Understanding Smart Sensors - Nomads.usp

18 Understanding Smart Sensorstechniques such as surface micromachining have been developed that achieveeven smaller structures. In addition, the processing techniques for surfacemicromachining are more compatible with the CMOS processes used to manufactureintegrated circuits.Silicon has many properties that make it ideal for mechanical structures.As indicated in Table 2.1, it has a modulus of elasticity (Young’s modulus)comparable to steel and a higher yield strength than steel or aluminum [1, 2].Silicon has essentially perfect elasticity, resulting in minimal mechanical hysteresis.(It is, however, a brittle material and will crack when stressed beyond itselastic limits.) Also, silicon’s electrical properties have made it the material ofchoice in most integrated circuits, providing established manufacturing techniquesfor many aspects of micromachined sensors. Micromachined semiconductorsensors take advantage of both the mechanical and electrical propertiesof silicon, but products that fully exploit the combination of the mechanicaland electrical properties are still in their infancy.Table 2.1Properties of Silicon Compared to Other MaterialsProperty 3C-SiC (6H-SiC) GaAs Si DiamondMelting point (°C) Sublimes at 1,825 1,238 1,415 Phase changeMax. operating temp. (°C) 873 (1,240) 460 300 1,100Thermal conductivity 4.9 0.5 1.5 20(W/cm °C)Thermal expansion coeff. 3.8 (4.2) 6.9 2.6 —(*10E-6 °C −1 )Young’s modulus (GPa) 448 75 190 1,035Physical stability Excellent Fair Good FairEnergy gap (eV) 2.2 (2.9) 1.42 1.12 5.5Electron mobility (cm 2 /V s) 1,000 (500) 8,500 1,350 2,200Hole mobility (cm 2 /V s) 40 (50) 400 600 1,600Sat. electron drift vel. 2.5 (2) 2 1 2.7(*10E7 cm/s)Breakdown voltage 3 (4-6) 0 .4 0.3 10(*10E7 cm/s)Dielectric constant 9.7 13.2 11.9 5.5Lattice constant (Å) 4.36 5.65 5.43 —-