Gray-scale photolithography using microfluidic photomasks
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Edited by George M. Whitesides, Harvard University, Cambridge, MA, and approved December 23, 2002 (received for review September 23, 2002)
Abstract
The ability to produce three-dimensional (3D) microstructures is of increasing importance in the miniaturization of mechanical or fluidic devices, optical elements, self-assembling components, and tissue-engineering scaffolds, among others. Traditional photolithography, the most widely used process for microdevice fabrication, is ill-suited for 3D fabrication, because it is based on the illumination of a photosensitive layer through a “photomask” (a transparent plate that contains opaque, unalterable solid-state features), which inevitably results in features of uniform height. We have devised photomasks in which the light-absorbing features are made of fluids. Unlike in conventional photomasks, the opacity of the photomask features can be tailored to an arbitrary number of gray-scale levels, and their spatial pattern can be reconfigured in the time scale of seconds. Here we demonstrate the inexpensive fabrication of photoresist patterns that contain features of multiple and/or smoothly varying heights. For a given microfluidic photomask, the developed photoresist pattern can be predicted as a function of the dye concentrations and photomask dimensions. For selected applications, microfluidic photomasks offer a low-cost alternative to present gray-scale photolithography approaches.
Footnotes
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↵ * To whom correspondence should be addressed. E-mail: afolch{at}u.washington.edu.
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This paper was submitted directly (Track II) to the PNAS office.
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↵ † The PDMS thin film is not strictly necessary, but it is convenient for making many exposures with the same μFPMs without having to refill the channels. For patterns that require maintaining flow in the microchannel during exposure (e.g., Fig. 2 D and E), it is more practical to apply the μFPMs directly (without PDMS thin film) on the photoresist.
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↵ ‡ It should be noted that the topography traces are the result of the convolution of the photoresist contour and the contour of the profilometer tip; therefore, walls with a slope higher than that of the tip appear to have the same slope as the tip, and valleys too deep for the tip to reach the bottom appear shallower than they really are.
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↵ § For example, we have serendipitously found that photoresist turns from highly hydrophobic to very hydrophilic when exposed to a brief (≈5 s) oxygen plasma. When the oxygen plasma exposure was masked with an elastomeric membrane containing holes in intimate contact with the photoresist surface, wettability patterns of dye solution with the same shape as the stencil holes formed onto the photoresist. A quick dip of the photoresist in dye solution resulted in the automatic assembly of microdroplets on the photoresist surface. The dye microdroplets then acted as nonaddressable μFPMs that, after photoresist development, resulted in dome-shaped photoresist structure (data not shown).
- Abbreviations:
- μFPM,
- microfluidic photomask;
- PDMS,
- polydimethylsiloxane
- Copyright © 2003, The National Academy of Sciences
