Molecular layer deposition (MLD) is of growing interest as a vapor-phase method for depositing ultrathin polymeric films. The amount of material deposited per cycle of MLD is believed to be strongly dependent on the number of available surface reaction sites. However, the factors that alter the number of reactive sites during cycling have not been well understood. In this work, we provide concrete evidence that although chain termination reactions may reduce the number of reaction sites, the adsorption or absorption of monomers allows for the introduction of new reaction sites. These absorbed monomers are strongly bound to the film and allow the films to recover from growth defects introduced by chain terminations. A mathematical model for growth is developed and suggests that these absorptions eventually lead to the characteristic steady-state growth rate seen for MLD processes. Fits of the model to the experimental data for polyurea MLD indicate that ∼3% of the chains experience termination events during each cycle at steady state, with any given chain having an ≈50% chance to be terminated before reaching 22 cycles; this termination is then compensated for by uptake of new monomers. Together, these results indicate that typical MLD growth involves the continuous occurrence of termination reactions and the renucleation of chains within the film via the absorption of monomers.

中文翻译：

---

分子层沉积中链终止和单体吸收的机理研究

分子层沉积（MLD）作为用于沉积超薄聚合物薄膜的气相方法越来越受到关注。据信每个MLD周期沉积的材料量在很大程度上取决于可用的表面反应位点的数量。然而，尚未充分了解在循环过程中改变反应位点数量的因素。在这项工作中，我们提供了具体的证据，尽管链终止反应可以减少反应位点的数量，但是单体的吸附或吸收可以引入新的反应位点。这些吸收的单体牢固地结合在薄膜上，并使薄膜从链终止引入的生长缺陷中恢复过来。建立了生长的数学模型，该模型表明这些吸收最终会导致MLD工艺所见的特征性稳态生长速率。模型与聚脲MLD实验数据的拟合表明，在每个循环中，约有3％的链在稳态下经历终止事件，任何给定的链在达到22个循环前都有≈50％的机会被终止；然后通过吸收新的单体来补偿该终止。在一起，这些结果表明典型的MLD生长涉及终止反应的连续发生和薄膜内链通过单体的吸收而重新成核。模型与聚脲MLD实验数据的拟合表明，在每个循环中，约有3％的链在稳态下经历终止事件，任何给定的链在达到22个循环前都有≈50％的机会被终止；然后通过吸收新的单体来补偿该终止。在一起，这些结果表明典型的MLD生长涉及终止反应的连续发生和薄膜内链通过单体的吸收而重新成核。模型与聚脲MLD实验数据的拟合表明，在每个循环中，约有3％的链在稳态下经历终止事件，任何给定的链在达到22个循环前都有≈50％的机会被终止；然后通过吸收新的单体来补偿该终止。在一起，这些结果表明典型的MLD生长涉及终止反应的连续发生和薄膜内链通过单体的吸收而重新成核。