Tham khảo tài liệu 'microsensors, mems and smart devices - gardner varadhan and awadelkarim part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | MONOLITHIC PROCESSING 73 conductivity. In addition SOI technology offers extremely low unwanted parasitic effects and excellent isolation between devices see Section . Bipolar Processing The bipolar process has evolved over many years as has its so-called standard process. Clearly this is an important issue and the integration of a microsensor or microactuator will depend on the exact details of the process that is employed. As stated earlier the possible approaches to microsensors and MEMS integration and the problems associated with compliance to a standard process are both discussed in some detail in later chapters. This section presents what may be regarded as the standard bipolar process which employs an epi-layer to make the two most important types of bipolar components that is vertical and lateral transistors. Bipolar n-p-n transistors are the most commonly used components in circuit design as both amplifiers and switches because of their superior characteristics compared with p-n-p transistors. Let us now consider in detail the process steps required to make a vertical n-p-n and lateral p-n-p transistors. A similar process can be defined to make vertical p-n-p transistors or the simpler substrate p-n-p transistors with slightly different device characteristics. Worked Example Vertical and Lateral Bipolar Transistors The standard bipolar process begins by taking a p-type substrate . single-crystal silicon wafer with the topside polished2. A buried n-layer is formed within the p-type substrate by first growing an oxide layer. The oxide is usually grown in an oxidation furnace using either oxygen gas dry oxidation or water vapour wet oxidation at a temperature in the range of 900 to 1300 C. The chemical reactions for these oxidation processes are as follows Heat Si s O2 g SiO2 s Heat Si s 2H2O g SiO2 s 2H2 g Other ways of forming an oxide layer such as CVD are discussed in Chapter 5. The thermal oxide layer is then patterned using a .
