Understanding Smart Sensors - Nomads.usp

Micromachining 21Table 2.2Attributes of Bulk Micromachining Etchants (After: [2])Etched Material Etch Rate (nm/min) Possible Etchant Comments silicon 1,250 EDP Anisotropic silicon 1,400 KOH AnisotropicSi 900–1,300 SF 6 IsotropicSi 10,000–300,000 HF IsotropicSiO 2 400–8,600 HF SurfaceCircuitsSiliconsubstrateDeepboron diffusionBoronburied layer1to20mm(a)4to40mmEpi layer(b)Active devicesp-n junctionelectrochemicaletch-stopnp2to40mmEpi layer(c)Figure 2.2 Etch-stop techniques in anisotropic etchants: (a) deep boron diffusion, (b) boronburied layer, and (c) p-n junction.as 15–50 mm, with oxide masking capable of protecting other areas of the chipfor adding circuitry.2.3 Wafer BondingIn addition to micromachining, different types of wafer bonding are neededto produce more complex sensing structures. The attachment of silicon to a