Editing Photolithography (section)

== Exposure ("printing") systems ==
[[Image:Wafertraksystem.jpg|thumb|200px|The wafer track portion of an [[Aligner (semiconductor)|aligner]] that uses 365 nm ultraviolet light]]
Exposure systems typically produce an image on the wafer using a [[photomask]]. The photomask blocks light in some areas and lets it pass in others.  ([[Maskless lithography]] projects a precise beam directly onto the wafer without using a mask, but it is not widely used in commercial processes.)  Exposure systems may be classified by the optics that transfer the image from the mask to the wafer.

Photolithography produces better thin film transistor structures than [[printed electronics]], due to smoother printed layers, less wavy patterns, and more accurate drain-source electrode registration.<ref name="Noh et al 2015">{{cite journal|last1=Noh|first1=Jinsoo|last2=Jung|first2=Minhoon|last3=Jung|first3=Younsu|last4=Yeom|first4=Chisun|last5=Pyo|first5=Myoungho|last6=Cho|first6=Gyoujin|title=Key Issues With Printed Flexible Thin Film Transistors and Their Application in Disposable RF Sensors|journal=Proceedings of the IEEE|date=April 2015|volume=103|issue=4|pages=554–566|issn=0018-9219|doi=10.1109/JPROC.2015.2410303|doi-access=free}}</ref>

=== 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.

=== Projection ===
{{See also|Stepper}}

[[Very-large-scale integration]] (VLSI) lithography uses projection systems. Unlike contact or proximity masks, which cover an entire wafer, projection masks (known as "reticles") show only one die or an array of dies (known as a "field") in a portion of the wafer at a time. Projection exposure systems (steppers or scanners) project the mask onto the wafer many times, changing the position of the wafer with every projection, to create the complete pattern, fully patterning the wafer. The difference between steppers and scanners is that, during exposure, a scanner moves the photomask and the wafer simultaneously, while a stepper only moves the wafer.  Contact, proximity and projection [[Aligner (semiconductor)|Mask aligners]] preceded steppers<ref>{{Cite web |title=GCA Mann 4800 Direct Step on Wafer |url=http://www.chiphistory.org/99-gca-mann-4800-direct-step-on-wafer-system |access-date=2023-12-30 |website=Chip History |language=en-us}}</ref>
<ref>{{cite web | url=https://www.chiphistory.org/154-perkin-elmer-micralign-projection-mask-alignment-system | title=Perkin-Elmer Series 100 Projection Scanning Aligners }}</ref> and do not move the photomask nor the wafer during exposure and use masks that cover the entire wafer. [[Immersion lithography]] scanners use a layer of [[Ultrapure water]] between the lens and the wafer to increase resolution. An alternative to photolithography is [[nanoimprint lithography]]. The maximum size of the image that can be projected onto a wafer is known as the reticle limit.