This paper reports the effect of positive substrate bias (V-s) varying from 0 to 180 V on the spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS) and X-ray Auger electron spectroscopy (XAES) studies of diamond-like carbon (DLC) films deposited using CH4 gas as a feedstock into a saddle field fast atom beam (FAB) source. The values of optical constants like refractive index (n) and extinction coefficient (k) of the deposited DLC films were determined using a two phase model. The values of 'n' were found to fall in the range from 1.505 to 1.720 and 'k' from 0.03 to 0.125 by application of different values of V-s. Value of these optical constants were found to decrease with the increase of substrate bias up to 90 V and then increase beyond this value. Position of C 1s peak evaluated from XPS data was found to occur at 286.09 +/- 0.18 eV in DLC films deposited by application of different values of V-s. Observation of full width at half maximum (FWHM) (tau) value (1.928 eV at V-s = 0 V, 2.0 eV at V-s = 90 V and 1.89 eV at V-s = 180 V) clearly hinted the existence of a point of inflection in the properties of DLC films deposited using FAB source this way. A parameter 'D' defined as the distance between the maximum of positive going excursion and the minimum of negative going excursion was calculated in the derivative XAES spectra. The values of D evaluated from XAES data for DLC films were found to be 14.8, 14.5 and 15.2 at V-s = 0, 90 and 180 V, respectively. The sp(2) percentage was calculated for samples deposited this way and was found to be low and lie approximately at 5.6, 2.8, 2.3, 5.7 and 11.5 for different values of V-s = 0, 50, 90, 150 and 180 V. The sp(3) content percentage and sp(3)/sp(2) ratio was found to be 94.4 and 16.7. 97.7 and 42.5 at V-s = 0 and 90 V, respectively. Beyond V-s = 90 V these values started decreasing. Mainly, a point of inflection in all the proper-ties of DLC films studied over here at around 90 V of applied substrate bias has been observed, which has been explained on the basis of existing theories in the literature. (C) 2003 Elsevier B.V. All rights reserved.