Understanding Smart Sensors - Nomads.usp

222 Understanding Smart Sensorsfreed from the substrate by a KOH etch and precision backsawing, as shown inFigure 9.17(e). Alternatively, back-masked anisotropic etching produces thestructure shown in Figure 9.17(f ). A variety of other materials, including silicondioxide, boron-doped silicon, polysilicon, refractory, and noble metals, canbe used for the nozzle.9.3.11 Interconnects for Stacked WafersWafer-on-wafer construction is being investigated to improve the density ofintegrated circuits. Micromachining with subsequent metalization can providea technique to interconnect a stack of various silicon technologies at the waferlevel. As shown in Figure 9.18 [32], anisotropically etched wafers are alignedwhere the interconnection is required. The pyramid-shaped structure has asquare opening at the top of 25 mm and is 120 mm at the bottom for thinwafers. A fine gold-plated wire mesh is used to fill the cavity. The wire mushroomsat the top and compresses at the bottom to form a gold-on-gold contact.9.3.12 NanoguitarIn case the reader’s imagination has not been sufficiently stimulated by the varietyof structures discussed so far, a final example is the nanoguitar fabricated byresearchers at Cornell University [33]. As shown in Figure 9.19, what has beencalled the world’s smallest guitar is only 10 mm long. The six strings are each50 nm, or 100 atoms, wide. The nanoguitar was micromachined from crystallinesilicon. The key to the technology is electron-beam (E-beam) lithography,Wafer segment 3Wafer segment 2Wafer segment 1PC boardFigure 9.18 Interconnects from pyramid-shaped vias for multiple layer structure. (After: [32].)