SnO2 gas sensor fabricated by low frequency alternating field electrophoretic deposition

SnO₂ thick film gas sensor has been prepared by applying low frequency (0.1 Hz) AC electric fields to a stable suspension of SnO₂ nanoparticles in acetylacetone. Parallel gold electrodes were used as the deposition substrate. Effect of CO, O₂ and H₂ gas exposure as well as ethanol vapor on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic-based gas sensors.     

Study on TiO2-doped ZnO thick film gas sensors enhanced by UV light at room temperature

The gas-sensing properties of titanium oxide (TiO₂)-doped zinc oxide (ZnO) thick film sensor specimens to typical ethanol vapor under UV light activation at room temperature have been investigated. Zinc nanoparticles were mixed with commercial TiO₂ in various weight percentage (0%, 1%, 5%, and 10%) and sintered at 650 °C for 2 h to prepare the thick film sensors. The sensors exhibit better photosensitivity and gas sensitivity to ethanol analyte. The response and recovery times are within 8 s. TiO₂ doping can improve the sensors stability and reproducibility. X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterization of the film materials revealed that Zn₂TiO₄ and TiO₂ phases hindered the rod- or needle-like structure growth and subsequently affected the gas sensitivity. UV absorption spectra of the sensing film material completely dispersed in ethanol solution exhibited that the red shifts were caused with the doping of a small amount of TiO₂ into ZnO then blue shift was caused with higher TiO₂ level. The results of the UV spectra are well consistent with the photosensitive performance. The maximum sensitivity can be achieved by doping the amount of TiO₂ (5 wt%).     

Solid phase epitaxial growth of Dy-germanide films on Ge(0 0 1) substrates

Dy thin films are grown on Ge(0 0 1) substrates by molecular beam deposition at room temperature. Subsequently, the Dy film is annealed at different temperatures for the growth of a Dy-germanide film. Structural, morphological and electrical properties of the Dy-germanide film are investigated by in situ reflection high-energy electron diffraction, and ex situ X-ray diffraction, atomic force microscopy and resistivity measurements. Reflection high-energy electron diffraction patterns and X-ray diffraction spectra show that the room temperature growth of the Dy film is disordered and there is a transition at a temperature of 300-330 °C from a disordered to an epitaxial growth of a Dy-germanide film by solid phase epitaxy. The high quality Dy₃Ge₅ film crystalline structure is formed and identified as an orthorhombic phase with smooth surface in the annealing temperature range of 330-550 °C. But at a temperature of 600 °C, the smooth surface of the Dy₃Ge₅ film changes to a rough surface with a lot of pits due to the reactions further.     

Effects of substrate temperature on electrical and structural properties of copper thin films

This paper addresses the effects of substrate temperature on electrical and structural properties of dc magnetron sputter-deposited copper (Cu) thin films on p-type silicon. Copper films of 80 and 500 nm were deposited from Cu target in argon ambient gas pressure of 3.6 mTorr at different substrate temperatures ranging from room temperature to 250 °C. The electrical and structural properties of the Cu films were investigated by four-point probe and atomic force microscopy. Results from our experiment show that the increase in substrate temperature generally promotes the grain growth of the Cu films of both thicknesses. The RMS roughness as well as the lateral feature size increase with the substrate temperature, which is associated with the increase in the grain size. On the other hand, the resistivity for 80 nm Cu film decreases to less than 5 μΩ-cm at the substrate temperature of 100 °C, and further increase in the substrate temperature has not significantly decreased the film resistivity. For the 500 nm Cu films, the increase in the grain size with the substrate temperature does not conform to the film resistivity for these Cu films, which show no significant change over the substrate temperature range. Possible mechanisms of substrate-temperature-dependent microstructure formation of these Cu films are discussed in this paper, which explain the interrelationship of grain growth and film resistivity with elevated substrate temperature.     

