Manufacturing Processes Multiple Choice Questions on “Electrochemical Etching – 9”.
1. Bending curvature of silicon wafers varies ______ with the specific area of the MpSi structure.
a) linearly
b) inversely
c) cubically
d) it not affected by specific area
Answer: a
Clarification: The structure of MpSi can result broken by oxide-induced stress in the course of this stage. Thinner MpSi layers minimise this problem since the bending curvature of silicon wafers relies linearly on the specific area of the MpSi structure.
2. Silicon oxide layer can be removed using HF solution.
a) True
b) False
Answer: a
Clarification: The silicon oxide layer is selectively removed from the backside of the silicon wafer by a wet chemical etching in a buffer HF solution (BHF). Finally, the remaining silicon is etched away by a wet chemical etching in a solution of TMAH 25 % wt at 85 °C.
3. Length of the silicon dioxide pillars is dependant on the etching time and the pore length.
a) True
b) False
Answer: a
Clarification: Silicon is etched away by a wet chemical etching in a solution of TMAH. After that, a new structure based on silicon dioxide micropillars rises from the backside of the wafer as the silicon dioxide layer covering the pore walls is not etched by TMAH. The length of the resulting silicon dioxide pillars can be established by the etching time and by the pore length in the MpSi structure.
4. Lithographic technique can be used for patterning the Si wafer.
a) True
b) False
Answer: a
Clarification: The backside of the silicon wafer can be patterned by lithographic methods after the oxidation stage, enabling a selective etching on the wafer backside by TMAH.
5. It possible to produce arrays of silicon dioxide pillars inside _____
a) long pipes
b) spherical cavities
c) truncated pyramids
d) thin shells
Answer: c
Clarification: Given that the etching process in TMAH is selectively performed over these areas without protective mask in an anisotropic manner, the approach makes it possible to produce arrays of silicon dioxide pillars inside truncated pyramids.
6. Arrays of cone-like silicon dioxide pillars can be produced using _____
a) truncated MpSi
b) conical MpSi
c) cylindrical MpSi
d) planner MpSi
Answer: b
Clarification: Arrays of cone-like silicon dioxide pillars can be produced when conical MpSi is used as the starting material. The features of these 3D structures can be precisely controlled by the etching parameters and the features of the lithographic patterning.
7. When the doping density in the silicon wafer is increased, then _____
a) electric field strength is increased
b) of depletion region is increased
c) hinderance to charge carries is increased
d) quality of the product is increased
Answer: a
Clarification: When the doping density in the silicon wafer is increased, the electric field strength increases, decreasing the of the depletion region and enabling charge carriers to pass through the space charge region (SCR) by tunnelling.
8. Avalanche breakdown is the dominant mechanism at high doping densities.
a) True
b) False
Answer: b
Clarification: At high doping densities (i.e. ≥1018 cm−3) tunneling mechanism dominates the charge transfer while avalanche breakdown is the dominant mechanism at low doping densities.
9. The electric field strength decreases around the depressions present on the silicon wafer surface.
a) True
b) False
Answer: b
Clarification: The electric field strength increases around the depressions and pits present on the silicon wafer surface. This results in a local reduction of the depletion region , which increases the tunneling probability of charge carriers, incrementing the local current density.
10. In the beginning of the electrochemical etching, some of these pits present on the surface of the silicon wafer develop into mesopores.
a) True
b) False
Answer: a
Clarification: In the course of the first seconds of the electrochemical etching, some of these pits present on the surface of the silicon wafer develop into mesopores, the density of which across the surface is established by the doping level and the etching conditions.