In semiconductor manufacturing, photolithography is often referred to as the “soul of chip fabrication.” Whether it is Deep Ultraviolet (DUV) lithography or Extreme Ultraviolet (EUV) lithography, both rely on high-precision optical systems to accurately transfer nanoscale patterns. In these optical systems, the choice of mask substrate material is critical. Synthetic fused silica has become the mainstream substrate material for both DUV and EUV lithography, and its dominant position is well justified by its unique performance advantages.

Key Advantages of Synthetic Fused Silica

1. Ultra-High Transmittance

Synthetic fused silica exhibits extremely high transmittance in the wavelengths commonly used in lithography (193 nm, 248 nm, 365 nm, and 436 nm). With double-sided coatings, its transmittance can exceed 99.5%. This ensures minimal light energy loss during exposure, improving energy efficiency and image resolution.

2. Exceptional Thermal Stability

Photolithography equipment inevitably generates heat during high-speed operation under intense illumination. If the substrate has a high coefficient of thermal expansion, dimensional distortion can occur, affecting pattern transfer accuracy. Synthetic fused silica has an extremely low thermal expansion coefficient, maintaining dimensional stability even under high-energy exposure, ensuring reliable lithography performance.

3. Excellent Optical Homogeneity

Produced from high-purity materials, synthetic fused silica has a highly uniform refractive index throughout its volume. This optical homogeneity minimizes distortion during light propagation and ensures precise beam focusing on the mask, maintaining high imaging quality.

4. Nanometer-Level Surface Precision

In semiconductor lithography, even minor surface defects can lead to faulty chips. Through nanometer- or even sub-nanometer-level polishing processes, synthetic fused silica substrates achieve exceptionally smooth surfaces. Parameters such as total thickness variation (TTV), bow (BOW), warp (WARP), and surface flatness reach ultra-high precision, providing a solid foundation for depth-of-focus control and accurate pattern transfer.

Applications and Industry Significance

In advanced photolithography, synthetic fused silica is nearly irreplaceable. It serves as the fundamental material for DUV photomasks and plays a key role in the development of EUV optical components. Without this material, achieving leading-edge process nodes such as 7 nm, 5 nm, or even 3 nm would be extremely challenging.

Future Outlook

Although synthetic fused silica is the current industry standard, ongoing device miniaturization will impose even stricter requirements on optical materials:

Higher transmittance to cover shorter wavelengths or new exposure sources;

Enhanced thermal and mechanical stability to support higher energy-density lithography tools;

Even finer surface polishing, potentially approaching sub-nanometer limits;

Emergence of new optical materials that may surpass fused silica in transmittance, stability, and cost efficiency.

Until such alternatives emerge, synthetic fused silica remains the cornerstone of DUV lithography.

Conclusion

Synthetic fused silica is more than just a material—it is a critical pillar of the semiconductor industry. Its exceptional transparency, stability, purity, and surface precision provide irreplaceable support for advanced lithography processes. While new materials may eventually appear, for now, synthetic fused silica remains the most reliable and robust choice for high-precision photolithography.