Metal-assisted Chemical Etching Of Silicon Research Articles

Catalytic nickel silicide as an alternative to noble metals in metal-assisted chemical etching of silicon.

Metal-assisted chemical etching (MACE) has received much attention from researchers because it can be used to fabricate plasma-free anisotropic etching profiles for semiconductors. However, the etching mechanism of MACE is based on the catalytic reaction of noble metals, which restricts its use in complementary metal oxide semiconductor (CMOS) processes. To obtain process compatibility, we developed catalytic Ni after alloying it with Si as a substitute for noble metals in the MACE of Si substrates. Nickel silicide is a material commonly used as a contact electrode in CMOS processes. When NiSi was used as the catalyst, the anisotropic etching of Si with a smooth surface was successfully demonstrated. Silicidation increased the standard reduction potential of the Ni alloy and enhanced the electrochemical stability in the MACE of Si. In contrast, when pure Ni was used as the catalyst, a rough-etched surface was fabricated because of the low standard reduction potential. Based on the experimental results, the factors affecting the MACE of Si were systematically analyzed to optimize the catalytic NiSi properties. The implementation of the NiSi alloy potentially eliminates the use of noble metals in MACE and allows the technology to be adopted in contemporary CMOS processes.

Doping density, not valency, influences catalytic metal-assisted plasma etching of silicon.

Metal-assisted plasma etching (MAPE) of silicon (Si) is an etching technique driven by the catalytic activity of metals such as gold in fluorine-based plasma environments. In this work, the role of the Si substrate was investigated by examining the effects of the dopant concentration in both n- and p-type Si and the dopant atom type in n-type Si in SF6/O2 mixed gas plasma. At the highest dopant concentrations, both n- and p-type Si initially exhibit inhibition of the MAPE-enhanced etching. As the etch progresses, MAPE initiates, resulting in catalytic etching of the underlying Si at the metal-Si interface. Interestingly, MAPE-enhanced etching increases with decreasing doping concentrations for both n- and p-type Si substrates, distinct from results for the similar but divergent, metal-assisted chemical etching of silicon in liquid. Our findings show that the metal-Si interface remains essential to MAPE, and surface enrichment of the dopant atoms or other surface chemistries and the size of metal nanoparticles play roles in modulating catalytic activity.

Study of metal-assisted chemical etching of silicon as an alternative to dry etching for the development of vertical comb-drives

Metal-assisted chemical etching (MaCEtch) has recently emerged as a promising technique to etch anisotropic nano- and microstructures in silicon by metal catalysts. It is an economical wet chemical etching method, which can be a good alternative to deep-reactive ion etching (DRIE) process in terms of verticality and etch depth. In the present study, gold is used as a metal catalyst and deposited using physical vapour deposition. It has already been demonstrated that (100) p-type Si wafer can be etched with vertical and smooth side walls. Effects of varying concentrations of etchant constituents and various other parameters, that is, porosity of deposited Au, surface contaminants, oxide formation, metal catalyst, etching time, role of surface tension of additives on the etch depth and surface defects are studied and discussed in detail. By increasing the hydrofluoric acid (HF) concentration from 7.5 M to 10 M, lateral etching is reduced and the microstructure’s width is increased from 17 µm to 18 µm. Porous defects are suppressed by decreasing the hydrogen peroxide (H2O2) concentration from 1.5 M to 1 M. On increasing the etching time from 30 min to 60 min, the microstructures are over-etched laterally. Smoother side walls are fabricated by using the low-surface-tension additive ethanol. The maximum etch depth of 2.6 µm is achieved for Au catalyst in 30 min. The results are encouraging and useful for the development of vertical comb-drives and Micro-Electro-Mechanical Systems (MEMS).

Length prediction of silicon nanowires (SiNWs) prepared by the MACE method using the ANN-COA-PSO algorithm for high supercapacitor applications

In this study, the metal-assisted chemical etching of silicon with the help of metal is used to fabricate large-scale silicon nanowire arrays (SiNWs). This method was optimised by artificial neural networks (ANN) and the cuckoo optimization algorithm (COA) was used to optimize ANN. Finally, the particle swarm optimization (PSO) method has been used to achieve the maximum length of silicon nanowires. As a result, the nanowires produced by the 1 M NaOH electrolyte had the properties of a capacitive cloud with a capacity of 758 mF/cm2. It has the potential to be an excellent candidate for supercapacitor applications.

