How SEM Works

The Scanning Electron Microscope (SEM) was developed mainly because of the limitations of optical microscopy. These limitations are caused by two inherent factors involved when using light.

Microscopes: The first is the rather large wavelength of visible light. In theory, an imaging source should be able to resolve an object the size of half the wavelength of the imaging energy.
Scanning Electron Microscope (SEM) - Seal Laboratories

Considering electrons have a much smaller wavelength than visible light, the potential for an instrument with a much higher resolution exists.

The next limitation of an optical microscope is its rather poor depth of field. The main parameter effecting depth of field is the aperture angle.

 Optical Aperture Angle SEM - Seal Laboratories




The aperture angle is defined as the angle formed between a line from the sample through the center of the lens and a line from the sample through the edge of the aperture opening. The problem with optical microscopy is that a high power objective lens has a short focal length, increasing the aperture angle and decreasing the depth of field.






This problem does not exist in a Scanning Electron Microscope (SEM). An SEM has a long working distance (the distance between the sample and the final lens) and a small aperture opening making a very shallow aperture angle and hence a good depth of field. 

To see examples of an SEM's better resolution and higher depth of field click here.

 SEM Aperture Angle - Seal Laboratories

Obviously, there are advantages to imaging using electrons instead of visible light. However, electrons have some limitations of their own. First of all, we can't see electrons, secondly, electrons will not freely travel through air - there are enough molecules in air to easily absorb an electron beam. Therefore, the electron source, lenses, and sample must all be under a vacuum. Another limitation is that since electrons are electrically charged, the sample needs to be conductive enough to dissipate this charge.

To image using electrons before the invention of the SEM, the Transmission Electron Microscope (TEM) was developed.

 Transmission Electron Microscope (TEM) - Seal Laboratories

The Transmission Electron Microscope takes electrons from a source and through condenser and objective electromagentic lenses, focuses the beam on an area of the sample. If the sample is thin enough for the electrons to travel through, the projector lens will project an interference pattern, (or the image) onto a phosphorus screen below.

 

A TEM can have extremely high resolution. There are research instruments which can see atoms. However, a TEM has some limitations. First of all, because the electron beam has to travel through the sample, lengthy sample preparation is usually required to make the sample thin enough. Plus, since the beam is traveling though the sample, the sample bulk and not the surface is being imaged.

To further understand how an SEM works, we must begin with the electrons. In a light microscope, light from a source (usually an incandescent light) is focused through lenses onto the sample. Theimage is formed when the sample reflects and absorbs different wavelengths of this light which is detected by our eyes and formed into an image by our brains. An electron microscope works in a similar fashion. Electrons from a source are focused on the sample. These electrons reflect off the sample (As will be explained later, these electrons don't really reflect off the sample, but for now, let's assume they do), they are then picked up by an electron detector and then processed into an image which is projected onto a CRT that our eyes can see.

To begin our understanding of how an SEM works, let's begin with the source of electrons, the electron gun.Most SEMs have what is called a hot cathode source, usually a tungsten filament similar to that in an incandescent light bulb. When such a filament is heated by passing current through it, it not only emits light, but an electron cloud forms around the filament. Left on their own, they remain in the cloud and are reabsorbed into the filament when the current is removed.

Electron Gun - Seal LaboratoriesPlace a positively charged plate (an anode) near the filament and the electrons (being negatively charged) will be attracted to it. Problem is, the electrons would not be well directed and would probably jump over to the anode plate in a series of arcs. But place a negatively charged cathode plate near the filament (which they are repelled by) with a hole in it and a positively charged anode (which they are attracted to) under this with another hole in it and we have the makings of an electron gun.

The electron cloud is attracted to the anode plate enough that they will travel through the hole in the cathode. But in doing so, they gain enough speed that most of them travel right through the hole in the anode plate.

Now we have an electron gun. The speed of the electrons emitted from this gun is controlled by the amount of potential (accelerating voltage) applied to the cathode and anode plates.

The electrons from the gun come out in almost a spray pattern, so we may have a flow of electrons, but this could hardly be called a beam. As in a light microscope, we need lenses to control the flow of electrons, however, the glass lenses of a light microscope will not work. Instead, an electron microscope uses electromagnetic lenses.

 SEM Electromagnetic Lens - Seal Laboratories

 SEM Column - Seal Laboratories

An electromagnetic lens is a relatively simple device. By applying current to wire coiled around aniron cylindrical core, a magentic field is created which acts as a lens.

