Question

A scientist would like to use thermal diffusion to dope the silicon wafer. He decides to deposit 10 nm Boron on the surface and heat up the wafer to allow Boron to diffuse into the wafer. What technique do you recommend him to use to deposit high purity Boron on the surface? Explain what are the machines he could use and what kind of precursors he needs to use.

Answer 1

Types of diffusion:

1.Empty space diffusion: Even in perfect single crystals there will be empty places which the dopnt can fill (B)

2. Inter lattice diffusion : The impurity atoms move into the void spaces between silicon atoms in the crystal lattice

3. Exchange with silcon atoms: The dopant atoms which occupy lattice positions can exhange with Si atoms

The rate of diffusion depends mainly on the following factors:

1. Nature and quantity of the substrate (Si)

2. Nature and quantity of the dopant (B) - If the source of the dopant is limited (exhaustible source) the dopant concentration on the surface decreases with time, because the dopant atoms on the surface penetrate into the crystal lattice and there are not sufficient dopant atoms to replace them.

On the other hand if the dopant is available in unlimited quntity (inexhaustible source) the concentration of the dopant atoms will remain constant because the impurity atoms that penetrate into the substrate crystal lattice are continually replenished. This technique should be followed for uniform deposition ob B on the surface.

3 .Temperature - Lesser the temperature. lesser will be the rate of diffusion

4 Concentration gradient

5. Crysallographic orientation of the substrate

DIFFUSION TECHNIQUES:

1.Gas phase diffusion:

A carrier gas like Argon or Nitrogen is enriched with diborane B₂H₆ (precursor to B atom) and led into a quartz tube containing silicon wafers kept at a certain temperature

2 .Solid source diffusion:

Slices containing the dopant B are placed in between silicon wafers in a quartz tube and the tube is heated. The dopant atoms in the solid matrix escape into the vapor phase from which they are deposited on the suface uniformly using a carrier gas lik Argon or Nitrogen.

3. Liquid source diffusion:

The liquid source for deposition of Boron is Boron tribromide BBr₃ (precursor). For transporting the dopant from the liquid phase to vapour phase a carrier gas is bubbled through the liquid. As in the other methods, the silicon wafers are kept inside a quartz tube which is heated gradually, usually at the rate of 10⁰ C per minute, until the temperature reaches 900⁰ C. The dopant is now led into the quartz tube and the temperature is raised to 1200⁰ C.Now diffusion takes place and B is deposited uniformly on the surface of the silicon wafer.