Interpretation and Theory of SEM

SEM is a technique for achieving high resolution images of surfaces. It involves scanning a fine beam of electrons (energies ranging from 15 eV to 30 eV) over a specimen and detecting emitted signals.

The electrons emitted from the sample surface are detected which results in a current, which is used to create an image.

Resolution is on the order of 2 nm. Compare this to a conventional optical microscope that has a resolution of about 1 micrometer!

Imaging in the SEM must be carried out under a vacuum, as electrons cannot travel through air.

Electrons are emitted by an electron gun typically at 20 kV.

Once emitted, the electrons pass through the condenser lens and the objective lens, and then through a set of scan coils and an aperture.

A scan is simultaneously generated on a computer monitor.

Electrons emitted by the specimen are detected by a detector. Once detected they are amplified and the signal is used to produce an image!

Electron Gun: Source of the electron beam which is accelerated down the column.

Lenses (Condenser and Objective): Control the diameter of the beam as well as to focus the beam on the specimen.

Apertures: Micron-scale holes in the metal film which the beam passes through.

Detector: Acquires the signal generated from the specimen.

Scan Generator: Controls detector coils, which raster the focused beam across the specimen surface.

Computer Monitor: Where image is displayed.

The electron gun refers to the top region of the SEM that generates a beam of electrons. Our SEM uses a heated tungsten wire (tungsten thermionic source) to produce electrons.

A very fine tungsten filament is surrounded by a Whenelt cylinder that closes over the filament and has a small hole in the centre through which electrons exit.

Energy spread around 1 - 2 eV

The tip of a tungsten wire hairpin filament is about 10 micrometers in diameter.

Electromagnetic lenses are NOT the same as optical lenses.

They are stationary, have variable focal points, and cause the image to be rotated.

Solenoid of coil copper wire inside an iron pole piece.

Because current through the coils produces a magnetic field at right angles, the field pushes inwards into the hole at the center. This acts to shape a beam of electrons traveling in their natural spiral path down the central hole.

With a higher accelerating voltage the electron beam penetration is greater and the interaction volume is larger.

The spatial resolution of micrographs created from those signals will be reduced.

There will be a brighter image due to the number of increased backscattered electrons, however resolution will be worse.

When high energy electrons hit the specimen, a multitude of signals are generated.

Backscattered Electrons: Originate deep in within the sample and interact more strongly with the sample. They provide compositional info, but give lower resolution images.

Secondary Electrons: Originate from within a few nm on specimen surface, and have a much lower energy than the backscattered electrons (<50V). They are very sensitive to surface structure, and provide topographic information.

If a vacancy due to the creation of a secondary electron is filled from a higher level orbital, an X-ray characteristic of that energy transition is produced.

X-rays: These give information about the elemental composition of the sample.

The detector for secondary electrons is the Everhart Thornley Detector (ETD)

Consists of a scintillator that emits photons when hit by high-energy electrons.

Emitted photons are collected by a light guide and transported to a photomultiplier for detection.

A metal grid known as a Faraday cage surrounds the scintillator. It is held at a positive potential to attract the secondary electrons.

In order to generate a SEM image, there must be a variation in signal from the different parts of the specimen.

Source of Signal Variation: Regions of the specimen that are not perpendicular to the electron beam.

These electrons are more likely to be scattered out of the specimen, rather than further into the specimen.

Result: Brighter regions in the secondary electron image.

Magnification is the enlargement of an image, or portion of an image.

In SEM is achieved by scanning a smaller area.

As a smaller region is scanned, the user can see the object getting bigger.

The aperture stops electrons that are off-axis or off-energy from progressing down the column.

The aperture can also narrow the beam below the aperture, depending on the size of the hole selected.

Charging is another SEM artifact that occurs when high-energy electrons land on an insulator sample and a charge build-up occurs on the sample surface due to a lack of grounding path

Charging manifests in may different ways:

Abnormal Contrast

Image Deformation

Image Shifting

Edges of objects can appear to be brighter because electrons can be emitted not only from the top but the side, artificially making that part of the image brighter.

Edge Effect appear on images as bright spots.

Lowering the electron beam kV can reduce edge effects in the imaging of a sample.

Backscattered electrons are strongly scattered back in the direction of the incident beam.

The detector for these electrons is placed around the final lens.

The electrons which impinge on the detector produce electron-hole pairs which produce a current which can be amplified.

Resolution the SEM is determined by the size of the incident beam.

This can be reduced by introducing an aperture onto the beam path and by reducing the probe size using the condenser lens.

Note: Reducing probe size also reduces beam current.

Resolution dependent on accelerating voltage. Higher energy electrons experience less spherical aberration when they pass through the lenses.

Resolution is also improved by reducing the working distance, up to a certain point. Beyond that point the lenses may be unable to focus the beam.

Note: Images obtained with backscattered electrons have a lower resolution than those obtained with secondary electrons, because the originate deeper within the specimen.

In the image to the left, bacteria have been placed on the head of a pin (a). When magnified (b) a small amount of detail can be seen. When the machine is adjusted to achieve high resolution (c), individual bacteria are easily seen even at higher mag.

Astigmatism is a problem that is commonly encountered in SEM.

It is an aberration of lenses that causes rays in a plane parallel to the optical axis to be focused at a different focal point from rays in a plane perpendicular to it.

Effect: Objects in the image generally appear "stretched" in one direction, and then in the other direction as you go through focus.

All electron microscopes are equipped with stigmators, which allow the user to correct the astigmatism.

The images show how astigmatism alters the image as you go through focus. The first two images are uncorrected. The third image is corrected with the stigmators.

A lens suffers from spherical aberration if it focuses rays more tightly. Aka if rays enter far from the optic axis rather than closer to the axis.

Therefore, it does not produce a perfect focal point.