Getting poor SEM images with bright spots, image drift, or strange contrast? If you’re imaging non-conductive samples like polymers, ceramics, biological specimens, or composites, the culprit is almost always charging. Here’s how to fix it with proper sample preparation.

Scanning electron microscopy requires samples to be electrically conductive. When electrons from the beam hit a non-conductive surface, they accumulate and create a negative charge that deflects incoming electrons, causing image artifacts. The solution is straightforward: make your sample conductive.

Why Non-Conductive Samples Cause Problems

In SEM, the electron beam continuously bombards your sample surface. Conductive materials like metals dissipate these electrons through the sample holder to ground. Non-conductive materials trap the electrons, causing:

• Charging artifacts: Bright areas that bloom and shift during imaging
• Image drift: The image moves as charge builds up
• Poor contrast: Surface details become washed out
• Beam deflection: The electron beam bends away from charged areas

The most reliable solution is sputter coating—depositing a thin conductive layer on your sample surface.

Sputter Coating: The Gold Standard

Sputter coating uses plasma to knock atoms off a metal target, which then deposit onto your sample. The result is a thin, uniform conductive layer that eliminates charging while preserving surface detail.

Choosing Your Coating Material

• Gold (Au): Most common choice. Excellent conductivity, easy to sputter, good for general imaging. Typical thickness: 5-20nm.
• Gold-Palladium (Au/Pd): Finer grain structure than pure gold. Better for high-magnification imaging where gold grains might be visible.
• Platinum (Pt): Even finer grain than Au/Pd. Ideal for high-resolution work. More expensive target material.
• Carbon (C): Essential for EDS analysis. Doesn’t interfere with X-ray detection of other elements. Lower conductivity than metals.

Step-by-Step Coating Process

Using a sputter coater like the MCM-100, the process takes just a few minutes:

1. Mount your sample: Secure the sample to an SEM stub using conductive carbon tape or silver paint. Ensure good contact between sample and stub.
2. Load the chamber: Place the stub in the sputter coater chamber at the recommended working distance (typically 50-60mm from the target).
3. Pump down: Evacuate the chamber to the required vacuum level. The MCM-100 reaches operating vacuum in about 2 minutes with its built-in rotary pump.
4. Introduce argon: Backfill with argon gas to the sputtering pressure (typically 0.05-0.1 mbar).
5. Apply plasma: Strike the plasma and sputter for 30-120 seconds depending on desired thickness.
6. Vent and remove: Vent the chamber and retrieve your coated sample.

How Thick Should the Coating Be?

Coating thickness depends on your application:

• 5-10nm: Minimum for charge elimination. Best for preserving fine surface detail.
• 10-20nm: Standard thickness for most applications. Good balance of conductivity and detail preservation.
• 20-30nm: Heavy coating for highly insulating samples or lower accelerating voltages.

Thicker isn’t always better. Excessive coating can obscure fine surface features, especially at high magnification where individual coating grains become visible.

Alternative: Low Vacuum Mode

Some SEMs, including the SNE-Alpha with the low vacuum option, can image non-conductive samples without coating. Low vacuum mode introduces a small amount of gas into the chamber, which helps neutralize surface charge.

Low vacuum is useful when:

• You can’t alter the sample surface
• You need to preserve the sample for other analysis
• Quick screening doesn’t require optimal resolution

However, sputter coating still provides better image quality for most applications. The thin conductive layer eliminates charging completely, while low vacuum mode only reduces it.

Common Mistakes to Avoid

• Coating too thick: Obscures fine detail. Start with less and add more if needed.
• Poor stub contact: If the sample isn’t grounded through the stub, coating won’t help. Use conductive tape or paint.
• Contaminated targets: Old or oxidized sputter targets produce uneven coatings. Replace targets periodically.
• Rushing pump-down: Incomplete vacuum leads to poor coating quality. Wait for proper vacuum before sputtering.

Additional Resources

• MCM-100 Ion Sputter Coater
• More SEM Tips & Best Practices
• Sputter Coating Fundamentals – ScienceDirect