Choosing between CaF₂ and fused silica for 193 nm or 248 nm optical systems depends on the specific performance demands of each optical element. At 193 nm, CaF₂ is evaluated for elements requiring ultra-low absorption or chromatic correction; at 248 nm, high-purity UV fused silica is the default starting point. The final choice must rest on grade-specific, supplier-verified data matched to your system's error budget.

Key Takeaways

• At 193 nm, CaF₂ offers intrinsically lower bulk absorption than fused silica and is used for select elements in ArF lithography systems, while fused silica remains the majority material for economic and processing reasons.
• Our CaF₂-A grade is qualified for 193 nm ArF applications (>99.8%/cm, LD-1); our CaF₂-B grade is qualified for 248 nm KrF applications (>99.8%/cm, LD-2). Do not cross-apply wavelength qualifications between grades.
• At 248 nm, begin material selection with high-purity UV fused silica; evaluate CaF₂ only when pulse energy, damage-lifetime, or dispersion requirements exceed what fused silica can deliver.
• CaF₂'s thermal expansion coefficient is roughly 34× higher than fused silica's (18.9 × 10⁻⁶/K vs. 0.55 × 10⁻⁶/K), making thermal management a critical design factor when substituting CaF₂ into a beam path.
• Always compare candidates under matched wavelength, test condition, and optical task — never extrapolate a qualification at one wavelength to another.

Why Does Material Selection Differ Between 193 nm and 248 nm Optical Systems?

The two wavelengths impose fundamentally different demands on optical materials, so treating them as a single selection problem leads to specification errors.

Semiconductor lithography exposure wavelengths evolved from g-line (436 nm) and i-line (365 nm) to KrF (248 nm) and ArF (193 nm), with each step tightening material requirements for absorption, homogeneity, and damage resistance (according to Corning). Fused silica has historically served as the workhorse material across both 248 nm and 193 nm systems because it combines adequate optical transmittance, laser-damage resistance, refractive-index uniformity, and extensive polishing know-how at a favorable cost point. CaF₂ is used for selected elements in 193 nm systems and was the only viable transmissive material at 157 nm, positioning it as a performance-driven complement rather than a universal replacement.

The transmission windows of the two materials differ significantly. CaF₂ transmits from the deep UV through the mid-IR (approximately 0.13 µm to 8 µm), while UV fused silica transmits from approximately 180 nm to 2.5 µm. This broader window matters when a system spans multiple wavelength bands or when mid-IR monitoring channels share the same optical path.

The comparison parameters — internal transmittance, LIDT, stress birefringence, index homogeneity, CTE, and post-processing stress — must be evaluated under the specific wavelength and operating conditions of the target system. A material qualified at 248 nm cannot be assumed adequate at 193 nm, and vice versa.

When Is CaF₂ Worth Evaluating Instead of Fused Silica in a 193 nm Optical System?

CaF₂ becomes the preferred candidate when the optical element requires ultra-low bulk absorption at 193 nm and fused silica's residual absorption is a binding constraint.

Transmission and absorption. Our CaF₂-A grade provides >99.8% internal transmittance per cm at 193 nm with LD-1 laser-durability classification. Fused silica's transmission at 193 nm is grade-dependent and sensitive to OH content and manufacturing route; not all fused silica grades are suitable for this wavelength. When even small residual absorption causes unacceptable thermal lensing or wavefront distortion, CaF₂ provides the lower-absorption alternative.

Laser-damage survivability. We have documented LIDT values for CaF₂ at 193 nm: 6.22 J/cm² at 6.7 ns pulse width and 7.5 J/cm² at 10 ns pulse width. These values are meaningful only when pulse duration, repetition rate, and beam profile are specified alongside them. In a 193 nm laser-lifetime test, a D25.4 × 4 mm CaF₂ sample irradiated at 600 Hz, 15 kV, 6.2 mJ energy, with a 5.5 mm × 3.5 mm spot size survived 2 billion pulses with no surface or bulk damage and maintained transmittance of 91.87% at 193 nm. This is a specific test configuration; confirm applicability to your own fluence and duty-cycle conditions.

