Nitrogen-doped chemical vapour deposited diamond-like carbon (DLC) films were treated by reactive ion etching system under various conditions of CHF3 gas successively after pre-treatment with oxygen. Atomic force microscopy and Raman spectroscopy were carried out in order to characterize the surface morphology and chemical bond, respectively. Scanning tunnelling microscopy was used in order to investigate the surface state of the DLC films at the nanoscale level. Scanning probe field emission current measurement was performed in order to obtain the emission current mapping. The emission sites appeared to a great extent after the surface treatment by CHF3 gas and a clear activation effect was observed. We confirmed that the surface treatment using CHF3 gas affected not only the appearance of emission sites but also the activation process.