A SimpIe Ionizing Radiation SpectrometerLDosimeter based on Radiation Sensing Field Effect Transistors (RadFETs)

The RadFETs discussed in this paper are based on our [2] dual dielectric design of SiN over SO, The RadFETs were fabricated at the Micrel Corp. in San Jose, CA. to specifications provided by the authors. Micrel was able to use their standard p-MOSFET process with the addition of a Low Pressure Chemical Vapor Deposition (LPCVD) SIN deposition after gate oxidation to fabricate the RadFETs. Micrel performs a high dose arsenic implant to provide a low resistance top-side contact to the (100) n on n+ Si epitaxial substrate. A 2.5 pm, 1 lOO”C, wet oxidation and the negative photoresist are used to mask the implant whose beam current is less than 400 pA. The resist is removed by an 0 2 plasma followed by a wet chemical cleaning step. After a 1000°C arsenic drive-in and a source-drain masking operation, BBr3 is diffused at 1030°C. A 1000°C field oxidation is performed to produce a field oxide of 12 pm and a p+ junction depth of 1.75 pm. After depositing and densifying an 8 pm, 2.5% phosphorous CVD oxide, negative resist is used to etch the gate region followed by a wet chemical resist strip. A 875”C, dry, sacrificial gate oxide is grown and etched before the gate oxidation and the nitride deposition.