Diffusion

Question 1

What is diffusion?

Answer 1

the transport of particles (atoms, ions, molecules, electrons ..) via a random mixing processes.

Question 2

What is interdiffusion?

Answer 2

in an alloy, atoms tend to migrate from regions of high conc. to regions of low concentration

Question 3

What is self-diffusion?

Answer 3

in an elemental solid, atoms also migrate

Question 4

When does self-diffusion occur?

Answer 4

during crystal growth and also in a process called annealing where a material is heated (increases diffusion and slowly cooled down – annealing helps to reduce defects.

Question 5

What are the two types of diffusion mechanisms?

Answer 5

vacancy diffusion

interstitial diffusion

Question 6

What is vacancy diffusion? (2)

Answer 6

• atoms exchange with vacancies

- applies to substitutional impurities atoms

Question 7

What does the rate of vacancy diffusion depend on?

Answer 7

rate depends on:

• -number of vacancies
• -activation energy to exchange

Question 8

What is interstitial diffusion? (2)

Answer 8

smaller atoms can diffuse between atoms

more rapid than vacancy diffusion

Question 9

What is case hardening in metals?

Answer 9

diffuse carbon atoms into the host iron atoms at the surface.
resulting in the presence of C atoms making iron (steel) harder

Question 10

What is carburisation?

Answer 10

process by which carbon is diffused into the surface of steel in order to increase its hardness.

Question 11

How does carburisation occur?

Answer 11

The carbon forms carbide precipitates (particularly if the steel contains carbide forming elements such as niobium, manganese or molybdenum) which pin dislocations and prevent slip, thus making the material harder.

Question 12

What happens in carburisation when increasing carbon content?

Answer 12

Increased carbon content reduces the toughness of the material. In most applications it is important that the surface of the steel is hard, but the bulk material can remain softer without detriment to the properties of the component. Thus, carbon is often diffused in from the outer surfaces to obtain a material that is hard on the surface but tough in the bulk.

Question 13

What does toughness mean?

Answer 13

toughness is the ability of a material to absorb energy and plastically deform without fracturing

Question 14

What does hardness mean?

Answer 14

Hardness means resistance to scratches and indents

Question 15

What is ion exchange?

Answer 15

Ion exchange is a chemical strengthening process where large ions are “stuffed” into the glass surface, creating a state of compression.
The potassium (K) on the surface exchanges with sodium (Na) in the glass through a diffusion process. The larger K ion creates a compression on the surface of the glass that stops cracks from spreading

Question 16

What is the effect of the compression due to ion exchange on average bond energy?

Answer 16

before compression the bond energy and length is normal

after compression the bond energy is lower and the length is shorter

Question 17

How is diffusion used in doping semiconductors?

For example, Doping Si phosphorous to make it n-type

Answer 17

1. Deposit P rich layers on surface.
[silicon]
2. Heat it.
3. Result: Doped semiconductor
regions.
[silicon]

Question 18

Which diffusion is greater interstitial or substitutional?

Answer 18

D(interstitial) > D(substitutional)

Question 19

What are the steps to make a the p-type Si on the left side of the MOSFET through diffusion?

Answer 19

• Clean silicon surface (very clean)
  • Deposit small amount of Boron (very small) on surface
  • Heat to about 1173K (very hot) for about 30min to increase to
  diffusion coefficient
• Heat to 1373 K (very very hot) to increase diffusion and drive in dopant

Question 20

How do you make the source metal region of a MOSFET?

Answer 20

• Because Cu has a larger electrical conductivity than Al it was decided by the semiconductor industry in the 1997 road map (International Technology Roadmap for Semiconductors) to replace Al with Cu
  • Required to keep on track with Moore’s law – number of MOSFETs on a chip doubles every 18 months
  • Less heat dissipated in the interconnects

Question 21

Why are diffusion barriers required in a MOSFET?

Answer 21

The diffusion barrier was required so that Cu did not diffuse into and the barrier material did not diffuse into the Si