Method for fabricating high-density semiconductor read-only memory device

A method for fabricating a ROM device comprising the steps of:

• preparing a semiconductor substrate of a first semiconductivity type;
• forming a plurality of parallel-spaced first diffusion regions of a second semiconductivity type on the substrate in a first direction, the first diffusion regions serving as a plurality of bit lines on which an array of memory cells are defined, the semiconductor substrate and the first diffusion regions defining a planar surface;
• forming an insulating layer over the substrate covering all of the first diffusion regions;
• forming a plurality of contact windows in the insulating layer to expose select locations on the first diffusion regions where a first group of the memory cells, set to a permanently-ON state, are located, wherein the unexposed portions of the first diffusion regions are associated with a second group of the memory cells that are set to a permanently-OFF state;
• using the insulating layer as a mask, performing an ion-implantation process to dope an impurity material of the first semiconductivity type into the exposed portions of the first diffusion regions to form a plurality of second diffusion regions of the first semiconductivity type in the associated first diffusion regions, each of the second diffusion regions and its associated first diffusion region in combination comprising a junction diode;
• and
• forming a plurality of parallel-spaced conductive layers, serving as a plurality of word lines, over the insulating layer in a second direction intersecting the bit lines, the conductive layers filling all of the contact windows in the insulating layer to provide electrical contact with all of the second diffusion regions associated with the ON-state memory cells.