Lenses, electrostatic immersion

The second type of SIMS Instrument the direct Imaging type Is based on the design of Castaing and Slodzlan (IQ). In the direct Imaging Instrument an area 1 to 300 pm In diameter Is bombarded by the primary Ions. The secondary Ions are extracted by an electrostatic Immersion lens which maintains a point to... 

Parent ions of internal energy equal to the nominal photon energy or their ionic reaction products are selectively detected in delayed coincidence with threshold electrons. For this purpose the threshold electron signal is used to trigger an electric pulse producing a 10 V/cm electric field for 15 ps i.e., longer than the total ion TOF. The primary ions are thus accelerated over 0.3 cm of the ionization chamber and refocused by a 3 element electrostatic immersion lens. onto an effusive jet of... 

The secondary Ion extraction optics should Include an Immersion lens above the sample surface to maximize secondary Ion transmission from the sample surface to the mass spectrometer. An electrostatic analyzer should again be used to filter out high-energy neutral particles and photons from the secondary Ion beam. Energy analysis of the secondary Ions Is generally not necessary, but can be used to vary the amount of fragmentation observed In the mass spectrum (9). 

An alternative design is based on electrostatic beam transport and focussing. Figure 3.4 shows schematically the Aarhus electrostatic positron beam line (Deutch et al. [3.9]). A 70mCi 8 mm diameter Na source delivers particles that are moderated in a laser-annealed W(100) foil. The transmitted slow positrons are collected and accelerated by a modified Soa immersion lens (Canter et al. [3.10]). They are then further accelerated and deflected through 90° in a cylindrical mirror analyzer and thus removed from the primary P" and 7 background of the source. The beam is then transported and focussed to a spot of 2 mm diameter on the target, where the positron intensity is 10 s ... 

Extraction of the secondary ion beam from the sputtered surface is optimally carried out through the immersion of the sample in an appropriate electrostatic field. This is carried out by ensuring a potential difference exists between the sample and some metallic plate or cone bearing an orifice of 1 mm in diameter. This plate/cone is typically situated in close proximity to the sample (in some instruments, this plate/cone is referred to as the immersion lens). This acts to... 

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