Technology Notes

The AXIS Charge Balance System

The analysis of insulating materials is an important capability for XPS spectrometers. Many materials generate a strong surface potential when exposed to X-rays. This effect can be particularly severe with monochromatic sources. In order to maximize sensitivity and resolution this charge must be reduced to a minimum, and be uniform in nature. A variable surface charge will result in differences in peak energy, which will subsequently degrade the energy resolution (FWHM). This effect can be significant with monochromatic X-ray sources where the X-ray flux density may not be uniform across the irradiated area.

The conventional method for compensating for the charge build up involves the use of a focussed electron flood gun which supplies electrons to the surface. These arrangements are typically non-normal to the sample surface, and therefore also generally have a variable flux density resulting in reduced performance.

An ideal charge neutralization system should therefore contain the following elements:

• A high flux of electrons to provide a uniform charge density.
• The electrons should be of low energy.
• The operation of the neutralizer should be "transparent" to the user.

Such a system is available with all AXIS spectrometers. The magnetic immersion lens which provides the high sensitivity and small area XPS performance is used as part of the charge neutralization system. Low energy electrons (typically 0.1eV), are injected into the magnetic field from a filament located at the base of the electron input lens. These electrons follow the field lines to the surface of the sample where they neutralize any surface charge build up. As these are the same magnetic field lines used to extract and focus the photoelectrons, the neutralizing electrons arrive at exactly the same point from which the photoelectrons are emitted. The design of the neutralizer enables excess electrons to become trapped in the magnetic field until required for the neutralization process. Creating a sufficiently high flux of electrons ensures that the whole analyzed area has a uniform surface charge, providing optimal XPS performance.

 

Electron Paths

 

The Figure, above, based on a real "iron filings" field plot of a real Axis magnetic lens, shows the field shape which simultaneously transports and traps electrons of the appropriate energies.

Electrons of the selected energy traverse the field into the imaging lens as shown. Higher energy electrons are "under focussed" and hit the (negative potential) charge balance plates, producing low energy secondaries which are trapped in the magnetic field and channelled back to the sample. Lower energy "over focussed" electrons from the sample follow similar paths and return to their point of origin. Adding more electrons from the neutralizer provides an charge balance cloud in which the trapped electrons oscillate up and down the magnetic field gradient depending on the sample potential.

 

More about the magnetic lens

For comments, suggestions or contributions or an Instrument Demonstration
e-mail to: dsurman@kratos.com
Last Update: 98/03/11

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