Fabrication method of topographically modulated microstructures using pattern homogenization with UV light
Inventors
Locascio, Laurie E. • Atencia-Fernandez, Francisco Javier • Barnes, Susan • Douglas, Jack F.
Assignees
National Institute of Standards and Technology NIST
Publication Number
US-8236480-B2
Publication Date
2012-08-07
Expiration Date
2028-05-27
Interested in licensing this patent?
MTEC can help explore whether this patent might be available for licensing for your application.
Abstract
A method for microfabrication of a microfluidic device having sub-millimeter three dimensional relief structures is disclosed. In this method, homogeneous surfaces, which do not exhibit apparent pixel geometry, emerge from the interaction of the overlapping of diffracted light under opaque pixels and the nonlinear polymerization properties of the photoresist material. The method requires a single photolithographic step and allows for the fabrication of microstructures over large areas (centimeters) with topographic modulation of features smaller than 100 micrometers. The method generates topography that is useful in a broad range of microfluidic applications.
Core Innovation
The invention is a photolithographic fabrication method for creating microstructures with arbitrary three-dimensional topography using ultraviolet (UV) light and a binary transparency mask designed to induce a homogenization effect in a photopolymer. This method enables the formation of homogeneous surfaces that do not display pixel geometry by leveraging the nonlinear polymerization properties of photoresist material and diffraction effects. The technique requires only a single photolithographic step and supports fabrication of microstructures with features smaller than 100 micrometers over large areas, enabling topographically modulated three-dimensional relief structures.
The problem addressed is the limitation of conventional microfluidic system fabrication, which is mostly restricted to two-dimensional structures or pseudo-3D devices with flat topography. Existing methods for creating true 3D microstructured devices involve compromises among resolution, cost, design complexity, and scalability. Grayscale masks or other photolithographic technologies for 3D fabrication are often expensive, have limited flexibility in structuring large areas with varied topographies, or lack precision for microfluidic applications requiring controlled channel height for flow and capillary effects.
This invention overcomes these limitations by employing binary pixel grayscale pattern homogenization to fabricate large-area microfluidic devices with modulated 3D topographies at micro-scale resolution. It utilizes the physical phenomenon where overlapping diffracted light under opaque pixels and the nonlinear response of photopolymerization cause emergence of homogenous surfaces from discrete pixel arrays or swatches. The hierarchical design approach using repetitive pixel motifs (swatches) facilitates ease of design and patterning complexity, enabling microchannels and other microstructures with controlled topography in all three spatial dimensions.
Claims Coverage
The patent includes multiple independent claims focused on inventive methods for fabricating three-dimensional microfluidic devices by photopolymerizing photoresist materials through binary transparency masks. The main inventive features cover the use of specific pixel density and size criteria, homogenization effects, and swatch-based hierarchical patterning.
Three-dimensional microfluidic device fabrication using binary transparency mask with homogenization threshold
A method comprising creating a binary transparency mask and exposing photoresist material to UV light through the mask while maintaining the product of transparent pixel density (n) and pixel side length (a) greater than 5500 μm per mm² of patterned area, thus transferring patterns as homogeneous smooth surfaces and fabricating at least one microchannel with x, y, and z dimensions.
Utilization of opaque pixel diffraction overlapping and nonlinear photopolymerization phenomenon
Providing opaque pixels in the mask that cause overlapping exponential decay in light intensity due to diffraction edges, triggering continuous polymerized structures in the photoresist resulting from the nonlinear nature of photopolymerization.
Use of tiling pattern units (swatches) as repetitive motifs to control UV transmission levels
Employing swatches—distinct arrays of pixels with controlled ratios of transparent to opaque pixels—to define areas transmitting the same UV intensity, enabling modulation of photopolymerized structure heights and complex microstructure patterning.
Fabrication of combinatorial polymerized structures via hierarchical swatch design
Designing masks with arrays of circles filled with differing pixel swatch patterns varying in average grayscale tone or pixel size to create arrays of combinatorial three-dimensional polymerized microstructures, including polymerized horns serving as microfluidic ejectors for monodisperse droplet generation.
Creating complex microchannel geometries by combining multiple swatch types and graphic software operations
Combining different swatch dimensions such as 8×4 and 5×1 pixel swatches and using graphic design techniques like skewing, stretching, rotating, and overlays to generate microchannels with modulated topographies, including zigzag structures and semicircular microchannels with semi-spiral ridges.
Fabrication method steps involving partial polymerization under diffracted UV light and creation of fully polymerized 3D microfluidic devices
Providing photoresist material with low contrast optical adhesive, subjecting it to UV light through the mask to promote partial polymerization in diffracted light areas, and transferring mask patterns as homogeneous or discrete polymerized microstructures to create functional microfluidic devices with topographically modulated microchannels.
The independent claims focus on a novel method of fabricating three-dimensional microfluidic devices using binary transparency masks designed to induce pattern homogenization through control of pixel density and size, nonlinear photopolymerization effects, utilization of swatch-based hierarchical pattern design, and specific UV exposure techniques to produce microstructures with controlled topography and functionalities including microchannels and ejector nozzles.
Stated Advantages
Ease of design and rapid turn-around times for mask design and fabrication due solely to control of exposure times.
Low cost of transparency masks, approximately $15 USD, making the method economically feasible for large area patterning.
Capability to pattern large areas and fabricate single structures simultaneously with topographic resolutions on the order of tens of microns.
The method allows mask pixels significantly larger than the optical resolution of the photolithographic system, enabling use of inexpensive masks and conventional equipment.
Documented Applications
Fabrication of microfluidic devices with three dimensional (3D) relief structures such as microchannels with modulated topography for controlling fluid flow, capillarity, and shear stresses.
Deterministic storage of liquid plugs with varied chemical concentrations in microfluidic devices employing microchannels with topographic constrictions and side channels.
Creation of semi-circular and zigzag microchannels with modulation in three spatial dimensions to influence mixing and flow patterns.
Fabrication of arrays of polymerized horns used as ejector plates in microfluidic devices for monodisperse liquid droplet generation and ultrasonic atomizers.
Manufacture of 3D microfluidic chip masters suitable for replica molding to produce microfluidic devices with precise control over channel geometry and surface topography for applications such as chemical libraries, cell manipulation, and acoustic fluidics.
Interested in licensing this patent?