Gas-sensing properties of sprayed films of (CdO)/sub x/(ZnO)/sub 1-x/ mixed oxide

A novel NO/sub 2/ sensor based on (CdO)/sub x/(ZnO)/sub 1-x/ mixed-oxide thin films deposited by the spray pyrolysis technique is developed. The sensor response to 3-ppm NO/sub 2/ is studied in the range 50/spl deg/C-350/spl deg/C for three different film compositions. The device is also tested for other harmful gases, such as CO (300 ppm) and CH/sub 4/ (3000 ppm). The sensor response to these reducing gases is different at different temperatures varying from the response typical for the p-type semiconductor to that typical for the n-type semiconductor. Satisfactory response to NO/sub 2/ and dynamic behavior at 230/spl deg/C, as well as low resistivity, are observed for the mixed-oxide film with 30% Cd. The response to interfering gas is poor at working temperature (230/spl deg/C). On the basis of this study, a possible sensing mechanism is proposed.     

Preparation of ferroelectric Ba(Ti/sub 0.85/Sn/sub 0.15/)O/sub 3/ thin films by metal-organic decomposition

Ferroelectric Ba(Ti/sub 0.85/Sn/sub 0.15/)O/sub 3/ (BTS/sub 15/) thin film is newly prepared on the Pt/Ti/SiO/sub 2//Si substrate by metal-organic decomposition. The firing condition is determined by thermogravimetric and differential thermal analysis. The BTS/sub 15/ thin film with a flat surface and uniform thickness is obtained by spin coating in N/sub 2/ atmosphere that avoids moisture. The BTS/sub 15/ film has a perovskite phase and a preferential [110] texture. It is also found that the crystalline structure is cubic at 24/spl deg/C with a lattice constant of 4.01 /spl Aring/, and a grain size of about 30 nm was estimated by Scherrer equation and SEM image. From P-E hysteresis loop at 20/spl deg/C, the polarization at E=0 and the electric field at P=0 are found to be 1.07 /spl mu/C/cm/sup 2/ and 24.0 kV/cm, respectively. It is observed that the dielectric constant decreases monotonously from about 830 to 630 with increasing temperature ranging from 20/spl deg/C to 50/spl deg/C. Finally, it is found that the BTS/sub 15/ thin film shows a sufficient ferroelectricity and is an attractive material for functional ferroelectric devices, such as thermal-type infrared sensors.     

CO-sensing properties of In/sub 2/O/sub 3/-doped SnO/sub 2/ thick-film sensors: effect of doping concentration and grain size

In/sub 2/O/sub 3/-doped SnO/sub 2/ nanoparticles were prepared using sol-gel technique from 0.1-M solutions of both stannic chloride (SnCl/sub 4/ 5H/sub 2/O) and indium nitrate. The doping concentration was varied from 7.718/spl times/10/sup -5/ to 3.859/spl times/10/sup -4/ moles. The average particle size, as measured from XRD, SEM, and TEM analyses, varies from 34-130 nm as a result of powder calcination at different temperatures ranging from 300/spl deg/C-900/spl deg/C. Thick-film samples with a thickness of /spl sim/15 /spl mu/m, were tested for low concentration (15-1000 ppm) of CO in air ambient. The optimal temperature for CO sensing is found to be 220/spl deg/C-240/spl deg/C. A blue shift in the sensing temperature and increase in sensitivity factor (S/sub f/) is observed with increasing doping concentration of indium oxide. Maximum sensitivity factor of /spl sim/5 is found for the highest doping concentration (3.859/spl times/10/sup -4/ moles) at 1000 ppm of CO concentration. The morphological and elemental studies of the film are carried out using SEM, TEM, XRD, and EDAX techniques. The results are discussed based on elemental analyses and available theories.     

