Silicon Nitride Thin Films for Nanofluidic Device Fabrication

Abstract

Silicon nitride is a ubiquitous and well-established nanofabrication material with a host of favourable properties for creating nanofluidic devices with a range of compelling designs that offer extraordinary discovery potential. Nanochannels formed between two thin silicon nitride windows can open up vistas for exploration by freeing transmission electron microscopy to interrogate static structures and structural dynamics in liquid-based samples. Nanopores present a strikingly different architecture—nanofluidic channels through a silicon nitride membrane—and are one of the most promising tools to emerge in biophysics and bioanalysis, offering outstanding capabilities for single molecule sensing. The constrained environments in such nanofluidic devices make surface chemistry a vital design and performance consideration. Silicon nitride has a rich and complex surface chemistry that, while too often formidable, can be tamed with new, robust surface functionalization approaches. We will explore how a simple structural element—a ∼100 nm-thick silicon nitride window—can be used to fabricate devices to wrest unprecedented insights from the nanoscale world. We will detail the intricacies of native silicon nitride surface chemistry, present surface chemical modification routes that leverage the richness of available surface moieties, and examine the effect of engineered chemical surface functionality on nanofluidic device character and performance.

Solution-Based Photo-Patterned Gold Film Formation on Silicon Nitride

“…The rich chemistry of the native SiNx surface, and of the palladium species, complicates the determination of the mechanism, and indeed may allow for multiple mechanisms to be simultaneously operational. 3,13,23,28 Figure S2 shows the results of several process chemistry variations, all delivering lower metallized pattern quality than seen in Figures 1e and 1f, but nevertheless highlighting a setting rich with opportunity for fundamental study and applications. For example, substrate photopatterning through an air layerlikely through a photochemical oxidation route similar to that seen on silicon 29 -instead of 1-octene ( Figure S2) yielded spatial selectivity degraded by smudges of gold across the surface.…”

“…We sought to develop a gentle, solution-based patterned metallization approach [16][17] capable of plating a range of even structured substrates, including inside existing (nano)fluidic channels. 3,7,[14][15]18 The horizons of single-molecule science have recently been dramatically expanded by the development of simple methods for fabricating nanopores: entirely solution-based processes requiring only uncomplicated instrumentation are removing barriers to the widespread use of nanopore methods. 19 To conserve the benefits of simple pore formation methods, our focus also included developing similarly widely-accessible, straightfoward solutionbased approaches to patterned metallization.…”

“…[1][2][3] Devices and applications exploiting these beneficial native features can be augmented and improved using designer metal overlayers that fulfill structural roles, serve as electrodes, and provide alternative surface chemistry options, including as a platform for subsequent thiol monolayer self-assembly. The field of nanopore single-molecule sensing offers compelling examples of the prospects of merging SiNx thin films and designer metal layers into devices, and does this within a nanofluidic context where the need for versatile metallizing approaches is clear.…”

“…The field of nanopore single-molecule sensing offers compelling examples of the prospects of merging SiNx thin films and designer metal layers into devices, and does this within a nanofluidic context where the need for versatile metallizing approaches is clear. [3][4][5][6][7] The most common solid-state nanopores are <100 nm-diameter nanofluidic channels formed through <100 nm-thick, free-standing SiNx films, and nanopore-integrated metal films can enhance sensing capabilities by serving as optical elements such as light shields and plasmonic films, as electrodes for tunneling and other molecular control and sensing functions, and as a means to tune nanopore size and surface chemistry. [3][4][5][6][7][8] The nanoscale dimensions of the SiNx film and pore can be significant barriers to efforts to incorporate such functional metal films, particularly when the interior of the pore must be metallized.…”

Through a Window, Brightly: A Review of Selected Nanofabricated Thin-Film Platforms for Spectroscopy, Imaging, and Detection

“…[17][18][19] Thin films with suitable properties and structures can also be used in a more active role in experimental studies: a $10 nm diameter channel through a < 100 nm thick thin film-a so-called nanopore-is the basis for a wide range of single molecule sensing and manipulation methods that emerge from the passage of a molecule into, or through, the nanofluidic channel. [20][21][22][23][24][25][26][27][28][29] Figure 1 offers two illustrations to illuminate the rich scope of experimental configurations and possibilities using just these two platforms. We will frame our discussion of the interplay between thin-film design-composition and structureand application by using these two quite different implementations: a sample cell with thin-film windows bracketing a nanofluidic channel; and a nanopore single molecule sensor platform where the robust thin film supports a nanofluidic through-channel.…”