Editing Photolithography (section)

== Light sources ==
[[Image:Lithography Wavelength vs Resolution.PNG|thumb|right|300px|One of the evolutionary paths of lithography has been the use of shorter wavelengths. It is worth noting that the same light source may be used for several technology generations.]]
Historically, photolithography has used ultraviolet light from [[gas-discharge lamp]]s using [[mercury (element)|mercury]], sometimes in combination with [[noble gas]]es such as [[xenon]]. These lamps produce light across a broad spectrum with several strong peaks in the ultraviolet range. This spectrum is filtered to select a single [[spectral line]]. From the early 1960s through the mid-1980s, Hg lamps had been used in lithography for their spectral lines at 436&nbsp;nm ("g-line"), 405&nbsp;nm ("h-line") and 365&nbsp;nm ("i-line"). However, with the semiconductor industry's need for both higher resolution (to produce denser and faster chips) and higher throughput (for lower costs), lamp-based lithography tools were no longer able to meet the industry's high-end requirements.

This challenge was overcome in 1982 when [[excimer laser]] lithography was proposed and demonstrated at IBM by Kanti Jain.<ref>Jain, K. ''"Excimer Laser Lithography"'', SPIE Press, Bellingham, WA, 1990.</ref><ref>Jain, K. et al., "Ultrafast deep-UV lithography with excimer lasers", IEEE Electron Device Lett., Vol. EDL-3, 53 (1982): https://ieeexplore.ieee.org/document/1482581/;jsessionid=66FBB98827D6C47335DB6E9D31D6000E?arnumber=1482581</ref><ref>Lin, B. J., ''"Optical Lithography"'', SPIE Press, Bellingham, WA, 2009, p. 136.</ref><ref>Basting, D., et al., "Historical Review of Excimer Laser Development," in ''"Excimer Laser Technology"'', D. Basting and G. Marowsky, Eds., Springer, 2005.</ref> Excimer laser lithography machines (steppers and scanners) became the primary tools in microelectronics production, and has enabled minimum features sizes in chip manufacturing to shrink from 800 nanometers in 1990 to 7 nanometers in 2018.<ref name=Samsung10nm>{{cite web|title=Samsung Starts Industry's First Mass Production of System-on-Chip with 10-Nanometer FinFET Technology|url=https://news.samsung.com/global/samsung-starts-industrys-first-mass-production-of-system-on-chip-with-10-nanometer-finfet-technology|date=October 17, 2016}}</ref><ref name=TSMC7nm>{{cite web |url=https://www.anandtech.com/show/12677/tsmc-kicks-off-volume-production-of-7nm-chips|title=TSMC Kicks Off Volume Production of 7nm Chips|publisher=AnandTech|date=2018-04-28|access-date=2018-10-20}}</ref> From an even broader scientific and technological perspective, in the 50-year history of the laser since its first demonstration in 1960, the invention and development of excimer laser lithography has been recognized as a major milestone.<ref>American Physical Society / Lasers / History / Timeline; http://www.laserfest.org/lasers/history/timeline.cfm</ref><ref>SPIE / Advancing the Laser / 50 Years and into the Future; http://spie.org/Documents/AboutSPIE/SPIE%20Laser%20Luminaries.pdf</ref><ref>U.K. Engineering & Physical Sciences Research Council / Lasers in Our Lives / 50 Years of Impact; {{cite web |title=Lasers in our lives: 50 years of impact |url=http://www.stfc.ac.uk/Resources/PDF/Lasers50_final1.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110913160302/http://www.stfc.ac.uk/Resources/PDF/Lasers50_final1.pdf |archive-date=2011-09-13 |access-date=2011-08-22}}</ref>

The commonly used deep ultraviolet excimer lasers in lithography systems are the [[krypton fluoride]] (KrF) laser at 248&nbsp;nm wavelength and the [[argon fluoride laser]] (ArF) at 193&nbsp;nm wavelength. The primary manufacturers of excimer laser light sources in the 1980s were Lambda Physik (now part of Coherent, Inc.) and Lumonics. Since the mid-1990s Cymer Inc. has become the dominant supplier of excimer laser sources to the lithography equipment manufacturers, with [[Gigaphoton Inc.]] as their closest rival. Generally, an excimer laser is designed to operate with a specific gas mixture; therefore, changing wavelength is not a trivial matter, as the method of generating the new wavelength is completely different, and the absorption characteristics of materials change. For example, air begins to absorb significantly around the 193&nbsp;nm wavelength; moving to sub-193&nbsp;nm wavelengths would require installing vacuum pump and purge equipment on the lithography tools (a significant challenge). An inert gas atmosphere can sometimes be used as a substitute for a vacuum, to avoid the need for hard plumbing.  Furthermore, insulating materials such as [[silicon dioxide]], when exposed to photons with energy greater than the band gap, release free electrons and holes which subsequently cause adverse charging.

Optical lithography has been extended to feature sizes below 50&nbsp;nm using the 193&nbsp;nm ArF excimer laser and liquid immersion techniques. Also termed [[immersion lithography]], this enables the use of optics with numerical apertures exceeding 1.0. The liquid used is typically ultra-pure, deionised water, which provides for a [[refractive index]] above that of the usual air gap between the lens and the wafer surface. The water is continually circulated to eliminate thermally-induced distortions. Water will only allow ''NA'''s of up to ~1.4, but fluids with higher [[refractive indices]] would allow the effective ''NA'' to be increased further.

[[Image:Photon Energy vs Resolution.PNG|thumb|left|300px|Changing the lithography wavelength is significantly limited by absorption. Air absorbs below {{circa|185&nbsp;nm}}.]]

