CaF₂ (Calcium Fluoride) Single Crystal

Advantages

• Optical transmission range: 0.13-10 μm.
• Excellent DUV performance: Internal transmittance >99.8%/cm at 193 nm; >90% at 157 nm, making it the material of choice for 193 nm lithography optics .
• Low refractive index and dispersion: n = 1.39908 at 5 μm; low chromatic aberration enables achromatic lens designs without complex correction .
• High laser damage threshold: Suitable for high-fluence excimer laser systems (ArF 193 nm, KrF 248 nm) .
• Low stress birefringence: Typically ≤5 nm/cm (P-V), with advanced grades ≤2 nm/cm.
• Non-hygroscopic and chemically stable: Resistant to climatic exposure and weakly acidic environments.
• Cubic structure: Optically isotropic (no intrinsic birefringence), available in large diameters up to 350 mm.

Typical Specifications

• Crystal structure: Cubic, fluorite type, space group Fm3m; cleavage plane (111)
• Lattice constant: 5.462 Å
• Transmission range: 0.13–10 μm
• Internal transmission: >99.8%/cm @ 193 nm; >99.9%/cm @ 248 nm
• Refractive index: n = 1.39908 @ 5 μm; n = 1.433864 @ 587.6 nm
• Refractive index homogeneity: ≤2 to ≤5 ppm (P-V) for premium grades
• Stress birefringence: ≤2 to ≤5 nm/cm (P-V)
• Density: 3.18 g/cm³ at 25°C
• Melting point: 1360–1420°C (sources vary)
• Thermal conductivity: 9.71 W/(m·K) at 25°C
• Thermal expansion coefficient: 18.85 × 10⁻⁶/K
• Knoop hardness: 158.3 (100) with 500 g indenter
• Young‘s modulus: 75.8 GPa
• Poisson’s ratio: 0.26
• Solubility in water: 0.0017 g/100 g H₂O at 20°C
• Dielectric constant: 6.76 at 1 MHz

Main Applications

• Deep UV lithography optics: Lenses and windows for 193 nm (ArF) and 157 nm (F₂) excimer laser lithography systems. CaF₂ is one of only two materials suitable for 193 nm immersion lithography (along with fused silica).
• Excimer laser windows: High-fluence windows for ArF (193 nm), KrF (248 nm), and XeCl (308 nm) lasers .
• IR spectroscopy and thermal imaging: Windows, prisms, and lenses for FTIR spectroscopy and thermal imaging systems (0.13–10 μm).
• Achromatic lenses and optical systems: Wide-band transmittance and low dispersion enable achromatic lens designs for telescopes, cameras, and imaging systems .