The current recording density on a rigid disk media increases at an annual rate of about 60%, and such a trend is seen to continue in the near future. This requires a continual shrinkage of the head to magnetic recording film spacing and hence a thinner and thinner overcoat on the magnetic film. Developing sub-100 Angstrom overcoats and establishing accurate measurement and control schemes are then a must. This paper will discuss the growth mechanism for diamond-like carbon (DLC) on magnetic alloy (CoCrTaPt) and non-magnetic alloy surfaces. The DLC films were deposited using a physical vapor deposition technique. Transmission electron microscopy (TEM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), angle resolved X-ray photon spectrometry (ARXPS) were used to analyze the film thickness. It was found that the growth mechanism depends on the substrate surfaces that could cause the variations of the DLC overcoat thickness by up to 14 Angstrom even with the exactly the same DLC deposition conditions. The thickness difference of DLC film on different substrates is determined at early stage of deposition due to the difference in initial nucleation, and possibly the density. An N & K analyzer, which is calibrated by TEM, is used to support the disk production and provide instant feedback.