0
Research Papers

Fabrication of Centimeter Long, Ultra-Low Aspect Ratio Nanochannel Networks in Borosilicate Glass Substrates OPEN ACCESS

[+] Author and Article Information
Marie Pinti

e-mail: pinti.3@osu.edu

Tanuja Kambham

e-mail: kambham.1@osu.edu

Bowen Wang

e-mail: wang.3161@osu.edu

Shaurya Prakash

e-mail: prakash.31@osu.edu
Mechanical and Aerospace
Engineering Department,
The Ohio State University,
Columbus, OH 43210

1Corresponding author.

Manuscript received July 25, 2013; final manuscript received August 15, 2013; published online October 3, 2013. Assoc. Editor: Abraham Wang.

J. Nanotechnol. Eng. Med 4(2), 020905 (Oct 03, 2013) (7 pages) Paper No: NANO-13-1042; doi: 10.1115/1.4025366 History: Received July 25, 2013; Revised August 15, 2013

Nanofluidic devices have a broad range of applications resulting from the dominance of surface-fluid interactions. Examples include molecular gating, sample preconcentration, and sample injection. Manipulation of small fluid samples is ideal for micro total analysis systems or lab on chip devices which perform multiple unit operations on a single chip. In this paper, fabrication procedures for two different ultra-low aspect ratio (ULAR) channel network designs are presented. The ULAR provides increased throughput compared to higher aspect ratio features with the same critical dimensions. Channel network designs allow for integration between microscale and nanoscale fluidic networks. A modified calcium assisted glass–glass bonding procedure was developed to fabricate chemically uniform, all glass nanochannels. A polydimethylsiloxane (PDMS)-glass adhesive bonding procedure was also developed as adhesive bonding allows for more robust fabrication with lower sensitivity to surface defects. The fabrication schemes presented allow for a broad array of available parameters for facile selection of device fabrication techniques depending on desired applications for lab on chip devices.

FIGURES IN THIS ARTICLE
<>

Nanofluidic devices have at least one critical dimension on the nanoscale (usually less than 100 nm) leading to a characteristic high surface area-to-volume ratio. This leads to dominance of surface-fluid interactions where transport is governed by surface charge and surface energy [1-4]. The surface-fluid interactions enable a broad range of applications including water desalination [5,6], molecular gating [7,8], energy transfer [9,10], and deoxyribonucleic acid (DNA) elongation [11,12] and sieving [13] among other applications [14].

Nanofluidics requires nanofabrication techniques to allow for miniaturization of functional components or systems. Miniaturization to the nanoscale allows for integration of many components with various functionalities onto a single platform, which as a whole can be a microchip. Such devices are known as micro total analysis systems (μ-TAS) or lab on a chip (LoC) [15] systems. μ-TAS or LoC systems can perform sample pretreatment or preconcentration, analyte separation, analyte detection and data analysis among other functionalities or unit operations within a single device [14]. The ability to perform multiple unit operations requires fluid manipulation and precise flow control over increasingly smaller sample volumes reaching the atto-liter (10−18 l) range [7]. Device designs need to allow for transfer of the fluid sample between different functional elements of the lab on a chip device and also allow for manipulation of the sample and fluid species within each functional component. Additionally, one of the desired features of LoC technology is real-time, rapid detection for quality performance, where high sample throughput may help decrease device operation time with potential benefits for high signal to noise ratios [16,17].

In this paper, fabrication of two different designs of ULAR, centimeter long, nanochannel networks with integration between microscale and nanoscale fluidic networks is reported. The ULAR allows for increased volumetric flow rate for a given critical dimension and fluid velocity compared to higher aspect ratio channels. The increased volumetric flow rate can be of particular interest in sensing applications where the interplay of diffusion, convection, and reaction time scales are of critical importance to detection limits, which in-turn are strongly influenced by critical dimension and fluid velocity [18]. Furthermore, ULAR channels can be modeled as ideal 1D systems allowing for computationally less challenging modeling of flow characteristics for various devices [19].

Microfluidic and nanofluidic devices find many substrates for fabrication driven by competing interests of cost, fabrication complexity, and the need to achieve optimal performance for each application. However, most devices are typically fabricated using silicon due to well established fabrication techniques developed for the semiconductor industry; glass (soda lime and borosilicate or pyrex are most common) and fused silica for their chemical stability, electrical isolation and optical properties; polymers such as PDMS or polymethylmethacrylate for their biocompatibility and additional compatibility with specific fabrication techniques (i.e., soft lithography) or a combination of these materials [20]. In this paper, all results are reported for fabrication of ULAR nanochannel fluidic networks in borosilicate glass substrates. The nanochannel networks are sealed with either a glass cover or a glass cover with a PDMS adhesion layer.

Operational devices comprise pathways for fluid and species transport. Such pathways, channels, or fluid conduits are patterned through lithography with eventual pattern transfer to the physical substrate performed through wet or dry etching methods to generate the specific device layouts. While many forms of lithography are available for pattern transfer [20], conventional UV lithography is a well-controlled, well established method for patterning features with micron scale widths, that does not require specialized equipment or significant time required for direct writing methods [21]. In order to enable nanochannel networks and repeatable fabrication of ULAR devices, achieving control with high degree of precision over channel dimensions that can be several mm to cm long has been a challenge [22,23].

