**3. Fabrication of porous silicon**

Since 1956 that porous silicon was discovered, more than 20 methods have been developed to fabricate porous silicon structures. These methods can be divided into two categories: topdown and bottom-up. In the top-down approach, clusters of a monocrystalline silicon wafer are removed to generate voids in an almost perfect crystal, and form a porous structure. This approach relies on chemical and/or physical removal of the atoms from a silicon wafer. The result would be a chip-based porous silicon layer. In contrary, bottom-up approach relies on putting silicon clusters together in such a way that while establishing a crystalline form, leaves empty spaces behind so that a porous structure can be synthesized. These approaches usually lead to porous silicon powders. In this section, all fabrication routes introduced for preparation of porous silicon will be discussed; however, it should be noted that among about 20 fabrication routes, less than a dozen has attracted attention.

#### **3.1. Anodic etching**

Before chemical mechanical polishing became dominant, electropolishing was used for planarization of silicon wafers. In electropolishing, a silicon wafer is placed in an electrochemical cell as the anode, a platinum electrode is utilized as the cathode, and hydrofluoric acid as the electrolyte. Passing electric current through the silicon wafer leads to dissolution of silicon atoms and removal of surface roughness if a critical current density (JPSL) is exceeded. In 1956, something went wrong during an electropolishing process at Bell Labs, and the current in the cell reduced leaving a matt black, brown, or red layer on the surface of the wafer [7]. For more than a decade, it was believed that the matt dark layer formed on the silicon surface was a subfluoride (SiF<sup>2</sup> )*x* grown during the anodic dissolution. Later, it was proposed that the dark film was a dissolution/precipitation product resulted from a two-step disproportionation reaction. Finally, in 1969, it was discovered that the layer indeed has a porous structure formed by dissolution of silicon atoms in an electrochemical etching process [5].

Anodic etching, which is also called electrochemical etching, has been the most common method for the fabrication of porous silicon over the last 60 years. During these years, three electrochemical cell configurations have been utilized for the formation of porous silicon: lateral cell, single cell, and double cell. Lateral cell, which is the simplest electrochemical cell used for anodic etching of silicon, is shown in **Figure 3(a)**. Silicon wafer about to be etched serves as the anode, platinum or any other conducting material resistant to hydrofluoric acid, like graphite, serves as the cathode electrode, and the cell body is made of acid-resistant polymers like PTFE. As the wafer is soaked in HF, any silicon surface that is exposed to the electrolyte is porosified as long as the current density remains below the critical value (J < J PSL). The main advantages of the lateral cell are its simplicity and ability to anodize silicon-on-insulator (SOI) wafers. Its drawback is the nonuniformity in both porosity and thickness of the resulting layer. This inhomogeneity is due to a lateral potential drop across the wafer which leads to nonuniform current density and therefore nonuniform porosity and thickness [8].

The second configuration, single cell, shown in **Figure 3(b)**, is the most common electrochemical cell used for porosification of silicon wafers. In order to provide uniform current density inside the silicon wafer, a back-side contact is used for the anode and the wafer is sealed so that only its front-side could be exposed to the electrolyte. Using this single cell configuration, acceptable porosity and thickness uniformity can be achieved for low resistivity silicon wafers. However, high resistivity wafers need high dose B or P ion implantation and subsequent annealing on their back-side to provide appropriate electrical contact to the external circuit. This implantation and subsequent annealing steps might be even followed by deposition of a thin layer of metal. Single cell configuration provides simultaneous control over porosity and thickness of the porous silicon film. Moreover, illumination which is necessary for n-type silicon wafers, can be easily performed in this cell. Using chemical pumps to circulate the electrolyte further improves the uniformity and minimizes the attachment of hydrogen bubbles to the silicon surface [9].

**3. Fabrication of porous silicon**

Electrical Resistivity 10−2–103

6 Porosity - Process, Technologies and Applications

Free electron mobility 1350 cm2

Hole mobility 480 cm2

Diffusivity 0.8 cm2

Emissive PL wavelength 1000–1200 nm 400–1300 nm

Physiochemical Isoelectric point pH 1.6–2.5 pH 1.6–7.7

**Table 1.** Tunable properties of mesoporous silicon in comparison with those of bulk silicon [4].

Thermal Conductivity 150 W/mK 0.03–20 W/mK

Dielectric constant 11.5 2–8

Specific heat 0.7 J/gK —

Melting point 1414°C 800–1414°C

PL efficiency 10−6 0.01–0.23 (films)

EL efficiency 10−8 0.01–0.1

Surface wettability 5–96° <0.5–167°

Zeta potential (pH 7) –(45–70) mV —

Biochemical Medical biodegradability — Months (implants)

**3.1. Anodic etching**

20 fabrication routes, less than a dozen has attracted attention.

Since 1956 that porous silicon was discovered, more than 20 methods have been developed to fabricate porous silicon structures. These methods can be divided into two categories: topdown and bottom-up. In the top-down approach, clusters of a monocrystalline silicon wafer are removed to generate voids in an almost perfect crystal, and form a porous structure. This approach relies on chemical and/or physical removal of the atoms from a silicon wafer. The result would be a chip-based porous silicon layer. In contrary, bottom-up approach relies on putting silicon clusters together in such a way that while establishing a crystalline form, leaves empty spaces behind so that a porous structure can be synthesized. These approaches usually lead to porous silicon powders. In this section, all fabrication routes introduced for preparation of porous silicon will be discussed; however, it should be noted that among about

**Property Bulk silicon Mesoporous silicon**

Ωcm 103

/s —

/Vs 0.1–30 cm2

/Vs 2–6 cm2

–1012 Ωcm

/Vs

0.01–0.6 (suspensions)

Days (microparticles) Hours (nanoparticles)

/Vs

Before chemical mechanical polishing became dominant, electropolishing was used for planarization of silicon wafers. In electropolishing, a silicon wafer is placed in an electrochemical The last configuration, double cell, is designed to optimize the uniformity of porous silicon layer. It is composed of two half-cells separated by the silicon wafer about to be etched as illustrated in **Figure 3(c)**. Large platinum electrodes which are immersed in both half-cells serve as anode and cathode. The electric current flows from one half-cell to the other through the wafer. Hence, the front-side and back-side of the wafer act as local anode and local cathode. Chemical pumps are used to circulate the electrolyte between the half-cells to prevent any decrease in the local concentration of the electrolyte and remove the hydrogen bubbles. Here, electrolytic contact to the wafer reduces the nonuniformities associated with the back-side metal contact in

were replaced by the Si─H ones, is ruled out. This led to the conclusion that if a surface silicon atom establishes a bond to a fluorine atom, it is immediately removed from the surface and the surface is passivated by hydrogen [13]. The dissolution of a surface atom begins when a hole traveling inside the silicon wafer reaches the interface of silicon and electrolyte (stage 1).

