Nanotechnology. 2017 Oct 27;28(43):435601. doi: 10.1088/1361-6528/aa8941.

Novel nanofluidic chemical cells based on self-assembled solid-state SiO2 nanotubes.

Zhu H1, Li H, Robertson JWF, Balijepalli A, Krylyuk S, Davydov AV, Kasianowicz JJ, Suehle JS, Li Q.

State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai 200433, People’s Republic of China.

Abstract

Novel nanofluidic chemical cells based on self-assembled solid-state SiO2 nanotubes on silicon-on-insulator (SOI) substrate have been successfully fabricated and characterized. The vertical SiO2 nanotubes with a smooth cavity are built from Si nanowires which were epitaxially grown on the SOI substrate. The nanotubes have rigid, dry-oxidized SiO2 walls with precisely controlled nanotube inner diameter, which is very attractive for chemical-/bio-sensing applications. No dispersion/aligning procedures were involved in the nanotube fabrication and integration by using this technology, enabling a clean and smooth chemical cell. Such a robust and well-controlled nanotube is an excellent case of developing functional nanomaterials by leveraging the strength of top-down lithography and the unique advantage of bottom-up growth. These solid, smooth, clean SiO2 nanotubes and nanofluidic devices are very encouraging and attractive in future bio-medical applications, such as single molecule sensing and DNA sequencing.

 

PMID: 28854152

Supplement

Since the first introduction of one-dimensional nanochannels/nanopores for single molecule detection, there has been increasing interest in modern nanofluidic system applications, e.g., ultra-fast DNA sequencing. [1] The recently developed artificial inorganic solid-state nanotubes (e.g., SiO2) have attracted certain attention due to their robustness of solid-state membranes, high aspect ratio, good mechanical properties, precise control of the nanotube size, and feasibility of surface functionalization. [2] However, SiO2 nanotubes prepared with conventional template-assisted sol-gel methods usually have poor morphology and porous walls, and can be easily damaged or contaminated during fabrication. [3]

 

We report a new and optimized process route to fabricate clean and smooth SiO2 nanotubes on silicon-on-insulator (SOI) substrate with a cavity towards direct chemical-/bio-sensing devices (Fig. 1). The SiO2 nanotubes are fabricated on SOI substrate consisting of a 400 μm handle Si (100) layer and a 4 μm top Si (111) layer with a sandwiched 1 μm buried oxide. Squares of different sizes on the back side are defined and formed by photolithography and reactive ion etching (RIE) after Si3N4 deposition, followed by an anisotropic etching of Si (100) in a 30% w/w potassium hydroxide (KOH) solution at 85 °C. After removing the buried oxide on top of the opened square window in hydrofluoric (HF) acid, the top Si3N4 layer on Si (111) surface is removed by RIE. Then, epitaxial synthesis of Si nanowire is carried out on the top Si (111) surface based on the vapor–liquid–solid growth mechanism (Fig. 2a). Immediately after the growth step, the Si nanowires are oxidized at 900 °C for 40 min in pure O2 to form a layer of high-quality and uniform 20 nm thick SiO2 shell. The substrate is then spin-coated with a layer of photoresist, and a brief RIE O2 plasma cleaning is performed to remove the photoresist off the nanowire surface extending above the photoresist level. The Au nanoparticle and the exposed SiO2 shell are etched by gold etchant and HF, respectively. XeF2 etchant gas is then used to selectively remove the Si nanowire cores and the suspended Si (111) layer beneath the nanowire after cleaning the photoresist (Fig. 2b). This finally opens a channel in the ultra-thin SiO2 membrane, forming a robust solid-state SiO2 nanotube (Fig. 3).

 

Fig. 1. Process flow of the fabrication of SiO2 nanotubes on SOI substrate from epitaxial Si nanowires.

 

 

Fig. 2. (a) SEM image of Au-catalyzed Si nanowires growth. (b) Cross-sectional SEM image of a nanotube on SOI substrate during XeF2 etching before thrilling through Si (111) layer.

 

The diameter of the as-grown Si nanowires is largely determined by the Au nanoparticle size but it can be further reduced to a desired value by repeating oxidation-HF etching process, enabling SiO2 nanotubes with controllable inner diameters. Critical to the development of scalable nanofluidic devices, our nanotube preparation did not involve focused ion beam which will heavily increase the cost and is not applicable for large batch fabrication, or milling process which will inevitably introduce contaminants and damage to the nanochannel. Eliminating the costly, serial fabrication steps offers a route to routine manufacturability of such nanofluidic devices. Additionally, our nanotube on SOI paradigm gives rise to in situ cell fabrication without nanotube harvesting and aligning on target substrates. Epitaxial growth of Si nanowires from patterned Au catalyst by electron-beam lithography on SOI substrates has also been tested in this work, which is very promising for robust nanofluidic device arrays and systems.

 

In summary, we developed a novel process of integration of solid-state SiO2 nanotubes on a SOI substrate which enables in situ fabrication of nanofluidic cells from the self-assembled SiO2 nanotubes without dispersion/aligning. This is attractive for large-scale integration of clean devices with rigid, uniform and high-quality oxide wall, and a hydrophilic and smooth surface. The inner diameter of the nanotube can be precisely controlled through multiple oxidation-etching steps. With further position-controlled nanowire growth using a self-assembly plus lithography integration technology, batch fabrication of nanotube arrays can be realized. Such a new and optimized nanofabrication process to enable well controlled SiO2 nanotubes for advanced nanofluidic device/system applications are very attractive for bio-medical application.

 

Fig. 3. (a) Tilted and (b) non-tilted top view of a fabricated nanotube with about 500 nm height and 20 nm inner diameter standing on the 20-nm thick rigid SiO2 membrane.

 

Reference

[1] Siwy Z, Trofin L, Kohli P, Baker L, Trautmann C and Martin C R 2005 J. Am. Chem. Soc. 127 5000

[2] Lee S B, Mitcell D T, Trofin L, Nevanen T K, Soderlund H and Martin C R 2002 Science 296 2198

[3] Kovtyukhova N I, Mallouk T E and Mayer T S 2003 Adv. Mater. 15 780