Once fluidic networks have been created in the physical substrate, devices are sealed to obtain closed pathways. Most commonly used bonding techniques include anodic bonding for silicon-glass devices, fusion bonding for glass–glass devices, and adhesive or interfacial layer-based bonding techniques. Channels as small as 2 nm (aspect ratio of 0.001) have been fabricated using anodic bonding [24] while channels down to 5 nm (aspect ratio of 1) have been successfully sealed through fusion bonding [25]. In addition to fabrication challenges described above, nanochannels are also prone to collapse during the bonding process due to bowing of the channel surfaces under high voltages required for anodic bonding and re-flow of glass for high temperature fusion bonding. Channels are more prone to collapse as the aspect ratio decreases [26] leading to fluidic conduits that are blocked and do not allow transport. Both anodic bonding and thermal fusion bonding require highly controlled and clean environments to yield defect-free devices [20].

Adhesive or interfacial layer-based bonding techniques have the advantage of a decreased dependence on the presence of surface contaminants and surface defects compared to anodic or fusion bonding due to the compliant nature of the interfacial layer used. However, the compliant nature of the adhesive materials limits the aspect ratio compared to glass–glass or glass-silicon devices. The lowest previously reported aspect ratio for glass‐glass devices was 0.00025 [27], which is an order of magnitude lower than previously reported 0.005 for PDMS-glass devices [28].

In this paper, two bonding strategies for sealing the fluidic networks that allow the device to be either fully glass or glass-PDMS are reported with both yielding nanochannel networks at cm-long length scales with ULAR nanochannels. A modified calcium assisted bonding approach [29] was used for sealing glass devices. The second bonding approach employs a PDMS adhesion layer spin-coated onto a borosilicate top cover with sealing performed through oxygen plasma bonding.

In this paper, fabrication of two different designs of ultra-low aspect ratio centimeter long channel networks that include integration between microscale and nanoscale fluidic networks are presented. The procedures presented allow for reliable fabrication of ULAR channel networks with integration between channels of various length scales as well as selection of bonding procedure to produce the desired configuration for a LoC device, while achieving some of the lowest aspect ratios reported till date. Therefore, the purpose of this paper is to demonstrate a broadly applicable fabrication scheme relying on conventional UV lithography, wet etching, and subsequent bonding to fabricate ULAR nanochannel networks in borosilicate glass substrates.

Borosilicate substrates were used for fabrication of fluidic channel networks shown in Figure 1 (Fisher Scientific Borosilicate Cover Slips, 12–50 C). Substrates were de-greased using a standard solvent degrease process then immersed in a piranha solution (4:1 ratio of 96% sulfuric acid to 30% hydrogen peroxide) for 10 mins to remove organic and metallic contaminants from the surface. Note: The piranha solution is a strongly oxidizing agent and must be used with extreme caution. A 20 nm Cr adhesion layer followed by an inert 100 nm Au masking layer was applied to cleaned substrates using an e-beam evaporator (CHA Solution System E-Gun Evaporator). A two-step Au evaporation controlled surface defects in the masking layer, reducing pinhole defects in the final device [30].

Standard UV lithography was used to pattern the microchannel portion of the integrated device (Design 1) and also to pattern the channel network in the purely nanoscale device (Design 2). Design 1 required a second UV lithography step to pattern interconnecting nanofluidic channels between the microchannels. As discussed below, fabrication parameters had to be adjusted in the second UV lithography step to prevent photoresist delamination and subsequent defect formation at the micronanochannel interface. A UV contact aligner (EV Group 620 Advanced Contact Aligner) and Shipley 1813 photoresist along with MF 319 developer were used in the photolithography process. Patterns were transferred to the substrate through wet etching using a 4:1 ratio of de-ionized (DI) water to 49% HF in the case of microchannels and a standard 10:1 buffered oxide etch for all nanochannels.

Channel networks were bonded using two different methods to yield either PDMS-glass devices or glass–glass devices forming the walls enclosing the nanochannels. Fluidic access ports were drilled into 1 mm thick glass covers using a diamond core drill bit. For PDMS-glass devices, uncured PDMS mixed with a standard 10:1 monomer to curing agent ratio was spun and cured onto the covers. Bonding between the borosilicate covers and the borosilicate substrates containing the fluidic networks was achieved through well-established oxygen plasma bonding [31].

Glass–glass devices were achieved by using an interfacial bonding procedure using calcium-assisted bonding [29]. In order to obtain bonded nanochannel networks, patterned borosilicate substrates and glass covers were first cleaned with a piranha solution for 10 mins. The cleaned glass cover was then scrubbed with a 1% Alconox (Alconox, Inc.,) solution in DI water for 2 mins. The cover and patterned substrate were rinsed with a 0.5% calcium acetate/0.5% Alconox slurry. The two device components were then brought into contact, separated, and rinsed with DI water for 20 seconds. Next, the two glass slides (i.e., the one with the etched channel network and the second one with the cover holes drilled) were aligned and dehydrated under pressure at 60 °C for 1 h. Devices were then inspected for defects where a defect was identified by the presence of Newton rings [32]. If defects were present i.e., an interference pattern was observed, the two glass slides were pulled apart, and then re-bonded. Once the temporary bonds with no visible defects were achieved, the device was fully dehydrated and irreversibly sealed under pressure at 115 °C for 2 h. This procedure modifies previously reported calcium assisted bonding [29] by adding the piranha cleaning step. While the original report for fabrication of micron depth channels does not require aggressive cleaning for device bonding, it was found at the length scales reported here successful devices could not be fabricated without the piranha cleaning step suggesting the need for better surface property control for nanochannel networks.

