Manufacturing Processes Multiple Choice Questions on “Electrochemical Etching – 8”.
1. Cone-like pores can be obtained by using Lehmann’s law.
a) True
b) False
Answer: a
Clarification: According to the Lehmann’s law, the increasing collection of photo-generated holes results in a progressive reduction of the pore diameter with depth, yielding cone-like pores with decreasing diameter from top to bottom.
2. Progressively modified illumination intensity approach can be used to develop some optical structures.
a) True
b) False
Answer: a
Clarification: This approach can be readily used to develop some optical structures such as photonic crystals and optical waveguides in the visible and NIR range as the refractive index of MpSi can be engineered in depth along the pore yielding a waveguide structure embedded in the array of macropores.
3. Progressively modified illumination intensity approach when combined with lithographic patterning, gives_____
a) straight macropores
b) straight micropores
c) sponge-like pSi
d) honeycomb like pattern
Answer: a
Clarification: Variable illumination intensity technique can be combined with lithographic patterning to produce 2D infrared photonic crystals featuring perfectly ordered and straight macropores.
4. In the technique mentioned in the previous question, arrays of macropores are _____
a) heated to the melting temperature
b) soaked in HF solution for several hours
c) coated by a lithographic mask
d) kept under UV rays
Answer: c
Clarification: In this process, arrays of macropores are coated by a lithographic mask after electrochemical etching. Next, MpSi is selectively removed from the unmasked areas by a wet chemical etching. This results in a set of deep bars of macropore rows with well-defined pore geometry.
5. Which of the following is true according to the Lehmann’s model for the formation of MpSi?
a) The current density is equal to the critical current density
b) The current density is half of the critical current density
c) The critical current density is equal to square of the current density
d) Lehmann’s model does not give relation between current density and critical current density
Answer: a
Clarification: According to the Lehmann’s model for the formation of MpSi, the current density at the pore bottom tips is equal to the critical current density. Therefore, an increment of the illumination intensity at the backside of the silicon wafer during etching leads to the generation of electronic holes, which directly contribute to the total current density.
6. The pore diameter in macroporous silicon can be periodically modulated.
a) True
b) False
Answer: a
Clarification: Following the approach based on the Lehmann’s model, the pore diameter in macroporous silicon can be periodically modulated (Fig. 1.5). This approach takes advantage from the fact that the porosity of the resulting MpSi structure is established by the ratio between the total current density and the critical current density (i.e. JPS—current density limit between the formation of pSi and electro-polishing of silicon).
7. Increase in the total current density produced by the generation of electronic holes increases the pore diameter.
a) True
b) False
Answer: a
Clarification: The critical current density is constant at the pore bottom tips and thus the increment of the total current density produced by the generation of electronic holes widens the pore diameter. Following this procedure, the pore structure of MpSi can be modulated to produce periodic ratchet-type (i.e. asymmetric) or circular (i.e. symmetric) pore modulations.
8. If the etching conditions are not controlled, then _____ decreases with pore depth.
a) the period length
b) diameter of the pores
c) orderliness of the pores
d) reflectivity of the material
Answer: a
Clarification: If the etching conditions are not controlled, the period length, which is defined as the distance between two consecutive pore modulations, decreases with the pore depth due to diffusion limitations (i.e. lower concentration of HF at the pore bottom tips). As a result, the critical current density decreases and the pore growth rate becomes slower.
9. Post-treatment starts with a thermal treatment of the MpSi structure at _____
a) 900°C
b) 1100 °C
c) 1570°C
d) 1650°C
Answer: b
Clarification: A post-treatment is required after the fabrication of MpSi. This post-treatment starts with a thermal treatment of the MpSi structure at 1100 °C for 100 min under oxygen atmosphere.
10. During post treatment of MpSi, a layer of silicon dioxide is formed.
a) True
b) False
Answer: a
Clarification: A thermal treatment of the MpSi structure at 1100 °C for 100 min under oxygen atmosphere generates a silicon dioxide layer of 200 nm along the inner surface of the pores of MpSi.