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Excimer laser

An excimer laser is a pulsed ultraviolet laser that generates high-intensity, short-wavelength light through stimulated emission from excited molecular dimers (excimers) formed in a gas mixture of noble gases and halogens, enabling precise material ablation and photochemical reactions without significant thermal damage.^[1] These lasers operate on the principle of electrical discharge or electron-beam pumping in a high-pressure gas medium, where excimers—stable only in their excited state—emit ultraviolet radiation upon returning to a repulsive ground state, preventing reabsorption and allowing high optical gain.^[2] Common wavelengths include 157 nm (F₂), 193 nm (ArF), 248 nm (KrF), and 308 nm (XeCl), with pulse durations of 10–30 ns, energies from millijoules to joules, and repetition rates up to 1 kHz.^[1] Invented in 1970 by Nikolai Basov, V. A. Danilychev, and Yu. M. Popov at the Lebedev Physical Institute in Moscow using a xenon dimer (Xe₂) emitting at 172 nm, the excimer laser marked a breakthrough in ultraviolet light sources, building on earlier maser and laser developments for which Basov shared the 1964 Nobel Prize in Physics.^[3] Early demonstrations involved electron-beam excitation of liquid xenon, followed by gas-phase systems in the early 1970s, with commercial availability achieved by 1976 for applications beyond initial research in laser fusion and plasma physics.^[2] Subsequent refinements, such as discharge-pumped designs, improved efficiency (up to 5%) and power output (average powers of 10–100 W, peak powers exceeding 100 kW), addressing challenges like gas mixture degradation and electrode erosion.^[1] Excimer lasers have transformed multiple fields due to their unique ability to deliver "cold" ablation via photon-induced bond breaking, with primary applications in semiconductor manufacturing—where ArF and KrF variants enable sub-10 nm photolithography for integrated circuits—and refractive eye surgery, such as LASIK and PRK using 193 nm light to reshape the cornea with micrometer precision.^[2] In medicine, 308 nm XeCl lasers treat dermatological conditions like psoriasis and vitiligo by targeting inflamed skin lesions, achieving over 75% improvement in many cases after fewer than 10 sessions.^[3] Additional uses span micromachining of polymers and metals, pulsed laser deposition for thin films, coronary angioplasty to vaporize plaques, and environmental monitoring via fluorescence spectroscopy, contributing to a global market exceeding $3 billion annually in systems and procedures.^[2]

Fundamentals

Definition and Terminology

An excimer laser is a form of ultraviolet laser that achieves optical amplification through stimulated emission in a gas mixture containing transient excimer molecules, typically dimers formed between a noble gas (such as argon, krypton, or xenon) and a halogen (such as fluorine or chlorine).^[1] These molecules exist only in an excited electronic state and rapidly dissociate upon returning to the ground state, enabling efficient lasing without significant reabsorption of the emitted light.^[1] The lasers produce short, high-energy pulses of coherent ultraviolet radiation in the wavelength range of approximately 157 to 351 nm.^[1] The term "excimer" is derived from "excited dimer," describing a diatomic species that is stable only when one or both atoms are in an electronically excited state, in contrast to stable ground-state dimers that persist without excitation.^[4] In practice, excimer lasers commonly utilize rare gas halide combinations, which are heterodimers of dissimilar atoms and thus technically exciplexes (short for "excited complex"), though the broader "excimer" terminology is conventionally applied to these systems.^[4] Representative examples include the KrF laser emitting at 248 nm, the ArF laser at 193 nm, and the XeCl laser at 308 nm.^[1] At their core, excimer lasers rely on the fundamental principle of stimulated emission, in which an incoming photon interacts with an excited molecule to trigger the release of an identical photon, resulting in amplification of light with high coherence and directionality.^[5] This process requires a population inversion, where more molecules are in the excited state than in the lower energy state, achieved through electrical discharge or other pumping mechanisms in the gas medium.^[5]

