Metal Ions’ Dynamic Effect on Metal-Assisted Catalyzed Etching of Silicon in Acid Solution

Abstract

Metal-assisted catalyzed etching (MACE) technology is convenient and efficient for fabricating large-area silicon nanowires at room temperature. However, the mechanism requires further exploration, particularly the dynamic effect of various ions in the acid-etching solution. This paper investigated the MACE of silicon wafers predeposited with metal nanofilms in an HF-M(NO₃)x-H₂O etching solution (where M(NO₃)x is the nitrate of the fourth-period elements of the periodic table). The oxidizing ability of Fe³⁺ and NO₃⁻ was demonstrated, and the dynamic influence of metal ions on the etching process was discussed. The results show that the MACE of silicon can be realized in various HF-M(NO₃)_x-H₂O etching solutions, such as KNO₃, Al(NO₃)₃, Cr(NO₃)₃, Mn(NO₃)₂, Ni(NO₃)₂, Co(NO₃)₂, HNO₃, and Ca(NO₃)₂. It is confirmed that the concentration and type of cations in the etching solution affect the etching rate and morphology of silicon. Fe³⁺ and NO₃⁻ act as oxidants in catalytic etching. The fastest etching rate is about 5~6 μm/h in Ni(NO₃)₂, Co(NO₃)₂, and Ca(NO₃)₂ etching solutions. However, a high concentration of K⁺ hinders silicon etching. This study expands the application of MACE etching solution systems.

1. Introduction

Silicon nanowires (SiNWs) have attracted widespread attention in the scientific community in recent decades because of their special physical and chemical properties [1,2,3,4,5,6]. There are various methods for synthesizing SiNWs. In 1996, hydrothermal technology was first used to prepare porous silicon [7,8]. The reactor was filled with the HF-Fe(NO₃)₃-H₂O solution and heated by a thermostat to provide high temperature and high pressure to promote the reaction. This hydrothermal technology enables the preparation of porous silicon without an external electric power supply. However, this method requires closed containers like autoclaves. In 2005, K.Q. Peng deposited a Ag or Au nanoparticle layer by electroless deposition on the surface of the silicon wafer, immersed the processed silicon wafer in an HF-Fe(NO₃)₃-H₂O solution at room temperature for half an hour, and successfully synthesized large-area SiNWs using metal-assisted catalyzed etching (MACE), which is more orderly [9]. In addition, Peng studied the mechanism of MACE and explained the flow of electrons and holes in the oxidation–reduction etching process of silicon driven by galvanic cells based on band bending theory [9,10,11]. The contact interface between noble metal particles and silicon can be considered a typical metal–semiconductor contact.

The technology generally adopts a two-step method to prepare nanosilicon by MACE in a liquid system. Firstly, the surface of the cleaned silicon wafer is coated with a noble Ag or Au nanoparticle layer [12,13,14,15,16,17,18,19]. Secondly, the noble-metal-coated silicon wafer is immersed in an etching solution containing HF and an oxidant. The currently reported etching solution systems include HF-H₂O₂-H₂O solution and HF-Fe(NO₃)₃-H₂O solution. It is believed that the above-mentioned electroless deposition of the metal nanoparticle layer and MACE of silicon are both localized microelectrochemical redox processes, including the cathode and anode reactions [20]. Taking the electroless deposition of the silver nanoparticle layer on the silicon wafer surface as an example, the first step of MACE in the liquid system, when the clean silicon wafer is immersed in HF-AgNO₃-H₂O solution, Ag⁺ is oxidation, which injects holes into adjacent silicon to gain electrons, which are reduced to Ag that gradually collects on the surface of the silicon wafer to form a uniform nanoparticle layer. The silicon injected with holes is oxidized and highly unstable in hydrofluoric acid solution and reacts with hydrofluoric acid to form the soluble H₂SiF₆ [21,22,23,24]. As the reaction goes on, the silver particles continually grow and accumulate on the surface of the silicon wafer. Finally, the silicon wafer in the plating solution is tightly covered by a uniform film consisting of silver nanoparticles on the surface. The second step of MACE in the liquid system is essentially an electrochemical etching process driven by the galvanic cell. The dispersed Ag nanoparticles deposited on the surface of the silicon wafer combine with the silicon substrate to form a galvanic cell. The galvanic cell can accelerate the etching of the silicon substrate covered by the metal particles, while the uncovered part of the substrate would be retained. Thereby, a neat array of silicon nanowires is formed on the surface. Taking the reaction in the HF-Fe(NO₃)₃-H₂O solution system as an example, with the catalysis of silver nanoparticles, the Fe³⁺ can inject a hole into the silicon, obtain electrons, and reduce to Fe²⁺. As with the reaction in the deposition of the silver nanoparticle layer, holes are injected into the silicon atoms covered by the silver nanoparticles, and the silicon atoms are oxidized. Therefore, the location of the silicon atom at the bottom of the silver nanoparticle becomes a small hole into which the silver nanoparticle “sinks”. As the reaction progresses, more Fe³⁺ ions in the solution inject holes into silicon atoms through the silver particles. The silicon atoms are continually oxidized to SiO₂, which reacts with HF to form silicofluoride radical dissolved in solution. The pits at the bottom of the silver nanoparticles continue to deepen, and the silver nanoparticles gradually sink into the interior of the silicon substrate as the pits grow.

