Understanding Smart Sensors - Nomads.usp

24 Understanding Smart SensorsSiliconelectricalcontactSilicon diaphragm6.5 mmVacuum cavityMetalized electrode0.9 mmHermetic sealPyrex glasssubstrateSolder bumpsFigure 2.4 SCAP sensor with silicon-on-glass (anodic) bonding.the support structure allows additional wet-etching techniques to be performedlater in the process.2.3.3 Silicon Fusion BondingA technique that bonds wafers at the atomic level without polymer adhesives, aglass layer, or an electric field is known as silicon fusion bonding (SFB) ordirect wafer bonding (DWB) [Figure 2.5(a)]. Before bonding, both wafers aretreated in a solution, such as boiling nitric acid or sulfuric peroxide [12]. Thisstep covers the surface of both wafers with a few monolayers of reactivehydroxyl molecules. Initial contact of the wafers holds them together throughstrong surface tension. Subsequent processing at temperatures from 900 to1,100°C drives off the hydroxyl molecules. The remaining oxygen reacts withthe silicon to form silicon dioxide and fuses the two surfaces.Silicon fusion bonding can be used to reduce the size of a micromachinedstructure. As shown in Figure 2.5(b), the anisotropically etched cavity can bemuch smaller, yet the diaphragm area is identical for an SFB pressure sensorcompared to a conventional pressure sensor. The bottom wafer with the anisotropicallyetched cavity is silicon fusion bonded to a top wafer. After bonding,the top wafer is etched back to form a thin diaphragm and the bottomwafer is ground and then polished to open the access to the diaphragm. For