For fabricated devices to be useful, the nanochannel networks must yield open channels with fluid transport. Operability of these channels was also verified. Successful fabrication of nanochannel networks was confirmed by a multitechnique analysis. Scanning electron microscopy (Zeiss Ultra 55 Plus FE-SEM) of device cross sections confirmed bonded dimensions along with atomic force microscopy (Asylum MFP-3D AFM) scanning to confirm depth of channels as etched. In addition, the SEM imaging showed open nanochannel cross sections suggesting clear pathways for fluid transport. Etch plots relating nanochannel depth to etching conditions have been reported previously [19]. Flow through the fabricated nanochannel networks was verified through fluidic transport of two types. Capillary filling was visualized through the entire channel network in glass–glass devices using isopropyl alcohol (IPA). Electrokinetic flow measurements in PDMS-glass devices were performed using 20 mM KCl solution along with a Keithley 6485 picoammeter to measure current flow through the channel network at applied voltages ranging from 10 V to 200 V.

Figure 1 shows the schematic representation of the two channel networks. Design 1 incorporated both microscale and nanoscale channels. Two 3 cm long microchannels, with measured widths at 131 μm ± 1 μm and a measured depth of 8.0 μm ± 0.1 μm were connected by a bank of 0.5 cm long nanometer depth channels. The nanochannel array consisted of 3–6 nanochannels that were measured to be 33 μm ± 1 μm wide. The measured depth of the nanochannels ranged from 16.0 nm ± 0.1 nm to 227 nm ± 5 nm. Therefore, aspect ratio for the bonded devices ranged from 0.0075 for 227 nm channels down to 0.0005 for 16 nm channels compared to previous lowest report of 0.005 using PDMS-stamp and stick bonding [28]. Design 2 featured a completely nanochannel network. The channel network was a simple “Y” shaped network with a single 1 cm long, 100 μm channel measured to be ∼106 μm wide which split into two 1 cm long, 50 μm channels of nominal width measured to be ∼56 μm, each at a 60 deg angle from the centerline of the single-wide channel. In this case, the bonded devices had aspect ratios ranging from 0.0042 for the 446 nm deep nanochannel device down to 0.0008 for the 77 nm deep nanochannel device.

Control over lateral channel dimensions required use of a metal etching mask as the wet etch used for glass channels was performed using an isotropic etchant (see methods section for details). Previous reports show use of a metal mask for etching relatively deep (micron scale or larger) features to prevent defects caused by undercutting and photoresist delamination [30,33-35]. During etching for deeper features, the HF-based etchants eventually attack the photoresist masking layer causing delamination. For hydrophilic substrates such as glass, the etchant can also wet the glass surface underneath the photoresist layer, causing delamination [34]. Inert metals such as Au are used to prevent feature defects caused by photoresist delamination when etching deep features [34].

During the fabrication of nanochannel networks such as those reported here, it was found that the metal mask was required for precise control of the lateral dimension even when patterning sub-50 nm features with exposure times of 1 min or less suggesting aspect ratio dependence for wet etchants. Such dependence has been investigated for dry etching previously [36]. Figure 2 shows representative microscope images of a 77 nm deep channel fabricated with and without a metal mask. The lateral dimension of the channel was poorly controlled for the case where no metal mask was used. Figures 2(a) and 2(b) show distortion of the lateral channel dimension in the 50 μm and a 100 μm nominal width channel, respectively, with the defects marked in the figure. Figure 2(c) shows a microscope image of Design 2 where defects in the lateral dimension are not observed.

Despite relatively short etching times (5 mins or less) significant undercutting was observed in the case without the metal mask The channel widths of Design 2 were measured at 162 μm for the nominal dimension of 100 μm without the metal mask as compared to 106 μm with the metal mask for a 77 nm deep nanochannel network indicating a metal mask was necessary for desired control over the feature geometry.

Lithography Modification.

Initial fabrication of the micronanochannel network (Design 1) resulted in delamination of the photoresist film between the nanochannels at the micronanochannel interface. Figure 3 shows a schematic representation of the ideal channel network fabrication (Fig. 3(a)) as well as fabrication that resulted in ridge formation at the micronanochannel interface (Fig. 3(b)). Photoresist delamination can be problematic when dealing with nonplanar surfaces as differences in the topography of the surface lead to nonuniform or uneven coating of the photoresist layer. The nonuniform substrate can lead to thin regions and/or photoresist pile-up during spinning depending on the surface being patterned. Thin regions are susceptible to photoresist delamination or over-exposure resulting in a clearing dose in an undesired area. Figure 3(b) shows a schematic representation of uneven photoresist coverage for the microchannels in Design 1. The uneven photoresist layer led to complete development of the region at the micronanochannel interface leading to all of the nanochannels being connected by a ridge of the same depth as the nanochannels. In the past, while dealing with nonplanar geometries requiring step-wise lithography for other features, several other approaches have been developed [37-39]. Here, a multistep photoresist spin was developed to allow for easy integration of existing process steps with the ability to pattern nonplanar surfaces. A slow spin step (∼60 rpm) allowed the photoresist to fully fill the patterned microchannels leading to a planar surface for subsequent photolithography; thus, eliminating the nanoscale fluidic connection between subsequent nanochannels (Fig. 4). Additional details on the use of slow spin speeds for patterning nonplanar surfaces have been reported previously [21].

Device Bonding.

It should be noted that while either bonding scheme can be used for each of the two designs, in the discussions to follow, Design 1 for PDMS-glass device discussions and glass–glass devices for Design 2 discussions for illustrative purposes are described. Table 1 summarizes aspect ratios achieved using each bonding scheme.

PDMS-Glass Bonding.

