Thick-film screen process

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. 

Fig 1 Scheme of the screen-printed process a typical thick-film screen consists of a finely woven mesh of stainless steel, nylon or polyester, mounted under tension on a metal frame, normally aluminium. The screen defines the pattern of the printed film and also determines the amount of paste which is deposited. The mesh is coated with a ultraviolet sensitive emulsion (usually a polyvinyl acetate or polyvinyl alcohol sensitized with a dichromate solution) onto which the circuit pattern can be formed photographically. The ink is placed at one side of the screen and a squeegee crosses the screen under pressure, thereby bringing it into contact with the substrate and also forcing the ink through the open areas of the mesh. The required circuit pattern is thus left on the substrate... 

The thick-film metallization process requires modest capital equipment and the process is relatively simple. Typical capital equipment for thick-film metallization includes a silk screening printer and a furnace for annealing and removal of the organic binder. This equipment is relatively modest in cost. In addition to silk screening on a flat substrate, thick-film techniques can be applied by dipping, spraying, or roller coating the non-flat surface [4], Thick-film techniques can also be applied in a continuous operation which can be cost effective. 

In order to address the needs of field sensing of explosives, it is necessary to move away from traditional bulky electrodes and cells (commonly used in research laboratories). The exploitation of advanced microfabrication techniques allows the replacement of conventional ( beaker-type ) electrochemical cells and electrodes with easy-to-use sensor strips. Both thick-film (screen-printing) and thin-film (lithographic) fabrication processes have thus been used for high-volume production of highly reproducible, effective and inexpensive electrochemical sensor strips. Such strips rely on... 

Copper conductors can be deposited and defined in either one of two processes thick-film screen printing and etched thick film. In the former process, thick-film inks are forced through a screen in a definite pattern, dried, and fired. In the latter process, a blanket coat of conductor ink is applied to the ceramic with a screen and fired. It is then etched to the final pattern using classical photolithographic techniques. Line widtirs in tiiis etch process can be as narrow as 0.001 in., whereas the classical screen-printed conductor can only be as narrow as 0.004 in. 

In the etched pattern process as shown in Figure 8.20, thick-film conductor ink is blanket-coated onto a ceramic substrate using standard thick-film screening. After drying and firing, the pattern is etched using a photolithographic process. Line widths as fine as 0.001 in. are made with this process. 

Screen printing — thick-film copper process flow for two conductor layers. 

With the current thick film printing processes, line widths of 80 pm are the limit. The development of new processes is desirable for achieving finer wiring. The reason for the difficulties in achieving miniaturization lies in the principles of the screen printing technique itself in which ink is pressed... 

For the formation of a metallic film in addition to thick film silk-screen technique, thin film metallization is another means for the film deposition. Deposition of thin film can be accomplished by either physical or chemical means, and thin film technology has been extensively used in the microelectronics industry. Physical means is basically a vapor deposition, and there are various methods to carry out physical vapor deposition. In general, the process involves the following 1) the planned deposited metal is physically converted into vapor phase and 2) the metallic vapor is transported at reduced pressure and condensed onto the surface of the substrate. Physical vapor deposition includes thermal evaporation, electronic beam assisted evaporation, ion-beam and plasma sputtering method, and others. The physical depositions follow the steps described above. In essence, the metal is converted into molecules in the vapor phase and then condensed onto the substrate. Consequently, the deposition is based on molecules and is uniform and very smooth. 

However, in the development of chemical and biosensors, new pastes must usually be developed. Polymer thick films can be screen printed on cheap polymer substrates with a thickness anywhere between 5 and 50 /xm. Importantly, no high-temperature steps are involved in the deposition process. The first commercial planar electrochemical glucose sensor (the ExacTech by MediSense) was a screen-printed sensor. A comparison of thick-film deposition against thin-film deposition is shown in table 3.10. 

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