Excimer laser

An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and micromachining.

An excimer laser

Terminology and history

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.)

The excimer laser was invented in 1970[1] by Nikolai Basov, V. A. Danilychev and Yu. M. Popov, at the Lebedev Physical Institute in Moscow, using a xenon dimer (Xe2) excited by an electron beam to give stimulated emission at 172 nm wavelength. A later improvement, developed by many groups in 1975[2] was the use of noble gas halides (originally XeBr). These groups include the Avco Everett Research Laboratory,[3] Sandia Laboratories,[4] the Northrop Research and Technology Center,[5] the United States Government's Naval Research Laboratory[6] who also developed a XeCl Laser[7] that was excited using a microwave discharge.[8] and Los Alamos National Laboratory.[9]

Construction and operation

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.[10][11]

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.

Wavelength determination

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

Excimer Wavelength Relative power
Ar2*126 nm
Kr2*146 nm
F2*157 nm
Xe2*172 & 175 nm
ArF193 nm60
KrCl222 nm25
KrF248 nm100
XeBr282 nm
XeCl308 nm50
XeF351 nm45

Excimer lasers, such as XeF and KrF, can also be made slightly tunable using a variety of prism and grating intracavity arrangements.[12]

Pulse repetition rate

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.[13] In contrast, discharge-pumped excimer lasers, also first demonstrated at the Naval Research Laboratory, are able to output a steady stream of pulses.[14][15] 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, Inc[16] in Santa Clara, California between 1980-1988. 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 stainlesss 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.[17]

Major applications

Photolithography

Excimer lasers are widely used in high-resolution photolithography machines, one of the critical technologies required for microelectronic chip manufacturing. Current state-of-the-art lithography tools use deep ultraviolet (DUV) light from the KrF and ArF excimer lasers with wavelengths of 248 and 193 nanometers (the dominant lithography technology today is thus also called "excimer laser lithography"[18][19][20][21]), 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.[22]

The most widespread industrial application of excimer lasers has been in deep-ultraviolet photolithography,[18][20] a critical technology used in the manufacturing of microelectronic devices (i.e., semiconductor integrated circuits or "chips"). Historically, from the early 1960s through the mid-1980s, mercury-xenon lamps had been 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.[18][19][20][23] With phenomenal advances made in equipment technology in the last two decades, and today microelectronic devices fabricated using excimer laser lithography totaling $400 billion in annual production, it is the semiconductor industry view[22] that excimer laser lithography has been a crucial factor in the continued advance of Moore's law, enabling minimum features sizes in chip manufacturing to shrink from 800 nanometers in 1990 to 7 nanometers in 2018.[24][25] 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 50-year history of the laser.[26][27][28]

Medical uses

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 eye surgery LASIK. 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[29] 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 of The United States Barack Obama for their work related to the excimer laser.[30] Subsequent work introduced the excimer laser for use in angioplasty.[31] Xenon chloride (308 nm) excimer lasers can also treat a variety of dermatological conditions including psoriasis, vitiligo, atopic dermatitis, alopecia areata and leukoderma.

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.

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.[32]

Scientific research

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.[33][34] 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.[35]

