CH基团与金刚石(111)面的碰撞反应及其对碳膜生长的影响

等离子体增强化学气相沉积技术中的碳膜选择性自组装机理是高性能碳膜制备过程中的挑战性基础课题.采用经典分子动力学方法,模拟了不同能量(1.625-65 eV)的CH基团在清洁金刚石和吸氢金刚石(111)面上的轰击行为,获得了吸附、反弹、反应等各类事件的发生概率,并据此探讨了含氢碳膜制备过程中CH基团的贡献.结果表明,随着入射能量的增加,CH基团对薄膜生长的贡献由单纯的吸附、反弹机理向反应、吸附混合机理转变,其中最主要的反应过程是释放一个或两个氢原子的反应,而释放氢分子的反应则很少发生.这些反应不仅使薄膜生长过程更均匀、薄膜表面更平整,还降低了薄膜的氢含量.生长机理的转变导致低能量条件下所成薄膜中的多数碳原子都包含一个氢原子作为配位原子,而高能量条件下的薄膜中的碳原子则很少有氢原子作为配位原子.另外,通过分析sp~3-C和sp~2-C数目的变化,研究了CH基团对金刚石基底的破坏作用.

Collision reactions of CH radical on diamond and their effects on the carbon film growth

The growth mechanism of hydrogenated carbon films in plasma-enhanced chemical vapor deposition (PECVD) is complicated and much attention has to be paid to it for the unique properties of carbon films. In this paper molecular dynamics simulations are carried out to illustrate the collision behaviors of CH radical on the clear and hydrogenated diamond (111) surface with varying incident energy (from 1.625 to 65 eV), aiming at the growth mechanism of hydrogenated carbon film by PECVD. Our simulations show that the behaviors of incident CH radical can be divided into adsorbing, rebounding, reaction releasing one H atom and reaction releasing two H atoms, while the reaction releasing one H₂molecule rarely occurs. At low incident energy, selective adsorption of CH at unsaturated surface C site is the dominated growth mechanism since no reactions can conduct. Such growth model results in films with rough surface, high hydrogen fraction, and loose structure. As the incident energy increases, two chemical reactions that one releases one H atom and the other releases two H atoms are important. Caused by these reactions, the saturated C site in the surface will be transferred into unsaturated one, so that it can further adsorb subsequently incident CH radicals. The occurrence of these reactions makes films grow more uniformly, leading to the smoothness and dense structure of the films. The hydrogen fraction in the films will be reduced by these reactions. To confirm the above growth mechanism, the carbon film growth from CH radicals are then simulated. The film obtained with low energy (3.25 eV) CH radicals is found to be loose, rough, and have many carbon chains with adsorbed hydrogen atoms on the surfaces, while the film produced with high energy (39 eV) radicals are dense, smooth and the chains on the surfaces are short and have less hydrogens. On the other hand, most of the C atoms in the films deposited with low energy have one H atom as coordination, while for high energy most of C atoms in the films have no H atom as coordination. These observations agree well with the proposed growth mechanism. The destruction effects caused by the incident CH radicals are also analyzed based on the variation of the sp²-C and sp³-C in the films.