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44934-G5
Fundamental Research in Plasma Enhanced Chemical Vapor Deposition of Amorphous Carbon-Based Films from Hydrocarbon Plasmas

Sumit Agarwal, Colorado School of Mines

Hydrogenated amorphous carbon (a-C:H) is a versatile material with a vast number of applications, such as a wide band gap semiconductor for high-performance electronic devices, protective coatings for infrared optical elements and magnetic storage disks, inert biocompatible coatings, and as cold cathode emitters for flat-panel displays. a-C:H films are generally deposited over large areas at relatively low substrate temperatures by plasma-enhanced chemical vapor deposition (PECVD) using hydrocarbon feed gases such as CH₄ or C₂H₂ diluted in H₂ or an inert gas, such as Ar. The properties of these films depend on the sp³-to-sp² hybridization ratio and the H content. Depending on the composition, these films demonstrate different characteristics, such as hardness, electrical resistivity, optical transparency and chemical inertness. Films with hardness and resistivity similar to diamond are referred to as diamond-like carbon. On the other hand, films with high hardness, high sp² content, and a low optical band gap are referred to as hard graphitic a-C:H. During PECVD, a variety of radicals are generated in the plasma, which impinge onto the growing film's surface, and react, leading to film growth. The interaction of H atoms with the a-C:H surface is particularly important in determining both the sp³-to-sp² hybridization ratio and the H content in the film, which is the focus of this study.

The specific interactions of H/D atoms with hard a-C:H films were investigated using a combination of atomistic simulations and experiments based on surface spectroscopic techniques. To study these interactions, realistic a-C:H films were created using molecular dynamics (MD) simulations, which were subsequently impinged with H atoms at thermal energies. A more accurate method was developed to characterize the sp² and sp³ hybridization states in the a-C:H film based not just on the number of nearest neighbors, as previously reported in the literature, but also on the bond energy. This facilitated a more accurate description of the reaction pathways of H atoms with a-C:H surfaces due to the ability to identify the creation of dangling  bonds and the corresponding change in hybridization. The analyses of the MD trajectories revealed that the hydrogenation of hard a-C:H films occurs primarily at the sp² sites via an Eley-Rideal mechanism. The hydrogenation reaction was highly exothermic, >2.5 eV, with a negligible activation energy barrier. We observed that hydrogenation at the sp² sites may or may not create a dangling bond: this depended on the first and second neighbors of the C atom to which the H atom was attached. The experimental data was consistent with the MD simulations. Both spectroscopic ellipsometry and Raman spectroscopy data indirectly showed that the sp³ content of the a-C:H film increased upon exposure to D atoms. The attenuated total reflection Fourier transform infrared spectroscopy data showed D addition to the a-C:H film and etching. While the set of reactions discussed herein show that interaction with H/D atoms led to an overall change from sp² to sp³ hybridization in these films, the films remained amorphous and did not show any increase in order towards a diamond-like structure. Thus the interaction of H atoms with a-C:H films is more complex than with amorphous films of other group IV elements such as Si and Ge, which show increased order upon H exposure. Therefore, we conclude that other reaction pathways must also exist that eventually lead to ordering of these films at temperatures much lower than those required for thermal annealing.

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