Approaching the Limits of Aspect Ratio in Free‐Standing Al₂O₃3D Shell Structures

Abstract

Nanoscale free‐standing membranes are used for a variety of sensors and other micro/nano‐electro‐mechanical systems devices. To tune performance, it is indispensable to understand the limits of aspect ratios achievable. Herein, vapor hydrofluoric (VHF) processes are employed to release 3D shell structures made of atomic‐layer‐deposited Al2O3 etch‐stop layers. Structure heights of 100–600 nm and widths of 1–200 nm are fabricated for membranes with 20 and 50 nm thickness. Undercut depths of and aspect ratios of 475:1 etch depth to structure width (50 nm films) and etch depth to membrane thicknesses of 495:0.02 (20 nm films) are achieved. The etch‐rate stagnates above a ratio of 31% hydrofluoric (HF), where decreasing EtOH shares reduce reproducibility. Etch rates reach 0.75 mm min−1 and are generally constant over vapor etch depth. For 100 nm heights and widths of , etch rates however stagnate for deeper depths. All explored structures remained stable with widths up to 5 μm independent of the height. Above width, top membranes deflect, likely from stress accumulated during deposition. Herein, exploring and understanding the limits of aspect ratio in future free‐standing membrane devices are helped.

<i>In Situ</i> ALD-TEM System to Study Atomic Nucleation Phenomena - Enabled by Ultrathin Large Aspect Ratio Free-Standing Shell Structures

“…They must (1) resist the mechanical load when exposed to differential pressure, (2) block the diffusion of chemical species, (3) benefit the nucleation process, and must be (4) chemically and (5) thermally stabile. Thin films of ALD Al2O3 benefit ALD growth [10] and can be released as ultrathin membrane using HF vapor etching [7]. In response, we realized an enclosed cavity sealed by ALD Al2O3 membranes on a single chip with integrated longitudinal tunnel like cavities that serves as microreactor allowing gas diffusion and film deposition inside its cavities.…”

“…The cavities led to a free-standing Al2O3 membrane capped deposition environment (Figure 2). To realize this architecture, we exploited the high selectivity of Al2O3 in HF vapor etch processes [7]. The process included the fabrication of a series of rib structures from sacrificial SiO2 encapsulated by an ALD A process flow chart of the fabrication steps is shown in Figure 3.…”