1)

a) A bare Si wafer is oxidized in dry O₂ at 1000 °C. The desired final thickness is 40 nm.

i) Find the time required to perform the oxidation, ignoring rapid growth.

ii) Find the time required to perform the oxidation, taking rapid growth into account.

Do not use oxide growth curves.

b) A <100> silicon wafer is oxidised using the following sequence

i) 40 mins @ 1000°C in dry O₂.

ii) 60 mins @ 1000°C in steam.

iii) Etch 0.3 microns of oxide.

iv) 20 mins @ 1100°C in dry O₂.

Use growth curves to calculate the oxide thickness after each step.

Nitride-

Silicon Wafer

Oxidation:

Pad Oxide

Oxide

Nitride removal:

"Bird's beak"

[10 1/3 marks]

[10 marks]

Figure 1. LOCOS process

c) The diagram above, Figure 1, shows a fully recessed LOCOS (Local Oxidation of

Silicon) process. Explain, with reference to the diagram, how the process is used to

produce areas of oxide on a wafer without a subsequent lithography and etch step.

[5 marks]

d) Discuss five of the features of SiO₂ which account for its widespread use in

microelectronics.

[5 marks]/n