Method for fabrication mask ROM

In a method for fabricating and programming a mask read only memory comprised of an array of a plurality of MOS transistors, certain designated transistors of said array operating in a depletion mode to indicate operation in one logic state and other designated transistors operating in the enhancement mode, a method for fabricating substantially the entire structure of the memory prior to performing the fabrication step for designation of those transistors selected for operation in the enhancement mode, said method comprising the steps of:

• forming a gate oxide film, a gate electrode and source and drain regions on one portion of a semiconductor substrate to form a MOS transistor;
• depositing a BPSG film over the entire exposed surface of the resultant structure after the formation of the MOS transistor to smooth the structure;
• forming a metal electrode on portions of the BPSG film, which portions are disposed over the source and drain regions;
• forming a pad on a portion of the BPSG film, which portion is disposed over another portion of the semiconductor substrate, the pad being adapted to provide a connection with an outer circuit;
• coating a nitride film over the entire exposed surface of the resultant structure after the formation of the pad and removing a portion of the nitride film disposed over the pad to expose the pad;
• coating a photoresist film over the entire exposed surface of the resultant structure after the partial removal of the nitride film and removing portions of the photoresist film, which portions are disposed over the gate electrode and portions of the source and drain regions of the MOS transistor to be implanted with ROM code ions, to partially expose the nitride film and the source and drain region portions disposed adjacent to the gate electrode;
• removing the exposed portion of the nitride film, to partially expose the BPSG film;
• selectively removing the exposed portion of the BPSG film to expose the gate electrode and the source and drain region portions disposed adjacent to the gate electrode;
• implanting ROM code ions in the exposed gate electrode and source and drain region portions;
• forming a PIQ (Poly Imide Iso Indro Quizolin Dione) film over the entire exposed surface of the resultant structure after the ion implantation and removing a portion of the PIQ (Poly Imide Iso Indro Quizolin Dione) film disposed over the remaining photoresist film;
• removing the remaining photoresist film;
• and
• curing the remaining PIQ (Poly Imide Iso Indro Quizolin Dione) film by a heat treatment and simultaneously forming an impurity diffusion region in a portion of the semiconductor substrate disposed beneath gate electrodes of MOS transistors selected to operate in the enhancement mode thereby causing the MOS transistor controlled by the gate electrodes wherein the impurity diffusion region is formed to operate in the enhancement mode.