Ion Beam Sculpting Solid-State Nanopores
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
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