**Figure 4.** Divalent electrochemical dissolution of a silicon atom in hydrofluoric acid solution [13].

with a Si─F bond. Since the electronegativity of hydrogen is close to that of silicon, Si─H bonds are effectively unpolarized; therefore, they could not be influenced by bifluoride anions unless a hole was present. After the first Si─F bond established, due to its polarizing effect, another bifluoride anion attacks the silicon atom releasing a hydrogen molecule as depicted in stage 3 of the figure. The polarization induced by the Si─F groups lowers the electron density of the silicon back-bonds and facilitates the dissolution of the loosely bounded silicon atom by HF. After the removal of a silicon atom, the remaining surface passivates with hydrides again

(stage 5). The overall reaction in the divalent process can be summarized as Eq. (1):

<sup>−</sup> <sup>+</sup> *<sup>h</sup>*<sup>+</sup>→Si F6

If the dissolved silicon atom was removed from a microscopically flat surface, its removal leaves a microroughness. This small topographical alteration changes the distribution of the electric field which increases the probability of presence of the holes. Hence, the etch rate at microroughness becomes greater than the surrounding flat areas. Accordingly, surface rough-

Anodic etching is the most common method for the fabrication of chip-based n-type and p-type porous silicon. All classes of porosity can be realized by anodic etching with proper control over the porosity and thickness. Although there has been success in integration of anodically etched porous silicon structures with electronic circuitry, this fabrication method

Shortly after observation of the matt dark layer on silicon wafer which had been subjected to electrochemical etching, when the porous nature of the material were still unknown, similar structures were realized by electroless chemical etching of silicon in mixture of hydrofluoric acid and concentrated nitric acid solution [15]. Due to the stains formed on the surface of the silicon wafer as a result of electroless chemical dissolution (**Figure 5**), this method became

¯) ion from the solution could attack Si─H bonds replacing one

<sup>2</sup><sup>−</sup> + 2HF + H2 + *e*<sup>−</sup> (1)

Porous Silicon

9

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

At this point a bifluoride (HF<sup>2</sup>

Si + 4H F2

**3.2. Stain etching**

known as stain etching.

ness increases and eventually a porous structure is formed.

is not compatible with standard ULSI technology [14].

**Figure 3.** Electrochemical cell configurations used for the fabrication of porous silicon: (a) lateral cell, (b) single cell, and (c) double cell [8, 10].

the single cell approach. It also resolves the necessity of metallization in high resistivity wafers. By contrast, the equipment setup is complicated in comparison to the other electrochemical cells used for porosification [10]. In this configuration, if anodic etching under illumination is needed, the cell body should be made of transparent acid-resistant materials like PMMA.

The dissolution of silicon atoms by anodic etching can be controlled by either the current or the voltage of the electrochemical cell. Generally, constant current is preferred due to reproducibility and better controllability of porosity and thickness of the porous layer [11]. In the dissolution process, hydrogen gas is freed. The generated hydrogen bubbles are attached to the surface for some time preventing the electro-active species to reach the surface and interrupt the dissolution process. Addition of a surfactant like ethanol or methanol improves the penetration of the electrolyte into the pores and minimizes the hydrogen bubbles evolution [9]. Besides aqueous, ethanolic, or methanolic HF electrolytes, porous silicon can also be formed in the mixture of HF with metal oxides like manganese(IV) oxide MnO2 , or certain organic compounds like acetonitrile CH3 CN and dimethylformamide (DMF).

Depending on the fact that electrochemical cell works either in electropolishing regime (J > J PSL) or porosification (J < J PSL), the charge transfer reactions that lead to the removal of the surface silicon atom, could be a tetravalent or divalent mechanism respectively. **Figure 4** illustrates the divalent mechanism which leads to formation of porous silicon structures. In situ spectroscopy of silicon samples immersed in HF-based solutions has shown that the silicon surface is passivated by Si─H bonds [12]. As the Si─F bond (6 eV) is much stronger than the Si─H bond (3.5 eV), the possibility that the surface had been passivated by fluorine and then Si─F bonds

**Figure 4.** Divalent electrochemical dissolution of a silicon atom in hydrofluoric acid solution [13].

were replaced by the Si─H ones, is ruled out. This led to the conclusion that if a surface silicon atom establishes a bond to a fluorine atom, it is immediately removed from the surface and the surface is passivated by hydrogen [13]. The dissolution of a surface atom begins when a hole traveling inside the silicon wafer reaches the interface of silicon and electrolyte (stage 1). At this point a bifluoride (HF<sup>2</sup> ¯) ion from the solution could attack Si─H bonds replacing one with a Si─F bond. Since the electronegativity of hydrogen is close to that of silicon, Si─H bonds are effectively unpolarized; therefore, they could not be influenced by bifluoride anions unless a hole was present. After the first Si─F bond established, due to its polarizing effect, another bifluoride anion attacks the silicon atom releasing a hydrogen molecule as depicted in stage 3 of the figure. The polarization induced by the Si─F groups lowers the electron density of the silicon back-bonds and facilitates the dissolution of the loosely bounded silicon atom by HF. After the removal of a silicon atom, the remaining surface passivates with hydrides again (stage 5). The overall reaction in the divalent process can be summarized as Eq. (1):

$$\mathrm{Si} + 4\mathrm{H}\,\mathrm{F}\_2^- + h^\* \longrightarrow \mathrm{Si}\,\mathrm{F}\_6^{\mathrm{z}-} + 2\mathrm{HF} + \mathrm{H}\_2 + e^- \tag{1}$$

If the dissolved silicon atom was removed from a microscopically flat surface, its removal leaves a microroughness. This small topographical alteration changes the distribution of the electric field which increases the probability of presence of the holes. Hence, the etch rate at microroughness becomes greater than the surrounding flat areas. Accordingly, surface roughness increases and eventually a porous structure is formed.

Anodic etching is the most common method for the fabrication of chip-based n-type and p-type porous silicon. All classes of porosity can be realized by anodic etching with proper control over the porosity and thickness. Although there has been success in integration of anodically etched porous silicon structures with electronic circuitry, this fabrication method is not compatible with standard ULSI technology [14].

#### **3.2. Stain etching**

, or certain

PSL)

the single cell approach. It also resolves the necessity of metallization in high resistivity wafers. By contrast, the equipment setup is complicated in comparison to the other electrochemical cells used for porosification [10]. In this configuration, if anodic etching under illumination is needed, the cell body should be made of transparent acid-resistant materials like PMMA.

