MEMS-based microheaters integrated gas sensors

Abstract

In the present work efforts have been made to develop microheater integrated gas sensors with low power consumption. The design and simulation of a single-cell microheater is carried out using ANSYS. Low power consumption (<35?mW) platinum micro-heater has been fabricated using bulk micromachining technique on silicon dioxide membrane (1.5?μm thin), which provided improved thermal isolation of the active area of 250?×?250?μm². The micro-heater has achieved a maximum temperature of ～950?°C at an applied dc voltage of 2.5 V. Fabricated mircro-heater has been integrated with SnO₂ based gas sensors for the efficient detection of H₂ and NO₂ gases. The developed sensors were found to yield the maximum sensing response of ～184 and ～2.1 with low power consumption of 29.18 and 34.53?mW towards the detection of 1?ppm of NO₂ gas and 500?ppm of H₂ gas, respectively.     

Low operating temperature CO sensor prepared using SnO<Subscript>2</Subscript> nanoparticles

A low operating temperature CO (carbon monoxide) sensor was fabricated from a nanometer-scale SnO₂ (tin oxide) powder. The SnO₂ nanoparticles in a size range 10–20 nm were synthesized as a function of surfactant (tri-n-octylamine, TOA) addition (0–1.5 mol%) via a simple thermal decomposition method. The resulting SnO₂ nanoparticles were first screen-printed onto an electrode patterned substrate to be a thick film. Subsequently, the composite film was heat-treated to be a device for sensing CO gas. The thermal decomposed powders were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffractometry (XRD), and surface area measurements (BET). The CO-sensing performance of all the sensors was investigated. The experimental results showed that the TOA addition significantly decreased the particle size of the resulting SnO₂ nanoparticle. However, the structure of the powder coating was crucial to their sensing performance. After heat-treatment, the smaller particle tended to cause the formation of agglomeration, resulting in the decline of surface area and reducing the reaction site during sensing. However, the paths for the sensed gas entering between the agglomerated structure may influence the sensing performance. As a CO sensing material, the SnO₂ nanoparticle (~12 nm in diameter) prepared with 1.25 mol% TOA addition exhibited most stable electrical performance. The SnO₂ coating with TOA addition >0.75 mol% exhibited sensor response at a relatively low temperature of <50°C.     

Effect of Zn2SnO4 on the sintering and electrical properties of SnO2 ceramics

SnO₂ ceramics with relative density about 98 % were obtained based on the addition of Zn₂SnO₄. The shrinkage of the ceramic samples increased sharply and got a saturated value about 13.3 % with doping more than 0.2 mol% Zn₂SnO₄. In the dielectric spectra, no relaxation peaks were observed and no deep trap states could be detected from 50–300 °C and 40–5 M?Hz. Thus, the oxygen vacancies may not be necessary for the densification of SnO₂ ceramics during sintering process. For all the samples, nonlinear electrical properties were observed and the breakdown electrical fields are in good agreement with the barrier height. With increasing Zn₂SnO₄ content, the activation energies E _a for O^? or O^2? adsorbed at grain boundary decreased and the doping of Zn₂SnO₄ may be an important reason for the improve of grain conductivity and formation of Schottky barrier.     

NOx Sensing Properties of Varistor-Type Gas Sensors Consisting of Micro p-n Junctions

NO _x sensing properties of SnO₂-xCr₂O₃ as a varistor-type gas sensor have been investigated in the temperature range of 200–600°C. The breakdown voltage of SnO₂ shifted to a higher electric field upon exposure to NO₂ at 300–500°C, and the largest breakdown voltage shift, i.e. the highest NO₂ sensitivity was observed at 400°C. In contrast, the direction of the breakdown voltage shift in NO varied with temperature: the breakdown voltage shifted to a lower electric field at 300°C, but to a higher electric field at 500°C, and remained almost unchanged at 400°C. The NO₂ sensitivity of SnO₂ was superior to the NO sensitivity at every temperature, and then the SnO₂ exhibited good selectivity to NO₂ at 400°C. The breakdown voltage of Cr₂O₃ shifted in the reverse direction upon exposure to NO and NO₂, in comparison with those observed with SnO₂, owing to its p-type semiconductivity. Thus, Cr₂O₃ also exhibited certain sensitivity to both NO and NO₂ at 200°C, being more sensitive to NO₂, though the sensitivities decreased drastically at temperatures higher than 300°C. The addition of 5.0 wt% Cr₂O₃ to SnO₂ resulted in a significant improvement of NO and NO₂ sensitivities at 600°C, being accompanied by an increase in the breakdown voltage in air. Especially, the NO sensitivity was superior to the NO₂ sensitivity in the concentration range of 20–100 ppm, and then SnO₂ mixed with 5.0 wt% Cr₂O₃ was found to be the most suitable candidate for a NO sensor among the sensors tested. The increase in the breakdown voltage in air induced by the Cr₂O₃ addition was confirmed to arise from both the decrease in the particle size of SnO₂ and the formation of micro p-n junctions at grain boundaries. The decrease in the particle size was also responsible for the increased NO and NO₂ sensitivities, but the p-n junctions were suggested to play a more important role in promoting and stabilizing the chemisorption of NO at higher temperatures.     

