The etching of high aspect ratio structures in silicon via the Bosch process is essential in modern technologies such as microelectromechanical systems (MEMS) and through‑silicon vias (TSV) fabrication. The process can be very demanding on the mask selectivity due to long etching times, and it has been shown that an Al₂O₃ hard mask is very suitable in this regard, as it offers significantly higher selectivity compared to the conventional SiO₂ or resist masks. In this work, we employ a combination of Scanning Electron Microscopy (SEM), Spectroscopic Ellipsometry (SE) and X-Ray Photoelectron Spectroscopy (XPS) depth profiling to scrutinize the Al₂O₃ mask etching mechanism and therefore the origin of the extraordinary high selectivity. We demonstrate that by increasing the passivation step time, a thicker fluorocarbon polymer layer is formed on the Al₂O₃, and Al₂O₃ is then removed with a minuscule average etch rate of ~0.01 nm/min. XPS depth profiling reveals that during Deep Reactive Ion Etching (DRIE) using the Bosch process, an AlF_x layer is formed between the polymer and Al₂O₃. As AlF_x is non-volatile, it requires sputtering to be removed. If the polymer layer is thick enough to attenuate the incoming ions such that their energy is not sufficient to lead to desorption of AlF_x, such as when using a longer passivation time, the mask is not eroded. By investigating the surface after different amounts of DRIE cycles, we also obtained information about the formation rate of AlF_x and the changes in the Al₂O₃ and polymer thicknesses over the course of a DRIE process. These findings further expand the knowledge of DRIE and can help process engineers to tailor the processes accordingly.

通过博世工艺蚀刻硅中的高纵横比结构对于微机电系统 (MEMS) 和硅通孔 (TSV) 制造等现代技术至关重要。由于蚀刻时间长，该工艺对掩模选择性的要求非常高，并且已经表明，Al ₂ O ₃硬掩模在这方面非常适合，因为与传统的 SiO ₂或抗蚀剂相比，它提供了显着更高的选择性面具。在这项工作中，我们结合使用扫描电子显微镜 (SEM)、光谱椭偏仪 (SE) 和 X 射线光电子能谱 (XPS) 深度分析来仔细检查 Al ₂ O ₃掩膜蚀刻机制，因此是非凡的高选择性的起源。我们证明，通过增加钝化步骤时间，在 Al ₂ O ₃上形成更厚的碳氟聚合物层，然后以 ~0.01 nm/min 的微小平均蚀刻速率去除Al ₂ O ₃。XPS 深度分析表明，在使用博世工艺进行深度反应离子蚀刻 (DRIE)期间，在聚合物和 Al ₂ O ₃之间形成了 AlF _x层。作为 AlF _x是非挥发性的，需要去除溅射。如果聚合物层足够厚以衰减进入的离子，使得它们的能量不足以导致AlF _x解吸，例如当使用更长的钝化时间时，掩模不会被腐蚀。通过研究不同数量的 DRIE 循环后的表面，我们还获得了关于 AlF _x的形成速率以及在DRIE 过程中Al ₂ O ₃和聚合物厚度变化的信息。这些发现进一步扩展了 DRIE 的知识，可以帮助工艺工程师相应地定制工艺。