See also

References
  1. Basov, N. G. et al., Zh. Eksp. Fiz. i Tekh. Pis'ma. Red. 12, 473(1970).
  2. Basting, D. et al., History and future prospects of excimer laser technology, 2nd International Symposium on Laser Precision Microfabrication, pages 14–22.
  3. Ewing, J. J. and Brau, C. A. (1975), Laser action on the 2 Sigma+ 1/2--> 2 Sigma+ 1/2 bands of KrF and XeCl, Appl. Phys. Lett., vol. 27, no. 6, pages 350–352.
  4. Tisone, G. C. and Hays, A. K. and Hoffman, J. M. (1975), 100 MW, 248.4 nm, KrF laser excited by an electron beam, Optics Comm., vol. 15, no. 2, pages 188–189.
  5. Ault, E. R. et al. (1975), High-power xenon fluoride laser, Applied Physics Letters 27, p. 413.
  6. Searles, S. K. and Hart, G. A., (1975), Stimulated emission at 281.8 nm from XeBr, Applied Physics Letters 27, p. 243.
  7. "High Efficiency Microwave Discharge XeCl Laser", C. P. Christensen, R. W. Waynant and B. J. Feldman, Appl. Phys. Lett. 46, 321 (1985).
  8. 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,
  9. A Comprehensive Study of Excimer Lasers, Robert R. Butcher, MSEE Thesis, 1975
  10. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "excimer laser". doi:10.1351/goldbook.E02243
  11. Basting, D. and Marowsky, G., Eds., Excimer Laser Technology, Springer, 2005.
  12. F. J. Duarte (Ed.), Tunable Lasers Handbook (Academic, New York, 1995) Chapter 3.
  13. Wolford, M. F.; Hegeler, F.; Myers, M. C.; Giuliani, J. L.; Sethian, J. D. (2004). "Electra: Repetitively pulsed, 500 J, 100 ns, KRF oscillator". Applied Physics Letters. 84 (3): 326–328. Bibcode:2004ApPhL..84..326W. doi:10.1063/1.1641513.
  14. Burnham, R. and Djeu, N. (1976), Ultraviolet-preionized discharge-pumped lasers in XeF, KrF, and ArF, Applied Physics Letters 29, p.707.
  15. 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).
  16. Personal notes of Robert Butcher, Laser Engineer at XMR, Inc.
  17. Appl. Phys. Lett. 36, 943 (1980);Lifetime extension of XeCl and KrCl lasers with additives,
  18. Jain, K. et al., "Ultrafast deep-UV lithography with excimer lasers", IEEE Electron Device Lett., Vol. EDL-3, 53 (1982): http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1482581
  19. Polasko et al., "Deep UV exposure of Ag2Se/GeSe2utilizing an excimer laser", IEEE Electron Device Lett., Vol. 5, p. 24(1984): http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1484194&tag=1
  20. Jain, K. "Excimer Laser Lithography", SPIE Press, Bellingham, WA, 1990.
  21. Lin, B. J., "Optical Lithography", SPIE Press, Bellingham, WA, 2009, p. 136.
  22. La Fontaine, B., "Lasers and Moore's Law", SPIE Professional, Oct. 2010, p. 20. http://spie.org/x42152.xml
  23. Basting, D., et al., "Historical Review of Excimer Laser Development," in Excimer Laser Technology, D. Basting and G. Marowsky, Eds., Springer, 2005.
  24. Samsung Starts Industry's First Mass Production of System-on-Chip with 10-Nanometer FinFET Technology; https://news.samsung.com/global/samsung-starts-industrys-first-mass-production-of-system-on-chip-with-10-nanometer-finfet-technology
  25. "TSMC Kicks Off Volume Production of 7nm Chips". AnandTech. 2018-04-28. Retrieved 2018-10-20.
  26. American Physical Society / Lasers / History / Timeline: http://www.laserfest.org/lasers/history/timeline.cfm
  27. SPIE / Advancing the Laser / 50 Years and into the Future: http://spie.org/Documents/AboutSPIE/SPIE%20Laser%20Luminaries.pdf
  28. U.K. Engineering & Physical Sciences Research Council / Lasers in Our Lives / 50 Years of Impact: "Archived copy" (PDF). Archived from the original (PDF) on 2011-09-13. Retrieved 2011-08-22.CS1 maint: archived copy as title (link)
  29. US 4784135, "Far ultraviolet surgical and dental procedures", issued 1988-10-15
  30. "IBM News Release". IBM. 2012-12-21. Retrieved 21 December 2012.
  31. R. Linsker; R. Srinivasan; J. J. Wynne; D. R. Alonso (1984). "Far-ultraviolet laser ablation of atherosclerotic lesions". Lasers Surg. Med. 4 (1): 201–206. doi:10.1002/lsm.1900040212. PMID 6472033.
  32. Li SM, Kang MT, Zhou Y, Wang NL, Lindsley K (2017). "Wavefront excimer laser refractive surgery for adults with refractive errors". Cochrane Database Syst Rev. 6 (6): CD012687. doi:10.1002/14651858.CD012687. PMC 6481747.CS1 maint: uses authors parameter (link)
  33. Duarte, F. J. and Hillman, L. W. (Eds.), Dye Laser Principles (Academic, New York, 1990) Chapter 6.
  34. 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.
  35. Chrisey, D.B. and Hubler, G.K., Pulsed Laser Deposition of Thin Films (Wiley, 1994), ISBN 9780471592181, Chapter 2.
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