Editing Photomask (section)

== History ==
For IC production in the 1960s and early 1970s, an opaque [[rubylith]] film laminated onto a transparent [[BoPET|mylar]] sheet was used. The design of one layer was cut into the rubylith, initially by hand on an illuminated [[drafting table]] (later by machine ([[plotter]])) and the unwanted rubylith was peeled off by hand, forming the master image of that layer of the chip,  often called "artwork". Increasingly complex and thus larger chips required larger and larger rubyliths, eventually even filling the wall of a room, and artworks were to be photographically reduced to produce photomasks (Eventually this whole process was replaced by the [[optical pattern generator]] to produce the master image). At this point the master image could be arrayed into a multi-chip image called a '''''reticle'''''. The reticle was originally a 10X larger image of a single chip.

The reticle was, by step-and-repeater photolithography and etching, used to produce a photomask with an image size the same as the final chip. The photomask might be used directly in the fab or be used as a master-photomask to produce the final actual working photomasks.

As feature size shrank, the only way to properly focus the image was to place it in direct contact with the wafer. These [[Contact lithography|contact aligners]] often lifted some of the [[photoresist]] off the wafer and onto the photomask and it had to be cleaned or discarded. This drove the adoption of reverse master photomasks (see above), which were used to produce (with contact photolithography and etching) the needed many actual working photomasks. Later, projection photo-lithography meant photomask lifetime was indefinite. Still later direct-step-on-wafer ''stepper'' photo-lithography used reticles directly and ended the use of photomasks.

Photomask materials changed over time. Initially [[soda glass]]<ref>{{cite book | url=https://books.google.com/books?id=cvoR9vmDJIQC&dq=soda+glass+photomask&pg=PA97 | isbn=978-1-119-99189-2 | title=Introduction to Microfabrication | date=29 October 2010 | publisher=John Wiley & Sons }}</ref> was used with [[silver halide]] opacity. Later [[borosilicate]]<ref>{{cite book | url=https://books.google.com/books?id=I-bLBQAAQBAJ&dq=soda+glass+photomask&pg=PA11 | isbn=978-1-4200-2878-2 | title=Handbook of Photomask Manufacturing Technology | date=3 October 2018 | publisher=CRC Press }}</ref> and then [[fused silica]] to control expansion, and [[chromium]] which has better opacity to [[ultraviolet light]] were introduced. The original pattern generators have since been replaced by [[electron beam lithography]] and [[laser]]-driven mask writer or [[maskless lithography]] systems which generate reticles directly from the original computerized design.