Read only memory (ROM) and method for forming the same

摘要

A read only memory (ROM) and method for forming the same. A first programming method comprises adjusting threshold voltage of ROM cell by Vt implantation and oxidation. In this programming method, four-level ROM is obtained. The first ROM code with threshold voltage Vt1 is performed with one Vt implantation and has a thin gate oxide layer. The second ROM code with threshold voltage Vt2 is performed with two Vt implantations and has a thin gate oxide layer. The third ROM code with threshold voltage Vt3 is performed with one Vt implantation and has a thick gate oxide layer. The fourth ROM code with threshold voltage Vt4 is performed with two Vt implantations and has a thick gate oxide layer. A second programming method comprises connecting the ROM cell to or disconnecting it from bit line through contact window, such that two-level ROM without implant misalignment is obtained. Furthermore, the first and second programming methods can be combined, and each ROM cell has five ROM codes for selection.