Lifetime and Stability of Silicon Nitride Nanopores and Nanopore Arrays for Ionic Measurements

Abstract

Nanopores are promising for many applications including DNA sequencing and molecular filtration. Solid-state nanopores are preferable over their biological counterparts for applications requiring durability and operation under a wider range of external parameters, yet few studies have focused on optimizing their robustness. We report the lifetime and durability of pores and porous arrays in 10 to 100 nm-thick, low-stress silicon nitride (SiN x ) membranes. Pores are fabricated using a transmission electron microscope (TEM) and/or electron beam lithography (EBL) and reactive ion etching (RIE), with diameters from 2 to 80 nm. We store them in various electrolyte solutions (KCl, LiCl, MgCl2) and record open pore conductance over months to quantify pore stability. Pore diameters increase with time, and diameter etch rate increases with electrolyte concentration from Δd/Δt ∼ 0.2 to ∼ 3 nm/day for 0.01 to 3 M KCl, respectively. TEM confirms the range of diameter etch rates from ionic measurements. Using electron energy loss spectroscopy (EELS), we observe a N-deficient region around the edges of TEM-drilled pores. Pore expansion is caused by etching of the Si/SiO2 pore walls, which resembles the dissolution of silicon found in minerals such as silica (SiO2) in salty ocean water. The etching process occurs where the membrane was exposed to the electron beam and can result in pore formation. However, coating pores with a conformal 1 nm-thick hafnium oxide layer prevents expansion in 1 M KCl, in stark contrast to bare SiN x pores (∼ 1.7 nm/day). EELS data reveal the atomic composition of bare and HfO2-coated pores.

In situsolid-state nanopore fabrication

“…51 However, several other dielectric membranes have also been used. For example Al 2 O 3 , 52,53 HfO 2 , 54 and TiO 2 55 have been used to improve the nanopore surface charge, pore stability, and membrane photoluminescence respectively. Two-dimensional membranes such as graphene, 56,57 MoS 2 , 58 hBN, 59 WS 2 60 and MXenes, 61 have also received attention for nanopore applications.…”

“…40,74,97 Drilling nanopores using charged particles can also lead to changes in the fundamental membrane composition which may decrease the pore stability. 54,92 Another technique used to fabricate solid-state nanopores relies on the well-developed nanofabrication techniques of electron beam lithography (EBL) and reactive ion etching (RIE). 40,98,99 Here, nanopores are patterned into an electron sensitive resist using EBL and this pattern is transferred to the membrane via RIE.…”

Sensing with Nanopores and Aptamers: A Way Forward

“…Such techniques are employed to monitor size and shape of the nanopore [ 82 , 83 , 84 ], or modify surface properties such as charge or hydrophobicity [ 82 , 83 ]. A good advantage of such a coating is that it can increase the stability of the membrane over time by preventing a slow etching by the electrolyte [ 85 ]. In particular, some studies have shown that the gas phase deposition of HfO 2 has inhibited Si 3 N 4 dissolution [ 86 ].…”

Four Aspects about Solid‐State Nanopores for Protein Sensing: Fabrication, Sensitivity, Selectivity, and Durability

“…However, in actual experiments, a great number of solid‐state nanopores tend to expand in salt solution with time. [ 61,133,134 ] As we explore thinner membrane for higher sensitivity, the stability and robustness of solid‐state nanopores should attract more attention in order to achieve commercialization of the device.…”

“…By carefully checking the 2D electron energy loss spectrum (EELS) spatial intensity maps of silicon, nitrogen, as well as oxygen elements from a TEM drilled SiN x nanopore, they concluded the expansion of SiN x membrane is a electrochemistry process, which resembles the erosion of silica in salt sea water. [ 133 ] The conclusion is counterintuitive at first sight, since the SiN x membranes are not expected to be etched in aqueous solutions. [ 136 ] However, they found the illumination of electron beams generated an N‐deficient region or Si/SiO 2 ‐rich region around pore edge with a diameter of 20–30 nm ( Figure a), which tend to erode in salt solutions governed by SiO 2 + 2H 2 O ↔ H 4 SiO 4 .…”