Multispectral Imaging using Polydimethylsiloxane (PDMS) Embedded Vertical Silicon Nanowires

Abstract

We demonstrate the use of vertical silicon nanowires for multispectral imaging. The eight filter functions of our filter array are defined in a single lithography step. We show both visible color and near-infrared imaging.OCIS codes: (110.4234) Multispectral and hyperspectral imaging; (220.4241) Nanostructure fabrication Multispectral imaging divides the spectrum into many bands, permitting materials and objects in a scene to be identified from their absorption or reflection characteristics. Multispectral imaging is used in many applications, including remote sensing [1] and food quality control [2]. However, most conventional multispectral imaging devices are expensive and bulky because they use motorized filter wheels or multiple cameras. Here we develop a compact multispectral image system using polydimethylsiloxane (PDMS) embedded vertical silicon nanowires. The system is based on filter array with eight different spectral bands and a reference channel. All filter functions are defined in a single lithography step, and cover visible to near-infrared (NIR) wavelengths. By attaching the device to a monochrome image sensor, we successfully demonstrate a compact multispectral imaging system. Figure 1 (a) Concept schematic of multispectral image system. (b) Fabrication steps for multispectral filter.Fig. 1(a) shows the schematic representation of our multispectral imaging system. It was recently demonstrated that vertical silicon nanowires show vivid colors, a consequence of the wavelength-dependence of the spatial distribution of the nanowire's waveguide mode, which modifies the coupling of the incident light to the mode [3, 4].The mode's spatial distribution depends on nanowire radius, meaning that the transmission spectrum displays a dip whose position is determined by the nanowire radius [3,4]. Here, we use this property to realize a filter array for multispectral imaging. The unit cell of the filter array contains eight different filters, each comprising nanowires of a particular radius. The unit cell also contains a transparent region in its center. The fabrication process [4] starts with a silicon wafer. (Fig. 1(b)) PMMA resist is spun-coated on the wafer and e-beam lithography is performed. An aluminum etch mask is fabricated using evaporation and the lift-off process. The wafer is then dry etched. The fabricated vertical silicon nanowires are 1.67 µm tall and on a pitch of 1 µm. PDMS (base and curing agent ratio 5:1) is then spun cast onto the etched wafer and cured (230 °C). The PDMS film is then mechanically cut from the substrate with a razor blade. After fabrication, devices are mounted on a monochrome image sensor. Fig. 2(a) shows a scanning electron microscope (SEM) image of the etched vertical silicon nanowires. Nanowires with eight different radii (45 nm to 80 nm in 5 nm steps) are fabricated. The array contains 20 × 20 unit cells. The unit cell is 75 × 75 µm and contains eight different filters. Each filter comprises 24 × 24 nanowires and corresponds to 4 × 4 pixels of the image sensor. The nanow...