Excimer laser

File:Electra Laser Generates 90K Shots.webm

The term excimer is short for 'excited dimer', while 'exciplex' is short for 'excited complex'. Most excimer lasers are of the noble gas halide type, for which the term excimer is, strictly speaking, a misnomer. (Although less commonly used, the proper term for such is an exciplex laser.)

Excimer laser was proposed in 1960 by Fritz Houtermans.{{cite journal | author=F.G. Houtermans | title=Über Massen-Wirkung im optischen Spektralgebiet un die Möglichkeit absolut negativer Absorption für einige Fälle von Molekülspektren (Licht-Lawine) | year=1960 | journal=Helvetica Physica Acta| volume=33 | page=939}} The excimer laser development started with the observation of a nascent spectral line narrowing at 176 nm  reported in 1971{{Cite journal|last1=Basov|first1=N G|last2=Danilychev|first2=V A|last3=Popov|first3=Yurii M|date=1971-01-31|title=Stimulated emission in the vacuum ultraviolet region|url=http://dx.doi.org/10.1070/qe1971v001n01abeh003011|journal=Soviet Journal of Quantum Electronics|volume=1|issue=1|pages=18–22|doi=10.1070/qe1971v001n01abeh003011|bibcode=1971QuEle...1...18B|issn=0049-1748|url-access=subscription}} by Nikolai Basov, V. A. Danilychev and Yu. M. Popov, at the Lebedev Physical Institute in Moscow, using liquid xenon dimer (Xe₂) excited by an electron beam. Spurred by this report, H.A. Koehler et al. presented a better substantiation of stimulated emission in 1972,{{Cite journal|last1=Koehler|first1=H.A.|last2=Ferderber|first2=L.J.|last3=Redhead|first3=D.L.|last4=Ebert|first4=P.J.|date=September 1972|title=Stimulated VUV emission in high-pressure xenon excited by high-current relativistic electron beams|url=http://dx.doi.org/10.1063/1.1654342|journal=Applied Physics Letters|volume=21|issue=5|pages=198–200|doi=10.1063/1.1654342|bibcode=1972ApPhL..21..198K|issn=0003-6951|url-access=subscription}} using high pressure xenon gas. Definitive evidence of a xenon excimer laser action at 173 nm using a high pressure gas at 12 atmospheres, also pumped by an electron beam, was first presented in March 1973, by Mani Lal Bhaumik of Northrop Corporation, Los Angeles. Strong stimulated emission was observed as the laser's spectral line narrowed from a continuum of 15 nm to just 0.25 nm, and the intensity increased a thousand-fold. The laser's estimated output of 1 joule was high enough to evaporate part of the mirror coatings, which imprinted its mode pattern. This presentation established the credible potential of developing high power lasers at short wavelengths.{{Cite journal|last1=Ault|first1=E.|last2=Bhaumik|first2=M.|last3=Hughes|first3=W.|last4=Jensen|first4=R.|last5=Robinson|first5=C.|last6=Kolb|first6=A.|last7=Shannon|first7=J.|date=March 1973|title=Xe Laser Operation at 1730 Ǻ|url=https://doi.org/10.1364/JOSA.63.000907|journal=Journal of the Optical Society of America|volume=63|issue=7|pages=907|doi=10.1364/JOSA.63.000907|issn=|url-access=subscription}}{{Cite journal|date=May 1973|title=The News in Focus|url=|journal=Laser Focus|volume=9|issue=5|pages=10–14}}{{Cite journal|last1=Ault|first1=E.|last2=Bhaumik|first2=M.|last3=Hughes|first3=W.|last4=Jensen|first4=R.|last5=Robinson|first5=C.|last6=Kolb|first6=A.|last7=Shannon|first7=J.|date=March 1973|title=Xenon molecular laser in the vacuum ultraviolet|url=http://dx.doi.org/10.1109/jqe.1973.1077396|journal=IEEE Journal of Quantum Electronics|volume=9|issue=10|pages=1031–1032|doi=10.1109/jqe.1973.1077396|bibcode=1973IJQE....9.1031A|issn=0018-9197|url-access=subscription}}

