The cryogenic process and the Bosch process are two widely used processes for reactive ion etching of high aspect ratio silicon structures. This paper focuses on the cryogenic deep etching of 400-nm pitch silicon gratings with various etching mask materials, including polymer, Cr, SiO2, and Cr-on-polymer. The undercut is found to be the key factor limiting the achievable aspect ratio for the direct hard masks of Cr and SiO2, while the etch selectivity responds to the limitation of the polymer mask. The Cr-on-polymer mask provides the same high selectivity as Cr and reduces the excessive undercut introduced by direct hard masks. By optimizing the etching parameters, we etched a 400-nm pitch grating to ≈10.6 μm depth, corresponding to an aspect ratio of ≈53.

中文翻译：

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高纵横比 400 nm 间距硅光栅的低温蚀刻

低温工艺和博世工艺是两种广泛使用的高纵横比硅结构反应离子蚀刻工艺。本文重点介绍了使用各种蚀刻掩模材料（包括聚合物、Cr、SiO2 和 Cr-on-polymer）对 400 nm 间距硅光栅进行低温深蚀刻。发现底切是限制 Cr 和 SiO2 直接硬掩模可实现的纵横比的关键因素，而蚀刻选择性响应聚合物掩模的限制。Cr-on-polymer 掩模提供与 Cr 相同的高选择性，并减少直接硬掩模引入的过度底切。通过优化蚀刻参数，我们将 400 nm 间距的光栅蚀刻到 ≈10.6 μm 深度，对应于 ≈53 的纵横比。