Study of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ solid solution for low-temperature resistive oxygen gas sensors

A noble type of oxygen-sensitive and electrical-conductive material, ZrO/sub 2/-based with /spl alpha/-Fe/sub 2/O/sub 3/ thick-film gas sensor, was investigated for low operating temperature. Amorphous-like solid solutions of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ powders were derived using the high-energy ball milling technique, and their physical and microstructural properties were characterized from DTA, XRD, TEM, and XPS. The oxygen gas-sensing properties of the screen-printed thick-film gas sensors fabricated from such mechanically-alloyed materials were characterized systematically. Very good sensing properties were obtained with a relative resistance value of 82 in 20% oxygen, and at a low operating temperature of 320/spl deg/C. AC impedance spectra and thermally stimulated current were characterized to investigate the conduction properties of the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, in air and nitrogen (carrier gas), respectively. It was found that the Arrhenius plots of /spl sigma/T versus 1000/T have two distinct gradients corresponding to two activation energies in the high and low temperature regions. The transition temperature occurs at about 320/spl deg/C that corresponds to an optimal operating temperature of the gas sensor. It is believed that the high oxygen vacancy concentration present in the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, and the dissociation of the associated oxygen vacancy defect complexes at 320/spl deg/C are the critical factors for the high relative resistance to oxygen gas at low operating temperature.     

Fabrication and characterization of nano-sized SrTiO/sub 3/-based oxygen sensor for near room-temperature operation

Nano-sized SrTiO/sub 3/-based oxygen sensors were fabricated from synthesized SrTiO/sub 3/ and commercial SrTiO/sub 3/ using the high-energy ball milling and the thick-film screen-printing techniques. The particle sizes, microstructural properties, oxygen-sensing properties, and humidity effects of the synthesized nano-sized SrTiO/sub 3/-based oxygen sensors were characterized using X-ray diffraction (XRD), transmission electron microscope, scanning electron microscope (SEM), and gas sensing measurements. Experimental results showed that the particle size of the powders was milled down to be around 27 nm. The effect of different annealing temperatures (400/spl deg/C, 500/spl deg/C, 600/spl deg/C, 700/spl deg/C, and 800/spl deg/C) on the gas sensing properties of the synthesized SrTiO/sub 3/ sensor from nitrogen to 20% oxygen was characterized. The commercial SrTiO/sub 3/ devices annealed at 400/spl deg/C, both with 0-h and 120-h milling time, were used for comparison. The optimal relative resistance (R/sub nitrogen//R/sub 20%oxygen/) value of 6.35 is obtained for the synthesized SrTiO/sub 3/ sample annealed at 400/spl deg/C and operating at 40/spl deg/C. This operating temperature is much lower than that of conventional metal oxide semiconducting oxygen gas sensors (300/spl deg/C-500/spl deg/C) and SrTiO/sub 3/ oxygen gas sensors (>700/spl deg/C). The response and recovery times are 1.6 and 5 min, respectively. The detected range is 1-20% oxygen. The impedance of the synthesized SrTiO/sub 3/ sensor with annealing at 400/spl deg/C and operating at 40/spl deg/C (from 1 mHz to 10 MHz) in 20% oxygen ambient was found to be independent of the relative humidity (dry, 20% RH, 80% RH, near 100% RH).     

Ultrahigh-sensitive tin-oxide microsensors for H/sub 2/S detection

Ultrahigh-sensitivity SnO/sub 2/-CuO sensors were fabricated on Si(100) substrates for detection of low concentrations of hydrogen sulfide. The sensing material was spin coated over platinum electrodes with a thickness of 300 nm applying a sol-gel process. The SnO/sub 2/-based sensors doped with copper oxide were prepared by adding various amounts of Cu(NO/sub 3/)/sub 2/.3H/sub 2/O to a sol suspension. Conductivity measurements of the sensors annealed at different temperatures have been carried out in dry air and in the presence of 100 ppb to 10-ppm H/sub 2/S. The nanocrystalline SnO/sub 2/-CuO thin films showed excellent sensing characteristics upon exposure to low concentrations of H/sub 2/S below 1 ppm. The 5% CuO-doped sensor having an average grain size of 20 nm exhibits a high sensitivity of 2.15/spl times/10/sup 6/ (R/sub a//R/sub g/) for 10-ppm H/sub 2/S at a temperature of 85/spl deg/C. By raising the operating temperature to 170/spl deg/C, a high sensitivity of /spl sim/10/sup 5/ is measured and response and recovery times drop to less than 2 min and 15 s, respectively. Selectivity of the sensing material was studied toward various concentrations of CO, CH/sub 4/, H/sub 2/, and ethanol. SEM, XRD, and TEM analyses were used to investigate surface morphology and crystallinity of SnO/sub 2/ films.     

