Single Molecule Analysis Using Single Nanopores - Nanofluidics: Nanoscience and Nanotechnology - page 117

planar lipid membranes can easily persist for periods in excess of 24 hours. Conceivably,
the membrane lifetime might be increased by forming the ultra-thin films on very small
apertures 35 or by using lipids that can be polymerized. 36
6.2.2 Formation of Solid-State Nanopores on Thin Films
6.2.2.1 Free Standing Thin Film Preparation
A prerequisite to making nanopores is the fabrication of solid-state free-standing
thin films. There are many ways to make thin films. We have used the following methods
to fabricate our free-standing thin films. Si 3 N 4 films can be deposited using low pressure
chemical vapour deposition (LPCVD) at a temperature of 825 C using ammonia and
dichlorosilane gases. 37,38 Ammonia to dichlorosilane are introduced at a flow rate ratio of
approximately 1:5. This results in a silicon-rich nitride film, with a tensile stress in the
range of 50 to 150 MPa. This stress is low enough to allow the formation of free standing
membranes. A 50 µm x 50 µm window is then fabricated on a ~ 350 µm thick silicon
substrate wafer using photolithography and standard KOH wet-etching (Figure 6.3). 39,40
Figure 6.3 An image of silicon nitride film / chip (a), a TEM image of 50 nm thick silicon nitride
membrane (b), a live fast Fourier transform (FFT) of silicon nitride membrane (c), and typical
solid-state nanopore drilled by TEM (d). The inset in (d) is a schematic of the silicon nitride
membrane supported by silicon. The scale bars are 5 nm in length. Adapted from M. J. Kim, et
al. , Nanotechnology, 2007, 18, 205302 and reproduced with permission.
The mechanical and electrical properties of the free-standing film directly affect
the efficiency and quality of the fabricated nanopores, as well as the performance of the
nanopores when studying macromolecular translocations. We have performed a systematic
evaluation of films that have different thicknesses, are made with different materials, and
are deposited under different sets of parameters, to find general deposition rules and
optimal conditions for thin film fabrication. The films can be characterized using
ellipsometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy
(XPS) on the silicon substrates.
Intense e-beams (~10 8 -10 9 e/nm 2 ) can be used to directly fabricate nanopores in
the range of 4-8 nm diameter as previously reported. 41-44 For example, Figure 6.3 shows an
8 nm diameter nanopore. Depending on the thickness of the membrane (20-50 nm), pore
formation times will vary between 5 and 60 seconds (with thicker membranes requiring