Silicon wafers, thin film sensors

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. 

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... 

Another variation on solution casting is spin coating. This technique borrows from the methods developed by the semiconductor industry to deposit very thin and uniform layers of photoresist onto silicon wafers. This method has been successfully used in the sensor industry to deposit polymer electrolyte membranes onto silicon-based gas sensors [21]. Some main advantages of spin coating are that very thin and reproducible films can be produced, and that an entire array of sensors can be coated simultaneously using batch fabrication methods. In addition, spin coating equipment is readily available fi"om the semiconductor industry. 

A fabrication and packaging process has been developed for the OSC sensor devices. The OSC material is vacuum deposited on oxide-coated silicon wafers to which gold electrodes are then added. The electrodes extend off the side of the OSC film area onto the bare oxide where a thin titanium layer is used to provide sufficient adhesion for wire bonding. The chips are mounted in ceramic DIP packages using a room temperature cure adhesive and then wire bonded to the chip lead frame. A picture of a typical chip is shown in Fig. 7.12. 

New techniques are being applied to develop miniaturized sensor arrays such as screen printing for thick film and electron beam evaporation, thermal vacuum deposition, and pulsed laser deposition for thin-film technique. Microfabrication techniques were used to prepare a sensor array for use in a voltammetric e-tongue by depositing gold (Au), platinum (Pt), iridium (Ir), and rhodium (Rh) on a silicon wafer. 

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