It is a microscope that uses a focused electron probe to extract structural and chemical information point-by-point from a region of interest in the sample.

WHAT IS SEM

SEM is a powerful magnification tool .The high-resolution, three-dimensional images produced by SEMs provide topographical, morphological and compositional information makes them invaluable in a variety of science and industry applications. 

PRINCIPLE

SEM DIAGRAM

Components of sem

• electron gun (filament)• Electron beam generation.

• electromagnetic optics• Controlling diameter of e beam.

• scan coils• Gives parralel e beam.

• sample stage• Hold sample

• detectors• Detects electronic signals

• vacuum system• computer hardware

Electromagnetic lens

 A coil of wire through which current flows. Because the current flow produces a magnetic field at right angles, the field pushes inwards into the hole in the centre. This acts to shape a beam of electrons travelling in their natural spiral path down the central hole.

The condenser lens is at the top and the objective lens at the bottom. Each does a different job. The condenser lens converges the cone of the electron beam to a spot below it, before the cone flares out again and is converged back again by the objective lens and down onto the sample.

Interaction of Electron with sample

Elastic interactions- No electron collision so no energy loss.

Inelastic interactionsCollision of electrons and energy

loss.

Interaction of Electron with sample

E collision scatteringAuger effect

SEM SIGNALS

Detectors

Secondary electrons: Everhart-Thornley Detector

Backscattered electrons: Solid State Detector

X-rays: Energy dispersive spectrometer (EDS)

SEM WORKING

http://www.youtube.com/watch?v=bfSp8r-YRw0

APPLICATIONS

Topography and morphology Chemistry Crystallography Orientation of grains In-situ experiments:

Reactions with atmosphere Effects of temperature

Topography and morphology

Chemistry

Fe Ce

Sr

In-situ imaging

oxidation of steel at high temperatures

800 °C, pH2O = 667 Pa

Formation of Cr2O3

2 min 10 min 90 min

Results

SEM Results: 30 nm crystalline

g

Element

Weight%

Atomic%

Ti 45.08 50.15Ni 54.92 49.85

Totals 100.00

The compositions are of 50.15 at.% Ti–49.85 at.% Ni.

A few particles of different sizes have been observed in this sample.

Limitations

Big samplesLiquid samplesInsulators (Usage after

experiment)Very high vacuumVibration freeLarge spaceMorphology Modification