Krypton fluoride laser
- For background information about krypton and fluorine, the two active elements in a krypton fluoride laser, see Krypton and Fluorine.
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.
- 2 Kr + F2 → 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 + F2
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.
The most widespread industrial application of KrF excimer lasers has been in deep-ultraviolet photolithography 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 IBM 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 joules. 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 lipids, nucleic acids and proteins, making it attractive for applications in medical therapy and surgery.
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 carcinogenic properties of the UV beam, and UV goggles are needed to protect the eyes.
See also 
- Argon fluoride laser
- Nike laser
- Krypton difluoride
- Excimer laser
- Moore's law
- Basting, D. and Marowsky,G., Eds., Excimer Laser Technology, Springer, 2005.
- Jain, K.; Willson, C.G.; Lin, B.J. (1982). "Ultrafast deep UV Lithography with excimer lasers". IEEE Electron Device Letters 3 (3): 53–55. doi:10.1109/EDL.1982.25476.
- Jain, K. “Excimer Laser Lithography”, SPIE Press, Bellingham, WA, 1990.
- La Fontaine, B., “Lasers and Moore’s Law”, SPIE Professional, Oct. 2010, p. 20.
- 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
- SPIE / Advancing the Laser / 50 Years and into the Future
- U.K. Engineering & Physical Sciences Research Council / Lasers in Our Lives / 50 Years of Impact