A later improvement was the use of noble gas halides (originally Xe Br) developed by many groups in 1975.{{cite book | doi=10.1117/12.456812 | chapter=History and future prospects of excimer lasers | title=Second International Symposium on Laser Precision Microfabrication | date=2002 | last1=Basting | first1=Dirk | last2=Pippert | first2=Klaus D. | last3=Stamm | first3=Uwe | volume=4426 | page=25 | editor-first1=Isamu | editor-first2=Yong Feng | editor-first3=Koji | editor-first4=Jan J. | editor-last1=Miyamoto | editor-last2=Lu | editor-last3=Sugioka | editor-last4=Dubowski }} These groups include the Avco Everett Research Laboratory,{{cite journal | doi=10.1063/1.88473 | title=Laser action on the ²Σ⁺_1/2→²Σ⁺_1/2 bands of KRF and XeCl | date=1975 | last1=Ewing | first1=J. J. | last2=Brau | first2=C. A. | journal=Applied Physics Letters | volume=27 | issue=6 | pages=350–352 | bibcode=1975ApPhL..27..350E }} Sandia Laboratories,{{cite journal | doi=10.1016/0030-4018(75)90281-3 | title=100 MW, 248.4 nm, KRF laser excited by an electron beam | date=1975 | last1=Tisone | first1=G.C. | last2=Hays | first2=A.K. | last3=Hoffman | first3=J.M. | journal=Optics Communications | volume=15 | issue=2 | pages=188–189 | bibcode=1975OptCo..15..188T }} the Northrop Research and Technology Center,{{cite journal | doi=10.1063/1.88496 | title=High-power xenon fluoride laser | date=1975 | last1=Ault | first1=E. R. | last2=Bradford | first2=R. S. | last3=Bhaumik | first3=M. L. | journal=Applied Physics Letters | volume=27 | issue=7 | pages=413–415 | bibcode=1975ApPhL..27..413A }} the United States Government's Naval Research Laboratory,{{cite journal | doi=10.1063/1.88409 | title=Stimulated emission at 281.8 nm from XeBr | date=1975 | last1=Searles | first1=S. K. | last2=Hart | first2=G. A. | journal=Applied Physics Letters | volume=27 | issue=4 | pages=243–245 | bibcode=1975ApPhL..27..243S }} which also developed a XeCl Laser{{cite journal | doi=10.1063/1.95617 | title=High efficiency microwave discharge XeCl laser | date=1985 | last1=Christensen | first1=C. P. | last2=Waynant | first2=R. W. | last3=Feldman | first3=B. J. | journal=Applied Physics Letters | volume=46 | issue=4 | pages=321–323 | bibcode=1985ApPhL..46..321C }} that was excited using a microwave discharge,Microwave discharge resulted in much smaller footprint, very high pulse repetition rate excimer laser, commercialized under U. S. Patent 4,796,271 by Potomac Photonics, Inc, and Los Alamos National Laboratory.{{cite thesis |last=Butcher |first=Rober R. |year=1975 |title=A Comprehensive Study of Excimer Lasers, Robert R. Butcher |type=MSc Thesis |publisher=University of New Mexico |url=https://digitalrepository.unm.edu/ece_etds/544/}}

Image:Nike laser amplifier.jpg

An excimer laser typically uses a combination of a noble gas (argon, krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation and high pressure, a pseudo-molecule called an excimer (or in the case of noble gas halides, exciplex) is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range.{{GoldBookRef|title=excimer laser|file=E02243}}Basting, D. and Marowsky, G., Eds., Excimer Laser Technology, Springer, 2005.

Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (dissociative) ground state. Noble gases such as xenon and krypton are highly inert and do not usually form chemical compounds. However, when in an excited state (induced by electrical discharge or high-energy electron beams), they can form temporarily bound molecules with themselves (excimer) or with halogens (exciplex) such as fluorine and chlorine. The excited compound can release its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly repulsive ground state molecule which very quickly (on the order of a picosecond) dissociates back into two unbound atoms. This forms a population inversion.{{citation needed|date=January 2022}}

The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet range of electromagnetic radiation:

Excimer lasers, such as XeF and KrF, can also be made slightly tunable using a variety of prism and grating intracavity arrangements.F. J. Duarte (Ed.), Tunable Lasers Handbook (Academic, New York, 1995) Chapter 3.

