Solid State Nanopores

Ion Beam Sculpting Solid-State Nanopores

Motivation:

We desire solid-state nanopores to manipulate and electronically register single DNA molecules in aqueous solution. A detector consisting of a single nanopore in a thin, insulating, solid-state membrane could mimic the function of a-hemolysin pores in lipid bilayers, while serving as a platform for integrated electronic detection devices. A new nanofabrication technique called "ion-beam sculpting" has been invented to reproducibly fabricate such molecular-scale pores in a variety of solid-state materials. The technique uses low-energy ion beams to slowly shape the surface of a material, while a feed-back loop enables single-nanometer control over pore dimension.

The Ion Beam Sculpting Process:

Low energy ion beams effect nanometer scale rearrangements at the surface of a material. When an ion beam with energies of several thousand electron-Volts impinges on a material surface, two distinctly different phenomena occur. An atomic-scale erosion process called sputtering removes atoms and molecules from outermost layers, causing a pore to open. The ion beam also stimulates the lateral transport of matter into pre-existing features, such as a pore, causing the pore to close. By changing the sample temperature or ion beam parameters during the process, we can control whether pore opening or closing dominates. Thus, nanopores can be fabricated in two ways: a nanopore can be created from a cavity in the membrane under conditions where the sputtering erosion process dominates (Figure 1a); alternatively, a nanopore can be made by filling in a larger pore under conditions where the lateral mass transport process dominates (Figure 1b). Thus, by controlling the conditions in the ion beam, a nanopore can be opened or closed at will.

Figure 1. Ion bean sculpting to make nanopores from a cavity (a) or from a through hole created by RIE or FIB in a 500 nm silicon nitride membrane (b). Either sputter erosion or lateral transport processes dominate, depending on the selected conditions used in the ion beam sculpting apparatus.

The feed-back controlled ion beam sculpting system that creates molecular scale pores. The apparatus (Figure 2) can (a) count the ion transmission rate through a pore in a solid-state membrane, which is a direct measure of the pore size, and (b) use the count signal to deflect the incident ion beam away when the desired pore size is obtained.

Figure 2. A schematic illustrating the main components of the ion beam sculpting apparatus and its feedback control loop that deflects the ion beam off the sample when the nanopore reaches its desired diameter.

The graph in Figure 3, below, shows the parameters and measured data used to develop a nanopore from a 70 nm through hole, as diagrammed in Figure 1b. The flow of matter to the developing pore shrinks the 70 nm diameter starting hole to attain a final diameter of 1.8 nm, approximately the diameter of a single strand of DNA.

Figure 3. Counting ions to close a 70 nm pore down to 1.8 nm in our sculpting apparatus.
Left: TEM images of a nanopore before (top) and after (bottom) ion beam exposure.
Right: Ar ion count rate and pore area vs. ion beam exposure time.