Accel/Decel system for the IMS 4, 5, 6 and 7f duoplasmatron - CAMECA - #1

It consists in maintaining an extraction voltage on the

DP source high enough to avoid space charge effects and decelerating ions at the entrance of the column in order to reach the target with low impact energy.

The Accel/Decel system developed for the DP source of the IMS 7f is schematically represented hereunder. The DP source itself is not modified, but the formerly grounded extraction electrode is electrically isolated, and is biased at a negative potential. An Einzel lens (L1) is also added, downstream, between the DP source and the Primary Beam Mass Filter (PBMF). As shown in the figure, primary ions are accelerated between DP

source anode (A) and the extraction electrode (EE),

then decelerated between (EE) and lens (L1). Finally, they reach the sample surface with a net impact energy

determined by the difference between source and

sample potentials. Lens (L1) focuses the beam in order to match the DP source emittance with the acceptance of the PBMF/

For positive secondary ion analysis on magnetic sector SIMS instruments, the sample (brought to a high potential in order to collect the secondary ions with a very high efficiency) acts as a deceleration electrode for the primary ions (PI). Thus, theoretically, low impact energies for PI can be reached while keeping a rather high extraction voltage on the duoplasmatron (DP) source and decelerating PI just before reaching the sample surface. Practically, due to the oblique incidence of the primary column, high amplitude deceleration effects yield to very large incidence angles (beyond 70° relative to the normal) of the primary beam. These large incidence angles make focusing the primary beam difficult. Moreover, large incident angles are not suitable for sputtering silicon with an oxygen beam because of ripple formation which deteriorates depth resolution

For the IMS 4-5-6-7f the main limiting factor in routine operating mode at sub-keV impact energies (< 1keV) was determined by the performance of the DP source itself. An O2+ primary beam with an impact energy of 500eV and an incidence angle of 44° is obtained by setting the DP source at 1 kV and the se­condary extraction voltage at + 500V. At 1 kV DP extraction vol­tage, space charge ef­fects reduce the source brightness and the pri­mary beam density is no longer high enough to maintain sputter rates at levels compati­ble with high through­put analyses.

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