Tham khảo tài liệu 'microsensors, mems and smart devices - gardner varadhan and awadelkarim part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | COMBINED IC TECHNOLOGY AND ANISOTROPIC WET ETCHING 163 passivating dielectric films poly-Si layers and metal layers. The poly-Si and metal layers constitute the active layers and are usually sandwiched between the dielectric films that are necessary for electrical insulation and component passivation. By special design windows are opened around the multilayer structures for removal of all dielectric layers thus exposing the silicon surface underneath. In the second so-called postprocessing phase the wafers are immersed in anisotropic silicon etchants. Thus the exposed silicon surface around the multilayer structure is removed and by under-etching the microstructures finally become freestanding. Because the active layers are completely contained within the dielectric layers they are protected against the silicon-etching process. An alternative approach is to etch anisotropically only the backside of the wafer that is use a single-sided etching bath. This technique may be used to make certain structures but tends to be a more time-consuming and therefore a more costly process. A worked example is now provided. Worked Example Integration of Air Gap Capacitor Pressure Sensor and Digital Readout13 Objective To fabricate a microstructure that consists of a top plate separated by a small air gap and a bottom plate that has an inlet for pressurised gas Figure . The gas pressure moves the top plate upward increasing the gap and hence decreasing the capacitance. Metal oxide semiconductor MOS electronics14 are integrated next to the capacitive sensor. Figure Schematic view of an air gap capacitive pressure sensor with integrated digital readout Kung and Lee 1992 13 For details see Kung and Lee 1992 . 14 Standard CMOS process has been described in Section . 164 SILICON MICROMACHINING SURFACE _ n diffusion MOSt-bT Uxidc - - 100 silicon substrate a Silicon nitride b Trench etch Trench etch c Polysilicon d Al-l Si Front-side IC processing completed e .
