Evaporation, thin film sensor fabrication

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. 

Over the last decades, there have been a lot of efforts to fabricate polymer thin film. However, it is difficult to obtain a conducting polymer in thin film or monolayer owing to lack of processabihty and solubility. Among the conducting polymers, PANI thin film has a great potential to apply for chemical sensor due to its sensing ability and conductivity. Similar to PPy, there have been several methods to fabricate PANI thin film LB technique, self-assembly, electropolymerization, evaporation, and plasma-mediated polymerization. 

Thermal evaporation of PANI has been also developed to fabricate PANI thin film by vacuum deposition of PANI powder on the reference electrode [310]. The chemically synthesized PANI powder was formed in pellet type and the pelletized PANI was evaporated on glass substrates at a pressure of 10 mm Hg to form PANI thin film. The pre-cleaned glass substrate was covered uniformly with PANI thin film and this thin film was utihzed as a carbon monoxide sensor. Thus, thermal evaporation method could be used for thin film formation of conducting polymer nanomaterials [311-313]. 

Microfabrication has been the topic of a recent review in which thin-film (<1 pm, based on vacuum evaporation, sputtering or chemical vapor deposition) and thick-film (>10pm, based on screen printing or lamination) technologies are described for the mass production of potentiometric sensors and sensor arrays [80]. Current challenges include the cost of fabrication, especially for thin-film devices, the control of physical dimensions of the sensing elements, the incorporation of liquid reservoirs, and the stability of the integrated reference electrodes. 

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