Low Vacuum Mode

Theory

Charging occurs in an SEM when the emitted electrons from a specimen do not equal the incident electrons from the beam. In conventional SEM, charging may be countered by changing the voltage or current, or by sputter coating a specimen with conductive material. These methods may not be sufficient for insulating materials or they may detract from the image quality.

Low Vacuum mode supplies water vapor to the specimen chamber to aid in imaging insulating specimens. Interactions between the incident electron beam, scattered electrons, and the water vapor amplify the secondary electron signal and neutralize charging on the specimen surface.

• The incident electron beam ionizes water vapor into H+ and OH-, which are drawn to and neutralize charged areas of the specimen surface.

• Emitted secondary electrons scatter in the water vapor, creating a cascade of secondary electrons which amplifies the signal.

• At high pressures, the water vapor may cause the beam to spread and be less focused. Low voltages increase this effect. It is important to find a good ratio of water vapor pressure and accelerating voltage for your specimen.

Charging

• Too high pressure (too much water vapor) increases scattering and reduces the contrast.

• Lower voltages and higher pressures are more likely to increase available ions for charge neutralization. Too low voltage or too high pressure may induce charging.

• Higher voltages tend to require higher pressures.

• The same principles of interaction volume apply: e.g. lower voltages are better for surface detail.

• Use the lowest pressure that yields a stable image.

• If the image is only charging at slow scan speeds, use the snapshot (F4) mode for capturing images. The parameters for snapshot can be set via Tools>Preferences>Scanning. Snapshot is the camera icon with the flash.