Ion implantation at ultra low energies (<lkeV) using deceleration of ions adjacent to the wafer surface appear to show energy contamination based on process parameters or design of the beamline of the implanter. Previously energy contamination was overwhelmed by diffusion of dopants when using soak or spike RTA processes. Use of ultra fast anneal techniques (e.g. flash/laser) for ultra shallow junction formation provides diffusion less activation of implanted dopants. Thus energy contamination is becomming a severe issue for precisely positioning pn-junctions. By use of deceleration modes, front of the wafer, fractions of the dose penetrate the wafer at non-decelerated energy and introduce undesired move of the junction depth. The fraction of decelerated to non-decelerated ions typically needs to be less than ∼0.1% so that those amounts will not influence device parameters.