For PDMS-glass devices, a PDMS adhesion layer was spin-coated onto a borosilicate cover with pre-drilled fluidic access ports. The thickness of the PDMS depends on the spin speed where a higher spin speed results in a thinner film [40]. The main limitation of this bonding approach is the uniformity of the PDMS layer. Glass covers were plasma treated at 200 W at a chamber pressure of 105 mTorr for 3 mins before spin coating. Due to the spin process and the degradation of the plasma treatment with time, nonuniformities in the PDMS layer can arise leading to leaking due to missing portions of the film or to device failure [19]. Spin speeds ranging from 1000 rpm to 6000 rpm were tested. The maximum spin speed that allowed a continuous, uniform, and defect-free film was 1500 rpm while further reducing the spin speed to 1000 rpm increased device bonding yield from approximately 20% to approximately 65%. The lowest aspect ratio achieved was 0.0005 as summarized in Table 1. Spin speeds higher than 1500 rpm yielded unreliable PDMS thin films with bonded devices yields well below ∼10%. Therefore, spin speeds beyond 1500 rpm were not considered further. Figure 5 shows a representative cross section image of a PDMS-glass based device showing the channel was fully sealed. From the cross section image, the PDMS layer in the bonded device was measured to be ∼ 2 μm for a 1000 rpm spin.

Additional confirmation of device operation was performed through I–V (current–voltage) measurements for bonded devices. Figure 6 shows a representative I–V curve for a 22 nm PDMS-glass device filled with 20 mM KCl solution where the applied voltage ranged from 10 V to 200 V in steps of 10 V. A finite measured current suggested that electrokinetic flow was occurring through the nanochannel.

Glass-Glass Bonding.

Glass–glass bonding results in a chemically uniform surface enclosing the nanochannels. The lowest aspect ratio achieved in a glass–glass sample was 0.0008. Since the device components could be separated and re-bonded, if defects were observed in the bond at the intermediate bonding stage, successful device yield was nearly 100% for aspect ratios reported. Figure 6 shows a cross section image of a 77 nm deep glass–glass nanofluidic channel. The cracks seen in the image arise due to cutting the glass slides for sample preparation for SEM imaging and are not present in the full-scale device used for flow testing (Figure 7).

The cross section image shows that after bonding the channel depth varies from 62 to 77 nm at different locations along the channel width; however, it is known that the action of cutting glass to measure nanochannel depths can alter the edge profile leading to a variable cross section image [28]. Therefore, a more reliable measurement for channel depth is the AFM trench depth and then channel operation verification by a flow measurement. Further confirmation of device operation was performed through capillary filling experiments of all glass channel networks. Figure 8 shows a representative condition for isopropyl alcohol in 77 nm, 242 nm, and 446 nm deep nanochannels respectively.

Two different nanochannel network designs in borosilicate substrates were fabricated using standard UV lithography followed by wet etching to yield centimeter long nanochannels. Two different bonding approaches were employed allowing for glass–glass bonding resulting in a chemically uniform channel surface or glass-PDMS adhesive bonding with less sensitivity to surface defects. The fabrication schemes presented allow for selection of device fabrication techniques depending on desired application for a lab on a chip device providing a facile set of tools to develop ULAR nanochannel networks in glass substrates.

The authors would like to acknowledge Eugene Sosnov for assistance during early stages of fabrication as well as the staff at Nanotech West Laboratories at The Ohio State University for assistance with equipment during fabrication and characterization of devices. We also acknowledge partial financial support from Defense Advanced Research Projects Agency (DARPA), through the US Army Research Office (ARO) grant number W911NF09C0079, and the National Science Foundation (NSF) through Grant No. CBET-1335946.