Chromatic-aberration correction. CaF₂'s Abbe number of 95.2 and ultra-low dispersion make it essential for apochromatic lens designs in all-refractive 193 nm projection systems. CaF₂ and fused silica can be combined in the same optical train for this purpose.

Athermal design. CaF₂'s negative dn/dT coefficient complements fused silica's positive coefficient, enabling "zero thermal shift" optical designs when the two materials are paired.

Birefringence considerations. CaF₂ is a crystalline material with orientation-dependent intrinsic birefringence. In high-NA, polarization-sensitive, or tight-imaging-error-budget systems, crystal orientation and birefringence must be explicitly modeled. Our grades offer customizable stress birefringence within 0–10 nm/cm.

Decision rule: Prioritize CaF₂ at 193 nm when the application demands ultra-low absorption, high laser-damage lifetime, or chromatic/thermal aberration correction. Prioritize fused silica when cost, thermal stability, and large-aperture availability dominate.

How Should You Compare CaF₂ and UV Fused Silica for a 248 nm Beam-Delivery System?

At 248 nm, begin with high-purity synthetic UV fused silica as the default material because it provides a well-characterized combination of transmission, damage resistance, and thermal stability with mature processing infrastructure.

Our CaF₂-B grade serves as the CaF₂ benchmark for 248 nm: >99.8% internal transmittance per cm at 248 nm, LD-2 laser-durability classification. CaF₂-B is specifically qualified for KrF applications — do not substitute CaF₂-A or CaF₂-C ratings. For beam-path optical-design calculations, the CaF₂ theoretical refractive index at 248.27 nm is 1.46794, with allowable deviation ≤20 × 10⁻⁶.

Conditions that trigger CaF₂ evaluation at 248 nm: extreme pulse energy exceeding fused silica's damage budget, strict cumulative-damage lifetime requirements, or specific dispersion needs that fused silica cannot meet.

Direct LIDT comparison data between CaF₂ and fused silica at 248 nm under matched test conditions is not available in our current dataset. Request supplier-specific LIDT data at 248 nm with matched pulse parameters for both materials before making a substitution decision.

Because CaF₂'s CTE is roughly 34× that of fused silica, thermal management, mounting design, and warm-up stability must be re-evaluated when introducing CaF₂ into a beam-delivery path originally designed for fused silica.

How Do Grade-Specific Transmission, LIDT, and Stress Birefringence Affect the Comparison?

"CaF₂" and "fused silica" are material classes, not specifications — performance varies dramatically by grade, manufacturer, and lot. Selection must be based on grade-specific, measured data.

Wavelength-specific qualification. Our grade-tier structure illustrates this principle: CaF₂-A for 193 nm (LD-1), CaF₂-B for 248 nm (LD-2), CaF₂-C for 365 nm/i-line (LD-3). A grade qualified at 365 nm must not be assumed adequate at 193 nm.

Internal transmittance. All three grades achieve >99.8%/cm at their rated wavelength. Across the 190–300 nm band, CaF₂ transmittance exceeds 90%; in the 300–800 nm band it is approximately 93–94%.

Optical homogeneity. We have measured a PV of 0.579 × 10⁻⁶ on a 15.5 mm thick CaF₂ sample. Index homogeneity directly affects wavefront error in imaging and interferometric applications — a parameter that must be specified for each clear aperture.

Stress birefringence. Our CaF₂ grades are customizable within 0–10 nm/cm, and our processing achieves ≤1 nm/cm low-stress capability. Birefringence control is critical for polarization-sensitive lithography and metrology systems.

Surface and sub-surface quality. Our processing achieves sub-surface damage ≤70 nm and surface roughness Rq ≤0.35 nm, which directly affect scatter loss and laser-damage initiation at DUV wavelengths.

For fused silica, grade-specific transmission and LIDT data should be obtained from the fused-silica supplier under matched wavelength and pulse conditions. Request these values explicitly rather than relying on generic datasheet ranges.

What Optical-Design and Environmental Factors Shift the Trade-Off?

System-level and environmental considerations often override raw optical performance in the material decision.

Thermal expansion. CaF₂'s CTE of 18.9 × 10⁻⁶/K versus fused silica's 0.55 × 10⁻⁶/K means CaF₂ elements require more careful thermal management, mounting compliance, and warm-up protocols.