Vertical-Aligned Silicon Nanowire Arrays with Strong Photoluminescence Fabricated by Metal-Assisted Electrochemical Etching

Metal-assisted chemical etching of silicon is a commonly used method to fabricate vertical aligned silicon nanowire arrays. In this report we show that if in the above method the chemical etching is replaced by the electrochemical one, we can also produce silicon nanowire arrays, but with a special characteristic-extremely strong photoluminescence. Further research showed that the huge photoluminescence intensity of the silicon nanowire arrays made by metal-assisted electrochemical etching is related to the anodic oxidation of the silicon nanowires which has occurred during the electrochemical etching. It is most likely that the luminescence of the silicon nanowire arrays made with metal-assisted electrochemical etching is the luminescence of silicon nanocrystallites (located on the surface of silicon nanowire fibers) embedded in a silicon oxide matrix, similar to that in a silicon rich oxide system.

Metal-assisted chemical etching of silicon and achieving pore sizes as small as 30 nm by altering gold thickness

Metal-assisted chemical etching is applied to fabricate deep, high aspect ratio nanopores in silicon. The authors’ simple and cost-effective fabrication process has proven capable of generating nanopores with diameters as small as 30 nm, over the whole wafer surface (50.8 mm in diameter). The process uses a thin layer of DC-sputtered gold and H2O2/H2O/HF treatment to generate Au nanoislands. The formation of these nanoislands is confirmed by scanning electron microscopy. In this paper, the authors study the effect of Au-layer thickness on the diameter and morphology of the fabricated nanopores. The resulting structures have wide applications in optical sensing and filtering.

Carbon induced galvanic etching of silicon in aerated HF/H2O vapor

Metal-assisted chemical etching of silicon in HF aqueous solutions has aroused great interest due to its potential applications in photovoltaic, thermoelectric, sensors, and batteries. In this work, an alternative carbon induced galvanic etching of silicon in the aerated HF/H2O vapor was demonstrated. The potentials of carbon materials including graphite particles and carbon nanotubes to promote silicon etching in aerated HF/H2O vapor are demonstrated. By substituting noble metals with earth abundant carbon materials, the present method offers an alternative low cost route to fabricate silicon micro/nanostructures while provide new insight into galvanic etching of silicon in the presence of HF.

Optical and Structural Properties of Ag and c-Si Nanostructures Formed During the Metal-Assisted Chemical Etching of Silicon

This study consisting of two parts is concerned with the features of the three-stage process of the metal-assisted chemical etching (MACE) of silicon. This process is used to fabricate silicon nanostructures. In the first part of this work, a layer of self-assembled Ag nanoparticles chemically deposited from a solution on the surface of single-crystal silicon (c-Si) (MACE stage 1) was studied, and the second part includes of investigation of Si nanostructures formed in stages 2 and 3. By means of spectroscopic ellipsometry (in the range of wavelengths λ = 250–900 nm), the pseudodielectric functions of the nanostructures were determined and compared for all the three stages of the MACE process. In addition, for the Si nanostructures, the parameters of layers (the thickness and void fraction) were calculated in the context of the multilayer optical model, with the use of Bruggeman’s effective-medium approximation and fitting procedures.

Surface Nanostructures Forming during the Early Stages of the Metal-Assisted Chemical Etching of Silicon. Optical Properties of Silver Nanoparticles

In this two-part work, nanostructures formed in a three-step process of metal-assisted chemical etching of silicon are investigated. In the first part (present publication), the process of the chemical deposition of a layer of self-assembled silver nanoparticles on the surface of a silicon wafer (the first stage of metalassisted chemical etching) is studied. This layer, on the one hand, serves as a catalyst for the subsequent etching of silicon, and, on the other hand, represents a kind of mask for the formation of a certain topology of the emerging Si nanowires. The morphology of the obtained 40- to 60-nm-thick silver nanoparticle layers is investigated by scanning electron microscopy. The spectral dependences of the ellipsometric angles Ψ and Δ are measured using spectroscopic ellipsometry (λ = 250–900nm), and the complex dielectric function of the silver nanolayers is determined from these spectra. The dielectric function features a characteristic plasmon resonance peak in the ultraviolet spectral range. The study of the optical properties of Si nanofilament layers which form during the early stages of metal-assisted chemical etching will be reported as the second part of this work in a separate publication.

Поверхностные наноструктуры, формирующиеся на ранних стадиях металл-стимулированного химического травления кремния. Оптические свойства наночастиц серебра

AbstractIn this two-part work, nanostructures formed in a three-step process of metal-assisted chemical etching of silicon are investigated. In the first part (present publication), the process of the chemical deposition of a layer of self-assembled silver nanoparticles on the surface of a silicon wafer (the first stage of metalassisted chemical etching) is studied. This layer, on the one hand, serves as a catalyst for the subsequent etching of silicon, and, on the other hand, represents a kind of mask for the formation of a certain topology of the emerging Si nanowires. The morphology of the obtained 40- to 60-nm-thick silver nanoparticle layers is investigated by scanning electron microscopy. The spectral dependences of the ellipsometric angles Ψ and Δ are measured using spectroscopic ellipsometry (λ = 250–900nm), and the complex dielectric function of the silver nanolayers is determined from these spectra. The dielectric function features a characteristic plasmon resonance peak in the ultraviolet spectral range. The study of the optical properties of Si nanofilament layers which form during the early stages of metal-assisted chemical etching will be reported as the second part of this work in a separate publication.

