A high voltage ion-implanted MOSFET

The drain breakdown voltage of usual p-channel MOS devices is limited to about 30 to 40 V by field crowding at the drain junction. By optimizing certain design parameters like diffusion depth, substrate resistivity and gate oxide thickness, 80 V breakdowns can be achieved. To date, higher breakdowns are only possible with special device geometries (e.g. the stacked gate tetrode) which are generally not compatible with common MOS processes. By adding an ion-implantation step to the conventional aluminum-gate MOS process, it is possible to fabricate MOSSFET\´s with source-drain breakdown potentials of greater than 240 V on the same chip with conventional MOS circuitry. The device looks identical to the conventional MOSFET except that an unmetalized gate oxide region is left between the gate and the drain juction. During a subsequent ion-implantation this portion of the channel is lightly doped with a p-type dopant in the case of p-channel devices. Since-both the metal and the field oxide are thick enough to stop all ions, the implantation affects only the high voltage devices on the wafer. The implanted channel has the effect of reducing the field in the vicinity of the drain junction, thereby increasing the source-drain breakdown voltage. P-channel devices were made on 10 Ω-cm material using 1µm diffused junctions. The aluminum gate was 10 µm long and the implanted channel was 25µm long. Breakdowns up to 240 V were achieved after an optimum dose of boron ions/cm²were implanted at 80 keV through 1300 Å of gate SiO2. The wafers were subsequently annealed for 15 minutes at 525°C in N2. Implanted resistors made at the same time had sheet resistivities of 25 kΩ/□. Of critical importance in obtaining best results are low threshold voltages and accurate implantation dose control To obtain long term reliability it is necessary to prevent charge build-up on the oxide over the implanted channel. This is accomplished by the deposition of a layer of phosphosilicate glass after ion-implantation. It is now possible to include decoding circuitry on the same chip as the high voltage drivers for display tube- s.