Understanding Smart Sensors - Nomads.usp

22 Understanding Smart Sensorssecond silicon wafer or silicon to glass is an important aspect of semiconductorsensors. In fact, a clear dependence on wafer bonding as the enabling technologyfor high-volume MEMS has been identified [8]. Four different approachesto wafer bonding are discussed in this chapter.2.3.1 Silicon-on-Silicon BondingA common approach for manufacturing semiconductor pressure sensors uses abulk micromachined diaphragm anisotropically etched into a silicon wafer. Piezoresistivesensing elements diffused or ion implanted into the thin diaphragmare either a four-element Wheatstone bridge or a single element positioned tomaximize the sensitivity to shear stress [9]. Two silicon wafers are often usedto produce the piezoresistive silicon pressure sensor. Figure 2.3 shows atwo-layer silicon-on-silicon pressure sensor [9]. The top wafer is etched until athin square diaphragm approximately 0.001 inch (25.6 mm) in thickness isachieved. The square area and the 54.7-degree angle of the cavity wall areextremely reproducible. In addition to a sealed reference cavity for absolutepressure measurements, the two-layer silicon sensor allows atmospheric or areference pressure to be applied to one side of the sensor by an inlet holemicromachined in the silicon bottom (constraint) wafer. Several methodsare used to attach the top wafer to the bottom, including anodic bonding,glass frit seal, and direct wafer (silicon-to-silicon) bonding or silicon fusionbonding.The sensor shown in Figure 2.3 uses a glass frit or paste to attach the topwafer to the bottom wafer. The paste is applied to the bottom (constraint)wafer, which is then thermocompression-bonded to the top wafer containingthe bulk micromachined pressure sensing structure. The bottom wafer, containingthe glass, provides stress isolation and allows a reference vacuum to besealed inside the combined structure.To extend the pressure capability to very low-pressure readings (≤2inches, or 5 cm, of water) and minimize nonlinearity, several different approachesare being pursued. Those approaches include silicon bosses used asstress concentrators in circular, square, and dual rectangular diaphragms; convolutedsquare diaphragm; and etch stop techniques to control the diaphragmthickness. Diaphragms as thin as 2.5 mm have been used to produce capacitivepressure sensors for 300 mtorr and lower pressure applications [10].Micromachining can be enhanced by using the electronics capability inherentfrom semiconductor manufacturing. That may provide an additional solutionfor low-pressure measurements.