An optical device reflecting a range of wavelengths comprised between 10 nm and 20 nm comprises alternate superposed first and second layers. The first layers are made of metal or metallic compound and the second layers are formed by an amorphous silicon compound chosen from a-Si—H_x, a-Si—CH_x, a-Si—C_x, a-Si—OH_x, a-Si—F_x, a-Si—FH_x, a-Si—N_x, a-Si—NH_x, x being comprised between 0.01 and 0.3. The use of second layers of amorphous silicon compound enables the mechanical stresses of the optical device to be stabilized up to at least 200 C. The optical device is preferably used as reflector for a lithography mask in the extreme ultraviolet (EUV).