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Additional process controls which have been implemented by semiconductor manufacturers in order to produce low threshold voltage CMOS integrated circuits are shown to significantly improve the radiation hardness of such devices. In particular, this improvement is directly correlated with reduced concentrations of the sodium impurity which results in smaller threshold voltage shifts for the p-channel (negatively biased) transistors. Further, it has been shown that n-channel devices can be hardened by the technique of ion implantation on production line integrated circuits. Thus, digital CMOS devices hard to greater than a Mrad(Si) have been realized.