Thin Metal Films in Gas Sensors

Korotcenkov, Handbook of Gas Sensor Materials Properties, Advantages and Shortcomings for Applications Volume 1 Conventional Approaches, Integrated Analytical Systems, 

Schottky barrier, metal oxide semiconductor (MOS), and held-effect transistors (MOSFETs) sensors are other types of gas sensor which involve metal films. These types of sensors were pioneered by Lundstrom et al. (1975). Besides silicon, compound semiconductors such as GaAs, InP, GaN, Ga Oj, and SiC have been alternatively employed as substrate materials for the above-mentioned 

Since the response of a hydrogen sensor is mainly governed by the hydrogen adsorption reaction on the palladium-semiconductor interface, the decrease in film thickness would lead to increased response and recovery rates. It was established that such a device with 10-100-nm thick palladium layer could easily detect 10 ppm of hydrogen in air in the range 27-300 °C (Lundstrom et al. 1975 Choietal. 1986). 

Sensing head Concentration rrmge Response time References 

SMF single-mode fiber, MMF multimode fiber, LPG long-period fiber grating, FBG fiber Bragg grating, SPR surface plasmon resonance 

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Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

Tang Z, Chan PCH, Sharma PK, Yan G, Hsing I-M, Sin JKO (2001) Investigation and control of microcracks in tin oxide gas sensor thin-film. Sens Actuators B Chem 79 39-47 Hemann M (2007) Porous metal oxides as gas sensors. Chem Eur J 13 8376-8388... 

Shim et al. (2011) have shown that conductive metal oxides can also be used as electrode materials in gas sensors. Analyzing the behavior of indium-tin oxide (ITO) and aluminum-doped zinc oxide (AZO) films in WO3- and SnO -based sensors, they established that ITO can replace Pt electrodes in these devices. Upon exposure to 50 ppm CO at 300 °C, WO3 or SnO thin-film sensors with ITO interdigitated electrodes (IDEs) on glass substrates displayed higher responses than sensors with Pt IDEs, attributed to the low-resistance ohmic contacts between the electrode (ITO) and the sensing material (WO3 or SnO ). It is necessary only to take into account that contacts should be stabilized at T 500 °C before using ITO (see Fig. 9.4). 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

Related to the plasmon resonance physics is the micromirror optical sensor for hydrogen (Butler, 1991). Like gold and silver, palladium is a free-electron gas metal in which charge groupings such as phonons or plasmons are likely to occur. As we have seen already, palladium has a natural selectivity due to its sorption of monoatomic hydrogen. In that sensor, the reflectivity of the thin Pd film mirror mounted at the end of cladded optical fiber (Fig. 9.19) is modulated by absorption of hydrogen. 

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