Gray-scale photolithography using microfluidic photomasks

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 and͞or 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. Photolithography is used to define critical feature size in the fabrication of the vast majority of microdevices including microelectronic circuits, microelectromechanical systems (MEMS), microfluidic devices, and nucleic acid͞protein microarrays (1). Essentially, photolithography consists of selectively illuminating a thin photosensitive layer (''photoresist'') with UV light through a mask containing opaque features (e.g., metal or ink emulsion) on a transparent background (e.g., glass or plastic). The photomasks impose two fundamental limitations on the features that can be produced. (i) The exposure is an all-or-none illumination process that results in photoresist features of uniform height; thus, the fabrication of threedimensional (3D) microstructures by traditional photolithography requires multiple exposure͞alignment steps. (ii) Photomask features are permanent, and thus design changes require the (costly and slow) fabrication of a new photomask, a major hurdle in research settings requiring fast-turnaround microdevice prototyping.To overcome the all-or-none illumination constraint of conventional photolithography, a number of ''gray-scale'' approaches capable of generating ranges of exposure levels have been developed. Presently, gray-scale photolithography can be realized with scanning lasers (2, 3), micromirror projection displays (4), high-energy beam-sensitive glass photomasks (www. canyonmaterials.com), ultra-high-resolution ''halftone'' photomasks (5), or metal-on-glass photomasks [where each gray-scale level is determined by a different metal thickness (6)]. Although extremely useful for producing 3D structures, these gray-scale approaches either (i) exacerbate the costs͞turnaround times of standard photolithography ...

Tunable microfluidic optical devices with an integrated microlens array

“…[19] The opacity of the photomask features can be tailored to an arbitrary gray-scale level to produce three-dimensional (3D) microstructures. Clearly such strategy can be applied to microlens arrays filled with desired dye solutions to control the degree of illumination through the lens.…”

“…This design feature is a key component of an adaptive optical device that provides transmission tunability, diaphragm action, wavelength selectivity, minimization of the "cross-talk" between the lenses, and improved angular selectivity. [18] The variable transmission and wavelength selectivity are of particular interest in applications, including electronic display [3], a high contrast spatial light modulator [7], and gray scale photolithography [19]. Inspired by the unique lens design and the consequent outstanding optical properties in brittlestars, we search for novel approaches to create a structure that combines microlens arrays with microfluidics.…”

Parallel mixing of photolithographically defined nanoliter volumes using elastomeric microvalve arrays

“…Standard SU-8 photolithography methods were used to create masters for the microfluidic layer and the valve control layer [12]. We designed the masks using Corel-Draw software, and printed transparency masks at 8000 dots per inch (CAT/Art services, Poway, CA, USA).…”

Reconfigurable phase-change photomask for grayscale photolithography

“…High-energy-beam sensitive (HEBS) glass masks selectively change metal-ion concentration on exposure to electron beams, resulting in transparency differences and allowing the manufacture of grayscale masks 5 . Other 3D micro-fabrication methods include using microfluidic photomasks 6 or a polymer photomask doped with laser dye 7 . Those techniques are not in widespread use in either industry or academic laboratories because they are either restricted to a limited range of shapes or not scalable to large-scale processing.…”