Wednesday, November 25, 2009

Lithographic Manufacturing Proposed for SERS Using Nano Void Metallodielectric Photonic Crystals

D3 Technologies Limited details its improvements in metrology by using a metal nano-void photonic crystal for surfaced enhanced Raman spectroscopy (SERS) in U.S. Patent Application 20090273779. The system provides a platform, system, and method for surface enhanced Raman spectroscopy that provides reproducible Raman signal enhancements for a low concentration of analyte molecules as well as the added functionality of pre- and post-processing which can be integrated on a single platform. Also provided is a method of mass fabrication of the platform using lithographic technology. 

Metallodielectric photonic crystals (PC) are used to extract a SERS signal from low concentrations of analyte molecules embedded inside the PC lattice, according to inventors Jeremy Baumberg, Sven Mahnkopf, Majd Zoorob, John Lincoln and James Wilkinson. The system encompasses four main aspects, namely a photonic crystal (PC)-based surfaced enhanced Raman spectroscopy (SERS) active platform, a system and method for obtaining a SERS signal from an analyte, and a fabrication method for the PC-based SERS platform.

Major improvements provided by the D3 Technologies system compared to many prior art systems include the following aspects. The topologies allow the launch of light from the side, hence facilitating easier and more reproducible means of launching light to the SERS active region. Secondly, the integration of many functional system blocks into one highly integrated chip will furthermore reduce the system complexity due to a reduced number of required external components and hence reduced system cost. The more compact system architecture will furthermore be appreciated for example in in-vivo endoscopy. Thirdly, the separation of light launch, Raman signal generation and light extraction can be used to optimize each functional block to its specific task (for example light extraction as opposed to maximum SERS enhancement).

Finally, compared to topologies that rely on colloidal crystals, the proposed fabrication method that defines the pattern by lithographic means as opposed to self-assembly promises greater reproducibility and increased yield. The scalability of the novel anisotropic wet and deep Silicon etching fabrication processes for the generation of SERS substrates provides a cost-effective method of mass producing identical SERS substrates while still maintaining high reproducibility of signals across different experiments; different substrates on the same wafer; and/or different substrates on different wafers

Raman spectroscopy is used for a variety of applications, most commonly to study vibrational quanta, such as vibrations in molecules or phonons in solids, although other quantized entities can also be studied. Raman spectroscopy can provide detailed information relating to the physical state of sample materials and can be used to distinguish various states of otherwise chemically identical molecules, such as various molecular isomers, from one another.

Raman spectroscopy finds wide-ranging use in numerous different industries. By way of example, Raman spectroscopy finds application in the pharmaceutical, chemical, bio-analysis, medical, materials science, art restoration, polymer, semiconductor, gemology, forensic, research, military, sensing and environmental monitoring fields.

Although Raman spectroscopy is an extremely useful analytical tool, it does suffer from a number of disadvantages. The principal drawbacks associated with Raman spectroscopy arise because of the small scattering cross-section. Typically, only 10.sup.-7 of the photons incident on the sample material will undergo Raman scattering. Hence, in order to detect Raman scattered photons, Raman spectrometers typically employ high power laser sources and high sensitivity detectors. Not only is the scattering cross-section small in an absolute sense, but it is small relative to Rayleigh scattering in which the scattered photon is of the same energy as the incident photon. This means that there are often problems related to separating out the small Raman signal from the large Rayleigh signal and the incident signal, especially when the Raman signal is close in energy to the incident signal.

High power sources are not only both bulky and expensive, but at very high power the intensity of the optical radiation itself can destroy the sample material, thus placing an upper limit on the optical radiation source intensity. Similarly, high sensitivity detectors are often bulky and expensive, and even more so where forced cooling, such as with liquid nitrogen, is necessary. Additionally, detection is often a slow process as long integration periods are required to obtain a Raman spectrum signal having an acceptable signal-to-noise ratio (SNR).

The problems associated with Raman spectrometry have been known long since C. V. Raman discovered the effect itself in 1928. Since that date, various techniques have been applied to improve the operation of Raman spectrometers

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