tailieunhanh - Giáo trình môn học công nghệ vi điện tử 9

Việc xử lý bề mặt sẽ giúp chúng ta loại bỏ những tạp vô cơ, hữu cơ hoặc sai hỏng trên bề mặt tấm silicon trước khi chuyển nó vào những bước công nghệ tiếp theo. | For the NMOS process the starting point is a polished p-type silicon disc. The thickness of the disc is typically around 500 ịx. A layer of SiO2 in the neighborhood of 1000 Ả thick is first added to the entire wafer using the oxidation process. On top of this a layer of SÌ3N4 about 1500 Â thick is applied by the CVD process. Following the application of a layer of photoresist Mask 1 is used to pattern the surface. Mask 1 which is often called the moat or n diffusion mask defines in photoresist the drain source and channel regions of all transistors as well as any other regions where n implants are desired. After exposure development removes the photoresist layer in areas that are not to be moat . the complement of the moat or the antimoaf A top Mask 1 pattern and cross-sectional view at this stage of what will be the two transistors appear in Fig. . The SÌ3N4 is then etched from the areas not protected by the photoresist. A high-energy implant of p-type impurities typically boron is then applied to the entire water The remaining photoresist protects the moat regions from this implant. This heavy implant is used to raise the threshold voltage in the antimoat region often called the field and to provide electrical isolation between adjacent devices. After this field implant and a drive in diffusion the remaining photoresist is stripped. A thick layer of S1O2 about 10 000 Ẳ is then thermally grown by the oxidation process over the wafer. This layer is formed in the field but no oxidation can take place in the region protected by the SÌ3N4 because SÌ3N4 does not oxidize. The thick field oxide layer is termed a local oxidation layer and is often called LOCOS. The oxidation consumes some of the substrate silicon. The second cross section in Fig. shows the state of the wafer following growth of the field oxide. This corresponds to Step 10 in the process scenario of Table 2A .1. Following removal of the SÌ3N4 the thin layer of S1O2 under the SỈ3N4 is stripped