Argon fluoride laser
The argon fluoride laser (ArF laser) is a particular type of excimer laser, which is sometimes (more correctly) called an exciplex laser. With its 193-nanometer wavelength, it is a deep ultraviolet laser, which is commonly used in the production of semiconductor integrated circuits, eye surgery, micromachining, and scientific research. "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.
ArF (and KrF) 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 800 nanometers in 1990 to 7 nanometer in 2018.
- 2 Ar + F
2 → 2 ArF
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 ArF → 2 Ar + F
The result is an exciplex laser that radiates energy at 193 nm, which lies in the far ultraviolet portion of the spectrum, corresponding with the energy difference of 6.4 electron volts between the ground state and the excited state of the complex.
The most widespread industrial application of ArF 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 invented and 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 ArF and KrF 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 has continued into this decade, with the smallest device feature sizes reaching 10 nanometers in 2016) and 7 nm in 2018.
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 UV light from an ArF laser is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the ArF laser dissociates the molecular bonds of the surface tissue, which disintegrates into the air in a tightly controlled manner through ablation rather than burning. Thus the ArF and other 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 such lasers well suited to precision micromachining organic materials (including certain polymers and plastics), and especially delicate surgeries such as eye surgery (e.g., LASIK, LASEK).
Recently, through the use of a novel diffractive diffuse system composed of two microlens arrays, surface micromachining by ArF laser on fused silica has been performed with submicrometer accuracy.
The light emitted by the ArF 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.
- Excimer laser
- Excimer lamp
- Krypton fluoride laser
- Nike laser
- Moore’s law
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- Jain, K. "Excimer Laser Lithography", SPIE Press, Bellingham, WA, 1990.
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