Reflective optics for EUV refers to the use of multilayer Bragg mirrors instead of conventional lenses to focus and image extreme ultraviolet (EUV) light at 13.5 nm wavelength in lithography systems. At EUV wavelengths, no practical transparent lens material exists, making reflection the only viable optical approach.

Why Mirrors Instead of Lenses?

• At 13.5 nm wavelength, virtually all materials absorb EUV light — including glass, quartz, and every material used in conventional optical lenses.
• Even air absorbs EUV strongly — the entire beam path must be in vacuum.
• Only specially engineered multilayer mirrors can reflect EUV light efficiently enough for practical use.

Multilayer Mirror Construction

• EUV mirrors consist of 40–50 alternating layers of molybdenum (Mo) and silicon (Si), each layer approximately 3.4 nm thick (half the wavelength).
• Each Mo/Si interface reflects a small percentage of light. When layers are spaced at the correct period, reflections from all interfaces constructively interfere (Bragg reflection), amplifying the reflected signal.
• Peak reflectivity of a single Mo/Si mirror is approximately 67–70% at 13.5 nm.

EUV Optical System

• A typical EUV scanner uses 6 mirrors in the projection optics (from mask to wafer). Each mirror reflects ~67%, so the total optical throughput is approximately $0.67^6 \approx 9\%$.
• Including the reflective mask (also a multilayer mirror), overall light efficiency from source to wafer is only ~2–4% — a major engineering challenge.
• Each mirror must be polished to sub-50 picometer RMS surface roughness — making them the most precise optical surfaces ever manufactured.

Mirror Challenges

• Surface Precision: Sub-angstrom figure accuracy over large areas. Any imperfection scatters light and degrades image quality.
• Contamination: Carbon deposition and oxidation on mirror surfaces degrade reflectivity over time. Active cleaning systems (hydrogen plasma) are used in the scanner.
• Thermal Management: EUV mirrors absorb ~30% of incident light as heat, requiring precise thermal control to prevent distortion.
• Coating Uniformity: The multilayer stack must have sub-angstrom thickness uniformity across the entire mirror surface.

EUV reflective optics represent one of the greatest precision engineering achievements in human history — enabling high-volume semiconductor manufacturing at wavelengths where no other optical approach is viable.