Scanning Electron Microscope (SEM) Function

What is SEM?

A surface imaging technique that provides topographical/composition information

What does the SEM produce?

A micrograph (black & white due to the electron wavelengths used)

How does the electron beam/probe scan the specimen surface?

In a raster pattern

What does a brighter spot/area on a SEM micrograph indicate?

An increase in electrons detected

What is Microscopy?

A technique that makes fine details visible

What is backscatter?

Backscatter is the emission of electrons that are deflected backwards from the surface, providing insights into a material's composition and topography.

What are the types of samples studied in SEM?

Biological specimens, materials analysis (metals, alloys, ceramics, polymers), and geological samples

What is magnification?

The ratio of image size to object size (image size/object size)

What is magnification in SEM?

The ratio of the area displayed on the screen to the area of the specimen scanned (Screen display area/scanned area). As the scanned area decreases, the magnification increases.

What is the SEM magnification range?

8x to 300,000x

What is resolution?

The shortest distinguishable difference between two points.

What is the SEM resolution compared to a typical microscope resolution?

SEM resolution is 3 nm

Regular microscope is 200 nm

What is the field of view?

The field of view is the area scanned in the XY plane that is visible. This is defined by the raster pattern and adjusted with magnification.

What is the depth of field?

The distance in the z-plane that is in focus at one time

What is the SEM’s depth of field compared to that of a regular microscope?

SEM:1000 nm

Regular microscope: 2 nm

What is contrast?

The ability to visually separate the specimen from the background/different parts of the specimen. The parameters for this include: tilt, acceleration voltage, and the detector type.

What is working distance?

The distance from the bottom of the objective lens to the focal plane. This is changed with focus using the software for the SEM.

What is z-distance?

The distance from the end of the objective lens to the top of the stud holder.

What is the Focus Equation?

(z-distance) - (height of stud above stud holder + sample height) = New optimal working distance

What are the 4 main components of the SEM?

1. The Electron Gun

2. The Electron Column

3. The Specimen Chamber

4. The Plinth

What does the vacuum provide to the system?

The vacuum decreases air interaction with the electron beam/probe to increase the resolution and increase the signal to noise ratio

What generates the electron beam/probe?

A tungsten hairpin filament acting as a cathode in the electron gun. This is generating primary electrons.

What does the Wehnelt cylinder do?

Focuses and shapes (controls/stabilizes) the electron beam by applying a negative voltage, ensuring better resolution and intensity of the beam

What does the anode do?

Attracts the electron beam by being positively charged. The anode sets the voltage of the electron beam (gives specific energy). The anode is grounded at 0 volts.

What is contained in the electron column? (In top to bottom order)

Gun alignment coils (EM), condenser lenses (EM), aperture (phys), Objective lens (EM), Stigmators (EM), Scan Coils (EM), and the Orifice

What do the gun alignment coils do?

Electromagnetically align electron beam

What does a condenser lens do?

Concentrate the electron beam and sets the spot size of the electron beam (in conjunction with the aperture, objective lens, and acceleration voltage)

What is an aperature?

A physical opening that restricts the size of the electron beam

What are the different aperture sizes used for? (largest to smallest)

0: troubleshooting

3: set up/general imaging

2: higher resolution imaging

1: highest resolution imaging

What does an objective lens do?

Electromagnetically focuses the electron beam and sets the focal plane, which can be adjusted with focus controls.

What do stigmators (stigs) do?

Electromagnetically corrects for asymmetry in the electron beam using an 8-pole system

What do scan coils do?

Electromagnetically deflects the electron beam to create a raster scan over the sample (determining the field of view) and sets the scan speed (how long the electron beam is in one spot). This is adjusted with the magnification controls on the SEM software.

What does the orifice do?

Allows for differential pressure between the electron column and the specimen chamber

What are the vacuum strengths of the specimen/sample chamber?

High vacuum: 10^-4 Pa (most common to use)

Low vacuum: 10^-3 Pa (used when specimen cannot be as dried, etc)

What does the sample stage do?

Holds the specimen in place and controls movement in x, y, z, t (tilt), and r (rotation) directions

What are the detectors of the SEM?

Secondary electron detector (Everhart Thornley detector) and the Backscatter detector

What does the Secondary (Everhart Thornley) detector do?

Provides topographical information of the specimen surface by detecting the secondary electrons emitted when/where the specimen is targeted by the electron beam. This is done through the primary electron colliding with the electrons in the specimen, resulting in a new secondary electron being emitted.

This is considered an inelastic collision (Secondary electron is lower energy than the primary electron)

The faraday cage is positively charged to collect the secondary electrons

The scintillator then translates the electrons to light (photons) which are multiplied by the photomultiplier (it is easier to multiply photons than electrons)

This detector produces a strong signal with shadowless illumination

What does the backscatter electron detector do?

Provides compositional information about a specimen’s surface.

1. The primary electron beam interacts with an atom in the specimen

2. The same primary electron (then called the backscattered electron) is read by the backscatter detector

This is an elastic collision that retains the original energy of the primary electron (elastic collision). The backscatter electrons help in identifying atomic number and material contrast. This produces lower signal with shadowed illumination.

How does a heavier element appear in the produced image of the backscatter detector? Why?

A heavier element is a brighter image because it has more gravity, meaning the electrons are more likely to hit the backscatter detector.

What is charging?

Charging is the build-up of the electron beam on the sample. This can cause artifacts/brightening in the image by displacing the electron beam and can damage a sample.

What is required of a sample to be viewed in the high vacuum mode of the SEM?

The sample must be conductive (naturally or through prep) and dry (naturally or through prep)

Compare the pros and cons of high vacuum and low vacuum methods in the SEM

High vacuum: higher resolution, higher signal to noise ratio but more prep time/skill is required

Low vacuum: No sample prep, but lower resolution and lower signal: noise ratio (more noise). The method for viewing can also be more time consuming.

What is a cold stage used for?

A cold stage is used to slow the sublimation of wet samples

What are 3 common image issues and how to fix them?

Grainy/black image that improves with slower scan speed means poor signal

Section of image that is blurry but improves with focus means adjusting the depth of field

A blurry image that does not improve with focus means poor resolution

What is comparative microscopy and why is it useful?

Using multiple imaging techniques to confirm the validity of what is seen in the image

This is useful for identifying artifacts and ensuring accurate interpretation of sample features.

Types: using different microscopes, modes, and/or detectors on a sample