A krypton fluoride laser (KrF laser) is a particular type of

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 microm ...

, which is sometimes (more correctly) called an exciplex laser. With its 248 nanometer wavelength, it is a deep ultraviolet laser which is commonly used in the production of semiconductor

integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

s, industrial micromachining, and scientific research. The term

excimer An excimer (originally short for excited dimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has a valence shell completely filled with electrons (for example, noble gases). In this case, form ...

is short for 'excited dimer', while exciplex is short for 'excited complex'. An excimer laser typically contains a mixture of: a noble gas such as argon, krypton, or xenon; and a halogen gas such as fluorine or chlorine. Under suitably intense conditions of electromagnetic stimulation and pressure, the mixture emits a beam of coherent stimulated radiation as

laser light 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 ...

in the ultraviolet range. KrF and ArF excimer lasers are widely incorporated into 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, to protec ...

machines, one of the critical tools required for microelectronic chip manufacturing in nanometer dimensions. Excimer laser lithographyJain, K. "Excimer Laser Lithography", SPIE Press, Bellingham, WA, 1990. has enabled transistor feature sizes to shrink from 800 nanometers in 1990 to 10 nanometers in 2016.La Fontaine, B.
"Lasers and Moore's Law"
SPIE Professional, Oct. 2010, p. 20.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

Theory

A krypton fluoride laser absorbs energy from a source, causing the

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 of ...

gas to react with the

fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...

gas producing the exciplex krypton fluoride, a temporary

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

in an excited energy state: :2 Kr + → 2 KrF The complex can undergo spontaneous or stimulated emission, reducing its energy state to a metastable, but highly repulsive ground state. The ground state complex quickly dissociates into unbound atoms: :2 KrF → 2 Kr + The result is an exciplex laser which radiates energy at 248 nm, near 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 radiation ...

portion of the

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

, corresponding with the energy difference between the ground state and the excited state of the complex.

Example Systems

There have been several of these lasers built for ICF experiments; examples include: * Los Alamos built a KrF laser in 1985 to prove test firing of beam with an energy level of 1.0 × 10⁴

joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force appli ...

s. This was part of the larger Aurora laser research effort that looked a CO2 lasers and other systems. * Nike Laser. The Laser Plasma Branch of 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, technologic ...

completed a KrF laser called the

Nike laser The Nike laser at the United States Naval Research Laboratory in Washington, DC is a 56-beam, 4–5 kJ per pulse electron beam pumped krypton fluoride excimer laser which operates in the ultraviolet at 248 nm with pulsewidths of a few ...

that can produce about 4.5 × 10³ joules of UV energy output in a 4

nanosecond A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...

pulse. The NIKE laser was switched to an Argon fluoride laser after 2013 to show the impact of going to shorter (193 nm) wavelengths. *

built the Electra laser and Nike to prove out both KrF and ArF lasers for ICF approaches. In 2013, Electra demonstrated 90,000 shots over 10 hours of operation. * Rutherford Appleton Laboratory built the Sprite and Titania KrF lasers * Japan's Electrotechnical Laboratory built the Ashura and Super Ashura KrF lasers. * China Institute for Atomic Energy had a laser before the middle-1990's * Livermore National Laboratory developed a KrF laser and amplifier known as a Raman Amplifier Pumped by Intensified Excimer Radiation (RAPIER) system system.

Applications

This laser has also been used to produce soft X-ray emission from a plasma, through irradiation by brief pulses of this laser light. Other important applications include manipulating of various materials such as plastic, glass, crystal, composite materials and living tissue. The light from this UV laser is strongly absorbed by

lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...

nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main ...

s and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s, making it useful for applications in medical therapy and surgery.

Microelectronics

The most widespread industrial application of KrF excimer lasers has been in deep-ultraviolet

for the manufacturing of microelectronic devices (i.e., semiconductor

s or "chips"). From the early 1960s through the mid-1980s, Hg-Xe lamps had been used for lithography at 436, 405 and 365 nm wavelengths. However, with the semiconductor industry's need for both finer resolution (for denser and faster chips) and higher production 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 demonstrated at IBM by K. Jain. With phenomenal advances made in equipment and technology in the last two decades, modern semiconductor electronic devices fabricated using excimer laser lithography now total more than $400 billion in annual production. As a result, it is the semiconductor industry view that excimer laser lithography (with both KrF and ArF lasers) has been a crucial factor in the predictive power of

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 ...

. 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.

Fusion Research

The KrF laser has been used in

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

energy research since the 1980s. This laser offers several advantages:Obenschain, Stephen, et al. "High-energy krypton fluoride lasers for inertial fusion." Applied optics 54.31 (2015): F103-F122. * High rate repetition shots - because the KrF is made using gas it does not heat up, allowing for higher shot rates. * Higher beam uniformity * Relatively shorter wavelength for improved ICF compression.

Safety

The light emitted by the KrF is invisible to the human eye, so additional safety precautions are necessary when working with this laser to avoid stray beams. Gloves are needed to protect the skin from the potentially

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...

ic properties of the UV beam, and UV goggles are needed to protect the eyes.

References

External links

Laser fusion energy
{{Excimer lasers Excimer lasers Krypton Fluorine