2)

a) A phosphorous doped n-type silicon wafer of 1 92-cm resistivity is subjected to a

boron diffusion at a temperature of 1150 °C. A constant surface concentration of 2 x

10¹⁹ cm³ is maintained throughout the diffusion.

i) Calculate the background concentration in the wafer.

ii) How long should the diffusion be carried out for to obtain a junction depth of 4

microns.

iii) Calculate the charge, Q.

[15 1/3 marks]

b) Describe with the aid of a diagram, a liquid source diffusion system.

[5 marks]

c) State Henry's Law. Explain why predeposition is performed at the solid solubility

limit in industrial practice.

[5 marks]

excitation:

primary ion beam:

ot, Art, O, Cst

ooooooooooooo

Figure 2. SIMS Schematic Diagram

analytical signal

secondary ions:

X*, X, X-Y*, X-Y-

not used:

neutrals,

excited atom

d) Figure 2, above, shows a schematic diagram illustrating Secondary Ion Mass

Spectroscopy (SIMS) Briefly describe the technique. Why would SIMS be a poor

method to use to profile dopant concentrations in a multi-quantum-well laser

consisting of several ultra-thin layers of different materials? How is this measurement

attempted in practice?

[5 marks]

e) Rapid Thermal Processors use several methods to compensate for higher radiant heat

losses at the periphery of the wafer. Briefly discuss one of these methods.

[3 marks]