Historical Development

The concept of excimer lasers originated from theoretical work in the 1960s, particularly by Soviet physicist Nikolay Basov, who explored the stimulated emission from excited dimers of rare gases, laying the groundwork for ultraviolet laser development. The first practical excimer laser was demonstrated in 1970 by Basov's group at the Lebedev Physical Institute in Moscow, using a xenon dimer (Xe₂) excited by an electron beam to produce lasing at 172 nm, marking the birth of excimer laser technology.^[6] In 1975, independent demonstrations of rare gas halide excimer lasers occurred: S.K. Searles and G.A. Hart at the Naval Research Laboratory reported lasing in XeBr at 282 nm using electron-beam pumping, while J.J. Ewing and C.A. Brau at Avco Everett Research Laboratory demonstrated lasing in KrF at 248 nm and XeCl at 308 nm.^[7]^[8] These early devices faced significant challenges, including low overall efficiency below 1% and short gas mixture lifetimes due to chemical reactions and electrode erosion, which limited output power and operational duration.^[9] Improvements in the late 1970s and 1980s addressed these issues through innovations like electron-beam-sustained discharge pumping for higher repetition rates and gas flow systems to refresh mixtures, boosting efficiency to several percent and enabling pulse energies up to hundreds of millijoules.^[6] The first commercial excimer laser, the EMG 500 from Lambda Physik, was introduced in 1976, operating at up to 20 Hz with KrF or other wavelengths, paving the way for industrial adoption.^[9] By the 1980s, excimer lasers were integrated into semiconductor photolithography, with IBM's 1982 demonstration of deep-UV excimer laser lithography using KrF at 248 nm enabling sub-micron feature sizes in chip manufacturing.^[10] In the 1990s, regulatory approvals expanded medical applications, including the FDA clearance of excimer lasers for LASIK vision correction procedures in 1999, following pioneering ablation studies in the 1980s.^[3]

Applications

Photolithography in Semiconductor Manufacturing

Excimer lasers serve as critical deep ultraviolet (DUV) light sources in photolithography, enabling the patterning of semiconductor features below 100 nm by providing coherent, high-intensity illumination at wavelengths such as 248 nm (KrF) and 193 nm (ArF).^[11] These lasers replaced earlier mercury lamp systems in the mid-1980s, allowing for shorter wavelengths that reduce diffraction limits and support denser integrated circuits.^[12] In the photolithography process, the excimer laser illuminates a photomask containing the desired circuit pattern, which is then projected through a reduction lens onto a photoresist-coated silicon wafer using a stepper or scanner tool.^[11] The step-and-scan method involves sequentially exposing portions of the wafer field by field, synchronizing the mask and wafer stages to achieve high-resolution imaging over large areas, typically 26 mm × 33 mm per exposure.^[12] For advanced nodes, ArF excimer lasers at 193 nm are employed in immersion lithography, where a thin layer of water between the lens and wafer increases the effective numerical aperture to 1.35, enhancing resolution by approximately 30% compared to dry systems.^[13] Historically, KrF excimer lasers at 248 nm, introduced in production tools during the late 1980s, enabled the transition from 250 nm to 130 nm features, supporting the fabrication of 16-Mbit and 64-Mbit DRAMs and sustaining Moore's Law through improved transistor densities.^[11] By the early 2000s, ArF lasers at 193 nm further advanced this progression, achieving resolutions down to 38 nm in high-volume manufacturing via ASML's TWINSCAN immersion scanners, which integrated these lasers for nodes like 45 nm and below.^[12] This evolution extended optical lithography's viability, delaying the need for more complex alternatives until the 7 nm node. Although extreme ultraviolet (EUV) lithography at 13.5 nm has emerged for sub-7 nm patterning since the mid-2010s, excimer lasers continue to play essential roles in semiconductor manufacturing for older technology nodes (above 28 nm) and critical support processes like photomask inspection, where their stable 193 nm output detects defects at nanometer scales.^[12]^[14]

Medical Procedures

Excimer lasers have revolutionized ophthalmology through procedures like photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK), where a 193 nm argon-fluoride (ArF) laser precisely reshapes the cornea to correct refractive errors such as myopia and astigmatism.^[15] In PRK, the laser directly ablates the corneal surface, while in LASIK, a flap is created prior to ablation of the underlying stroma, enabling faster recovery.^[16] The 193 nm wavelength is strongly absorbed by corneal proteins and glycosaminoglycans, enabling non-thermal photoablation that minimizes collateral damage to adjacent tissue.^[17] Ablation occurs at typical fluences of 0.25–0.5 J/cm² per pulse, removing approximately 0.25–0.7 µm of corneal tissue per pulse, allowing surgeons to control depth with high precision for customized corneal profiles.^[18]^[19] The U.S. Food and Drug Administration (FDA) approved excimer lasers for PRK in 1995 and for LASIK in 1999, marking the clinical adoption of these techniques for vision correction.^[16] In dermatology, the 308 nm xenon-chloride (XeCl) excimer laser treats localized psoriasis and vitiligo by delivering targeted ultraviolet B (UVB) radiation to affected skin areas, inducing repigmentation in vitiligo and reducing plaque thickness in psoriasis without exposing healthy skin.^[3] Hand-held excimer devices facilitate precise application to small or irregular lesions, often requiring sessions of 2–3 times weekly for 10–12 weeks to achieve significant clearance rates of 75% or more in responsive cases.^[20] The FDA cleared the first such system, the XTRAC excimer laser, for psoriasis in 2000 and vitiligo in 2001, establishing it as a standard targeted phototherapy option.^[21] In cardiovascular applications, excimer laser coronary atherectomy (ELCA) uses 308 nm XeCl lasers delivered via optical fiber catheters to ablate calcified plaques, thrombi, and in-stent restenosis in coronary arteries, facilitating percutaneous coronary interventions. Approved by the FDA in 1993, ELCA is particularly effective for complex lesions uncrossable by balloons, with success rates over 90% in debulking and reduced procedural complications compared to mechanical atherectomy.^[22]^[23] Pulse energy control in both fields ensures ablation depths of microns per pulse, with real-time adjustments via eye-tracking in ophthalmology and dosimeters in dermatology to optimize outcomes while preventing overexposure.^[15] A notable recent advancement is the TENEO Excimer Laser Platform by Bausch + Lomb, FDA-approved in January 2024—the first new U.S. excimer laser in over a decade—featuring higher pulse repetition rates up to 500 Hz for faster procedures and a compact design reducing the system footprint by 40% compared to prior models.^[24]^[25]