Structural, luminescent, and NO2 sensing properties of SnO2-core/V2O5-shell nanorods

SnO₂-core/V₂O₅-shell nanorods were synthesized using a two-step process: thermal evaporation of Sn powders and sputter-deposition of V₂O₅. The core-shell nanorods were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and photoluminescence spectroscopy. The diameters of these core-shell nanorods ranged from 80 to 200 nm with a shell layer thickness in the range of 7–13 nm. The cores and shells of the annealed core-shell nanorods consisted of a single crystal tetragonal-structured SnO₂ and a single crystal orthorhombic-structured V₂O_5, respectively. Photoluminescence measurements revealed the SnO₂ nanorods to have a yellow emission band centered at approximately 590 nm, which was enhanced significantly by the V₂O₅ coating and further by thermal annealing. The sensitivity of the networked SnO₂-core/V₂O₅-shell nanorod sensor to NO₂ gas was slightly higher than that of the bare SnO₂ nanorod sensor. The enhanced sensitivity of the SnO₂ nanorods by the V₂O₅ coating was attributed to the modulation of electron transport by the SnO₂-V₂O₅ heterojunction with an adjustable energy barrier height.     

Sintering and varistor behaviour of SnO2-Zn2SnO4 composite ceramics

Densified SnO₂-Zn₂SnO₄ composite ceramics were prepared by conventional ceramic processing and the sintering, electrical properties were investigated. The X-ray diffraction results and sintering curves showed that the pellets pressed by ZnO and SnO₂ mixed powders began to shrink after Zn₂SnO₄ was synthesized at about 950 °C. The results suggest that the densification of SnO₂-Zn₂SnO₄ composite ceramics cannot be attributed to the oxygen vacancies created by acceptor doping as traditional viewed. The measurement of J-E curves showed that the SnO₂-Zn₂SnO₄ composite ceramics have good nonlinear properties (α?~?3.9–4.5) without any other doping. Another interesting result is that the composite ceramics have low breakdown electrical field (E _B?~?10 V/mm) with high relative dielectric constant (1 kHz, ε _r?~?6?×?10³). Further studies demonstrate that the varistor behavior is also a grain boundary barrier effect and the barrier height is about 0.84 eV.     

NO2 adsorption behaviour on LaFeO3 electrodes of YSZ-based non-nernstian electrochemical sensors

X-ray photoelectron spectroscopy (XPS) was used to examine the NO₂ adsorption behaviour on the LaFeO₃ and Pt electrodes of planar yttria stabilized zirconia non-Nernstian gas sensors. The electrochemical sensors were exposed to the same gas atmosphere containing 1000 ppm NO₂ at 650°C. XPS of the as-prepared sensors and sensors after exposure to NO₂ revealed bonded nitrogen peaks on the surface of the semiconducting oxide but no nitrogen peaks on the Pt electrode. Therefore, NO₂ adsorption on a LaFeO₃ electrode plays an important role in the NO₂ detection mechanism.     

