1. In the pore formation mechanism of MpSi, electronic holes initiate the dissolution process of silicon.
a) True
b) False
Explanation: As far as the pore formation mechanism of MpSi is concerned, electronic holes initiate the dissolution process of silicon and these are minority charge carriers in n-type silicon. Therefore, the concentration of electronic holes in n-type silicon under equilibrium conditions is very low.
2. The neutral state is achieved when the concentration of electrons is equal to the concentration of electronic holes and ionized donors.
a) True
b) False
Explanation: Three types of charges are present in the bulk silicon: namely, electronic holes, electrons and ionized donors. The system is a neutral state when the concentration of electrons is equal to the concentration of electronic holes and ionized donors together.
3. For a given level of doping, the growth of MpSi in n-type silicon substrates can be controlled by_____
a) amount of electric current passed
b) current density
c) voltage drop
d) processing time
Explanation: For a given level of doping, the growth of MpSi in n-type silicon substrates can be led by the current density and the illumination applied in the course of the etching process.
4. A pre-treatment by a lithographic patterning stage enables the production of ordered pore distribution.
a) True
b) False
Explanation: As-produced MpSi has a random pore distribution since pores nucleate uniformly on the Si wafer surface. A pre-treatment by a lithographic patterning stage enables the production of MpSi structures with perfectly ordered pores featuring square or triangular arrangement.
5. Among all the etching parameters, the etching current density (J) is the most critical factor.
a) True
b) False
Explanation: The growth of well-defined cylindrical macropores from top to bottom in MpSi can be precisely controlled through the etching parameters (e.g. etching current density, HF concentration and its temperature, wafer doping, illumination intensity, etc.). In particular, among these parameters, the etching current density (J) is a critical factor to lead a homogeneous pore growth as the longer the pore the more effective the collection of photo-generated holes.
6. Which of the following is true about chemical dissolution of hydrogen-terminated silicon in HF based solutions.
a) Si atoms are replaced by F atoms
b) H atoms are replaced by F atoms
c) F− ions are replaced by Si atoms
d) H atoms are replaced by F− ions
Explanation: As far as the chemical dissolution of hydrogen-terminated silicon is concerned, this process requires the replacement of superficial H atoms by fluoride ions (F−), which are incorporated from the etching electrolyte solution.
7. To create a neutralized Si–F bound, an electronic hole (h+) is generated.
a) True
b) False
Explanation: Under open circuit conditions, the silicon surface remains passivated. Therefore, an electronic hole (h+) must be generated to create a neutralized Si–F bound. The Si–F bound can be created under certain anodic bias by the polarisation effect induced by the F atom over the Si atom.
8. Si + 4HF2— + h+ -> SiF62- + 2HF + _____ + e—. Complete the reaction for dissolution of hydrogen-terminated Si in HF based solution.
a) F2
b) SiF4
c) H2
d) Si-H
Explanation: A new F− atom bounds the Si atom at a different position and a gas hydrogen molecule (H2) is generated. The progressive repetition of this process weakens the Si–Si bounds by the strong electronegativity of F atoms through nucleophilic attack. In this way, Si atoms are etched away by reaction with HF and pores are generated.
9. In the fabrication of pSi structures, wet chemical etching with alkaline etchants is used.
a) True
b) False
Explanation: An important aspect in the fabrication of pSi structures is the use of wet chemical etching with alkaline etchants, which are mainly used for chemical polishing or anisotropic etching of silicon. These pre- and post-treatments make it possible to design and engineer a broad range of pSi structures
10. Which of the following alkaline etchant is most widely used to etch silicon?
a) Tetramethyl ammonium hydroxide
b) Aluminium hydroxide
c) Calcium hydroxide
d) Formaldehyde
Explanation: The most widespread alkaline etchants used to etch silicon are potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH), although other inorganic and organic compounds such as lithium hydroxide (LiOH), sodium hydroxide (NaOH), rubidium hydroxide (RbOH), caesium hydroxide (CsOH), ammonium hydroxide (NH4OH), cholin and ethylenediamine have been used as well.