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3-D nanofluidic device developed

Posted: 13 Apr 2009     Print Version  Bookmark and Share

Keywords:nanofluidic  3-D surfaces  photolithography 

Researchers at the Commerce Department's National Institute of Standards and Technology (NIST) and Cornell University have capitalised on a process for manufacturing ICs at the nanometer level and used it to develop a method for engineering the first-ever nanoscale fluidic (nanofluidic) device with complex 3-D surfaces.

The Lilliputian chamber is a prototype for future tools with custom-designed surfaces to manipulate and measure different types of nanoparticles in solution.

Among the potential applications for this technology: the processing of nanomaterials for manufacturing; the separation and measuring of complex nanoparticle mixtures for drug delivery, gene therapy and nanoparticle toxicology; and the isolation and confinement of individual DNA strands for scientific study as they are forced to unwind and elongate (DNA typically coils into a ball-like shape in solution) within the shallowest passages of the device.

Nanofluidic devices are usually fabricated by etching tiny channels into a glass or silicon wafer with the same lithographic procedures used to manufacture circuit patterns on computer chips. These flat rectangular channels are then topped with a glass cover that is bonded in place. Because of the limitations inherent to conventional nanofabrication processes, almost all nanofluidic devices to date have had simple geometries with only a few depths. This limits their ability to separate mixtures of nanoparticles with different sizes or study the nanoscale behaviour of biomolecules (such as DNA) in detail.

To solve the problem, NIST's Samuel Stavis and Michael Gaitan teamed with Cornell's Elizabeth Strychalski to develop a lithographic process to fabricate nanofluidic devices with complex 3-D surfaces.

As a demonstration of their method, the researchers constructed a nanofluidic chamber with a "staircase" geometry etched into the floor. The "steps" in this staircase—each level giving the device a progressively increasing depth from 10 nanometers (approximately 6,000 times smaller than the width of a human hair) at the top to 620 nanometers (slightly smaller than an average bacterium) at the bottom—are what give the device its ability to manipulate nanoparticles by size in the same way a coin sorter separates nickels, dimes and quarters.

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The NIST-Cornell nanofabrication process utilises greyscale photolithography to build 3-D nanofluidic devices. Photolithography has been used for decades by the semiconductor industry to harness the power of light to engrave microcircuit patterns onto a chip. Circuit patterns are defined by templates, or photomasks, that permit different amounts of light to activate a photosensitive chemical, or photoresist, sitting atop the chip material, or substrate.

Conventional photolithography uses photomasks as "black-or-white stencils" to remove either all or none of the photoresist according to a set pattern. The "white" parts of the pattern—those that let light through—are then etched to a single depth into the substrate. Greyscale photolithography, on the other hand, uses "shades of grey" to activate and sculpt the photoresist in three dimensions. In other words, light is transmitted through the photomask in varying degrees according to the "shades" defined in the pattern. The amount of light permitted through determines the amount of exposure of the photoresist, and, in turn, the amount of photosensitive chemical removed after development.

The NIST-Cornell nanofabrication process takes advantage of this characteristic, allowing the researchers to transfer a 3-D pattern for nanochannels of numerous depths into a glass substrate with nanometer precision using a single etch.

The result is the "staircase" that gives the 3-D nanofluidic device its versatility.

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