High temperature electrical characterization of nano-crystalline BaTiO3 synthesized using Ba(NO3)2 and TiO2

The reaction of Ba(NO₃)₂ with TiO₂ was studied by thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques up to 1000°C and in nitrogen atmosphere. It was found that the formation of BaTiO₃ takes place above 600°C. BaTiO₃ powder was prepared by calcination of Ba(NO₃)₂ and TiO₂ precursor mixture at 800°C for 8 h. X-ray diffraction analysis of the synthesized BaTiO₃ confirmed the formation of tetragonal phase. Average crystallite size was found to be 44 nm, For the electrical and morphological characterization pellets of the obtained powder were sintered at 1000 °C for 12 h. Scanning electron micrograph (SEM) exhibits spherical and rod shaped grains. The dielectric constant, dissipation factor, complex plane impedance and ac conductivity of the sintered pellet has been measured in the temperature range of 40–600°C and frequency range of 100 Hz–2 MHz. DC conductivity of the sample was obtained from the impedance data. The conductivities (both ac and dc) and relaxation time (τ) exhibit two regions of temperature dependence, namely region I, which represents (280–450°C) and region II, which governs (450-600°C). Conduction and relaxation in both the temperature regions are explained in terms of hopping of electrons and doubly ionized oxygen vacancies (V_O^??).     

WO3/BTO heterostructures based NO2 sensor with enhanced response characteristics

Abstract

In the present work, an efficient NO₂ gas sensor has been realised using single phase Barium titanate, BaTiO₃, (BTO) thin film, grown by chemical solution deposition technique (CSD). The gas sensing characteristics of BTO thin film were enhanced by integrating WO₃ modifier in the form of uniformly distributed circular nano-clusters and continuous overlayer. The WO₃ nanoclusters/BTO sensing element exhibited enhanced sensor response (～156) with fast response speed (16?s) at a relatively low operating temperature (140?°C) towards 50?ppm NO₂ gas. An attempt has been made to explain the sensing mechanism involving the twin effect of “Fermi-level exchange mechanism” and “spill over mechanism” upon interaction with target NO₂ gas. The obtained results in the present work are encouraging for the realization of hand-held NO₂ gas sensor.     

Gas-phase driven nano-machined TiO2 ceramics

A simple, inexpensive gas phase reaction termed as “nanocarving process” converts TiO₂ grains into arrays of single crystal nanofibers by selective and anisotropic etching. This process is conducted by exposing dense polycrystalline TiO₂ to a H₂/N₂ environment at 700 °C. The dimensions of nanofibers are around 20 nm in diameter and 1 μm in length. The preferred crystallographic orientation for the nanocarving process is the <001> direction. Nanoparticles composed of Fe and Ni were observed on the surface of TiO₂ that formed nanofiber tips. Sintering parameters before the nanocarving treatment play a critical role in the formation of nanofibers. As sintering temperature and time increased, the rate of nanofiber generation decreased. Moreover, it was observed that by varying the heat treatment conditions, it is possible to create other structures like nanowhiskers and nanofilaments. Nanowhiskers were formed by reoxidation of nanofiber-formed TiO₂ over 600 °C. Nano-filaments were generated by heat treating sintered TiO₂ in N₂-carrying water vapor at 700 °C.     

Nanostructured TiO2-based mixed metal oxides prepared using microemulsions for carbon monoxide detection

Nanostructured powders of Nb-doped TiO₂ (TN) and SnO₂ mixed with Nb-doped TiO₂ in two different atomic ratios—10 to 1 (TSN 101) and 1 to 1 (TSN 11)—were synthesized using the reverse micelle microemulsion of a nonionic surfactant (brine solution/1-hexanol/Triton X-100/cyclohexane). The powders were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Thick films were fabricated for gas sensors and characterized by XRD analysis and field emission scanning electron microscopy (FE-SEM). The effects of the film morphology and firing temperature in the range 650–850 °C on CO sensitivity were studied. The best gas response, expressed as the ratio between the resistance in air and the resistance under gas exposure (R _air/R _gas), was measured for TSN 11 at 11 for 1,000 ppm CO exposure. All types of sensors showed good thermal stability. The electrochemical impedance spectroscopy (EIS) measurements were performed in different gas atmospheres (air, O₂, CO and NO₂) to better understand the electrical properties of the nanostructured mixed metal oxides.     

