Effects of package geometry, materials, and die design on energydependence of pMOS dosimeters

This paper presents the results of further studies of dose enhancement in dual and single-dielectric pMOSFET dosimeters for various package and die designs. Eight different MOSFET designs and package types were investigated over a photon energy range from 14 to 1250 keV. Seven X-ray effective energies and two radioactive sources of cesium and cobalt provided the radiation. As in a previous study, Rutherford back-scattered electrons were primarily responsible for the dose enhancement factors which achieved values as high as 20. Packages filled with silicon grease, aluminum oxide, or paraffin eliminated the contribution of back-scatter to the enhanced dose. These modifications allowed measurements of the usual dose enhancement at the aluminum or polysilicon gate-silicon nitride (dual dielectric devices), or silicon dioxide interfaces (single dielectric parts), and at the silicon nitride-silicon dioxide interface. In addition to the primary peak in the DEF (dose enhancement factor) curve versus energy at 45.7 keV, there is a second peak at about 215 keV. This peak might be due to enhancements at the interfaces of a MOSFET. These interface effects were small in the single-insulator parts in standard ceramic packages, and significantly larger in the dual-insulator devices. The effects were reduced by filling the packages with the materials as previously described. The geometry of the package, for example, the size of the air gap between the die's surface, and the lid of the package impacts the value of the DEF