Patent ID: 7388646

Claim:
A process of making optical systems by immersion lithography, comprising passing a light beam through a liquid layer between an optical element and a substratum at a wave length lower than 250 nm, wherein the liquid layer comprises perfluoropolyethers having the following formula: X 1 —O(CF 2 O) n (CF 2 CF 2 O) m (CF 2 CF 2 CF 2 O) p (CF 2 CF 2 CF 2 CF 2 O) q —X 2 (I) wherein: the repeating units —CF 2 O—, —CF 2 CF 2 O—, —CF 2 CF 2 CF 2 O—, —CF 2 CF 2 CF 2 CF 2 O— are statistically distributed along the chain; —X 1 and —X 2 are perfluoroalkyl chain end groups equal to or different from each other, having formula —(CF 2 ) z CF 3 wherein z is an integer from 0 to 3; n, m, p, q are integers comprised between 0 and 100, with the proviso that: at least one among n, m, p and q is different from zero; the ratio between the oxygen atoms and the carbon atoms O/C is higher than 0.33; and wherein chlorine atoms, determined by 19 F-NMR are present in an amount lower than 1 meg/kg perfluoropolyethers of formula (I).