  Diamond-like carbon (DLC) films are synthesized on Si using plasma enhanced
chemical vapor deposition. The role of substrate bias and nitrogen doping on
the structural evolution and local elastic modulus of DLC films are
systematically investigated. Raman spectroscopic studies reveal that the amount
of graphitic C=C sp2 bonding increases with substrate bias and nitrogen doping.
The density and hydrogen concentration in the films are found to vary from 0.7
to 2.2 g/cm3 and 16 to 38 atomic %, respectively, depending upon the substrate
bias and nitrogen concentration in the DLC films. Atomic force acoustic
microscopic (AFAM) analysis shows a direct correlation between local elastic
modulus and structural properties estimated by Raman spectroscopy, Rutherford
back scattering and elastic recoil detection analysis. AFAM analysis further
confirms the evolution of soft second phases at high substrate biases (> -150V)
in undoped DLC films. Further, N doping leads to formation of such soft second
phases in DLC films even at lower substrate bias of -100 V. The AFAM studies
provide a direct microscopic evidence for the "sub-implantation growth model"
which predicts the formation of graphitic second phases in DLC matrix at high
substrate biases.
