Getting high aspect ratio (HAR) structures is a frequent request in directional etching of silicon using mainstream plasma tools. HAR features are useful either directly (e.g., photonic devices) or as a template for constructing more complicated structures (e.g., metamaterials). The latter is possible by adding postetch procedures such as atomic layer deposition. In this study, a procedure to fabricate ultra-HAR nanofeatures is demonstrated. It is built on a recently developed highly directional plasma etch procedure operating at room temperature called CORE (meaning clear, oxidize, remove, and etch) in which the usual fluorocarbon (FC) inhibitor of the Bosch process is replaced by oxygen. The effect of different CORE parameters on the etch rate and profile is investigated and optimized with respect to low mask undercut and high directionality. Due to the self-limiting property of the oxidation step, the CORE sequence is different from FC-based sequences, particularly concerning what type of etch mask is preferable. We show that 60 nm of chromium masking is well suited for ultra-HAR etching without complicating the plasma process or compromising the overall fabrication procedure. The nanopillar arrays (200 nm diameter, 400 nm pitch and 60 nm diameter, 500 nm pitch) have smooth straight sidewalls with aspect ratios beyond 55 for gaps and up to 200 for pillars. Due to the very mild plasma condition (less than 40 W RIE power), the mask selectivity with respect to silicon can be tuned above 500. In addition, the clean operation of the CORE sequence (no FC pileup as is typical in the Bosch process) prevents time-consuming profile tuning and enables process freedom and reproducibility.

中文翻译：

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SF6-O2等离子体中硅的超高深宽比蚀刻：透明氧化去除蚀刻（CORE）序列和铬掩模

使用主流等离子工具进行硅的定向刻蚀时，经常需要获得高深宽比（HAR）结构。HAR功能既可以直接使用（例如光子设备），也可以用作构建更复杂结构的模板（例如超材料）。后者可以通过添加后蚀刻程序（例如原子层沉积）来实现。在这项研究中，展示了一种制造超HAR纳米功能的程序。它建立在最近开发的高度定向的等离子刻蚀程序的基础上，该程序在室温下运行，称为CORE（意思是清除，氧化，去除和刻蚀），其中Bosch工艺中常用的碳氟化合物（FC）抑制剂被氧气取代。针对低掩模底切和高方向性，研究并优化了不同CORE参数对蚀刻速率和轮廓的影响。由于氧化步骤的自我限制特性，CORE序列与基于FC的序列不同，特别是在哪种蚀刻掩模是优选的方面。我们表明60 nm的铬掩膜非常适合进行超HAR蚀刻，而不会使等离子体工艺复杂化或损害整个制造过程。纳米柱阵列（直径200 nm，间距400 nm，直径60 nm，间距500 nm）具有光滑的直侧壁，间隙的纵横比超过55，而柱的纵横比则高达200。由于等离子体条件非常温和（RIE功率低于40 W），因此相对于硅的掩模选择性可以调整到500以上。此外，CORE序列的清洁操作（没有Bosch工艺中典型的FC堆积） ）可避免耗时的配置文件调整，并实现过程的自由度和可重复性。