Gas-to Solid Processing 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 Chapter 6 Selected Materials Processing Technologies

2 As illustrated in Fig , the gas atmosphere can be a mixture of CO and CO 2, with or without an inert gas such as N 2, to cause carburization by the following reaction: 2CO(g)  CO 2 (g) + C(s) [6.3-1] The equilibrium constant for the reaction is as follows [6.3-2] Where P CO2 and P CO are the partial pressures of CO 2 and CO in the gas mixture, respectively. The activity of carbon a C is a function of the carbon concentration w C as follows: a c = f c w c [6.3-3]

3 Where f c is the activity coefficient. The equilibrium constant K P 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 w C depends on both temperature T and parameter K defined by [6.3-5] Figure can be used to find w C from T and K ; the total pressure of the gas mixture is 1 atm. Similar information is also available for carburization by the reaction CH 4 (g)  2H 2 (g) + C(s)[6.3-6]

Semiconductor Device Fabrication The fabrication of silicon devices is illustrated in Fig

Chemical Vapor Deposition Chemical vapor deposition is a widely used process illustrated in Fig 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 tetrachloride. The basic reaction is [6.3-7]

6 The Si single-crystal thin film grows on the substrate with the same lattice structure and orientation as the substrate ； this is, epitaxial growth. Chemicals containing the atoms to be doped in the thin film are introduced in the inlet gas ； examples are phosphine (PH 3 ) for n-type doping and diborane (B 2 H 6 ) for P-type doping. Figure illustrates the formation of a P-doped Si film Thermal Oxidation Thermal oxidation in Si device fabrication is to form a SiO 2 layer (Fig c) that can protect the device surface and/or provide a mask for selective diffusion. Either a dry or a steam oxidation process can be used, as shown by Dry oxidation: Steam oxidation: [6.3-8] [6.3-9]

7 The steam oxidation process is illustrated on Fig As shown in Fig , the growth mechanism of the SiO 2 layer is such that the oxidant, either O 2 (g) or H 2 O(g), diffuses through the layer to the SiO 2 /Si interface and react with Si to form SiO 2.

Thermal Diffusion Thermal diffusion in semiconductor device fabrication consists of two steps: predeposition and drive-in diffusion. In predeposition the wafer is exposed briefly to a dopant-containing gas atmosphere at an elevated temperature so that its surface is saturated with the dopant, as illustrated in Fig a. A Fig ; the furnace temperature ranges from 800 to 1200 o C.The liquid dopant source can be boron tribromide BBr 3 for boron diffusion in silicon. The BBr 3 vapor, which is produced by bubbling an inert carrier gas (e.g., N2) through the liquid source, is allowed to react with oxygen according to the following reaction: 4BBr3(g) + 3O2(g)  2B2O3(g) + 6Br2(g) [6.3-10]

9 The gaseous B2O3 then reacts with silicon as follows: 2B 2 O 3 (g) + 3Si(s)  4B(s) + 3SiO 2 (g) [6.3-11] The boron so produced is incorporated into silicon, whereas the SiO2 forms a thin layer on the surface. The concentration of the dopant at the surface of the wafer is nominally equal to the solubility of the dopant in silicon, which is given in Fig as a function of temperature for several dopants in silicon. After predeposition, extended thermal diffusion can be applied to reduce the surface dopant concentration and push the dopant deeper into the bulk of the substrate. This step, called drive-in diffusion, is illustrated in Figs b

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11 Chapter 9 Mass Transfer in Materials Processing 9.2 One-dimensional mass transfer Surface heat treating: Carburizing The surface of a carbon steel of an initial carbon level w Ai is to be carburized (Section 6.3.1). The steel is heated to the desired temperature in a furnace. At time = 0 the steel is exposed to a gas mixture containing CO2 and CO, which keeps its surface at a constant carbon level w AS throughout carburizing, as illustrated in Fig the carburized layer is much thinner than the steel itself and the latter can thus be considered semiinfinite.

12 We assume that the overall density  and the diffusion coefficient of carbon in steel D A are both constant. Since the steel is stationary and there are no chemical reactions in it, the species continuity equation reduces to the following Eq. [9.2-1] Since  A =  w A and  is constant, this equation becomes [9.2-2] The initial and boundary conditions arew A (x,0) =w Ai [9.2-3] [9.2-4] [9.2-5] w A (0,t) =w AS w A (∞,t) =w Ai The solution is as follows: [9.2-6]

Semiconductor device fabrication: Dopant diffusion Doping by diffusion is usually conducted in two steps: predeposition and drive-in. Let us consider the predeposition of a dopant A into an initially dopant free substrate. Assume that the diffusion coefficient of the dopant D A and the density  are constant, and that the doped layer is much thinner than the substrate, that is, the substrate is seminfinite. Since W Ai =0, from Eq. [9.2-6] [9.2-7] or [9.2-8] Let M be the amount of dopant predeposited per unit area [9.2-9] Substituting Eq.[9.2-8] into [9.2-9] [9.2-10]

14 [9.2-11] using We obtain w A (x,0) =w Ai [9.2-15] [9.2-16] w A (∞,t) =w Ai Let us now consider the drive-in of dopant A. We assume that the depth of diffusion in predeposition is much smaller that that in drive-in, and that the latter is in turn much smaller than the thickness of the substrate. From Eq.[9.2-2] The initial and boundary conditions are [9.2-14] [9.2-13] [9.2-12]

15 And the mass conservation requirement is Where D A is the diffusion coefficient of the dopant at the drive-in temperature and t is the drive-in time. Eq. [9.2-18] describes the concentration profile of dopant A in the substrate. The amount of the dopant predeposited, M, can be determined from Eq. [9.2-13]. [9.2-18] [9.2-17] The solution is listed as follows