**Figure 3.** Electrochemical cell configurations used for the fabrication of porous silicon: (a) lateral cell, (b) single cell, and

The dissolution of silicon atoms by anodic etching can be controlled by either the current or the voltage of the electrochemical cell. Generally, constant current is preferred due to reproducibility and better controllability of porosity and thickness of the porous layer [11]. In the dissolution process, hydrogen gas is freed. The generated hydrogen bubbles are attached to the surface for some time preventing the electro-active species to reach the surface and interrupt the dissolution process. Addition of a surfactant like ethanol or methanol improves the penetration of the electrolyte into the pores and minimizes the hydrogen bubbles evolution [9]. Besides aqueous, ethanolic, or methanolic HF electrolytes, porous silicon can also be

CN and dimethylformamide (DMF).

PSL), the charge transfer reactions that lead to the removal of the surface

formed in the mixture of HF with metal oxides like manganese(IV) oxide MnO2

Depending on the fact that electrochemical cell works either in electropolishing regime (J > J

silicon atom, could be a tetravalent or divalent mechanism respectively. **Figure 4** illustrates the divalent mechanism which leads to formation of porous silicon structures. In situ spectroscopy of silicon samples immersed in HF-based solutions has shown that the silicon surface is passivated by Si─H bonds [12]. As the Si─F bond (6 eV) is much stronger than the Si─H bond (3.5 eV), the possibility that the surface had been passivated by fluorine and then Si─F bonds

organic compounds like acetonitrile CH3

or porosification (J < J

(c) double cell [8, 10].

8 Porosity - Process, Technologies and Applications

Shortly after observation of the matt dark layer on silicon wafer which had been subjected to electrochemical etching, when the porous nature of the material were still unknown, similar structures were realized by electroless chemical etching of silicon in mixture of hydrofluoric acid and concentrated nitric acid solution [15]. Due to the stains formed on the surface of the silicon wafer as a result of electroless chemical dissolution (**Figure 5**), this method became known as stain etching.

The oxidized silicon atom is then removed by hydrofluoric acid (Eq. (4)), and the residual

Si O2 <sup>+</sup> 6HF→H2 Si F6 <sup>+</sup> <sup>2</sup> H2 <sup>O</sup> (4)

3Si <sup>+</sup> 4HN O3 <sup>+</sup> 18HF→<sup>3</sup> H2 Si F6 <sup>+</sup> 4NO <sup>+</sup> <sup>8</sup> H2 <sup>O</sup> <sup>+</sup> 3(4 <sup>−</sup> *<sup>n</sup>*) *<sup>h</sup>*<sup>+</sup> <sup>+</sup> 3(4 <sup>−</sup> *<sup>n</sup>*)*e*<sup>−</sup> (5)

the mixture of hydrofluoric acid with other compounds such as sodium nitrite NaNO<sup>2</sup>

to formation of porous silicon structures [17–19]. The reaction pathways that lead to the dis-

oxidizing agent of the solution oxidizes silicon atoms and the oxidized atoms are removed by HF, forming a soluble complex. It is also possible to use acetic acid as a surfactant. It does not participate in the chemical reactions; it only dilutes the solution and decreases the surface

Stain etching is the simplest, most straightforward, and most inexpensive way to fabricate porous silicon. It almost needs nothing but a plastic beaker. It can be used to prepare porous silicon on a SOI wafer in which buried oxide layer makes electrochemical etching difficult. The main disadvantage of this method is that there is an upper limit (about 1.5 μm) for the

Fabrication of mesoporous silicon by photoetching has been introduced in 1993 [20]. In the proposed method, silicon wafer is immersed in the aqueous HF solution under illumination of He-Ne laser as shown in **Figure 6**. Although coherent light sources have usually been utilized, photoetching can also be performed by incoherent sources such as Xe lamps and W lamps [21, 22]. Due to the built-in electric field existed in silicon, near its interface with the solution, formation of porous structures by this method is restricted to n-type silicon substrates. Indeed, illumination of both n-type and p-type silicon with proper photon energies (hυ > E<sup>g</sup>

breaks the Si–Si bonds and generates free electron/hole pairs. The charge carriers generated near the surface are drifted by the built-in electric field, but the direction of the built-in electric field is different for n-type and p-type silicon. In the n-type silicon, holes are pushed toward the surface where they can facilitate the removal of a nearby silicon atom, while the opposite direction of the built-in electric field in p-type silicon wafers drives the holes away. Hence,

It should be noted that it is not impossible to prepare p-type porous silicon by photoetching; however, the rate of the

photoetching can only remove silicon atoms from the n-type silicon wafers.2

where *n* being the average number of holes needed to remove one silicon atom [16].

Although porous silicon formation by stain etching usually occurs in HF/HNO3

, iron(III) chloride FeCl3

tension so that better wetting and a smoother surface would be achieved [16].

solution of silicon atoms are similar to those that occur in HF/HNO3

and repeat the cycle.

, and potassium iodate KIO3

solution,

Porous Silicon

11

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can also lead

solution. The strong

,

),

) at the anode site react with HNO3

The overall reaction can be written as Eq. (5):

chromium(VI) oxide CrO3

thickness of the porous silicon layer.

process is extremely disappointing [23].

**3.3. Photoetching**

2

protons (H+

**Figure 5.** The stains formed on the silicon surface after being subjected to electroless chemical etching.

Although an external potential is not applied to the silicon wafer to be etched by this route, dissolution of silicon atoms has a localized electrochemical mechanism. Indeed, sites of the silicon surface temporarily act as local cathode and local anode. Complex charge transfer reactions taking place between these local electrodes lead to the removal of an atom at the local anode site. Few moments later, these local electrode sites will no longer have their previous roles; they might become neutral areas or might even turn into new local electrodes; yet, it is probable that a local anode site will become a local electrode again. When a silicon atom is removed from the surface, the resulting microroughness attracts the electric charge carriers passing by. If this charge carrier was a hole, the site would act as a local anode again. Continuation of this local electrochemical process leads to formation of a porous layer. Due to the electrochemical mechanism of stain etching, holes do have a critical role in the removal of silicon atoms and formation of pores, as they do in the anodic etching of silicon. Hence, p-type silicon which has at least thousands of holes in every cubic micrometers can be easily porosified, while n-type silicon which has negligible holes usually needs illumination.1 Incident photons with proper energy (hυ > E<sup>g</sup> ) generate holes in the n-type silicon and promote stain etching.

