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Scanning Electron Microscopy: Instrumentation & Analysis

Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

This lesson will teach you how a scanning electron microscope works by breaking the instrument down into several sections, and going over what each one does. Then you'll explore what makes this special type of microscope useful by going over some of the main types of analysis it can do.

The Scanning Electron Microscope

The electron microscope was invented in 1931 by Max Knoll and Ernst Ruska in Berlin, Germany. They created the electron microscopes with the desire to get higher resolution images than an optical microscope would allow. Optical microscopes have a theoretical resolution limit of 200 nanometers, and by 1944 SEM's were operating with a theoretical limit of just 2 nanometers. This means they could get more detailed images of objects than optical microscopes would allow.

Electron Microscope from 1950
SEM 1950

A scanning electron microscope (SEM) works by firing a beam of electrons at a sample target, and then collecting the signals given off by their interaction. Throughout this lesson we're going to look in detail how an SEM works by learning about the main components of the microscope. We'll then see what this scientific instrument can be used for by going over some of the types of analysis that can be done by an SEM.

SEM Components

A scanning electron microscope is a complicated instrument. It takes a high amount of precision to manipulate a beam of electrons to create these incredibly detailed magnified images. However, as complicated as the microscope is, it can be broken down into several distinct sections. Let's look at each of them.

SEM diagram
SEM diagram

Electron Gun

The first part of the SEM is the electron gun. As the name suggests, an electron gun fires electrons at the sample you're magnifying. The electrons can be created a few different ways, but the most common method heats up a tungsten wire to produce the electrons.

Condenser Lens

The second part of an SEM is the condenser lens. This is used to narrow the electron beam given off by the electron gun. This lens isn't made of glass like you might expect. Instead it is a lens made of coils of wire that create an electromagnetic field which compresses the electrons as they travel through it.

Apertures

Next are the apertures. These allow you to control the diameter of the electron beam being passed through them. The aperture consists of a metal rod with different size holes cut into it. The diameter of the electron beam is controlled by changing which hole it travels through. The aperture also blocks off any extra electrons that didn't get fully condensed into the beam from hitting the sample.

Objective Lens and Sample Chamber

After the apertures is another electromagnetic lens called the objective lens. This is the final lens that focuses the electron beam down onto the sample.

Once passing through the objective lens, the electron beam passes into the sample chamber. This chamber holds the sample under a vacuum to eliminate interference of unwanted particles. The target itself needs to be conductive to prevent charging, and allow for better image quality. If the target isn't made of a naturally conductive material it can be coated in one, such as gold.

Detectors

Finally, there are the detectors. These are used to create magnified images, and collect other data. What they detect are the various signals given off by the sample as it is struck by electrons from the beam scanning over it. These signals include secondary electrons, backscattered electrons, and x-rays among others.

SEM Analysis

The most widely known form of analysis performed by an SEM is morphological and topographical analysis of a sample. In layman's terms, this means studying a samples structure and physical features under extreme magnification. This information is gathered by the SEM using the secondary and backscattered electron signals; however, it is the secondary electrons that give the most detailed view of the sample. The uses for high resolution magnification are extremely vast, and span many different scientific disciplines. This can include anything from assessing the length of nanowires, viewing cellular structure, analyzing minute surface fractures in substances, and much more.

SEM Image of Blood Cells
SEM blood cells

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