Lhadi Merhari

Surface chemistry of TiO2 nanoparticles: influence on electrical and gas sensing properties

The modification of the surface chemistry of semiconducting nanoparticles is often required for optimising their performance. For example, surface modifications of semiconductor-based sensors can be envisaged to tailor the device selectivity. However, surface chemical modifications should deteriorate neither the bulk characteristics nor the electrical properties of the material. This becomes critical for nanoparticles due to their high surface-to-bulk ratio. In this work, surface modifications of titanium oxide nanoparticles by grafting hexamethyldisilazane (HMDS) are monitored in situ by Fourier transform infrared spectroscopy. The HMDS grafting decreases the density of the hydroxyl groups at the titanium oxide surface and, therefore modifies the surface affinity to water molecules. The consequences of these surface modifications on the gas sensing properties of the nanomaterial are discussed. In particular, it is shown how moisture adsorption subsequently alters these new grafted chemical species, resulting in a decrease of the cross-sensitivity to humidity. The variations of the infrared background absorption versus gas exposures are demonstrated to follow a λ2 dependence in agreement with the Drude–Zener theory, thus indicating that they are essentially due to the free carrier absorption. Therefore, the variations of the infrared absorption versus gas exposures can be directly correlated to the electrical conductivity variations.

Advances in Air Quality Monitoring via Nanotechnology

Urban air pollution has become an inescapable issue due to its serious consequences on public health and, therefore, needs more accurate tracking through denser networks of air quality monitoring (AQM) stations. A higher density of these networks can be afforded by cities only if the costs of future individual AQM stations decrease. We review here the outcome of two European projects where our objective was to provide an alternative approach consisting in the development of cost-effective mobile microstations based on semiconductor sensors and capable of complementing the expensive and bulky current AQM stations. Improvement of the sensor sensitivity to detect very low levels of pollutants (CO, NO, NO2, O3) in air was the major challenge to take up. This was achieved by using metal oxide nanosized particles with both controlled size and surface chemistry, and by adapting the screen-printing process to the nanometer size specificity. The detection thresholds for NO2, NO and O3 of our nanoparticles-based sensors have been decreased by a factor of 3–5 compared to currently commercialized sensors. The lowest detectable concentration of CO has been reduced from 5 to 3 ppm without affecting the selectivity. In terms of sensitivity performance, our sensor prototypes can now meet the criteria for outdoor AQM whereas the commercial semiconductor and electrochemical sensors still cannot. As for the implementation of the network as a whole, our technological approach is outlined.

Nanocomposite resist systems for next generation lithography

A novel nanocomposite resist system was developed for sub-100 nm resolution e-beam lithography by dispersing surface-treated silica nanoparticles in a commercial ZEP520® resist. At 4.0 wt.% loading of silica nanoparticles, the system exhibited a much higher resolution than ZEP520® without sacrificing the intrinsic sensitivity and contrast of the starting polymer. The first major result is that 46 nm-wide isolated lines were obtained in the nanocomposite system (∼250 nm-thick layer), whereas comparatively 130 nm-wide lines were obtained in ZEP520® under the same experimental conditions. Contrary to standard e-beam resists, this important reduction of line broadening already occurred at 20 keV while higher energy e-beams (up to 100 keV) did not lead to further line broadening reduction. Moreover, it was shown that the addition of silica nanoparticles resulted in a higher resistance of the nanocomposite to plasma etching with 02 gas. Subjecting this nanocomposite resist to 75-keV Xe+ ion irradiation showed that it is also particularly suitable for ion projection lithography as a preliminary resolution of 114 nm (1/s) was obtained while the sensitivity increased by a factor of 40 compared to 30-keV electrons. Extending the nanocomposite approach to KRS-XE®, a chemically amplified resist, led to both enhanced resolution and mechanical stability for electron beam lithography. The major resolution and etch resistance improvements in both resist systems indicate that nanocomposite systems are promising candidates not only for sub-100 nm resolution e-beam lithography but also for ion projection lithography. Supported by preliminary Monte Carlo simulations a tentative mechanism highlighting the electron-nanocomposite interactions as the explanation for line broadening reduction is proposed.

Nanocomposite resists for electron beam nanolithography

Abstract A novel nanocomposite resist system was developed for sub-100 nm resolution e-beam lithography by dispersing surface-treated silica nanoparticles in a commercial ZEP520 ® resist. At 4.0 wt % loading of silica nanoparticles, the system exhibited a much higher resolution than ZEP520 ® without sacrificing the intrinsic high sensitivity and contrast of the starting polymer. The first major result is that 46 nm-wide isolated lines were obtained in the nanocomposite system, whereas comparatively 130 nm-wide lines were obtained in ZEP520 ® under the same experimental conditions. Moreover, it was shown that the addition of silica nanoparticles resulted in a higher resistance of the nanocomposite to plasma etching with O 2 gas. The major resolution improvement indicates that the nanocomposite is a promising candidate resist for sub-100 nm resolution e-beam lithography.