Understanding the influence of thermal annealing of the metal catalyst on the metal assisted chemical etching of silicon

The metal-assisted chemical etching (MACE) is emerging as one of the cost-effective technique for anisotropic etching of silicon (Si) where the metal catalyst plays a key role in the etching process. Although intensive research has been carried out in this field, the effect of thermal annealing of the metal catalyst on MACE is not well understood. Therefore, a systematic study of the thermal annealing effect of the metal catalyst on the MACE of silicon (Si) in conjunction with gold (Au) micro stripes as a catalyst has been carried out in this work. We have demonstrated that the post-deposition thermal annealing of Au micro stripes has a direct relationship to the thickness of the metal catalyst for Si etching. The experimental results indicate that Si etching occurs when an annealed thick (140 nm) and wider (> 3 µm) Au micro stripe is applied as the metal catalyst. However, Si etching does not occur for an annealed thin (50 nm) and wider (> 3 µm) Au micro stripe is used as a metal catalyst. Furthermore, the direct effect of the oxidizing agent on the MACE Si etching by using annealed thick Au micro stripes has also been investigated. We show that a higher concentration of the oxidizing agent only polishes the Si surface with formation of craters in different locations of the Si. Here in this paper, the probable reasons for these results are also discussed.

How the Oxidation Stability of Metal Catalysts Defines the Metal-Assisted Chemical Etching of Silicon

Metal-assisted chemical etching (MACE) is done with different metal species. The resulting silicon nanostructures appear strongly dependent on the choice of metal, but a deeper understanding of the MACE process is still missing. We report here direct evidence that the etching solution composition plays a major role in the chemical stability of the metal catalyst used. We show from an elemental analysis of post-MACE etch baths that dissolved silver is found in the bath with concentrations up to 3 orders of magnitude larger than when gold is used. Furthermore, the dissolved silver content also correlates with the amount of H2O2, either in different initial conditions, or as would be expected from its decomposition over time. We also show that silver dissolution leads to unintended etching elsewhere on the substrate. This species-dependent behavior of the metal catalyst is responsible for the different kinds of control possible over the nanostructures produced with silver- and gold-based MACE.

Evidences for redox reaction driven charge transfer and mass transport in metal-assisted chemical etching of silicon.

In this work, we investigate the transport processes governing the metal-assisted chemical etching (MacEtch) of silicon (Si). We show that in the oxidation of Si during the MacEtch process, the transport of the hole charges can be accomplished by the diffusion of metal ions. The oxidation of Si is subsequently governed by a redox reaction between the ions and Si. This represents a fundamentally different proposition in MacEtch whereby such transport is understood to occur through hole carrier conduction followed by hole injection into (or electron extraction from) Si. Consistent with the ion transport model introduced, we showed the possibility in the dynamic redistribution of the metal atoms that resulted in the formation of pores/cracks for catalyst thin films that are ≲30 nm thick. As such, the transport of the reagents and by-products are accomplished via these pores/cracks for the thin catalyst films. For thicker films, we show a saturation in the etch rate demonstrating a transport process that is dominated by diffusion via metal/Si boundaries. The new understanding in transport processes described in this work reconcile competing models in reagents/by-products transport, and also solution ions and thin film etching, which can form the foundation of future studies in the MacEtch process.

Effects of silver and gold catalytic activities on the structural and optical properties of silicon nanowires

The metal-assisted chemical etching of silicon in an aqueous solution of hydrofluoric acid and hydrogen peroxide is established for the fabrication of large area, uniform silicon nanowire (SiNW) arrays. In this study, silver (Ag) and gold (Au) are considered as catalysts and the effect of different catalysts with various thicknesses on the structural and optical properties of the fabricated SiNWs is investigated. The morphology of deposited catalysts on the silicon wafer is characterized by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). It is shown that the morphology of the fabricated silicon nanostructures remarkably depends upon the catalyst layer thickness, and the catalyst etching time directly affects the structural and optical properties of the synthesized SiNWs. FESEM images show a linear increment of the nanowire length versus time, whereas the etching rate for the Au-etched SiNWs was lower than the Ag-etched ones. Strong light scattering in SiNWs caused the total reflection to decrease in the range of visible light, and this decrement was higher for the Ag-etched SiNW sample, with a longer length than the Au-etched one. A broadband visible photoluminescence (PL) with different peak positions is observed for the Au- and Ag-etched samples. The synthesized optically active SiNWs can be considered as a promising candidate for a new generation of nano-scale opto-electronic devices.

