Radiation Protection Considerations of Ion Implantation Systems

Ion implanters produce beams of positive ions that impinge on target materials located in an enclosed vacuum system and are not intended to produce external beams or any other external ionizing radiation. The deceleration of positive ions produces negligible bremsstrahlung external to the implanters. The deceleration of electrons when striking beam-tube components produces bremsstrahlung resulting in significant quantities of unintended external radiation. These electrons are created by the interaction of positive ions with component parts of the implanter and with the residual gas and are subsequently accelerated in the implanter beam tube) At implanter ion energies (< 500 keV), the bremsstrahlung is emitted in lobes essentially perpendicular to the electron stream (1800 from the ion beam) modified by self-shielding from implanter structures. Experimentally, the radiation exposure rate is generally proportional to the electron current and to the square of the electron energy in essential agreement with theory. Implanter x-ray spectra are also in general agreement with spectra taken under laboratory conditions. Principles for reducing external radiation include the judicious combination of implanter design, materials with low atomic number for electrons to strike, cleanliness in assembly, use of suppression fields to reduce the number of electrons, shielding, interlocks, and operation. The exposure rate limits can vary from the federal and state value of 0.5 milliroentgens per hour for electronic products to various lower values which may be established for other countries or by individual manufacturers.