Swaminathan, V. V., Gibson, L. R., II, Pinti, M., Prakash, S., Bohn, P. W., and Shannon, M. A., 2012, “Ionic Transport in Nanocapillary Array Membranes,” J. Nanopart. Res., 14, p. 951. [CrossRef]
Prakash, S., Piruska, A., Gatimu, E. N., Bohn, P. W., Sweedler, J. V., and Shannon, M. A., 2008, “Nanofluidics: Systems and Applications,” IEEE Sens. J., 8(5), pp. 441–450. [CrossRef]
Prakash, S., Karacor, M. B., and Banerjee, S., 2009, “Surface Modification in Microsystems and Nanosystems,” Surf. Sci. Rep., 64(7), pp. 233–254. [CrossRef]
Prakash, S., and Karacor, M. B., 2011, “Characterizing Stability of “Click” Modified Glass Surfaces to Common Microfabrication Conditions and Aqueous Electrolyte Solutions,” Nanoscale, 3(8), pp. 3309–3315. [CrossRef] [PubMed]
Shannon, M. A., 2012, “Water Desalination: Fresh for Less,” Nat. Nanotechnol., 5, pp. 248–250. [CrossRef]
Kim, S. J., Ko, S. H., Kang, K. H., and Han, J., 2010, “Direct Seawater Desalination by Ion Concentration Polarization,” Nat. Nanotechnol., 5, pp. 297–301. [CrossRef] [PubMed]
Kuo, T. C., Cannon, D. M.Jr., Chen, Y., Tulock, J. J., Shannon, M. A., Sweedler, J. V., and Bohn, P. W., 2003, “Gateable Nanofluidic Interconnects for Multilayed Microfluidic Separation Systems,” Anal. Chem., 75(8), pp. 1861–1867. [CrossRef] [PubMed]
Pardon, G., and Wijingaart, W. V. D., “Modelling and Simulation of Electrostatically Gated Nanochannels,” Adv. Colloid Interface Sci. (in press).
Goswami, P., and Chakraborty, S., 2010, “Energy Transfer through Streaming Effects in Time-Periodic Pressure-Driven Nanochannel Flows With Interfacial Slip,” Langmuir, 26(1), pp. 581–590. [CrossRef] [PubMed]
Pennathur, S., Eijkel, J. C., and Berg, A. V. D., 2007, “Energy Conversion in Microsystems: Is There a Role for Micro/Nanofluidics,” Lab Chip, 10, pp. 1234–1237. [CrossRef]
Matteucci, M., Christiansen, T. L., Tanzi, S., Ostergaard, P. F., and Larsen, S. T., 2013, “Fabrication and Characterization of Injection Molded Multi Level Nano and Microfluidic Systems,” Microelectron. Eng., 111, pp. 294–298. [CrossRef]
Yasuri, T., Rahong, S., Motoyama, K., Yanagida, T., Wu, Q., Kaji, N., Kanai, M., Doi, K., Nagashima, K., Tokeshi, M., Taniguchi, M., Kawano, S., Kawai, T., and Baba, Y., 2013, “DNA Manipulation and Separation in Sublithographic-Scale Nanowire Array,” ACS Nano, 7(4), pp. 3029–3035. [CrossRef] [PubMed]
Fu, J., Schoch, R. B., Stevens, A. L., Tannenbaum, S. R., and Han, J., 2007, “A Patterned Anisotropic Nanofluidic Sieving Structure for Continuous-Flow Separation of DNA and Proteins,” Nat. Nanotechnol., 2(2), pp. 121–128. [CrossRef] [PubMed]
Prakash, S., Pinti, M., and Bhushan, B., 2012, “Theory, Fabrication and Applications of Microfluidic and Nanofluidic Biosensors,” Philos. Trans. R. Soc. London, 370, pp. 2269–2303. [CrossRef]
Mark, D., Haeberle, S., Roth, G., Stetten, F. V., and Zengerle, R., 2012, “Microfluidic Lab-on-a-Chip Platforms: Requirements, Characteristics, and Applications,” Chem. Soc. Rev., 39, pp. 1153–1182. [CrossRef]
Kovarik, M. L., Gach, P. C., Ornoff, D. M., Wang, Y., Balowski, J., Farrag, L., and Allbritton, N. L., 2012, “Micro Total Analysis Systems for Cell Biology and Biochemical Assays,” Anal. Chem., 84, pp. 516–540. [CrossRef] [PubMed]
Rios, A., Zougagh, M., and Avila, M., 2012, “Miniaturization Through Lab-on-a-Chip: Utopia or Reality for Routine Laboratories? A Review,” Anal. Chim. Acta, 740, pp. 1–11. [CrossRef] [PubMed]
Squires, T. M., Messinger, R. J., and Manalis, S. R., 2008, “Making it Stick: Convection, Reaction and Diffusion in Surface-Based Biosensors,” Nat. Biotechnol., 26(4), pp. 417–426. [CrossRef] [PubMed]
Pinti, M., and Prakash, S., 2013, “Fabrication of Hybrid Micro-Nanofluidic Devices With Centimeter Long Ultra-Low Aspect Ratio Nanochannels,” Proceedings of ASME 2013 International Mechanical Engineering Congress and Exposition, IMECE2013_65763, November 17–21, San Diego, CA (in press).
