Characterization of diamond-like carbon thin films produced by pulsed-DC low pressure plasma monitored by a Langmuir probe in time-resolved mode

Abstract

The deposition process and physical properties (mechanical and surface) of diamond-like carbon (DLC) films prepared at room temperature by asymmetric bipolar pulsed-DC methane plasma-enhanced CVD (PECVD) are described in this study. The pulse frequency ranged from 100 to 200 kHz and voltage peaks ran between −600 and −1400 V. Plasma parameters within the pulse cycle were measured by a Langmuir probe with a time resolution of 1 μs. The results show a splitting of two electron populations at high positive bias voltage of the I– V plasma characteristics, the hotter one reaching a temperature of 10 eV. Plasma potential varied smoothly around 30 V and ion density increased promptly in the low-voltage region of the pulsed-DC cycle. Indentation and surface profilometry provided the results of hardness and stress, respectively. The behavior of the friction coefficient was analyzed by the scratch test method, using a 200 μm-diameter diamond spherical tip. Film characteristics are discussed as a function of technological parameters and compared to the characteristics of films grown using methane-glow discharge excited by rf power. For industrial purposes, pulsed-DC PECVD is a promising alternative to the more commonly used rf PECVD, because of the lower cost of its technology, lower film stress and higher deposition rate. DLC films grown by pulsed-DC PECVD with improved mechanical properties (high adherence and wear resistance, low stress, low roughness and low friction coefficient) are a real alternative for use as protective coatings on magnetic storage devices and sliding surfaces.