Biomolecule Separation, Concentration, and Detection using Nanofluidic Channels - Nanofluidics: Nanoscience and Nanotechnology - page 34

2.2.2 Sacrificial Layer Etching Techniques

Another method for generating nanochannels involves the use of sacrificial layers,
Such an approach has been widely-used for creating suspended MEMS structures. Fluidic
channels (nanochannels and microchannels) can be defined by depositing a sacrificial layer
which is sealed with another capping layer. Subsequently, the sacrificial layer is removed
by a highly-selective etching process. The greatest advantage of this technique is that there
is no need for substrate-bonding (which is often very sensitive to wafer flatness), since the
sealing of the fluidic channel is achieved as part of processing. This approach can allow
fabrication of complex, three-dimensional fluidic networks with multiple channels crossing
each other. Disadvantages of the methodology include extended etching times (especially
when the enclosed fluidic channels are long) and defect generation due to stresses on the
capping layer.
Since the thickness of the deposited sacrificial layer can be controlled very
accurately, one can make highly thin, planar nanochannels using this method. 5,6 For
example, Turner et al. used poly-Si as a sacrificial layer to build complex
nano/microchannels, with elaborate openings and supporting structures that prevent
channel collapse due to stress. 7 Additionally, Cao et al . used nanoimprint lithography and
non-uniform deposition techniques to generate sealed nanochannels as small as 10 nm×50
nm. 8 (Figure 2.1(B))

2.2.3 Other Fabrication Methods

The issue of limited sample throughput can only be resolved by creating parallel
arrays of nanopores / nanofilters, either by relying on polymeric nanoporous structures
(such as gels) or relying on alternative, non-standard fabrication methods. Many different
techniques to build regular nanopore / nanochannel structures with good pore size control
have been developed. Nuclear track-etch nanopore membranes, 9-11 and the honeycomb
structure of A nodized A lumina O xide (AAO) 12 are the two well-known techniques. Non-
traditional methods have also been used to generate the same effect. For example, Jeon et
al. combined a PDMS phase mask and photopolymerization to create three-dimensional
submicron pore systems. 13 Alternatively, close-packed micro- or nanoparticles could be
used as a regular nanoporous structure, or as a template to fabricate one. 14 These
techniques are generally economical methods for generating parallel nanoporous systems,
but their integration into monolithic systems is challenging, although recent efforts have
been quite successful. 15,16 In addition, only periodic structures can be created using these
techniques, with somewhat limited long-range order.

2.3 BIOMOLECULE SEPARATION USING NANOCHANNELS

2.3.1 Molecular Sieving using Nanofluidic Filters

One obvious application of nanochannels is their use as a filter for molecules and
particles. In fact, this was the first application (and motivation) for building nanofluidic
structures, i.e. 'artificial gels' for biomolecule separation. 17 While it is now possible to
make a nanofilter or nanopore as large as a single protein, the 'filtration' phenomena that
occur within a nanofilter can be quite complicated. Biomolecules are usually flexible 3D