Duan, C., Wang, W., and Xie, Q., 2013, “Fabrication of Nanofluidic Devices,” Biomicrofluidics, 7, p. 026501. [CrossRef]
Pinti, M., and Prakash, S., 2011, “A Two-Step Wet Etch Process for the Facile Fabrication of Hybrid Micro-Nanofluidic Devices,” Proceedings of ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE2011_64508, November 11–17, Denver, CO, pp. 647–651.
Huang, X. T., Gupta, C., and Pennathur, S., 2010, “A Novel Fabrication Method for Centimeter-Long Surface-Micromachined Nanochannels,” J. Micromech. Microeng., 20, p. 015040. [CrossRef]
Han, A., Rooij, N. F. D., and Staufer, U., 2006, “Design and Fabrication of Nanofluidic Devices by Surface Micromachining,” Nanotechnology, 17(10), pp. 2498–2503. [CrossRef] [PubMed]
Duan, C., and Majumdar, A., 2010, “Anomalous Ion Transport in 2-nm Hydrophillic Nanochannels,” Nat. Nanotechnol., 5, pp. 848–852. [CrossRef] [PubMed]
Menard, L. D., and Ramsey, J. M., 2011, “The Fabrication of Sub-5 nm Nanochannels in Insulating Substrates Using Focused Ion Beam Milling,” Nano Lett., 11, pp. 512–517. [CrossRef] [PubMed]
Mao, P., and Han, J., 2005, “Fabrication and Characterization of 20 nm Planar Nanofluidic Channels by Glass-Glass and Glass-Silicon Bonding,” Lab Chip, 5, pp. 837–844. [CrossRef] [PubMed]
Haneveld, J., Tas, N. R., Brunets, N., Jansen, H. V., and Elwenspoek, M., 2008, “Capillary Filling of Sub-10 nm Nanochannels,” J. Appl. Phys., 104, p. 014309. [CrossRef]
Duan, C., Karnik, R., Liu, M. C., and Majumdar, A., 2012, “Evaporation-Induced Cavitation in Nanofluidic Channels,” Proc. Natl. Acad. Sci. U.S.A., 109(10), pp. 3688–3693. [CrossRef] [PubMed]
Allen, P. B., and Chiu, D. T., 2008, “Calcium-Assisted Glass-to-Glass Bonding for Fabrication of Glass Microfluidic Devices,” Anal. Chem., 80, pp. 7153–7157. [CrossRef] [PubMed]
Iliescu, C., Chen, B., and Miao, J., 2008, “On the Wet Etching of Pyrex Glass,” Sens. Actuators, A, 143, pp. 154–161. [CrossRef]
Bhattacharya, S., Datta, A., Berg, J. M., and Gangopadhyay, S., 2005, “Studies on Surface Wettability of Poly(Dimethyl) Siloxane (PDMS) and Glass Under Oxygen-Plasma Treatment and Correlation with Bond Strength,” J. Microelectromech. Syst., 14(3), pp. 590–597. [CrossRef]
Knight, R. D., 2003, Physics for Scientists and Engineers: A Strategic Approach, Addison-Wesley, San Francisco, CA.
Tay, F., Iliescu, C., Jing, J., and Miao, J., 2006, “Defect-Free Wet Etching Through Pyrex Glass Using Cr/Au Mask,” Microsyst. Technol., 12, pp. 935–939. [CrossRef]
Zhu, H., Holl, M., Ray, T., Bhushan, S., and Meldrum, D. R., 2009, “Characterization of Deep Wet Etching of Fused Silica Glass for Single Cell and Optical Sensor Deposition,” J. Micromech. Microeng., 19, p. 065013. [CrossRef]
Prakash, S., Long, T. M., Selby, J. C., Moore, J. S., and Shannon, M. A., 2007, ““Click” Modification of Silica Surfaces and Glass Microfluidic Channels,” Anal. Chem., 79(4), pp. 1661–1667. [CrossRef] [PubMed]
Yeom, J., Wu, Y., Selby, J. C., and Shannon, M. A., 2005, “Maximum Achievable Aspect Ratio in Deep Reactive Ion Etching of Silicon Due to Aspect Ratio Dependent Transport and the Microloading Effect,” J. Vac. Sci. Technol. B, 23(6), pp. 2319–2329. [CrossRef]
Yeom, J., and Shannon, M. A., 2009, “Detachment Lithography of Photosensitive Polymers: A Route to Fabricating Three-Dimensional Structures,” Adv. Funct. Mater., 20, pp. 289–295. [CrossRef]
Jackman, R. J., Wilbur, J. L., and Whitesides, G. M., 1995, “Fabrication of Submicrometer Features on Curved Substrates by Microcontact Printing,” Science, 269, pp. 664–666. [CrossRef] [PubMed]
Leong, T. G., Lester, P. A., Koh, T. L., Call, E. K., and Gracias, D. H., 2007, “Surface Tension-Driven Self-Folding Polyhedra,” Langmuir, 23, pp. 8747–8751. [CrossRef] [PubMed]
Mata, A., Fleischman, A. J., and Roy, S., 2005, “Characterization of Polydimethylsiloxane (PDMS) Properties for Biomedical Micro/Nanosystems,” Biomed. Microdevices, 7(4), pp. 281–293. [CrossRef] [PubMed]
Copyright © 2013 by ASME
View article in PDF format.

