Sputtering 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. 

Figure 2 shows a radio frequency magnetron sputtering apparatus for the fabrication of the thin-film sensor shown in Figure 1. The substrate (2) which is engine frictional part is fixed onto the substrate holder (1) in the top of the sputter chamber. The target (3) consists of sputter materials such as AI2O3, Si02 and sensor alloys. The film was deposited under the conditions listed in Table 1.

With the aid of a particular class of materials (thin-film piezoelectrics), incorporation of AW devices and conventional integrated circuit components on the same silicon substrate is in fact possible. Under the proper conditions, a number of piezoelectric materials can be deposited in thin-film form, typically by RF sputtering, and retain their piezoelectric nature. For this to occur, the crystallites that grow during deposition must be predominantly oriented in a single, piezo-electrically active crystallographic direction. Two such materials are 2 0 and AIN the former has been used as an overlayer on Si wafers to fabricate all of the FPW devices studied for sensor applications to date, and also for SAW resonators. Because extremely thin piezoelectric films are readily fabricated, both ZnO and AIN have been used to make bulk resonators that operate at much higher... 

NS-Ti02 materials have been used for ammonia detection [277-280]. Suh etal. [277] proposed a thin film gas sensor planar structure of fabricated with Ti02 to monitor ammonia. They deposited a thin sensitive Ti02 film by a DC reactive magnetron sputtering technique onto a cleaned silicon substrate equipped with interdigitated comb shaped... 

It is noted that all of the semiconductor gas sensors so far in use are of the thick film (or layer) type, prepared through the wet processes discussed above. Thin film type devices, especially those fabricated via physical methods such as sputtering, have frequently shown interesting sensing performances in the short term, but little use is currently made of these devices. 

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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