Tailoring sheet resistance through laser fluence and study of the critical impact of a V-shaped plasma plume on the properties of PLD-deposited DLC films for micro-pattern gaseous detector applications

Diamond-like carbon (DLC) is an attractive inexpensive alternative to diamond. The control of the C (sp3)/C(sp2) ratio, influencing the material properties, is fundamental for its use in dedicated applications like, for example, the fabrication of a particular class of gas detectors for ionizing particles termed Micro-Pattern Gaseous Detectors (MPGDs). A deposition technique enabling control on the amount of C(sp3) hybridized bonds, hydrogen content and disorder is pulsed lased deposition (PLD). In this paper, we report on the preparation of hydrogen-free DLC films by nanosecond excimer laser-based ablation of a graphite target correlating the laser fluence with the electrical properties of the DLC films to be used as protective resistive layers in MPGD detectors. Moreover, unprecedented physical insight and discussion are provided about the critical aspects of both occurrence of a V-branched plume and its composition on the spatial distribution of C(sp3) bonds and film uniformity. Improvement strategies are demonstrated by comparing and discussing two classes of PLD experiments: ON-axis (coaxial plasma expansion axis and substrate symmetry axis) and OFF-axis (substrate-axis shifted with respect to the plume-axis and rotating substrate). Optimally working OFF-axis configuration conditions are shown to lead to fine tuning of resistivity and uniformity of composition of C(sp3) bonding over a few cm2 extended areas. Sheet resistance (3.1 ± 0.3) × 109 Ω/sq., corresponding to C(sp3) bonding concentration of ~86%, is reported in OFF-axis experiments at a fluence of 5.5 J/cm2 onto kapton substrates which are used for the realization of MPGDs.