Influence of rapid thermal annealing temperature on structure and electrical properties of high permittivity HfTiO thin film used in MOSFET

HfTiO thin films were prepared by r.f. magnetron co-sputtering on Si substrate. To improve the electrical properties, HfTiO thin films were post heated by rapid thermal annealing (RTA) at 400 °C, 500 °C, 600 °C and 700 °C in nitrogen. It was found that the film is amorphous below 700 °C and at 700 °C monoclinic phase HfO₂ has occurred. With the increase of the annealing temperature, the film becomes denser and the refractive index increases. By electrical measurements, we found at 500 °C annealed condition, the film has the best electrical property with the largest dielectric constant of 44.0 and the lowest leakage current of 1.81 × 10⁻⁷ A/cm², which mainly corresponds to the improved microstructure of HfTiO thin film. Using the film annealed at 500 °C as the replacement of SiO₂ dielectric layer in MOSFET, combining with TiAlN metal electrode, a 10 μm gate-length MOSFET fabricated by three-step photolithography processes. From the transfer (I_DS–V_G) and output (I_DS–V_DS) characteristics, it shows a good transistor performance with a threshold voltage (V_th) of 1.6 V, a maximum drain current (I_ds) of 9 × 10⁻⁴ A, and a maximum transconductance (G_m) of 2.2 × 10⁻⁵ S.     

Fabrication and characterisation of high quality ZnO thin films by reactive electron beam evaporation technique

High quality zinc oxide thin films have been deposited on silicon substrates by reactive e-beam evaporation in an oxygen environment. The effect of the growth temperature and air annealing on the structural, optical and electrical properties has been investigated. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented and that the linewidth of the (002) peak is sensitive to the variation of substrate temperature. The optimum growth temperature has been observed at 300 °C. Raman spectroscopy has been found to be an efficient tool to evaluate the residual stress in the as-grown ZnO films from the position of the E₂ (high) mode. On the other hand, the vanishing of the 574 cm⁻¹. Raman feature after annealing has been explained as due to an increase of grain size and the reduction of O-vacancy and Zn interstitial. The SEM images have shown that the surfaces of the electron beam evaporated ZnO became smoother for the growth temperatures higher than 300 °C. The optical transmittance is the highest at 300 °C and has been increased after annealing in air showing an improvement of the optical quality. Finally, the maximum electrical resistivity has been found at 300 °C, which explains its relation with the crystal quality and increased from 5.8×10⁻² Ω cm to reach an approximate value of 10⁹ Ω cm after annealing at 750 °C.     

Low-temperature carbon monoxide gas sensors based gold/tin dioxide

Tin dioxide nanocrystals were synthesized by a precipitation process and then used as the support for 2 wt.% gold/tin dioxide preparation via a deposition–precipitation method, followed by calcination at 200 °C. Thick films were fabricated from gold/tin dioxide powders, and the sensing behavior for carbon monoxide gas was investigated. The gold/tin dioxide was found to be efficient carbon monoxide gas-sensing materials under low operating temperature (83–210 °C). The Au/SnO₂ sensor with SnO₂ calcined at 300 °C exhibited better CO gas-sensing behavior than the SnO₂ calcined at other temperatures. The experimental results indicated the potential use of Au doped SnO₂ for CO gas sensing.     

Flexible metal-insulator-metal capacitor using plasma enhanced binary hafnium-zirconium-oxide as gate dielectric layer

We have used a sol-gel spin-coating process to fabricate a new metal-insulator-metal capacitor comprising 10-nm thick binary hafnium-zirconium-oxide (Hf_xZr_1−xO₂) film on a flexible polyimide (PI) substrate. The surface morphology of this Hf_xZr_1−xO₂ film was investigated using atomic force microscopy and scanning electron microscopy, which confirmed that continuous and crack-free film growth had occurred on the PI. After oxygen plasma pre-treatment and subsequent annealing at 250 °C, the film on the PI substrate exhibited a low leakage current density of 3.22 × 10⁻⁸ A/cm² at −10 V and maximum capacitance densities of 10.36 fF/μm² at 10 kHz and 9.42 fF/μm² at 1 MHz. The as-deposited sol-gel film was oxidized when employing oxygen plasma at a relatively low temperature (∼250 °C), thereby enhancing the electrical performance.     

