Understanding Smart Sensors - Nomads.usp

Micromachining 2325.4 micron (0.001") diaphragmReference vacuumsealed in etched cavityGlass sealConstraintwaferFigure 2.3 Pressure sensor with silicon-to-silicon bonding.2.3.2 Silicon-on-Glass (Anodic) BondingElectrostatic, or anodic, bonding is a process used to attach a silicon top waferto a glass substrate and also to attach silicon to silicon. Anodic bonding attachesa silicon wafer, either with or without an oxidized layer, to a borosilicate(Pyrex ® ) glass heated to about 400°C when 500V or more is applied across thestructure [4]. An example of a product manufactured with anodically bondedsilicon to glass is the silicon capacitive absolute pressure (SCAP) sensing elementshown in Figure 2.4 [11].The micromachined silicon diaphragm with controlled cavity depth isanodically bonded to a Pyrex ® glass substrate that is much thicker than thesilicon die. Feed-through holes are drilled in the glass to provide a preciseconnection to the capacitor plates inside the unit. The glass substrate is metallizedusing thin-film deposition techniques, and photolithography defines theelectrode configuration. After attaching the top silicon wafer to the glass substrate,the drilled holes are solder-sealed under vacuum. That provides acapacitive sensing element with an internal vacuum reference and solderbumps for direct mounting to a circuit board or ceramic substrate. The valueof the capacitor changes linearly from approximately 32 to 39 pF with appliedpressure from 17 to 105 kPa. The capacitive element is 6.7 mm by 6.7 mmand has a low temperature coefficient of capacitance (−30 to 80 ppm/°C),good linearity (≈1.4%), fast response time (≈1 ms), and no exposed bondwires.Silicon is also bonded to a second silicon wafer using anodic bonding. Athin (≈4 mm) layer of Pyrex ® glass is sputtered on one of the layers, and a muchlower voltage of approximately 50V is applied [4]. Silicon instead of glass for