However, it is worth noting that the noble Ag nanoparticle layers are not as stable as they are supposed to be, especially when the oxidant is strong, as there is a dynamic equilibrium of deposition and dissolution of the noble metal particle films in the etching solution. Furthermore, various ions lead to a complex reaction equilibrium, as the Fe³⁺ and the NO₃⁻ ions in the etching solution are also oxidizing but neglected. Focusing on these questions, the objective of this work is to verify the role of Fe³⁺ as the oxidant in the MACE process and to further discuss whether NO₃⁻ can inject holes into the silicon substrate as the oxidant in MACE. This work also focuses on the influence of the dynamic effect of metal ions in M(NO₃)_x on the MACE process to further explore the mechanism of the MACE process, which helps the expansion of the application and building of low-cost and easy-to-operate systems to prepare large-area SiNW arrays.

2. Materials and Methods

One-side-polished single crystalline N-type (100) 2–2.7 Ω·cm silicon wafers were selected and purchased from Beijing General Research Institute for Nonferrous Metals (GRINM). CH₃COCH₃, C₂H₅OH, and HF were purchased from Beijing Sinopharm Chemical Reagent Co., Ltd., Beijing, China, and deionized water was prepared. The other chemicals used were purchased from Sigma–Aldrich (St. Louis, MO, USA) and used without further treatment. The silicon wafers were cut into 2 × 2 cm² and cleaned ultrasonically in deionized (DI) water, CH₃COCH_3, and C₂H₅OH for 10 min, respectively. Then, the silicon wafers were rinsed three times with DI water and immersed in H₂SO₄-H₂O₂ solution at 80 °C for 30 min. Finally, enough DI water was used to clean the remaining acid on the silicon wafers. The silicon wafers were placed in HF-AgNO₃-H₂O or HF-AuCl₄H-H₂O solution for 2 min in a fume cupboard. Then, the silicon wafer was removed and cleaned with DI water, and then transferred into sealed Teflon-lined autoclaves at a temperature of 50 °C for one hour. The total volume of the etching solution used in all the experiments in this paper was 50 mL. The concentration of HF in the etching solution was 4.6 M if not specially stated in this paper. After the reaction, the silicon wafers were taken out, cleaned, and dried with the SiNWs on the surface. The silicon wafers were cut into 1 × 1 cm², and the cross- and top-sectional morphologies were characterized by a high-resolution field-emission scanning electron microscope (SEM, HITACHI, S-4800, Hitachi of Japan, Tokyo, Japan). SiNWs were scraped off with a razor blade, dispersed ultrasonically in ethanol for 10 min, and then dropped on copper mesh to dry before being characterized by transmission electron microscopy (TEM, JEOL JEM-2100, Japan Electronics, Tokyo, Japan). The crystal composition was investigated by X-ray diffraction (XRD, Cu kα, SmartLab SE, Rigaku, Corporation, Tokyo, Japan).

3. Results and Discussion

3.1. Oxidation Validation of Fe³⁺ and NO₃⁻

Because of the catalysis of Ag (or Au and Pt), neat SiNW array structures were successfully prepared on both P-type and N-type silicon substrates, according to previous research [25,26,27,28]. However, the etching morphology of the P-type and N-type silicon wafers with normal doping levels in the HF-Fe(NO₃)₃-H₂O are almost the same, and the crystal orientations of the SiNW arrays are determined by the crystal orientation of the substrates. Therefore, an N-type (100) silicon wafer with a resistivity of 2.2 Ω·cm was used as the research object, with Ag as the catalyst. First, the cleaned silicon wafers without silver plating were directly immersed in an etching solution consisting of 4.6 M HF and 1.5 M FeCl₃ (HF-FeCl₃-H₂O) and reacted at 50 °C for 30 min. As shown in Figure 1a, a porous layer about 200 nm thick appeared on the surface of the silicon wafer. This indicates that a redox reaction occurred, and a small amount of silicon was dissolved to form a porous layer, which is consistent with the research results of porous silicon. Next, a silver nanoparticle layer was coated on the surface of the cleaned silicon wafer by the electroless deposition technique described above, and the sample was then put into the HF-FeCl₃-H₂O solution at 50 °C for 30 min. As shown in Figure 1b,c, there was no sign of etching on the silicon wafer surface, but some micro- and nanospheres appeared. The surfaces of the spheres were not smooth and seemed to have some attachments.

