Molecular-level fundamentals for ultra-planarization and ultra-cleaning processes

在衬底的超平坦化和超清洁工艺中，经常使用微细颗粒作为磨料，即化学机械抛光(CMP)。要获得更好的抛光效果，对化学机械抛光机理的了解是必不可少的。因此，有必要了解颗粒与衬底之间以及颗粒之间的表面力和摩擦力。在本研究项目中，我们在分子水平上阐明了水溶液中二氧化硅颗粒与硅片之间的摩擦力机制。结果表明：(1)对于单价电解液，摩擦力随电解液浓度的增加而显著减小，其大小与阳离子的水化能绝对值相对应，表明摩擦力与阳离子周围的自由水含量有关；(2)摩擦力对二氧化硅表面的分子级粗糙度极为敏感，与表面的清洗过程密切相关；(3)摩擦力随阳离子价态的变化复杂，随二价阳离子的水化能先减小后增大；(4)对于三价阳离子，由于阴离子与阳离子的相互作用，摩擦力的机理更加复杂；(5)随着溶液pH的升高，摩擦力显著降低，这是由于在pH=10时，在二氧化硅表面形成了厚度为1.0-1.5 nm的毛状层；(6)毛状层与电解液之间的关系比较复杂。通过计算机模拟，我们研究了(1)颗粒与光滑衬底之间的摩擦力是如何随力载荷的变化而变化的，以及(2)如何去除光滑衬底上的颗粒污染物。

Fine particles are frequently used as abrasives in ultra-planarization and ultra-cleaning processes for substrates, that is, chemical mechanical polishing (CMP). Understanding of the mechanism of CMP is indispensable for finer polishing. It is therefore necessary to understand surface forces and frictional forces between a particle and a substrate and between the particles. In this research project, we have elucidated at the molecular level the mechanism of frictional forces between a silica particle and a silicon wafer in aqueous electrolyte solutions. It is found that (1)as for monovalent electrolytes, the frictional force significantly decreases with increasing the electrolyte concentration and this order corresponds to the absolute value of hydration energy of the cations, indicating that the frictional force depends on the amount of free water around the cations ; (2)the frictional force is extremely sensitive to the molecular-scale roughness of the silica surfaces and significantly depends on the cleaning procedure of the surfaces ; (3)the frictional force complicatedly varies with the cationic valency, where it decreases with the hydration energy of divalent cations and then increases ; (4)as for trivalent cations, the mechanism of the frictional force is more complex due to the interaction of anions with the cations ; (5)the frictional force significantly decreases with increasing the solution pH, due to the presence of hairy layer with thickness of 1.0-1.5 nm formed at the silica surfaces at pH>10 ; (6)the relationship between the hairy layer and the electrolytes is rather complicated. Using computer simulations, we have investigated (1)how the frictional force between a particle and a smooth substrate depends on the force load and (2)how to remove the particulate contaminants on the smooth substrate.