Articles

Raman spectroscopy is to be used as one of the analytical techniques for the European Space Agency’s ExoMars mission to identify the geological and biogeological spectral signatures that could herald the presence of extinct or extant life on Mars. The article looks at the benefits of Raman spectroscopy for this and the research on Earth to build knowledge of the spectra of organisms living in extreme conditions here.

The problem of detecting, recognising and identifying explosives at significant standoff distances has proved one of the most difficult—and most important—challenges during recent years, being today, one of the most demanding applications of spectroscopic techniques. The limited number of sophisticated available techniques potentially capable of standoff detection of minimal amounts of explosives is based on laser spectroscopy. Of the recently developed techniques, Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) are considered significant for their potential for homeland defence applications.

Interest in Raman spectroscopy as an analytical technique that can be applied in a wide variety of fields continues to increase. The main reason for this interest is that no special sample preparation is required. However, the Raman signal is typically very weak, with only one in every 10⁶–10⁸ photons being scattered. This has driven the development of several enhancement techniques, e.g. Resonance Raman (RR), Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Resonance Raman Spectroscopy (SERRS), which can be used for dilute samples.

A number of analytical applications in the area of security screening, medical diagnosis, drug authentication and quality control often require non-invasive probing of diffusely scattering (turbid) media in order to obtain chemical characterisation of deep-lying sample regions. Examples include non-invasive disease diagnosis, the detection of concealed explosives and illicit materials, the identification of counterfeit drugs and quality control applications in the pharmaceutical industry. Raman spectroscopy holds particular promise in this area due to its inherently high chemical specificity [exceeding that of near infrared (NIR) absorption spectroscopy and comparable with mid-infrared and THz methods], the ability to probe samples in the presence of water (the Raman scattering cross-section of water is very low) and its high penetration depth into turbid non-absorbing or weakly absorbing samples. On the downside, the technique is restricted to samples that do not exhibit strong fluorescence emission although this problem can, in the majority of cases, be avoided by using NIR excitation. Until recently, Raman techniques have generally been confined to applications involving surface layers of turbid media due to limitations imposed by the backscattering collection geometry common to the majority of commercial Raman probes. In principle, confocal Raman microscopy can potentially resolve objects to depths of up to several hundred micrometres. Deeper layers cannot be readily resolved and, typically, are overwhelmed by Raman and fluorescence signals emanating from the surface layer.

Fernando Rull Pérez and Jesus Martinez-Frias

Centro de Astrobiología, Unidad Asociada CSIC-Universidad de Valladolid, Facultad de Ciencias, 47006 Valladolid, Spain

Simon FitzGerald

Horiba Jobin Yvon Ltd, 2 Dalston Gardens, Stanmore, Middlesex HA7 1BQ, UK. E-mail: [email protected]

Philippe Colomban

LA​DIR-UMR 7075 CNRS & Université P. & M. Curie, 2 rue Henry Dunant, 94320 Thiais, France

Moore’s law* dictates microelectronics researchers to make integrated circuit (IC) devices smaller and to put them as close to each other as possible on a chip. This results in a better performance and a larger functionality of the chips. However, these devices also require a good electrical isolation from each other. This is in general done by the formation of a thick local oxide in the “field region” between the devices. In 1970, researchers from Philips¹ invented the so-called LOCOS (LOCal Oxidation of Silicon) technique to achieve this isolation. Using a Si₃N₄ mask, the silicon is thermally oxidised in the nitride-free field regions. Figure 1 (left) shows a typical LOCOS structure. Although LOCOS seemed a perfect solution at that time, it came with a lot of problems, many of them related to mechanical stresses. Thermal oxidation of Si to SiO₂ occurs together with a 125% volume expansion. As a result, the oxide grown in the field region, called the “field oxide,” exerts large forces on the surrounding silicon. Another major drawback of this technique is the so-called “bird’s beak,” caused by the lateral growth of the oxide under the nitride mask. This bird’s beak not only affects the intended device length, it also introduces large local mechanical stresses in the silicon, because of volume expansion, and it also deforms the nitride film. These stresses often resulted in the generation of dislocations in the silicon, which are quite harmful for the devices.