Understanding Smart Sensors - Nomads.usp

30 Understanding Smart Sensors2.4.3 Particulate ControlOne of the design problems that must be solved in working with structuresseparated by only a few microns is avoidance of contamination. Wafer-levelpackaging is an attractive solution because it provides a low-cost, protective,and safe environment for moving parts that require additional electrical testingand assembly processes. The package protects the device from microscopic particulatesand handling and provides an ambient atmosphere for adjustingdamping. A hermetically sealed accelerometer chip that can be overmolded in alow-cost conventional epoxy package is one example of an approach to avoidingcontamination.The general concept of a sensing die with moving parts that needs protectionfrom the environment is shown in Figure 2.7 [15]. The three polysiliconlayers are sealed inside a protective cavity formed by the silicon substrate, a bulkmicromachined top (cover) wafer, and a glass layer that entirely surrounds thepolysilicon structure. The glass is spaced a distance from the polysilicon structureto avoid the possibility of mechanical interference. The glass serves notonly as the bonding medium, but also as the “mechanical” spacer that providesthe elbow room for the movable structure. A 0.015-inch silicon wafer is usedas the top or cover wafer. The glass is applied to the top wafer, which isthen thermocompression-bonded to the bottom wafer, which contains themicromachined accelerometer structures. The top-wafer design provides a hermeticenvironment, physical protection, and access to the bond pads. Whenbonded, a sealed cavity for controlled squeeze-film damping is achieved.Other techniques to minimize particle contamination use metal can orceramic packaging. In those cases, the final package provides a hermetic environmentfor the structure. Prior to packaging, attention must be given to otherprocessing steps that could allow particles to be trapped in the structure.2.4.4 Combinations of Surface and Bulk MicromachiningAnother combination of surface and bulk micromachining has been reported.An airgap capacitive pressure sensor has been demonstrated that combines bulkand surface micromachining on a single wafer [18]. The structure in Figure 2.8used standard IC processing to create n-channel metal oxide semiconductor(NMOS) circuits with an additional polysilicon layer to produce a capacitorwith a 0.6-mm-thick dielectric. Surface micromachining allowed a smaller gapto be produced. MOS circuitry on the top of the wafer was not exposed to thepressure medium. The inlet for the pressure source and the release for the surfacemicromachined structure were bulk micromachined into the silicon substrateusing a KOH etch. A sensitivity of 0.93 mV/kPa (6.4 mV/psi) was