According to relevant reports, these spheres may be AgCl. They were formed from silver nanoparticle layers that dissolved into Ag⁺ and combined with Cl⁻ in the system to form AgCl spheres [29,30,31,32]. Then, the sample was soaked in concentrated nitric acid for half an hour. As shown in Figure 1d,e, these small balls still existed and became smoother. It proves that these balls were indeed AgCl balls, and the surface attachments may be silver dissolved in concentrated nitric acid. For further verification, the crystal composition of the samples was characterized by XRD. The clean silicon wafer coated with a silver nanoparticle layer was directly tested. The XRD curve is shown in Figure 1f(a), and only the characteristic peaks of silver appear around 38° and 44.5°. After the silver-plated silicon wafer was immersed in the HF-FeCl₃-H₂O solution at 50 °C for 30 min, the characteristic peaks of silver (JCPDS#: 41-1402; the grazing incidence angle was 5°) became weak (Figure 1f(b)), indicating that most silver particles had disappeared.

Compared with the standard XRD pattern, several new characteristic peaks corresponded exactly to those of AgCl (JCPDS#: 31-1238; the grazing incidence angle was 5°), indicating that the small balls in the above pictures were AgCl. Although the silver-plated silicon wafers did not become SiNW arrays when immersed into the HF- FeCl₃-H₂O solution, as was initially expected, this does not prove the inability of Fe³⁺ to oxidize. It was suspected that the primary reason for the failure of the expected effect was that silver is unstable in the environment of Fe³⁺ and becomes Ag⁺ combined with Cl⁻. The results may be different when changing the silver-coated silicon wafer for the gold-coated silicon wafer.

To verify that Fe³⁺ ions can act as oxidants to etch silicon using electroless deposition technology, a clean N-type silicon wafer (100) with 2–2.7 Ω·cm resistivity was immersed in a plating solution mixed with AuCl₄H and HF for 1 min to plate a gold nanoparticle layer on the surface. Then, the gold-coated silicon wafer was placed in the etching solution mentioned above, consisting of 4.6 M HF and FeCl₃ etching solution with concentrations of 0 M, 1 M, 1.5 M, and 2 M, respectively, and reacted at 50 °C for 30 min. As shown in Figure 2a–d, the one-dimensional SiNW array structures were successfully prepared, and with the increase of the concentration of FeCl₃, SiNW array structures with different lengths were obtained. The above results indicate that Fe³⁺ is more electronegative than silicon and acts as an oxidant in the HF-FeCl₃-H₂O system. When clean silicon wafers are immersed in HF-AuCl₄H-H₂O solution, Au³⁺ is oxidizing and able to inject holes into the adjacent silicon to gain electrons and reduce to Au, which gradually gathers on the surface of silicon wafers to form uniform nanoparticle layers. The silicon injected with holes is oxidized and is extremely unstable in HF solution, reacting with HF to form soluble H₂SiF₆ [21,22,23,24] and resulting in the structure of SiNWs (Figure 2a). Then, the gold-plated silicon wafer was immersed in the HF-FeCl₃-H₂O system. With gold particles as the reactive center, the Fe³⁺ ions in the solution injected holes into the silicon atoms under the gold nanoparticles, extracted electrons, and were reduced to Fe²⁺ ions. The silicon under the gold nanoparticles was oxidized and quickly dissolved in HF, leaving small pits. With the continuous progress of the etching, the small pits kept getting deeper, and the gold nanoparticles also moved to the inside of the silicon substrate along with the pits, leaving the uncoated silicon wafer as a silicon wire array structure.

