Thick film sensors

Volume 1 Thick Film Sensors (edited by M. Prudenziati)... 

When the development of a scientific field expands, the need for handbooks arises, wherein the information that appeared earlier in journals and conference proceedings is systematically and selectively presented. The sensor and actuator Held is now in this position. For this reason, Elsevier Science took the initiative to develop a series of handbooks with the name "Handbook of Sensors and Actuators" which will contain the most meaningful background material that is important for the sensor and actuator field. Titles like Fundamentals of Transducers, Thick Film Sensors, Magnetic Sensors, Micromachining, Piezoelectric Crystal Sensors, Robot Sensors and Intelligent Sensors will be part of this series. 

The results suggest SSTTXs as new sensor elements for glycoside, pharmaceutical, prostaglandin and steroid sensors. Indeed, a 2 pm thick film sensor (i) provides 0.3 pM detection limits for 9-anthrol (ii) yields a 45 s response time and (iii) is completely reversible (6% relative... 

A. Heilig, N. Birsan, U. Weimar, M. Schweizer-Berberich, J.W. Gardner, and W. Gopel. Gas identification by modulating temperatm-es of Sn02-based thick film sensors . Sensors and Actuators B43 (1997), 45-51. 

S. Hahn. Sn02 thick film sensors at ultimate limits Performance at low O2 and H2O concentrations Size reduction by CMOS technology, Ph.D. thesis. University of Tubingen, Germany (2002). 

Fig. 6.2. Screen-printed (thick-film) sensors for decentralized metal testing. WE working reference RE reference electrode CE counter electrode.

Moreover, a thick-film sensor array suitable for automation combined to readout based on intermittent pulse amperometry (IPA) has been commercialised by Alderon Biosciences [27,28]. These genosensors and the readout instruments provide a simple, accurate and inexpensive platform for patient diagnosis. 

Before dealing with the central topic, I would like to raise some further issues pertinent to it, and indeed to the development of thick-film sensors in general. Thick-film sensors are an important part of biosensor research because some blood glucose sensors for use in the home are made this way—if these are successful surely others can be Further, thick-film technology is not expensive and allows research laboratories to produce quickly, reasonably uniform devices in sufficient numbers for well replicated experiments. At the same time, some insight can be gained into the nature and demands of an industrial production process. 

Developers of thick-film sensors have removed the interference from the sample by chemical oxidation [12,13], by-passed it by selection of applied potentials and electrode materials, including the use of redox mediators [14,15], and kept it from electrode surfaces by inner and outer membranes [16-19]. 

The author of this review was asked if electrochemical thick-film sensors could be used in field determinations of the level of organo-phosphate residues in sheep wool. Wool must be the epitome of matrices where sample preparation is absolutely essential before any progress with analysis using electrochemical biosensors is conceivable. Even with meat, one can imagine that analysis by a sensor of the fluid exuded into the space made by an incision might be possible indeed this has been attempted as noted above [36]. The question as to whether wool could be analysed using a biosensor brought the problems of sensors and difficult samples into sharp and inescapable focus. 

J. Atkinson, Screen printing the art of thick-film sensors. In K.T.V. Grattan (Ed.), Sensors Technology, Systems and Applications, Adam Hilger, Bristol, 1992, pp. 541-546. 

S. H. Hahn, N. Barsan, U. Weimar, S. G. Ejakov, J. H. Visser, and R. E. Soltis, CO sensing with Sn02 thick film sensors, role of oxygen and water vapour, Thin Solid Films 436,17-24 (2003). 

Sharma, R.K. and M.C. Bhatnagar (1999). Improvement of the oxygen gas sensitivity in doped Ti02 thick films. Sensors and Actuators B-Chemical, 56(3), 215-219. 

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]. 

Prudenziati M (ed) (1994) Thick Film Sensors. Elsevier, Amsterdam... 

Schmidt-Zhang. P.. Sandow. K.-P., Adolf, F.. Copel. W. and Guth. U, (2000) A novel thick film sensor for simultaneous O2 and NO monitoring in exhaust gases. Sens. Actuators B, 70, 25-9. 

An interesting electrochemical method for the determination of bound sialic acid has been developed, making use of a potentiometric four-channel thick-film sensor [236]. The sialidase sensor consists of a bilayer of a membrane containing Clostridium perfringens sialidase immobilized in a poly(vinyl acetate)-polyethylene copolymer, which is placed on top of an fT -selective poly(vinyl chloride)-poly(vinyl acetate) indicator membrane. The enzyme-induced release of bound sialic acid leads to a concomitant decrease in pA a of the carboxyl function of sialic acid. This decrease affords a local pH change inside the sialidase-containing sensor membrane, which is monitored by the H -selective indicator membrane. The pH optimum of the sialidase sensor was pH 4 for sialyllactose, mucin and colominic acid. 

Resolution and minimum feature size are particularly superior for thin films. Also the porosity, roughness, and purity of deposited metals is less reproducible with thick films compared to thin films. Finally, the geometric accuracy is poorer with thick films. On the other hand the thick-film method is very versatile and when small size is not too important, but cost in relatively small production volumes is, silk screening forms an excellent alternative. The size limitations clear from table 3.10 make thick-film sensors more appropriate for in vitro applications than in vivo applications. For in vivo applications where size is... 

In comparing different sensor technologies it should be mentioned that 60-80% of sensor fabrication cost consists of packaging costs, an aspect not addressed frequently enough. In comparison with CMOS-compatible sensors, the packaging of thick-film sensors is often easy. Since packaging expenses overshadow all other costs, this is a very important decision criterion. 

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. 

The performance of the thick-film sensor was similar to that of the thin-film model. Although not verified, the response time may actually be faster however, the greater surface area of the thick film may actually slow response because of... 

Figure 14.9 (a) A thin-film hydrogen sensor and (b) a thick-film sensor based upon the same sensing principles. 

Thin gold or aluminum wire, with a diameter of 0.025-0.075 mm, is used to make a connection between the bonding pad and the sensor. This wire bonding is performed using standard microelectronic techniques such as thermal compression or ultrasound bonding. The external lead wire is then bonded onto the bonding pad. Because of the thinness of the metallic film of the pad, the external lead wire connection is usually made by thermal compression. Similar to the connection of the thick-film sensor, the conductive epoxy is first applied to the connecting joint and then covered with insulation epoxy or silicone. 

The gas-sensitive properties of thin and thick film sensors (Figure 1) based on both Fc203-In203 and Fe203-Sn02 (Fe In/Sn 9 1 1 1 1 9) with respect to C2H5OH (100-500 ppm), CH4 (50 ppm), CO (50 ppm) ozone (200 ppb) and NO2 (0.5-5 ppm) were investigated. Besides, electrical conductivity of some layers in air was also estimated. 

Figure 11. Temperature-dependent response of a-Fe203-Sn02 and y-Fe203-Sn02 thick film sensors to 0.05%o of C2H5OH.

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