Inertial Confinement Fusion

Excimer lasers play a critical role in inertial confinement fusion (ICF) through direct-drive approaches, where short-wavelength ultraviolet beams rapidly compress and heat fuel pellets to achieve ignition conditions. Krypton fluoride (KrF) excimer lasers operating at 248 nm deliver high-energy pulses, typically in the megajoule range over nanoseconds, to ablate the outer layer of a spherical fuel capsule containing deuterium-tritium, generating inward shock waves that implode the core to densities exceeding 1000 times liquid density. This process aims to create the extreme temperatures and pressures necessary for thermonuclear fusion, with the laser's deep ultraviolet wavelength enabling higher ablation pressures and reduced hydrodynamic instabilities compared to longer-wavelength drivers.^[26]^[27] Key experimental systems have advanced KrF laser technology for ICF, including the Nike facility at the Naval Research Laboratory, operational since the late 1980s and delivering up to 3 kJ in 56 beams for hydrodynamic and plasma studies. Nike employs induced spatial incoherence (ISI), a beam-smoothing technique that broadens the laser spectrum to over 1 THz, achieving irradiation nonuniformities below 1% RMS and enabling uniform target illumination essential for stable implosions. Complementing Nike, the Electra laser demonstrates high-repetition-rate operation, producing 700 J pulses at 5 Hz for over 50,000 shots, supporting the development of drivers for inertial fusion energy plants. These systems highlight KrF lasers' scalability, with proposals for multi-megajoule facilities to test high-gain targets.^[26]^[27]^[26] A primary challenge in KrF-driven ICF is achieving uniform illumination to suppress Rayleigh-Taylor instabilities, which arise at the ablation front and can disrupt implosion symmetry if nonuniformities exceed 0.5%. ISI and techniques like beam zooming mitigate this by smoothing intensity variations and adjusting focal profiles, reducing seed perturbations that amplify during compression. The fusion gain, defined as Q = \frac{E_{\text{fusion}}}{E_{\text{laser}}}, must exceed 100 for viable energy production, with KrF systems targeting values above 140 through optimized pulse shapes and low implosion velocities of 200-250 km/s to control instability growth. Recent advancements include the 2025 Long Pulse Kinetics (LPK) platform by Xcimer Energy, the first private-sector electron-beam-pumped KrF laser, which achieved a record 3-microsecond pulse length and demonstrated efficiencies informing scalable ICF drivers.^[27]^[26]^[27]^[28]

