Engineering Questions with Answers - Multiple Choice Questions

Manufacturing Processes – Electrochemical Etching – 2

1 - Question

Etching process should be selective to the material that has to be removed.
a) True
b) False
View Answer Answer: a
Explanation: Etching process should be selective to the material that has to be removed because this helps to protect the material under the mask (within limits of isotropic etching) and also the mask material itself (oxide, nitride, or resists).


2 - Question

_______________ is commonly used as a mask for Si etching.
a) Silicon dioxide
b) Silicon nitride
c) Silicone gel
d) Silicon sulphate
View Answer Answer: b Explanation: If silicon nitride is used as mask, its etch rate, under the same conditions, is 1 nm/hr., nearly 105 times slower than the etch rate for Si (100) at 90 °C using 30 % KOH which is ~ 100 µm/hr. Thus, silicon nitride is commonly used as a mask for Si etching (especially for making Si cantilever based devices).


3 - Question

The etching rate and time are crucial to prevent over etching.
a) True
b) False
View Answer Answer: a
Explanation: The etching rate and time are crucial to prevent over etching since resist removal can cause damage to portions of the wafer that have to be protected from the etchant.


4 - Question

Silicon dioxide etch rate at 90 °C using 30 % KOH is approximately ______________
a) 1 µm/hr
b) 5 µm/hr
c) 10 µm/hr
d) 100 µm/hr
View Answer Answer: a
Explanation: Silicon dioxide etch rate at 90 °C using 30 % KOH is ~ 1 µm/hr. So, using silicon oxide as a mask for Si etching will not be good enough or a very thick oxide layer is required.


5 - Question

For silicon oxide etching, usually ______________ is used.
a) nitric acid and hydrofluoric acid (HF)
b) mixture of HF and ammonium fluoride (NH4F)
c) H3PO4
d) Mixture of HPO4, HNO (nitroxyl), Acetic acid, water
View Answer Answer: b
Explanation: For silicon oxide etching, usually a mixture of HF and ammonium fluoride (NH4F) is used, that produces a etch rate of ~0.1 µm/hr at room temperature. This mixture does not etch Si, so it provides very good selectivity.


6 - Question

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
View Answer Answer: c
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.


7 - Question

Dry etching produces more vertical side walls compared to wet etching.
a) True
b) False
View Answer Answer: a
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.


8 - Question

In dry etching ______________ are used for removing material.
a) solvent etchants
b) gaseous etchants
c) carbide tools
d) powder etchants
View Answer Answer: b
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.


9 - Question

In plasma etch, the chemical etchant is introduced in the gas phase.
a) True
b) False
View Answer Answer: a
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.


10 - Question

Plasma etching provides poor thickness control.
a) True
b) False
View Answer Answer: b
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.

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