Weckman Timo, Laasonen Kari
Aalto University, Kemistintie 1, Espoo, Finland.
Phys Chem Chem Phys. 2015 Jul 14;17(26):17322-34. doi: 10.1039/c5cp01912e.
Atomic layer deposition (ALD) is a coating technology used to produce highly uniform thin films. Aluminiumoxide, Al2O3, is mainly deposited using trimethylaluminium (TMA) and water as precursors and is the most studied ALD-process to date. However, only few theoretical studies have been reported in the literature. The surface reaction mechanisms and energetics previously reported focus on a gibbsite-like surface model but a more realistic description of the surface can be achieved when the hydroxylation of the surface is taken into account using dissociatively adsorbed water molecules. The adsorbed water changes the structure of the surface and reaction energetics change considerably when compared to previously studied surface model. Here we have studied the TMA-H2O process using density functional theory on a hydroxylated alumina surface and reproduced the previous results for comparison. Mechanisms and energetics during both the TMA and the subsequent water pulse are presented. TMA is found to adsorb exothermically onto the surface. The reaction barriers for the ligand-exchange reactions between the TMA and the surface hydroxyl groups were found to be much lower compared to previously presented results. TMA dissociation on the surface is predicted to saturate at monomethylaluminium. Barriers for proton diffusion between surface sites are observed to be low. TMA adsorption was also found to be cooperative with the formation of methyl bridges between the adsorbants. The water pulse was studied using single water molecules reacting with the DMA and MMA surface species. Barriers for these reactions were found to reasonable in the process conditions. However, stabilizing interactions amongst water molecules were found to lower the reaction barriers and the dynamical nature of water is predicted to be of importance. It is expected that these calculations can only set an upper limit for the barriers during the water pulse.
原子层沉积（ALD）是一种用于制备高度均匀薄膜的涂层技术。氧化铝（Al2O3）主要使用三甲基铝（TMA）和水作为前驱体进行沉积，是迄今为止研究最多的ALD工艺。然而，文献中仅报道了少数理论研究。先前报道的表面反应机理和能量学主要集中在类三水铝石表面模型，但当使用解离吸附的水分子考虑表面羟基化时，可以实现对表面更真实的描述。与先前研究的表面模型相比，吸附的水改变了表面结构，反应能量学也有很大变化。在此，我们使用密度泛函理论在羟基化氧化铝表面研究了TMA-H2O工艺，并重现了先前的结果以作比较。给出了TMA和随后水脉冲过程中的机理和能量学。发现TMA以放热方式吸附到表面。与先前给出的结果相比，发现TMA与表面羟基之间的配体交换反应的反应势垒要低得多。预计TMA在表面的解离会在单甲基铝处饱和。观察到表面位点之间质子扩散的势垒较低。还发现TMA吸附与吸附剂之间甲基桥的形成具有协同作用。使用单个水分子与DMA和MMA表面物种反应研究了水脉冲。发现在工艺条件下这些反应的势垒是合理的。然而，发现水分子之间的稳定相互作用会降低反应势垒，并且预计水的动力学性质很重要。预计这些计算只能为水脉冲期间的势垒设定一个上限。
