Krypton fluoride laser
Encyclopedia
A krypton fluoride laser (KrF laser) is a particular type of excimer laser
, 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 circuits, industrial micromachining, and scientific research. The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'. An excimer laser typically uses a mixture of a noble gas (argon, krypton, or xenon) and a halogen gas (fluorine or chlorine), which under suitable conditions of electrical stimulation and high pressure, emits coherent stimulated radiation (laser light) in the ultraviolet range.
KrF (and ArF) excimer lasers are widely used in high-resolution photolithography
machines, one of the critical technologies required for microelectronic chip manufacturing. Excimer laser lithography has enabled transistor feature sizes to shrink from 0.5 micrometer in 1990 to below 45 nanometers in 2010.
gas to react with the fluorine
gas producing krypton fluoride, a temporary complex
, in an excited energy state:
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:
The result is an exciplex laser
that radiates energy at 248 nm, which lies in the near ultraviolet
portion of the spectrum
, corresponding with the energy difference between the ground state and the excited state of the complex.
for the manufacturing of microelectronic devices (i.e., semiconductor integrated circuits 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 I.B.M. by K. Jain. With phenomenal advances made in equipment technology in the last two decades, today semiconductor electronic devices fabricated using excimer laser lithography total $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 continued advance of the so-called Moore’s law (that describes the doubling of the number of transistors in the densest chips every two years – a trend that is expected to continue into this decade, with the smallest device feature sizes approaching 10 nanometers). 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.
The KrF laser has been of great interest in the nuclear fusion
energy research community in inertial confinement experiments. This laser has high beam uniformity, short wavelength, and the ability to modify the spot size to track an imploding pellet.
In 1985 the Los Alamos National Laboratory
completed a test firing of an experimental KrF laser with an energy level of 1.0 × 104 joule
s. The Laser Plasma Branch of the Naval Research Laboratory completed a KrF laser called the Nike laser
that can produce about 4.5 × 103 joules of UV energy output in a 4 nanosecond
pulse. Kent A. Gerber was the driving force behind this project. This later laser is being used in laser confinement experiments.
This laser has also been used to produce soft X-ray emission from a plasma
irradiated by brief pulses of this laser light. Other important applications include micromachining of a variety materials such as plastic, glass, crystal, composite materials and organic tissue (see more detailed information under excimer laser
). The light from this UV laser is strongly absorbed by lipid
s, nucleic acid
s and protein
s, making it attractive for applications in medical therapy and surgery.
ic properties of the UV beam, and UV goggles are needed to protect the eyes.
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
, 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 circuits, industrial micromachining, and scientific research. The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'. An excimer laser typically uses a mixture of a noble gas (argon, krypton, or xenon) and a halogen gas (fluorine or chlorine), which under suitable conditions of electrical stimulation and high pressure, emits coherent stimulated radiation (laser light) in the ultraviolet range.
KrF (and ArF) excimer lasers are widely used in high-resolution photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
machines, one of the critical technologies required for microelectronic chip manufacturing. Excimer laser lithography has enabled transistor feature sizes to shrink from 0.5 micrometer in 1990 to below 45 nanometers in 2010.
Theory
A krypton fluoride laser absorbs energy from a source, causing the kryptonKrypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
gas to react with the fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
gas producing krypton fluoride, a temporary complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
, 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
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
that radiates energy at 248 nm, which lies in the near ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
portion of the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
, corresponding with the energy difference between the ground state and the excited state of the complex.
Applications
The most widespread industrial application of KrF excimer lasers has been in deep-ultraviolet photolithographyPhotolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
for the manufacturing of microelectronic devices (i.e., semiconductor integrated circuits 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 I.B.M. by K. Jain. With phenomenal advances made in equipment technology in the last two decades, today semiconductor electronic devices fabricated using excimer laser lithography total $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 continued advance of the so-called Moore’s law (that describes the doubling of the number of transistors in the densest chips every two years – a trend that is expected to continue into this decade, with the smallest device feature sizes approaching 10 nanometers). 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.
The KrF laser has been of great interest in the nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
energy research community in inertial confinement experiments. This laser has high beam uniformity, short wavelength, and the ability to modify the spot size to track an imploding pellet.
In 1985 the Los Alamos National Laboratory
Los Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...
completed a test firing of an experimental KrF laser with an energy level of 1.0 × 104 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
s. The Laser Plasma Branch of the Naval Research Laboratory completed a KrF laser called the Nike laser
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 nanoseconds. Nike was completed in the late 1980s and is used...
that can produce about 4.5 × 103 joules of UV energy output in a 4 nanosecond
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
pulse. Kent A. Gerber was the driving force behind this project. This later laser is being used in laser confinement experiments.
This laser has also been used to produce soft X-ray emission from a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
irradiated by brief pulses of this laser light. Other important applications include micromachining of a variety materials such as plastic, glass, crystal, composite materials and organic tissue (see more detailed information under excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
). The light from this UV laser is strongly absorbed by lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s, nucleic acid
Nucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...
s and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, making it attractive for applications in medical therapy and surgery.
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 flesh from the potentially carcinogenCarcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
ic properties of the UV beam, and UV goggles are needed to protect the eyes.
See also
- Nike laserNike laserThe 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 nanoseconds. Nike was completed in the late 1980s and is used...
- LaserLaserA laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
- KryptonKryptonKrypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
- FluorineFluorineFluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
- Excimer laserExcimer laserAn excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
- PhotolithographyPhotolithographyPhotolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
- Moore's lawMoore's LawMoore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....
- ExcimerExcimerAn excimer is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which is in an electronic excited state. Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is...