s5_Three-Dimensional Nanochannels Formed by Nanoimprint

Three-Dimensional Nanochannels Formed by Nanoimprint

S. W. Pang, R. M. Reano, C. Peng, Y. P. Kong, H. Y. Low, and A. F. Yee

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

A technique to create sealed three-dimensional nanochannel networks is developed using sequentially stacked thermal nanoimprint lithography (NIL) on planarized self-supporting dielectric sealing material over polymer sacrificial layers. Void-free plasma enhanced chemical vapor deposited (PECVD) SiO₂ encloses and seals nanochannels that are formed upon the removal of the sacrificial polymer. Planarization of the SiO₂ surface allows the utilization of the vertical dimension to sequentially apply nanoimprint lithography for the formation of multiple-level nanochannel networks. Removal of the sacrificial polymer is performed with a high-power and high-pressure O₂ plasma. Wet chemical processes using common solvents are found to be ineffective in removing the sacrificial polymer. Two level nanochannels with cross-sectional dimensions of 300 nm X 200 nm and lengths of 65 µm that are aligned offset from one another and aligned on top of one another are demonstrated.

Polymer nanostructures were patterned on Si substrates using optical lithography or nanoimprint lithography, followed by oxide deposition to form the sealed channels. A high-speed dry etching technique for removing the sacrificial polymer was developed using an O₂ plasma at high power, high pressure, and elevated temperature. This dry etching technique provides a fast lateral etch rate of 3.91 µm/min for the polymer inside nanochannels, which is an order of magnitude higher than conventional reactive ion etching. High selectivity of 1200 was obtained between the lateral etch rate of polymer inside the nanochannels and the vertical etch rate of oxide. Etch rate dependence on pressure, temperature, and channel width were studied. It was found that the etch rate increases with pressure and temperature. To form multiple levels of nanochannels, the oxide covering the channels was planarized by a photoresist coating and etch-back process. After oxide planarization, the channel formation process is repeated and multiple levels of nanochannels can be stacked to build three dimensional (3D) nanostructures. A two-level channel structure was demonstrated. Interconnect openings between channels of adjacent levels were also demonstrated. With such technique, complex 3D system can be fabricated. Since oxide is transparent to visible light and the channels have hydrophilic surfaces, therefore such a 3D nanofluidic system is suitable for various biomedical studies.

A new method of imprinting 3D polymeric micro- and nanostructures using a duo-mold process is presented. In this method, two patterned Si molds are surface treated to have different surface energies. A polymer solution is spin-coated onto one of the molds, forming a planarized thin film on the mold. The two molds are pressed together at an appropriate temperature and pressure and then released. The patterned thin film adhering to one of the molds is then pressed onto a substrate at an appropriate temperature and pressure to form supported 3D structures. Alternatively, the patterned thin film on the mold may be released to form freestanding 3D structures. A key success factor in this process is the silane-based surface treatments to enable the patterning of the thin polymer film and final release from the mold to form 3D structures. We demonstrate various polymethyl methacrylate 3D structures with combinations of grating, circular-, and square-patterned molds. The duo-mold process is a potentially low-cost and high-throughput method to fabricate 3D polymeric micro- and nanostructures.

Figure 1(a) shows void-free SiO₂ deposited on the grating imprints for a deposition pressure of 9.5 Torr. A 1.6 µm photoresist layer spin-coated onto the nonplanar SiO₂ capping layer is shown in Figure 1(b). The SiO₂ layer is planarized by conducting an etch-back using an O₂ plasma followed by a CF₄ plasma. The resulting planarized SiO₂ surface is shown in Figure 1(c). To clear the channels of sacrificial resist, the entire structure is exposed to a high-pressure and high-power O₂ plasma. The etch process is shown in Figure 1(d).

Figure 1. (a) Void-free PECVD SiO₂ capping layer; Planarized SiO₂ surface formed by (b) imprint with SiO₂ capping layer and planarizing resist and (c) etch back in O₂ and CF₄ plasma to achieve void-free sealed nanochannels with planarized top surface; (d) Top-view optical micrograph of etch progress into nanochannels by O₂ plasma.

To build up the structure in the vertical dimension, a new layer of polymer photoresist (SC1813) is spin-coated and imprinted using thermal NIL and a second SiO₂ capping layer is deposited and planarized, prior to clearing out the channels. Figure 2 shows the two-level nanochannels.

Figure 2. Two-level nanochannels: (a) Micrograph of two-level nanochannels offset from one another and (b) micrograph of two-level nanochannels aligned on top of one another.

By repeating polymer patterning and PECVD oxide deposition, 3D nanostructures with multiple levels can be fabricated. In addition, to form a complex fluidic system, interconnects between channels at different levels are needed and such a network is demonstrated in Figure 3.

Figure 3. Micrographs of (a) channel with interconnect opening etched through the oxide layer, (b) two levels of channels without interconnect openings, and (c) two levels of channels with interconnect opening between the upper and lower channels.

Duo-mold imprinting has the unique feature of forming 3D structures with completely sealed cavities. The structures shown in Figure 4 are examples of 3D structures with closed cavities that are defined by the polymer and the underlying substrate.

Figure 4. (a) Scanning electron micrograph of a duo-mold imprinted 3D structure consisting of 700 nm pitch gratings with 1:1 duty cycle on the top and 3 µm square cavities on the bottom of the structure; (b) Optical micrograph and (c) Scanning electron micrograph of a duo-mold imprinted 3D structure consisting of 20 µm square cavities on the top and 5 µm diameter circular cavities on the bottom of the structure.

Figure 5 shows a 3D structure on Si imprinted using a duo-mold imprinting process. This 3D structure is comprised of a 700 nm grating with a 1:1 duty cycle on both the top and bottom.

Figure 5. A duo-mold imprinted 3D structure consisting of 700 nm pitch gratings with 1:1 duty cycle on both the top and bottom of the structure.

References

Last Updated: November 19, 2007

E-Mail: pang@umich.edu

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