Tech TipBy Karl Kersten

Applications Engineer FEI Company

Take Aways

• Capturing images from nonconductive

samples with a traditional scanning

electron microscope (SEM) can cause

electron charging, resulting in poor

quality images with bright and dark

distorted regions.

• Phenom uses a unique combination of

features to minimize sample charging:

low acceleration voltage, a low vacuum,

and a superior electron source.

• You can virtually eliminate sample charg-

ing by using a few key tips outlined in this

article, resulting in better-quality images.

PHENOM

Successful Imaging of Nonconductive Materials with PhenomThe Problem of Analyzing Nonconductive MaterialsElectron charging of nonconductive samples is a common problem with traditional

scanning electron microscopes (SEMs). Sample charging can ruin your image by

producing unwanted bright (or dark) regions and image distortion.

Charging can occur in both conductive and nonconductive materials that do not have a

good electrical leakage path to ground. Since electrons are negatively charged particles, and

the number of signal electrons coming out of the sample is typically smaller than the

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

image. In addition, when charging disturbs the electron signal to the microscope’s detector,

the image data is lost, resulting in bright or dark regions. As you zoom in on your

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 nonconduc-

tive 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 SolutionThe 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.

Page 2

PHENOM

Tips for Reducing Sample ChargingEvery 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

to progress to the next level of sample preparation before you can

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 ExamplePhenom 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 1Magnification 1,000X. No charging visible.

Example 2Magnification 2,000X. No charging visible.As magnification increases, the scanned area decreases. The electron current is constant but applied to a decreasing area.

Page 3

PHENOM

Phenom Yields Better Data Faster by Reducing Sample ChargingGreater 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

Higher Quality Results:• Better surface images with no distortion

Example 3Magnification 3,500X. Some charging visible.Charging deflects electrons inconsistently from the sample surface. Much like shining a flash-light on a glossy surface, artifacts make image resolution more difficult.

Example 4Magnification 5,000X. Charging area expands.Charging in this image cannot be corrected by adjusting contrast or brightness.

Example 5Magnification 10,000X. Sample completely charged.The white areas in the image cannot be corrected. Most data is lost to charging distortion.

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HBD/05.09

Example 7Magnification 10,000X. No charging.Example 6

Magnification 3,500X. No charging.Due to the sputter coating, the elemental composition contrast is less noticeable.

For Phenom examples 6 and 7, samples were coated with a layer of approximately 10 nm of gold.

ConclusionA 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.