This set of VLSI Multiple Choice Questions & Answers (MCQs) focuses on “nMOS Fabrication”.
1. nMOS fabrication process is carried out in ____________
a) thin wafer of a single crystal
b) thin wafer of multiple crystals
c) thick wafer of a single crystal
d) thick wafer of multiple crystals
Explanation: nMOS fabrication process is carried out in thin wafer of a single crystal with high purity.
2. ______________ impurities are added to the wafer of the crystal.
a) n impurities
b) p impurities
Explanation: p impurities are introduced as the crystal is grown. This increases the hole concentration in the device.
3. What kind of substrate is provided above the barrier to dopants?
Explanation: Above a layer of silicon dioxide which acts as a barrier, an insulating layer is provided upon which other layers may be deposited and patterned.
4. The photoresist layer is exposed to ____________
a) Visible light
b) Ultraviolet light
c) Infra red light
Explanation: The photoresist layer is exposed to ultraviolet light to mark the regions where diffusion is to take place.
5. In nMOS device, gate material could be ____________
Explanation: In nMOS device, the gate material could be metal or polysilicon. This polysilicon layer has heavily doped polysilicon deposited by CVD.
6. Which is the commonly used bulk substrate in nMOS fabrication?
a) silicon crystal
Explanation: In nMOS fabrication, the bulk substrate used can be either bulk silicon or silicon-on-sapphire.
7. In nMOS fabrication, etching is done using ____________
b) hydrochloric acid
c) sulphuric acid
d) sodium chloride
Explanation: In nMOS fabrication, etching is done using hydrofluoric acid or plasma. Etching is a process used to remove layers from the surface.
8. Heavily doped polysilicon is deposited using ____________
a) chemical vapour decomposition
b) chemical vapour deposition
c) chemical deposition
d) dry deposition
Explanation: The polysilicon layer consists of heavily doped polysilicon deposited by chemical vapour deposition.
9. In diffusion process ______ impurity is desired.
a) n type
b) p type
c) np type
d) none of the mentioned
Explanation: Diffusion is carried out by heating the wafer to high temperature and passing a gas containing the desired ntype impurity.
10. Contact cuts are made in ____________
c) metal layer
d) diffusion layer
Explanation: Contact cuts are made in the desired polysilicon area, source and gate. COntact cuts are those places where connection has to be made.
11. Interconnection pattern is made on ____________
a) polysilicon layer
b) silicon-di-oxide layer
c) metal layer
d) diffusion layer
Explanation: The metal layer is masked and etched to form interconnection pattern. The metal layer was formed using aluminium deposited over the formed surface.
12. SIlicon-di-oxide is a good insulator.
Explanation: SIlicon-di-oxide is a very good insulator so a very thin layer is required in the fabrication of MOS transistor.
13. _______ is used to suppress unwanted conduction.
Explanation: Boron is used to suppress the unwanted conduction between transistor sites. It is implanted in the exposed regions.
14. Which is used for the interconnection?
Explanation: Aluminium is the suitable material used for the circuit interconnection or connecting two layers.
Sanfoundry Global Education & Learning Series – VLSI.
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