1. Wet etching is used for removal of material from large areas.
a) True
b) False
Explanation: Wet etching is used for removal of material from large areas (trench sizes > 3 µm). For smaller areas, where greater precision in the removal of material is required, dry etch is preferred.
2. The wet etching process is ___________
a) isotropic
b) anisotropic
c) isotropic for few materials
d) isobaric process
Explanation: The wet etching process is anisotropic i.e. the etch rate depends on the plane of the atoms in the material, from which atoms are being removed
3. Incomplete etch occurs due to ___________
a) high concentration of the chemicals
b) high pressure
c) insufficient temperature
d) low chemical activity
Explanation: In incomplete etch, the time is not sufficient for complete material removal. This is usually due to concentration or temperature not being sufficient. The concentration profile left behind is usually a rough surface, due to local variations in material removal.
4. Product after etching of Si wafer with KOH is ______________ shape.
a) square
b) circular at the end
c) trapezoidal
d) oval
Explanation: Because of the difference in the etch rates of Si along the different crystallographic layers the final profile is trapezoidal, with the angle determined by the etch rates.
5. In case of over-etching, material under protective layer gets removed.
a) True
b) False
Explanation: When the etch time is larger than the required etch time, due to isotropic etching, material under the photoresist can get removed. This is called over etching and in extreme cases, it can also lead to liftoff the resist layer
6. Etching process should be selective to the material that has to be removed.
a) True
b) False
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).
7. _______________ is commonly used as a mask for Si etching.
a) Silicon dioxide
b) Silicon nitride
c) Silicone gel
d) Silicon sulphate
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).
8. The etching rate and time are crucial to prevent over etching.
a) True
b) False
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.
9. 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
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.
10. 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
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