The comparative effect of two different annealing temperatures and times on the sensitivity and long-term stability of WO/sub 3/ thin films for detecting NO/sub 2/

We have deposited 150-nm-thick WO/sub 3/ films on Si/sub 3/N/sub 4//Si substrates provided with platinum interdigital electrodes and annealed in static air at 300/spl deg/C and 500/spl deg/C temperatures for 24 h and 200 h. The morphology, crystalline phase, and chemical composition of the films have been characterized using AFM, grazing incidence XRD and high resolution XPS techniques. The sensor resistance response curve has been obtained in the 0.2 -4 ppm NO/sub 2/ gas concentration range in humid air (50% relative humidity), varying the operating temperature between 25 and 250/spl deg/C. By plotting both sensor resistance and gas concentration logarithmically, the response is linear over the investigated dynamic range. Sensor sensitivities, here defined as the ratio of sensor resistance in gas to that in air (i.e., S=R/sub Gas//R/sub Air/), have been compared at a given NO/sub 2/ gas concentration (0.2 ppm). The long-term stability properties have been evaluated by recording film sensitivity for 1 yr under standardized test conditions. Increasing the annealing temperature from 300 to 500/spl deg/C causes the sensitivities to decrease. The 300/24h film is shown to be the most sensitive at S=233, but with poor long-term stability properties. The 300/200h film with S=32 is stable over the examined period. The 500/24 and the 500/200 films are shown to be less sensitive with S=16 and S=14, respectively. The longer the annealing time and the higher the temperature, the poorer the sensitivity, but with positive effects upon the long-term stability of the electrical response. The influence of the annealing conditions on sensitivity and long-term stability has been correlated with the concentration of surface defects, like reduced WO/sub 3/ phase (i.e., W/sup 4+/), which resulted in a strong effect on the sensors' response.     

High-temperature single-crystal 3C-SiC capacitive pressure sensor

Single-crystal 3C-silicon carbide (SiC) capacitive pressure sensors are proposed for high-temperature sensing applications. The prototype device consists of an edge-clamped circular 3C-SiC diaphragm with a radius of 400 /spl mu/m and a thickness of 0.5 /spl mu/m suspended over a 2-/spl mu/m sealed cavity on a silicon substrate. The 3C-SiC film is grown epitaxially on a 100-mm diameter <100> silicon substrate by atmospheric pressure chemical vapor deposition. The fabricated sensor demonstrates a high-temperature sensing capability up to 400/spl deg/C, limited by the test setup. At 400/spl deg/C, the device achieves a linear characteristic response between 1100 and 1760 torr with a sensitivity of 7.7 fF/torr, a linearity of 2.1%, and a hysterisis of 3.7% with a sensing repeatability of 39 torr (52 mbar). A wide range of sensor specifications, such as linear ranges, sensitivities, and capacitance values, can be achieved by choosing the proper device geometrical parameters.     

CMOS monolithic metal-oxide sensor system comprising a microhotplate and associated circuitry

A gas sensor system fabricated in industrial CMOS technology is presented, which includes, for the first time, a microhotplate and the necessary driving and control circuitry on a single chip. Post-complementary-metal-oxide-semiconductor (CMOS) fabrication steps, such as micromachining of the membrane structure, the deposition of noble metal on the electrodes, and the processing of the sensitive metal-oxide layer, have been developed to be fully compatible with the industrial CMOS process. Temperatures up to 350/spl deg/C were reached on the hotplates using a low-voltage power supply (5 V). A symmetric hotplate design with a temperature homogeneity of better than 2% in the heated area was realized. The integrated temperature controller regulates the membrane temperature with a resolution of /spl plusmn/0.3/spl deg/C in the tracking mode. The temperature increase on the bulk chip owing to heat transfer through the membrane is less than 2% of the respective membrane operation temperature (6/spl deg/C at 350/spl deg/C membrane temperature). The gas sensing performance of the sensor was assessed by test measurements with carbon monoxide (CO). The gas tests evidenced a limit of detection of less than 5 ppm CO.     

