wire electrodes immersed in the test solution and the current which is generated by the reduction/oxidation reaction of the species of interest is measured. The current is proportional to the concentration of the reduc- g/oxidizing species. A selective response is obtained if no other available species has a lower redox potential. Because the selectivity of peroxide over oxygen is poor, some glucose sensing schemes employ a second enzyme called catalase which converts peroxide to oxygen and hydroxyl ions. The latter produces a change in the local pH. As described earlier, an ISE can then be used to convert the ph to a measurable voltage. In this latter example, glucose sensing involves two chemical-to-chemical transductions followed by a chemical-to-electrical transduction mechanism 5 Microsensors Microsensors are sensors that are manufactured using integrated circuit fabrication technologies and/or micro- machining Integrated circuits are fabricated using a series of process steps which are done in batch fashion meaning that thousands of circuits are processed together at the same time in the same way. The patterns which define the components of the circuit are photolithographically transferred from a template to a semiconducting abstrate using a photosensitive organic coating. The coating pattern is then transferred into the substrate or nto a solid-state thin film coating through an etching or deposition process. Each template, called a mask, can contain thousands of identical sets of patterns, with each set representing a circuit. This"batch"method of manufacturing is what makes integrated circuits so reproducible and inexpensive. In addition, photoreduction enables one to make extremely small features, on the order of microns, which is why this collection of process steps is referred to as microfabrication. The resulting integrated circuit is contained in only the top few microns of the semiconductor substrate and the submicron thin films on its surface. Hence, integrated is said to consist of a set of planar, microfabrication processes. Micromachining refers to the set of processes which produce three-dimensional microstructures using the same photolithographic techniques and batch rocessing as for integrated circuits. Here, the third dimension refers to the height above the substrate of the deposited layer or the depth into the substrate of an etched structure Micromachining produces third dimen sions in the range of 1-500 um(typically). The use of microfabrication to manufacture sensors produces the same benefits as it does for circuits: low cost per sensor, small size, and highly reproducible behavior. It also enables the integration of signal conditioning, compensation circuits and actuators, i. e entire sensing and control systems, which can dramatica iprove sensor performance for very little increase in cost. For these reasons, there is a great deal of research and development activity in microsensors The first microsensors were integrated circuit components, such as semiconductor resistors and p-n junction diodes. The piezoresistivity of semiconductors and optical sensing by the photodiode were already discussed Diodes are also used as temperature-sensing devices. When forward-biased with a constant diode current, the resulting diode voltage increases approximately linearly with increasing temperature. The first micromachined microsensor to be commercially produced was the silicon pressure sensor. It was invented in the mid-to-late 1950s at Bell Labs and commercialized in the 1960s. This device contains a thin silicon diaphragm(10 um) which is produced by chemical etching. The diaphragm deforms in response to a pressure difference across it (Fig. 56.3). The deformation produces two effects: a position-dependent displacement which is maximum at ne diaphragm center and position-dependent strain which is maximum near phragm edge. Both of these effects have been used in microsensors to produce an electrical output which is proportional to differential sensor. The strain is sensed in another by placing a piezoresistor, fabricated in the same silicon substrate, along one edge of the diaphragm. The two leads of the piezoresistor are connected to a wheatstone bridge. The latter type of sensor is called a piezoresistive pressure sensor and is the commercially more common type of pressure microsensor. Pressure microsensors constituted about 5% of the total U.S. consumption of pressure sensors in 1991. Most of them are used in the medical industry as disposables due to their low cost and small, rugged construction. Many other types of microsensors are commercially under development, including accelerome ters, mass flow rate sensors, and biosens c 2000 by CRC Press LLC© 2000 by CRC Press LLC wire electrodes immersed in the test solution and the current which is generated by the reduction/oxidation reaction of the species of interest is measured. The current is proportional to the concentration of the reduc￾ing/oxidizing species. A selective response is obtained if no other available species has a lower redox potential. Because the selectivity of peroxide over oxygen is poor, some glucose sensing schemes employ a second enzyme called catalase which converts peroxide to oxygen and hydroxyl ions. The latter produces a change in the local pH. As described earlier, an ISE can then be used to convert the pH to a measurable voltage. In this latter example, glucose sensing involves two chemical-to-chemical transductions followed by a chemical-to-electrical transduction mechanism. 56.5 Microsensors Microsensors are sensors that are manufactured using integrated circuit fabrication technologies and/or micro￾machining. Integrated circuits are fabricated using a series of process steps which are done in batch fashion, meaning that thousands of circuits are processed together at the same time in the same way. The patterns which define the components of the circuit are photolithographically transferred from a template to a semiconducting substrate using a photosensitive organic coating. The coating pattern is then transferred into the substrate or into a solid-state thin film coating through an etching or deposition process. Each template, called a mask, can contain thousands of identical sets of patterns, with each set representing a circuit. This “batch” method of manufacturing is what makes integrated circuits so reproducible and inexpensive. In addition, photoreduction enables one to make extremely small features, on the order of microns, which is why this collection of process steps is referred to as microfabrication. The resulting integrated circuit is contained in only the top few microns of the semiconductor substrate and the submicron thin films on its surface. Hence, integrated circuit technology is said to consist of a set of planar, microfabrication processes. Micromachining refers to the set of processes which produce three-dimensional microstructures using the same photolithographic techniques and batch processing as for integrated circuits. Here, the third dimension refers to the height above the substrate of the deposited layer or the depth into the substrate of an etched structure. Micromachining produces third dimen￾sions in the range of 1–500 mm (typically). The use of microfabrication to manufacture sensors produces the same benefits as it does for circuits: low cost per sensor, small size, and highly reproducible behavior. It also enables the integration of signal conditioning, compensation circuits and actuators, i.e., entire sensing and control systems, which can dramatically improve sensor performance for very little increase in cost. For these reasons, there is a great deal of research and development activity in microsensors. The first microsensors were integrated circuit components, such as semiconductor resistors and p-n junction diodes. The piezoresistivity of semiconductors and optical sensing by the photodiode were already discussed. Diodes are also used as temperature-sensing devices. When forward-biased with a constant diode current, the resulting diode voltage increases approximately linearly with increasing temperature. The first micromachined microsensor to be commercially produced was the silicon pressure sensor. It was invented in the mid-to-late 1950s at Bell Labs and commercialized in the 1960s. This device contains a thin silicon diaphragm (ª10 mm) which is produced by chemical etching. The diaphragm deforms in response to a pressure difference across it (Fig. 56.3). The deformation produces two effects: a position-dependent displacement which is maximum at the diaphragm center and position-dependent strain which is maximum near the diaphragm edge. Both of these effects have been used in microsensors to produce an electrical output which is proportional to differential pressure. The membrane displacement is sensed capacitively as previously described in one type of pressure sensor. The strain is sensed in another by placing a piezoresistor, fabricated in the same silicon substrate, along one edge of the diaphragm. The two leads of the piezoresistor are connected to a Wheatstone bridge. The latter type of sensor is called a piezoresistive pressure sensor and is the commercially more common type of pressure microsensor. Pressure microsensors constituted about 5% of the total U.S. consumption of pressure sensors in 1991. Most of them are used in the medical industry as disposables due to their low cost and small, rugged construction. Many other types of microsensors are commercially under development, including accelerome￾ters, mass flow rate sensors, and biosensors