The above results fully verify that Fe³⁺ can work as the oxidant in the MACE process [9]. To verify the effect of NO₃⁻, the silicon wafer with a deposited silver nanoparticle layer was prepared in a solution containing 0.01 M AgNO₃ and 4.6 M HF, and the HF-Fe(NO₃)₃-H₂O etching solution system was replaced with HF-HNO₃-H₂O, keeping the concentration of NO₃⁻ consistent. The silicon wafer with silver coating was immersed in two sealed Teflon-lined autoclaves containing HF-Fe(NO₃)₃-H₂O and HF-HNO₃-H₂O solution, respectively, with a treatment temperature of 50 °C for one hour. The cross-sectional SEM mages of SiNW etching in HF-Fe(NO₃)₃-H₂O and HF-HNO₃-H₂O solution for 1 h are shown in Figure 3a and Figure 3b, respectively. Figure 3a shows that the silicon wafer was exactly etched in the HF-HNO₃-H₂O system. The etching rate of Si in the HF-Fe(NO₃)₃-H₂O solution was faster than that in the HF-HNO₃-H₂O solution, with a consistent molarity of NO₃⁻.

3.2. Metal Ions’ Dynamic Effect Analysis

Based on this experimental phenomenon, it was believed that NO₃⁻ also had an oxidizing effect in addition to Fe³⁺. To discuss the influence of metal cations and expand the system range of the etching solution, the fourth-period element corresponding nitrate (M(NO₃)_x) and HNO₃ in the periodic table of elements were researched in this section in the hope that each M(NO₃)_x would probably represent each main group element’s corresponding M(NO₃)_x. Considering the metal elements in the fourth period of the periodic table, KNO₃, Al(NO₃)₃, Cr(NO₃)₃, Mn(NO₃)₂, Ni(NO₃)₂, Co(NO₃)₂, HNO_3, and Ca(NO₃)₂ were finally selected to form the etching solution with HF. After the reaction was completed, the cross-sectional morphology of the silicon wafer was observed with SEM, and the crystal orientation of the silicon nanostructure was observed with TEM.

The cross-sectional view of the SEM in Figure 4 shows that the silver nanoparticle layer is the catalyst, and the etching solution containing M(NO₃)_x and HF can achieve regular SiNW arrays on the surface of a single-crystal silicon wafer. Most of the arrays are dense, one-dimensional arrays with a line diameter of 100–200 nm. However, the lengths of the SiNW arrays are different in different M(NO₃)_x etching solutions, which means that the etching rate of silicon is also significantly different in different M(NO₃)_x etching solutions. After one hour of etching reaction at 50 °C, the length of the SiNW arrays prepared in KNO₃ and Al(NO₃)₃ etching solution was about 3 μm (Figure 4a,b). The length of the SiNW arrays prepared in Mn(NO₃)₂ and Cr(NO₃)₃ etching solutions was the shortest, about 1 μm (Figure 4c,d). The lengths of the SiNW arrays prepared in Ni(NO₃)₂, Co(NO₃)₃, and Ca(NO₃)₃ etching solutions were about 5–6 μm (Figure 4e–g). As shown in Figure 4h, observing the microscopic surface morphology of a single silicon wire with a high-power TEM can find that the diameter of a single silicon wire is between 100 and 200 nm, which is also the (100) crystal orientation.

The above results show that in different M(NO₃)_x systems, the etching rate is also different, although the NO₃⁻ is an oxidant that continuously promotes the etching reaction. However, when only the types of metal ions in the M(NO₃)_x are different, there are still some differences in the SiNW arrays. Therefore, metal ions are still one of the factors that dynamically affect the etching process. Considering the strong complexing ability of F⁻ in the etching system, the types of ions are very complex, the toxicity of HF is strong, and the characterization methods of ion species are limited. It was also found that cation concentration has a great influence on the etching results. KNO₃ was selected to research the effect of metal ions on the etching reaction, and the influence of different concentrations of KNO₃ on the system was also investigated.

Keeping the concentration of HF unchanged, when using 0.48 M KNO₃ etching solution, the surface of the silicon wafer was gray-white after reacting at 50 °C for three hours, and, observed under an SEM, as shown in Figure 5a, there was a layer of granular material attached. As shown in the cross-section shown in Figure 5b, the silicon wafer was not etched. The essence of MACE is that the galvanic cell drives the reaction of silicon in an HF solution system with an oxidant. The cathode and anode, the electrolyte, and the connected circuit are indispensable. During the etching reaction, K⁺ will combine with F⁻ to form an insoluble K₂SiF₆ film that is closely attached to the surface of the silicon wafer, preventing the etching solution from contacting the silicon wafer and thereby inhibiting the occurrence of etching. With 0.05 M KNO₃, after the F⁻ was completely complexed with the K⁺, there was still a large amount of residue, which was enough to support the occurrence of subsequent etching reactions. The SiNW arrays are shown in Figure 5c,d. When adding 0.48 M KNO₃, most of the F⁻ ions in the system were complexed by K⁺ and formed insoluble K₂SiF₆, which adhered to the surface of the silicon wafer and inhibited the occurrence of the etching reaction. To verify the crystal composition of the gray-white substance on the surface of the silicon wafer, X-ray diffraction was used to characterize it. As shown in Figure 5f, the positions of the peaks in the XRD pattern correspond exactly to those of standard K₂SiF₆ (the power diffraction file 52-1831). Therefore, the small particles on the SiNW array side wall shown in Figure 5e are K₂SiF₆.

