Excimer Laser
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An excimer laser, sometimes more correctly called an exciplex laser, is a form of
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiati ...
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
which is commonly used in the production of microelectronic devices,
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
based integrated circuits or "chips", eye surgery, and micromachining. Since 1960s excimer lasers are widely used in high-resolution
photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer (electroni ...
machines, one of the critical technologies required for microelectronic chip manufacturing.


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.) Excimer laser was proposed in 1960 by Fritz Houtermans. The excimer laser development started with the observation of a nascent spectral line narrowing at 176 nm  reported in 1971 by Nikolai Basov, V. A. Danilychev and Yu. M. Popov, at the Lebedev Physical Institute in
Moscow Moscow ( , US chiefly ; rus, links=no, Москва, r=Moskva, p=mɐskˈva, a=Москва.ogg) is the capital and largest city of Russia. The city stands on the Moskva River in Central Russia, with a population estimated at 13.0 million ...
, using liquid
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
dimer (Xe2) excited by an
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...
beam. Spurred by this report, H.A. Koehler et al. presented a better substantiation of stimulated emission in 1972, 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. A later improvement was the use of
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low che ...
halides (originally Xe Br) developed by many groups in 1975. These groups include the Avco Everett Research Laboratory, Sandia Laboratories, the Northrop Research and Technology Center, the United States Government's
Naval Research Laboratory The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps. It was founded in 1923 and conducts basic scientific research, applied research, technological ...
, which also developed a XeCl Laser that was excited using a microwave discharge, and Los Alamos National Laboratory.


Construction and operation

An excimer laser typically uses a combination of a
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low che ...
(
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as a ...
,
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
, or
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
) and a reactive gas ( fluorine or
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...
). Under the appropriate conditions of electrical stimulation and high pressure, a pseudo-
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
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 A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
light in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiati ...
range. Laser action in an excimer molecule occurs because it has a bound (associative)
excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers t ...
, but a repulsive (dissociative) ground state. Noble gases such as xenon and
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
are highly
inert Inert may refer to: * Chemically inert, not chemically reactive ** Inert gas ** Noble gas, historically called inert gas * Inert knowledge, information which one can express but not use * Inert waste, waste which is neither chemically nor biol ...
and do not usually form
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...
s. 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 Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...
. The excited compound can release its excess energy by undergoing
spontaneous Spontaneous may refer to: * Spontaneous abortion * Spontaneous bacterial peritonitis * Spontaneous combustion * Spontaneous declaration * Spontaneous emission * Spontaneous fission * Spontaneous generation * Spontaneous human combustion * Spontan ...
or stimulated emission, resulting in a strongly repulsive ground state molecule which very quickly (on the order of a
picosecond A picosecond (abbreviated as ps) is a unit of time in the International System of Units (SI) equal to 10−12 or (one trillionth) of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000&nbs ...
) dissociates back into two unbound atoms. This forms a population inversion.


Wavelength determination

The
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
of an excimer laser depends on the molecules used, and is usually in the ultraviolet range of
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...
: Excimer lasers, such as XeF and KrF, can also be made slightly ''tunable'' using a variety of prism and grating intracavity arrangements.


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. In contrast, discharge-pumped excimer lasers, also first demonstrated at the Naval Research Laboratory, are able to output a steady stream of pulses. 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 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.


Major applications


Photolithography

Since 1960s the most widespread industrial application of excimer lasers has been in deep-ultraviolet
photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer (electroni ...
,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=1482581Jain, 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 Kanti may refer to: * Kanti, Muzaffarpur, a town in Muzaffarpur district, Bihar, India **Kanti railway station Kanti railway station is a railway station on Muzaffarpur–Gorakhpur main line under the Samastipur railway division of East Centr ...
.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 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. 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"), 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 Moore's law is the observation that the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and projection of a historical trend. Rather than a law of physics, it is an empi ...
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 (EUL) has started to replace excimer laser lithography to further improve resolution of semiconductor circuits lithography process.


