Understanding Smart Sensors - Nomads.usp

32 Understanding Smart Sensorsmeasurements that can be made, or enhance the capabilities of previously discussedapproaches. The newer techniques include the LIGA process, severalmethods of dry etching, micromilling, and laser micromachining.2.5.1 LIGA ProcessOne of the newer micromachining processes is the LIGA (derived from Germanterms for lithography, electroforming, and molding) process, which combinesX-ray lithography, electroforming, and micromolding techniques [19].The LIGA process allows high-aspect-ratio (height/width) structures to befabricated. X-ray–patterned photoresist molds are chemically etched in a metalplate. A polyimide layer a few microns thick acts as a sacrificial layer. A complementarystructure is built up by electrodepositing a metal layer, such as nickel.After the final etching process, portions of the microstructure remain attachedto the substrate and are able to move freely. Temperatures are under 200°C forthe entire process. This process has made tiny 100-mm gears (about the diameterof a human hair). Up to a dozen metal gears have been driven by a low-levelmagnetic field powered micromotor. The LIGA process greatly expandsmicromachining capabilities, making possible vertical cantilevers, coils,microoptical devices, microconnectors, and actuators [14]. However, the LIGAprocess is difficult to integrate with electronics and has a high capital investmentcost compared to bulk and surface micromachining. Table 2.4 comparesother aspects of the three micromachining processes [20].A differential pressure sensor with double-sided overload protection hasbeen fabricated with a modified LIGA process [21]. As shown in Figure 2.9,this design combines isotropic bulk micromachining for the cavity and flowchannels, LIGA processing for the electroplated nickel structure (which is100-mm thick and has a gap of 0.80 mm), and sacrificial etching for the polysilicondiaphragm. The high-aspect-ratio metallic stop limits motion and suppressesdiaphragm stress, as well as facilitates the option of a second signal toverify performance.2.5.2 Dry-Etching ProcessesPlasma etching and reactive ion etching can produce structures that are notpossible from the wet chemical etching processes. Plasma etching is an etchingprocess that uses an etching gas instead of a liquid to chemically etch a structure.Plasma-assisted dry etching is a critical technology for manufacturingultra-large-scale integrated circuits [22]. Plasma-etching processes are dividedinto four classes: sputtering, chemical etching, energetic or damage-driven ion