a-C:H thin films deposited by radio-frequency plasma: influence of gas composition on structure, optical properties and stress levels

a-C:H thin films are deposited by plasma-enhanced chemical vapor deposition (PE-CVD) at 13.56 MHz at room temperature. Three different precursor gas mixtures are used (CH4, CH4/He, CH4/Ar). Structure, optical properties and stress levels are evaluated by elastic recoil detection analysis (ERDA), IR absorption, UV/vis spectrometry, Raman spectroscopy. We observe a loss of hydrogen content (bonded and not bonded) from 38 to 24 at.%, as well as an increasing of sp2 content (from 14 to 29%) with the increase of self-bias voltage for all mixtures. Argon and helium addition to methane induce a greater graphitization of a-C:H thin films. These modifications induce a decrease of the optical gap which is set between 1.4 and 1.1 eV and an increase of the Urbach gap from 0.6 to 0.8 eV. The internal stresses are controlled by subplantation model and decrease from 4 to 1 GPa with the increase of the bias voltage. The use of argon and helium as carrier gas induce lower stress in the films.