Athermal pairing. CaF₂'s negative dn/dT combined with fused silica's positive dn/dT enables passively athermalized optical designs — a reason to use both materials together rather than choosing one exclusively.

Hygroscopicity. CaF₂ slowly absorbs moisture in humid environments, leading to surface degradation. Storage, handling, and operational environment controls (dry nitrogen purge, desiccated enclosures) are necessary for CaF₂ elements, adding to total cost of ownership.

Mechanical sensitivity. CaF₂ is softer and more sensitive to thermal shock than fused silica, affecting cleaning protocols, mounting stress limits, and shipping procedures.

UV-induced degradation. CaF₂'s primary degradation mechanism under UV irradiation is color-center (F-center) formation, which increases absorption bands. According to PatSnap Eureka, this effect is most pronounced below 157 nm but should be monitored at 193 nm under high cumulative dose.

Intrinsic birefringence. In high-NA systems, the crystal cut direction must be specified and modeled as part of the polarization error budget.

In summary: environmental robustness and thermal stability favor fused silica; ultra-low dispersion, athermal pairing, and extended UV/IR transmission favor CaF₂. The weight of each factor depends on the specific system architecture and operating environment.

How Do Processing, Coating, and Lead-Time Considerations Differ?

Practical manufacturing and supply-chain factors influence total cost of ownership beyond raw material performance.

Fused silica benefits from decades of accumulated polishing, figuring, and coating experience across the optics industry, giving it shorter lead times and broader supplier availability for standard specifications. CaF₂ requires specialized crystal-growth and processing expertise; we offer CaF₂ blanks and finished optics up to 600 mm in diameter, with standard and custom crystal orientations.

DUV coatings on CaF₂ substrates use fluoride thin-film materials whose adhesion, stress, and damage behavior differ from coatings on fused silica substrates — consult your coating supplier for material-specific guidance. Our CaF₂ coating material page provides detailed substrate-coating interaction guidance.

Our documented processing benchmarks — sub-surface damage ≤70 nm, surface roughness Rq ≤0.35 nm, and low stress ≤1 nm/cm — directly affect coating adhesion, scatter, and laser-damage initiation. Lead-time and cost comparisons between CaF₂ and fused silica are project-specific and should be confirmed directly with suppliers; no generic cost ratio is reliable across grades, sizes, and quantities.

Which Material Fits Which Optical Task?

The following decision framework organizes starting-point recommendations by optical task and wavelength.

This framework provides starting-point guidance. Final selection requires system-level error-budget analysis and supplier-verified batch data.

What Should You Require from Your Supplier?

A structured data-request checklist helps procurement teams and quality engineers build defensible specifications for DUV-grade CaF₂ or UV fused silica.

Transmission

• Internal transmittance at the operating wavelength, measured per cm, with measurement method and instrument specified.

Laser Damage

• LIDT at the operating wavelength with pulse duration, repetition rate, beam size, and number of test sites specified. For CaF₂, request cumulative-dose lifetime data if available.

Birefringence

• Stress birefringence in nm/cm, with measurement aperture and method.

Index Homogeneity

• PV and/or RMS over the clear aperture, with interferometric measurement conditions.

Refractive Index

• Measured value at the operating wavelength with stated uncertainty; for CaF₂, confirm crystal orientation.

Surface and Sub-Surface Quality

• Surface roughness (Rq), sub-surface damage depth, scratch-dig specification.

Thermal Properties

• CTE and dn/dT at the operating temperature range.

Traceability and Standards

• Ask whether the material or process is traceable to a recognized standard. We participated in drafting the CaF₂ national standard GB/T 44567-2024 in China and hold ISO 9001:2015 and ISO 14001:2015 certifications.

Environmental Handling (CaF₂)

• Confirm supplier recommendations for storage humidity, cleaning agents, and thermal-shock limits.

Specific pricing and lead-time data must be obtained directly from suppliers, as no reliable generic benchmarks exist for DUV-grade materials. The final material decision should be validated against the system-level error budget using the supplier's batch-specific measured data, not catalog typical values.

FAQ

What is the key difference between CaF₂ and fused silica for 193 nm optical systems?

