Chemical structure and mechanical properties of Si-containing a-C:H and a-C thin films and their Cr- and W-containing derivatives

Abstract

Si-containing a-C:H and a-C thin films with nitrogen, oxygen and transition metal (Cr and W) additives were deposited on polished single crystalline silicon substrates at room temperature by plasma activated CVD, magnetron-sputtering PVD and also by combined PACVD–PVD techniques.In particular Si–, Si–O– and Si–N-containing a-C:H films (denoted as a-C–Si, a-C–Si–O and a-C–Si–N) were deposited respectively from tetramethylsilane (TMS), hexamethyldisiloxane and hexamethyldisilazane vapourised precursors in He carrier gas by electron cyclotron wave resonance RF plasma. Cr-containing a-C:H films, further denoted as a-C–Si–Cr, were deposited with combined PVD–PACVD by sputtering chromium and carbon targets in argon and introducing TMS vapour. Wcontaining a-C films (denoted as a-C–Si–W) were deposited by PVD with simultaneous sputtering of the constituents in Ar. Detailed characterisation of the composition and chemical state of the elements present in the films were done by X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy. Mechanical properties, hardness (H), reduced modulus (E) and scratch behaviour were studied by depth-sensing nanoindentation and scratch tests.In the a-C:H films, the C/Si ratio varied between 1.5 and 3.5 showing a significant deficiency of C as compared to the composition of the precursors. The Cr and W content in the a-C–Si–Cr and a-C–Si–W films varied in a very broad 2–50at.% range.The chemical state of carbon was primarily of sp2 and partly of sp3 type C–C with XPS chemical shifts for C 1s at 284.3eV and 285.0eV, respectively. The Si in the films is bonded predominantly to carbon (Si 2p at 100.8eV BE) or to nitrogen in N-containing films (Si 2p at 101.3eV BE). In the a-C–Si–Cr films Cr–C bonding states were determined (Cr 2p3/2 at 574.5eV and C 1s at 282.8eV). Si formed predominantly Si–C and also Si–Cr bonds. In the a-C–Si–W, films C–Si and C–W (W 4f7/2 at 32.3eV) chemical bonds could be identified.It was discovered that the modified Auger parameter for silicon, αSi (derived from the Si 2p electron and Si KLL Auger line energy), sensitively reflects the entire chemical structure of these films, including crosslinking and densification. These spectroscopic data, supported further by the increase of the bulk plasmon loss energy of the C 1s peak, were directly connected with the mechanical properties of these films. The amorphous nature of the films was deduced from the chemical state analysis and was verified also by transmission electron microscopy (TEM) and electron diffraction (ED) studies.Hardness and elastic modulus of the a-C–Si–(O,N) films could be adjusted in a wide range of approx. 5–15GPa and 40–140GPa, respectively. Systematic alteration of these values with the composition (C/Si, O/Si and N/Si atomic ratio) and with the chemical structure (αSi) was established and their interrelations are discussed. Whilst incorporation of Cr does not alter the mechanical properties, the addition of W increases H and E up to 19GPa and 210GPa, respectively.