Ceramics and glasses · Workhorse

ULE and Zerodur (near-zero CTE)

ULEvalues describe: ULE 7973

Why it wins

TiO2-doped fused silica and glass ceramic with near-zero thermal expansion: EUV mask substrates and scanner metrology frames, because nanometer patterns cannot ride thermal expansion.

Why not the alternative

Standard silica CTE is too high by orders of magnitude.

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	optical clean only: detergent, DI, solvent drag-wipe, no immersion of coated surfaces
Forbidden	anything that could touch the figure or the coating
Limit	surface figure in nanometres, so cleaning is an optical operation

Vacuum and outgassing

Outgassing, unbaked (10 h)	1e-9 mbar·L/s/cm²
Outgassing, baked	1e-12 mbar·L/s/cm²
Vapour pressure	none

Temperature

Bake, assembled	200 °C
Vacuum degas	300 °C
Metallurgical limit	Zerodur ceramises above 600 °C; ULE anneals ~1000 °C

Thermal

CTE	0 ± 0.03 ppm/K
Thermal conductivity	1.3 W/m·K
Specific heat	770 J/kg·K
Emissivity	0.9
Melting / softening	1490 soft. °C

Mechanical

Strength	50 flex MPa
Elongation	nil
Young's modulus	67–91 GPa
Hardness	600 HV
Density	2.21 ULE, 2.53 Zerodur g/cm3

Electrical and magnetic

Relative permeability	1.0
Resistivity	>1e15 Ω·cm

Engineering

Corrosion	inert
Joining	optically contacted or low-stress bonded
Process notes	the substrate for EUV mask blanks and mirrors
Availability and cost	Corning and Schott, long lead, high cost

Grades

Corning ULE 7972	EUV mask and mirror substrates
Zerodur class 0 / 1 / 2	expansion class per metrology budget

Sources · 1

1. EUV lithography overview, LTEM substrates

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