Kozole Joseph, Wucher Andreas, Winograd Nicholas
Department of Chemistry, Penn State University, 104 Chemistry Building, University Park, Pennsylvania 16802, USA.
Anal Chem. 2008 Jul 15;80(14):5293-301. doi: 10.1021/ac8002962. Epub 2008 Jun 13.
The role of the location of energy deposition during cluster ion bombardment on the quality of molecular depth profiling was examined by varying the incident angle geometry. Cholesterol films approximately 300 nm in thickness deposited onto silicon substrates were eroded using 40-keV C60(+) at incident angles ranging from 5 degrees to 73 degrees with respect to the surface normal. The erosion process was evaluated by determining at each incident angle the total sputtering yield of cholesterol molecules, the damage cross section of the cholesterol molecules, the altered layer thickness within the solid, the sputter yield decay in the quasi-steady-state sputter regime, and the interface width between the cholesterol film and the silicon substrate. The results show that the total sputtering yield is largest relative to the product of the damage cross section and the altered layer thickness at 73 degrees incidence, suggesting that the amount of chemical damage accumulated is least when glancing incident geometries are used. Moreover, the signal decay in the quasi-steady-state sputter regime is observed to be smallest at off-normal and glancing incident geometries. To elucidate the signal decay at near-normal incidence, an extension to an erosion model is introduced in which a fluence-dependent decay in sputter yield is incorporated for the quasi-steady-state regime. Last, interface width calculations indicate that at glancing incidence the damaged depth within the solid is smallest. Collectively, the measurements suggest that decreased chemical damage is not necessarily dependent upon an increased sputter yield or a decreased damage cross section but instead dependent upon depositing the incident energy nearer the solid surface resulting in a smaller altered layer thickness. Hence, glancing incident angles are best suited for maintaining chemical information during molecular depth profiling using 40-keV C60(+).
通过改变入射角几何形状，研究了团簇离子轰击过程中能量沉积位置对分子深度剖析质量的影响。使用40 keV的C60(+)以相对于表面法线5度至73度的入射角对沉积在硅衬底上的厚度约为300 nm的胆固醇薄膜进行侵蚀。通过在每个入射角确定胆固醇分子的总溅射产率、胆固醇分子的损伤截面、固体内部的改变层厚度、准稳态溅射区域中的溅射产率衰减以及胆固醇薄膜与硅衬底之间的界面宽度来评估侵蚀过程。结果表明，在73度入射角时，相对于损伤截面与改变层厚度的乘积，总溅射产率最大，这表明当使用掠入射角几何形状时积累的化学损伤量最少。此外，在非垂直和掠入射角几何形状下，观察到准稳态溅射区域中的信号衰减最小。为了阐明近垂直入射角处的信号衰减，引入了侵蚀模型的扩展，其中为准稳态区域纳入了与注量相关的溅射产率衰减。最后，界面宽度计算表明，在掠入射角时，固体内部的损伤深度最小。总体而言，测量结果表明，化学损伤的减少不一定取决于溅射产率的增加或损伤截面的减小，而是取决于将入射能量沉积在更靠近固体表面的位置，从而导致改变层厚度更小。因此，掠入射角最适合在使用40 keV C60(+)进行分子深度剖析时保持化学信息。
