SRAM system having very lightly doped SRAM load transistors for improving SRAM cell stability and method for making the same

摘要

A static random access memory (SRAM) cell is given increased stability and latch-up immunity by fabricating the PMOS load transistors of the SRAM cell to have a very low drain/source dopant concentration. The drain/source regions of the PMOS load transistors are formed entirely by a P−− blanket implant. The PMOS load transistors are masked during subsequent implant steps, such that the drain/source regions of the PMOS load transistors do not receive additional P-type (or N-type) dopant. The P−− blanket implant results in PMOS load transistors having drain/source regions with dopant concentrations of 1e17 atoms/cm³ or less. The dopant concentration of the drain/source regions of the PMOS load transistors is significantly lower than the dopant concentration of lightly doped drain/source regions in PMOS transistors used in peripheral circuitry.