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PHENOM
Tech Tip
By Karl Kersten
Applications Engineer
FEI Company
Successful Imaging of Nonconductive
Materials with Phenom
The Problem of Analyzing Nonconductive Materials
Take Aways
Electron charging of nonconductive samples is a common problem with traditional
• Capturing images from nonconductive
scanning electron microscopes (SEMs). Sample charging can ruin your image by
producing unwanted bright (or dark) regions and image distortion.
samples with a traditional scanning
electron microscope (SEM) can cause
Charging can occur in both conductive and nonconductive materials that do not have a
electron charging, resulting in poor
good electrical leakage path to ground. Since electrons are negatively charged particles, and
quality images with bright and dark
the number of signal electrons coming out of the sample is typically smaller than the
distorted regions.
number of incoming primary beam electrons, the sample will charge negatively if there
is not a sufficient leakage path to ground through the sample. Such electron build-up on
the sample’s surface is known as charging and can deflect the electron beam, distorting the
• Phenom uses a unique combination of
features to minimize sample charging:
image. In addition, when charging disturbs the electron signal to the microscope’s detector,
low acceleration voltage, a low vacuum,
the image data is lost, resulting in bright or dark regions. As you zoom in on your
and a superior electron source.
image, the charging becomes more localized and more distortion will typically occur
until the image is completely obscured, yielding no data. Data loss due to sample
charging is common and can be reduced or prevented by using a few key tricks.
How much sample charging will occur in a nonconductive sample? This depends on the
material’s composition, the scan speed, the scan area, the electron beam voltage and the
vacuum level, to name a few. Many factors affect image quality when viewing nonconductive samples. In this tool tip, you will learn how Phenom™ is designed specifically to
reduce sample charging with no sample modification. You’ll also learn how to eliminate
sample charging at any magnification by sputter coating your sample.
The Phenom Solution
The Phenom personal electron microscope provides a perfect solution for microscopists,
whether you’re accustomed to using a light microscope or a laboratory SEM for imaging.
If you are currently using a light microscope, it is important to know that Phenom is
designed to minimize sample charging while maintaining the best image quality. And,
with a few simple steps in sample preparation, you will experience no sample charging
with Phenom up to its maximum magnification of 24,000X.
On the other hand, if you use a traditional SEM for routine imaging, Phenom will accelerate
time-to-data by allowing you to perform routine imaging tasks quickly, thus lightening
the load on your lab SEM.
Phenom is designed with a versatile combination of low acceleration voltage (5 kV), a low
vacuum (0.1 mbar), and a high-brightness/long-life electron source (CeB6). FEI chose this
unique combination to allow the capture of high-quality images on non-conductive
samples without affecting the electron source life, a key concern for future maintenance and
repair. Some SEM systems use a low vacuum leak combined with a less-powerful source to
reduce charging. This atmosphere degrades the image’s signal-to-noise ratio by disrupting
the electron beam, and actually shortens the source lifetime, increasing cost of ownership.
Phenom’s low vacuum design prevents damage and lengthens source lifetime while
providing a path for electron leakage.
• You can virtually eliminate sample charging by using a few key tips outlined in this
article, resulting in better-quality images.
PHENOM
Tips for Reducing Sample Charging
.
Every sample is different. That said, it can be difficult to predict how
a nonconductive sample will image. Every sample behaves differently,
and ultimately, trial and error instructs a user on the best practice for
capturing high-quality images of their sample. To reduce charging
effects without sample preparation, try these simple steps first.
1. Increase the working distance between the sample and the detector.
By lowering your sample, you increase the chance of the electrons
escaping to the surrounding atmosphere and this may decrease
the charge.
2. Adjust Phenom’s brightness and/or contrast controls. You may
find your feature of interest is present even though you are
experiencing charging.
3. Use carbon or silver paint, available from any microscope supplier,
and paint a line from the top of the sample to the sample holder.
