Method for manufacturing a master disc for optical discs

A method for making a master disc used for manufacturing a double-sided optical disc of reflection type each comprising a pair of light-transmissive circular substrates adhered to each other, one light-transmissive circular substrate being an information recording disc having signal portions concentrically or spirally formed as a pit-row of recorded track and, the other light-transmissive circular substrate being a dummy disc having a displaying pattern region having a diffraction grating region generating diffracted lights due to an incident light and a non-diffraction region, the master disc is a master stamper for replication of the dummy disc, the method comprising the steps of:

• making a circular substrate on which a photoresist layer bearing an image of a spiral or concentric dummy pit-row or pregroove at a predetermined constant pitch;
• depositing a metal electro-conductive layer on the imaged photoresist layer;
• plating continuously and electrically a metal layer on the metal electro-conductive layer in an electroforming bath so as to form a preliminary stamper of the metal;
• separating the preliminary stamper and the substrate thereby preparing the preliminary carrying the dummy pit-row or pregroove forming the diffraction grating region;
• and
• forming the non-diffraction region in said diffraction grating region of said preliminary stamper in such a manner that a portion of the dummy pit-row or pregroove corresponding to a main object pattern to be displayed is selectively removed from said diffraction grating region, wherein said forming step of the non-diffraction region is performed by a laser-marking so that the portion of the dummy pit-row or pregroove is crushed and melted, thereby preparing a master stamper.