The advantage an electromagnetic lens in an electron microscope has over its glass counterparts in a light microscope is that by varying the current through the wires, the lens can have a variable focal length.

We now can arrange the electron gun and lenses in a column mounted on a sample chamber.  The condenser lens controls the size of the beam, or the amount of electrons traveling down the column. Increasing the size of the beam achieves a better signal to noise ratio, but because the beam diameter is larger, gives a lower resolution. Depending on the magnification, a compromise between signal to noise and resolution achieves the best image quality.

The objective lens focuses the beam into a spot on the sample. This is necessary to have an image in proper focus.

So far, the column we have designed will just focus the electron beam into a spot on the sample. This is fine for welding or if we wanted the beam to pass through the sample as in a TEM, but for an SEM to work, we need the beam to scan.

 SEM Scan Coils - Seal Laboratories

 SEM Scan Generator - Seal Laboratories

By placing sets of plates around the beam and varying the potential between them, the electron beam can be deflected. If these plates are attached to a scan generator, the beam can be made to scan lines across the sample similar to the way a television tube scans. But if this scan generator is not only controlling the scan coils, but is also controlling the beam of a CRT, the image formed on the CRT will be synched to the electron beam scanning the sample.

So now we have a beam that is scanning across the sample surface and this beam is synched to the beam of a  CRT. But how is the image formed? To understand this, we need to know what happens when the electron beam interacts with the atoms of the sample.

The incident beam electrons (from the electron gun) do not simply reflect off the sample surface. As the beam travels through the sample it can do three things: First, it can pass through the sample without colliding with any of the sample atoms (matter is mostly space). Second, it can collide with electrons from the sample atoms, creating secondary electrons. Or third, it can collide with the nucleus of the sample atom, creating a backscattered electron.

 Secondary Electrons SEM - Seal Laboratories

The incident beam is composed of highly energized electrons. If one of these electrons collides with a sample atom electron, it will knock it out of its shell. This electron is called a secondary electron and is weak in energy (nearly 100 volts). If these secondary electrons are close enough to the sample surface, they can be collected to form an SEM image.

The incident beam electron loses little energy in this collosion. In fact, a single electron from the beam will produce a shower of thousands of secondary electrons until it doesn't have the energy to knock these electrons from their shells.

If the incident beam collides with a nucleus of a sample atom, it bounces back out of the sample as a backscattered electron. These electrons have high energies and because a sample with a higher density will create more of them, they are used to form backscattered electron images, which generally can discern the difference in sample densities

To see examples of the difference between secondary and back scattered electron images click here.

 Backscattered Electrons SEM - Seal Laboratories
An electron detector is placed in the sample chamber. By having a 10 keV positive potential on its face, it attracts the secondary electrons emitted from the sample surface. One advantage of this biased detector is that it can attract secondary electrons emitted from sides of the sample which are physically blocked from the detector face. This greatly reduces shadowing effects in SEM images.  Secondary Electron Detector - Seal Laboratories

So how is the contrast formed?

SEM Contrast  - Seal Laboratories

In secondary imaging mode, as the incident beam scans across the sample's surface topography, secondary electrons are emitted from the sample. If the beam travels into a depression or hole in the sample, the amount of secondary electrons that can escape the sample surface is reduced and the image processing places a corresponding dark spot on the screen. Conversely, if the incident beam scans across a projection or hill on the sample, more secondary electrons can escape the sample surface, and the image processing places a bright spot on the screen.

This form of image processing is only in gray scale which is why SEM images are always in black and white.

Seal Laboratories

Incident Beam Electrons - Seal Laboratories

In backscattered imaging mode, as the incident beam scans across the sample's surface topography, backscattered electrons are emitted from the sample. A low atomic weight area of the sample will not emit as many backscattered electrons as a high atomic weight area of the sample. In reality, the image is mapping out the density of the sample surface.

Some of the sample topography does affect the amount of backscattered electron emission, so the image formed shows some of the topography mixed with the sample density.

To see examples of the difference between secondary and backscattered electron images, click here.

So how does an SEM change the magnification of an image?

SEM Magnification - Seal Laboratories

By reducing the size of the area scanned by the scan coils, the SEM changes the magnification of the image.

The previous pages have given a simplified version as to how an SEM works and the instruments advantages over light microscopy. It is suggested that you click on the EDX analysis button to learn about elemental X-ray analysis which can be simultaneously performed when imaging a sample in an SEM.

 Below, we invite you to view examples of an SEM's better resolution and higher depth of field.

Also, be sure to check out the images below,  which detail the difference between secondary and back scattered electron images.

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