Paper PS-WeM6
Molecular Dynamics Simulation for Hydrogen Plasma Processing of Graphene

Wednesday, October 30, 2013, 9:40 am, Room 104 C

Graphene is a two-dimensional material with unique physical, chemical and mechanical properties, promising for novel applications in industrial scale. The successful development of graphene-based thin film technologies relies on the capability to grow and integrate this material into sophisticated devices, but the nm-scale control of graphene processing challenges current technology, especially in plasma treatment.

The main issue associated with plasma/graphene processes is the few- to mono-atomic thickness of the material: graphene is easily damaged upon exposure to reactive plasma. This precludes the use of conventional plasma technologies to clean, dope and pattern graphene layers in a controlled way as is done for other materials in the microelectronic industry. Pulsed inductively coupled plasmas (ICPs) could in principle alleviate this issue by reducing ion bombardment energy while retaining active radicals. Hydrogen plasma has been shown to be promising for graphene treatment with minimal damage, but little is known of the fundamental mechanisms.

In this work, we applied classical molecular dynamics (MD) simulations of H2 plasma / graphene interaction to assist the development of two important processes: graphene surface cleaning (selective removal of polymeric PMMA residues from its surface) and graphene nanoribbon (GNR) patterning with well controlled edges. Using MD we investigate the impact of the graphene temperature and incident H species energy on nanoribbon modification. We found that on the ribbon basal plane, H species experienced a repulsive force due to delocalized π-electrons, which prevents them from chemisorption if their energy is below ~0.6 eV. By contrast, there is no barrier for H chemisorption on GNR edges and the graphene border can be rapidly hydrogenated by H radicals without damaging the basal plane. MD simulations further suggest that lateral etching will not occur unless the graphene temperature is raised above ~ 600 K and that etching probability slows above about 800K. This result is in good agreement with experiments. We also show that exposure of graphene to energetic H above a threshold of ~12 eV leads to H penetration through graphene. Severe damage of the graphene basal plane (i.e. C-C bond breaking) is observed at incident H energies higher than ~15 eV. This suggests that ions and fast neutrals from pulsed ICPs, which impact surfaces at ~ 1-10 eV, may be well suited for graphene cleaning and GNR trimming. This result has now been confirmed experimentally. XPS, AFM, Raman and electrical measurements show that pulsed H2 plasmas clean PMMA residues from graphene surface with almost no damage after annealing.