Advantages of Sol Gel Process

Sol-gel technique, as a method for synthesizing thin- or thick-film gas sensor, offers several advantages [1, 2]: (i) The sol-gel technique is simpler and cheaper than others, (ii) it allows an easy coating of large and complex shaped substrates (if the angles are not too sharp), (iii) the process is inexpensive with respect to the initial investment of the production plant and the running cost, (iv) it produces no perturbations of devices in the case of deposition on top, (v) it offers easy control of film thickness and porosity, (vi) it can produce ultrafine films, (vii) it is a low-temperature process, (viii) the large surface area can be obtained on both films and powders that enhance the sensing properties,

(ix) it can easily modify the composition with uniformly dispersed dopants and/or modifier that can enhance the sensitivity and the selectivity towards specific gas species, (x) the process is potentially easy to scale up, (xi) the technology is well suited for the production of thin-film sensors because of its relatively low processing cost and the ability to control the film morphology, (xii) it is well suited for the preparation of thin sensitive layers for MOS-based gas sensors, (xiii) it offers easy deposition of thin films by spin- or dip-coating methods, (xiv) sol-gel produced thin films consist of porous interconnected nanosized particles, while PVD and sputtering produce compact films with reduced gas sensitivity, (xv) sol-gel spin-coated SnO2 can be patterned onto the silicon-based sensor structure; these spin-coating and patterning steps are compatible with silicon-based microfabrication processes, (xvi) particle size can be controlled through careful control of the film calcination temperature; good porosity, and improved surface to volume ratio, due to nanosized particles, result in high sensitivity to various reducing gases, (xvii) highly energetic procedures such as RF sputtering and vacuum evaporation deposit SnO2 crystallites in high energy state, which take a long time to stabilize; in contrast to this, the sol-gel process deposits more stable SnO2 thin films on an alumina or silica glass by dip-coating or spin-coating from a SnO2 sol.

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