This set of VLSI Questions and Answers for Campus interviews focuses on “GaAs Fabrication -2”.
1. The ohmic contacts are deposited by
Explanation: The ohmic contacts between the metal interconnect and the source and the drain are deposited by evaporation using E-beam technology.
2. Which has high parasitic gate resistance?
Explanation: Titanium provides a good, high barrier, Schottky contact and has a high parasitic gate resistance.
3. Which is used as the top layer?
Explanation: To reduce the parasitic resistance, gold is used as the top layer with platinum or tungsten as the intermediate layer.
4. First layer metallization is accomplished by plasma etching.
Explanation: First layer metallization is accomplished by delineating photoresist patterns, plasma etching, deposition of metal on GaAs wafer and photoresist lift-off.
5. Deposition rate is given as
a) width per unit time
b) thickness per unit time
c) sputtering rate per unit time
d) depositing rate per unit time
Explanation: Deposition rate is given as thickness per unit time. It depends upon the sticking coefficient of the depositing material and the nature of sputtering eequipment.
6. Passivation is used to protect against contamination.
Explanation: The last step for fabrication in GaAs is passivation. This process is used to protest the device against contamination and moisture.
7. Plasma-enhanced chemical vapour deposition process is used for fabrication of
a) conducting films
b) insulating films
c) both of the mentioned
d) none of the mentioned
Explanation: Plasma-enhanced chemical vapour deposition process is a chemical deposition technique used for the fabrication of both insulating and conducting films.
8. Which method uses plasma excitation?
b) low pressure CVD
c) high pressure CVD
Explanation: PECVD (plasma-enhanced chemical vapour deposition) method uses plasma excitation in addition to usual thermal energy.
9. Which causes degradation of transconductance?
a) low source resistance
b) high source resistance
c) low drain resistance
d) high drain resistance
Explanation: The very thin undepleted n- layer causes high source resistance and this causes the degradation of the transconductance gm.
10. Cuts are not needed for
a) ohmic contacts
b) schottky barriers
c) interconnect metallizations
d) joining two layers
Explanation: Cuts are made in dielectric only where ohmic contacts, schottky barriers and interconnect metallizations are required and not for joining any two layers.
11. Which is the less costly material that can be used for first-level metal?
Explanation: Gold is the more costly material used for first-level and second-level metal layer whereas aluminium is the less costly material that can be used.
12. ________ is controlled by varying ion flux and velocity
a) doping density
b) doping thickness
c) doping rate
d) doping material
Explanation: Doping density and dopants distribution in the semi-insulating material are controlled by varying the ion flux and velocity.
13. The extent of damage to crystal depends on
a) target mass
b) mass of the implanted ion
d) all of the mentioned
Explanation: The extent of damage to the crystal depends on several factors such as mass of the implanted ion, target mass, energy associated with the ion, dose, temperature and displacement energies.
Sanfoundry Global Education & Learning Series – VLSI.
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