The reaction in the local cathode consists of a series of complicated reaction pathways that lead to the reduction of HNO3 , generation of NO, and injection of holes into the silicon. The first stage of these reaction pathways is the formation of HNO<sup>2</sup> which is the rate-limiting step of overall reaction. The cathode reaction can be summarized as Eq. (2):

$$\mathrm{HNO\_3 + 3H^+ \to NO + 2H\_2O + 3h^+} \tag{2}$$

If the injected holes do not recombine with free electrons of the silicon wafer, they could attach to a silicon atom, which then turns to be a local anode and oxidized as depicted by Eq. (3):

$$\text{Si} + 2\,\text{H}\_2\text{O} + n\,h^\* \to \text{Si}\,\text{O}\_2 + 4\,\text{H}^+ + (4-n)\,e^- \tag{3}$$

<sup>1</sup> Stain etching under illumination should not be confused with photoetching which will be discussed in the following subsection.

The oxidized silicon atom is then removed by hydrofluoric acid (Eq. (4)), and the residual protons (H+ ) at the anode site react with HNO3 and repeat the cycle.

$$\text{SiO}\_2 + 6\text{HF} \rightarrow \text{H}\_2\text{SiF}\_6 + 2\text{H}\_2\text{O} \tag{4}$$

The overall reaction can be written as Eq. (5):

$$\text{3Si} + 4\text{HNO}\_3 + 18\text{HF} \rightarrow \text{3H}\_2\text{SiF}\_6 + 4\text{NO} + 8\text{H}\_2\text{O} + \text{3(4-n)}\,h^\* + \text{3(4-n)}\,e^- \tag{5}$$

where *n* being the average number of holes needed to remove one silicon atom [16].

Although porous silicon formation by stain etching usually occurs in HF/HNO3 solution, the mixture of hydrofluoric acid with other compounds such as sodium nitrite NaNO<sup>2</sup> , chromium(VI) oxide CrO3 , iron(III) chloride FeCl3 , and potassium iodate KIO3 can also lead to formation of porous silicon structures [17–19]. The reaction pathways that lead to the dissolution of silicon atoms are similar to those that occur in HF/HNO3 solution. The strong oxidizing agent of the solution oxidizes silicon atoms and the oxidized atoms are removed by HF, forming a soluble complex. It is also possible to use acetic acid as a surfactant. It does not participate in the chemical reactions; it only dilutes the solution and decreases the surface tension so that better wetting and a smoother surface would be achieved [16].

Stain etching is the simplest, most straightforward, and most inexpensive way to fabricate porous silicon. It almost needs nothing but a plastic beaker. It can be used to prepare porous silicon on a SOI wafer in which buried oxide layer makes electrochemical etching difficult. The main disadvantage of this method is that there is an upper limit (about 1.5 μm) for the thickness of the porous silicon layer.

#### **3.3. Photoetching**

Although an external potential is not applied to the silicon wafer to be etched by this route, dissolution of silicon atoms has a localized electrochemical mechanism. Indeed, sites of the silicon surface temporarily act as local cathode and local anode. Complex charge transfer reactions taking place between these local electrodes lead to the removal of an atom at the local anode site. Few moments later, these local electrode sites will no longer have their previous roles; they might become neutral areas or might even turn into new local electrodes; yet, it is probable that a local anode site will become a local electrode again. When a silicon atom is removed from the surface, the resulting microroughness attracts the electric charge carriers passing by. If this charge carrier was a hole, the site would act as a local anode again. Continuation of this local electrochemical process leads to formation of a porous layer. Due to the electrochemical mechanism of stain etching, holes do have a critical role in the removal of silicon atoms and formation of pores, as they do in the anodic etching of silicon. Hence, p-type silicon which has at least thousands of holes in every cubic micrometers can be easily porosified, while n-type

**Figure 5.** The stains formed on the silicon surface after being subjected to electroless chemical etching.

) generate holes in the n-type silicon and promote stain etching. The reaction in the local cathode consists of a series of complicated reaction pathways that

HN O3 <sup>+</sup> <sup>3</sup> H+→NO <sup>+</sup> <sup>2</sup> H2 <sup>O</sup> <sup>+</sup> <sup>3</sup> *<sup>h</sup>*<sup>+</sup> (2)

If the injected holes do not recombine with free electrons of the silicon wafer, they could attach to a silicon atom, which then turns to be a local anode and oxidized as depicted by Eq. (3):

Si <sup>+</sup> <sup>2</sup> H2 <sup>O</sup> <sup>+</sup> *<sup>n</sup> <sup>h</sup>*<sup>+</sup>→Si O2 <sup>+</sup> <sup>4</sup> H+ <sup>+</sup> (4 <sup>−</sup> *<sup>n</sup>*)*e*<sup>−</sup> (3)

Stain etching under illumination should not be confused with photoetching which will be discussed in the following

, generation of NO, and injection of holes into the silicon. The

Incident photons with proper

which is the rate-limiting step

silicon which has negligible holes usually needs illumination.1

first stage of these reaction pathways is the formation of HNO<sup>2</sup>

of overall reaction. The cathode reaction can be summarized as Eq. (2):

energy (hυ > E<sup>g</sup>

1

subsection.

lead to the reduction of HNO3

10 Porosity - Process, Technologies and Applications

Fabrication of mesoporous silicon by photoetching has been introduced in 1993 [20]. In the proposed method, silicon wafer is immersed in the aqueous HF solution under illumination of He-Ne laser as shown in **Figure 6**. Although coherent light sources have usually been utilized, photoetching can also be performed by incoherent sources such as Xe lamps and W lamps [21, 22]. Due to the built-in electric field existed in silicon, near its interface with the solution, formation of porous structures by this method is restricted to n-type silicon substrates. Indeed, illumination of both n-type and p-type silicon with proper photon energies (hυ > E<sup>g</sup> ), breaks the Si–Si bonds and generates free electron/hole pairs. The charge carriers generated near the surface are drifted by the built-in electric field, but the direction of the built-in electric field is different for n-type and p-type silicon. In the n-type silicon, holes are pushed toward the surface where they can facilitate the removal of a nearby silicon atom, while the opposite direction of the built-in electric field in p-type silicon wafers drives the holes away. Hence, photoetching can only remove silicon atoms from the n-type silicon wafers.2

<sup>2</sup> It should be noted that it is not impossible to prepare p-type porous silicon by photoetching; however, the rate of the process is extremely disappointing [23].