File:Electra Laser System NRL 2013.png

While electron-beam pumped excimer lasers can produce high single energy pulses, they are generally separated by long time periods (many minutes).  An exception was the Electra system, designed for inertial fusion studies, which could produce a burst of 10 pulses each measuring 500 J over a span of 10 s.{{Cite journal|last1=Wolford|first1=M. F.|last2=Hegeler|first2=F.|last3=Myers|first3=M. C.|last4=Giuliani|first4=J. L.|last5=Sethian|first5=J. D.|year=2004|title=Electra: Repetitively pulsed, 500 J, 100 ns, KRF oscillator|journal=Applied Physics Letters|volume=84|issue=3|pages=326–328|bibcode=2004ApPhL..84..326W|doi=10.1063/1.1641513}} In contrast, discharge-pumped excimer lasers, also first demonstrated at the Naval Research Laboratory, are able to output a steady stream of pulses.{{cite journal | doi=10.1063/1.88934 | title=Ultraviolet-preionized discharge-pumped lasers in XeF, KRF, and ArF | date=1976 | last1=Burnham | first1=R. | last2=Djeu | first2=N. | journal=Applied Physics Letters | volume=29 | issue=11 | pages=707–709 | bibcode=1976ApPhL..29..707B }}Original device acquired by the National Museum of American History's Division of Information Technology and Society for the Electricity and Modern Physics Collection (Acquisition #1996.0343). Their significantly higher pulse repetition rates (of order 100 Hz) and smaller footprint made possible the bulk of the applications listed in the following section. A series of industrial lasers were developed at XMR, IncPersonal notes of Robert Butcher, Laser Engineer at XMR, Inc. in Santa Clara, California between 1980 and 1988. Most of the lasers produced were XeCl, and a sustained energy of 1 J per pulse at repetition rates of 300 pulses per second was the standard rating. This laser used a high power thyratron and magnetic switching with corona pre-ionization and was rated for 100 million pulses without major maintenance. The operating gas was a mixture of xenon, HCl, and Neon at approximately 5 atmospheres. Extensive use of stainless steel, nickel plating and solid nickel electrodes was incorporated to reduce corrosion due to the HCl gas. One major problem encountered was degradation of the optical windows due to carbon build-up on the surface of the CaF window. This was due to hydro-chloro-carbons formed from small amounts of carbon in O-rings reacting with the HCl gas. The hydro-chloro-carbons would slowly increase over time and absorbed the laser light, causing a slow reduction in laser energy. In addition these compounds would decompose in the intense laser beam and collect on the window, causing a further reduction in energy. Periodic replacement of laser gas and windows was required at considerable expense. This was significantly improved by use of a gas purification system consisting of a cold trap operating slightly above liquid nitrogen temperature and a metal bellows pump to recirculate the laser gas through the cold trap. The cold trap consisted of a liquid nitrogen reservoir and a heater to raise the temperature slightly, since at 77 K (liquid nitrogen boiling point) the xenon vapor pressure was lower than the required operating pressure in the laser gas mixture. HCl was frozen out in the cold trap, and additional HCl was added to maintain the proper gas ratio. An interesting side effect of this was a slow increase in laser energy over time, attributed to increase in hydrogen partial pressure in the gas mixture caused by slow reaction of chlorine with various metals. As the chlorine reacted, hydrogen was released, increasing the partial pressure. The net result was the same as adding hydrogen to the mixture to increase laser efficiency as reported by T.J. McKee et al.{{cite journal | doi=10.1063/1.91658 | title=Lifetime extension of XeCl and KRCL lasers with additives | date=1980 | last1=McKee | first1=T. J. | last2=James | first2=D. J. | last3=Nip | first3=W. S. | last4=Weeks | first4=R. W. | last5=Willis | first5=C. | journal=Applied Physics Letters | volume=36 | issue=12 | pages=943–945 | bibcode=1980ApPhL..36..943M }}

