Nanopore
technology has been extensively investigated for analysis
of biomolecules, and a success story in this field concerns DNA sequencing
using a nanopore chip featuring an array of hundreds of biological
nanopores (BioNs). Solid-state nanopores (SSNs) have been explored
to attain longer lifetime and higher integration density than what
BioNs can offer, but SSNs are generally considered to generate higher
noise whose origin remains to be confirmed. Here, we systematically
study low-frequency (including thermal and flicker) noise characteristics
of SSNs measuring 7 to 200 nm in diameter drilled through a 20-nm-thick
SiN
x
membrane by focused ion milling.
Both bulk and surface ionic currents in the nanopore are found to
contribute to the flicker noise, with their respective contributions
determined by salt concentration and pH in electrolytes as well as
bias conditions. Increasing salt concentration at constant pH and
voltage bias leads to increase in the bulk ionic current and noise
therefrom. Changing pH at constant salt concentration and current
bias results in variation of surface charge density, and hence alteration
of surface ionic current and noise. In addition, the noise from Ag/AgCl
electrodes can become predominant when the pore size is large and/or
the salt concentration is high. Analysis of our comprehensive experimental
results leads to the establishment of a generalized nanopore noise
model. The model not only gives an excellent account of the experimental
observations, but can also be used for evaluation of various noise
components in much smaller nanopores currently not experimentally
available.