A window module includes: a window; a first anti-reflection layer disposed on the window; and a second anti-reflection layer disposed on the first anti-reflection layer, including magnesium fluoride and having a refractive index smaller than a refractive index of the first anti-reflection layer.

An optical system includes a bulk material including a fluorine (F)-containing optical material. The bulk material is exposed to an environment at a pressure ranging from atmospheric to vacuum when the bulk material is under extreme ultra-violet (EUV), vacuum ultra-violet (VUV), deep ultra-violet (DUV) and/or UV radiation. The environment includes at least one type of gas or vapor. The at least one type of gas or vapor includes polar molecules.

An optical system includes a bulk material including a fluorine (F)-containing optical material. The bulk material is exposed to an environment at a pressure ranging from atmospheric to vacuum when the bulk material is under extreme ultra-violet (EUV), vacuum ultra-violet (VUV), deep ultra-violet (DUV) and/or UV radiation. The environment includes at least one type of gas or vapor. The at least one type of gas or vapor includes polar molecules.

A method for producing nickel sulfate according to exemplary embodiments may prepare a raw material containing nickel. A first leachate may be produced by selectively leaching nickel from the raw material containing nickel using an oxidant. A second leachate may be produced by separating a residual metal from the first leachate. Thereafter, nickel sulfate may be obtained from the second leachate. Accordingly, unnecessary byproducts may be reduced by omitting a separate nickel extraction step.

A method for producing nickel sulfate according to exemplary embodiments may prepare a raw material containing nickel. A first leachate may be produced by selectively leaching nickel from the raw material containing nickel using an oxidant. A second leachate may be produced by separating a residual metal from the first leachate. Thereafter, nickel sulfate may be obtained from the second leachate. Accordingly, unnecessary byproducts may be reduced by omitting a separate nickel extraction step.