ADVANCED TFT SRAM CELL TECHNOLOGY USING A PHASE-SHIFT LITHOGRAPHY

An advanced TFT memory cell technology has been developed for making h igh-density and high-speed SRAM cells, The cell is fabricated using a phase-shift lithography that enables patterns with spaces of less than 0.25 mu m to be made using the conventional stepper, Cell area is als o reduced by using a small cell-ratio and a parallel layout for the tr ansistor, Despite the small cell-ratio, stable operation Is assured by using advanced polysilicon PMOS TFT's for load devices, The effect of the Si3N4 multilayer gate insulator on the on-current and the influen ce of the channel implantation are also investigated, To obtain stable operation and extremely low stand-by power dissipation, a self-aligne d offset structure for the polysilicon PMOS TFT is proposed and demons trated, A leakage current of only 2 fA/cell and an on-/off-current rat io of 4.6 x 10(6) are achieved with this polysilicon PMOS TFT in a mem ory cell, which is demonstrated in a experimental 1-Mbit CMOS SRAM chi p that has an access time of only 7 ns.