Method for making late programmable read-only memory devices

DETAILED DESCRIPTION It is, therefore, an object of this invention to provide a method of programming a ROM late in its manufacturing method without large ROMs needing expansion in size to obtain highest ROM operating speed.
Another object of the invention is to provide a method of programming a hexagonal-type ROM by ion implantation through openings in an overlying phosphosilicate glass layer immediately prior to metallization, wherein ROM drain lines need not cross the implant openings.
This invention comprehends coding a unique ROM array by ion implantation through windows in a phosphosilicate glass layer, immediately prior to metallization.
The ROM is unique in that it is formed of an orthogonal grid of gate thickness dielectric on a silicon surface within an area covered by a field thickness dielectric.
Diagonally disposed gate lines cross grids of vertical and horizontal areas to form a matrix of gates with interjacent silicon regions covered by a thin silicon dioxide layer.
Alternate points of intersection in the matrix alternately form source regions and drain regions, with each region communicating with four gates.
Alternate source and drain lines run parallel to one another diagonally on the matrix but oppositely from the diagonal gate lines, so that they are orthogonal to the gate lines.