The great interest in the use of diamond-like carbon (DLC) films as a coating material is justified by the superior wear resistance and hardness, chemical inertness, and very low friction coefficients of these coatings. Vacuum arc is well suited to prepare super-hard films with high sp3/sp2 ratios. However, the high level of internal stresses originating during growth prevents the deposition of thick films and limits their applications. To study in-situ the development of stress in the growing film, we developed a simple optical technique based on the deflection of cantilever. Our system can measure stresses as high as few tens of GPa with a precision of few kPa. The influence on stress of bias voltage and duty cycle, and of the pulse length was studied as well as the effect of highly charged ions. The results of these studies show the key role of the energy input per ion on the stress level. Strategies to deposit thick hard films with minimum stress are proposed.