Understanding Smart Sensors - Nomads.usp

34 Understanding Smart SensorsIon+SputteringNeutralChemicalNeutralIon+Volatile productIon-enhancedenergeticNeutralIon+Volatile productIon-enhancedinhibitorInhibitorFigure 2.10 Different structures from plasma etching. (After: [22].)Manufacturing control of a plasma-etching process addresses etch-ratecontrol, selectivity control, critical-dimension control, profile control, and controlof surface quality and uniformity. The control of these parameters allowsnew structural elements to be achieved. Plasma etching was used in the surfacemicromachined sensor in Figure 2.8. SF 6 was used to etch low-pressure chemicalvapor deposition (LPCVD) silicon nitride and polysilicon; CF 4 was used toetch boro-phospho-silicate glass (BPSG); and CH 4 was used to etch BPSG andthe nitride. Plasma etching was also performed on the aluminum metal [22].Ion-beam milling is a dry-etching process that uses an ion beam toremove material through a sputtering action. It can be used separately or withplasma etching. When it is combined with plasma etching, it is also known asreactive ion etching (RIE).The RIE process has been used for mechanical structures and capacitorsin single-crystal silicon [23]. The RIE process was used to achieve high-aspectratiomechanical structures difficult to achieve using surface micromachiningand wet-etch bulk micromachining. Structures with feature sizes down to