Scientific and Industrial Uses

Excimer lasers play a significant role in scientific research, particularly in spectroscopy, where their tunable ultraviolet output enables high-resolution studies of molecular structures and electronic transitions. Narrow-linewidth emissions from variants like KrF (248 nm) and ArF (193 nm) facilitate selective photoionization and absorption spectroscopy, allowing precise probing of atomic and molecular energy levels without thermal interference. In environmental monitoring, excimer lasers support fluorescence spectroscopy for detecting air and water pollutants, such as polycyclic aromatic hydrocarbons, by exciting samples and analyzing emission spectra for trace-level identification.^[29] In photoelectron spectroscopy, these lasers provide high-photon-energy pulses (e.g., 6.4 eV for ArF and 7.9 eV for F2) to ionize samples, enabling detailed analysis of valence electron distributions and surface states in materials like semiconductors and polymers.^[30] Time-resolved experiments on molecular dynamics also benefit from excimer lasers' nanosecond pulse durations (typically 10–30 ns) and high peak powers, supporting pump-probe setups to observe ultrafast processes such as bond breaking, energy transfer, and relaxation in photochemical reactions. For example, ArF excimer lasers have been used to initiate and monitor transient species in ablation plumes, revealing dynamics on picosecond to nanosecond timescales.^[31] These capabilities make excimer lasers indispensable for investigating transient phenomena in gas-phase and condensed-matter systems. In industrial settings, excimer lasers excel in micromachining polymers through photon-induced ablation, which removes material with minimal heat-affected zones, ideal for creating microstructures in medical devices and electronics. ArF excimer lasers at 193 nm ablate poly(L-lactide) to fabricate biodegradable stents, achieving clean cuts that maintain mechanical integrity and biocompatibility.^[32] Similarly, they drill microvias in printed circuit boards (PCBs), enabling high-density interconnects with yields exceeding 99.99% and aspect ratios up to 10:1.^[33] For display manufacturing, excimer laser annealing converts amorphous silicon to low-temperature polycrystalline silicon (LTPS) on large glass substrates, boosting carrier mobility for active-matrix OLED backplanes while avoiding substrate deformation.^[34] Beyond core processing, excimer lasers contribute to environmental remediation by driving photochemical oxidation of volatile organic compounds (VOCs), such as toluene and trichloroethylene, in air and water streams when paired with oxidants like hydrogen peroxide. Their intense UV pulses (e.g., from KrF at 248 nm) achieve destruction efficiencies over 90% in short irradiation times, offering a non-thermal alternative to conventional methods.^[35] In printing, high-repetition-rate excimer UV sources accelerate curing of inks and coatings, supporting speeds up to 1000 m/min for flexible packaging without solvents. XeCl excimer lasers (308 nm) extend utility to visible-range applications by pumping optical parametric oscillators, generating tunable output from 400–700 nm for spectroscopy and remote sensing.^[36] Emerging applications include pulsed laser deposition (PLD) of thin films for high-temperature superconductors, where excimer lasers ablate targets like YBa₂Cu₃O₇₋ₓ to deposit stoichiometric layers on substrates such as MgO or SrTiO₃, yielding films with critical temperatures above 90 K and smooth morphologies. KrF excimer lasers at 248 nm are preferred for their ability to vaporize complex oxides congruently, advancing superconducting device fabrication.^[37]

Advantages, Limitations, and Safety

Key Advantages

Excimer lasers offer the shortest commercially available wavelengths in the ultraviolet spectrum, typically below 200 nm, such as 157 nm from F₂ excimers and 193 nm from ArF mixtures, which enable superior resolution in processes requiring fine detail due to the diffraction limit scaling with wavelength.^[38]^[2] This UV output provides a distinct advantage over infrared lasers like Nd:YAG (1064 nm) or CO₂ (10.6 μm), which cannot access deep ultraviolet regimes for applications demanding high spatial precision.^[39]^[1] A key benefit is their capability for cold ablation, where high-photon-energy UV pulses directly break molecular bonds through photochemical processes, minimizing thermal effects and eliminating heat-affected zones that could damage surrounding material.^[40]^[41] This contrasts with thermal ablation from longer-wavelength lasers, allowing precise material removal from sensitive substrates without collateral heating.^[39] These lasers deliver exceptionally high peak intensities, often exceeding 1 GW/cm² in nanosecond pulses, facilitating nonlinear optical interactions and rapid energy deposition for efficient processing.^[42] Additionally, wavelength tunability is achieved by varying gas mixtures—such as ArF for 193 nm or KrF for 248 nm—offering flexibility across the UV spectrum without mechanical adjustments.^[1]^[43] Over time, excimer laser efficiency has improved significantly, from approximately 0.1–1% in early 1970s demonstrations to wall-plug efficiencies of up to 10% as of 2025 in modern systems, particularly for high-repetition-rate applications in inertial confinement fusion, enhancing their practicality for high-volume operations.^[1]^[44]^[45]^[46] This progression underscores their superiority in UV power delivery compared to alternatives like frequency-doubled solid-state lasers, which struggle with comparable output at short wavelengths.^[47]