Effect of reoxidation on positive temperature coefficient of resistance behavior for BaTiO3-K0.5Bi0.5TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, (1-x)BaTiO₃-xK_0.5Bi_0.5TiO₃ (BT-KBT, 0.05≦ x ≦0.15) ceramics without any donor doping were prepared by a conventional oxide mixing method. All samples were sintered in an Ar atmosphere at 1280?~?1350°C, subsequently, reoxidized at 800?~?1100°C in a gas mixture (99 %Ar–1 %O₂). The PTCR behavior of BT-KBT ceramics were investigated in terms of KBT content, reoxidation temperature and time. The results showed that the BT-KBT ceramics exhibited an abrupt increase in their resistivity near the Curie temperature (Tc) after annealing in gas mixture, Tc of 0.9BT-0.1KBT ceramic was shifted to a higher temperature (~150°C). Furthermore, the room-temperature resistivity (ρ_RT) of ceramic samples sintered in Ar and reoxidized in a gas mixture decreased to 10² Ω·cm. The jump in resistivity (maximum resistivity [ρ_max]/minimum resistivity [ρ_min]) was enhanced by three orders of magnitude through a suitable reduction–reoxidation method without sacrificing the ρ_RT.     

Electrical and optical properties of fluorine-doped tin oxide (SnOx:F) thin films deposited on PET by using ECR–MOCVD

The electrical, optical, structural and chemical bonding properties of fluorine-doped tin oxide (SnO_x:F) films deposited on a plastic substrate prepared by Electron Cyclotron Resonance–Metal Organic Chemical Vapor Deposition (ECR–MOCVD) were investigated with special attention to the process parameters such as the H₂/TMT mole ratio, deposition time and amount of fluorine-doping. The four point probe method, UV visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic emission spectroscopy (AES), X-Ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the films. Based on our experimental results, the characteristics of the SnO_x:F thin films were significantly affected by the process parameters mentioned above. The amount of fluorine doping was found to be one of the major parameters affecting the surface resistivity, however its excess doping into SnO₂ lead to a sharp increase in the surface resistivity. The average transmittance decreased with increasing film thickness. The lowest electrical resistivity of 5.0?×?10^?3 Ω^.cm and highest optical transmittance of 90% in the visible wavelength range from 380 to700 nm were observed at an H₂/TMT mole ratio of 1.25, fluorine-doping amount of 1.3 wt.%, and deposition time of 30 min. From the XRD analysis, we found that the SnO_x:F films were oriented along the (2 1 1) plane with a tetragonal and polycrystalline structure having the lattice constants, a?=?0.4749 and c?=?0.3198 nm.     

Microstructures and dielectric properties of Zn2SnO4 thin films by sputtering from a ZnO doped ceramic target

The dielectric properties of Zn₂SnO₄ thin films with various degrees of ZnO dopant concentration were investigated. Zn₂SnO₄ thin films were prepared using the radio frequency magnetron sputtering. The X-ray diffraction patterns of the 0 and 75 mole % ZnO doped Zn₂SnO₄ thin films revealed that Zn₂SnO₄ is the main crystalline phase, which is accompanied by a little SnO₂ as the second phase. The second phase SnO₂ in specimens vanished when the extent of ZnO additive was increased to 100 mole%. A dielectric constant of 15–40 and a loss factor of 0.10–0.14 of Zn₂SnO₄ thin films were measured at 1 MHz with ZnO dopant concentration in the range of 0–100 mole%.     

Characterization of the phases and morphology in synthesizing Cu2-xSe and CuSe films

ABSTRACT

Copper Selenides are important semiconductor materials with excellent performance. Cu_2-xSe and CuSe films were synthesized by spin-coating and chemical co-reduction method. The phases of product films were analyzed by X-ray diffraction (XRD) and the morphology was characterized by scanning electron microscopy (SEM), and the compositions of products were analyzed by energy dispersive spectroscopy (EDS). The surface resistance of the product film was measured using a four-probe resistance instrument. When Copper chloride was chosen as copper source, the products contain Cu_2-xSe, CuSe and a small amount of NaCl. When the reacting temperature is below 180 °C, the XRD intensity of impurity phase NaCl is obviously increased, while it is easier to produce Cu₂Se film at 200 or 220 °C. The sample obtained at 160 °C for 20 h consists of about 0.3 ～ 0.5 µm particles, while the sample obtained at 220 °C shows about 2 ～ 4 µm flake crystals; When the copper nitrate is used as a raw material, the XRD peaks of the product obtained at 200 °C for 20 h are much high and sharp, the phases obtained are mainly CuSe. The sample obtained at 200 °C consists of hexagonal flaky crystals with about 2 ～ 3 µm diameters, while it consists of particles with about 0.3 ～ 0.5 µm diameters for the sample obtained at 220 °C. In addition, the longer reaction time is conducive to the copper selenide formation, for example the single phase CuSe film can be obtained at 220 °C. The average resistivity of Cu-Se films synthesized at 200 °C for 20 h is 2.12E-3 Ω·cm.     

