Fabrication of solid-state nanopores (d < 10 nm)
One of the possible field of interest for a bio-medical engineer or a biologist would be the label-free detection of a molecule directly at the site (sample source) with the lab-on-chip technology. Current measurement through nanopore is one of the approaches towards label free detection of moleule which opens a broader perspective towards development of cheaper bio-sensors. Resistive pulse sensing technique (RPS) is applied to detect ingoing molecules through a nanopore sensor. A nanopore conventionally used to be the biological nature, which includes a micele or α-Haemolysin pore. However, the diametre of a pore is not small enough to detect a single DNA sequence, despite the fact that they are quite noise-resistant. After rapid development of different micro and nanofabrication techniques, stable and size-controlled solid-state nanopores are possible. Solid state nanopores are fabricated on the crystalline materials like silicon dioxide, silicon nitride and gold (Au) substrates. These substrates are stable compared to biological nanopores.
Scopes of solid state nanopores are broader in range, like: DNA sequencing , detection of proteins or polypeptides or nucleic acids or antibody-virus binding [2,3], nanoparticles , synthetic polymers , antibody-antigen [6,7] interaction and ions . In RPS technique, the incoming molecule displaces the conductive liquid flowing through the nanopore, which gives decrement in current or increment in resistance as the electrical signal. In this article, two major solid-state nanopore fabrication techniques would be discussed. The two techniques are: Focused electron beam method (introduced at Kavli Institute of Nanoscience, Delft University of Technology) and Ion Sculpting Method (introduced at Harvard University). Nanopores with the size in the orders of nanometres (d = 1-10 nm) can be fabricated on the substrates with these following methods.
(a) Focused electron beam method:
Focused electron beam method was first proposed by a team at Kavli Institute of Nanoscience, Delft University of Technology, where controlled nanopores (size upto 0.4nm) on SiN within gold electrodes were fabricated .
Figure 1: (a - d) Major steps involving in the fabrication of a nanopore in SiN layer by focused electron beam method. (e) Fabricated nanopores with diameter of 12, 2.5 and 0.4nm as observed in SEM .
In this method a focused electron beam is applied on the silicon nitride membrane with an intensity of 10-8 to 10-9 e/nm2. High intensity electron beam (100 to 300 KeV) breaks the covalent bond and causes Si and N atoms to be sputtered away into the vacuum. The major steps involves are: (a) Chemical vapor deposition (CVD) of SiN, SiO2 and SiN layers on both surface of Silicon substrate. The back surface is carefully etched by KOH resulting in a large 50 µm exposed SiN surface. (b) Etching away 3 µm of SiN and SiO2 layer from front surface of silicon substrate. (c) It is followed by applying a tight focused beam to SiN surface resulting approximately 0.4 - 100 nm of SiN hole. (d) Final step involves is the deposition of SiO2 layer by sputtering and evaporation of Au layer. When SiO2 sacrificial layer is etched away by the buffered HF we get a pyramidal nanopore structure. The major advantage of this method is on spot observation of the nanopore during e-beam lithography process.
(b) Ion sculpting method:
Ion beam sculpting method was first proposed by the team at Harvard University in 2001 . The method applies a feedback-controlled sputtering system which provides the fine control over ion beam exposure and temperature of the sample.
Figure 2: (a) Sputtering involves exposure of high energy argon beam to the Si3N4 sample. This high energy argon beam removes a thin layer of Si3N4 resulting a small nano pore. (b) Experimental setup involves sputtering unit and detector unit. Single ion detector sends the feedback signal for deflection of high energy argon source on the sample .
Figure 3: (a) Initial 60 nm pore on Si3N4,(b) 1.8nm pore formed after sputtering with argon ions .
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