3.3. Mechanism Analysis

Based on the above experimental results, the influence of Fe³⁺ and K⁺ in solution is further understood, and its mechanism is now updated. As shown in Figure 6, during the etching process, NO₃⁻ and Fe³⁺ are both oxidants; as oxidated silicon is dissolved by HF, the silver particles gradually sink as the reaction proceeds, while keeping dissolution and deposition in balance at the same time. In the HF-KNO₃-H₂O system, as the concentration of KNO₃ in the system increases, a layer of K₂SiF₆ will be formed on the surface of the silicon wafer, which hinders the continuation of the reactions. According to our previous work [10], it is suggested that the anodic dissolution of silicon during MACE of silicon follows both the divalent and tetravalent dissolution processes. The cathode is the reduction of the oxidant. Although Fe³⁺ and NO₃⁻ are cations and anions, respectively, both Fe³⁺ and NO₃⁻ are oxidizing in the etching solution; as they are all reacting at the cathode, they attack the same sections.

In the HF-Fe(NO₃)₃-H₂O system, Fe³⁺ is reduced to Fe²⁺, and NO₃⁻ is reduced to NO₂⁻. In the HF-KNO₃-H₂O system, most of the F⁻ ions in the system are complexed by K⁺ and form insoluble K₂SiF₆, which adheres to the surface of the silicon wafer and inhibits the occurrence of etching reactions. The anodic and cathodic half-cell reactions in HF-Fe(NO₃)₃-H₂O and HF-KNO₃-H₂O system are described as follows [20,33,34]:

In the HF-Fe(NO₃)₃-H₂O system (as shown in Figure 6a)

Anodic:

(1) Si + 2F⁻ +2h⁺ $\to$ SiF₂

SiF₂ + 4F⁻ +2H⁺ $\to$ SiF₆²⁻ +H₂

(2) Si + 2H₂O + 4h⁺ $\to$ SiO₂ + 4H⁺

SiO₂ + 6HF $\to$ 2H⁺ + SiF₆²⁻ + 2H₂O

Cathodic:

NO₃⁻ + 2H⁺ +2e⁻ $\to$ NO₂⁻ + H₂O

Fe³⁺ + e⁻ $\to$ Fe²⁺

In the HF-KNO₃-H₂O system (as shown in Figure 6b)

Anodic:

(1) Si + 2F⁻ +2h⁺ $\to$ SiF₂

SiF₂ + 4F⁻ + 2K⁺ +2H⁺ $\to$ K₂SiF₆ +H₂

(2) Si + 2H₂O + 4h⁺ $\to$ SiO₂ + 4H⁺

SiO₂ + 2K⁺ + 6HF $\to$ 2H⁺ + K₂SiF₆ + 2H₂O

Cathodic:

NO₃⁻ + 2H⁺ +2e⁻ $\to$ NO₂⁻ + H₂O

4. Conclusions

In this paper, the oxidative roles of Fe³⁺ and NO₃⁻ during the MACE of silicon were proved, and metal ions’ dynamic effect on MACE of silicon in the HF-M(NO₃)_x-H₂O etching acid solution (where M(NO₃)x is: KNO₃, Al(NO₃)₃, Cr(NO₃)₃, Mn(NO₃)₂, Ni(NO₃)₂, Co(NO₃)₂, HNO₃, and Ca(NO₃)₂) were discussed. The results showed that the type of cation affects the morphology of SiNW arrays and the rate of etching. Crystal composition analysis revealed that the influence of cation type and concentration on MACE is realized by the stability of the metal and its reaction products in the etching solutions. In HF-KNO₃-H₂O solution, the combination of K⁺ and F⁻ affects the dissolution of SiO₂ or directly generates an insoluble substance attached to the wafer surface, thus inhibiting the etching of silicon. As the concentration of KNO₃ in the system increases, a layer of K₂SiF₆ is formed, which hinders the continuation of the etching reactions. This work is further helpful in revealing the metal ions’ dynamic effect on MACE of silicon in the HF-M(NO₃)_x-H₂O etching acid solution, paves the way for further study of the mechanism of MACE, and provides a basis for us to find convenient and low-cost MACE methods.