Fabric transducers

The two metallizations most commonly used to fabricate transducers on AW devices are gold-on-chromium and aluminum. Au is often chosen for chemical detection applications because of its inertness and resistance to corrosion a layer 100-200 nm diick is necessary to provide adequate electrical conductivity. Unfortunately, the inertness of Au also prevents its adhesion to quartz and other oxides utilized for AW device substrates. Therefore, an underlayer of Cr (2-10 nm thick) is utilized to promote the adhesion of Au to the substrate the electropositive (reactive) nature of Cr allows it to form strcxig bonds with oxide surfaces, while alloying between the Cr and Au chemically binds the two metal layers... 

Figure 17.7 Schematic of test apparatus used to characterize the fabricated transducers.

The micro reactor properties concern process control in the time domain and process refinement in the space domain [65]. As a result, uniform electrical fields are generated and efficiency is thought to be high. Furthermore, electrical potential and currents can be directly measured without needing transducer elements. The reactor fabrication methods for electrical connectors employ the same methods as used for microelectronics which have proven to satisfy mass-fabrication demands. 

A shadow-mask technique has been applied for the local metal deposition to exclude metal residues on other designs processed on the same wafer (Fig. 4.2b). Such metal residues may be caused by imperfections in the patterned resist due to topographical features on the processed CMOS wafers or dust particles. The metal film is only deposited in those areas on the wafer, where it is needed for electrode coverage on the microhotplates. This also renders the lift-off process easier since no closed metal film is formed on the wafer, so that the acetone has a large surface to attack the photoresist. Another advantage of the local metal lift-off process is its full compatibility with the fabrication sequence of chemical sensors based on other transducer principles [20]. 

The thermal resistance of the circular hotplate was measured to be 5.8 °C/mW for the coated and uncoated transducer. An increased thermal resistance is desirable for sensor arrays with one approach being the reduction of the heated membrane area. At the moment the smallest possible diameter of drop-deposited tin-oxide is 100 pm. A microhotplate with a heated area of 100 pm in diameter was fabricated and featured an increased thermal resistance of approximately 10 °C/mW. 

U. Milnch, D. Jaeggi, N. Schneeberger, A. Schaufelbuhl, O. Paid, H. Baltes, and J. Jasper. Industrial fabrication technology for CMOS infrared sensor arrays , Dig. Tech. Papers Transducers 97, Vol. 1, Chicago, IL, USA, (1997), 205-208. 

The characteristics of transducer including the physical properties, electronics, device fabrication, and the specification for the application have recently been reviewed by Sethi (15) and is not covered in this chapter. 

One of the most promising applications of MIPs is the fabrication of chips with microarrays containing several selective polymers against different targets. Fabrication of such platforms will extend the application and the commercial impact of such materials to many different areas where multianalyte capability is a must. Nevertheless, further efforts are still needed to optimize fabrication of localized polymer structures, avoiding problems related to long term stability of the biomi-metic material, MIP deposition, and transducer coupling. 

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