Experimental tools using the 157&nbsp;nm wavelength from the F2 excimer laser in a manner similar to current exposure systems have been built. These were once targeted to succeed 193&nbsp;nm lithography at the 65&nbsp;nm feature size node but have now all but been eliminated by the introduction of immersion lithography. This was due to persistent technical problems with the 157&nbsp;nm technology and economic considerations that provided strong incentives for the continued use of 193&nbsp;nm excimer laser lithography technology. High-index immersion lithography is the newest extension of 193&nbsp;nm lithography to be considered. In 2006, features less than 30&nbsp;nm were demonstrated by IBM using this technique.<ref>{{cite web | first=Aaron | last=Hand | title=High-Index Lenses Push Immersion Beyond 32 nm | url=http://www.reed-electronics.com/semiconductor/article/CA6319061 | url-status=dead | archive-url=https://web.archive.org/web/20150929113253/http://www.reed-electronics.com/semiconductor/article/CA6319061 | archive-date=2015-09-29 }}</ref> These systems used CaF<sub>2</sub> calcium fluoride lenses.<ref>{{cite web | url=https://www.laserfocusworld.com/optics/article/16548707/microelectronics-processing-lithography-at-157-nm-gains-momentum | title=MICROELECTRONICS PROCESSING - Lithography at 157 nm gains momentum | date=August 1999 }}</ref><ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S0167931703000571 | doi=10.1016/S0167-9317(03)00057-1 | title=157-nm lithography with high numerical aperture lens for sub-70 nm node | date=2003 | last1=Itani | first1=Toshiro | last2=Wakamiya | first2=Wataru | last3=Cashmore | first3=Julian | last4=Gower | first4=Malcolm | journal=Microelectronic Engineering | volume=67-68 | pages=39–46 }}</ref> Immersion lithography at 157&nbsp;nm was explored.<ref>{{cite journal | url=https://pubs.aip.org/avs/jvb/article-abstract/19/6/2353/1073785/Immersion-lithography-at-157-nm?redirectedFrom=fulltext | doi=10.1116/1.1412895 | title=Immersion lithography at 157 nm | date=2001 | last1=Switkes | first1=M. | last2=Rothschild | first2=M. | journal=Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena | volume=19 | issue=6 | pages=2353–2356 | bibcode=2001JVSTB..19.2353S }}</ref>

UV excimer lasers have been demonstrated to about 126&nbsp;nm (for Ar<sub>2</sub>*). Mercury arc lamps are designed to maintain a steady DC current of 50 to 150 Volts, however excimer lasers have a higher resolution. Excimer lasers are gas-based light systems that are usually filled with inert and halide gases (Kr, Ar, Xe, F and Cl) that are charged by an electric field. The higher the frequency, the greater the resolution of the image. KrF lasers are able to function at a frequency of 4&nbsp;kHz . In addition to running at a higher frequency, excimer lasers are compatible with more advanced machines than mercury arc lamps are. They are also able to operate from greater distances (up to 25 meters) and are able to maintain their accuracy with a series of mirrors and antireflective-coated lenses. By setting up multiple lasers and mirrors, the amount of energy loss is minimized, also since the lenses are coated with antireflective material, the light intensity remains relatively the same from when it left the laser to when it hits the wafer.<ref>{{cite web | first=Matteo | last=Martini | title=Light Sources Used in Photolithography | url=http://www.martini-tech.com/light-sources-used-photolithography | access-date=2014-10-28 | archive-url=https://web.archive.org/web/20141029031238/http://www.martini-tech.com/light-sources-used-photolithography/ | archive-date=2014-10-29 | url-status=dead }}</ref>

Lasers have been used to indirectly generate non-coherent extreme UV (EUV) light at 13.5&nbsp;nm for [[extreme ultraviolet lithography]]. The EUV light is not emitted by the laser, but rather by a tin or xenon plasma which is excited by an excimer or {{CO2}} laser.<ref>{{Cite web|url=https://www.laserfocusworld.com/blogs/article/14039015/how-does-the-laser-technology-in-euv-lithography-work|title=StackPath|date=29 August 2019 }}</ref> This technique does not require a synchrotron, and EUV sources, as noted, do not produce coherent light. However vacuum systems and a number of novel technologies (including much higher EUV energies than are now produced) are needed to work with UV at the edge of the X-ray spectrum (which begins at 10&nbsp;nm). As of 2020, EUV is in mass production use by leading edge foundries such as TSMC and Samsung.

Theoretically, an alternative light source for photolithography, especially if and when wavelengths continue to decrease to extreme UV or X-ray, is the [[free-electron laser]] (or one might say xaser for an X-ray device). Free-electron lasers can produce high quality beams at arbitrary wavelengths.

Visible and infrared femtosecond lasers were also applied for lithography. In that case photochemical reactions are initiated by multiphoton absorption. Usage of these light sources have a lot of benefits, including possibility to manufacture true 3D objects and process non-photosensitized (pure) glass-like materials with superb optical resiliency.<ref>{{Cite journal|last1=Jonušauskas|first1=Linas|last2=Gailevičius|first2=Darius|last3=Mikoliūnaitė|first3=Lina|last4=Sakalauskas|first4=Danas|last5=Šakirzanovas|first5=Simas|last6=Juodkazis|first6=Saulius|last7=Malinauskas|first7=Mangirdas|date=2017-01-02|title=Optically Clear and Resilient Free-Form μ-Optics 3D-Printed via Ultrafast Laser Lithography|journal=Materials|language=en|volume=10|issue=1|pages=12|doi=10.3390/ma10010012|pmid=28772389|pmc=5344581|bibcode=2017Mate...10...12J|doi-access=free}}</ref>