Fabrication of Silica Nanofluidic Tubing for Single Molecule Detection - Nanofluidics: Nanoscience and Nanotechnology - page 91

Other lithographic techniques such as interferometric lithography 50 and
nanoimprint lithography 51,52 have been assessed as tools for fabricating nanofluidic
channels. Interferometric lithography uses a standing wave pattern generated by multiple
coherent optical beams to expose a photoresist layer. One of the primary challenges when
using this approach is realizing pattern flexibility, since two-beam interferometric
lithography will only produce periodic lines and spaces. Multiple beams and multiple
exposures are currently being explored to create more complex structures. 53,54 Nanoimprint
lithography is a simple process which transfers a pattern from a template to an imprint
resist by mechanical deformation. Its utility depends on the structure of template, which is
usually fabricated using ion beam lithography or electron beam lithography.
In addition to lithographic processing, an etching step is required to transfer the
lithographically patterned nano-scale tubing on the resist into a substrate such as fused
silica or silicon. In this step, a tubing depth in the nanometer range must be established.
For instance, reactive ion etching can be controlled to produce a nanofluidic channel with a
sub-100 nm depth on the surface of borosilicate 55 and silicon. 56 Wet anisotropic etching
has also been used to develop nanofluidic channels as shallow as 50nm on <110> silicon
wafer using native oxide as the mask and Olin OPD 4262 positive resist developer as the
etchant. 43
The last step in top-down fabrication of a nanofluidic channel is sealing or
bonding. This process is used to enclose the etched trenches. There are a variety of ways to
achieve a seal such as anodic bonding, fusion bonding, polymer bonding and eutectic
bonding. 42 The selection of a sealing or bonding method depends sensitively on the
substrate material being used. In addition, some sealing methods can narrow down the
channel size. For example, Austin and co-workers reported the sealing of a nanofluidic
channel array generated by nanoimprint lithography by depositing SiO 2 over the trenches
at a wide distribution of angles to create a capping layer. The local shadowing effects
inherent in the deposition process reduced the cross section to approximately 10 nm. 51
Self-sealed nanofluidic tubes have been fabricated by many researchers. For
instance, sacrificial layer methods have been commonly used. 46,57 This approach involves
patterning sacrificial materials on the surface of a substrate using high resolution pattering
techniques such as e-beam or nanoimprint lithography. Subsequently a capping layer is
deposited to cover the patterned sacrificial structure. In the final step, the sacrificial layer is
removed using chemical solvents or thermal decomposition. By using this approach,
nanofluidic tubes with uniform height can be produced. However the removal of sacrificial
material normally occurs over an extended time period, with conduit dimensions being
sensitive to the pattering techniques used.
In general, top-down nanochannel fabrication methods are normally costly and
generate rectangular or triangular shape cross-sectional profiles. Additionally, the success
of device fabrication relies completely on the lithography process. Interestingly, reports
relating to the bottom-up fabrication of nanofluidic tubes are rarer, due to the difficulty in
controlling the self-assembly process. In this process, atoms and molecules are initiated to
arrange themselves into more complex nanostructures. Bottom-up approaches to
nanochannel formation are however attractive due to their low cost and ability to create
nanostructures with extremely small dimensions. Electrospinning, which is a simple and
viable technique to fabricate continuous micro/nanofibers directly from a
polymer/polymer-blended solution, is a classical self-assembly process. However opinions
are split about whether electrospinning is a top-down or bottom-up fabrication method. 58,59
Details concerning this technique are discussed in the proceeding paragraphs.