6.3.2.3.ThermalDiffusion.pptVIP

* 6.3. Gas-to Solid Processing 6.3.1. surface Heat Treating Carburizing is a surface heat treating process in which the carbon content of the surface of a steel is increased, usually to between 0.8 and 1 wt%, by exposure to a gas atmosphere at an elevated temperature, often between 850 and 950°C . Subsequent rapid cooling allows the high-carbon surface layer to transform to martensite, thus producing a hardened surface layer for wear resistance, as shown in the gear in Fig. 6.3-1 Chapter 6 Selected Materials Processing Technologies As illustrated in Fig. 6.3-2, the gas atmosphere can be a mixture of CO and CO2 , with or without an inert gas such as N2, to cause carburization by the following reaction: 2CO(g) ? CO2(g) + C(s) [6.3-1] The equilibrium constant for the reaction is as follows [6.3-2] Where PCO2 and PCO are the partial pressures of CO2 and CO in the gas mixture, respectively. The activity of carbon aC is a function of the carbon concentration wC as follows: ac= fc wc [6.3-3] Where fc is the activity coefficient. The equilibrium constant KP has been determined to be a function of temperature T as follows : [6.3-4] From Eqs. [6.3-2] through [6.3-4], it is seen that the surface carbon concentration wC depends on both temperature T and parameter K defined by [6.3-5] Figure 6.3-3 can be used to find wC from T and K ; the total pressure of the gas mixture is 1 atm. Similar information is also available for carburization by the reaction CH4(g) ? 2H2(g) + C(s) [6.3-6] 6.3.2. Semiconductor Device Fabrication The fabrication of silicon devices is illustrated in Fig. 6.3-4 6.3.2.1 Chemical Vapor Deposition Chemical vapor deposition is a widely used process illustrated in Fig. 6.3-5. The Si wafer, placed on a rotatable graphite susceptor to typically above 1000℃ with an induction heater. The vapor does not deposit on the quartz tube as quartz cannot be induction-heated. The inlet gas is hydrogen containing a controlled concentration of silicon tetr