Limitations and Challenges

Excimer lasers exhibit low wall-plug efficiencies, typically ranging from 0.2% to 10% as of 2025, which contributes to high operating costs due to substantial electrical power requirements for generating the necessary high-voltage discharges.^[1]^[45] This inefficiency stems in part from energy losses in the pumping process, such as those in discharge excitation methods.^[48] High operating costs are further exacerbated by frequent gas consumption and component maintenance; the halogen-containing gas mixtures degrade rapidly due to chemical reactions, necessitating replacement every 30 million pulses or so to maintain performance.^[1] Halogen corrosion attacks the laser chamber walls and electrodes, leading to material degradation and requiring periodic electrode replacement to mitigate erosion from the reactive environment.^[49]^[50] The systems are inherently bulky, often requiring large high-voltage power supplies capable of delivering tens of kilovolts and extensive cooling infrastructure—typically water-based for high-power models—to dissipate the significant heat generated during operation.^[1]^[51] This combination limits portability, confining most excimer lasers to fixed industrial or laboratory installations rather than mobile applications.^[52] Scalability poses significant challenges, particularly for applications like inertial confinement fusion that demand repetition rates exceeding 1 kHz; achieving such rates increases thermal loads, gas depletion, and electrode wear, while maintaining stable discharge uniformity becomes difficult.^[53] Beam quality is often compromised by amplified spontaneous emission, which broadens the output and reduces spatial coherence, necessitating additional optics for homogenization in precision uses.^[54] Environmentally, the use of toxic halogens such as fluorine or chlorine in the gas mixtures generates hazardous waste upon disposal, requiring specialized handling to prevent release of corrosive and poisonous byproducts.^[1]^[55] While post-2020 research has explored gas purification and material coatings to extend lifetimes and reduce consumption, no widely adopted fluorine-free alternatives have emerged to fully address toxicity concerns.^[56]

Safety Considerations

Excimer lasers emit high-intensity ultraviolet (UV) radiation, typically in the UV-B and UV-C ranges (e.g., 193–351 nm), which presents severe hazards to human eyes and skin. Direct or scattered exposure to the beam can cause acute photokeratitis, a painful corneal inflammation akin to severe sunburn, while prolonged or repeated exposure increases the risk of cataracts and other ocular damage. Skin contact with the radiation may result in erythema, blistering, or thermal burns, depending on the dose and duration.^[57]^[58] As Class 4 lasers capable of causing irreversible injury from even diffuse reflections, excimer systems necessitate comprehensive engineering controls, including fail-safe interlocks on protective housings to prevent access during operation.^[57]^[59] The gas mixtures used in excimer lasers, such as rare-gas halides involving fluorine, chlorine, or other halogens, introduce additional chemical hazards. These gases are highly corrosive and toxic, capable of causing severe respiratory irritation, chemical burns to mucous membranes, or even lethality upon inhalation in concentrated forms. Safe handling protocols mandate the use of specialized gas cabinets, continuous monitoring of concentrations to remain below OSHA permissible exposure limits (PELs) and ACGIH threshold limit values (TLVs), and immediate access to emergency gas shutoff valves. Adequate exhaust ventilation systems are required to dilute and remove any leaks or byproducts, with regular leak detection protocols enforced.^[59]^[60]^[57] High-voltage electrical systems in excimer lasers, often exceeding several kilovolts for discharge pumping, pose electrocution and arc flash risks during operation or maintenance. These hazards are mitigated through compliance with ANSI Z136.1 standards, which require proper grounding of all components, insulated barriers, and lockout/tagout procedures before servicing. Only trained personnel may access high-voltage areas, with clear labeling of potential shock points.^[61]^[57]^[59] Operational use of excimer lasers can generate ozone as a byproduct when UV radiation interacts with atmospheric oxygen, particularly at wavelengths below 240 nm, leading to potential respiratory irritation or explosions in confined spaces. Mitigation involves enclosing the laser path to minimize air exposure and employing dedicated ventilation to maintain ozone levels below 0.10 ppm (OSHA PEL). Personal protective equipment (PPE) is essential across all hazards: wavelength-specific UV-blocking goggles or face shields with sufficient optical density (e.g., OD ≥4+ at the laser wavelength), full-body coverings to shield skin, and respirators or gas masks rated for halogen and ozone exposure during gas handling or high-risk procedures.^[57]^[59]^[60]

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... laser and comparison to ablation by a 193-nm excimer laser ... Photorefractive keratectomy (PRK) is usually performed by an excimer laser at 193 nm wavelength.
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However, their high-voltage power supplies generate significant heat (with electrical-to-optical conversion typically below 10%). Inefficient heat dissipation ...
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The Broadest Portfolio of Excimer Lasers ; ExciStar · High Repetition Rate - Up to 1 kHz. Tabletop - 650 x 300 mm footprint. ; IndyStar · High Pulse Rate - Up to 2 ...
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### Summary of UV Laser Hazards and Related Information from OSHA Technical Manual (OTM) Section III: Chapter 6
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