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A Bilayer Resist Method for Creating Silica Microfluidics

Benjamin R. Cipriany, Rob Ilic, and Harold G. Craighead

Abstract

In support of a growing collection of lab-on-a-chip applications utilizing inexpensively formed microfluidics [1-2], we have demonstrated a new method for creating silica microfluidic networks. Unlike some existing bilayer resist processes involving Hydrogen silsesquioxane (HSQ) [3-4], this process utilizes a single photolithographic step. The resulting silica microfluidics offer several advantageous material properties over polydimethylsiloxane (PDMS).

Research Summary

We formed microfluidics using various thicknesses of HSQ, which were spun and exposed to oxygen plasma to cross-link a 10nm thin barrier layer. This barrier was robust against photoresist solvents, allowing a bilayer stack to be formed without altering the underlying HSQ bulk. Photoresist was then spun, patterned with optical lithography, and used as a mask layer. A wet-chemical etch was used to transfer the pattern into the barrier layer, followed by development to isotropically dissolve the HSQ bulk. Microfluidic networks formed with this developer-based transfer are self-terminated on the underlying substrate without inducing surface damage. Cross-sectional electron micrographs of these channels revealed a sponge-like film composition, which was compacted into a dense silica film during a subsequent high-temperature anneal [Figure 1]. This annealed film exhibited autofluorescence in the visible spectrum comparable to thermally grown silicon dioxide [Figure 2] and excellent chemical solvent resistance. The resulting microfluidic channels have widths 1.5-3.1 micrometers and heights of 80-520nm, respectively [Figure 3].

Figure 1: (LEFT) Cross-sectional electron micrograph of HSQ film prior to annealing, exhibiting porous structure. (RIGHT) HSQ film structure following a high temperature anneal. The previously porous structure is collapsed into a dense, amorphous film.

The microfluidic networks were sealed and used to directly observed flow of fluorophore-labeled deoxyribonucleic-acid (DNA) using fluorescence videomicroscopy. Future applications of this fabrication method may include integration with other components such as MEMS/NEMS or nanowire sensors.

Figure 2: Autofluorescence for materials commonly used in the fabrication of microfluidic structures, as measured with photon counting modules affixed to an inverted microscope. Fluorescence is induced with a 488nm wavelength laser operating at 1 milliwatt.


Figure 3: Electron micrograph of a microfluidic channel constriction formed in HSQ with cross sectional dimensions 1.5 (w) by 0.08 (h) micrometers.

References

  1. SK Sia and GM Whitesides. Microfluidic devices fabricated in poly(dimethylsiloxane) for biological studies. Electrophoresis, Vol 24, 2003
  2. H. Wu, B. Huang, and R. Zare. Construction of Microfluidic Chips Using Polydimethylsiloxane for Adhesive Bonding. Lab on a Chip, Vol 5, 2005.
  3. Falco van Delft, et al. Hydrogen silsequioxane/novolak bilayer resist for high aspect ratio nanoscale electron-beam lithography. J Vacuum Science B, Vol 18, 2000.
  4. D.M. Tanenbaum, et al. Dual exposure glass layer suspended structures: A simplified fabrication process for suspended nanostructures on planar substrates. J Vacuum Science B, Vol 19, 2001.