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Photomasks are the master tooling used by chip manufacturers and other industries to optically transfer patterns onto wafers and other substrates. Photomask plates are made of high-purity quartz, borofloat or sodalime glass that has been polished to micron flat tolerances. These glass plates are uniformly coated on one side with a very thin layer of metal such as chrome or iron oxide. This thin layer of metal has a uniformed photosensitive compound called photoresist applied over the entire metal layer. The finished product is called a photomask blank.
This photomask blank is then exposed to high intensity light or electron beams that expose a precise pattern into the photoresist on the photomask blank. The pattern that is exposed on the photoresist has broken the chemical bonds in the photoresist where the pattern was exposed. This allows the photoresist that was expose to be developed and removed from the photomask blank.
After the photoresist has been removed the area under the removed photoresist is exposed metal on glass. This metal is then treated to a metal etch process that removes this exposed metal. The metal that is covered by the remaining photoresist is left unchanged. The remaining photoresist is then removed which leaves the unexposed metal on glass with the precise pattern etched out of the metal. This etched out pattern on glass becomes the finished master tooling called a photomask.
This master tooling photomask is then used in lithography tools such as printers or steppers. These tools project high intensity light through the etched out metal of the photomask and then through a high precision aperture lens. This etched out patterned image is exposed over an entire silicon wafer that has been coated with metal or other materials and covered with photoresist exactly like the photomask blank. The wafer is then developed and etched similarly to the photomask blank describe above.
A single wafer may use 1 to 70 photomasks to build an entire circuit or device. Each of these photomasks will have a unique pattern with features as small as 0.5 microns. Photomasks are truly the key to the cutting edge technologies that we expect today.
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