Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Electron beam evaporated carbon nanotube dispersed SnO2 thin film gas sensor

Carbon nanotube (CNT) is a useful material for gas-sensing applications because of its high surface to volume ratio structure. In this work, multi-wall CNTs are incorporated into tin oxide thin film by means of powder mixing and electron beam evaporation and the enhancement of gas-sensing properties is presented. The CNTs were combined with SnO₂powder with varying concentration in the range of 0.25–5% by weight and electron beam evaporated onto glass substrates. From AFM and TEM characterization, CNT inclusion in SnO₂thin film results in the production of circular cone protrusions of CNT clusters or single tube coated with SnO₂layer. Experimental results indicate that the sensitivity to ethanol of SnO₂thin film increases by the factors of 3 to 7, and the response time and recovery time were reduced by the factors of 2 or more with CNT inclusion. However, if the CNT concentration is too high, the sensitivity is decreased. Moreover, the CNT doped film can operate with good sensitivity and stability at a relatively low temperature of 250–300^∘C. The improved gas-sensing properties should be attributed to the increasing of surface adsorption area of metal oxide produced by CNT protrusion.     

Highly selective hydrogen sensing of Pt-loaded WO3 synthesized by hydrothermal/impregnation methods

Unloaded and 0.25–1.0 wt% Pt-loaded WO₃ nanoparticles were synthesized by hydrothermal method using sodium tungstate dihydrate and sodium chloride as precursors in an acidic condition and impregnated using platinum acetylacetonate. Pt-loaded WO₃ films on an Al₂O₃ substrate with interdigitated Au electrodes were prepared by spin-coating technique. The response of WO₃ sensors with different Pt-loading concentrations was tested towards 0.01–1.0 vol% of H₂ in air as a function of operating temperature (200–350 °C). The 1.0 wt% Pt-loaded WO₃ sensing film showed the highest response of ∼2.16 × 10⁴ to 1.0 vol% H₂ at 250 °C. Therefore, an operating temperature of 250 °C was optimal for H₂ detection. The responses of 1.0 wt% Pt-loaded WO₃ sensing film to other flammable gases, including C₂H₅OH, C₂H₄ and CO, were considerably less, demonstrating Pt-loaded WO₃ sensing film to be highly selective to H₂.     

Acetylene sensor based on Pt/ZnO thick films as prepared by flame spray pyrolysis

ZnO nanoparticles loaded with 0.2-2.0 at.% Pt have been successfully produced in a single step by flame spray pyrolysis (FSP) technique using zinc naphthenate and platinum(II) acetylacetonate, as precursors dissolved in xylene and their acetylene sensing characteristics have been investigated. The particle properties were analyzed by XRD, BET, TEM, SEM and EDS. Under the 5/5 (precursor/oxygen) flame condition, ZnO nanoparticles and nanorods were observed. The crystallite sizes of ZnO spherical and hexagonal particles were found to be ranging from 5 to 20 nm while ZnO nanorods were seen to be 5-20 nm in width and 20-40 nm in length. In addition, very fine Pt nanoparticles with diameter of ∼1 nm were uniformly deposited on the surface of ZnO particles. From gas-sensing characterization, acetylene sensing characteristics of ZnO nanoparticles is significantly improved as Pt content increased from 0 to 2  at.%. The 2 at.% Pt loaded ZnO sensing film showed an optimum C₂H₂ response of ∼836 at 1% acetylene concentration and 300 °C operating temperature. A low detection limit of 50 ppm was obtained at 300 °C operating temperature. In addition, Pt loaded ZnO sensing films exhibited good selectivity towards hydrogen, methane and carbon monoxide.     

The effect of carbon nanotube dispersion on CO gas sensing characteristics of polyaniline gas sensor

Polyaniline is one of the most promising conducting polymers for gas sensing applications due to its relatively high stability and n or p type doping capability. However, the conventionally doped polyaniline still exhibits relatively high resistivity, which causes difficulty in gas sensing measurement. In this work, the effect of carbon nanotube (CNT) dispersion on CO gas sensing characteristics of polyaniline gas sensor is studied. The carbon nanotube was synthesized by Chemical Vapor Deposition (CVD) using acetylene and argon gases at 600 degrees C. The Maleic acid doped Emeradine based polyaniline was synthesized by chemical polymerization of aniline. CNT was then added and dispersed in the solution by ultrasonication and deposited on to interdigitated AI electrode by solvent casting. The sensors were tested for CO sensing at room temperature with CO concentrations in the range of 100-1000 ppm. It was found that the gas sensing characteristics of polyaniline based gas sensor were considerably improved with the inclusion of CNT in polyaniline. The sensitivity was increased and response/recovery times were reduced by more than the factor of 2. The results, therefore, suggest that the inclusion of CNT in MA-doped polyaniline is a promising method for achieving a conductive polymer gas sensor with good sensitivity, fast response, low-concentration detection and room-operating-temperature capability.     

