1. Etching rate of silicon is independent of the crystal planes.
a) True
b) False
Explanation: The etching rate of silicon in these etchants is highly dependent on the crystal planes, the doping level of the silicon crystal and the etchant concentration and its temperature.
2. Porous materials are classified according to_____
a) total area
b) difference between initial and final mass
c) pore size
d) manufacturing method
Explanation: Following the International Union of Pure and Applied Chemistry (IUPAC) nomenclature used to classify porous materials as a function of their pore size (dp).
3. In how many categories porous silicon can be divided?
a) 2
b) 3
c) 4
d) 5
Explanation: pSi structures can be divided into three categories: (i) microporous silicon (μpSi) with dp < 2 nm, mesoporous silicon (mpSi) with 2 nm < dp < 50 nm and macroporous silicon (MpSi) with dp > 50 nm
4. According to morphological details, pSi can be divided as sponge-like pSi and pSi featuring cylindrical pores.
a) True
b) False
Explanation: pSi can present a variety of morphological details, these can be divided into two main categories: (i) sponge-like pSi, which features densely and randomly distributed branched pores and (ii) pSi featuring cylindrical pores, which can have rough or smooth walls. While μpSi and mpSi structures feature sponge-like morphology, MpSi structures have cylindrical pores.
5. MpSi structures have a random distribution of pores.
a) True
b) False
Explanation: As far as the spatial distribution of pores, MpSi structures present a random distribution. However, these structures can be produced featuring a perfectly ordered spatial distribution of pores. To this end, the surface of the silicon wafer must be patterned by lithographic techniques before the electrochemical etching process is carried out.
6. Cone-like pores can be obtained by using Lehmann’s law.
a) True
b) False
Explanation: 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.
7. Progressively modified illumination intensity approach can be used to develop some optical structures.
a) True
b) False
Explanation: 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.
8. 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
Explanation: Variable illumination intensity technique can be combined with lithographic patterning to produce 2D infrared photonic crystals featuring perfectly ordered and straight macropores.
9. 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
Explanation: 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.
10. 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
Explanation: 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.