Understanding Smart Sensors - Nomads.usp

The Nature of Semiconductor Sensor Output 673.6.1 ImpedanceMany mechanical transducers have a low impedance. Strain gauges, for example,are typically 350Ω. That impedance has been achieved in semiconductordevices, especially where direct replacement of a mechanical unit is required. Insemiconductor circuits, low impedance is desired for noise purposes. However,high impedance is required to minimize the current draw for portable applications,to use existing interface circuits, and to prevent loading on amplificationstages [20]. For high-impedance pressure sensors, input impedance in the rangeof approximately 5 kΩ is common. Output impedance for those devices is alsoapproximately 5 kΩ. The sensor must be designed to achieve the results thatthe particular system requires.3.7 Analysis of Sensitivity ImprovementLow-pressure measurements (< 0.25 kPa or 0.036 psi) are good examples of theproblems that can occur and the solutions that have been developed to copewith the low-level signal. Low-pressure measurements are limited by existingsilicon sensor designs. The sensitivity (and maximum stress) of a silicon diaphragmis directly related to the area and inversely related to the square of thethickness of the diaphragm. Three different approaches have been pursued toimprove sensitivity.3.7.1 Thin DiaphragmOne approach for increasing the sensitivity for low-pressure measurements isthinning the diaphragm. The typical thickness for silicon diaphragms rangesfrom2to12mm. However, a thin diaphragm can have unacceptable linearity.A number of researchers have investigated a variety of stress concentrators orbosses designed into the diaphragm structure to minimize the nonlinearity.The bosses provide a locally stiffer structure and limit the overall deflectionof the diaphragm. A thinner diaphragm (4 mm), shallow resistor geometriesin the submicron area, and advanced silicon micromachining have beenused in a piezoresistive sensor to achieve a sensitivity of 50 mV/V/psi (7.3mV/V/kPa) [21].3.7.2 Increased Diaphragm AreaA second approach to low sensitivity involves increasing the sensitivity byincreasing the size of the diaphragm. An area increase of over 2.5 times results