Deposition and gas sensing properties of tin oxide thin films by inductively coupled plasma chemical vapor deposition

Tin oxide thin films have been deposited by a custom-designed inductively coupled plasma chemical vapor deposition (ICP-CVD) system in order to explore its application as an alternative approach for thin film gas sensor preparation. The as-deposited SnO₂ films were of polycrystalline structure with nano-size grains of 12 nm. The SnO₂ films exhibited a maximum sensitivity of 43 to 1000 ppm H₂ at an optimum operating temperature of 350^∘C. The response time of the SnO₂ films was 12 s and full recovery was achievable.     

Electrical characteristics of Cu-doped In2O3/Sb-doped SnO2 ohmic contacts for high-performance GaN-based light-emitting diodes

We characterized the electrical and chemical properties of Cu-doped In₂O₃(CIO) (2.5 nm thick)/Sb-doped SnO₂(ATO) (250 nm thick) contacts to p-type GaN by means of current-voltage measurement, scanning transmission electron microscope (STEM) and x-ray photoemission spectroscopy (XPS). The CIO/ATO contacts show ohmic behaviors, when annealed at 530 and 630°C. The effective Schottky barrier heights on diodes made with Ni (5 nm)/Au (5 nm) contacts decrease with increasing annealing temperature. STEM/energy dispersive x-ray (EDX) profiling results exhibit the formation of interfacial In-Ga-Sn-Cu-oxide. XPS results show a shift of the surface Fermi level toward the lower binding energy side upon annealing. Based on the STEM and XPS results, the ohmic formation mechanisms are described and discussed.     

Preparation and Characterization of Electrospun TiO2 Nanofibers via Electrospinning

In this work, titanium dioxide (TiO₂) nanofibers were synthesized by a combination of electrospinning and calcinations process using a solution that contained poly(vinyl pyrolidone)(PVP) and titanium isopropoxide. Titanium isopropoxide/PVP composite fibers were investigated at different titanium isopropoxide concentrations and voltages. The fibers were characterized by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). SEM images showed that the morphology of as-spun TiO₂ nanofibers are smooth, uniform and good continuous fiber morphology with fiber diameters of 150–290 nm when calcined of 600°C for 1 h in air. XRD patterns revealed that the crystallinity corresponded to TiO₂ in the form of rutile structure. The fiber diameters were slightly decreased with increase of the applied voltages and the titanium tetraisopropoxide concentration.     

Spherical shape Ba-based glass powders prepared by spray pyrolysis for MLCCs

Spherical shape BaO–B₂O₃–SiO₂ glass powders were directly prepared by high temperature spray pyrolysis at >1000 °C. The thermal and morphological characteristics of the prepared glass powders were investigated. The glass powders prepared at temperature of 1000 °C had spherical shape and hollow inner structure. On the other hand, the powders prepared at high temperature of 1300 °C had complete spherical shape and dense inner structure by complete melting. The mean size of the glass powders was 0.9 μm. The glass transition temperatures (T _g) of the glass powders obtained by spray pyrolysis at preparation temperatures between 1000 and 1300 °C were 601.1 °C regardless of the preparation temperatures. The specimen of the glass powders obtained by spray pyrolysis at the preparation temperature of 1300 °C had small number of voids even at low sintering temperature of 700 °C. In addition, the specimen sintered at temperature of 800 °C had dense microstructure without voids.     

A study on the characteristics of hydrated La2O3 thin films with different oxidation gases on the various annealing temperature

In order to investigate the structural and electrical properties of La₂O₃ films deposited by O₂ and O₃, films were hydrated in DI-water and annealed at 600 and 900 °C. La₂O₃ films deposited by O₃ showed better hydration resistance than those deposited by O₂. The thickness of both hydrated films decreased after annealing at 600 °C and increased after annealing at 900 °C. The dielectric constants of the La₂O₃ films deposited by O₃ were greater than films deposited by O₂ after annealing at 600 °C and slightly less after annealing at 900 °C. The leakage current density of the La₂O₃ films deposited by O₃ was lower than those by O₂ after annealing at 900 °C. To this end, La₂O₃ films deposited by O₃ showed better dissolution resistance than O₂ for hydration experiment as a function of dipping time.