Reduction of Radiation Produced in Ion Implantation Devices, and Measurement of Some Relevant Cross-Sections

Abstract

Ion implantation devices, machines which can very precisely dope semiconductors using beams of accelerated charged particles, have in recent years begun to be used in implanting high energy light ions, with energies greater than 1 MeV. This has caused unprecedented production of neutron and gamma radiation, particularly of neutrons from the ¹³C(alpha,n)¹⁶O reaction, creating an unacceptable radiation hazard. To address this issue, we undertake dose mapping and modeling efforts to create simulation tools in Geant4 which can accurately predict dose rates on the Axcelis VXE LT.

Existing physics tools for modeling nuclear reactions have been shown to produce non-physical results at incident particle energies of 1 - 2 MeV, as these tools are frequently used for modeling reactions which may have energies into the GeV or even TeV range. To address these deficiencies, we construct a new drop-in physics model which uses relativistic kinematic equations to precisely predict the energy and angular distributions of secondary particles produced in Geant4 at low energies. This model relies on accurate cross-section data to describe the reaction; to address gaps in the literature on the two neutron producing reactions of interest to this work, we measure the angular dependent cross-section of the ¹³C(alpha,n)¹⁶O reaction over 7 angles, at the 2.605 and 2.670 MeV resonances, and we measure the total cross-section of the ²⁹Si(alpha,n)³²S reaction at 2.6 and 2.7 MeV.

By implementing the new physics model and adding new cross-section data to the model of the ion implantation device, we are able to produce a high-fidelity simulation of radiation production and transport in ion implantation devices. Using this tool, we then propose solutions to mitigate radiation production within the ion implanter, reducing the radiation hazards of high energy ion implantation devices.