N-Channel MOSFET field protection by nonmasked boron ion implantation

A boron ion implant protects the thick-oxide field regions of n-channel MOSFETs by penetrating the thick oxide and increasing the doping at the silicon interface. Active devices, with thin oxide in their gate regions, are not greatly affected because the implant lies buried beneath their channels. The method is compatible with standard four-mask FET processing, adding no new masking steps. Measurements of field properties were made on FET devices with thick oxides in their gate regions. Measurements are also reported for normal thin-oxide devices. The implant effect is characterized by the difference in properties between implanted and unimplanted devices on the same wafers. the degree of protection as a function of both implant dose and energy is reported over a wide range. The results are compared with calculations that accurately account for the nonuniform profile, with good agreement. Field protection, as measured by the thick-oxide threshold voltage, has a sharp peak at a well-defined energy. At energies below the peak, too many ions stop in the oxide; above the peak, they lie too deep in the silicon. Threshold voltages and transconductance of thin-oxide devices are unaffected, but the threshold voltage sensitivity to substrate bias is increased somewhat.