**Figure 6.** Porous silicon formation by photoetching.

Porosification of silicon also depends on the wavelength of the incident light. If the energy of the incident photons is less than the silicon bandgap (hυ < E<sup>g</sup> ), the photons are not absorbed by the wafer and the light travels through the material. In this case, neither free electron/hole pairs are generated, nor dissolution of silicon atoms takes place. In contrast, if the energy of the photons is greater than the bandgap of silicon, the light is absorbed and generated holes facilitate the dissolution of silicon atoms and formation of porous structures. Nevertheless, the formation of porous structures does not necessarily lead to the formation of a porous layer. Indeed, the porous layer cannot be formed unless porous structures survive and porosification continues. If the energy of the incident photons is less than the bandgap of the fabricated porous structures, the light reaches the underlying substrate and generates free electron/hole pairs there (**Figure 7(a)**). Generated holes are then promote the removal of silicon atoms; therefore, porosification continues and the porous layer is formed. However, if the energy of the photons is not only greater than the bandgap of silicon but also greater than that of porous silicon structures, photons are absorbed in the porous structures and lead to their removal (**Figure 7(b)**).

optical-power distribution of the incident laser beam and the synthesized porous layer. The dip at the center shows the complete removal of silicon atoms and the marked area illustrates the porous region. It is observed that the thickness of the porous layer linearly increases with the photoetching duration regardless of the crystal orientation of the specimen [20]. Addition

**Figure 8.** Porous silicon layer synthesized by photoetching: (a) micrograph, (b) relation between the power distribution

In addition to the aqueous HF solution, porous silicon structures have been photoetched using aqueous solutions of sodium fluoride NaF and potassium fluoride KF [25]. The chemical reactions lead to the removal of silicon atoms are almost the same. For instance, potassium

KF <sup>+</sup> H2 <sup>O</sup>→K+ <sup>+</sup> <sup>O</sup> <sup>H</sup><sup>−</sup> <sup>+</sup> HF (6)

The holes, generated by illumination, dehydrogenates the surface silicon atoms and facilitate the establishment of surface Si─F bonds. The atoms are then attacked by bifluoride anions and

It should be noted that stain etching can also be performed under illumination in which the incident light generates free electron/hole pairs and increases the etch rate. However, such a photo-assisted chemical dissolution of silicon should not be considered photoetching unless the solution was unable to dissolve silicon by itself and the light was the key factor. However, this classification has not always been followed and photo-assisted stain etching processes

to the HF solution leads to stable formation of porous

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and F¯ ions. Due to negligible solubility of F¯, the dissociation of

O2 or I2

of a mild oxidizing agent like H2

fluoride dissociates into K<sup>+</sup>

KF would be as follows:

layers in much shorter periods of time [24].

of the incident beam and the synthesized porous layer [20].

removed leading to formation of the porous layer.

were incorrectly addressed photoetching [18, 19, 26].

**Figure 8(a)** shows a micrograph of a porous structure formed in HF solution under illumination of the He-Ne laser beam for 60 minutes. Part (b) of this figure depicts the relation of

**Figure 7.** Formation of porous silicon layer depends on the energy of incident photons: (a) porosification continues and a porous layer is formed, and (b) porous structures are dissolved once being formed [20].

**Figure 8.** Porous silicon layer synthesized by photoetching: (a) micrograph, (b) relation between the power distribution of the incident beam and the synthesized porous layer [20].

optical-power distribution of the incident laser beam and the synthesized porous layer. The dip at the center shows the complete removal of silicon atoms and the marked area illustrates the porous region. It is observed that the thickness of the porous layer linearly increases with the photoetching duration regardless of the crystal orientation of the specimen [20]. Addition of a mild oxidizing agent like H2 O2 or I2 to the HF solution leads to stable formation of porous layers in much shorter periods of time [24].

In addition to the aqueous HF solution, porous silicon structures have been photoetched using aqueous solutions of sodium fluoride NaF and potassium fluoride KF [25]. The chemical reactions lead to the removal of silicon atoms are almost the same. For instance, potassium fluoride dissociates into K<sup>+</sup> and F¯ ions. Due to negligible solubility of F¯, the dissociation of KF would be as follows:

$$\mathrm{KF} + \mathrm{H}\_{2}\mathrm{O} \rightarrow \mathrm{K}^{+} + \mathrm{OH}^{-} + \mathrm{HF} \tag{6}$$

The holes, generated by illumination, dehydrogenates the surface silicon atoms and facilitate the establishment of surface Si─F bonds. The atoms are then attacked by bifluoride anions and removed leading to formation of the porous layer.

It should be noted that stain etching can also be performed under illumination in which the incident light generates free electron/hole pairs and increases the etch rate. However, such a photo-assisted chemical dissolution of silicon should not be considered photoetching unless the solution was unable to dissolve silicon by itself and the light was the key factor. However, this classification has not always been followed and photo-assisted stain etching processes were incorrectly addressed photoetching [18, 19, 26].

**Figure 7.** Formation of porous silicon layer depends on the energy of incident photons: (a) porosification continues and

Porosification of silicon also depends on the wavelength of the incident light. If the energy of

the wafer and the light travels through the material. In this case, neither free electron/hole pairs are generated, nor dissolution of silicon atoms takes place. In contrast, if the energy of the photons is greater than the bandgap of silicon, the light is absorbed and generated holes facilitate the dissolution of silicon atoms and formation of porous structures. Nevertheless, the formation of porous structures does not necessarily lead to the formation of a porous layer. Indeed, the porous layer cannot be formed unless porous structures survive and porosification continues. If the energy of the incident photons is less than the bandgap of the fabricated porous structures, the light reaches the underlying substrate and generates free electron/hole pairs there (**Figure 7(a)**). Generated holes are then promote the removal of silicon atoms; therefore, porosification continues and the porous layer is formed. However, if the energy of the photons is not only greater than the bandgap of silicon but also greater than that of porous silicon structures, photons are absorbed in the porous structures and lead to their removal (**Figure 7(b)**). **Figure 8(a)** shows a micrograph of a porous structure formed in HF solution under illumination of the He-Ne laser beam for 60 minutes. Part (b) of this figure depicts the relation of

), the photons are not absorbed by

the incident photons is less than the silicon bandgap (hυ < E<sup>g</sup>

**Figure 6.** Porous silicon formation by photoetching.

12 Porosity - Process, Technologies and Applications

a porous layer is formed, and (b) porous structures are dissolved once being formed [20].

Photoetching can be utilized for the realization of porous silicon on SOI wafers, micromachined wafers and wafers with microelectronic circuitry [24]. If HF solution is used, instead of NaF and KF solutions, the technique does not introduce any metallic impurities to the substrate. One of the main limitations of this route is that it only works for n-type silicon wafers. The other is the nonuniform thickness of the realized porous layer.

