Fabrication method of a read-only semiconductor memory device

Abstract

A fabrication method of a semiconductor memory device in which the punch-through phenomenon is difficult to occur in uncoded or unwritten memory cells. First impurity-doped regions for coding are formed in channel areas of selected ones of first MOS transistors in the memory cell area, using a patterned photoresist film as a mask, respectively. Second impurity-doped regions for threshold adjustment are formed in channel areas of unselected ones of the first transistors and second transistors in the peripheral circuit area, using a patterned photoresist film as a mask, respectively. During the process of forming the second impurity-doped regions, the dopant doped for threshold adjustment through the selected ones of the first transistors into the substrate forms third impurity-doped regions apart from the channel areas of the selected ones of the first transistors, respectively. The third impurity-doped regions serve to restrain the first dopant doped into the selected ones of the first transistors from laterally diffusing in the substrate, preventing the punch-through phenomenon.