Barrier and seed repair performance of thin RuTa films for Cu interconnects

Thin (<4 nm) Physical Vapor Deposited (PVD) Ru-10 at.% Ta films were evaluated as diffusion barriers and seed enhancement layers for Cu metallization in sub 25 nm trenches. The ratio of Ru/Ta on blanket wafers could be influenced by changing the process conditions. However, a difference in Ru/Ta ratio did not influence the thermal stability of the layers during High Temperature X-ray Diffraction (HT-XRD) measurements as all RuTa films exhibited good thermal properties since no Cu-silicide formation was observed for temperatures below 500 °C. The RuTa films also passed an 85 °C/85% relative humidity (RH) test of one week of storage in order to test the H₂O barrier integrity of the films. Furthermore no difference was found when testing the O₂ barrier integrity during 300 s anneals at various temperatures between 250 °C and 500 °C. Good Cu fill of 20 nm trenches (AR 4:1) patterned in oxide was achieved when combining the RuTa films with PVD Cu seed layers with thicknesses ranging from 7 to 20 nm and Cu plating. When compared to a Ta(N)/Ta barrier, relatively high electrical yields (60-80%) were obtained for structures with CDs <30 nm when combining RuTa films with PVD Cu seed layers as thin as 7 nm (on field), hence evidencing the seed enhancement ability of these layers.     

Hydrogen Gas Sensing Based on SnO<Subscript>2</Subscript> Nanostructure Prepared by Sol–Gel Spin Coating Method

A novel H₂ gas sensor based on a SnO₂ nanostructure was operated at room temperature (RT) (25°C). The SnO₂ nanostructure was grown on Al₂O₃ substrates by a sol–gel spin coating method. The structural characteristics, surface morphology, and gas sensing properties of the SnO₂ nanostructure were investigated. Thin film annealing at 500°C produced a high-quality SnO₂ nanostructure with a crystallite size of 33.98 nm. A metal–semiconductor–metal gas sensor was fabricated using the SnO₂ nanostructure and palladium metal. The gas sensor exhibited a sensitivity of 2570% to 1000 ppm H₂ gas at RT. The sensing measurements for H₂ gas at different temperatures (RT to 125°C) were repeatable?for 50 min. Sensor sensitivity was tested under different H₂ concentrations (150 ppm, 250 ppm, 375 ppm, 500 ppm, and 1000 ppm) at different operating temperatures. Adding glycerin to the sol solution increased the porosity of the SnO₂ nanostructure surface, which increased the adsorption/desorption of gas molecules which leads to the high sensitivity of the sensor. Therefore, this H₂ gas sensor is a suitable?portable?RT gas sensor.     

Synthesis and characterization of CuO–SnO2 nanocomposite and its application as liquefied petroleum gas sensor

Present paper reports the synthesis of CuO–SnO₂ nanocomposite via sol–gel route as a sensing material for a liquefied petroleum gas (LPG). X-ray diffraction analysis confirmed the formation of CuO–SnO₂ nanocomposite. Crystallite size was found 5 nm. The optical band gap of the nanocomposite was found 4.1 eV. The thin/thick films were fabricated using spin coating and screen printing technology respectively and investigated with the exposition of LPG at room temperature (25 °C). Surface morphology of the thin film exhibits that it has a number of gas adsorption sites. The sensitivities of the thick and thin film sensors were found 4.1 and 42 respectively. The response and recovery times of the fabricated film sensor were 180 and 200 s respectively. Maximum sensor response of thin film sensor was found 4100. Better sensitivity and percentage sensor response, small response and recovery times, and good reproducibility and stability recognize the fabricated thin film of CuO–SnO₂ as a challenging material for the detection of LPG.     