#### **3.4. Metal-assisted etching**

The authors of first reports on the fabrication of porous silicon by metal-assisted etching were not aware of the catalyst role in porous silicon formation mechanism and gave incorrect speculations for the appearance of the pores [27]. It was discovered later that the catalytic behavior of metallic nanoparticles is the key factor in realization of porous silicon layers in this technique. Here, metal catalysts such as Al, Ag, Au, Pd, Pt, Fe, or Au-Pd alloy are deposited and patterned on the surface of the silicon wafer. The wafer is then immersed in a solution consisting of hydrofluoric acid and a mild oxidizing agent. The oxidizing agents used in metal-assisted etching are not strong like those utilized in stain etching, so that silicon dissolution only takes place in the presence of the metal catalyst. While hydrogen peroxide H2 O2 is the most common oxidizing agent in metalassisted etching of silicon, porosification can also been performed using sodium persulfate Na2 S2 O8 , iron(III) nitrate Fe(NO3 )3 , potassium dichromate K2 Cr2 O7 , and potassium permanganate KMnO4 [28–30]. The metallic nanoparticle absorbs an electron and injects a hole into the silicon substrate. This hole injection facilitates the oxidation of a nearby silicon atom at the surface. The oxidized silicon atom is then attacked by bifluoride ions and dissolved by a divalent charge transfer reaction.

The injected holes accelerate the silicon atoms removal by a practical etch rate. **Figure 9** demonstrates an SEM image of a sample in which the gold catalyst layer was selectively removed

**Figure 9.** An SEM image of a sample subjected to metal-assisted etching after selective lithographic pattering. Only

coated area of the wafer, there has not been any visible dissolution in the uncoated regions.

(*aq*)→Si F2

(*aq*)→Si F6

By proper adjustment in the ratio of oxidizing agent to hydrofluoric acid, it is possible to control the class of porosity and fabricate mesoporous and macroporous structures [32]. As porous silicon formation only takes place in the area coated with the metal catalysts, any desirable pattern for the porous area can be easily achieved. The introduction of metallic

In 1999, Bessaïs and his colleagues observed that porous silicon can be fabricated on top of a solar cell structure if the device is being sprayed by HF droplets [33]. The dissolution of silicon atoms was attributed to the low velocity of HF droplets initiating the idea of etch-

ing. **Figure 10** schematically demonstrates the experimental setup used for preparation of porous silicon by vapor etching [34]. The polypropylene container partially filled with HF/

 solution is placed in a thermostatic bath. The silicon substrate is positioned inside the container's lid, a few centimeters above the liquid level. The lid must be sealed so that the empty space above the solution level becomes saturated with acid vapor. As the temperature

2−

(*aq*) + 2 H+

which the surface Si─H bonds are replaced by Si─F bonds as depicted in Eq. (8):

−

−

impurities to the substrate is the main disadvantage of metal-assisted etching.

The dissolution of the silicon atom after hole injection is a divalent charge transfer reaction in

solution. While significant etching occurred in the metal-

(*s*) + H2(*g*) + 2HF(*l*) + *e*<sup>−</sup> (8)

(*s*) (9)

Porous Silicon

15

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

(*aq*) + Si H2

droplets that later became known as vapor etch-

O2

(*s*) + *h*<sup>+</sup> + 2H F2

(*s*) + Si(*s*) + 2H F2

The silicon atom is then removed as a result of bifluoride ions attack:

prior to be immersed in HF/H2

regions coated with the metal catalyst were etched [32].

SiH2

SiF2

**3.5. Vapor etching**

HNO3

ing silicon with low velocity HF/HNO3

One way to perform metal-assisted etching is to deposit and pattern metal catalysts and then immerse the predeposited specimen in the etchant consisting of HF and the oxidizing agent. The other way is to immerse the bare silicon wafer in a solution composed of HF and certain metal salts such as silver nitrate AgNO3 , potassium tetrachloroaurate(III) KAuCl4 , and potassium hexachloroplatinate(IV) K2 PtCl6 [30, 31]. Here, metal catalysts are precipitated on the silicon surface, and initiate the dissolution process.

Similar to anodic etching, stain etching, and photoetching discussed in the previous subsections, holes have a key role in dissolution of silicon atoms in the metal-assisted etching. For instance, in gold-assisted porous silicon formation in HF/H2 O2 solution, holes necessary for silicon atoms removal are generated from the reduction of hydrogen peroxide. Due to the enormous difference between the electrochemical potential of hydrogen peroxide and silicon, H2 O2 injects few holes into the valence band of silicon even in the absence of a metal catalyst. Since silicon atoms removal in HF-based solutions stems from the presence of holes near the silicon surface, dissolution of silicon wafers is possible in the HF/H2 O2 solution; however, the etch rate is only few nanometers per hour [13]. The presence of gold catalytically promotes the reduction of H2 O2 and significantly increases the number of injected holes as depicted in Eq. (7):

$$\text{H}\_{2}\text{O}\_{2}\text{\text{-}1}\text{-}\text{H}\_{2}\text{-}\text{-}2\text{H}^{\*}\xrightarrow{\text{H}\_{2}}2\text{H}\_{2}\text{O}\text{\text{-}2}\text{\text{H}}\tag{7}$$

**Figure 9.** An SEM image of a sample subjected to metal-assisted etching after selective lithographic pattering. Only regions coated with the metal catalyst were etched [32].

The injected holes accelerate the silicon atoms removal by a practical etch rate. **Figure 9** demonstrates an SEM image of a sample in which the gold catalyst layer was selectively removed prior to be immersed in HF/H2 O2 solution. While significant etching occurred in the metalcoated area of the wafer, there has not been any visible dissolution in the uncoated regions.

The dissolution of the silicon atom after hole injection is a divalent charge transfer reaction in which the surface Si─H bonds are replaced by Si─F bonds as depicted in Eq. (8):

$$\mathrm{SiH\_2(s)} + \hbar^\* + 2\mathrm{H}\,\mathrm{F\_2^-}\mathrm{aq} \longrightarrow \mathrm{Si}\,\mathrm{F\_2(s)} + \mathrm{H\_2(g)} + 2\mathrm{HF(l)} + e^- \tag{8}$$

The silicon atom is then removed as a result of bifluoride ions attack:

$$\mathrm{SiF}\_2(\mathrm{s}) + \mathrm{Si}(\mathrm{s}) + 2\mathrm{HF}\_2^-(\mathrm{aq}) \longrightarrow \mathrm{Si}\,\mathrm{F}\_6^{2-}(\mathrm{aq}) + 2\,\mathrm{H}^\*(\mathrm{aq}) + \mathrm{Si}\,\mathrm{H}\_2(\mathrm{s})\tag{9}$$

By proper adjustment in the ratio of oxidizing agent to hydrofluoric acid, it is possible to control the class of porosity and fabricate mesoporous and macroporous structures [32]. As porous silicon formation only takes place in the area coated with the metal catalysts, any desirable pattern for the porous area can be easily achieved. The introduction of metallic impurities to the substrate is the main disadvantage of metal-assisted etching.