The key difference is bulk absorption. CaF₂ offers intrinsically lower absorption at 193 nm — our CaF₂-A grade achieves >99.8% internal transmittance per cm at 193 nm — while fused silica's 193 nm performance is highly grade-dependent and limited by residual OH and impurity absorption. Fused silica remains the majority material in 193 nm systems because of its cost, thermal stability, and processing maturity, but CaF₂ is used for elements where absorption or chromatic correction is the binding constraint.

When should I choose CaF₂ over UV-grade fused silica for a 193 nm ArF lithography lens?

Choose CaF₂ when the lens element requires ultra-low absorption, high cumulative laser-damage survivability, or serves as the complementary dispersion element for chromatic aberration correction — CaF₂'s Abbe number of 95.2 makes it essential for apochromatic designs. Our CaF₂ has demonstrated survival of 2 billion pulses at 193 nm under specified test conditions without damage. For elements where thermal stability and cost dominate, fused silica is typically the better starting point.

How do CaF₂ and fused silica compare for 248 nm KrF excimer laser beam delivery?

At 248 nm, high-purity UV fused silica is the recommended starting material for most beam-delivery tasks because it provides a well-proven balance of transmission, damage resistance, and thermal stability. CaF₂ — such as our CaF₂-B grade with >99.8%/cm transmittance and LD-2 durability at 248 nm — should be evaluated only when pulse energy, cumulative damage lifetime, or dispersion requirements exceed fused silica's capabilities. CaF₂'s CTE is substantially higher than fused silica's (18.9 × 10⁻⁶/K vs. 0.55 × 10⁻⁶/K), requiring thermal-management redesign.

Does fused silica transmit well enough at 193 nm for high-power DUV applications?

Yes, but only for the right grade. Transmission at 193 nm is sensitive to OH content, metallic impurity levels, and manufacturing process. High-purity synthetic grades can serve many 193 nm roles and remain the majority material in ArF systems. However, for elements where even small residual absorption causes unacceptable heating or wavefront distortion, CaF₂ provides a lower-absorption alternative. Request grade-specific internal transmittance data at 193 nm from your fused-silica supplier.

What grade specifications should procurement teams request when sourcing CaF₂ or UV fused silica for DUV optics?

Request internal transmittance per cm at the operating wavelength, LIDT with full pulse-condition disclosure, stress birefringence in nm/cm, refractive-index homogeneity (PV over clear aperture), surface roughness, and sub-surface damage depth. For CaF₂, also confirm crystal orientation, hygroscopic handling requirements, and whether the supplier's process is traceable to a recognized standard such as GB/T 44567-2024. Validate all values against your system's error budget using batch-specific data.

Can CaF₂ and fused silica be used together in the same 193 nm optical system?

Yes — combining CaF₂ and fused silica in the same optical train is standard practice in all-refractive 193 nm lithography designs. CaF₂'s ultra-low dispersion (Abbe number 95.2) complements fused silica for chromatic aberration correction, and CaF₂'s negative dn/dT paired with fused silica's positive dn/dT enables passively athermalized "zero thermal shift" designs. The two materials are complementary, not mutually exclusive.

What environmental and handling factors affect CaF₂ performance compared to fused silica in DUV systems?

CaF₂ is hygroscopic and will slowly absorb moisture in humid environments, causing surface degradation, so it requires dry-nitrogen storage and controlled handling. It is also softer and more sensitive to thermal shock than fused silica, demanding gentler cleaning protocols and compliant mounting designs. Its CTE of 18.9 × 10⁻⁶/K versus fused silica's 0.55 × 10⁻⁶/K means thermal stabilization time and mounting stress must be carefully managed. Under prolonged UV irradiation, CaF₂ can develop color centers that increase absorption, particularly at wavelengths below 157 nm.

Discuss Your 193 nm or 248 nm Material Requirements with DESIOPTOE

The next step is to bring your specific wavelength and system requirements to our engineering team. We can help you select the right CaF₂ grade, specify custom birefringence targets, and size blanks up to 600 mm in diameter for your application.

Contact us at desioptoe.com to request batch-specific transmittance, LIDT, and homogeneity data for the CaF₂ grade that matches your operating wavelength. If you are exploring coating options on CaF₂ substrates, our CaF₂ coating material page provides substrate-coating interaction guidance to support your design.