Practice Diffusion Problems

# Practice Diffusion Problems - PRACTICE DIFFUSION PROBLEMS 1...

This preview shows pages 1–2. Sign up to view the full content.

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

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 3

Practice Diffusion Problems - PRACTICE DIFFUSION PROBLEMS 1...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online