When Does it Make Sense to Use A Scanning Electron Microscope?

Being a materials scientist for the past 20 years, I've had a lot of real world experience in determining what type of microscopy or analytical technique should be used in different situations. However, many people are unfamiliar with Scanning Electron Microscopy (SEM) and when to use it. In this short article I'll include a few tips about how to know when you may want to switch from a traditional optical microscope to a SEM.

When we talk about different types of microscopy, the wavelength or size of the source is extremely important. In optical microscopy, the resolution of the system is ultimately limited by the wavelength of the light we're using to image the sample. Because of this, most optical microscopes have a resolution limit between 0.2-0.4 microns. If you have features in your sample that are smaller than 1 micron in size that you need to image in detail then you should really be switching over to using a SEM. Unlike optical microscopes, SEMs are currently limited by how well we can control the energy of the electrons from the source and the lenses which focus them. As an example, a 20 kV electron beam has a wavelength of approximately 8.5 picometers...YES that's not a typo, I did say PICOMETERS! However, due to chromatic and spherical aberration effects in SEMs, most commercial SEM systems have a usable resolution between 0.8-20 nm.

Another factor that often times determines whether to use an optical microscope or a SEM is what sort of depth of field is required. Optical microscopes have a very narrow depth of field, which means only a very narrow plane of the sample in the image will be in focus. SEMs provide a much higher depth of field in addition to their much higher resolution and magnification. In addition, most SEMs come equipped with 2-3 detectors to analyze and image the sample in different modes. Backscattered electrons show us elemental contrast or differences in local density of the sample. Secondary electrons provide extremely detailed topographical information which is useful for imaging fractures or irregular surfaces. Finally, Energy Dispersive Spectrometers (EDS) integrated into most SEMs provide the ability to perform elemental analysis where most elements can be detected and quantified to detection limits as low as 0.1 wt.%. This is a must have feature for failure analysis and quality control.

While SEMs are great, they also have their drawbacks. First, they are much more expensive than conventional optical microscopes because they are much more complex and difficult to build. They also require the sample to be under vacuum while imaging which means if you have wet or sensitive samples this can be a challenge. In addition, for part inspection, users can often physically hold a specimen and spin it to quickly see the location they're interested in and they can see what they're looking at in color. In a SEM the user has to manipulate the sample stage to get to the desired location and because most SEMs are greyscale, there's some work that needs to be done to determine where exactly that small rust colored stain is located on your sample in the greyscale SEM image. To summarize, here's a list of some Pros/Cons of both Optical Microscopy and SEM:

OPTICAL MICROSCOPY PROS AND CONS:

PROS:

1. Optical microscopes are non-destructive
2. Typically cost less than SEMs
3. Faster for working at low magnifications
4. Provide color images

CONS:

1. Limited depth of field
2. Resolution is limited to 0.2-0.4 microns
3. Inability to perform chemical analysis

SCANNING ELECTRON MICROSCOPY PROS AND CONS:

PROS:

1. High depth of field
2. Usable resolution of 0.8-20 nm for most commercial SEMs
3. Ability to see compositional differences using backscattered electrons
4. High resolution topographical imaging using secondary electron imaging
5. Local chemical analysis and detection of most elements down to 0.1 wt.%

CONS:

1. Higher cost than optical microscopes
2. Requires the sample to be under vacuum
3. More difficult to manipulate samples
4. Images are greyscale and often require experience to interpret

If you find yourself with additional questions about whether your specific sample could benefit from using a SEM, feel free to contact us and we can help steer you in the right direction. We offer both hourly SEM services in addition to desktop SEM rentals which can be delivered right to your office. Feel free to post any questions you may have in the comments below!

Contact Us:

(203) 978-3648

www.emitLLC.com

[email protected]