1. Structural features of metal-assisted etched pSi are different for different oxidant.
a) True
b) False
Explanation: 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.
2. Concentration and composition of the etchant electrolyte can affect the features of the etching product.
a) True
b) False
Explanation: 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.
3. 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
Explanation: 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.
4. 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%
Explanation: 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.
5. High concentrations of oxidant result in isotropic etching.
a) True
b) False
Explanation: 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.
6.Noble metal used in metal-assisted approach affects the formation of μpSi structures on the side walls of pores.
a) True
b) False
Explanation: Another interesting phenomenon dependent on the noble metal is the formation of μpSi structures on the side walls of pores or wires during the etching process. A detailed explanation of the origin of these effects is yet to come although some scientists have pointed out towards the role of the noble metal type in the catalytic activity for the reduction of the oxidant.
7. Etching rate can be improved by_____
a) lowering the temperature
b) using less active oxidant and highly active etchant
c) increasing the number of electronic holes
d) reducing the surface area of the pores so that less time will be required for the etching
Explanation: The more the injected electronic holes the faster the etching rate, resulting in a more efficient diffusion of electronic holes towards the side walls during the etching process and enabling the formation of μpSi structures on the side walls of pores or wires.
8. Noble metals affect the etching rate but not the geometric features of the resulting pSi.
a) True
b) False
Explanation: The geometric features and morphology of the resulting pSi structures are mainly established by the shape of the noble metal deposited on the surface of the silicon wafer as a result of its catalytic effect, which results in a much faster etching rate at the interface noble metal-silicon.
9. Wire-like pSi structures are produced when the distance between noble metal particles is reduced.
a) True
b) False
Explanation: Usually, well-defined and separated pores are produced from single particles distributed across the silicon surface. In contrast, when the distance between adjacent particles is reduced, wire-like or wall-like pSi structures are produced.
10. Pores produced by metal assisted etching feature _____ structure.
a) square-like
b) cylindrical
c) cone-like
d) needle-like
Explanation: Typically, pores produced by metal assisted etching feature a cone-like structure with pore diameter decreasing with depth. This conical effect can be more or less pronounced, depending on the etching conditions.