Manufacturing Processes Quiz on “Electrochemical Etching – 13”.
1. In metal-assisted etching of porous silicon, the oxidant is reduced at the noble metal surface.
a) True
b) False
Answer: a
Clarification: The oxidant is reduced at the noble metal surface due to its catalytic activity. The reduction of the oxidant generates electronic holes, which are subsequently injected into the silicon substrate through the noble metal interface.
2. Electronic holes in metal-assisted etching of porous silicon oxidise silicon atoms.
a) True
b) False
Answer: a
Clarification: Electronic holes oxidise silicon atoms at the interface noble metal-silicon, which are then etched away by HF molecules. The concentration of electronic holes is maximum at the noble metal-silicon interface and thus its dissolution rate is much faster there.
3. Which of the following is used as an oxidant in metal-assisted etching?
a) Sulphuric acid
b) Nitric acid
c) Oxygen bubbles
d) Peroxy-di-sulfuric acid
Answer: c
Clarification: Regarding the type of oxidant, hydrogen peroxide is the most commonly used although other oxidants such as oxygen bubbles, oxygen dissolved in water, silver nitrate (AgNO3), chloroauric acid (HAuCl4), potassium gold (III) chloride (KAuCl4), chloroplatinic acid (H2PtCl6) have been explored as well.
4. The holes which are left out after oxidation of silicon atoms enter the bulk silicon region.
a) True
b) False
Answer: a
Clarification: The remaining electronic holes (i.e. those not consumed during the dissolution of silicon) diffuse from the interface noble metal-silicon to the bulk silicon.
5. The structural features of pSi produced by metal-assisted method independent on illumination.
a) True
b) False
Answer: a
Clarification: The resulting structural features of pSi produced by metal-assisted approach rely on different parameters such as the etchant composition and its temperature, the nature of the noble metal, the illumination and the intrinsic properties of the silicon substrate (e.g. doping type and level, crystallographic orientation, etc.).
6. Structural features of metal-assisted etched pSi are different for different oxidant.
a) True
b) False
Answer: a
Clarification: The resulting morphologies and structural features of metal-assisted etched pSi differ from one oxidant to another as they can change the morphology of the metal or produce different precipitates in the course of the etching process.
7. Concentration and composition of the etchant electrolyte can affect the features of the etching product.
a) True
b) False
Answer: a
Clarification: Important parameter that directly affects the geometric features of the resulting pSi produced by metal-assisted etching is the concentration and composition of the etchant electrolyte. For instance, etching of silicon wafers coated with platinum particles in an etchant with low concentration of HF (HF(50 %): H2O2 (30 %):H2O = 2:1:8; v:v:v) yields pores with cone-like morphology from top to bottom.
8. At high concentrations of HF, the etching rate is determined by _____
a) oxidant
b) oxidant concentration
c) temperature of the oxidant
d) material to be etched
Answer: b
Clarification: Change of pore morphology and geometric features with the ratio acid/oxidant can be explained by the fact that at high concentrations of HF (the ratio acid/oxidant is between 100 – 70% ), the etching rate is determined by the oxidant concentration and the electronic holes generated at the interface noble metal-silicon are rapidly consumed as a result of the excess of HF available to dissolve silicon.
9. The etching rate is determined by the HF concentration when the ratio acid/oxidant is lower than _____
a) 50%
b) 60%
c) 68%
d) 70%
Answer: d
Clarification: When the ratio acid/oxidant is lower than 70 %, the etching rate starts to be determined by the HF concentration. In such a scenario, the generation of electronic holes is higher than their consumption rate and thus the excess of electronic holes diffuse away to the pore walls, where μpSi structures are generated.
10. High concentrations of oxidant result in isotropic etching.
a) True
b) False
Answer: a
Clarification: At high concentrations of oxidant (i.e. 20 % > [HF]/([HF] + [H2O2]) > 9 %) electronic holes diffuse massively towards the surface of silicon in contact with HF, resulting in an isotropic etching independent on the silver particles present on the wafer surface.