Editing Photolithography (section)

=== Exposure and developing ===
After prebaking, the photoresist is exposed to a pattern of intense light. The exposure to light causes a chemical change that allows some of the photoresist to be removed by a special solution, called "developer" by analogy with [[photographic developer]].  Positive photoresist, the most common type, becomes soluble in the developer when exposed; with negative photoresist, unexposed regions are soluble in the developer.

A post-exposure bake (PEB) is performed before developing, typically to help reduce [[standing wave]] phenomena caused by the destructive and constructive [[Interference (wave propagation)|interference]] patterns of the incident light. In deep ultraviolet lithography, chemically amplified resist (CAR) chemistry is used. This resist is much more sensitive to PEB time, temperature, and delay, as the resist works by creating acid when it is hit by photons, and then undergoes an "exposure" reaction (creating acid, making the polymer soluble in the basic developer, and performing a chemical reaction catalyzed by acid) which mostly occurs in the PEB.<ref>{{cite web | first=Omkaram | last=Nalamasu | title=An Overview of Resist Processing for DUV Photolithography | url=https://www.jstage.jst.go.jp/article/photopolymer1988/4/3/4_3_299/_pdf | display-authors=etal }}</ref><ref>{{Cite web|url=https://semiengineering.com/euvs-new-problem-areas/|title=EUV's New Problem Areas|first=Mark|last=LaPedus|date=March 19, 2018|website=Semiconductor Engineering}}</ref>

The develop chemistry is delivered on a spinner, much like photoresist. Developers originally often contained [[sodium hydroxide]] (NaOH). However, [[sodium]] is considered an extremely undesirable contaminant in [[MOSFET]] fabrication because it degrades the [[Electrical insulation|insulating]] properties of gate oxides (specifically, sodium ions can migrate in and out of the gate, changing the threshold voltage of the transistor and making it harder or easier to turn the transistor on over time). Metal-ion-free developers such as [[tetramethylammonium hydroxide]] (TMAH) are now used. The temperature of the developer might be tightly controlled using jacketed (dual walled) hoses to within 0.2&nbsp;°C.<ref name="Levinson-2005"/> The nozzle that coats the wafer with developer may influence the amount of developer that is necessary.<ref>{{cite book | s2cid=54991701 | doi=10.2991/ICMRA-15.2015.256 | chapter=A summary of the current development of developing technology in the field of integrated circuit manufacturing | title=Proceedings of the 3rd International Conference on Mechatronics, Robotics and Automation | date=2015 | last1=Wang | first1=Han | last2=Ning | first2=Feng | last3=Xu | first3=Qiang | last4=Liu | first4=Xue-Ping | volume=15 | isbn=978-94-62520-76-9 }}</ref><ref name="Handbook of VLSI Microlithography"/>

The resulting wafer is then "hard-baked" if a non-chemically amplified resist was used, typically at 120 to 180&nbsp;°C<ref>{{Cite web|url=https://cores.research.asu.edu/nanofabrication-and-cleanroom/techniques-lithography|title=Techniques - lithography {{!}} Core Facilities|website=cores.research.asu.edu|access-date=2020-02-04}}</ref> for 20 to 30 minutes. The hard bake solidifies the remaining photoresist, to make a more durable protecting layer in future [[ion implantation]], [[Chemical milling|wet chemical etching]], or [[plasma etching]].

From preparation until this step, the photolithography procedure has been carried out by two machines: the photolithography stepper or scanner, and the coater/developer. The two machines are usually installed side by side, and are "linked" together.<ref>{{cite book | chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/6154/61544L/Wafer-management-between-coatdeveloper-track-and-immersion-lithography-tool/10.1117/12.656303.short | doi=10.1117/12.656303 | chapter=Wafer management between coat/Developer track and immersion lithography tool | title=Optical Microlithography XIX | date=2006 | editor-last1=Flagello | editor-first1=Donis G. | last1=Fujiwara | first1=Tomoharu | last2=Shiraishi | first2=Kenichi | last3=Tanizaki | first3=Hirokazu | last4=Ishii | first4=Yuuki | last5=Kyoda | first5=Hideharu | last6=Yamamoto | first6=Taro | last7=Ishida | first7=Seiki | volume=6154 | pages=1553–1562 | s2cid=110508653 }}</ref><ref name="Wakamizu-2008"/><ref>{{cite book | url=https://books.google.com/books?id=Sx39H8XR1FcC&dq=photolithography+track+system&pg=PA2 | isbn=9780819475572 | title=Advanced Processes for 193-nm Immersion Lithography | date=2009 | publisher=SPIE Press }}</ref>