Understanding Smart Sensors - Nomads.usp

Micromachining 25absolute pressure sensors, such as the units in Figure 2.5(b), a support wafer isattached to the structure containing the diaphragm produced by any of the previouslydescribed methods.Table 2.3 is a comparison of the typical process conditions for silicon-tosiliconbonding, anodic bonding, and silicon fusion bonding.2.3.4 Wafer Bonding for More Complex Structures and Adding ICsWafer bonding is being investigated as a means to integrate other materials andto combine micromachined structures with microelectronics. Researchers at theUniversity of California, Berkeley, have used an epoxy bond for attaching a sapphirewafer with a gallium-nitride (GaN) film to a silicon wafer. The sapphirewafer is bonded faced down to the silicon wafer. Short, high-intensity pulsesfrom a laser scanned back and forth across the sapphire wafer separated theGaN from the sapphire. The GaN that remained attached to the silicon waferdid not have irregularities in the film [13].Sandia National Laboratories has grown a GaN film on a nonstandard silicon wafer. A hydrogen implant on the wafer causes small bubblesto form when the wafer is attached to a traditional (high-volume) wafer using silicon-to-silicon bonding. When the wafer is heated during bonding,the small bubbles expand, breaking off a thin layer of silicon. Theresulting silicon on insulator system has GaN that can also integrate CMOScircuits [13].2.4 Surface MicromachiningThe selective etching of multiple layers of deposited thin films, or surfacemicromachining, allows movable microstructures to be fabricated on siliconwafers [14]. With surface micromachining (shown in Figure 2.6), layers ofstructural material, typically polysilicon, and a sacrificial material, such as silicondioxide, are deposited and patterned. The sacrificial material acts as anintermediate spacer layer and is etched away to produce a freestanding structure.Surface micromachining technology allows smaller and more complexstructures with multiple layers to be fabricated on a substrate. However, annealingor special deposition process control is required to reduce stresses in thelayers that can cause warping. In contrast, bulk micromachining typically isstress free.Surface micromachining has been used to manufacture an accelerometerfor automotive air bag applications. A three-layer differential capacitor iscreated by alternate layers of polysilicon and phosphosilicate glass (PSG) on a