Diffusion Explained!

Diffusion

1. How does diffusion occur?
2. Why is diffusion an important part of processing?
3. How can the rate of diffusion be predicted for some simple cases?
4. How does diffusion depend on structure and temperature?

Definition - Mass transport by atomic motion. Diffusion is a consequence of the constant thermal motion of atoms, molecules, and particles that results in material moving from areas of high to low concentration.

Mechanisms

• Brownian motion is the seemingly random movement of particles suspended in a liquid or gas.
• Solids – vacancy diffusion or interstitial diffusion.
• Atoms in solid materials are in constant motion, rapidly changing positions.
• For an atom to move, 2 conditions must be met:
• There must be an empty adjacent site, and the atom must have sufficient (vibration) energy to break bonds with its neighboring atoms and then cause lattice distortion during the displacement.
• At a specific temperature, only a small fraction of the atoms is capable of motion by diffusion. This fraction increases with rising temperature.
• There are 2 dominant models for metallic diffusion:
• Vacancy Diffusion
• Interstitial Diffusion 

Doping by Diffusion:

• Integrated circuits (ICs), found in numerous electronic devices have been fabricated using doping techniques.
• The base material for these ICs is silicon that has been “doped” with other materials.
• More precisely, controlled concentrations of impurities have been diffused into specific regions of the device to change the properties (improve electrical conductivity).

Application of Diffusion:

• Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by diffusing elements into the material's surface, forming a thin layer of a harder alloy.
• Carbon atoms diffuse into the iron lattice atoms at the surface. This is an example of interstitial diffusion. The C atoms make iron (steel) harder.

 ~Jay Mehta

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