References

Swaminathan, V. V., Gibson, L. R., II, Pinti, M., Prakash, S., Bohn, P. W., and Shannon, M. A., 2012, “Ionic Transport in Nanocapillary Array Membranes,” J. Nanopart. Res., 14, p. 951. [CrossRef]
Prakash, S., Piruska, A., Gatimu, E. N., Bohn, P. W., Sweedler, J. V., and Shannon, M. A., 2008, “Nanofluidics: Systems and Applications,” IEEE Sens. J., 8(5), pp. 441–450. [CrossRef]
Prakash, S., Karacor, M. B., and Banerjee, S., 2009, “Surface Modification in Microsystems and Nanosystems,” Surf. Sci. Rep., 64(7), pp. 233–254. [CrossRef]
Prakash, S., and Karacor, M. B., 2011, “Characterizing Stability of “Click” Modified Glass Surfaces to Common Microfabrication Conditions and Aqueous Electrolyte Solutions,” Nanoscale, 3(8), pp. 3309–3315. [CrossRef] [PubMed]
Shannon, M. A., 2012, “Water Desalination: Fresh for Less,” Nat. Nanotechnol., 5, pp. 248–250. [CrossRef]
Kim, S. J., Ko, S. H., Kang, K. H., and Han, J., 2010, “Direct Seawater Desalination by Ion Concentration Polarization,” Nat. Nanotechnol., 5, pp. 297–301. [CrossRef] [PubMed]
Kuo, T. C., Cannon, D. M.Jr., Chen, Y., Tulock, J. J., Shannon, M. A., Sweedler, J. V., and Bohn, P. W., 2003, “Gateable Nanofluidic Interconnects for Multilayed Microfluidic Separation Systems,” Anal. Chem., 75(8), pp. 1861–1867. [CrossRef] [PubMed]
Pardon, G., and Wijingaart, W. V. D., “Modelling and Simulation of Electrostatically Gated Nanochannels,” Adv. Colloid Interface Sci. (in press).
Goswami, P., and Chakraborty, S., 2010, “Energy Transfer through Streaming Effects in Time-Periodic Pressure-Driven Nanochannel Flows With Interfacial Slip,” Langmuir, 26(1), pp. 581–590. [CrossRef] [PubMed]
Pennathur, S., Eijkel, J. C., and Berg, A. V. D., 2007, “Energy Conversion in Microsystems: Is There a Role for Micro/Nanofluidics,” Lab Chip, 10, pp. 1234–1237. [CrossRef]
Matteucci, M., Christiansen, T. L., Tanzi, S., Ostergaard, P. F., and Larsen, S. T., 2013, “Fabrication and Characterization of Injection Molded Multi Level Nano and Microfluidic Systems,” Microelectron. Eng., 111, pp. 294–298. [CrossRef]
Yasuri, T., Rahong, S., Motoyama, K., Yanagida, T., Wu, Q., Kaji, N., Kanai, M., Doi, K., Nagashima, K., Tokeshi, M., Taniguchi, M., Kawano, S., Kawai, T., and Baba, Y., 2013, “DNA Manipulation and Separation in Sublithographic-Scale Nanowire Array,” ACS Nano, 7(4), pp. 3029–3035. [CrossRef] [PubMed]
Fu, J., Schoch, R. B., Stevens, A. L., Tannenbaum, S. R., and Han, J., 2007, “A Patterned Anisotropic Nanofluidic Sieving Structure for Continuous-Flow Separation of DNA and Proteins,” Nat. Nanotechnol., 2(2), pp. 121–128. [CrossRef] [PubMed]
Prakash, S., Pinti, M., and Bhushan, B., 2012, “Theory, Fabrication and Applications of Microfluidic and Nanofluidic Biosensors,” Philos. Trans. R. Soc. London, 370, pp. 2269–2303. [CrossRef]
Mark, D., Haeberle, S., Roth, G., Stetten, F. V., and Zengerle, R., 2012, “Microfluidic Lab-on-a-Chip Platforms: Requirements, Characteristics, and Applications,” Chem. Soc. Rev., 39, pp. 1153–1182. [CrossRef]
Kovarik, M. L., Gach, P. C., Ornoff, D. M., Wang, Y., Balowski, J., Farrag, L., and Allbritton, N. L., 2012, “Micro Total Analysis Systems for Cell Biology and Biochemical Assays,” Anal. Chem., 84, pp. 516–540. [CrossRef] [PubMed]
Rios, A., Zougagh, M., and Avila, M., 2012, “Miniaturization Through Lab-on-a-Chip: Utopia or Reality for Routine Laboratories? A Review,” Anal. Chim. Acta, 740, pp. 1–11. [CrossRef] [PubMed]
Squires, T. M., Messinger, R. J., and Manalis, S. R., 2008, “Making it Stick: Convection, Reaction and Diffusion in Surface-Based Biosensors,” Nat. Biotechnol., 26(4), pp. 417–426. [CrossRef] [PubMed]
Pinti, M., and Prakash, S., 2013, “Fabrication of Hybrid Micro-Nanofluidic Devices With Centimeter Long Ultra-Low Aspect Ratio Nanochannels,” Proceedings of ASME 2013 International Mechanical Engineering Congress and Exposition, IMECE2013_65763, November 17–21, San Diego, CA (in press).
Duan, C., Wang, W., and Xie, Q., 2013, “Fabrication of Nanofluidic Devices,” Biomicrofluidics, 7, p. 026501. [CrossRef]
Pinti, M., and Prakash, S., 2011, “A Two-Step Wet Etch Process for the Facile Fabrication of Hybrid Micro-Nanofluidic Devices,” Proceedings of ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE2011_64508, November 11–17, Denver, CO, pp. 647–651.
Huang, X. T., Gupta, C., and Pennathur, S., 2010, “A Novel Fabrication Method for Centimeter-Long Surface-Micromachined Nanochannels,” J. Micromech. Microeng., 20, p. 015040. [CrossRef]
Han, A., Rooij, N. F. D., and Staufer, U., 2006, “Design and Fabrication of Nanofluidic Devices by Surface Micromachining,” Nanotechnology, 17(10), pp. 2498–2503. [CrossRef] [PubMed]
Duan, C., and Majumdar, A., 2010, “Anomalous Ion Transport in 2-nm Hydrophillic Nanochannels,” Nat. Nanotechnol., 5, pp. 848–852. [CrossRef] [PubMed]
Menard, L. D., and Ramsey, J. M., 2011, “The Fabrication of Sub-5 nm Nanochannels in Insulating Substrates Using Focused Ion Beam Milling,” Nano Lett., 11, pp. 512–517. [CrossRef] [PubMed]
Mao, P., and Han, J., 2005, “Fabrication and Characterization of 20 nm Planar Nanofluidic Channels by Glass-Glass and Glass-Silicon Bonding,” Lab Chip, 5, pp. 837–844. [CrossRef] [PubMed]
Haneveld, J., Tas, N. R., Brunets, N., Jansen, H. V., and Elwenspoek, M., 2008, “Capillary Filling of Sub-10 nm Nanochannels,” J. Appl. Phys., 104, p. 014309. [CrossRef]
Duan, C., Karnik, R., Liu, M. C., and Majumdar, A., 2012, “Evaporation-Induced Cavitation in Nanofluidic Channels,” Proc. Natl. Acad. Sci. U.S.A., 109(10), pp. 3688–3693. [CrossRef] [PubMed]
Allen, P. B., and Chiu, D. T., 2008, “Calcium-Assisted Glass-to-Glass Bonding for Fabrication of Glass Microfluidic Devices,” Anal. Chem., 80, pp. 7153–7157. [CrossRef] [PubMed]
Iliescu, C., Chen, B., and Miao, J., 2008, “On the Wet Etching of Pyrex Glass,” Sens. Actuators, A, 143, pp. 154–161. [CrossRef]
Bhattacharya, S., Datta, A., Berg, J. M., and Gangopadhyay, S., 2005, “Studies on Surface Wettability of Poly(Dimethyl) Siloxane (PDMS) and Glass Under Oxygen-Plasma Treatment and Correlation with Bond Strength,” J. Microelectromech. Syst., 14(3), pp. 590–597. [CrossRef]
Knight, R. D., 2003, Physics for Scientists and Engineers: A Strategic Approach, Addison-Wesley, San Francisco, CA.
Tay, F., Iliescu, C., Jing, J., and Miao, J., 2006, “Defect-Free Wet Etching Through Pyrex Glass Using Cr/Au Mask,” Microsyst. Technol., 12, pp. 935–939. [CrossRef]
Zhu, H., Holl, M., Ray, T., Bhushan, S., and Meldrum, D. R., 2009, “Characterization of Deep Wet Etching of Fused Silica Glass for Single Cell and Optical Sensor Deposition,” J. Micromech. Microeng., 19, p. 065013. [CrossRef]
Prakash, S., Long, T. M., Selby, J. C., Moore, J. S., and Shannon, M. A., 2007, ““Click” Modification of Silica Surfaces and Glass Microfluidic Channels,” Anal. Chem., 79(4), pp. 1661–1667. [CrossRef] [PubMed]
Yeom, J., Wu, Y., Selby, J. C., and Shannon, M. A., 2005, “Maximum Achievable Aspect Ratio in Deep Reactive Ion Etching of Silicon Due to Aspect Ratio Dependent Transport and the Microloading Effect,” J. Vac. Sci. Technol. B, 23(6), pp. 2319–2329. [CrossRef]
Yeom, J., and Shannon, M. A., 2009, “Detachment Lithography of Photosensitive Polymers: A Route to Fabricating Three-Dimensional Structures,” Adv. Funct. Mater., 20, pp. 289–295. [CrossRef]
Jackman, R. J., Wilbur, J. L., and Whitesides, G. M., 1995, “Fabrication of Submicrometer Features on Curved Substrates by Microcontact Printing,” Science, 269, pp. 664–666. [CrossRef] [PubMed]
Leong, T. G., Lester, P. A., Koh, T. L., Call, E. K., and Gracias, D. H., 2007, “Surface Tension-Driven Self-Folding Polyhedra,” Langmuir, 23, pp. 8747–8751. [CrossRef] [PubMed]
Mata, A., Fleischman, A. J., and Roy, S., 2005, “Characterization of Polydimethylsiloxane (PDMS) Properties for Biomedical Micro/Nanosystems,” Biomed. Microdevices, 7(4), pp. 281–293. [CrossRef] [PubMed]

