Broadband optical monitoring

A method of in situ monitoring a thin film deposition process on a substrate, the method comprising the steps of

• a) defining (10) a desired spectrum (T), said desired spectrum being a transmission or a reflection spectrum defined in a range of wavelengths;
• b) at least once:
• b1) illuminating (21) said substrate by means of a light source emitting light at least within said range of wavelengths,
• b2) receiving (22) light reflected from said substrate or transmitted through said substrate,
• b3) determining (23) at least one of a transmission and a reflection spectrum out of said received light within said range of wavelengths;
• c) defining a spectrum or a combination of spectra in dependency of said at least one spectrum determined in step b3) to be a current spectrum (C);
• d) determining (30) a weight spectrum (W) as a function of at least said current spectrum (C);
• e) calculating (40) a real number (K) as a function of said current spectrum (C), of said desired spectrum (T) and of said weight spectrum (W);
• f) exploiting (45) said real number (K) as indication for a deviation of said current spectrum (C) from said desired spectrum (T).