Ion bombardment during plasma‐enhanced chemical vapor deposition of hydrocarbon films mainly governs the properties of the films. The range of this ion‐induced modification of the optical properties of hydrocarbon films was determinedinsituby monochromatic ellipsometry. The hydrocarbon films were deposited by an electron cyclotron resonance methane plasma onto silicon substrates and additional rf bias was applied to vary the kinetic energy of the impinging ions. The ion‐induced modification of the film properties was investigated by means of a double layer consisting of a polymerlike film with low optical absorption and a hard carbon film with high absorption on top. The deposition of this double layer was monitoredinsituby ellipsometry during the growth and during the erosion of this film system in an oxygen plasma at floating potential. From these data it is possible to determine with high accuracy the range of the ion‐induced modification of the optical properties. This layer ranges from 6 A˚ at 30 V dc self‐bias to 40 A˚ at 100 V dc self‐bias, which is consistent withTRIM.SPcalculations for the bombardment of polymerlike films by hydrogen ions. ©1995 American Institute of Physics.