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Fabrication of Nanoscale Plasmonic Sensing Structures Over Large Areas

Written by Jeff Morse, PhD
March 29, 2011
Plasmonic sensors provide a means of detecting chemical and biological species through the observation of spectral features by measurement techniques such as surface enhanced Raman spectroscopy (SERS) or surface plasmon resonance (SPR). In order to develop a full-scale plasmonic sensor, periodic nanostructures must be replicated over large-area surfaces, such as six-inch silicon wafers. By incorporating hybrid top-down and bottom-up synthesis, a versatile approach has been demonstrated for fabricating large-area plasmonic sensors. The ability to fine tune nanostructure features over large areas renders this technique highly adaptable, thereby opening up new opportunities for plasmonic sensor applications.


Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network

  • Dhawan JW,  Du Y, Batchelor D, Wang HN, Leonard D, Misra V, Ozturk M, Vo-Dinh T. 2011. Hybrid Top-Down and Bottom-Up Fabrication Approach for Wafer-Scale Plasmonic Nanoplatforms. Small7(6):727-731. DOI: 10.1002/smll.201002186.

Plasmonic sensors provide a means of detecting chemical and biological species through the observation of spectral features by measurement techniques such as surface enhanced Raman spectroscopy (SERS) or surface plasmon resonance (SPR). Plasmonically active SERS substrates, such as metallic gratings or periodic metal nanowire arrays, have sub-wavelength features that enable direct coupling of normally incident electromagnetic (EM) radiation to surface plasmons. The high detection sensitivity of these methods is facilitated by EM field enhancements in the vicinity of the metallic nanostructures. Field enhancements can be achieved by controlling the spacing between nanostructures as well as the features along the length of individual nanostructures. In order to develop a full-scale plasmonic sensor, periodic nanostructures must be replicated over large-area surfaces, such as six-inch silicon wafers. One of the key challenges here is developing a cost effective approach to fabricate nanoscale structures over macroscale length scales with adequate uniformity and fidelity. Furthermore, process approaches must enable fine-tuning of the nanostructures in order to optimize field enhancement and reproducibility for the final sensor device configuration.

Dhawan Figure 2
A) Schematic of Si1-xGex nanowires (light grey color) with a diamond-shaped structure, epitaxially grown from silicon nanowires (magenta color). The Si1-xGex nanowires were coated with gold film (yellow color). When laser radiation is incident on the gold-coated DNWs, SERS hotspots H are produced between the diamond-shaped and the triangular nanowire structures. B) Scanning electron microscopy (SEM) image of 2D gold-coated Si1-xGex nanowires. C,D) TEM cross-section image showing small triangular sections formed in between the diamond NWs. E) TEM cross-section image showing ALD of platinum (black color) on the diamond-shaped Si1-xGex nanowires (dark grey color) formed on SOI wafers. F) High-resolution TEM image of the Si1-xGex nanowires.
Recently Dhawan et. al. reported on a hybrid synthesis approach to achieving a full-scale plasmonic sensor that incorporates top-down fabrication techniques—including deep-UV lithography, plasma etching of nanoscale features, and vacuum deposition of metal thin films—with bottom-up approaches—such as directed crystal growth and atomic layer deposition (ALD). The authors’ method created a unique diamond-shaped nanowire (DNW) structure with features on the order of <10 nm, providing increased field enhancements compared to cylindrical nanowire structures. The DNW structures were fabricated by first patterning silicon nanowires on silicon and silicon-on-insulator (SOI) substrates using 193 nm deep UV lithography, followed by directional reactive ion etching of the exposed silicon to a depth of approximately 100 nm. The resulting silicon nanowires had spacings as small as 100 nm, with diameters of 20-40 nm. To synthesize the DNW structures, silicon-germanium (SixGe1-x) films were epitaxially grown using a high-vacuum raid thermal chemical vapor deposition process at 550°C. For silicon nanowires fabricated on <100> silicon wafers, the SiGe epitaxial growth preferentially occurs on the <111> surface, resulting in the unique diamond-shaped nanowire structures. The DNWs formed on the silion substrates exhibit a triangular  SiGe feature at the bottom of the gap between the nanowires. For those formed on the SOI substrate, no such feature is formed as the oxide layer is exposed in the bottom of the gap, thereby precluding epitaxial growth. The different structures provide unique field enhancements that may be beneficial for the detection of different chemical or biological species.

Finally, the DNW structures are coated with a 20-100 nm film of plasmonically active metal, such as gold or silver, by sputter deposition or electron beam evaporation. Utilizing the precise control of the crystal growth process, spacings between the DNW structures of <10 nm have been observed. Further control of nanowire spacing is facilitated by the use of conformal ALD coatings applied after the crystal growth step. The rate limited growth provided by ALD enables highly uniform, precise control of the DNW spacing in order to optimize the plasmonic structures for a given application. Experimental and modeled performance of the resulting plasmonic sensor exhibited both high sensitivity and specificity. By incorporating hybrid top-down and bottom-up synthesis, a versatile approach has been demonstrated for fabricating large-area plasmonic sensors. The ability to fine tune nanostructure features over large areas renders this approach highly adaptable, thereby opening up new opportunities for plasmonic sensor applications.


Image reproduced with permission from Dhawan JW,  et. al. 2011. Small7(6):727-731. DOI: 10.1002/smll.201002186

Last updated: April 01, 2011
 

DOI: 10.4053/er509-110329

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Tags: Nanowires, diamond-shaped nanowire structures, deep UV Lithography, Nanopatterning/Lithography, directed crystal growth, Atomic layer deposition (ALD)

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