Etching mechanism of diamond by Ni nanoparticles for fabrication of nanopores
Introduction
Like their biological counterparts [1], artificially engineered nanopores in solid-state membranes constitute versatile and robust devices to investigate molecular properties of biomolecules [2] and biomolecular interactions [3]. The nanopore sensing concept utilizes chips (typically made in silicon (Si)) that contain a thin free-standing membrane into which a nanometer-sized hole (nanopore) is drilled [2], [4]. When a nanopore chip is immersed in a conducting electrolyte solution containing analyte molecules and an electric field is applied across the membrane, the molecules could partially block the ionic current upon passage through the pore [5]. Individual molecule could then be detected by measuring the time duration and magnitude of current blockage. Notable examples include nanopores in SiO2 and Si3N4 membranes fabricated by using either an Ar+ beam [5], [6] or an electron beam (e-beam) in a transmission electron microscope [7], [8].
Such nanopore sensing systems offer high stability against pressure variations and mechanical vibrations and thus give more reproducible results. However, analyzing molecules in harsh conditions such as extreme temperatures, voltages or pH conditions could lead to the malfunctioning or in extreme cases to the destruction of the chips with conventional membranes. The problem can be solved by establishing nanopores in diamond since this material possesses exceptional physio-chemical properties such as the highest hardness and Young’s modulus (1200 GPa) of all known solids, a very high thermal conductivity (20–22 W cm−1 K−1) and a low thermal expansion coefficient [9], [10], [11], [12]. Furthermore, because of its wide band gap, the electrochemical potential window of diamond is significantly larger and the background current within this regime considerably lower than conventional electrodes made from metals or graphite [13]. This behavior enables detection with low noise over a wide potential range.
Since diamond is chemically inert in gases and liquids at room temperature, the etching of diamond required for the device fabrication is performed almost exclusively by the reactive ion etching (RIE) technique with the help of lithographical processes, lithography being used for mask deposition [14], [15], [16]. However, with the RIE, fabrication of very narrow (10–50 nm in lateral size) and long (typically longer than 300 nm) pores has become problematic due to the limitation in lithographic resolution and high aspect ratio etching profiles. In addition, an unavoidable use of the e-beam lithography for fabricating high resolution nanostructures makes the RIE process costly and time consuming.
We have previously reported on the metal catalyst-assisted etching of polycrystalline and nanocrystalline diamond in hydrogen atmosphere for nanopore generation in the diamond membrane [17]. The role of different process parameters such as metal type, process temperature and time, hydrogen pressure, and diamond microstructure was investigated extensively. However, the etching mechanism was not fully understood. In this article, we extended our investigation to get further insight into the etching process and thereby to develop a model explaining the mechanism. For this purpose, we emphasized the influence of etching time, nanoparticle size and annealing atmosphere in the etching process and measured the chemical composition of the gas during etching. Surface chemistry of the samples was analyzed by XPS, while their morphology was examined using scanning electron microscopy (SEM). Ni was used as a potential catalyst since it showed the highest etching activity among the studied metals (Co, Pt and Au) [17].
The etching mechanism was investigated based on the reported works that dealt with catalytic formation of nano-holes and nano-channels (open tube) in the surface layers of highly oriented pyrolytic graphite and diamond [18], [19], and patterning of diamond surfaces [20]. The etching of diamond was suggested to be accompanied with the formation of a carbide-like layer containing hydrogen at the diamond–metal interface and the successive formation of this layer was thought to assist the moving of nanoparticles resulting in the channeling of diamonds [19]. In contrast, Ralchenko et al. [20] interpreted the etching according to a mechanism involving carbon dissolution in metal, followed by diffusion from the diamond–metal to the metal–gas interface and finally desorption of carbon as hydrocarbons.
Section snippets
Etching of diamond films
The etching technique consists of two steps, (i) the deposition of thin Ni layers on a diamond substrate, and (ii) the subsequent annealing of the substrate in hydrogen atmosphere. The Ni layer with thickness in the range of 1–15 nm was deposited by e-beam evaporation, the thickness being measured by a quartz oscillator. The annealing step was carried out in a chemical vapor deposition (CVD) reactor, usually dedicated to diamond growth, at 800–850 °C in 60 Torr flowing hydrogen with 100 standard
Formation of nanopores into diamond films
For this study, samples were processed in the temperature range 800–850 °C as no sign of etching was observed below 800 °C in 60 Torr hydrogen with increasing thickness of evaporated Ni film [17]. Fig. 2 shows SEM images of (1 0 0) surface of annealed polycrystalline diamond in the case of 3 nm Ni layers.
After 30 s of annealing at 800 °C we observed self-organized Ni nanoparticles on the surface (Fig. 2a). Etched structures started to appear after 3 min (Fig. 2b), and nanopores with flat sidewall faces
Conclusion
The etching mechanism of diamond using Ni nanoparticles as catalyst was investigated through SEM observation, surface chemistry by XPS and gas composition analysis. A simplified model was proposed to explain the etching mechanism. According to this model, carbon atoms are dissolved into the nickel particle and then transferred to the gas phase as methane with the help of hydrogen. Depending on the kinetics at the diamond surface and the hydrogen in- and methane out-diffusion between the chamber
Acknowledgements
The authors would like to thank Vo-Ha for atmospheric pressure experiments. The work was supported by ANR 2010-BLAN-812-03 (VLOC) program.
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