What Material is the EUV Mask Made of?

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The mask of EUV is reflective, but our common mask is transmissive, why is this happening?

Why is the mask of EUV reflective?

The wavelength of EUV light is 13.5 nm, and this extremely short wavelength is strongly absorbed when passing through any material, resulting in no suitable transparent material to effectively transmit EUV light, making traditional transmission masks unfeasible in EUV lithography. Structure of the EUV mask

As shown in the figure above, the EUV mask adopts a multi-layer film structure that uses the Bragg reflection principle to reflect EUV light. It can achieve a reflectivity of up to about 70%, which improves the accuracy and efficiency of lithography.
Anti-reflective coating (ARC) (Anti-Reflective Coating): Reduces the reflection on the surface of the reticle and increases the efficiency of light absorption. Absorber: The actual cambium layer of the lithography pattern that absorbs unreflected EUV light. Generally, the absorbing layer materials are Ta, TaN, TaBN and so on. Ru capping: Protects the multi-layer membrane structure against oxidation and damage.

Mo/Si multilayer mirror(Molybdenum/Silicon Multilayer Mirror): 40 to 50 alternating layers of silicon and molybdenum are deposited on top of the LTEM substrate as a Bragg reflective layer for maximum reflection of EUV at the 13.5nm wavelength.
Low thermal expansion material (LTEM)(Low thermal expansion material): Mask substrate material with high thermal stability and minimal dimensional change in the face of drastic temperature changes.

Conductive backside coating(Conductive Back Coating): Typically composed of chromium nitride to reduce static build-up and facilitate electrostatic holding.