Maintaining specific temperature is a key parameter for most of the gas sensing materials, particularly for metal oxide-based thin film layers, to operate them more efficiently to detect different gaseous (or vapor) species at ppm and ppb concentration levels. For field applications, battery-operated micro-gas-sensors, power dissipation and the required temperature stability are to be maintained with a close tolerance for better signal stability and also to analyze the generated data from the sensing element. This chapter mainly focuses on several metal oxide films to highlight the base temperature and its creation on silicon-based platforms. Air-suspended structures are highlighted, and a comparison is drawn between simple and MEMS-based structures from the power dissipation point of view. Ease of fabrication and operation limitations are explained with fabrication issues.

Silicon, as a single-crystal semiconductor, has sparked a revolution in the field of electronics and touched nearly every field of science and technology. Though available abundantly as silica and in various other forms in nature, silicon is difficult to separate from its chemical compounds because of its reactivity. As a solid, silicon is chemically inert and stable, but growing it as a single crystal creates many technological challenges. Crystal Growth and Evaluation of Silicon for VLSI and ULSI is one of the first books to cover the systematic growth of silicon single crystals and the complete evaluation of silicon, from sand to useful wafers for device fabrication. Written for engineers and researchers working in semiconductor fabrication industries, this practical text: Describes different techniques used to grow silicon single crystals Explains how grown single-crystal ingots become a complete silicon wafer for integrated-circuit fabrication Reviews different methods to evaluate silicon wafers to determine suitability for device applications Analyzes silicon wafers in terms of resistivity and impurity concentration mapping Examines the effect of intentional and unintentional impurities Explores the defects found in regular silicon-crystal lattice Discusses silicon wafer preparation for VLSI and ULSI processing Crystal Growth and Evaluation of Silicon for VLSI and ULSI is an essential reference for different approaches to the selection of the basic silicon-containing compound, separation of silicon as metallurgical-grade pure silicon, subsequent purification, single-crystal growth, and defects and evaluation of the deviations within the grown crystals.

Presently, a lot of research effort has been directed toward the development of small dimensional gas sensing devices, based on metal oxides, for practical applications ranging from toxic gas detection to pollution monitoring in the air ambience. The demand for better environmental control and safety has increased research activities of microgas sensors and their development. For this purpose, MEMS-based sensors in the form of thin or thick films seem to be more promising. Metal oxide-based sensors work on the principle of electrical conduction variation and for this purpose a predetermined temperature is crucial for the sensing films for best sensitivity values. Creating correct temperature and the proper temperature distribution is of paramount importance for these sensors. Creating thin film platforms for this purpose is crucial and practically limits the yield. This paper presents the results obtained for thick air-suspended platform, using ANSYS, for a PRT (platinum resistance thermometer) embedded microheater that is suitable to operate by a dc battery source. Both the microheater and PRT were defined in a single photolithography step and are realized on an oxidized silicon substrates (SiO2/Si). Realistic values are used to simulate the data and to obtain the optimum temperature distribution, of the order of 400 °C, over an area of 5 mm × 4 mm of air-suspended platform. These studies help greatly to visualize the microheater design to help in understanding the heat distribution on the membrane, where the sensing layer will be placed and also to get the optimum sensing properties of microgas sensors. Using embedded PRT, it is easy to confirm and control the required temperature necessary for the sensing layer. This approach simplifies the process to achieve a simple and practical device.

In this paper, the fabrication and results of zinc oxide (ZnO) thin-film CO gas sensor are discussed. The thin film of c-axis oriented ZnO is deposited on Si/SiO 2 substrate at room temperature using RF magnetron sputtering technique. The deposited ZnO film is annealed at different temperatures starting from 200 °C to 400 °C. The crystalline structural properties are analyzed by X-ray diffraction technique and the preferred orientation of ZnO thin film (along 002 direction) is found at 400 °C. Further, ZnO thin film is characterized by scanning electron microscopy and surface profilometer to analyze the surface properties and thickness. The deposited ZnO film is supported by Au electrodes and Pt microheater to couple the sensing signal with outer circuitry. A measurement system is also developed to measure the gas sensing properties and the results are discussed in detail.

The photophysical properties of a hemicyanine dye, 4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide (DASPC22), have been studied in homogeneous media of pure solvents and mixed solvents using UV-vis absorption and fluorescence spectroscopies. These properties were explored to study the binding interactions between DASPC22 and nanotubes of beta-cyclodextrin (beta-CD) using UV-vis absorption, steady-state fluorescence and fluorescence anisotropy, and time-correlated single-photon-counting (TCSPC) fluorescence measurements of the dye. DASPC22 molecules form H-aggregates in pure water. beta-CD forms a simple inclusion complex (1:2 stoichiometry) below its critical aggregation concentration (cac) by encapsulating the chromophoric part of the dye. The H-aggregate dissociates significantly to the monomeric form of the dye only when the nanotubes of beta-CD molecules start to form above its cac. The dye molecule exists in its monomeric form upon inclusion of its chromophoric part along with the aliphatic tail inside the hydrophobic nanotubular cavity of beta-CD. A 350-fold increase in fluorescence intensity of DASPC22 inside the nanotubular cavities formed by an 8 mM concentration of beta-CD compared to the fluorescence intensity in the form of a simple inclusion complex was observed. The effects of a Hofmeister series of potassium salts, namely, KClO4, KI, KCl, and KF, in both low and high concentration ranges on the binding strength between host and guest molecules were studied. Salts in their very low concentration ranges enhanced the stability of the host-guest complexes, resulting in a further increase in fluorescence intensity. The fluorescence properties can be tuned by the selective addition of potassium salts with various anions. The tuning of the optical properties of the dye in beta-CD nanochannels could help materials scientists to develop novel supramolecular materials.