Fusion

The
Naval Research Laboratory The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps. It was founded in 1923 and conducts basic scientific research, applied research, technological ...
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 Inertial confinement fusion (ICF) is a fusion energy process that initiates nuclear fusion reactions by compressing and heating targets filled with thermonuclear fuel. In modern machines, the targets are small spherical pellets about the size o ...
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 The National Ignition Facility (NIF) is a laser-based inertial confinement fusion (ICF) research device, located at Lawrence Livermore National Laboratory in Livermore, California, United States. NIF's mission is to achieve fusion ignition wi ...
and the Omega Laser. Electra demonstrated 90,000 shots in 10 hours; ideal for a
Inertial fusion power plant Inertial Fusion Energy is a proposed approach to building a nuclear fusion power plant based on performing inertial confinement fusion at industrial scale. This approach to fusion power is still in a research phase. ICF first developed shortly af ...
.


Medical uses

The ultraviolet light from an excimer laser is well absorbed by biological matter and
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s. 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 Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft material for a ...
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
polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...
s and plastics), or delicate surgeries such as eye surgery LASIK. In 1980–1983,
Rangaswamy Srinivasan Rangaswamy Srinivasan (born February 28, 1929 in Madras, India) is a physical chemist and inventor with a 30-year career at IBM Research. He has developed techniques for ablative photodecomposition and used them to contribute to the development of ...
,
Samuel Blum Samuel E. Blum (August 28, 1920 – January 9, 2013) was an American chemist and physicist. He was a researcher at the Battelle Memorial Institute in Columbus, Ohio, working for government and private companies. He worked with semiconductor mate ...
and
James J. Wynne James J. Wynne (born March 19, 1943, in Brooklyn) is an American physicist at the IBM Watson Research Center in Yorktown Heights, NY. Wynne pioneered the use of excimer lasers for medical applications, most notably LASIK. He received the Nat ...
at IBM's
T. J. Watson Research Center The Thomas J. Watson Research Center is the headquarters for IBM Research. The center comprises three sites, with its main laboratory in Yorktown Heights, New York, U.S., 38 miles (61 km) north of New York City, Albany, New York and wit ...
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 and Srinivasan, Blum and Wynne were elected to the
National Inventors Hall of Fame The National Inventors Hall of Fame (NIHF) is an American not-for-profit organization, founded in 1973, which recognizes individual engineers and inventors who hold a U.S. patent of significant technology. Besides the Hall of Fame, it also oper ...
in 2002. In 2012, the team members were honored with National Medal of Technology and Innovation by the President
Barack Obama Barack Hussein Obama II ( ; born August 4, 1961) is an American politician who served as the 44th president of the United States from 2009 to 2017. A member of the Democratic Party (United States), Democratic Party, Obama was the first Af ...
for their work related to the excimer laser. Subsequent work introduced the excimer laser for use in angioplasty. 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 Refractive eye surgery is optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea ( keratomil ...
and wavefront-guided or wavefront-optimized refractive surgery, as wavefront methods may better correct for higher-order aberrations.


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 A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...
, mainly to excite laser dyes emitting in the blue-green region of the spectrum. These lasers are also commonly used in
pulsed laser deposition Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the t ...
systems, where their large
fluence In radiometry, radiant exposure or fluence is the radiant energy ''received'' by a ''surface'' per unit area, or equivalently the irradiance of a ''surface,'' integrated over time of irradiation, and spectral exposure is the radiant exposure per un ...
, 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), , Chapter 2.


See also

* Beam homogenizer * Electrolaser * Excimer lamp * Krypton fluoride laser *
Moore's law Moore's law is the observation that the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and projection of a historical trend. Rather than a law of physics, it is an empi ...
* Nitrogen laser *
Photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer (electroni ...


References

{{Lasers LASIK