Review article: Fabrication of nanofluidic devices

Abstract

Thanks to its unique features at the nanoscale, nanofluidics, the study and application of fluid flow in nanochannels/nanopores with at least one characteristic size smaller than 100 nm, has enabled the occurrence of many interesting transport phenomena and has shown great potential in both bio- and energy-related fields. The unprecedented growth of this research field is apparently attributed to the rapid development of micro/nanofabrication techniques. In this review, we summarize recent activities and achievements of nanofabrication for nanofluidic devices, especially those reported in the past four years. Three major nanofabrication strategies, including nanolithography, microelectromechanical system based techniques, and methods using various nanomaterials, are introduced with specific fabrication approaches. Other unconventional fabrication attempts which utilize special polymer properties, various microfabrication failure mechanisms, and macro/microscale machining techniques are also presented. Based on these fabrication techniques, an inclusive guideline for materials and processes selection in the preparation of nanofluidic devices is provided. Finally, technical challenges along with possible opportunities in the present nanofabrication for nanofluidic study are discussed.

Figure 1

Schematic of nanochannel fabrication based on electron beam lithography. Step 1: Electron beam exposure. Steps a2-a5: Negative resist patterning, mold etching, soft lithography, and bonding. Steps b2-b4: Positive resist patterning, nanochannel etching and bonding.

Figure 2

Schematic of nanopore/nanochannel fabrication based on focused ion beam technology. Nanopore fabrication: Step 1: Thin film deposition and back chamber etching. Step 2: FIB milling. Step 3: Nanopore shrinkage using isotropic deposition. Nanochannel fabrication: Option (I) Direct FIB scanning, Option (II) Introduction of a sacrificial layer, followed by FIB scanning and sacrificial layer etching. The additional sacrificial layer in option II can help remove ridges formed during FIB scanning.

Figure 3

Schematic of nanochannel fabrication based on nanoimprint lithography. Typically, this method includes two major processes, i.e., nanochannel patterning and nanochannel sealing. The patterning process consists of four steps: Step 1: Imprint resist coating. Step 2: Mold pressing. Step 3: Mold removal. Step 4: Residual resist etching. Available sealing options include direct thermal bonding, solvent vapor sealing, melting reflow sealing, and direct template sealing.

Figure 4

Basic fabrication process of nanopore array by sphere lithography. After the preparation of a close-packed nanoparticle monolayer (step 1), two fabrication approaches are available and lead to different final structures. Option I: Step I2: Dry etching the substrate. Step I3: Releasing nanoparticles. Option II: Step II2: Dry etching the nanoparticles. Step II3: Depositing thin film. Step II4: Releasing nanoparticles. Step II5a: Isotropically etching the substrate. Step II5b: anisotropically etching the substrate. Substrate with a pre-etched back chamber was used here. The back chamber can also be created after step I3 or step II5 to form the final structure.

Figure 5

Schematic of nanochannel fabrication based on sacrificial layer releasing method. Step 1: Deposition of the bottom layer. Step 2: Deposition of the sacrificial layer. Step 3: Pattern sacrificial layer to create the male form of the nanochannel. Step 4: Deposition of the capping layer. Step 5: Formation of access reservoirs. Step 6: Nanochannel releasing.

Figure 6

Schematic of nanochannel fabrication based on etching and bonding method. Step 1: Nanochannel patterning and etching. Step 2: Microchannel patterning and etching. Step 3: Deposition of the protection layer. Step 4: Backside reservoir patterning and etching. Step 5: Removal of the protection layer. Step 6: Uniform growth/deposition of an insulation layer, e.g., a thermal oxide layer (optional). Step 7: Anodic bonding with a glass substrate.

Figure 7

Schematic of nanochannel fabrication based on etching and deposition method. Non-conformal deposition results in self-sealing nanochannels.

Figure 8

Schematic of nanochannel fabrication based on edge lithography technique. Step 1: Deposition and patterning of the first metal layer. Step 2: Formation of undercuts using isotropic etching. Step 3: Deposition of the second metal layer. Step 4: Photoresist lift-off. Step 5: Formation of open nanochannels using DRIE. Step 6: Removal of the metal mask. Step 7: Nanochannel sealing using deposition (7 I) or bonding (7 II).

Figure 9

Schematic of nanochannel fabrication based on spacer technique. Step 1: Patterning and etching of vertical trenches with micrometer sized openings. Step 2: Deposition of the spacer layer using CVD technique. Step 3: Vertical anisotropic etching to form the male form of nanochannels. Step 4: Deposition of the capping layer. Step 5: Surface planarization using CMP. Step 6: Nanochannel formation. Option I. Fabrication of nanochannel membrane by selectively etching nanochannels and back reservoir. Option II. Fabrication of sealed nanochannels using etching and bonding/deposition technique. Option III. Fabrication of sealed nanochannel using sacrificial layer etching. Step 5 is not required in this process.

Figure 10

Typical SEM photos of various nanoporous materials, top: top-view; bottom: cross-sectional view. (a) AAO membrane. Reprinted with permission from Vajandar et al., Nanotechnology 18(27), 275705 (2007). Copyright 2007 Institute of Physics. (b) Track etched nanoporous membrane. Reprinted with permission from Ali et al., ACS Nano 33, 603–608 (2009). Copyright 2009 American Chemical Society. (c) BCP nanoporous matrix. Reprinted with permission from Uehara et al., ACS Nano 34, 924-932 (2009). Copyright 2009 American Chemical Society.

Figure 11

Schematic of nanofluidic devices fabricated by self-assembling nanoparticles. Step 1: Fabrication of microchannels with reservoirs on both ends. Step 2: Introduction of nanoparticle suspension from one reservoir. Step 3: Self-assembly of nanoparticle crystal based on capillary evaporation. Step 4: Removal of extra nanoparticles by exchanging the buffer solution.

Figure 12

Schematic of nanochannel fabrication using nanowires. (a) Hot embossing based nanochannel fabrication. Step 1: Hot embossing with nanowire (NW) as master. Step 2: Removing the nanowire. Step 3: Bonding to substrate with predefined microchannels; (b) molding based nanochannel fabrication. Step 1: Assembling nanowire onto microstructures. Step 2: Casting polymer. Step 3: Releasing/removing the nanowire and microstructures and bonding with another substrate; and (c) lithography based nanochannel fabrication. Step 1: Assembling nanowire on the wafer. Step 2: Patterning photoresist. Step 3: Releasing nanowire and bonding with another substrate.

Figure 13

Schematic of nanochannel fabrication using nanotubes. (a) Suspended nanotube membrane. Step 1: Growing nanotubes (NTs). Step 2: Filling nanotubes with supporting material. Step 3: Releasing the NTs-embedded polymer membrane and opening the nanotubes; (b) nanotube embedded micro/nanofluidic device. Step 1: Growing nanotube on top of wafer. Step 2: Patterning photoresist or other protecting layer by lithography. Step 3: Opening the nanotube and packaging the structure with another wafer.

Figure 14

Achievable geometries and feature sizes of nanofluidic devices using current nanofabrication approaches. (a) Nanolithography based techniques; (b) MEMS based techniques; (c) Nanomaterials based techniques. This figure is based on the authors' understanding of the state-of-the-art of all the nanofabrication approaches in this review.