Metal additives, conductance sensor materials

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

Recently, organic conducting polymers have become the focus of much of the materials research in chemosensing devices. Synthetic flexibility allows the chemical and physical properties of polymers to be tailored over a broad range of values for any given application. In addition, polymers exhibit tunable specificity to volatile organic compounds, which makes them ideal candidates for replacing canonical sensor materials such as metal oxide semiconductors. 

In addition to the modified electrodes described in the previous sections, which usually involve a conductive substrate and a single film of modifying material, more complicated structures have been described. Typical examples (Figure 14.2.4) include multiple films of different polymers (e.g., bilayer structures), metal films formed on the polymer layer (sandwich structures), multiple conductive substrates under the polymer film (electrode arrays), intermixed films of ionic and electronic conductor (biconductive layers), and polymer layers with porous metal or minigrid supports (solid polymer electrolyte or ion-gate structures) (6,7). These often show different electrochemical properties than the simpler modified electrodes and may be useful in applications such as switches, amplifiers, and sensors. 

Oxides are normally stable at the operating temperatures necessary to enhance the interaction between their surface and the gas phase, much more stable compared to organic materials. They are normally operated between 500 and 800 K where the conduction is electronic and oxygen vacancies are doubly ionized. Different oxides have been proposed for conductometric chemical sensors, the most studied is by far tin dioxide that has also been commercialized in form of thick film sensors. Other oxides studied are titanium oxide, tungsten oxide, zinc oxide, indium oxide and iron oxide, first in form of thick and then in form of thin films. Furthermore, the use of mixed oxides, as well as the addition of noble metals, has been studied to improve not only selectivity but also stability. 

For all-printed thin film transistors (TFT), various organic and inorganic metal electrode materials, such as conductive polymer, carbon nanotube (CNT), organic metal compound, or metal nano-particles, have been used as gate and source/drain electrodes [6-11] in a combination with inkjet- and laser-based printing methods. One of the immediate applications for all-printed TFT would be flexible or rugged display backplane and disposable radio frequency identification (RFID) tags. In addition, printed metal electrodes and TFT have also been used to fabricate passive circuit components, power transmission sheets and sensors for ambient electronics and electronic skin [12-13]. 

The methods for miniaturization of chemical and biosensors are based on an extension of VLSI fabrication techniques, however with a broader range of materials [1-6], The range of materials is beyond what is normal for IC electronic devices because additional functionality is needed. These materials include electrochemi-cally active metals with catalytic properties, conductive oxides, and high-temperature materials. Examples of metal oxides include Sn02, WO3, and Ti02, and other catalytic metals include Pt, Ru, Ir, Pd, and Ag needed for electrochemical sensors [7,8]. As the dimensions of semiconductor devices continue to move to smaller gate lengths, nanoscale fabrication techniques are now developed. Hence, stmctures for sensors... 

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