ch06-Diffusion

ch06-Diffusion - Chapter 6 - c06cof01 c06cof01 Chapter 6:...

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Unformatted text preview: Chapter 6 - c06cof01 c06cof01 Chapter 6: Diffusion (Spring 2009) Steel gear Case hardened High temperature Carbon diffusion Chapter 6 - 2 ISSUES TO ADDRESS... How does diffusion occur? Why is it an important part of processing? How can the rate of diffusion be predicted for some simple cases? How does diffusion depend on structure and temperature? Chapter 6: Diffusion 6.1 Introduction Chapter 6 - 3 Adapted from chapter-opening photograph, Chapter 6, Callister & Rethwisch 3e. (Courtesy of Surface Division, Midland-Ross.) Case Hardening :-- Diffuse carbon atoms into the host iron atoms at the surface.-- Example of interstitial diffusion is a case hardened gear. Result: The presence of C atoms makes iron (steel) harder. Processing Using Diffusion Chapter 6 - 4 Doping silicon with phosphorus for n-type semiconductors: Process: 3. Result: Doped semiconductor regions. silicon Processing Using Diffusion magnified image of a computer chip 0.5 mm light regions: Si atoms light regions: Al atoms 2. Heat it. 1. Deposit P rich layers on surface. silicon Adapted from chapter-opening photograph, Chapter 12, Callister & Rethwisch 3e. Chapter 6 - 5 Diffusion Diffusion- Mass transport by atomic motion Mechanisms Gases & Liquids random (Brownian) motion Solids vacancy diffusion or interstitial diffusion Chapter 6 - 6 Interdiffusion : In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Adapted from Figs. 6.1 and 6.2, Callister & Rethwisch 3e. Diffusion After some time Chapter 6 - 7 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms Diffusion A B C D After some time A B C D Chapter 6 - c06f09 c06f09 Why aluminum interconnets in silicon ICs? Chapter 6 - c06tf03 c06tf03 Chapter 6 - c06f10 c06f10 Chapter 6 - c06f03 c06f03 6.2 Diffusion Mechanisms Chapter 6 - 12 Diffusion Mechanisms Vacancy Diffusion: atoms exchange with vacancies applies to substitutional impurities atoms rate depends on:-- number of vacancies-- activation energy to exchange. increasing elapsed time Chapter 6 - 13 Simulation of interdiffusion across an interface: Rate of substitutional diffusion depends on:-- vacancy concentration-- frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation QuickTime and a Cinepak decompressor are needed to see this picture. Chapter 6 - 14 Diffusion Mechanisms Interstitial diffusion smaller atoms can diffuse between atoms. More rapid than vacancy diffusion Adapted from Fig. 6.3 (b), Callister & Rethwisch 3e. Chapter 6 - Flux : J = 1 A dM dt kg m 2 s or atoms m 2 s Directional Quantity Flux can be measured for:--vacancies--host (A) atoms--impurity (B) atoms J x J y J z x y z x-direction Unit area A through which atoms move....
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This note was uploaded on 03/08/2010 for the course CHEN 313 taught by Professor Seminario during the Fall '07 term at Texas A&M.

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ch06-Diffusion - Chapter 6 - c06cof01 c06cof01 Chapter 6:...

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