**3.9. Oxidation of Rochow reaction byproduct**

Macroporous silicon powders can be synthesized by acid washing and oxidation of Rochow reaction byproducts. Metallurgical-grade silicon powders are ground with Cu-based particles to form a contact mass. The obtained material then reacts with chloromethane CH3 Cl to produce organosilicon compounds (Rochow reaction3 ). The waste contact mass byproduct is composed of unreacted silicon, metal compounds, and deposited carbon. Recovering the metal components by acid washing leaves porous Si/C composite as depicted in **Figure 14**. Such a porous Si/C composite has been successfully used as the anode in lithium-ion batteries [44]. Porous silicon can be obtained by oxidation of the Si/C composite in air at 400°C for 1 h.

### **3.10. Ion implantation**

Sn, and Te [39]. As demonstrated in **Figure 12**, the silicon substrate is placed near a tip and eroded by sparks for at least several hours so that porous structures are formed on its surface. Spark erosion does not involve any chemical reactions in the dissolution of silicon atoms and the removal mechanism is purely physical. The discharges between the tip and the silicon substrate ionize the gaseous environment and the silicon surface is then eroded due to the colliding ions. During their first studies, Hummel and his colleagues used silicon tips and

**Figure 12.** Schematic illustration of porous silicon formation by spark erosion [39].

[38]. However, later studies showed that using a tungsten tip or performing the erosion in air does not increase the impurity level of the substrates [40]. The voltage used for spark processing is in the range of several kilovolts; the currents flow during erosion are in the range of tens of milliamperes; and the average time interval between the sparks is few milliseconds [41]. Although in spark processing chip-based porous silicon structures are formed, the method is incompatible with standard fabrication technology. Moreover, the nonuniform thickness of

Another physical process that has been utilized to remove silicon atoms and synthesize porous layers is the generation of the air optical breakdown plasma near a silicon target which its surface is about to be porosified. As depicted in **Figure 13**, optical breakdown is initiated

repetition rate of 3 Hz) by a Fresnel lens near the silicon target. The intensity of the laser used by Kabashin and Meunier was not enough to initiate the optical breakdown; however, the presence of the silicon target reduces the threshold of the optical breakdown and generates

and contact of the silicon surface with such a plasma lead to localized melting, evaporation, vapor redeposition, recrystallization, erosion of silicon surface, and formation of the porous

to prevent unwanted impurities entering the specimen

laser beam (wavelength of 10.6 μm, pulse energy of 1 J, and

A) [42]. Apparently, light action

performed spark erosion in pure N2

18 Porosity - Process, Technologies and Applications

**3.8. Laser-induced plasma erosion**

by focusing a pulsed TEA CO2

silicon layer [42].

the porous layer is inevitable in this technique.

a high temperature plasma (104 K) with intense currents (106

Macroporous silicon structures can be fabricated by low-energy high-dose ion implantation of silver into monocrystalline silicon wafers without any thermal annealing process [45]. The implantation is carried out at the energy of 30 keV and doses above 1 × 1016 cm−2. Pore formation is presumably driven by microexplosion and voids clustering. It is believed that microexplosion initiates a void and lowers the energy required for the formation of nearby voids; the neighboring voids then cluster to minimize the dangling bonds density [45, 46]. The fabricated porous layer has an average pore size of about 120 nm. It has been observed that during ion implantation, silver atoms are agglomerated inside the pore walls forming nanoparticles with dimen-

<sup>3</sup> Rochow reaction is the most common route to synthesize organosilane monomers in the chemical industry [43].

**Figure 14.** Synthesis and SEM micrographs of porous Si/C composite synthesized by acid washing of Rochow reaction byproduct [44].

fully compatible with standard ULSI technology. Moreover, as it is a low-temperature process (below 400°C), it can be considered as a post-fabrication treatment to implement luminescent porous silicon devices on a chip with microelectronic circuitry. The limitation of this fabrica-

Porous Silicon

21

http://dx.doi.org/10.5772/intechopen.72910

Another bottom-up approach to realize porous silicon is collecting laser-ablated silicon clusters [52]. In this technique, a silicon target is irradiated with a pulsed laser beam in a vacuum chamber as illustrated in **Figure 16**. The laser-ablated silicon clusters are collected by placing the substrate in the vicinity of the target where the ablation plume could reach. The substrate is usually heated to increase adhesion of the porous layer. It is also rotated to increase the uniformity of the deposited film. The porosity and thickness of the porous silicon layer can be controlled by the power of the incident laser, the distance between the substrate and the target, and duration of the ablation. This technique has not attracted much attention for chip-

Mesoporous silicon structures can be fabricated by deposition of high void density crystalline silicon films using low-temperature high-density plasma. An electron cyclotron resonance (ECR)-PECVD tool with hydrogen diluted silane at about 100°C has been utilized for the preparation of thin films composed of nanoscale silicon columns in a void matrix [53]. Porous layers has been realized in the pressure ranges between 5 and 12 mTorr corresponding to microwave power between 640 and 340 W. Formation of the porous structure stems from low mobility and therefore low diffusion length of the deposition species compared to the average distance between the physisorption sites. The low mobility of deposition species can be attributed to the low substrate temperature as well as the low kinetic energy of the impinging ions

tion route is that only thin layers of porous silicon can be synthesized.

**Figure 15.** Schematic illustration of fabrication process of porous silicon by plasma hydrogenation.

based applications due to its incompatibility with standard technology.

**3.13. High-density plasma deposition of silicon**

**3.12. Laser ablation**

sions between 5 and 10 nm. Although the electromagnetic coupling between localized surface plasmon of silver nanoparticles and porous silicon structures promotes the optical behavior of the material, the presence of metallic nanoparticle is undesirable in other application areas.

#### **3.11. Plasma hydrogenation**

This method has been introduced in 2005 for the fabrication of luminescent microporous and mesoporous silicon [47, 48]. In contrast to the fabrication routes already discussed in this chapter, plasma hydrogenation is a bottom-up approach. It starts with the deposition of a thin amorphous silicon (a-Si) layer with a thickness of about 200 nm. The amorphous layer is deposited by physical vapor deposition techniques like evaporation [47, 48] or sputtering [49, 50] instead of chemical vapor deposition to increase the number of voids. Later, the specimens are placed in a DC plasma-enhanced chemical vapor deposition (DC-PECVD) setup to be exposed to DC hydrogen plasma as illustrated in **Figure 15**. After the hydrogenation, a thermal annealing step is performed. It is believed that hydrogen radicals of the plasma replace the dangling bonds of the amorphous silicon layer during the hydrogenation step; then, in the annealing step, the silicon surface is depassivated and H2 is exhausted from the specimen. The energy freed from breaking Si─H bonds promotes the rearrangement of silicon atoms of the specimen and a porous crystalline structure is formed [51]. Although porous silicon can be realized by performing one hydrogenation followed by one annealing step, breaking the process duration into three consecutive repetition of hydrogenation and annealing steps provide more controllability over the properties of the synthesized porous layer. This fabrication process is

**Figure 15.** Schematic illustration of fabrication process of porous silicon by plasma hydrogenation.

fully compatible with standard ULSI technology. Moreover, as it is a low-temperature process (below 400°C), it can be considered as a post-fabrication treatment to implement luminescent porous silicon devices on a chip with microelectronic circuitry. The limitation of this fabrication route is that only thin layers of porous silicon can be synthesized.
