Physical Mechanisms of Radiation Hardening of MOS Devices by Ion Implantation

This paper reports experimental and analytical studies performed on Al+ ion-implanted MOS capacitors. Electrons and holes were excited in SiO2 by a low-energy electron beam, the resultant current measured, and the results then compared to findings for unimplanted specimens. Comparisons were also made of flatband voltage shifts for implanted and unimplanted devices. Analytical expressions, based on a physical model, are developed which explain the experimental results. In this model, decreases in collected current and changes in flatband voltage shift are attributed to both hole and electron trapping in the implanted region. Thus, the model contains three critical parameters: the mobility-lifetime products for both holes and electrons in the implanted region and the depth of this region. While a range of these variables satisfy either the current vs voltage measurements or the flatband-voltage-shift results, considered jointly these experiments provide specific values for all three parameters. The depth of the implanted region so determined agrees well with experimental observation, and the mobility-lifetime products calculated for different experimental conditions are consistent. The product for holes (~3.5 × 10-13 cm2/V) is substantially smaller than that for electrons (~2 × 10-12 cm2/V), indicating that holes are more easily trapped in the implanted zone than electrons. The fact that more electrons than holes are actually trapped in the case of gamma irradiation under positive bias is explained by the model. Bias dependence is also explained, as well as the fact that there is an optimum implantation energy for minimizing flatband shifts.