As used in photolithography, a photomask is typically an optically transparent fused quartz blank imprinted with a pattern defined with chrome metal. A complete set of photomasks, each defining a pattern layer in integrated circuit fabrication, is fed into a photolithography stepper or scanner and individually selected for exposure.
In photolithography for the mass production of integrated circuit devices, the more correct term is usually photoreticle or simply reticle. In the case of a photomask, there is a one-to-one correspondence between the mask pattern and the wafer pattern. This was the standard for the 1:1 mask aligners that were succeeded by steppers and scanners with reduction optics.
As used in steppers and scanners, the reticle only contains a portion of the full wafer pattern. This is projected and shrunk by four or five times onto the wafer surface. To achieve complete wafer coverage, the wafer is repeatedly 'stepped' from position to position under the optical column until full exposure is achieved.
Leading commercial photomask manufacturers, 2003
- Dai Nippon Printing Company , Japan, $499 mln (revenue)
- Photronics , USA, $349 mln
- DuPont Photomasks , USA, $326 mln
- Toppan Printing Company , Japan, $321 mln
In October, 2004, DuPont was acquired by Toppan, pending approval by US regulators.
Major chipmakers, such as Intel, IBM, NEC, TSMC, Samsung, etc., have their own, large maskmaking facilities.
The cost to set up a modern mask shop is $200-500 mln, a very high threshold for entering this market.