Aligner (semiconductor)

An aligner, or mask aligner, is a system that produces integrated circuits (IC) using the photolithography process. It holds the photomask over the silicon wafer while a bright light is shone through the mask and onto the photoresist. The "alignment" refers to the ability to place the mask over precisely the same location repeatedly as the chip goes through multiple rounds of lithography.

Aligners were a major part of IC manufacture from the 1960s into the late 1970s, when they began to be replaced by the stepper.^[1][2] Currently, mask aligners are still used in academia and research, as projects often involve devices made using photolithography in smaller batches.^[3] In a mask aligner, there is a one-to-one correspondence between the mask pattern and the wafer pattern. The mask covers the entire surface of the wafer which is exposed in its entirety in one shot. This was the standard for the 1:1 mask aligners that were succeeded by steppers and scanners with reduction optics.^[4]

There are several distinct generations of aligner technology. The early contact aligners placed the mask in direct contact with the top surface of the wafer, which often damaged the pattern when the mask was lifted off again. Used only briefly, proximity aligners held the mask slightly above the surface to avoid this problem, but were difficult to work with and required considerable manual adjustment. Finally, the Micralign projection aligner, introduced by Perkin-Elmer in 1973, held the mask entirely separate from the chip and made the adjustment of the image much simpler.^[1][2] Through these stages of development, yields improved from perhaps 10% to about 70%, leading to a corresponding reduction in chip prices.^[1][2]