Dependence of MOS Device Radiation-Sensitivity on Oxide Impurities

The role of various impurities in the thermally grown silicon dioxide films of metal-oxide-semiconductor (MOS) structures has been investigated. It has been determined that aluminum and sodium within such films strongly influence the radiation-sensitivity of the corresponding MOS devices. The results of the present investigation indicate that much of the radiation-induced positive space charge accumulated within the silicon dioxide films is directly related to the drift and accumulation of ions such as those of sodium. Ionizing radiation, as fran cobalt-60 gamna rays, liberates sodium ions, which are bonded by coulombic forces to non-bridging oxygens, and consequently, the sodium ions drift in the applied gate fields. The space charge due to the accumulation of sodium near the silicon dioxide-silicon interface then causes a perturbation in the surface properties of the silicon. The insight afforded by the above model provides a basis for the fabrication of surface-controlled devices less sensitive to radiation, as well as for the fabrication of radiation dosimeters where the converse in radiation sensitivity is desired. Utilizing appropriate dopants in the silicon dioxide films the desired radiation hardening and dosimetry objectives can be realized.