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Micro Electro Mechanical System Design - James J. Allen Part 4

Tham khảo tài liệu 'micro electro mechanical system design - james j. allen part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | MEMS Technologies 71 SĨƠ2 mask silicon b. Isotropic wet etching with agitation a. Isotropic wet etching with no agitation FIGURE 3.3 Wet etching of crystalline silicon. due to the many other variables in a wet-etch process such as temperature chemical agitation purity and concentration. If this is not satisfactory etch stops can be used in wet etching to define a boundary on which the etch can stop. Several etch stops methods can be utilized in wet etching p boron diffusion or implant etch stop Material-selective etch stop Electrochemical etch stop For example the etch rate of boron-doped silicon p-silicon by KOH or EDP can be up to 100 times less than the etch rate in undoped silicon 1-3 . Therefore boron-doped regions produced by diffusion or implantation have been used to form features or as an etch stop Figure 3.4 . A material-selective etch stop can be produced by a thin layer of a material such as silicon nitride which has a greatly reduced etch rate in etchants such as KOH EDP and TMAH Table 2.6 . For example a thin layer silicon nitride can be deposited on a silicon device to form a membrane on which the etch will stop. An electrochemical etch stop can also be used Figure 3.5 . Silicon readily forms a silicon oxide layer that will impede etching of the bulk material Table 2.6 . The formation of the oxide layer is a reduction oxidation reaction which can be impeded by a reversed biased p-n junction that prevents the current flow necessary for the reduction oxidation reaction to occur. The p-n junction can be formed on a p-type silicon wafer with an n-type region diffused or implanted with an n-type dopant e.g. phosphorus or arsenic to a prescribed depth. With the p-n junction reverse biased the p-type silicon will be etched 2005 by Taylor Francis Group LLC 72 Micro Electro Mechanical System Design B B B B B B B B B B B B B B B B B B B B B B B B uiiiiiuiiiiuiuuuii Single Crystal Silicon a. Implant Boron in Single Crystal Silicon wafer SiO2 mas 100 B B B B B B