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Applying and exposing photoresist to create a device layer on an actual wafer is similar to the process used to create reticles. In fact, laser pattern generators and e-beams are actually used to do prototyping of wafers and one-offs by exposing wafer device layers. For actual production use though, a tool called stepper is used. |
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| A stepper exposes a photoresist coated wafer to single wavelength UV light passing through a reticle which contains the image of a single device layer. The term "stepper" comes from the "step-and-repeat" action of moving the wafer on its x and y axes to align the reticle with each individual device position. |
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| UV light is used because modern semiconductor device features are so small that the actual wavelength of the exposing light is a limiting factor. UV has a shorter wavelength (less than 500 nm) than visible light which allows the creation of smaller features. Even broadband (or multi-wavelength) UV is only capable of producing features down to about 2.0 µm. Today's semiconductors have features as small as 0.25 µm. This requires the use of single-wavelength UV. Typical wavelengths used are 436 nm (called G-Line), 405 nm (H-line), 365 nm (I-line) and 248 nm (called Deep UV). Semiconductor manufactuers are experimenting with 193 nm UV and also with x-ray (which has an even shorter wavelength, but presents personnel hazards in actual production.) |
Steppers are extremely high-precision devices and can cost upwards of $5,000,000 each.