Floating Low-Energy Ion Gun
 
FLIG 5
                             
    The FLIG 5 is an ion gun system ideal for use on SIMS depth profiling instruments. It has a floating column which transports O2+ ions at relatively high energy prior to deceleration in the final lens. Thus, it can deliver a probe of high current density at beam energies as low as 200eV. This low energy performance makes the FLIG 5 a powerful tool for shallow depth profiling. Realistic analysis time can be achieved at low energies with high depth resolution and high dynamic range.  
The FLIG 5 also operates conventionally (non-floating) at higher energy with no sacrifice in performance.
         
 
The need for low energy profiling                
The requirement for high depth resolution dynamic SIMS arises from the reduction in device size in the semiconductor industry. With the use of low implantation energies and new technology dependent on delta-doping and sharp interfaces, it is increasingly important to have access to depth profiling techniques which can quantify these structures both accurately and reproducibly.


When profiling with energetic oxygen beams, the kinetic energy of the incident particles is transferred to the near surface region of the sample, creating an altered layer in which atomic mixing has occurred. The depth of this layer is approximately 4nm per keV for an O2+ beam, and this imposes an absolute limit on depth resolution. Hence, it is necessary to use energies well below 1keV for profiling of shallow junctions.

Another important factor in the analysis of shallow implants is the transient region which occurs at the surface and at matrix interfaces. At the beginning of a shallow profile, the ion and sputter yields vary rapidly as probe atoms are incorporated into the analysed surface, and the surface chemistry changes. Similar effects occur at matrix interfaces. Whilst this behaviour persistes, the profile depth profile is not quantifiable, and any features lying within the transient will be distorted. The thickness of the region, and hence, the amount of lost information, can be reduced by using low impact energies.

      
    The Principle of the Floating Ion Gun
In a conventional ion gun, ions are transported through almost the whole ion-optical column at an energy determined by the anode voltage. Thus, to attain a 250eV impact energy (on a grounded sample) the anode must be set to 250V and the beam travels through the column at this energy. At such low energy, space charge effects and aberrations of the wide beam seriously limit the final intensity of the probe and impair the probe shape.

In the floating ion gun, almost all of the column is floated to a negative potential and the beam is accelerated to a more viable transport energy between the extraction region and the final lens. Inside the final lens, the beam is decelerated to the desired impact energy. Thus, for a 250eV impact energy, the anode is set to 250V and the float could be -3kV giving a transport energy of 3.25keV. This provides a signicant reduction in beam aberrations. In the FLIG, the Wien filter electrostatic plates and alignment units (including a bend to reject neutrals) are all referenced to the float voltage. 		 
                             
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