Micropatterned Porous-Silicon Bragg Mirrors by Dry-Removal Soft Lithography

Multilayer structures of nanoporous silicon have garnered significant interest as components of optical and sensing devices due to their low cost, ease of fabrication, and large-scale synthesis. Porous silicon (PSi) layer formation via electrochemical etching offers a simple platform to tune the refractive index (i.e., porosity) of stacked layers, which has led to a host of applications based on the optical properties of thin PSi layers. Recent studies have demonstrated the application of PSi superlattices in chemical sensors, waveguides, interference filters, distributed Bragg reflectors, and optical microcavities. Due to the high surface area of their porous structure (200–500 m cm), PSi Bragg reflectors (Bragg mirrors) have shown great potential as biological and chemical sensors based on optical reflectance spectroscopy. Such chemical sensors function via infiltration of a chemical species into the pore structure, which alters the effective refractive index of the dielectric medium. The shift in the Bragg wavelength (i.e., rejected light) caused by the index change can be probed using simple reflectance spectroscopy or colorimetry. Since the inclusion of chemicals into the porous layers is reversible, the films can be cycled through multiple detection runs without additional preparation steps. A significant challenge faced in integrating PSi Bragg mirrors into miniature optical devices is the reduction of functional PSi multilayer structures to the microand nanometer scales. Active length scales below one micrometer would be ideal for a multiplex sensor based on PSi, which would require multiple chemical-specific PSi Bragg mirrors on a single platform. To date, there exist several techniques to produce spatially confined PSi microstructures either by photolithography or electron-beam lithography, however, there have been no reports on PSi superlattices fabricated to microor nanoscale dimensions. Recent work by Sailor and co-workers has shown that PSi photonic crystals with dimensions less than 1 mm can be produced using a lithographically patterned photoresist. This technique, although effective, requires additional processing steps for each sample. Therefore it is advantageous to develop simple one-step patterning techniques that retain cost-effectiveness and minimize preparation times. Microcontact printing, a subfield of soft lithography, is a simple and inexpensive method of fabricating relatively large quantities of microstructured materials. This method utilizes a microprinting technique whereby thin-film structures are patterned on a micrometer scale by direct lift-off. Previous efforts in our group have demonstrated the effectiveness of PSi pattern formation by dry-removal soft lithography that uses nothing more than a clean polydimethylsiloxane (PDMS) elastomer stamp. Furthermore, this stamping procedure can be followed with an additional transfer step that deposits patterned microstructures from the PDMS stamp onto a flexible polymer film. In this communication, we build upon previous work by showing that our dry-removal technique can be used to produce ordered arrays of freestanding PSi multilayers without damaging the optical integrity of the film. This method allows relatively large-scale production of uniform nanostructures of optically tunable PSi Bragg mirrors in a variety of shapes and dimensions. In addition, we demonstrate the chemical-sensing capability of a PSi Bragg mirror with various levels of exposure to an infiltrating polymer, as well as the reversible detection of a volatile substance within our micropatterned photonic structures. Micrometer-sized PSi Bragg mirrors were fabricated by direct contact and subsequent lift-off using a patterned PDMSelastomer stamp. A schematic representation of the patterning procedure is illustrated in Figure 1. A patterned PDMS stamp was brought into contact with a PSi Bragg mirror composed of alternating layers of high and low porosity. After 5– 10 min of slight applied pressure (a 50 g weight), the stamp was peeled away, resulting in a negative pattern formation of the stamp. Regions where the stamp touched the PSi film were subsequently removed from the surface without damaging the material that resided under the structured side of the stamp (Fig. 1C). A contrast-enhanced optical image revealed green, reflecting 100 lm diameter discs affixed to a Si substrate (Fig. 1D). A secondary step was employed to transfer the removed PSi Bragg mirror pattern to a freestanding polymer film. To ensure proper transfer, a solution of the hydrophobic polymer poly[(vinyl butyral)-co-(vinyl alcohol)-co-(vinyl acetate)] (PVB) in ethanol was drop-cast directly on the stamp surface and allowed to cure (Fig. 1F). After curing, the polymer was gently peeled from the stamp resulting in complete transfer of the PSi microstructures to the mechanically C O M M U N IC A TI O N