An Introduction to mems (Micro-electromechanical Systems)
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| An Introduction to MEMS
Prime Faraday Technology Watch – January 2002 38
• Chemicapacitors are similar to chemiresistors but the capacitance of the sensitive layer is measured. Chemicapacitors have found application in sensing humidity.
• Chemomechanical sensors rely on direct chemical-to-mechanical transduction (for example, the expansion of a thin-film polymer in the presence of a substance being analysed).
• Calorimetric sensors measure heat generated by chemical reactions.
i) Work function based sensors This class of sensors, including the ion sensitive field effect transistor (ISFET) and the metal oxide semiconductor field effect transistor (MOSFET), utilizes metal-insulator-semiconductor junctions and the fact that the work function of the material at the interfaces can be chemically modulated. The ISFET was developed as a direct result of the fact that metal- oxide-semiconductor transistors were so sensitive to surface contaminants during their manufacture. As a large proportion of chemical sensors are based on the ISFET, a more detailed description of their operation is outlined.
ISFETs sense the concentration or activity level of a particular ion in solution. The ISFET is a derivative of a common electronic component called a MOSFET. This consists of a silicon semiconductor substrate (doped with impurities to make it p-type) and two electrical contacts (source and drain) doped with impurities so that negatively charged electrons are the main carriers in these small n-type silicon regions. A small distance separates source from drain (Figure 34). Overlaying the substrate between the source and drain is a silicon dioxide insulator which itself is overlaid with a metal electrode called a gate. When a potential is applied to the gate of the MOSFET, the induced electrical field changes the freedom with which the current flows between the source and the drain. In the case of an ISFET however, there is no gate electrode and the insulator is in direct contact with an electrolyte solution to be measured. With the ISFET, electric current flows from the source to the drain via a channel. As in the MOSFET the channel resistance depends on the electric field perpendicular to the direction of the current. Also it depends on the potential difference over the gate oxide. Therefore the source-drain current is influenced by the interface potential at the oxide/aqueous junction. When SiO 2 is used as the insulator, the chemical nature of the interface oxide is reflected in the measured source-drain current. With the selection of other appropriate insulator material, such as silicon nitride or aluminium oxide, hydrogen ions will reside at the surface of the insulator in proportion to the pH. Their positive charge produces an electric field that modulates the current between the source and drain. In order to quantify this effect, the control voltage is measured that must be applied (via a reference electrode) to maintain the drain-source current at a constant value.
The chemical sensitivity of the ISFET is completely controlled by the properties of the electrolyte/insulator interface. One significant problem in the design and fabrication of ISFETs is ensuring that the selective membrane adheres to the device. If the integrity of the membrane is compromised, then the device is useless.
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