Patent ID: 7035000

Claim:
An optical element comprising: a quartz substrate; an antireflection film comprising three low-refractive-index layers having a refractive index ns satisfying the expression 1.50≦ns≦1.60 and an extinction coefficient ks satisfying the expression 0<ks≦1×10 −5 , for ultraviolet light having a wavelength of 193 nm; and three high-refractive-index layers having a refractive index na satisfying the expression 1.74≦na≦1.77 and an extinction coefficient ka satisfying the expression 1×10 −5 ≦ka≦1×10 −3 , for ultraviolet light having a wavelength of 193 nm, a total geometrical thickness thereof being in a range of about 55 to about 65 nm, wherein the low-refractive-index layers and high-refractive-index layers are alternately layered on the quartz substrate, wherein one of the high-refractive index layers is a most proximal layer of the antireflection film to the quartz substrate, wherein the antireflection film has a design center wavelength λ 0 in a range of 180 nm to 220 nm, and wherein respective optical thicknesses d 1 , d 2 , d 3 , d 4 , d 5 , and d 6 of the high-refractive-index and low-refractive-index layers at first to sixth positions from a substrate side satisfy the following expressions: 0.02λ o ≦d1≦0.10λ o , 1.00λ o ≦d2≦2.00λ o , 0.22λ o ≦d3≦0.30λ o , 0.22λ o ≦d4≦0.30λ o , 0.22λ o ≦d5≦0.30λ o , and 0.22λ o ≦d6≦0.30λ o .