{{Main|Semiconductor device fabrication}}

Since the 1960s the most widespread industrial application of excimer lasers has been in deep-ultraviolet photolithography,{{cite journal | url=https://ieeexplore.ieee.org/document/1482581 | doi=10.1109/EDL.1982.25476 | bibcode=1982IEDL....3...53J | title=Ultrafast deep UV Lithography with excimer lasers | last1=Jain | first1=K. | last2=Willson | first2=C. G. | last3=Lin | first3=B. J. | journal=IEEE Electron Device Letters | date=1982 | volume=3 | issue=3 | page=53 | url-access=subscription }}Jain, K. "Excimer Laser Lithography", SPIE Press, Bellingham, WA, 1990. a critical technology used in the manufacturing of microelectronic devices. Historically, from the early 1960s through the mid-1980s, mercury-xenon lamps were used in lithography for their spectral lines at 436, 405 and 365 nm wavelengths. However, with the semiconductor industry's need for both higher resolution (to produce denser and faster chips) and higher throughput (for lower costs), the lamp-based lithography tools were no longer able to meet the industry's requirements. This challenge was overcome when in a pioneering development in 1982, deep-UV excimer laser lithography was proposed and demonstrated at IBM by Kanti Jain.{{cite journal | url=https://ieeexplore.ieee.org/document/1484194 | doi=10.1109/EDL.1984.25818 | bibcode=1984IEDL....5...24P | title=Deep UV exposure of Ag2Se/GeSe2utilizing an excimer laser | last1=Polasko | first1=K. J. | last2=Ehrlich | first2=D. J. | last3=Tsao | first3=J. Y. | last4=Pease | first4=R. F. W. | last5=Marinero | first5=E. E. | journal=IEEE Electron Device Letters | date=1984 | volume=5 | issue=1 | page=24 | url-access=subscription }}{{cite book | doi=10.1007/3-540-26667-4_2 | chapter=Historical Review of Excimer Laser Development | title=Excimer Laser Technology | date=2005 | last1=Basting | first1=D. | last2=Djeu | first2=N. | last3=Jain | first3=K. | pages=8–21 | isbn=3-540-20056-8 }} From an even broader scientific and technological perspective, since the invention of the laser in 1960, the development of excimer laser lithography has been highlighted as one of the major milestones in the history of the laser.American Physical Society / Lasers / History / Timeline: http://www.laserfest.org/lasers/history/timeline.cfm{{Cite report|url=http://spie.org/Documents/AboutSPIE/SPIE%20Laser%20Luminaries.pdf|title=SPIE / Advancing the Laser / 50 Years and into the Future|date=Jan 6, 2010}}U.K. Engineering & Physical Sciences Research Council / Lasers in Our Lives / 50 Years of Impact: {{cite web |url=http://www.stfc.ac.uk/Resources/PDF/Lasers50_final1.pdf |title=Archived copy |access-date=2011-08-22 |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 }}

Current lithography tools (as of 2021) mostly use deep ultraviolet (DUV) light from the KrF and ArF excimer lasers with wavelengths of 248 and 193 nanometers (called "excimer laser lithography"Lin, B. J., "Optical Lithography", SPIE Press, Bellingham, WA, 2009, p. 136.), which has enabled transistor feature sizes to shrink to 7 nanometers (see below). Excimer laser lithography has thus played a critical role in the continued advance of the so-called Moore's law for the last 25 years.La Fontaine, B., "Lasers and Moore's Law", SPIE Professional, Oct. 2010, p. 20. http://spie.org/x42152.xml By around 2020, extreme ultraviolet lithography (EUV) has started to replace excimer laser lithography to further improve the resolution of the semiconductor circuits lithography process.{{cite web |date=19 October 2019 |title=Samsung 5 nm and 4 nm Update |url=https://fuse.wikichip.org/news/2823/samsung-5-nm-and-4-nm-update/ |access-date=29 October 2021 |publisher=WikiChip Fuse}}

The Naval Research Laboratory built two systems, the Krypton fluoride laser (248 nm) and the Argon fluoride laser (193 nm) to test approaches to prove out Inertial Confinement Fusion approaches. These were the Electra and Nike laser systems. Because the excimer laser is a gas-based system, the laser does not heat up like solid-state systems such as National Ignition Facility and the Omega Laser. Electra demonstrated 90,000 shots in 10 hours; ideal for a Inertial fusion power plant.{{cite journal | doi=10.1364/AO.54.00F103 | title=High-energy krypton fluoride lasers for inertial fusion | date=2015 | last1=Obenschain | first1=Stephen | last2=Lehmberg | first2=Robert | last3=Kehne | first3=David | last4=Hegeler | first4=Frank | last5=Wolford | first5=Matthew | last6=Sethian | first6=John | last7=Weaver | first7=James | last8=Karasik | first8=Max | journal=Applied Optics | volume=54 | issue=31 | pages=F103-22 | pmid=26560597 | bibcode=2015ApOpt..54F.103O }}

