Practice Diffusion Problems

Practice Diffusion Problems - PRACTICE DIFFUSION PROBLEMS...

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ChE 378 Spring 2008 1 P RACTICE D IFFUSION P ROBLEMS 1. The formation of a silicon oxide (SiO 2 ) thin film on a silicon (Si) wafer surface is an important step in the fabrication of solid-state microelectronic devices. A thin film of SiO 2 serves as a barrier to dopant diffusion or as a dielectric insulator to isolate various devices being formed on the wafer. In one common process, silicon is oxidized by exposure to oxygen (O 2 ) gas a temperatures above 700° C according to the reaction () ( ) ( ) 22 Si s +O g SiO s Molecular O 2 dissolves into the SiO 2 solid, diffuses through the SiO 2 film, and then reacts with Si at the Si/SiO 2 interface as shown below. Assuming that the diffusion of O 2 through the SiO 2 film limits the oxidation process, develop a model to predict the thickness of the SiO 2 layer ( δ ) as a function of time at 1000° C . The density of solid SiO 2 ( ρ B ) is 2.27 gm / cm 3 and the molecular weight of SiO 2 ( M B ) is 60. The mass diffusivity of O 2 in SiO 2 is 2.7·10 -9 cm 2 / s at 1000° C , and the maximum solubility of O 2 in SiO 2 ( c As ) is 9.6·10 -8 gmole O 2 / cm 3 solid at 1000° C and 1 atm O 2 partial pressure. 2. Ammonia (NH 3 ) is being selectively removed from an air-ammonia mixture by absorption into water. In this steady-state process, ammonia is transferred by molecular diffusion through a stagnant gas layer 5 mm thick immediately adjacent to the interface and then through a stagnant water layer 0.1 mm thick. The concentration of ammonia at the outer boundary of the gas layer is 3.42 mol % and the concentration at the lower boundary of the water layer is essentially zero. The temperature of the system is 288 K and the total pressure is 1 atm . Neglecting water evaporation, determine the rate of diffusion of ammonia. Assume that the gas and
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Practice Diffusion Problems - PRACTICE DIFFUSION PROBLEMS...

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