Figures

Grahic Jump Location
Fig. 1

A schematic representation of the two fluidic network designs. Design 1 had two micron depth channels connected by a bank of nanometer depth channels. The depth of each feature was controlled by the etching time. The microchannels were 3 cm long and 131 μm ± 1 μm wide with a typical depth of 8.0 μm ± 0.1 μm. The nanochannel array consisted of 3–6 nanochannels that were 0.5 cm long and 33 μm ± 1 μm wide. The depth of the nanochannels ranged from 16.0 nm ± 0.1 nm to 227 nm ± 5 nm. Design 2 had nanoscale depth features only. The upper branch was 1 cm long and 106 μm wide. At the Y-junction the channel split into two 56 μm wide branches. Depths of the “Y” nanochannel network ranged from 77 nm ± 6 nm to 446 nm ± 5 nm.

Grahic Jump Location
Fig. 2

(a) A microscope image showing channel deformation in a 77 nm deep, 50 μm nominal width channel. (b) A microscope image showing channel deformation in a 77 nm deep, 100 μm nominal width channel fabricated without a metal mask. (c) A microscope image showing a visually uniform channel width for a 77 nm deep channel fabricated with a metal mask as marked by the white arrows.

Grahic Jump Location
Fig. 3

(a) A flow chart of channel network fabrication for Design 1. Briefly, a metal etch mask was applied to a piranha cleaned borosilicate substrate. UV lithography was used to pattern microchannel features. UV lithography with a modified spin process was used to create nanochannel pattern. (b) A schematic representation of the defect formed at the micronanochannel interface using standard UV lithography without a modified spin process. The photoresist did not fully fill the microchannel, leading to delamination at the micronanochannel interface. The nanochannels were fluidically connected by a ridge of the same depth as the nanochannels.

Grahic Jump Location
Fig. 4

SEM images of the micro-nanochannel interface of the hybrid channel network (Design 1). Figure 4(a) shows the nanoscale defect caused by photoresist delamination resulting in fluidically connected nanochannels. The ridge connecting the channels was the same depth as the channel itself [21]. Figure 4(b) shows a device fabricated using the modified spin procedure, resulting in elimination of the defect.

Grahic Jump Location
Fig. 5

An SEM image of a 227 nm deep 33 μm wide nanochannel bonded using a 2 μm thick PDMS adhesive layer [19]. The inset shows a zoomed in image of the channel cross section.

Grahic Jump Location
Fig. 6

A plot of measured current as a function of applied voltage for a 22 nm PDMS-glass device. A linear trend was observed suggesting that the in the voltage range tested the nanochannel permits ions to move across and is evidence for electrokinetic transport through nanochannels.

Grahic Jump Location
Fig. 7

A cross section image of a 56 μm wide and 77 nm deep nanofluidic channel bonded using calcium assisted bonding. The channel height varies at different points in the channel from 62 to 77 nm, presumably due to cutting the nanochannels for imaging [28].

Grahic Jump Location
Fig. 8

Microscope images of nanochannels of various depths filling with isopropyl alcohol in a capillary fill. Lines denoting the channel walls have been added to the image of the 77 nm deep channel for visual clarity.

Tables

Table Grahic Jump Location
Table 1 Summary of aspect ratios achieved using each bonding scheme

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In