Semiconducting metal oxides as sensors for environmentally hazardous gases

This article extensively reviews the recent development of semiconductor metal oxide gas sensors for environmentally hazardous gases including NO₂, NO, N₂O, H₂S, CO, NH₃, CH₄, SO₂ and CO₂. The gas sensing properties of differently-prepared metal oxides and loaded metal oxides towards nine environmentally hazardous gases have been individually compared and digested. Promising materials for sensitive and selective detection of each hazardous gas have been identified. For instance, unloaded WO₃ nanostructures are the most promising candidates for NO₂ sensing while metal catalyst loaded WO₃ and gold-loaded SnO₂ sensors are among the most effective for NO and N₂O sensing, respectively. Moreover, related gas-sensing mechanisms are comprehensively discussed.     

Symmetrical PolyMUMPs-Based Piezoresistive Microcantilever Sensors With On-Chip Temperature Compensation for Microfluidics Applications

Microelectromechanical systems (MEMS)-based cantilever beam sensors for microfluidics applications with on-chip temperature sensors for temperature drift compensation were developed. The stress induced on gold surface with polysilicon piezoresistive sensing is demonstrated. In principle, adsorption of biochemical species on a functionalized surface of the microfabricated cantilever will cause surface stress and, consequently, cantilever bending. The sensing mechanism relies on the piezoresistive properties of the doped polysilicon wire encapsulated in the beam. The beam is constructed through multiusers MEMS Process (PolyMUMPs) foundry with postprocessing silicon etching. Bending analysis is performed so that the beam tip deflection can be predicted. The piezoresistor designs on the beams were varied, within certain constraints, so that the sensitivity of the sensing technique could be measured by external read-out circuit. The mass detection of 0.0058-0.0110 g is measured by the beam resistor series as a balanced Wheatstone bridge configuration. The voltage output of the bridge is directly proportional to the amount of bending in the MEMS cantilever. The temperature dependency and sensor performance have been characterized in experiments. Compensation by resisters on the substrate significantly reduces the temperature dependence.     

Low-cost and flexible printed graphene–PEDOT:PSS gas sensor for ammonia detection

This work presents a simple, low-cost and practical inkjet-printing technique for fabricating an innovative flexible gas sensor made of graphene–poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film with high uniformity over a large area. An electronic ink prepared by graphene dispersion in PEDOT:PSS conducting polymer solution is inkjet-printed on a transparency substrate with prefabricated electrodes and investigated for ammonia (NH₃) detection at room temperature. Transmission electron microscopy, Fourier transform infrared spectroscopy, UV–visible spectrometer and Raman characterizations confirm the presence of few-layer graphene in PEDOT:PSS polymer matrix and the present of π–π interactions between graphene and PEDOT:PSS. The ink-jet printed graphene–PEDOT:PSS gas sensor exhibits high response and high selectivity to NH₃ in a low concentration range of 25–1000 ppm at room temperature. The attained gas-sensing performance may be attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH₃ molecules via π electrons network. The NH₃-sensing mechanisms of the flexible printed gas sensor based on chemisorbed oxygen interactions, direct charge transfers and swelling process are highlighted.     

An electronic nose for amine detection based on polymer/SWNT-COOH nanocomposite

An electronic nose (e-nose) system based on polymer/carboxylic-functionalized single-walled carbon nanotubes (SWNT-COOH) was developed for sensing various volatile amines. The SWNT-COOH dispersed in the matrix of different polymers; namely, polyvinyl chloride (PVC), cumene terminated polystyrene-co-maleic anhydride (cumene-PSMA), poly(styrenecomaleic acid) partial isobutyl/methyl mixed ester (PSE), and polyvinylpyrrolidon (PVP), were deposited on interdigitated gold electrodes to make the gas sensors. The response of these sensors to volatile amines was studied by both static and dynamic flow measurements. It was found that all sensors exhibited behaviors corresponding to Plateau-Bretano-Stevens law (R2 = 0.81 to 0.99) as the response to volatile amines. Real-world application was demonstrated by applying this e-nose to monitor the odor of sun-dried snakeskin gourami that was pre-processed by salting-preservation. This electronic nose can discriminate sun-dried fish odors with different stored days using a simple pattern recognition based on the principal component analysis (PCA).     

A high-performance,disposable screen-printed carbon electrode modified with multi-walled carbon nanotubes/graphene for ultratrace level electrochemical sensors

Journal of Applied Electrochemistry - In this work, a screen-printed carbon paste electrode (SPCE) combined with multi-walled carbon nanotubes (MCNT) and graphene (GP) in different mixing ratios...