High aspect ratio (HAR) structures have many promising applications such as biomedical detection, optical spectroscopy, and material characterization. Bottom-up self-assembly is a low-cost method to fabricate HAR structures, but it remains challenging to control the structure dimension, shape, density, and location. In this paper, an optimized top-down method using a combination of pseudo-Bosch etching and wet isotropic thinning/sharpening is presented to fabricate HAR silicon (Si) nanopillar and nanocone arrays. To achieve these structure profiles, electron beam lithography and reactive ion etching were carried out to fabricate silicon pillars having a nearly vertical sidewall, followed by thinning or sharpening by wet etching with a mixture of hydrofluoric (HF) acid and nitric acid (HNO₃). For the dry etching step using the pseudo-Bosch process, the sidewall angle is largely dependent on the SF₆/C₄F₈ gas flow ratio, and it was found that a vertical profile can be attained with a ratio of 22/38. For the wet etching process, a very large HNO₃/HF volume ratio is shown to give smooth etching with a slow and controllable etching rate. The final structure profile also depends on the pattern density/array periodicity. When the array period is large, silicon nanopillar is thinned down, and its aspect ratio can reach 1:135 with a sub-100 nm apex. When the pillar array becomes very dense (periodicity much smaller than height), a very sharp nanocone structure is obtained after wet etching with an apex diameter under 20 nm.

中文翻译：

---

超高纵横比硅纳米柱和纳米锥阵列的制备

高纵横比 (HAR) 结构具有许多有前途的应用，例如生物医学检测、光谱学和材料表征。自下而上的自组装是一种制造 HAR 结构的低成本方法，但控制结构尺寸、形状、密度和位置仍然具有挑战性。在本文中，提出了一种结合伪博世蚀刻和湿各向同性减薄/锐化的优化自上而下方法来制造 HAR 硅 (Si) 纳米柱和纳米锥阵列。为了实现这些结构轮廓，进行了电子束光刻和反应离子蚀刻以制造具有几乎垂直侧壁的硅柱，然后通过使用氢氟酸 (HF) 酸和硝酸 (HNO ₃). 对于使用伪博世工艺的干法蚀刻步骤，侧壁角很大程度上取决于SF ₆ /C ₄ F ₈气体流量比，并且发现可以以22/38的比值获得垂直剖面。对于湿法蚀刻工艺，非常大的 HNO ₃ /HF 体积比表明可以以缓慢且可控的蚀刻速率提供平滑的蚀刻。最终的结构轮廓还取决于图案密度/​​阵列周期性。当阵列周期较大时，硅纳米柱变细，纵横比可达1:135，顶点小于100 nm。当柱阵列变得非常密集（周期性远小于高度）时，湿法刻蚀后会得到非常尖锐的纳米锥结构，其顶点直径小于 20 nm。