This technique makes it easier for electrons to bleed off the sample
surface near the line.
4. Use copper tape with adhesive backing, also available from any
microscope supplier, and lay a track between the top of the sample
and the holder. Again, this technique makes it easier for electrons to
bleed off the sample surface near the tape.
If these steps do not result in reduced sample charging, you will need
Example 1
to progress to the next level of sample preparation before you can
Magnification 1,000X. No charging visible.
obtain a quality image. Depending upon your application, sputter
coating may be the best solution. A sputter coater device will apply a
thin conductive layer on top of the sample. Gold or carbon are commonly
deposited to provide an electron leakage path. Both will eliminate charging
on non-conductive samples and can increase image sharpness on
many samples.
1. Place your sample(s) in the sputter coater device and set the device
for an approximately 10 nm thick conductive (gold) layer. You can
find the current and time settings to do this in your sputter system
manual. The 10 nm thickness has no effect on the image, as the
Phenom is specified to 30 nm resolution.
2. A sputter coater device can handle multiple samples at one time
and the process takes no more than four minutes.
3. View your sample. The conductive layer will eliminate most
of the charging effect, allowing you to view your sample at the
maximum magnification.
Sample Charging Example
Phenom example images 1-5 show charging using plain copy paper,
a nonconductive sample. This series of images demonstrates how
charging increases as magnification increases. With Phenom’s low
acceleration voltage and low vacuum at the sample, it is easy to
capture high-quality images from nonconductive samples.
Example 2
Magnification 2,000X. No charging visible.
As magnification increases, the scanned area decreases.
The electron current is constant but applied to a decreasing area.
Page 2
PHENOM
Phenom Yields Better Data Faster by
Reducing Sample Charging
Greater Efficiency:
• Quick 30 second load and unload cycles deliver high-resolution
images as fast as using a light microscope
• Reduces bottleneck on the a traditional SEM
• No sputter coating necessary in most cases
• Speeds up workflow for sample analysis for about the same cost
and footprint of a high-end light microscope
• Operates at least 10X beyond a light microscope’s
magnification range
Greater Effectiveness:
• Reduces sample charging without the source lifetime being affected
• Easy to use—anyone on your staff can learn to operate a Phenom
with minimal training
• Accommodates flat and 3D samples
Example 3
Magnification 3,500X. Some charging visible.
Charging deflects electrons inconsistently from
the sample surface. Much like shining a flashlight on a glossy surface, artifacts make image
resolution more difficult.
Higher Quality Results:
• Better surface images with no distortion
Example 4
Magnification 5,000X. Charging area expands.
Charging in this image cannot be corrected by
adjusting contrast or brightness.
Example 5
Magnification 10,000X. Sample completely charged.
The white areas in the image cannot be corrected.
Most data is lost to charging distortion.
Page 3
For Phenom examples 6 and 7, samples were coated with a layer of approximately 10 nm of gold.
Example 7
Example 6
Magnification 3,500X. No charging.
Due to the sputter coating, the elemental composition contrast
is less noticeable.
Magnification 10,000X. No charging.
Conclusion
A Phenom personal electron microscope improves the efficiency and
effectiveness of your analytical lab. Personnel who haven’t been trained
to use high-resolution SEMs will find Phenom as easy to use as a light
microscope. You no longer have to wait for specially trained personnel
or a high-resolution lab SEM to obtain excellent images.
Phenom empowers its users to obtain the images they need,
when they need them. Phenom helps you to speed up your
work and improve the quality of your results.
See Beyond at phenom-world.com
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Phone: +1.503.726.7500
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Phone: +31.40.23.56000
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Phone: +81.3.3740.0970
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Phone: +86.21.5027.8805
© 2009. We are constantly improving the performance of our products, so all specifications are subject to change without notice.
FEI is a registered trademark, and Phenom and the FEI logo are trademarks of FEI Company. Other trademarks belong to their respective owners.
HBD/05.09