The ultraviolet light from an excimer laser is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning. Thus excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the material which is left intact. These properties make excimer lasers well suited to precision micromachining organic material (including certain polymers and plastics), or delicate surgeries such as LASIK eye surgery. In 1980–1983, Rangaswamy Srinivasan, Samuel Blum and James J. Wynne at IBM's T. J. Watson Research Center observed the effect of the ultraviolet excimer laser on biological materials. Intrigued, they investigated further, finding that the laser made clean, precise cuts that would be ideal for delicate surgeries. This resulted in a fundamental patent{{Ref patent|country=US|number=4784135|title= Far ultraviolet surgical and dental procedures|gdate=1988-10-15}} and Srinivasan, Blum and Wynne were elected to the National Inventors Hall of Fame in 2002. In 2012, the team members were honored with National Medal of Technology and Innovation by the President Barack Obama for their work related to the excimer laser.{{cite web|title=IBM News Release|url=http://www-03.ibm.com/press/us/en/pressrelease/39829.wss|archive-url=https://web.archive.org/web/20121231063032/http://www-03.ibm.com/press/us/en/pressrelease/39829.wss|url-status=dead|archive-date=December 31, 2012|publisher=IBM|access-date=21 December 2012|date=2012-12-21}} Subsequent work introduced the excimer laser for use in angioplasty.{{cite journal | title = Far-ultraviolet laser ablation of atherosclerotic lesions |author1=R. Linsker |author2=R. Srinivasan |author3=J. J. Wynne |author4=D. R. Alonso | journal = Lasers Surg. Med. | volume=4 | issue=1 | pages=201–206 | year=1984 | doi = 10.1002/lsm.1900040212|pmid=6472033 |s2cid=12827770 }} Xenon chloride (308 nm) excimer lasers are also used to treat a variety of dermatological conditions including psoriasis, vitiligo, atopic dermatitis, alopecia areata and leukoderma.{{citation needed|date=August 2024}}

As light sources, excimer lasers are generally large in size, which is a disadvantage in their medical applications, although their sizes are rapidly decreasing with ongoing development.{{citation needed|date=January 2022}}

Research is being conducted to compare differences in safety and effectiveness outcomes between conventional excimer laser refractive surgery and wavefront-guided or wavefront-optimized refractive surgery, as wavefront methods may better correct for higher-order aberrations.{{cite journal |vauthors=Li SM, Kang MT, Zhou Y, Wang NL, Lindsley K |title= Wavefront excimer laser refractive surgery for adults with refractive errors |journal=Cochrane Database Syst Rev|volume=6 |issue= 6 |pages= CD012687 |date=2017 |doi= 10.1002/14651858.CD012687|pmc=6481747 }}

Excimer lasers are also widely used in numerous fields of scientific research, both as primary sources and, particularly the XeCl laser, as pump sources for tunable dye lasers, mainly to excite laser dyes emitting in the blue-green region of the spectrum.Duarte, F. J. and Hillman, L. W. (Eds.), Dye Laser Principles (Academic, New York, 1990) Chapter 6.Tallman, C. and Tennant, R., Large-scale excimer-laser-pumped dye lasers, in High Power Dye Lasers, Duarte, F. J. (Ed.) (Springer, Berlin, 1991) Chapter 4. These lasers are also commonly used in pulsed laser deposition systems, where their large fluence, short wavelength and non-continuous beam properties make them ideal for the ablation of a wide range of materials.Chrisey, D.B. and Hubler, G.K., Pulsed Laser Deposition of Thin Films (Wiley, 1994), {{ISBN|9780471592181}}, Chapter 2.

{{Scholia}}

• Beam homogenizer
• Electrolaser
• Excimer lamp
• Moore's law
• Nitrogen laser

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Category:LASIK