Reliability and failure analysis of fine copper wire bonds encapsulated with commercial epoxy molding compound

The small outline transistor (SOT) devices which were interconnected with 20 μm copper bonding wire and encapsulated with commercial epoxy molding compound (EMC) have been used in a series of reliability tests which including the thermal shock test, the electrical service life test, and the isothermal aging test. Isolated IMC spots were found at the bonding interface during the thermal shock test. No void or crack was observed even after 1500 cycles thermal shock test. No electrical failure was happened. The isolated IMC spots also occurred at the Cu/Al bonds interface after 500 h electrical operation. After 1000 h electrical operation, the sizes of the IMC spots were about 0.5 μm. No layered IMC was observed. The IMCs were formed at the bonding interface when the aging temperature was between 150 °C and 250 °C. Micro cracks and Kirkendall voids were observed with the aging time of 9 days at 200 °C and the aging time of 9 h at 250 °C. The minor element in the EMC, Sb, has reacted with Cu wire and Cu bond surface at 250 °C when the aging time was more than 16 h. Cu₃Sb was the main product of the diffusion reaction. With the aging time of more than 49 h, the Cu wire was crashed into pieces and the Cu bond periphery has been severely corroded.     

Tin oxide nanosensors for highly sensitive toxic gas detection and their 3D system integration

We present nanosensors based on ultrathin SnO₂ films, which are very sensitive to the highly toxic gases SO₂ and H₂S. The SnO₂-sensing films are fabricated by a spray pyrolysis process on Si substrates with a thickness of 50 nm. The sensor resistance is decreased in the presence of the toxic gases. Exposure to 50 ppm SO₂ leads to a sensor resistance drop of ∼40% whereas a H₂S gas concentration of only 2.5 ppm decreases the resistance by ∼85%, which demonstrates the extraordinary sensitivity of the nanosensors. With respect to further system integration a CMOS technology based micro-hotplate containing heating element and sensing layer has been simulated. Preliminary results show that the micro-hotplates can provide operating temperatures of 400 °C with a power consumption of less than 5 mW. A concept for 3D system integration of the nanosensor chip and a CMOS chip based on Through-Silicon-Via (TSV) technology is proposed as potential roadmap towards smart nanosensor systems for daily life applications.     

Characterization of direct patterned Ag circuits for RF application

We investigated the effects of sintering temperature on microstructural evolution and electrical characteristics of screen printed Ag patterns. A commercial conducting paste containing Ag nanoparticles was screen printed onto a Si substrate passivated with SiO₂ and sintered under a sintering temperature range from 150 °C to 300 °C. Four point probe method was used to measure the DC resistance, while a network analyzer and Cascade’s probe system in the frequency range from 10 MHz to 30 GHz were employed to measure the S-parameters of the sintered Ag conducting patterns. The resistivity under the application of a DC decreased from 398 μΩ cm to 9 μΩ cm with increasing sintering temperature from 150 °C to 300 °C. From the measured S-parameters, the electrical losses in high frequencies also decreased with increasing sintering temperature (about 1.2 dB at 30 GHz) due to the formation of an interparticle neck after heat treatment at high temperatures.     

Improved properties of Al-doped ZnO film by electron beam evaporation technique

High-quality Al-doped ZnO (AZO) thin films have been fabricated by electron beam evaporation technique. The effect of the growth temperature on the optical and electrical properties of the electron-beam (e-beam) evaporated AZO film is investigated. X-ray diffraction measurements have shown that e-beam evaporated films are highly c-axis oriented at appropriate growth temperature. Transmittance measurement showed that the best optical and structural quality of the e-beam evaporated AZO film occurred at 200 °C. The scanning electron microscope images have shown that the surfaces of the e-beam evaporated AZO became smoother for the growth temperature at and above 200 °C. Finally, the maximum electrical resistivity of 2.5×10⁻⁴ Ω cm and optical transmittance of more than 85% has been found at 200 °C growth temperature, which explains its relation with the crystal quality of the film.     

