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

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 photoresist 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.


Supplementary
. Calculated positions of diffracted beams identifying exposures zones with overlap of all and partial number of diffraction orders for various beam diameters and working distances (z) from the phase mask. For 10 mm beam diameter, all diffraction orders strongly overlap at (a) z=40 µm exposure distance (99%) but (b) separate to yield 77% areal overlap at z=1 mm distance from the phase mask. Scaling to smaller beam diameter, the areal beam overlap of all diffraction orders for z=40 µm exposure distance reduces to 56% and 1% for (c) 200 µm and (d) 60 µm beam diameters, respectively. The color coding of zones with 5-beam (dark red), 4-beam (orange), 3-beam (yellow), 2-beam (cyan) and 1-beam (light blue) overlap is adapted from Fig. 1 in the main text.

Supplementary Note 1.
One significant objective in developing the laser scanning method of holographic interference lithography (HIL) was determining the highest writing resolution possible in thick photoresist (40 µm) as the laser beam diameter was scaled down in size. A systematic procedure of writing grid lines at varying exposure conditions was followed as beam diameter was reduced from 2 mm to 30 µm to confirm formation of a porous, uniform, and bicontinuous photonic crystal structure that bonded well with the substrate. Contrasting examples of 200 µm and 30 µm beam diameters are summarized here. 10% beam diameter as found optimal in generating uniform stop bands (Section 4). The PC structure was verified by SEM imaging (not shown) to mark the narrow laser exposure window labeled as 'PC' in Fig. S3a for 200 µm beam diameter, which was separated by 'underexposed' and 'overexposed' zones also identified. The threshold for forming single-scan solid (vertical) lines within the mesh design were seen at scan speeds slower than 1.8 mm/s, but speeds slower than 1.2 mm/s were preferred for more reliable substrate bonding. The formation of bicontinuous PC templates were initiated at speed of 18 mm/s, while lower exposures at 25 mm/s also proved sufficient when stabilized by the closely positioned crossed (vertical) lines. The optimized exposure conditions of around 15 mm/s and 50 mW were then applied in the demonstration of the optofluidic microsystem in Fig. 5, where bicontinuous 3D PC structure is seen in the SEM images of Fig. 5c and d.
With a further reduction in beam diameter to 30 µm, a similar velocity varying pattern of single exposure lines crossed by bands of 20 exposure lines (10% offset) revealed the formation of solid photoresist as verified in the SEM image of Fig. S3b. However, delamination is more prominent at the threshold exposure conditions here due to the narrower contacting surface of this smaller diameter beam. The ~30 µm wide solid lines first appeared at a threshold scan velocity of 2.4 mm/s, but higher exposures of less than 1.6 mm/s scan speed offered more reliable bonding to the substrate. Higher resolution examination of the horizontal bands formed by parallel scans were found at a threshold exposure of 28 mm/s scan speed, but yielded only a random 3D porous structure as shown in the inset SEM image of Fig. S3b (upper-right). The open structures do not follow the expected periods (Λx=Λy=570 nm) produced by the phase mask interference, as further discussed in the main article. The formation of this random 3D structure existed in a very narrow exposure window, labelled as 'porous' in Fig. S3b, before giving way to formation of solid structure with higher exposures, as verified by the inset SEM image in Fig. S3b ( upper-left).
Beam diameters between 30 and 200 µm were also examined, providing only random or distorted 3D nanostructures, such as the random porous structure reported for 40 µm diameter (Fig. S3b upper-right inset). The onset for creating well ordered periodic 3D nanostructure was therefore found at 200 µm beam diameter for the present case of proximity exposure of 40 µm thick photoresist with a 570-µm period phase mask.
In conclusion, the formation of mesh lines by laser scanning 3D direct-writing holography revealed a narrow process window for writing lines of solid structure and bicontinuous 3D PC nanostructure. In the limit of smallest 200 µm beam diameter, a 80 mW power exposure provided solid lines below 1-2 mm/s scan speed for single lines and bicontinuous 3D PC structure at 10-25 mm/s scan speed for parallel scans (10% diameter offset). These exposure conditions were adopted for writing various patterned designs of optofluidic devices as shown in Fig. 5d, providing a single-exposure method for monolithic integration of diverse components within a single chip.