Editing Photolithography (section)

=== Contact and proximity ===
{{Main|Contact lithography}}

A contact aligner, the simplest exposure system, puts a photomask in direct contact with the wafer<ref name="chiphistory.org">{{cite web | url=https://www.chiphistory.org/90-canon-pla-501f-fa-proximity-aligner | title=Canon PLA 501F/FA Proximity Aligner }}</ref> and exposes it to a uniform light. A proximity aligner puts a small gap of around 5 microns between the photomask and wafer.<ref name="chiphistory.org"/> In both cases, the mask covers the entire wafer, and simultaneously patterns every die.

Contact printing/lithography is liable to damage both the mask and the wafer,<ref name="chiphistory.org"/> and this was the primary reason it was abandoned for high volume production. Both contact and proximity lithography require the light intensity to be uniform across an entire wafer, and the mask to align precisely to features already on the wafer. As modern processes use increasingly large wafers, these conditions become increasingly difficult.

Research and prototyping processes often use contact or proximity lithography, because it uses inexpensive hardware and can achieve high optical resolution. The resolution in proximity lithography is approximately the square root of the product of the wavelength and the gap distance. Hence, except for projection lithography (see below), contact printing offers the best resolution, because its gap distance is approximately zero (neglecting the thickness of the photoresist itself). In addition, [[nanoimprint lithography]] may revive interest in this familiar technique, especially since the cost of ownership is expected to be low; however, the shortcomings of contact printing discussed above remain as challenges.