NMOS integrated circuit capacitors using poly-silicon gates - uses auxiliary ion implant for capacitors and auto alignment diffusion for telephone analogue=to=digital converter

摘要

Thin oxide windows are formed on the P type substrate (10) and a boron ion implantation through a mask is used to form the enhancement morle transistors. A first phospharons or arsenic ion implant through a mask is performed in the areas for the depletion capacitors and transistors until the transistors are formed. A supplementary implant through a further mask in used to complete the doping for the capacitors. A polysilicon coating (19) is put over the gates (20) etched by selective removal of the oxide layer and then an auto alignment diffusion is used with the polysilicon as a mask to form the transistor sources and drains (21,22) and the connections to the capacitor plates (23). The substrate is coated by a thick oxide layer (24) and diverse connections (25) added. The capacitors are surrounded by an annular difusion and to prevent under cut errors they are laid out in groups of concentric elemental capacitors with the larger values on the outer rim. Successive capacitor elements in a matrix vary in value by a power of two.