Engineering Questions with Answers - Multiple Choice Questions
Manufacturing Processes – Electrochemical Etching – 5
1 - Question
Electronic grade silicon wafers are produced by Czochralski growth method.
Explanation: Typically, electronic grade silicon wafers (high purity silicon 99.9999999 %—9 N) are produced by Czochralski growth method. In this process, a cylindrical ingot of high purity monocrystalline silicon is formed by pulling a seed crystal from a bath composed of polysilicon.
2 - Question
Porous silicon structures are produced by electrochemical etching of silicon wafers in organic solutions of_____
Explanation: Typically, porous silicon structures are produced by electrochemical etching of silicon wafers in organic solutions of acetonitrile (CH3CN) or dimethylformamide (C3H7NO) containing hydrofluoric acid (HF).
3 - Question
It is mandatory to consider all the safety aspects related to HF handling during etching of Si.
Explanation: HF is highly corrosive towards living tissues and its inhalation, ingestion or skin contact are all extremely hazardous and can be lethal. HF should be handled under a hood with proper ventilation, wearing personal HF gas monitor with audible alarm, safety sensor for liquids and proper personal protective equipment such as safety goggles, face shield, chemically resistant butyl rubber gloves and apron.
4 - Question
The oxide later formed on commercial Si does not affect the etching of Si.
Explanation: The native oxide layer formed on commercial silicon wafers under ambient atmospheric conditions must be removed. The most widespread method to remove this oxide layer is the so-called RCA cleaning process. This process is divided into three steps: namely; (i) removal of organic contaminants, (ii) removal of the native oxide layer and (iii) removal of ionic contaminations.
5 - Question
Which of the following can lead to broken porous silicon samples?
a) High current density
b) High concentration of electrolyte
c) Inappropriate design of cell
d) High temperature of the cell
Explanation: An inappropriate design of the cell can lead to broken samples, leaky set-ups and corroded contacts. Note that aspects such as materials, contacts and sealants are critical in the design of the electrochemical cell used to produce porous Si by electrochemical or chemical etching approaches.
6 - Question
Cell material must be resistant to HF solution.
Explanation: since most common acid electrolytes used to prepare porous Si are HF-based solutions, the materials of choice used to fabricate the body of the etching cells are resistant to HF. The most representative examples of these are polyvinyl chloride (PVC) and polytetrafluoroethylene (PTFE).
7 - Question
Which of the following materials is suitable for making sealant while working with HF based solutions?
a) Silicone rubber
d) Acrylonitrile-butadiene copolymer
Explanation: Sealants are standard black O-rings made of acrylonitrile-butadiene copolymer (Perbunan) or vinylidene fluoride-hexafluoropropylene (Viton) are preferred as they are stable in HF at concentrations up to 50 %.
8 - Question
Which of the following materials is the better choice for making conductors in cells used to produce pSi by electrochemical etching?
Explanation: It is critical to consider the resistance of the metal used as a contact towards corrosion since a continuous exposure to the etching electrolyte can degrade its integrity with use and thus its performance. Typically, noble metals such as platinum or gold are the best choice as they are inert. Other materials such as stainless steel, brass, tungsten or aluminium can also be used.
9 - Question
Hydrogen-terminated silicon surfaces are ____________
Explanation: After the cleaning step and immersion in HF solutions, the silicon wafer surface is terminated with hydrogen atoms (Si–H). Hydrogen-terminated silicon surfaces are hydrophobic, showing a large contact angle for a drop of water [34, 35]. Silicon surfaces dipped in HF or an ammonium fluoride (NH4F) show equivalent hydrophobic character.
10 - Question
Roughness of the hydrophobic surfaces formed after dipping silicon in HF solution is dependent on the type of etchant.
Explanation: The roughness of these surfaces is highly dependent on the chemical treatment and type of etchant used. While NH4F provides atomically flat surfaces for (111) and (100) oriented silicon surfaces, a treatment with HF creates surfaces with nanometric roughness.