Excimer laser
An excimer laser, sometimes more correctly called an exciplex laser, is a form of
Since the 1960s, excimer lasers have been widely used in high-resolution
Terminology and history
The term
Excimer laser was proposed in 1960 by
A later improvement was the use of
Construction and operation
An excimer laser
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]
Wavelength determination
The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet range of electromagnetic radiation:
Excimer | Wavelength | Relative power |
---|---|---|
Ar2* | 126 nm | |
Kr2* | 146 nm | |
F2* | 157 nm | |
Xe2* | 172 & 175 nm | |
ArF | 193 nm | 60 |
KrCl | 222 nm | 25 |
KrF | 248 nm | 100 |
XeBr | 282 nm | |
XeCl | 308 nm | 50 |
XeF | 351 nm | 45 |
Excimer lasers, such as XeF and KrF, can also be made slightly tunable using a variety of prism and grating intracavity arrangements.[17]
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.[18] In contrast, discharge-pumped excimer lasers, also first demonstrated at the Naval Research Laboratory, are able to output a steady stream of pulses.[19][20] 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[21] 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.[22]
Major applications
Photolithography
Since the 1960s the most widespread industrial application of excimer lasers has been in deep-ultraviolet
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"[23][25][24][30]), 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.[31] 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.[32]
Fusion
The
Medical uses
The ultraviolet light from an excimer laser is well absorbed by
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]
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.[37]
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
See also
- Beam homogenizer
- Electrolaser
- Excimer lamp
- Krypton fluoride laser
- Moore's law
- Nitrogen laser
- Photolithography
References
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- ^ Basting, D. et al., History and future prospects of excimer laser technology, 2nd International Symposium on Laser Precision Microfabrication, pages 14–22.
- ^ 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.
- ^ 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.
- ^ Ault, E. R. et al. (1975), High-power xenon fluoride laser, Applied Physics Letters 27, p. 413.
- ^ Searles, S. K. and Hart, G. A., (1975), Stimulated emission at 281.8 nm from XeBr, Applied Physics Letters 27, p. 243.
- ^ "High Efficiency Microwave Discharge XeCl Laser", C. P. Christensen, R. W. Waynant and B. J. Feldman, Appl. Phys. Lett. 46, 321 (1985).
- ^ 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,
- ^ A Comprehensive Study of Excimer Lasers, Robert R. Butcher, MSEE Thesis, 1975
- ^ Basting, D. and Marowsky, G., Eds., Excimer Laser Technology, Springer, 2005.
- ^ F. J. Duarte (Ed.), Tunable Lasers Handbook (Academic, New York, 1995) Chapter 3.
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- ^ Burnham, R. and Djeu, N. (1976), Ultraviolet-preionized discharge-pumped lasers in XeF, KrF, and ArF, Applied Physics Letters 29, p.707.
- ^ 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).
- ^ Personal notes of Robert Butcher, Laser Engineer at XMR, Inc.
- ^ Appl. Phys. Lett. 36, 943 (1980);Lifetime extension of XeCl and KrCl lasers with additives,
- ^ a b c 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
- ^ a b c Jain, K. "Excimer Laser Lithography", SPIE Press, Bellingham, WA, 1990.
- ^ a b 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
- ^ Basting, D., et al., "Historical Review of Excimer Laser Development," in Excimer Laser Technology, D. Basting and G. Marowsky, Eds., Springer, 2005.
- ^ American Physical Society / Lasers / History / Timeline: http://www.laserfest.org/lasers/history/timeline.cfm
- ^ SPIE / Advancing the Laser / 50 Years and into the Future (PDF) (Report). Jan 6, 2010.
- ^ 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.
{{cite web}}
: CS1 maint: archived copy as title (link) - ^ Lin, B. J., "Optical Lithography", SPIE Press, Bellingham, WA, 2009, p. 136.
- ^ La Fontaine, B., "Lasers and Moore's Law", SPIE Professional, Oct. 2010, p. 20. http://spie.org/x42152.xml
- ^ "Samsung 5 nm and 4 nm Update". WikiChip Fuse. 19 October 2019. Retrieved 29 October 2021.
- ^ Obenschain, Stephen, et al. "High-energy krypton fluoride lasers for inertial fusion." Applied optics 54.31 (2015): F103-F122.
- ^ US 4784135, "Far ultraviolet surgical and dental procedures", issued 1988-10-15
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