Optical element and exposure apparatus
An optical element is configured to have a base composed of a LuAg substrate having a refractive index of 2.14±0.01 for a wavelength of 193 nm and an antireflection film formed on the base, and the optical element being configured to contact a liquid having a refractive index of 1.64±0.01 for the wavelength of 193 nm. The antireflection film includes a high refractive index layer that is formed on the base, contains Al 2 O 3 having a refractive index of 1.87 to 1.92 for the wavelength of 193 nm, and has an optical film thickness of 0.21λ to 0.34λ for a design center wavelength λ of 193 nm and a low refractive index layer that is closer to the liquid than the high refractive index layer, contains Al 2 O 3 having a refractive index equal to or smaller than 1.78 but larger than the refractive index of the liquid for the wavelength of 193 nm, and has an optical film thickness of 0.29λ to 0.52λ at the design center wavelength λ.
1. An optical element which includes a base composed of a LuAg substrate having a refractive index of 2.14±0.01 for a wavelength of 193 nm and an antireflection film formed on the base, and the optical element being configured to contact a liquid having a refractive index of 1.64±0.01 for the wavelength of 193 nm, said the antireflection film comprising:
a high refractive index layer that is formed on the base, contains Al 2 O 3 having a refractive index of 1.87 to 1.92 for the wavelength of 193 nm, and has an optical film thickness of 0.21λ to 0.34λ for a design center wavelength λ of 193 nm; and
a low refractive index layer that is closer to the liquid than the high refractive index layer, contains Al 2 O 3 having a refractive index equal to or smaller than 1.78 but larger than the refractive index of the liquid for the wavelength of 193 nm, and has an optical film thickness of 0.29λ to 0.52λ at the design center wavelength λ.
2. An optical element according to claim 1 , wherein the high refractive index layer is formed by magnetron sputtering, and has a film density of Al 2 O 3 of 2.94 g/cm 3 or greater.
3. An optical element according to claim 1 , wherein the low refractive index layer is formed by magnetron sputtering with a CF 4 gas that has a flow amount between sccm and 3 sccm.
4. An optical system having the optical element according to claim 1 .
5. An exposure apparatus having the optical element according to claim 1 .
6. A device manufacturing method comprising the steps of:
exposing a substrate by using the exposure apparatus according to claim 5 ; and
developing the substrate that has been exposed.