#### **3.5. Vapor etching**

Photoetching can be utilized for the realization of porous silicon on SOI wafers, micromachined wafers and wafers with microelectronic circuitry [24]. If HF solution is used, instead of NaF and KF solutions, the technique does not introduce any metallic impurities to the substrate. One of the main limitations of this route is that it only works for n-type silicon wafers.

The authors of first reports on the fabrication of porous silicon by metal-assisted etching were not aware of the catalyst role in porous silicon formation mechanism and gave incorrect speculations for the appearance of the pores [27]. It was discovered later that the catalytic behavior of metallic nanoparticles is the key factor in realization of porous silicon layers in this technique. Here, metal catalysts such as Al, Ag, Au, Pd, Pt, Fe, or Au-Pd alloy are deposited and patterned on the surface of the silicon wafer. The wafer is then immersed in a solution consisting of hydrofluoric acid and a mild oxidizing agent. The oxidizing agents used in metal-assisted etching are not strong like those utilized in stain etching, so that silicon dissolution only takes place in the presence of the metal

assisted etching of silicon, porosification can also been performed using sodium persul-

a hole into the silicon substrate. This hole injection facilitates the oxidation of a nearby silicon atom at the surface. The oxidized silicon atom is then attacked by bifluoride ions

One way to perform metal-assisted etching is to deposit and pattern metal catalysts and then immerse the predeposited specimen in the etchant consisting of HF and the oxidizing agent. The other way is to immerse the bare silicon wafer in a solution composed of HF and certain

Similar to anodic etching, stain etching, and photoetching discussed in the previous subsections, holes have a key role in dissolution of silicon atoms in the metal-assisted etching. For

silicon atoms removal are generated from the reduction of hydrogen peroxide. Due to the enormous difference between the electrochemical potential of hydrogen peroxide and silicon,

etch rate is only few nanometers per hour [13]. The presence of gold catalytically promotes

H2 O2 <sup>+</sup> <sup>2</sup> H+ *Au* ⎯→<sup>2</sup> H2 <sup>O</sup> <sup>+</sup> <sup>2</sup> *<sup>h</sup>*<sup>+</sup> (7)

 injects few holes into the valence band of silicon even in the absence of a metal catalyst. Since silicon atoms removal in HF-based solutions stems from the presence of holes near the

, potassium dichromate K2

[28–30]. The metallic nanoparticle absorbs an electron and injects

, potassium tetrachloroaurate(III) KAuCl4

O2

and significantly increases the number of injected holes as depicted in

[30, 31]. Here, metal catalysts are precipitated on the

O2

)3

PtCl6

O2 is the most common oxidizing agent in metal-

Cr2 O7

, and potassium

, and potas-

solution, holes necessary for

solution; however, the

The other is the nonuniform thickness of the realized porous layer.

**3.4. Metal-assisted etching**

14 Porosity - Process, Technologies and Applications

catalyst. While hydrogen peroxide H2

metal salts such as silver nitrate AgNO3

O2

sium hexachloroplatinate(IV) K2

, iron(III) nitrate Fe(NO3

and dissolved by a divalent charge transfer reaction.

silicon surface, and initiate the dissolution process.

instance, in gold-assisted porous silicon formation in HF/H2

silicon surface, dissolution of silicon wafers is possible in the HF/H2

fate Na2

H2 O2

Eq. (7):

the reduction of H2

S2 O8

permanganate KMnO4

In 1999, Bessaïs and his colleagues observed that porous silicon can be fabricated on top of a solar cell structure if the device is being sprayed by HF droplets [33]. The dissolution of silicon atoms was attributed to the low velocity of HF droplets initiating the idea of etching silicon with low velocity HF/HNO3 droplets that later became known as vapor etching. **Figure 10** schematically demonstrates the experimental setup used for preparation of porous silicon by vapor etching [34]. The polypropylene container partially filled with HF/ HNO3 solution is placed in a thermostatic bath. The silicon substrate is positioned inside the container's lid, a few centimeters above the liquid level. The lid must be sealed so that the empty space above the solution level becomes saturated with acid vapor. As the temperature

**3.6. Reactive-ion etching**

this SiO*<sup>x</sup>*

of view.

F*y*

**3.7. Spark erosion**

subsequences (oxidation and fluorination) [36].

Chemically stable mesoporous and macroporous silicon structures can be formed as a result of a

[36]. The sequential process is composed of one etching and two passivation subsequences:

radicals of the plasma remove the silicon atoms by a combination of chemical and physical etching mechanisms. During the oxidation subsequence, a very thin oxide layer is formed on

quence, fluorination, the dangling bonds of the silicon/oxide interface are replaced by Si─F

been the inevitable weakness of porous silicon structures formed by HF-based processes [37].

By proper adjustment of the gases flow rates, plasma powers, durations of the subsequences, and repetition, it is possible to tailor the porosity and thickness of the mesoporous or macroporous material [36]. Since the fabrication process is performed at room temperature, it can be used as a post-fabrication treatment, which is very important in a technological point

Spark erosion that later became known as spark processing is another top-down porous silicon fabrication route introduced in the early 1990s [38]. This method can be used to prepare not only porous silicon but also other porous materials such as As, Bi, Ge, GaAs, Sb, Se,

**Figure 11.** The sequential RIE process to synthesize porous silicon is composed of one etching and two passivation

layer stabilizes the chemical properties of the final porous structure, which always

F*y*

oxidation and fluorination, as depicted in **Figure 11**. In the etching subsequence, SF6

The porous silicon structures evolve by the repetition of these three subsequences.

, O2

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

plasma. In the following subse-

passivating film. Formation of

, and SF6

plasma

Porous Silicon

17

ions and

maskless sequential reactive-ion etching (RIE) of silicon wafers using H2

the surface of the pores as a result of their exposure to the O2

bonds, transforming the thin oxide layer into a stable SiO*<sup>x</sup>*

**Figure 10.** Schematic illustration of vapor etching setup used for porous silicon fabrication [34].

increases, supersaturation leads to the formation of droplets on the container walls and the silicon surface. In case of extremely rough silicon surfaces, the droplets will attach and form a liquid film which destroys the porous layer [34].

