Ruvalues describe: 2 nm Ru cap
Semiconductor equipment & lithography
Why it wins
The 2 nm skin that keeps Mo/Si EUV mirrors alive: resists oxidation and the hydrogen ambient while transmitting 13.5 nm light.
Why not the alternative
A bare silicon top layer oxidizes and the reflectivity is gone.
Watch out
Tin debris on Ru drops the hydrogen penetration barrier (about 1.06 eV to 0.28 eV with stannane), hydrogen recombines under the layers and blisters the mirror. This is an open materials war.
Properties
The values below are candidate: compiled from the sources named, not yet individually validated.
Provisional provenance: Semiconductor chamber materials and plasma-resistant coating literature.
Wet cleaning
| Recipe | hydrogen radical cleaning in situ, not wet cleaning: the standard EUV method for removing carbon |
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| Forbidden | any wet chemistry on a multilayer; hydrogen also causes blistering, the current research problem |
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| Limit | the trade between carbon removal and hydrogen-induced blistering |
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Vacuum and outgassing
| Outgassing, unbaked (10 h) | 5e-10 mbar·L/s/cm² |
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| Outgassing, baked | 5e-13 mbar·L/s/cm² |
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| Vapour pressure | negligible |
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Temperature
| Bake, assembled | 400 °C |
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| Vacuum degas | substrate |
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| Metallurgical limit | oxidises to volatile RuO₄ under oxidising plasma |
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Thermal
| CTE | 6.4 ppm/K |
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| Thermal conductivity | 117 W/m·K |
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| Specific heat | 238 J/kg·K |
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| Emissivity | 0.2 |
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| Melting / softening | 2334 °C |
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Mechanical
| Strength | coating |
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| Elongation | nil |
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| Young's modulus | 447 GPa |
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| Density | 12.45 g/cm3 |
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Electrical and magnetic
| Relative permeability | ~1.0 |
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| Resistivity | 7.1 µΩ·cm |
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Engineering
| Corrosion | oxidation resistant in the EUV environment |
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| Joining | magnetron sputtered, nm scale |
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| Process notes | hydrocarbon partial pressure is its enemy, not outgassing |
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| Availability and cost | inside the EUV supply chain only |
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Grades
| Sputtered Ru, about 2 nm | the production cap: oxidation block versus EUV absorption trade |
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| Alloyed and oxide caps (research) | candidates against blistering and Sn uptake, not production |
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Sources · 3
- Tin the enabler, hydrogen diffusion into ruthenium
- DIFFER, Hotel Ruthenium
- Chen, oxidation, outgassing and blistering in EUV
← The Matter