Nanosized Nb-TiO/sub 2/ gas sensors derived from alkoxides hydrolization

Nanocrystalline TiO/sub 2/ modified with Nb has been produced through the sol-gel technique. Nanopowders have been obtained by means of the hydrolysis of pure alkoxides with deionized water and peptization of the resulting hydrolysate with diluted acid nitric at 100/spl deg/C. The addition of Nb stabilizes the anatase phase to higher temperatures. XRD spectra of the undoped and the Nb-doped samples show that the undoped sample has been almost totally converted to rutile at 600/spl deg/C, meanwhile the doped samples present still a low percentage of rutile phase. Nanocrystalline powders stabilized at 600/spl deg/C with grain sizes of about 17 nm have successfully been synthesized by the addition of Nb with a concentration of 2% at., which appears to be an adequate additive concentration to improve the gas sensor performances, such as it is suggested by the catalytic conversion efficiency experiments performed from FTIR measurements. FTIR absorbance spectra show that catalytic conversion of CO occurs at lower temperatures when niobium is introduced. The electrical response of the films to different concentrations of CO and ethanol has been monitored in dry and wet environments in order to test the influence of humidity in the sensor response. The addition of Nb decreases the working temperature and increases the stability of the layers. Also, large enhancement of the response time is obtained even with lower working temperatures. Moreover, humidity effects on the gas sensor response toward CO and ethanol are less important in Nb-doped samples than in the undoped ones.     

Enhanced catalytic activity of ultrathin CuO islands on SnO/sub 2/ films for fast response H/sub 2/S gas sensors

H/sub 2/S gas-sensing properties of a novel SnO/sub 2/-CuO structure consisting of ultrathin (/spl sim/10 nm) CuO dotted islands (600 /spl mu/m diameter) on 120-nm thick, sputtered SnO/sub 2/ film are compared with a pure SnO/sub 2/ and a SnO/sub 2/-CuO bilayer sensor. The SnO/sub 2/-CuO-dotted sensor exhibited a high sensitivity of 7.3/spl times/10/sup 3/ at a low operating temperature of 150/spl deg/C. A fast response time of 14 s for 20 ppm of H/sub 2/S gas and a recovery time of 118 s under flowing air have been measured. The electronic interaction due to modulation of the space charge regions between the distributed p-type CuO islands on the n-type SnO/sub 2/ thin-film surface and the presence of adsorbed oxygen on the SnO/sub 2/ support have been analyzed. Dissociated hydrogen available from the CuO-H/sub 2/S interaction spills over and its chemical interaction with the adsorbed oxygen on the SnO/sub 2/ surface is found to play a dominant role in the observed fast response characteristics.     

Ceramic temperature sensors for harsh environments

A ceramic thermocouple based on indium-tin-oxide (ITO) thin films is being developed to measure the surface temperature of gas turbine engine components employed in power and propulsion systems that operate at temperatures in excess of 1500/spl deg/C. By fabricating ITO elements with substantially different charge carrier concentrations, it was possible to construct a robust ceramic thermocouple. A thermoelectric power of 6.0 /spl mu/V//spl deg/C, over the temperature range 25-1250/spl deg/C, was realized for an unoptimized ITO ceramic thermocouple. The charge carrier concentration difference in the legs of the ITO thermocouple was established by r.f. sputtering in oxygen-rich and nitrogen-rich plasmas. SEM micrographs revealed that after high-temperature exposure, the surfaces of the nitrogen prepared ITO films exhibited a partially sintered microstructure with a contiguous network of ITO nanoparticles. Thermal cycling of ITO films in various oxygen partial pressures showed that the temperature coefficient of resistance was nearly independent of oxygen partial pressure at temperatures above 800/spl deg/C and eventually became independent of oxygen partial pressure after repeated thermal cycling below 800/spl deg/C. Based on these results, a versatile ceramic sensor system has been envisioned where a ceramic thermocouple and strain sensor can be combined to yield a multifunctional ceramic sensor array.     

