What is a photomask?
- A glass plate coated with an opaque surface, which can be etched with highly detailed patterns.
- The most common type of photomasks are quartz glass plates with an opaque chrome layer.
- Less common types with opaque layers are iron oxide or photographic emulsion.
In the semiconductor industry most photomasks also have a thin chrome oxide layer, which gives the photomask better anti-reflective properties. Advanced photomasks, such as Phase Shift masks or EUV reticles have additional layers present.
How is a photomask used?
A good analogy is to compare a process step in the semiconductor industry with traditional photography (i.e. pre-digital).
The photomask is like the negative, which will be used in the stepper/scanner/aligner (cf. enlarger) to produce a new pattern on the wafer (cf. the photographic print).
The significant differences are that the photomask process is lithographic (i.e. only opaque or clear - not shades of grey) and the feature sizes are orders of magnitude smaller.
Download Basic Principles of the photolithography process for a detailed diagram of how a photomask is used.