Photothermally Assisted Thinning of Silicon Nitride Membranes for Ultrathin Asymmetric Nanopores

Abstract

Sculpting solid-state materials at the nanoscale is an important step in the manufacturing of numerous types of sensor devices, in particular solid-state nanopore sensors. Here we present mechanistic insight into laser-induced thinning of low-stress silicon nitride (SiN x ) membranes and films. In a recent study, we observed that focusing a visible wavelength laser beam on a SiN x membrane results in efficient localized heating, and we used this effect to control temperature at a solid-state nanopore sensor. A side-effect of the observed heating was that the pores expand/degrade under prolonged highpower illumination, prompting us to study the mechanism of this etching process. We find that SiN x can be etched under exposure to light of ∼10 7 W/cm 2 average intensity, with etch rates that are influenced by the supporting electrolyte. Combining this controlled etching with dielectric breakdown, an electrokinetic process for making pores, nanopores of arbitrary dimensions as small as 1−2 nm in diameter and thickness can easily be fabricated. Evidence gathered from biomolecule-pore interactions suggests that the pore geometries obtained using this method are more funnel-like, rather than hourglass-shaped. Refined control over pore dimensions can expand the range of applications of solid-state nanopores, for example, biopolymer sequencing and detection of specific biomarkers.

Automated, Ultra‐Fast Laser‐Drilling of Nanometer Scale Pores and Nanopore Arrays in Aqueous Solutions

“…These nanopores were shown to exhibit noise levels matching their TEM‐drilled counterparts and translocate both nucleic acids and proteins. Subsequent reports have exploited the thinning effect of the laser drilling in Si x N membranes in order to accelerate CBD pore creation . However, the physical process governing laser‐drilling in thin, water‐immersed membranes, particularly in the absence of any dielectric breakdown application, has remained obscure.…”

“…Specifically, the relative contributions of direct heating versus polarization of the thin membrane by the laser light, have to date remained unclear. While heating has already been implicated in speeding up the chemical dissolution of the Si x N membrane, the role of charge generation at the membrane due to incident irradiation has yet to be elucidated. At the same time, laser‐induced microchemistry in semi‐conductors has widely been attributed to the optical creation of an electron–hole pair and the migration of the latter to the solid–liquid interface where it catalyzes chemical interactions .…”

Nanopore Formation via Tip‐Controlled Local Breakdown Using an Atomic Force Microscope

“…In Figure d we compare the average time‐to‐breakdown for our tip‐controlled approach versus classical dielectric breakdown. We find that our approach gives pore formation times two orders of magnitude lower than classical breakdown, by comparison with a wide‐range of experimental studies exploring classical breakdown for different film thickness (10–30 nm, 75 nm), pH (2–13.5), and voltage (1–24 V).…”

“…Thus, not only can TCLB drive novel nanopore sensing applications, TCLB can simultaneously drive the industrial scaling of these applications. As an example, consider combining TCLB with photothermally assisted thinning . In a photothermally assisted thinning process, a laser beam is focused on a silicon nitride membrane, leading to formation of a locally thinned out region, with thinning achieved down to a few nanometers .…”

“…As an example, consider combining TCLB with photothermally assisted thinning. [27,42,48] In a photothermally Figure 6. a) Semilog plot of the mean breakdown time (〈t BD 〉) versus voltage for 12-14 nm thick silicon nitride membrane with an exponential fit.…”