Masking of repeated overlay and alignment marks to allow reuse of photomasks in a vertical structure

A monolithic three dimensional semiconductor device structure comprising:

• a first layer including a first occurrence of a first reference mark at a first location;
• a second layer including a second occurrence of the first reference mark at a second location, wherein the second location is substantially directly above the first location;
• and
• an intermediate layer between the first layer and the second layer, the intermediate layer including a blocking structure, wherein the blocking structure is vertically interposed between the first occurrence of the first reference mark and the second occurrence of the first reference mark.

A method for avoiding interference between recurring reference marks when reusing photomasks in formation of a vertically stacked structure, the method comprising:

• using a first photomask to form a first reference mark at a first height above a substrate and at a first position;
• forming a blocking structure at a second height above the first height and at a second position, the second position substantially directly above the first position;
• using the first photomask to form a second reference mark at a third height above the second height and at a third position, the third position substantially directly above the second position,
• wherein, during the step of forming the second reference mark, the blocking structure serves to substantially render the first reference mark substantially undetectable when viewed from above.

A monolithic three dimensional memory array comprising

• a) a first memory level, the first memory level comprising a first layer at a first height, the first layer including a first occurrence of a first reference mark;
• b) a second memory level, the second memory level comprising a second layer at a second height above the first height, the second layer including a second occurrence of the first reference mark, the second occurrence of the first reference mark formed substantially directly above the first reference mark;
• and
• c) an intervening layer at a third height between the second height and the first height, the intervening layer including a blocking structure, the blocking structure vertically interposed between the first occurrence of the first reference mark and the second occurrence of the first reference mark,
• wherein the second memory level is monolithically formed above the first memory level.

A method for forming a monolithic three dimensional memory array, the method comprising:

• a) forming a first memory level, the step of forming the first memory level comprising using a first photomask to form a first layer at a first height, the first layer comprising a first reference mark;
• b) forming a second layer at a second height above the first height, the second layer comprising a blocking structure;
• and
• c) forming a second memory level, the step of forming the second memory level comprising using the first photomask to form a third layer at a third height above the second height, the third layer comprising a second reference mark,
• wherein the blocking structure is vertically interposed between the first reference mark and the second reference mark.