Understanding Smart Sensors - Nomads.usp

MEMS Beyond Sensors 219achieved with emission currents of about 10 nA. The thermionic emitters areused as the first stage on a miniature SEM. The SEM is less than 2 cm 3 in volumeand is formed by stacking five silicon dice.Those recent developments in thermionics require examining terms thatare commonly used for semiconductors. The invention and volume productionof semiconductors or solid-state devices displaced vacuum tubes. However,thermionic devices made using semiconductor processes have now createdgaseous-state microdevices. Volume production of such devices is certain toraise comments if they are referred to as solid-state devices.9.3.8 Field Emission DisplaysA number of field emission displays (FEDs) are being developed usingmicromachining techniques [25]. An FED consists of an array of emittingmicrotips. In one approach, the microtips are formed by etching chambersinside a masked silicon wafer. Molybdenum tips are vacuum deposited in thechamber. Several hundred emitter tips are fabricated for each pixel, allowingdozens of tips to fail without discernible loss of brightness.Another technique to produce the microtips uses selective etching of apolycrystalline silicon substrate. This self-aligned process uses the crystal structureof silicon to produce atomically sharp silicon tips. It may be possible toeliminate lithography in this approach due to the self-aligning nature of theprocess. (The application of FEDs is discussed further in Chapter 11.)9.3.9 Unfoldable MicroelementsMicrostructures that have elastic joints have been used to make movable threedimensionalstructures from planar surface micromachined structures. Microrobotsare among the possibilities for such devices. The basis of the movablestructure is shown in Figure 9.16(a) [26]. Polyimide provides the flexible connectionto polysilicon plates or skeletonlike structures. The PSG is sacrificiallyetched to free the structure for the substrate. Released structures have actuallybeen folded at the hinges, like paper, creating microcubes and insectlike threedimensionalstructures.An SEM of a robotic bug example is shown in Figure 9.16(b) [27]. Thestructure was fabricated using MCNC’s multiuser MEMS process (MUMPS).The bug includes an actuator, electronics, sensors, an IR diode, and a solar cell.The microrobot is made from a silicon chip 1 cm by 0.5 cm by 0.5 mm.A surface-micromachined MEMS hinge designed for foldout structuresuses two structural layers of polysilicon to form both the hinge and the movableplate [28]. The most recent development is an improvement over an earlier