EFFECTS OF PACKAGE GEOMETRY, MATERIALS, AND DIE DESIGN ON ENERGY-DEPENDENCE 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 radiactive sources of cesium and cobalt provided the radiation. As in a previous study [1], Rutherford back-s cattered electrons were primarily responsible for the dose enhancement factors which achieved values as high as 20. Packages filled with sil icon grease, aluminum oxide, or paraffin eliminated the contribution o f back-scatter to the enhanced dose. These modifications allowed measu rements of the usual dose enhancement at the aluminum or polysilicon g ate-silicon nitride (dual dielectric devices), or silicon dioxide inte rfaces (single dielectric parts), and at the silicon nitride-silicon d ioxide interface. In addition to the primary peak in the DEF (Dose Enh ancement Factor) curve versus energy at 45.7 keV, there is a second pe ak at about 215 keV. This peak might be due to enhancements at the int erfaces 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 th e packages with the materials as previously described. The geometry of the package, for example, the size of the air gap between the die's s urface, and the lid of the package impacts the value of the DEF.