Improving the sensitivity of PMOS dosimeters using dual dielectrics

Recent developments in PMOS dosimetry have concentrated on improving the radiation sensitivity of the RADFET device, with the ultimate aim of RADFET operation in the milli-rad range. This is essential for successful use in the clinical and personnel environments. This paper details the research in to improving radiation sensitivity of RADFET devices by examining a range of dual-gate dielectric RADFETs for a number of key dosimetric parameters. The existing 0.4 μm and 1 μm thermal gate oxides in the PMOS process at the NMRC have been optimised for RADFET use, and are used as the base of a number of dual dielectrics which use Chemical Vapour Deposition (CVD) oxides and nitrides as the extra layer. Over thirty different dielectric combinations were fabricated and extensive radiation testing has led to results on radiation sensitivity, read-time stability and long-term stability. The use of a 0.6 μm layer of TEOS based CVD oxide on a 0.4 μm thermal oxide shows an impressive sensitivity of 7.54 mV/rad (Irradiation bias Vir=+5 V, 5 krads) and also performs well in other important dosimetric areas such as the read-time drift and post-irradiation fading