Kelvin Probe (KP), a non-contact, non-destructive vibrating capacitor device, was used to measure the work function (WF) difference of thin Pt films, deposited on oxidized silicon substrates, with highly diluted H2 gas, in ppm levels, in the presence of with and without relative humidity (RH). Response times were extracted from the behavior of WF shift as a function of H2 concentration values. Measurements were compared for zero and non-zero RH conditions at a fixed temperature of 30°C. Changes in WF were evaluated by using HP VEE program, suitably modified for the present measurements. The events were executed step by step for every second time interval through an input formatted file. The data was recorded at each event and was analyzed for the shift in WF with respect to the H2 concentration. The shift in the WF and the response time (τ90) at different concentration levels for the Pt films is reported in this paper. The presence of humidity shows saturation of response time after 500 ppm whereas zero humidity conditions show a continuous reduction at higher concentration values.

Amorphous silicon nitride (a-SiNx:H) films were deposited at 300 degrees C by plasma-enhanced chemical vapour deposition. Silane/ammonia (SiH4/NH3) and silane/nitrogen (SiH4/N-2) gas compositions were used at various flow rates to study the effect of hydrogen passivation of the films using the photoluminescence (PL) spectroscopy. Fourier transform infrared (FTIR) spectroscopy was employed to derive the relative changes in the total bonded hydrogen (TBH) concentration with increasing flow rates. The composition and the refractive indices of the as-deposited films were also extracted using the bond density calculations from FTIR spectra. The calculated refractive indices of the silicon nitride films were consistent with the ellipsometry measurements. The PL spectra were observed to be free from any interference effect and this was attributed to the nitrogen related defects in all the a-SiNx:H films. The films deposited using SiH4/NH3 showed a higher PL intensity than those deposited in a SiH4/N-2 environment. A direct relation between the PL intensity and TBH content of the films was found.

A suitably sized charge transfer probe of an elongated geometry can induce the formation of alpha-cyclodextrin nanotubular suprastructures, a rare event because of size restriction of the host. The specific molecular structure is found to be responsible only for the 1:2 guest-host complex formations. No evidence of the formation of the 1:1 complex is found. Steady-state fluorescence anisotropy and atomic force microscopy show that the nanocomposites club with other such species very efficiently to form nanotubes and nanoclusters because of primary interactions through hydrogen bonding and develop nanotubular suprastructures due to secondary interactions. The degree of formation of the suprastructures is found to be very much controlled by the probe concentration. In aqueous environment, 2 mu M is observed to be the best concentration for the fluorophore form of large rodlike aggregates. Concentrations as low as 1 mu M and as high as 4 mu M induce the formation of relatively smaller structures. The findings encourage the applications of cyclodextrin nanotubular clusters toward nanotechnology and pharmaceutical research. The concentration dependent phenomenon will dictate the drug dosage as also the extent of formation of the nanostructures in the formation of insulated nanowires. The work is purely indicating the anchoring capability of the used molecule to form nanotubular alpha-CD suprastructures which otherwise does not form so frequently.

ABSTRACT In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or multi-component metal oxides. They cover gases such as CO, CO2, CH4, C2H5OH, C3H8, H2, H2S, NH3, NO, NO2, O2, O3, SO2, acetone, dimethylamine (DMA), humidity, liquid petroleum gas (LPG), petrol, trimethylamine (TMA), smoke, and many others. Both doped and undoped oxides are analyzed for the compatibility with silicon processing conditions and hybrid microcircuit fabrication techniques. In silicon processing conditions, they are further analyzed for the suitability for simple silicon surfaces, silicon-on-insulator (SOI) surfaces, and micromachined silicon geometries for different operating temperatures. Discussion on gas-sensing properties of each material and its applications are described in the text in alphabetical order of the elemental oxides. Further, the gas-sensing properties like sensitivity, detection limits, operating temperature, and so on are summarized in tables al ong with relevant references. The figures incorporated in the present review are primarily based on discussions and data in tables. However, these figures provide a qualitative comparison and present a pictorial view to examine suitability of a material for a particular application. From the known parameters, the present study clearly indicates the suitability of certain materials and the gases that they cover for the development of integrated micro gas sensors. A clear picture has been brought out for the development of silicon-based processing technology. Various parameters are discussed for the selection of these materials, to examine their suitability and practical problems that are being associated. Etching of these metal oxides and the reliability of devices are also discussed.