US Patent 7466581 SRAM design with separated VSS

An array of static random access memory (SRAM) cells arranged in a plurality of rows and a plurality of columns includes a plurality of VSS lines connected to VSS nodes of the SRAM cells, with each VSS line connected to the SRAM cells in a same column. The plurality of VSS lines includes a first VSS line connected to a first column of the SRAM cells; and a second VSS line connected to a second column of the SRAM cells, wherein the first and the second VSS lines are disconnected from each other.