Definition of overlay (OVL)- 1n : a computer program segment not permanently maintained in in ternal storage. [SEMATECH] 2 n : the technique of repeatedly using the same areas of internal storage during different stages of a computer program. [SEMATECH] 3 v : to load a segment of a computer program into a storage area occupied by parts of the program not currently needed. [SEMATECH] 4 n : the precision with which successive masks can be aligned with previous patterns on a silicon wafer. [1994 National Technology Roadmap for Semiconductors] 5 n : in semiconductor wafers and flat panel display substrates, a vector quantity defined at every point on the wafer or substrate. It is the difference, O, between the vector position,
 , of a substrate geometry, and the vector position of the corresponding point,  , in an overlaying pattern, which may consist of photoresist. [Adapted from SEMI P18-92 and D8-94]
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