A monolithic CMOS microhotplate-based gas sensor system

A monolithic CMOS microhotplate-based conductance-type gas sensor system is described. A bulk micromachining technique is used to create suspended microhotplate structures that serve as sensing film platforms. The thermal properties of the microhotplates include a 1-ms thermal time constant and a 10/spl deg/C/mW thermal efficiency. The polysilicon used for the microhotplate heater exhibits a temperature coefficient of resistance of 1.067/spl times/10/sup -3///spl deg/C. Tin(IV) oxide and titanium(IV) oxide (SnO/sub 2/,TiO/sub 2/) sensing films are grown over postpatterned gold sensing electrodes on the microhotplate using low-pressure chemical vapor deposition (LPCVD). An array of microhotplate gas sensors with different sensing film properties is fabricated by using a different temperature for each microhotplate during the LPCVD film growth process. Interface circuits are designed and implemented monolithically with the array of microhotplate gas sensors. Bipolar transistors are found to be a good choice for the heater drivers, and MOSFET switches are suitable for addressing the sensing films. An on-chip operational amplifier improves the signal-to-noise ratio and produces a robust output signal. Isothermal responses demonstrate the ability of the sensors to detect different gas molecules over a wide range of concentrations including detection below 100 nanomoles/mole.     

Chemical warfare agent detection using MEMS-compatible microsensor arrays

Microsensors have been fabricated consisting of TiO/sub 2/ and SnO/sub 2/ sensing films prepared by chemical vapor deposition (CVD) on microelectromechanical systems array platforms. Response measurements from these devices to the chemical warfare (CW) agents GA (tabun), GB (sarin), and HD (sulfur mustard) at concentrations between 5 nmol/mol (ppb) and 200 ppb in dry air, as well as to CW agent simulants CEES (chloroethyl ethyl sulfide) and DFP (diisopropyl fluorophosphate) between 250 and 3000 ppb, are reported. The microsensors exhibit excellent signal-to-noise and reproducibility. The temperature of each sensor element is independently controlled by embedded microheaters that drive both the CVD process (375/spl deg/C) and sensor operation at elevated temperatures (325/spl deg/C-475/spl deg/C). The concentration-dependent analyte response magnitude is sensitive to conditions under which the sensing films are grown. Sensor stability studies confirm little signal degradation during 14 h of operation. Use of pulsed (200 ms) temperature-programmed sensing over a broad temperature range (20/spl deg/C-480/spl deg/C) enhances analyte selectivity, since the resulting signal trace patterns contain primarily kinetic information that is unique for each agent tested.     

Low-temperature catalyst adding for tin-oxide nanostructure gas sensors

Thick-film gas sensors are successfully fabricated using the nanostructure tin-oxide powder. In order to suppress the coarsening of the nanostructure tin-oxide particles during the adding process, the low-temperature catalyst adding (LTCA) method is proposed in this paper. LTCA is an adding method of noble Pd catalyst onto the nanostructure tin-oxide particles under the lower temperature below 300/spl deg/C by excluding chloride. It turned out that the adding without particle coarsening is successfully carried out by means of LTCA. Applying LTCA to the fabrication of the thick film using nanostructure tin-oxide powder having a size smaller than 5 nm leads to an excellent performance with respect to the methane gas sensing. After aging at 400/spl deg/C, a good sensitivity (R/sub 3500//R/sub 1000/) of 0.66 is obtained for the sensor doped with 5 wt% of Pd catalyst. Also, the sensitivity of the sensor is so stable that the deviation of the electrical resistance is within 3% after 400 h of aging.     

Harsh environments minimally invasive optical sensor using free-space targeted single-crystal silicon carbide

To the best of our knowledge, for the first time, a single-crystal silicon carbide (SiC)-based minimally invasive smart optical sensor suited for harsh environments has been designed and demonstrated. The novel sensor design is based on an agile wavelength source, instantaneous single-wavelength strong two-beam interferometry, full optical power cycle data acquisition, free-space targeted laser beams, multiple single-crystal-thick SiC optical front-end chips, and multiwavelength signal processing for unambiguous temperature measurements to form a fast and distributed smart optical sensor system. Experiments conducted using a 1550-nm eye-safe band-tunable laser and a 300-/spl mu/m coating-free thick SiC chip demonstrate temperature sensing from room temperature to 1000/spl deg/C with an estimated average 1.3/spl deg/C resolution. Applications for the proposed sensor include use in fossil fuel-based power systems, aerospace/aircraft systems, satellite systems, deep-space exploration systems, and drilling and oil mining industries.     

