Dielectric Oxide vs Fluoride Deep-UV Coatings at 193 nm

Argon fluoride (ArF) excimer lasers are deep ultraviolet lasers with a 193 nm wavelength, which is commonly used in semiconductor lithography, eye surgery, micromachining, and scientific research. However, operating at this wavelength presents unique challenges: high-energy photon exposure accelerates optics degradation, making high reflectivity and low absorption essential. In addition, maintaining environmental stability is critical to ensure consistent laser system performance and uptime. 

Meeting these optical performance demands requires the use of specialized coating technologies. Two primary approaches dominate: fluoride-based coatings, valued for their intrinsic UV transparency, and dielectric oxide-based coatings, known for their durability and environmental resilience. While both can deliver high reflectivity at 193 nm, they diverge in performance in real-world conditions.

This whitepaper addresses the growing demand for long-lifetime, low-maintenance, and high-stability optical coatings in 193 nm deep ultraviolet (DUV) laser systems. We present the advantages of dielectric oxide coatings across real-world ArF lithography systems.

Substrate Considerations: SiO2 vs CaF2 

Two substrate materials have emerged as the primary choices for the 193-nm wavelength: fused silica and calcium fluoride. Well characterized, easy to fabricate, and thermally stable, fused silica is in many ways the material of choice for 193 nm. While fluoride crystals like CaF₂ are ultra-transparent in the DUV, dielectric substrates, especially UV-grade fused silica, offer superior readiness and operational advantages. 

CaF2 presents several notable drawbacks. It is pressure sensitive and exhibits a high coefficient of thermal expansion. These properties, combined with its single-crystal structure, make CaF₂ extremely difficult to fabricate to the required levels of optical smoothness. According to John Bruning of Tropel (Fairport, NY), the material “polishes terribly”. While it is possible to achieve a good surface finish, doing so is challenging and costly.

Fused silica substrates, by contrast, are robust, cost-effective, and widely available. When paired with IBS dielectric coatings, they enable long-lifetime, easy-to-maintain optics. However, the performance and longevity of the optical system are primarily dictated by its coating technology. 

The comparison between fused silica and calcium fluoride substrates 

Property	Fused Silica	Calcium Fluoride
Transparency (193 nm)	UV-grade fused silica transmits down to ~180 nm; absorption <0.002 cm⁻¹	CaF₂ absorption <0.0005 cm⁻¹ excellent DUV  performance
Availability & Cost	Abundant large blanks, mature manufacturing, lower cost	Brittle, hygroscopic, high cost, limited supply
Polishing	Easily polished to a few-angstrom surface roughness	Difficult and slow polishing, higher defect risk
Mechanical Stability	Excellent robustness, low birefringence, widely used	Brittle, mechanical risk under stress
Laser Exposure Stability	Prone to compaction over time — mitigated   by  coatings and grade selection	No compaction due to crystalline structure

Coating Technologies at 193nm: Dielectric Oxide vs Fluoride

Dielectric coatings for 193 nm optics offer a combination of high reflectivity, damage resistance, and environmental durability that makes them ideal for demanding DUV applications. Multi-layer stacks, typically composed of Al₂O₃ and SiO₂, can achieve reflectivity levels of up to 97–99%, suitable for both mirrors and anti-reflection applications. These coatings demonstrate strong resistance to DUV laser-induced damage, and enhanced versions have shown long operational lifetime at high repetition rates and moderate fluence levels. When deposited via Ion Beam Sputtering (IBS), oxide coatings also offer superior mechanical resilience, low moisture sensitivity, and require minimal maintenance. 

Fluoride-based coatings, typically made from LaF₃ and MgF₂, also show high reflectivity (~99%) at 193 nm when properly deposited at elevated temperatures (up to 400 °C). However, they face significant challenges, including higher film stress, lower packing density, and sensitivity to environmental conditions. Fluoride coatings exhibit lower resistance to air humidity exposure and require more complex maintenance and upkeep, particularly under tightly controlled operating conditions. Unlike fluoride-based coatings, IBS oxide coatings are less sensitive to UV exposure and moisture.

