A novel self-aligned isolation process for VLSI

We have developed a novel NMOS process for VLSI isolation. The process employs an RIE of the field oxide followed by a metal liftoff and a high-energy boron sheet implant. The energy of the implant and the metal thickness are selected such that the boron penetrates the field oxide, but not the metal. Performing the isolation doping after growing the field oxide eliminates encroachment of the isolation doping into the channel end while simultaneously providing a self-aligned channel stop. The doping concentration in the channel region is determined by implants that are independent of the doping concentration in the channel stop area. With this technique, we have obtained excellent device performance as well as rigorous device isolation. Even at the isolation spacing of 0.7 µm, there was <1-pA leakage at 5 V. The active devices exhibit minimal body effect coefficients (0.1-0.2), good subthreshold behavior (80 mV/DEC), and low junction capacitance. The experimental data is confirmed by two-dimensional analysis.