Fabrication of resonant-tunneling diodes by Sb surfactant modified growth of Si films on CaF/sub 2//Si

Molecular beam epitaxy growth of Si thin films on CaF/sub 2//Si(111) substrates has been studied. A surfactant-modified solid-phase epitaxy method, where the room temperature Si deposition was followed by annealing under Sb flux, resulted in a continuous, smooth epitaxial crystalline Si film with a sharp (/spl radic/3/spl times//spl radic/3)R30/spl deg/ reconstruction and a surface roughness of 0.15-nm rms for a 2.8-nm Si thin film. This growth technique was used to fabricate CaF/sub 2//Si/CaF/sub 2/ double-barrier resonant tunneling diodes in SiO/sub 2/ windows patterned on Si(111) substrates. A negative differential resistance (NDR) peak was found at /spl sim/0.35 V at 77 K, and the current density at the NDR peak was estimated to be 3-4 orders of magnitude higher than in earlier reports.     

A Pt/Ga/sub 2/O/sub 3/-ZnO/SiC Schottky diode-based hydrocarbon gas sensor

In this paper, a novel metal-reactive insulator-silicon carbide device with a catalytic layer for hydrocarbon gas-sensing is presented. This structure, employed as a Schottky diode, utilizes sol-gel prepared Ga/sub 2/O/sub 3/-ZnO layer as the reactive insulator. The sensor has been exposed to propene gas, which lowers the barrier height of the diode. The responses were stable and repeatable at operating temperatures between 300 and 600/spl deg/C. The response to propene in different ambients was examined. The effect of diode bias has been investigated by analyzing the sensors response to various propene concentrations when held at constant currents of 2 and 8 mA.     

Iridium oxide as low temperature NO/sub 2/-sensitive material for work function-based gas sensors

This paper presents the results on work function-based NO/sub 2/-sensing properties of iridium-oxide thin films at 130/spl deg/C. Films of 20-nm and 100-nm thickness were deposited on silicon substrates using dc sputtering followed by annealing in oxygen ambient. Sensitivity of these films to different concentrations of NO/sub 2/, H/sub 2/, CO, Cl/sub 2/, and NH/sub 3/ in synthetic air was measured using a Kelvin probe. It was observed that work function of 20-nm-thick iridium-oxide film changed by /spl sim/100 mV on exposure to 5-ppm NO/sub 2/ (German safety limit). Cross sensitivity to other gases (except NH/sub 3/) and interference of humidity was found to be negligibly small. The film was incorporated as a gate electrode in a hybrid suspended gate field effect transistor (HSGFET) structure to examine its suitability in FET-type sensors. The films were characterized using Rutherford backscattering spectroscopy, X-ray diffraction analysis, and scanning electron microscopy to determine their composition, phase, and surface morphology. The results suggest that iridium-oxide film is a promising material for the realization of a FET-based NO/sub 2/ sensor.     

Evanescent wave optical-fiber sensing (temperature, relative humidity, and pH sensors)

Sensitive and versatile evanescent wave-sensing systems featuring polished optical fiber-based sensor designs with low-cost light sources have been developed for temperature, relative humidity, and pH measurements. The work herein contained describes the fabrication of three types of sensors based on standard silica, single-mode fibers previously subjected to a lateral polishing of the cladding. Temperature sensing through oils whose refractive index varied linearly with temperature showed applicability with up to 5 dB//spl deg/C for a 5/spl deg/ range. Polyvinyl alcohol films on the fibers showed almost 10-dB linear variation from 70% to 90% relative humidity. Sol-gel trapped dyes as thin films on the polished surface were capable of performing 15-dB output variation (although not linearly) for pH ranging from 2 to 11.