Understanding Smart Sensors - Nomads.usp

42 Understanding Smart SensorsAluminumSealed air cavityPolysiliconZnOSiliconnitrideSilicon substrateToxFigure 2.15 Piezoelectric surface micromachined sensor. (After: [35].)200 Hz to 40 kHz. The dimensions of the sensor range from 50 by 50 mm 2 to250 by 250 mm 2 .Surface micromachining techniques have been applied in a GaAs metalsemiconductorfield effect transistor (MESFET) process to form both sensorsand electronic circuits [36]. The sensors incorporate a 1-mm-thick sputteredZnO capacitor supported by a 2-mm aluminum membrane formed on theGaAs substrate. The piezoelectric pressure sensor is 80 mm by80mm. Circuitsusing 4-mm GaAs MESFETs are fabricated in a planar, direct ion-implantedprocess. The sensors and circuits both operate at temperatures up to 200°C.2.6.3 Films on MicrostructuresThe sensitive layers for chemical sensors can be deposited over a bulkmicromachined structure. As shown in Figure 2.16, one approach to a semiconductorgas sensor uses a thin metal oxide semiconductor deposited over anintegrated heating element [37]. The resistivity of the metal oxide depends onthe reducing or oxidizing environment being sensed. The sensitive area is thermallyinsulated from the silicon substrate to minimize power consumption.Response time of the device is less than 1 second with an operating temperaturerange of 250–400°C. The structure is contained in a chip 3.5 mm by 3.5 mmby 0.3 mm.Polymer films are also being deposited on both surface and bulkmicromachined structures. The films act as a membrane in biosensors. Multiplesites can have a variety of films sensitive to different levels of a particularsubstance or to different substances. These sensors call for a very small samplematerial to provide an analysis, making them ideal for usage on small childrenor any application where only a small sample is available.