SIMS Instrumentation: Ion Energy Analyzers

Electrostatic energy analyzers bend lower energy ions more strongly than higher energy ions. The sputtering process produces a range of ion energies. An energy slit can be set to intercept the high energy ions (shown in green).

Voltage offset is a strategy for enhancing monatomic ions over multiatomic. The monoatomic ions have higher energy distributions. If the accelerating voltage is lowered (offset), more of the atomic ions still have enough energy to pass through the energy slits. In a typical SIMS experiment, the accelerating voltage is 4.5 kV, and the offset is 50 V. The inner jaw of the slits intercepts most (low energy) multiatomic ions. Both monatomic and multiatomic ion intensities are reduced in a voltage offset measurement, but multiatomic ions relatively more than monoatomic.

The inner and outer sector electrodes have voltages of opposite polarity. Their magnitude is about 10% of the ion accelerating voltage. The ion image comes into focus, producing a virtual image inside the electrostatic sector behind the field aperture. The active surfaces of the electrostatic sector are spherical. This geometry transfers the image to the mass analyzer with minimal distortion. The spectrometer lens adjusts the ion beam focus (cross over) to meet the input requirements of the mass analyzer.

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