Laser Scanning Holographic Lithography for Flexible 3D Fabrication of Multi-Scale Integrated Nano-structures and Optical Biosensors

Liang Yuan, Peter R. Herman
2016 Scientific Reports  
Three-dimensional (3D) periodic nanostructures underpin a promising research direction on the frontiers of nanoscience and technology to generate advanced materials for exploiting novel photonic crystal (PC) and nanofluidic functionalities. However, formation of uniform and defect-free 3D periodic structures over large areas that can further integrate into multifunctional devices has remained a major challenge. Here, we introduce a laser scanning holographic method for 3D exposure in thick
more » ... resist that combines the unique advantages of large area 3D holographic interference lithography (HIL) with the flexible patterning of laser direct writing to form both micro-and nano-structures in a single exposure step. Phase mask interference patterns accumulated over multiple overlapping scans are shown to stitch seamlessly and form uniform 3D nanostructure with beam size scaled to small 200 μm diameter. In this way, laser scanning is presented as a facile means to embed 3D PC structure within microfluidic channels for integration into an optofluidic lab-on-chip, demonstrating a new laser HIL writing approach for creating multi-scale integrated microsystems. The top-down approach to nano-structuring 1 has greatly evolved over the decades to underpin today's most important trends in science and technology. Highly resolving beams of electrons 2 , ions 3 , and photons 4,5 are regularly applied in high resolution surface patterning, for example, providing the narrow 16-nm (and beyond) transistor gate widths as required in today's commercial microchips with laser lithography 5 . Smaller 4 nm dimensions are otherwise available at much slower processing speed by direct writing with electron or ion beams 2,3 . An emerging opportunity here centers on new approaches that can transform rapid nano-structuring into the third dimension. Assembled layering of 2D-structures is exceptionally tedious and slow 6 and direct 3D nano-structuring is highly challenging, as charged particles cannot propagate without scattering inside solid material. Hence, high resolution 3D patterning has been best met with optical approaches 4,7 to provide advanced nanostructures that underlie omnidirectional stopbands 7 and superprism effects 8 in photonic crystals (PCs), negative refraction 9 and broadband circular polarizers 10 in metamaterials, engineered tissue growth 11 and controlled drug release 12 in scaffolds and carbon nano-tube composites 13 in 3D templates. Direct writing with tightly focused laser beams has been attractive for flexible and high resolution structuring of 3D features to sizes as small as 9 nm, representing a small fraction of the laser wavelength (~λ /90) in photoresist 4 . This patterning approach has underpinned many broadly based nanostructure applications 4,7-13 , while offering the flexibility to machine fully-3D-shaped microdevices 14 , optical waveguides embedded in 3D PC templates 15 and optical cloaking metamaterials 16 . On the other hand, today's highly powered lasers are highly favored in holographic interference lithography (HIL) for expanding the fabrication area and speeding the process time of 3D PC templates with resolution at one half optical wavelength 7 . Various configurations of free space and phase mask beam splitting have permitted flexible tuning of the crystal symmetry between Tetragonal (TTR) 17 , woodpile 18,19 and diamond-like 20-22 structures, and from simple Bravais lattices 23-25 to compound 26 , chiral 27 and quasi-crystalline structures 28 . However, traditional free-space methods for beam splitting and combining 7,20,29 have given way to the more stable methods of beam interference with prismoids 18,26 , transmission gratings 30 , and proximity phase mask 17,31-34 techniques that firmly lock together the phases of multiple diffracted beams to stabilize the 3D interference fringe pattern [17] [18] [19] 26, [29] [30] [31] [32] [33] [34] . Despite the advantages of rapid parallel processing, the HIL methods have been limited to static beam exposure with 3D periodic structures forming only within the area of the overlapping laser beams. Further, the areal uniformity is highly dependent on the laser beam quality. A more flexible means for embedding multi-functional components within such uniform periodic nanostructures have also been pursued by various groups 15,32,35-43 towards integration of nano-and micro-structured systems 36 . In one approach, laser direct writing, when following an HIL exposure step, has enabled optical defect waveguides to be registered within the pre-existing 3D periodic structure 15, 40, 41 . Alternatively, multi-scale devices have been formed by multi-exposure steps of shadow and phase masks to embed 3D PCs within microfluidic channels 32,37-39 . The addition of such functionality would be more appealing if possible in a single exposure step. In another approach, a spatial light modulator 43 has generated ~5 μm periodic structures over a small ~100 μm exposure zone. More robust and higher throughput approaches are therefore desirable in multi-scale and monolithic fabrication that can offer high speed parallel processing of 3D nano-structures over large area in a single exposure step. In this paper, the concept of beam scanning in 2D laser projection lithography 5 is extended to proximity phase masks by introducing direct-write laser scanning as a new hybrid direction for micro-scale structuring in 3D HIL nanopatterning. Here, for the first time, the advantage of phase-locked multi-beam interference is applied with a direct-write beam scanning to flexibly pattern and integrate 3D photonic nanostructures with microfluidic structures. The hybrid approach opens a new unexplored domain where elements of direct-writing and 3D structuring are blended for intermediate benefits of high-resolution nano-structuring with overall 2D pattern control (refer to Supplementary Fig. S1 ). This approach builds on the overlapping scanning exposure we first introduced in ref. 44 to overcome the prior limitation found in the uniformity and areal size of 3D PCs fabricated by static exposure 17-43 . This scanning approach has been further extended here to higher resolution and non-uniform beam exposure to present a wider breadth of application. Flexible exposure conditions were examined theoretically and matched with experimental demonstration in forming uniform 3D periodic templates with multi-scanned beams of various uniform and non-uniform profiles. The interference pattern of the moving diffracted beams remains phase-locked to the phase mask to permit the seamless stitching of nanostructure in thick photoresist film. Body-centered tetragonal (BCT)-like nanostructure is presented with low variance in the optical Γ -Z stopbands (∆ λ peak < 1%) across multiple overlapping exposure zones. These benefits are further scaled to small beam diameter, assessing against the challenges of small working distance as imposed by the widely diverging diffraction orders from the phase mask that lead to incomplete interference and subsequent distortion of the nano-structure motif in thick photoresist 45 . In this way, velocity controlled direct-write exposure of a single phase-mask is illustrated to embed 3D PC nanostructures flexibly within wells and microfluidic channels, with micro-scale resolution of ~200 μm. This novel laser writing method enables PC functionality in laboratory on chip devices, where waveguides, microfluidic channels and 3D PC structure are presented for on-chip fluorescence detection of low dye concentration (4 × 10 −9 mol/ml). This top-down writing method is highly flexible in forming high-resolution patterns of 3D nanostructures through a simple phase mask that can be scaled with high power lasers to high writing speeds, attractive for opening new directions in high resolution 3D nanofabrication.
doi:10.1038/srep22294 pmid:26922872 pmcid:PMC4770283 fatcat:6lkqlvi7gvdkrgceletbv75wdm