This set of Manufacturing Processes test focuses on “Electrochemical Etching – 6”.
1. 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.
2. To create a neutralized Si–F bound, an electronic hole (h+) is generated.
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.
3. Si + 4HF2— + h+ -> SiF62- + 2HF + _____ + e—. Complete the reaction for dissolution of hydrogen-terminated Si in HF based solution.
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.
4. In the fabrication of pSi structures, wet chemical etching with alkaline etchants is used.
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.
5. Which of the following alkaline etchant is most widely used to etch silicon?
a) Tetramethyl ammonium hydroxide
b) Aluminium hydroxide
c) Calcium hydroxide
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.
6. Etching rate of silicon is independent of the crystal planes.
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.
7. 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).
8. In how many categories porous silicon can be divided?
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.
9. According to morphological details, pSi can be divided as sponge-like pSi and pSi featuring cylindrical pores.
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.
10. MpSi structures have a random distribution of pores.
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.
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