Simultaneous Growth of Different Thickness Gate Oxides in Silicon CMOS Processing

A method is proposed that allows the growth of gate oxides of different thicknesses on a single wafer. The method does not require masking the gate oxide during oxidation with its inherent risk to the oxide quality, but rather relies on the implant of nitrogen into the silicon wafer before both oxide growth and preoxidation cleans. This implant is performed at the same step as the normal threshold voltage implants, avoiding possible contamination. Using Nitrogen implant doses of the order of 3 x 1014-3 X 1015 cm-2, it is shown that it is possible to grow oxides of 30–70 Å for a process with a nominal oxide thickness of 90 Å.