In order to achieve homogenous porous layers, the kinetics of the process and hence the determining factors, i.e., concentration and temperature of the solution as well as duration of vapor exposure, must be controlled. Increasing either the HNO3 content of the solution or the temperature of the process significantly increases the etch rate. For temperatures between 20 and 30°C, the etch rate is rather low and homogenous porous layers can be achieved by exposure times up to 30 min. However, increasing the temperature to 40°C leads to speedy condensation of droplets on the silicon surface and destruction of porous layers unless the exposure time remains below 20 min. At 60°C and beyond, the speed of condensation is so fast that porous layers can only be synthesized in exposure times less than 2 min.

The vapor etching method is rather simple and inexpensive. It can be used for preparation of luminescent silicon structures and antireflection coatings in solar cells where thin layers of porous silicon are required. The concentration of the solution, temperature, and duration of the vapor etching process have to be controlled precisely or ammonium hexafluorosilicate (NH<sup>4</sup> ) 2 SiF6 , and Si/SiO*<sup>x</sup>* nanoparticles will be formed instead of porous silicon [35]. The method is unable to produce highly porous structures as the condensation of the droplets leads to the formation of a liquid film on the surface, which in turn results in the removal of the porous layer. The area of specimen should be chosen small in comparison to the diameter of the container or the uniformity in porosity and thickness will be lost.

### **3.6. Reactive-ion etching**

Chemically stable mesoporous and macroporous silicon structures can be formed as a result of a maskless sequential reactive-ion etching (RIE) of silicon wafers using H2 , O2 , and SF6 plasma [36]. The sequential process is composed of one etching and two passivation subsequences: oxidation and fluorination, as depicted in **Figure 11**. In the etching subsequence, SF6 ions and radicals of the plasma remove the silicon atoms by a combination of chemical and physical etching mechanisms. During the oxidation subsequence, a very thin oxide layer is formed on the surface of the pores as a result of their exposure to the O2 plasma. In the following subsequence, fluorination, the dangling bonds of the silicon/oxide interface are replaced by Si─F bonds, transforming the thin oxide layer into a stable SiO*<sup>x</sup>* F*y* passivating film. Formation of this SiO*<sup>x</sup>* F*y* layer stabilizes the chemical properties of the final porous structure, which always been the inevitable weakness of porous silicon structures formed by HF-based processes [37]. The porous silicon structures evolve by the repetition of these three subsequences.

By proper adjustment of the gases flow rates, plasma powers, durations of the subsequences, and repetition, it is possible to tailor the porosity and thickness of the mesoporous or macroporous material [36]. Since the fabrication process is performed at room temperature, it can be used as a post-fabrication treatment, which is very important in a technological point of view.

### **3.7. Spark erosion**

increases, supersaturation leads to the formation of droplets on the container walls and the silicon surface. In case of extremely rough silicon surfaces, the droplets will attach and form

In order to achieve homogenous porous layers, the kinetics of the process and hence the determining factors, i.e., concentration and temperature of the solution as well as duration

the temperature of the process significantly increases the etch rate. For temperatures between 20 and 30°C, the etch rate is rather low and homogenous porous layers can be achieved by exposure times up to 30 min. However, increasing the temperature to 40°C leads to speedy condensation of droplets on the silicon surface and destruction of porous layers unless the exposure time remains below 20 min. At 60°C and beyond, the speed of condensation is so

The vapor etching method is rather simple and inexpensive. It can be used for preparation of luminescent silicon structures and antireflection coatings in solar cells where thin layers of porous silicon are required. The concentration of the solution, temperature, and duration of the vapor

 nanoparticles will be formed instead of porous silicon [35]. The method is unable to produce highly porous structures as the condensation of the droplets leads to the formation of a liquid film on the surface, which in turn results in the removal of the porous layer. The area of specimen should be chosen small in comparison to the diameter of the container or the unifor-

fast that porous layers can only be synthesized in exposure times less than 2 min.

etching process have to be controlled precisely or ammonium hexafluorosilicate (NH<sup>4</sup>

content of the solution or

) 2 SiF6 , and

a liquid film which destroys the porous layer [34].

16 Porosity - Process, Technologies and Applications

mity in porosity and thickness will be lost.

Si/SiO*<sup>x</sup>*

of vapor exposure, must be controlled. Increasing either the HNO3

**Figure 10.** Schematic illustration of vapor etching setup used for porous silicon fabrication [34].

Spark erosion that later became known as spark processing is another top-down porous silicon fabrication route introduced in the early 1990s [38]. This method can be used to prepare not only porous silicon but also other porous materials such as As, Bi, Ge, GaAs, Sb, Se,

**Figure 11.** The sequential RIE process to synthesize porous silicon is composed of one etching and two passivation subsequences (oxidation and fluorination) [36].

Using laser-induced plasma erosion, luminescent mesoporous silicon has been prepared on both p-type and n-type silicon wafers regardless of the dopant type or concentration of the silicon wafers. The porosity of the layers is typically between 40 and 70% with thicknesses up to 500 nm. However, the thickness of the porous region shows significant nonuniformity. The area of the porous region varies between hundreds of micrometers and several millimeters depending on the position of the silicon target with respect to the focal plane of the lens.

**Figure 13.** Schematic illustration of the experimental setup used for laser-induced plasma erosion (The additional target

shown in the figure was used for control tests and was not necessary for porous silicon formation.) [42].

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

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.

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-

Rochow reaction is the most common route to synthesize organosilane monomers in the chemical industry [43].

Cl to

Porous Silicon

19

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

). The waste contact mass byproduct

**3.9. Oxidation of Rochow reaction byproduct**

**3.10. Ion implantation**

3

produce organosilicon compounds (Rochow reaction3

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

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 performed spark erosion in pure N2 to prevent unwanted impurities entering the specimen [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 the porous layer is inevitable in this technique.

#### **3.8. Laser-induced plasma erosion**

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 by focusing a pulsed TEA CO2 laser beam (wavelength of 10.6 μm, pulse energy of 1 J, and 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 a high temperature plasma (104 K) with intense currents (106 A) [42]. Apparently, light action 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 silicon layer [42].

**Figure 13.** Schematic illustration of the experimental setup used for laser-induced plasma erosion (The additional target shown in the figure was used for control tests and was not necessary for porous silicon formation.) [42].

Using laser-induced plasma erosion, luminescent mesoporous silicon has been prepared on both p-type and n-type silicon wafers regardless of the dopant type or concentration of the silicon wafers. The porosity of the layers is typically between 40 and 70% with thicknesses up to 500 nm. However, the thickness of the porous region shows significant nonuniformity. The area of the porous region varies between hundreds of micrometers and several millimeters depending on the position of the silicon target with respect to the focal plane of the lens.
