Beam Energy Purity on Axcelis XE High Energy Ion Implanter

By Shu Satoh and Jonathan David
Axcelis Technologies, Inc.

The ability to create a precise dopant depth profile reliably and repeatedly has been the one of the big promoting factors for ion implantation over other doping technologies. Beam energy purity, or lack of energy contamination (EC), is one of the important performance criteria on ion implanters.

Axcelis high energy serial ion implanter Purion XE is based on the RF linear accelerator (RF linac) technology [1].  From its acceleration principle, the RF linac acts as a strong velocity filter, to accelerate only those ions which happen to be at the right place at the right time at all the 24 RF accelerating gaps.   When ions reach the wafer, the ions have already gone through three filters, first from AMU magnet (magnetic mass analysis),  second from the velocity filtering of the RF linac and third through the magnetic energy filter immediately after the RF linac.  All these three filters help to purify the ion beam in energy and mass spectra before reaching the process wafer.  

Detection of EC is not straightforward. Sweeping an energy filter, either a magnetic filter, as usually found on high energy implanters, or electrostatic filter found on some medium current implanters, does not reveal the presence of EC.  It is because either filters filter the ions by rigidity, magnetic rigidity or electrostatic rigidity, not by pure energy value, and EC pass through the filters by possessing the same rigidity as the desired main beam. Because of this difficulty, detection of EC has been done almost exclusively with the SIMS depth profile analysis.  Although SIMS is a very powerful method with the fantastic wide sensitivity dynamic range, its off-line nature makes it quite unpractical, or inconvenient, tool for implanter setup or tuning.

Purion XE has a beam diagnostic tool, called ETS (Energy Tracking System) which measures beam energy, or more precisely electrostatic rigidity of the beam, using the precision beam scanner, independently from the beam energy reading by the magnetic energy filter [2]. 

In this study, we have used the ETS as an on-tool on-line EC detection tool. 

Typical output energy spectrum from RF Linac

Fig. 1.  Typical Output Energy Spectrum from RF Linac


Basic electrostatic deflector
Fig. 2.  Basic electrostatic deflector


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