1. For ____________ usually a strong acid like hot phosphoric acid is used at high temperatures.
a) silicon carbide
b) poly Si
c) silicon nitride
d) aluminium
Explanation: For silicon nitride, usually a strong acid like hot phosphoric acid is used at high temperatures (180°C) since it is a very good passivating layer and hard to remove under normal conditions.
2. Dry etching produces more vertical side walls compared to wet etching.
a) True
b) False
Explanation: Dry etching, as the name suggest, is a removal of material in the absence of solvent. Because of the anisotropic nature of etching, dry etching produces more vertical side walls compared to wet etching, but the removal rate is slower.
3. In dry etching ______________ are used for removing material.
a) solvent etchants
b) gaseous etchants
c) carbide tools
d) powder etchants
Explanation: Dry etching is removal of material in the absence of solvent. Here, etchant gases are the primary medium for the removal of material. There are three main types of dry etching-
1. Plasma etch
2. Ion beam milling
3. Reactive ion etch.
4. In plasma etch, the chemical etchant is introduced in the gas phase.
a) True
b) False
Explanation: In plasma etch, the chemical etchant is introduced in the gas phase. For etching silicon oxide, CF4 (tetrafluoromethane) is used. The chamber is first evacuated before introducing the gas. Radio frequency (RF) electrodes are then used to generate the plasma that ionizes the gas
5. Plasma etching provides poor thickness control.
a) True
b) False
Explanation: In plasma etching, the ionised gas attacks the oxide layer, removing the layer. Etch rates in plasma etch are ~1—10 µm/hr, much smaller than wet etching. So, it is more suitable for thin layers, but it also provides greater thickness control.
6. In reactive ion etching, argon gas is ionized by bombarding with electrons.
a) True
b) False
Explanation: Ar gas is introduced into the vacuum chamber where they are ionized by bombarding with electrons. These ions are then directed on to the wafer where they remove material by physical bombardment.
7. In deposition process, material is added unlike etching where material is removed.
a) True
b) False
Explanation: The deposition process is the opposite of etching. Here, material is added to the wafer surface. The layers different from grown layers like oxide and nitride, where the underlying Si is consumed during a high temperature furnace process
8. Which of the following does not hold true?
a) In epitaxial layers, poly Si is grown using deposition
b) For making trench capacitors deposited films are used
c) Deposited layers can also be used as passivation layers
d) Deposition is same as etching
Explanation: Some of the layers, where deposited films are used, are
(1) Epitaxial layers – usually poly Si is grown for use as a gate.
(2) Dielectric layers – intermetallics (high k capacitors)
(3) Trench capacitors
(4) Intermetal conducting plugs
(5) Metal layers – conductors
(6) Passivation layers.
9. In deposition, the Si from the wafer is consumed.
a) True
b) False
Explanation: In deposition, the Si from the wafer is not consumed and the wafer can be maintained at room temperature or at elevated temperatures.
10. How many deposition techniques are there?
a) 2
b) 3
c) 4
d) 5
Explanation: There are two main growth techniques-
(1) Physical deposition
(2) Chemical deposition
Depending upon few parameters, the deposition method is selected