A new cell for high capacity mask ROM by the double LOCOS technique

A new cell for use in an extremely high-capacity mask ROM has been developed by using the Double LOCOS (DL) technique. This cell, called a SADL cell (Self-Aligned cell by DL technique), realizes area (F: minimum feature size) using a fully self-aligned fabrication process. The area of the new cells can be reduced to 1/2 of that of ordinary ROM cells, even when fabricated with conventional silicon IC technology. A 4K-bit SADL cell array was fabricated using 2-µm (diffusion line width) and 3-µm (field line width) rules, realizing a cell size of 25 µm2, the smallest reported so far. In the SADL cell, conductance is large because it is determined by the forward-biased diode characteristic. A high-speed readout method may be adopted for further speed improvement.