Alumina film sensors

Alumina film sensors have been used in various industries such as medicine and foodstuffs. Figure 20-4 (a) shows a schematic view of an alumina film sensor which is formed by the anodic oxidation of an aluminum plate in an acid solution and then the deposition of a Au film as an electrode. The equivalent circuit of this sensor element is shown in Figure 20-4 (b). The resistance and capacitance are changed by the adsorption and desorption of water. This alumina film sensor has fast response times, and even a few ppm of water in the atmosphere can be detected. However, the use of this element is restricted to dry atmospheres because of the hysteresis and the change of sensor characteristics which occur during operation [11]. 

Figure 20-4. Alumina film sensor (a) and its equivalent circuit (b).

Dickey E. C., Varghese O. K., Ong K. G., Gong D. W., Paulose M., and Grimes C. A., Room temperature ammonia and humidity sensing using highly ordered nanoporous alumina films. Sensors, 2, 91-110, 2002. 

Instead of the system silica/silicate also other systems such as titania/titanate, zirconia/zirconate can be used as a reference system [xiv]. The response time of freshly fabricated thick-film sensors based on thin-film /3-alumina is very short (about 15 ms at 650 °C). After several weeks of operating this time increases 10 times (150 ms) [xv]. Solid electrolyte C02 sensors using Ni/carbonate composite as measuring electrode are suited for measuring of C02 in equilibrated water gases [xiv]. Using semiconducting oxides and carbonates like ITO (indium tin oxide) Nasicon-based C02 sensors are able to measure at room temperature [xvi]. 

The thick-film design consists of four layers, to be separately screen printed and fired on a 1 in square alumina substrate (figure 14.9). Commercial formulations were used for electrodes, bridge trimming resistors, and passivation layers. The first attempted sensor layer was a commercial silver/palladium paste modified by the addition of palladium powder. Based on the performance of the first thick-film sensors, DuPont Electronics (Research Triangle Park, NC) specifically formulated a palladium-based thick-film paste for this application. 

For thin-film metallization, a thin metallic film is first deposited onto the surface of the substrate. The deposition can be accomplished by thermal evaporation, electronic-beam- or plasma-assisted sputtering, or ion-beam coating techniques, all standard microelectronic processes. A silicon wafer is the most commonly used substrate for thin-film sensor fabrication. Other substrate materials such as glass, quartz, and alumina can also be used. The adhesion of the thin metallic film to the substrate can be enhanced by using a selected metallic film. For example, the formation of gold film on silicon can be enhanced by first depositing a thin layer of chromium onto the substrate. This procedure is also a common practice in microelectronic processing. However, as noted above, this thin chromium layer may unintentionally participate in the electrode reaction. 

Chen Z et al 1990 Humidity sensors with reachvely evaporated alumina films as... 

Another reported material for thick-film sensors is Sn02 with La compounds (Yoshioka et al., 1991 Kim et al., 2000 Diagne and Lumbreras, 2001 Marsal et /., 2003). In particular, Kim et aL (2000) used alumina substrates with platinum electrodes and a heater printed on the same side of the sensing device, as seen in Fig. 16.7. The sensing material was tin dioxide with a... 

Chatzandroulis S, Tegou E, Goustouridis D, Polymenakos S, Tsoukalas D (2004) Capadtive-type chemical sensors using thin sUicon-polymer bimorph membranes. Sens Actuators B 103 392-396 Chen Z, Jin M-C (1992) An alpha-alumina moisture sensor for relative and absolute humidity measurement In Proceedings of the 27th annual conference of IEEE Industry Application Society, Houston, TX, vol 2, pp 1668-1675 Chen Z, Lu C (2005) Humidity sensors a review of materials and mechanisms. Sens Lett 3(4) 274—295 Chen Z, Jin M-C, Zhen C (1990) Humidity sensors with reactively evaporated Al Oj films as porous dielectrics. Sens Actuators B 2 167-171... 

Khanna VK (2012) Detection mechanisms and physico-chemical models of solid-state humidity sensors. In Korotcenkov G (ed) Chemical sensors simulation and modelling, Vol. 3 Solid state devices. Momentum, New York Khanna VK, Nahar RK (1984) Effect of moisture on the dielectric properties of porous alumina films. Sens Actuators 5 187-198... 

Sensors based on solid electrolytes are also known for C02. The general setup of a thick film sensor is given in Fig. 19.4. As a solid electrolyte sodium ion conductors like B-alumina or Nasicon together with sodium carbonate are used. The electrochemical reactions take place at temperatures between 350 and 550 °C. The electrode reactions of the measuring and reference electrode are... 

Although insulators other than aluminum oxide have been tried, aluminum is still used almost universally because it is easy to evaporate and forms a limiting oxide layer of high uniformity. To be restricted, therefore, to adsorption of molecules on aluminum oxide might seem like a disadvantage of the technique, but aluminum oxide is very important in many technical fields. Many catalysts are supported on alumina in various forms, as are sensors, and in addition the properties of the oxide film on aluminum metal are of the greatest interest in adhesion and protection. 

Detailed studies were conducted by infrared, TPD, XPS but also by more sophisticated techniques such as CP-MAS solid state NMR or EXAFS, on the various steps by which molybdenum can be deposited on alumina supports starting from [Mo(CO)e]. Indeed, thin films of molybdenum or of its oxides have wide application as gas sensors or solar cell catalysts. 

A cross-section schematic drawing of the newly-developed thick film oxygen sensor is shown in Figure 2. The platinum film heater is embedded in the alumina substrate. Electrical resistance of the heater is about 6 ohms at room temperature. 

The multilayer sensor structure consists of cermet and polymer based layers sequentially deposited on a 96% alumina ceramic substrate using a thick film screen printing process. The cermet layers are of ceramic-metal composition which require firing at a temperature of 850°C and the polymer layers are cured at temperatures below 100°C. Layout of this multilayer sensor structure is shown in Figure 1. 

Thin films, to attain enough sensitivity and response time, of oxide materials normally deposited on a substrate are typically used as gas sensors, owing to their surface conductivity variation following surface chemisorption [183,184], Surface adsorption on a Sn02 film deposited on alumina produces a sensitive and selective H2S gas sensor [185]. In addition, a number of perovskite-type compounds are being used as gas sensor materials because of their thermal and chemical stabilities. BaTi03, for example, is used as sensor for C02 [183],... 

Fig. 9 Schematic cross-section of a R thick film electrode on a Na+-p-alumina solid electrolyte of a type III potentiometric COg sensor.

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