**3. Fabrication of silicon nanostructures**

Silicon nanostructures are fabricated using chemical/electrochemical technique [43]. Samples of p-type <100> silicon wafers (380 ± 10 μm thickness with resistivity about 5–10 Ω cm from Silicon Material Inc.) were cut in 1 × 1 cm2 pieces and cleaned using RCA technique. In the first step of etching, samples were textured by pyramidal structures through anisotropic etching in tetramethylammonium hydroxide (TMAH) based solution. The solution for anisotropic etching, made of equal amount of TMAH and isopropyl alcohol (IPA) (5 wt% each), was used to etch the samples for 20 minutes. The temperature of the solution was kept constant at 90°C using an oil bath system. A condenser covered the etchant container in order

#### **Figure 3.**

*Experimental results of the evolution of the nanostructures. (a) Anisotropic etching of <100> p-type silicon at 90°C for 20 minutes in 5 wt% TMAH and 5 wt% IPA resulted in formation of pyramids that covers the surface completely. (b) Top view of the electrochemically etched textured silicon after 10 minutes of etching. (c) Top view of the electrochemically etched silicon after 70 minutes. (d) Top view of the electrochemically etched silicon after 70 minutes. (d-inset) Tilted view of the final nanostructures illustrates the existence of residual walls around the structures with the thickness of 100 nm.*

**105**

cm3

anodic etching.

*Miniaturized Gas Ionization Sensor Based on Field Enhancement Properties of Silicon…*

to keep the etchant concentration constant throughout the experiment. **Figure 3a** shows the SEM image of textured Si sample containing square-based pyramidal structures after anisotropic etching step. As it is shown in the figure, the average tip-to-tip separation of the large pyramids is more than 5 μm. The samples were subsequently exposed to electrochemical isotropic etching. In this step the textured silicon samples, are placed in a two-electrode cell of anodic etching. The etching was done in an electrolyte consisting of 1:3 hydrofluoric acid and ethanol for 70 minutes

electrochemical etching, all around the pyramids. **Figure 3b** shows the SEM image of electrochemically etched sample after 10 minutes of etching. As the etching continues, the pyramid faces are etched away, while lateral edges remain unetched. **Figure 3c** and **d** shows the top view and tilted view SEM image of the sample after 70 minutes isotropic etching. To narrow down the lateral edges, samples are subjected to another anisotropic etching in a weak etchant solution (1 wt% of TMAH and 1 wt% of IPA) for 30 seconds. The final structures are shown in the inset of **Figure 3d**. They are arrow shape structures with remaining lateral edges with the

Electrochemical dissolution of Si, in the second step of etching, is highly influenced by hole current density at the interface of Si/electrolyte. Hole drift current

Where e is electric charge (1.6 × 10<sup>−</sup>19 coulomb), p is the concentration of holes/

Due to uneven textured Si surfaces, the electric field is not uniform at the interface. As a result, the hole drift current density is not uniform over the surface of the samples and different areas are experiencing different dissolution rates during the

The electric field intensity generated at textured Si (anode) is modeled and simulated utilizing COMSOL, electrostatics module. To study the evolution mechanism we have considered the 3D schematic geometry and initial electric field distribution shown in **Figure 4** when voltage is applied to the backside of the textured Si. Simulation results shows that the electric field is not uniform and the lowest belongs to tips and lateral edges of the pyramids, and highest is for pyramid perimeters. To present the development of the structures, COMSOL deformed geometry (dg) physics interface is applied to all domains. A prescribed mesh velocity is assigned to the boundary between Si and electrolyte (pyramidal texture) and a prescribed mesh displacement is assigned to boundaries all around the block. Moving

**Figure 5** clearly shows that the etching and developing porous silicon start at the

edges of the pyramids bases where the electric field has the maximum value. As the etching extends, pyramid faces are etched away while lateral edges remain as connecting walls between the pyramids. **Figure 6** clearly illustrates the

Eq. 4 confirms the dependence of the etching rate with the electric field strength. According to **Figure 3b** the pyramid perimeters are experiencing a higher etching rate at the beginning of the etching process. This can be attributed to

\_\_\_\_ *A*

/V s) and E is the applied electric field (V/cm).

*cm*<sup>2</sup>) (4)

applied anodic current density. Porous silicon starts to form by

*DOI: http://dx.doi.org/10.5772/intechopen.84264*

under a 15 mA/cm2

thickness about 100 nm.

**4. Formation mechanism of the nanostructures**

*Jp* <sup>=</sup> *<sup>e</sup>* <sup>×</sup> *<sup>p</sup>* <sup>×</sup> *μp* <sup>×</sup> *<sup>E</sup>*(

maximum electric field intensity at those regions.

mesh velocity is defined proportional to electric field.

residual walls between the structures.

density in semiconductors can be found by,

, *μp* is the hole mobility (cm<sup>2</sup>

*Miniaturized Gas Ionization Sensor Based on Field Enhancement Properties of Silicon… DOI: http://dx.doi.org/10.5772/intechopen.84264*

to keep the etchant concentration constant throughout the experiment. **Figure 3a** shows the SEM image of textured Si sample containing square-based pyramidal structures after anisotropic etching step. As it is shown in the figure, the average tip-to-tip separation of the large pyramids is more than 5 μm. The samples were subsequently exposed to electrochemical isotropic etching. In this step the textured silicon samples, are placed in a two-electrode cell of anodic etching. The etching was done in an electrolyte consisting of 1:3 hydrofluoric acid and ethanol for 70 minutes under a 15 mA/cm2 applied anodic current density. Porous silicon starts to form by electrochemical etching, all around the pyramids. **Figure 3b** shows the SEM image of electrochemically etched sample after 10 minutes of etching. As the etching continues, the pyramid faces are etched away, while lateral edges remain unetched. **Figure 3c** and **d** shows the top view and tilted view SEM image of the sample after 70 minutes isotropic etching. To narrow down the lateral edges, samples are subjected to another anisotropic etching in a weak etchant solution (1 wt% of TMAH and 1 wt% of IPA) for 30 seconds. The final structures are shown in the inset of **Figure 3d**. They are arrow shape structures with remaining lateral edges with the thickness about 100 nm.
