Understanding Smart Sensors - Nomads.usp

230 Understanding Smart Sensors• Magnetic sensitivity;• Integration level.A passivation layer deposited near the end of the wafer fabrication processprotects the active area of semiconductor devices. Table 10.1 [2] lists the commonterms used to describe that layer. Silicon dioxide and silicon nitride dopedwith boron, phosphorus, or both are two materials used for the passivationprocess. For the semiconductor sensor, the mechanical properties of the layersalso must be taken into account. For example, in silicon pressure sensors,implanted or diffused elements are protected by a passivation layer, but the diaphragmarea is masked to avoid the dissimilar material interface and dampeningeffect that would be caused by the glass layer.For semiconductor components, the package provides protection fromenvironmental factors, which can include moisture and gaseous or liquidchemicals. The packages are further protected in applications like automotiveunderhood-mounted modules by additional epoxy and silica potting compoundsor conformal coatings (e.g., acrylics, polyurethane, silicone, and ultravioletcuring compounds) that cover the printed circuit to which thecomponent is mounted. However, semiconductor sensors frequently have tointerface to the environment. Pressure sensors, for example, that respond tostatic and dynamic pressures must have protection techniques that allow thepressure signal to be transmitted to the force-collecting diaphragm with minimaldamping and distortion of the signal. Distortion can be caused by compressiblefluids.Table 10.1Silicon Wafer Passivation Layer Terms (After: [2])MaterialSiloxVapoxPyroxGlassivation layerPSGBSGPBSG (phospho-borosilicate glass)ParyleneDimethyl-siliconeAssembly LevelWaferWaferWaferWaferWaferWaferWaferPackagePackage