Optimization of oxygen-implanted silicon substrates for CMOS devices by electrical characterization

The use of oxygen-implanted silicon substrates for CMOS SOI device technology has great potential for use in VLSI and radiation-hardened circuits. The electrical characterization of such substrates is described by reference to CMOS devices fabricated directly into them; no epitaxial silicon was grown. Electrical parameters were related to the oxygen-implantation conditions of dose and temperature. Thermally generated oxygen donors in the top silicon layer were identified as being responsible for threshold voltage shifts and resistivity changes that altered transistor characteristics. Suitable boron implants enabled electrical parameter control to be maintained. Full island-to-substrate electrical isolation was only achieved for oxygen doses greater than 1.6 × 10¹⁸cm^-2, a larger dose from that required to create stoichiometric SiO2. Channel mobilities and NMOS back-channel leakage currents were found to be dependent on oxygen implant temperature; as a result a favorable implant window of 460-510 °C was established to fabricate ring oscillators twice as fast as bulk silicon counterparts for the same power dissipation.