The primary ion bombardment leads to the emission of secondary ions and electrons.
In the case of non-conducting samples the emission of the secondary electrons leads to a localised positive charge at the surface.
As a consequence of this sample charging, the secondary ion energy will change and hence the respective flight time. Futhermore, the trajectories of the secondary ions will be influenced which, in the worst case, may lead to a loss of transmission.
In addition the positive charge at the sample surface may become the driving force for migration effects of electro-positive surface species (e.g. alkali metals) into the bulk. Therefore, the positive surface charge has to be compensated.
In order to neutralise the positive surface charge, low energy electrons (< 20 eV) are applied. The advantage of using low energy electrons is that sample damage is minimised.
Moreover, low energy electrons only go to such surface areas where the charging occurs resulting in a self-adjusting charge compensation.
During the compensation the high voltage extraction field is switched off.
The TOF.SIMS 5 can compensate sample charging with a repetition rate of up to 50 kHz. The analysis and the compensation operate quasi simultaneously to maintain highest data rates even on insulating samples.
High resolution mass spectrum of a bulk PET sample. The spectrum was acquired using Bi3++ as primary ion projectile.