These oxide-based coatings, particularly those deposited via Ion Beam Sputtering (IBS), exhibit excellent mechanical durability. They are scratch-resistant, humidity-resistant, and require minimal maintenance, preserving optical performance under challenging environmental conditions. From the measurement we have done, we observe that the dielectric-oxide coating, developed and produced by OPTOMAN, remains stable over time and has a higher reflectance value than the fluoride coating deposited using E-beam technology. 

Fluoride		Dielectric
High performance: highly transparent at 193 nm in controlled environment conditions;		High reflectivity: Multi-layer stacks form dielectric mirrors and anti-reflection coatings with reflectivity up to 97–99% at 193 nm;
Anti-compaction: is a single-crystal material and therefore not subject to compaction;		Operational lifetime: Our LIDT comparison showed that the lifetime of a dielectric-coated mirror under DUV laser exposure was longer compared to a fluoride-coated example.
Chromatic correction: CaF₂ is the only practical material for chromatic correction alongside fused silica in all-refractive systems;		Enhanced durability: Oxide coatings are durable, scratch-resistant, and humidity-resistant, requiring minimal maintenance. They maintain optical integrity under demanding conditions.
Performance in vacuum: Fluoride coatings maintain stable performance in vacuum down to 10⁻⁸ mbar and support ~10 W intracavity power, but require oxygen “conditioning” to prevent rapid degradation.		No Special Conditions: IBS oxide coatings are less sensitive to UV and humidity than fluoride coatings.

Practical Advantages of Dielectric IBS Coatings in DUV Optics

DUV optics with Ion Beam Sputtering (IBS) oxide coatings are durable, scratch-resistant, and humidity-resistant, requiring minimal maintenance. They maintain optical integrity under demanding conditions, making them highly suitable for precision-driven lithography systems. The ease of handling, cleaning, and storing enhances the overall operational reliability and cost-effectiveness of dielectric optics in DUV systems, especially when uptime and maintenance predictability are critical. 

• Handling Guidelines 

Dielectric optics with IBS coatings are robust and user-friendly. Safe handling requires standard powder-free nitrile gloves, with optics manipulated at the edges using non-scratching tweezers or gloved hands. The mechanical resilience of these coatings significantly reduces the risk of accidental damage during handling compared to more fragile alternatives like fluoride-coated components. 

• Cleaning Made Simple 

Maintenance is straightforward and forgiving. Particulate contaminants can be removed using clean, oil-free air or nitrogen blowers. For smudges or residue, ethyl acetate serves as an effective solvent, applied gently with lint-free wipes or optical swabs made from polyurethane, polyester, or microfiber. IBS coatings exhibit strong resistance to cleaning-induced damage, unlike the more delicate fluoride-coated optics. 

• Storage Recommendations 

Dielectric optics demand no special storage infrastructure. Simply keeping them in their original, sealed packaging within a clean, dry, room-temperature environment is sufficient to prevent contamination. While it is advisable to avoid prolonged exposure to extreme humidity or UV, IBS oxide coatings are far less sensitive to such factors compared to fluoride-based coatings, further simplifying storage and logistics. 

Conclusions

When it comes to 193 DUV laser optics, UV-grade fused silica substrates, combined with advanced dielectric coatings offer a superior balance of performance, reliability, and manufacturing repeatability. While fluoride coatings require strict environmental control and conditioning, dielectric coatings offer robust, long-term reliability in real-world DUV laser systems.

Applied using Ion Beam Sputtering (IBS) technology, these coatings exhibit exceptional durability, scratch resistance, and environmental stability, making them ideally suited for high-volume laser lithography systems. With proper material selection and coating design, fused silica optics are well equipped to consistently meet the rigorous demands of ArF excimer laser systems, both in terms of optical performance and long-term system reliability. Explore OPTOMAN’s in-stock Deep-UV Laser Mirrors to find the best option for your laser system here.