Investigation of the Gas Detection Mechanism in Semiconductor Chemical Sensors by FTIR Spectroscopy

In this review article, we focus on two complementary approaches of Fourier transform infrared (FTIR) spectroscopic analysis of semiconductors, in an attempt to bridge a gap between the “atomistic model” and the “rigid band model.” First, we report on selected papers describing the chemical reactions occurring at the surface of semiconductor chemical gas sensors. Then, we mention different studies in which FTIR spectroscopy has been used to monitor the free carrier absorption in semiconductors. Finally, to correlate surface chemistry with electronic properties, we describe our approach combining both possibilities offered by FTIR spectroscopy. This approach allows one to clarify the fundamentals of the gas detection mechanisms and to obtain an unequivocal and rapid correlation between the electrical response of the chemical sensors and the infrared absorption of sensing semiconductors.

Ion Projection Direct-Structuring For Nanotechnology Applications

Large-field ion-optics has been developed for reduction printing. Sub-100nm ion projection direct-structuring (IPDS) of patterned magnetic media discs has been demonstrated, extending over 17mm diameter exposure fields, in a single exposure. First results of IPDS patterning of nanocomposite resist material are presented. Information about a novel 200x reduction projection focused ion multi-beam (PROFIB) tool development is provided. Further IPDS nanotechnology applications are discussed.

FTIR Analysis of the Mechanisms of NO x Detection by Semiconductor Metal Oxide Nanoparticles

The chemical reactions occurring at the surface of tin oxide nanoparticles under NO x adsorption have been investigated by Fourier transform infrared spectroscopy and correlated with the variations of the electrical conductivity of tin oxide. These surface reactions have been found dependent on the particle size and on the presence of oxygen. The strong coordination of the newly formed chemical species onto the nanoparticle surface may result in a poor reversibility of the sensor response.

New advances in resist system for next-generation lithography

A novel nanocomposite resist system was developed for sub-100 nm resolution e-beam lithography by dispersing surface-treated silica nanoparticles in a commercial ZEP520 resist. At 4.0 wt % loading of silica nanoparticles, the system exhibited a much higher resolution than ZEP520 without sacrificing the intrinsic sensitivity and contrast of the starting polymer. The first major result is that 46 nm-wide isolated lines were obtained in the nanocomposite system (~ 250 nm thick layer), whereas comparatively 130 nm-wide lines were obtained in ZEP520 under the same experimental conditions. Interestingly, this dramatic reduction of line broadening already occurred at 20 keV while higher energy e-beams (up to 100 keV) did not lead to further line broadening reduction. Moreover, it was shown that the addition of silica nanoparticles resulted in a higher resistance of the nanocomposite to plasma etching with O2 gas. Extending the nanocomposite approach to the KRS-XE resist led to both enhanced resolution and mechanical stability. The major resolution improvement in both systems indicates that nanocomposite systems are promising candidates for sub-100 nm resolution e-beam lithography. A mechanism, explaining the electron-nanocomposite interactions at the origin of line broadening reduction, is proposed and tentatively backed by preliminary Monte Carlo simulations.

Dual contribution of FTIR spectroscopy to nanoparticles characterization: Surface chemistry and electrical properties

In a combined approach toward the optimization of chemical gas sensors, Fourier transform infrared spectroscopy is used to investigate in situ the surface reactions taking place at the surface of semiconductor nanoparticles and to simultaneously monitor the variations of the free-carrier density. The correlation between the surface reactions and the changes in the infrared absorbance under gas adsorption/desorption cycles gives information on the chemical phenomena responsible for electrical conductivity variations and therefore for the gas detection. Interaction of CO and NOX with tin oxide nanoparticles is presented and discussed. While the chemical reactions leading to the increase of the electrical conductivity under CO adsorption are relatively straightforward, the adsorption of NOX is much more complex. It is demonstrated that, although generating a strong increase of the electrical conductivity, the NOX adsorption on a fresh tin oxide surface is not fully reversible and actually poisons the surface. Subsequent NOX adsorptions lead to reversible chemical reactions even though the electrical response of the sensor is weaker.

A Versatile Approach for Biomaterial Patterning: Masked Ion Beam Lithography

We describe a new approach for biomaterial patterning, viz, masked ion beam lithography. Poly (methyl methacrylate) (PMMA) film was used as a model system and subjected to Ca + and P + ion implantations through masks. Ca + ion implantation was performed at an energy of 85 keV with a fluence of 1x10 14 ions/cm 2 . P + ion implantation was done at an energy of 85 keV with fluences of 1x10 15 and 1x10 16 ions/cm 2 . Arrays of holes were generated during these processes. AFM showed that the depth of the holes is in the nanoscale region. The surface hydrophobicity of the exposed PMMA films was investigated by contact angle measurement. The results indicated that ion implantation changed the surface hydrophobicity.