| Many thin film deposition processes supply varying amounts of energetic particle bombardment during growth that can influence the morphology, composition, and properties of the deposited film. This can be highly advantageous when the available thermal energy is low and cannot produce desirable film properties and morphology.; One such case of insufficient thermal energy is the standard processing of amorphous hydrogenated silicon (a-Si:H) on low-temperature polymeric substrates. The resulting material exhibits inhomogeneous morphology and surface roughness that is undesirable for optical applications. Deliberate energetic particle (mostly ions) bombardment during growth may help, but has been avoided due to the resulting poor electrical properties that are unacceptable for photovoltaic applications---the primary end-use for a-Si:H. Materials with poor electrical properties may still be quite useful for passive optical applications, leaving a-Si:H on polymers for optical applications a relatively unexplored area.; It is shown here that the evolving morphology can be modified substantially by ion bombardment due to the self-bias of the plasma during Plasma-Enhanced Chemical Vapor Deposition (PECVD) at substrate temperatures of 75°C. By modifying the PECVD parameters, smooth surfaces and dense morphology as quantified by atomic force microscopy (AFM) and spectroscopic ellipsometry, can be achieved. Quantification of the ion bombardment dose was obtained from Monte Carlo simulations of the ion dynamics within the sheath. Thresholds of ∼0,2 and 2 eV per deposited atom were found for void fraction modification and surface roughness suppression, respectively.; Additionally, the ion type was found to be important in modifying the final film morphology and material properties. Helium or argon were equally effective in increasing low-energy surface adatom diffusion. As a result, shadowing instabilities during growth could be suppressed, and homogeneous morphologies obtained. However, high-energy processes such as surface dehydrogenation could only be affected by argon ions, which translate a higher fraction of energy to the surface versus helium. By modifying the ion energy and type (global parameters that are used in many deposition processes), a wide range of materials suitable for optical applications on polymers can be realized. |
