A blanket boron implant that functions as both a punchthrough and field-isolation implant for sub-micron N-channel CMOS devices. The boron impurity is implanted with high energy subsequent to field oxide growth in order to position the impurity below the field oxide regions and below the future channel region of the N-channel devices. In order to avoid significant counter-doping of the substrate in the N-well regions, the phosphorus dosage during the N-well implant is at a much higher dosage level than the dosage level used for the punchthrough/field isolation implant.