Understanding Smart Sensors - Nomads.usp

Micromachining 294-inch (100-mm) wafer that is 0.015 inch (0.38 mm) thick [15]. A siliconwafer serves as the substrate for the mechanical structure. The trampolineshapedmiddle layer is suspended by four support arms. That movable structureis the seismic mass for the accelerometer. The upper and lower polysilicon layersare fixed plates for the differential capacitor. The PSG is sacrificially etchedby an isotropic etch, such as hydrofluoric acid (HF).Because of the small spacing (≈2 mm) possible with surface micromachining,new issues arise that affect both the sensor design and the manufacturingprocess. Squeeze-film damping, stiction, and particulate control must beaddressed in each new design. The next three sections describe those areas andsome approaches being used to deal with them.2.4.1 Squeeze-Film DampingThe movement of structures separated by only a few microns can be greatlyaffected by the actual spacing and ambient (gas or vacuum) between the structures.That effect is known as squeeze-film damping. Squeeze-film dampingcan be significant in bulk micromachined capacitive structures, in which closerspacing is needed to achieve higher capacitance values. It is inherent in surfacemicromachining, in which spacing is only a few microns. For a particularstructure, the gas that separates the layers has a viscous damping constantthat increases with the inverse cube of the spacing [16]. Incorporating holesin the surface micromachined structure allows the damping to be tuned fordesired characteristics. Holes also provide distributed access for the etchantto reduce etching time and the possibility of overetching portions of thestructure.2.4.2 StictionStiction (static friction) is a phenomenon that occurs in surface micromachiningresulting from capillary (van der Waals) forces generated during the wetetching of the sacrificial layers [17]. Under certain fabrication conditions, themicrostructures can collapse and permanently adhere to the underlying substrate.The failure is catastrophic and must be prevented to achieve highprocessyield and a reliable design. Preventing the top structure from contactingthe bottom structure requires minimizing the forces acting on the device whenthe liquid is removed, or minimizing the attractive forces between the structuresif they contact each other. Techniques used to prevent stiction depend onthe manufacturer, the product design, and the process flow [17]. Many othersolutions for stiction are being developed.