Design of a micromachined thermal accelerometer: thermal simulation and experimental results

This paper describes numerical simulation of a micromachined thermal accelerometer and experimental measurements. The sensor principle consists of a heating resistor, which creates a symmetrical temperature profile, and two temperature detectors symmetrically placed on both sides of the heater. When an acceleration is applied, the free convection is modified, the temperature profile becomes asymmetric and the two detectors measure the differential temperature. This temperature profile and sensor sensitivity according to the distance heater-detector have been studied using numerical resolution of fluid dynamics equations with the commercial code CFD2000/STORM: it shows that the optimum distance between the temperature detectors and the heater is about 300 μm. A thermal accelerometer with 3 pairs of detectors placed at 100, 300 and 500 μm from the heater was manufactured using the techniques of micromachining silicon and experimental measurements have shown a good agreement with the numerical simulations: the experimental optimum distance between heater and detectors seems to be close to 400 μm and the differential temperature of detectors is about 3 °C/g for an operating heater power of 54 mW and an heater temperature rise ΔT of 238 °C. The electrical sensitivity is then 2.5 mV/g.     

Co gas detection by ThO2-Doped SnO2

The selective detection of CO gas by stannic oxide incorporated with ThO2(l − 10 wt%), in the presence of H2 and petroleum gases such as C₃H₈ and iso-C₄H₂ has been studied. Materials mixed with 5 wt% ThO₂ showed high selectivity to CO gas at a sample temperature of 200−250‡C. The effects of hydrophobic or hydrophilic silica present in the samples on the detection sensitivity to CO gas have been investigated. From the results it is apparent that the removal of the hy-droxyl radical from the surface of SnO₂ enhances the sensitivity to CO gas.     

Comparison of PVD, PECVD & PEALD Ru(-C) films as Cu diffusion barriers by means of bias temperature stress measurements

The diffusion barrier properties of PVD Ru and PECVD / PEALD Ru-C films, deposited by RuEtcp₂ precursor and N₂/H₂ plasma, were compared on the basis of bias temperature stress measurements. An MIS test structure was used to distinguish between thermal diffusion induced by annealing and a Cu field drift due to applied electric fields. BTS-CV, TZDB and TDDB measurements revealed that the barrier performance is significantly better for PEALD and PECVD Ru-C films. This improvement is associated with carbon impurities in the Ru films with a concentration in the order of several percent according to ToF-SIMS and ERDA. The TDDB mean time to failure at 250 °C, +5 MV/cm was 7 s for PVD Ru samples, ≈500 s for PECVD Ru-C, ≈800 s for PEALD Ru-C and >3600 s for PVD TaN. Triangular voltage sweep measurements at 300 °C, 0.1 V/s confirmed the presence of Cu ions inside the SiO₂ for degraded dots, in contrast to the Al reference sample and to PVD TaN, which performed best among all the Cu barriers under test. XRD data suggests that PEALD and PECVD Ru-C films are only weakly crystalline.     

Study of Ni-Si thin-film interfacial reactions by coupling differential scanning calorimetry measurements and transmission electron microscopy analyses

In this paper, the solid-state interactions between a 500 nm thick Ni layer and a Si wafer are studied for temperatures up to 500 °C by coupling Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM). The phase transformation temperatures determined by DSC are about 250, 300, 350 and 410 °C. Dedicated samples were prepared to identify phase transformations occurring during heating up to these temperatures. TEM analyses show that the reaction product always consists of a continuous layer so that the nature of phase(s) formed at the interface can be determined. The reaction layer thickness is about 25, 50 and 150 nm for samples heated to 250, 300 and 350 °C, respectively. Moreover, from TEM diffraction patterns, it is shown that, for such a thick layer of Ni deposited on Si substrate, the first phase forming at the Ni/Si interface is the metastable Ni₃Si compound.     

Low temperature synthesis and characterization of NTC powder and its ‘lead free’ thick film thermistors

A general and simple chemical approach has been proposed to prepare negative temperature coefficient (NTC) powder material using commercially available tetra hydrated acetates of Mn, Co, Ni and oxalic acid. The thermal decomposition of Mn-Co-Ni oxalate at 700 °C leads to formation of spinel phases with fine powder material. Lead free thick film thermistor pastes were formulated using the synthesized spinel powders. Planar thick film thermistor patterns of the formulated pastes were screen printed on alumina substrates. The microstructure and electrical properties of these thick film thermistors were determined. Depending on the powder preparation, the prepared thick film NTC thermistors showed room temperature resistance in the range of 12-29 MΩ. The values of thermistor constant, β_25/300 ranged from 4014 to 4223 K.