Catalytic Activity of Ru for Metal-Assisted Etching of Si

Metal-assisted chemical etching of Silicon (Si) in hydrofluoric acid (HF) solution is an electroless method to produce porous Si (1, 2). Such etching generally uses not only metal-modified Si but also an oxidizing agent as hydrogen peroxide, metal ions and oxygen. We reported that etching rate of platinum (Pt), rhodium (Rh), gold (Au) and silver (Ag)-particle-modified Si depended on catalytic activity of metal for oxygen reduction (3), and palladium (Pd) exhibits high catalytic activity for the etching of Si without an oxidizing agent even under dissolved oxygen-free and dark conditions (4). This etching reaction using Pd is explained by electron injection into the conduction band of Si due to Pd assisted anodic oxidization of Si (Fig. 1). In this study, we have investigated the catalytic activity of ruthenium (Ru), which is in the same platinum group as Pd, for etching of Si in a HF solution. Single-crystalline n-Si wafers (CZ, (100), ca. 10 Ω cm) were used as substrates. Si wafers were immersed in 1 mM (M = mol dm-3) ruthenium chloride (RuCl3) solution including 0.15 M HF at 313 K for electroless displacement metal deposition. Metal deposited Si wafers were immersed in a 7.3 M HF solution at 298 K under dark conditions for 24 h. Argon or oxygen gas bubbling was applied to the HF solution before and during etching to remove or saturate the dissolved oxygen in the solution, respectively. The etching rate of Si was measured by a gravimetric procedure. No metal deposition occurred on mirror-polished Si wafers even by immersing in the deposition solution for even long time of 24 h. On the other hand, on Si wafers roughed with sand paper (#1200), Ru particles were deposited by immersing in the solution for 5 min. The etching rates of such Ru-particle-deposited Si wafers in 7.3 M HF solution were 1.1×10-5 and 2.0×10-4 g cm-2 h-1 under oxygen-free and -saturated conditions, respectively. The etch rate in the oxygen-free case is much higher than 1.2×10-6 g cm-2 h-1 of Pt-particle-modified case and one order lower than 3.4×10-4 g cm-2 h-1 the of Pd case. Figure 2 shows the SEM images of Ru-particle-deposited Si wafers before and after etching with the oxygen free HF solution. Ru particles were localized on scratches before etching (fig. 2a). After etching, a porous layer covered whole surface of Si (Fig. 2b and c). Pores, which have Ru particle at the bottom, were formed. The depth of pores, ca. 0.2 mm, is much shorter than 1 mm of depth calculated using the gravimetric procedure. These results indicate that Ru has much lower activity for electroless displacement deposition than that of other noble metals and high catalytic activity for metal assisted etching of Si similar to Pd. ACKNOWLEDGEMENTS The present work was partly supported by JSPS KAKENHI (26289276). REFERENCES Z. Huang, N. Geyer, P. Werner, J. Boor, U Gösele, Adv. Mater., 23,285 (2011).S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, and H. Matsuda, Electrochem.Commun., 5, 632 (2003).S. Yae, Y. Morii, N. Fukumuro, H. Matsuda, Nanoscale Res. Lett., 7, 352 (2012).S. Yae, M. Tashiro, M. Abe, N. Fukumuro, and H. Matsuda, J. Electrochem. Soc., 157, D90 (2010). Figure 1

Biodegradable silicon nanoneedles delivering nucleic acids intracellularly induce localized invivoneovascularization.

The controlled delivery of nucleic acids to selected tissues remains an inefficient process mired by low transfection efficacy, poor scalability because of varying efficiency with cell type and location, and questionable safety as a result of toxicity issues arising from the typical materials and procedures employed. High efficiency and minimal toxicity in vitro has been shown for intracellular delivery of nuclei acids by using nanoneedles, yet extending these characteristics to in vivo delivery has been difficult, as current interfacing strategies rely on complex equipment or active cell internalization through prolonged interfacing. Here, we show that a tunable array of biodegradable nanoneedles fabricated by metal-assisted chemical etching of silicon can access the cytosol to co-deliver DNA and siRNA with an efficiency greater than 90%, and that in vivo the nanoneedles transfect the VEGF-165 gene, inducing sustained neovascularization and a localized sixfold increase in blood perfusion in a target region of the muscle.

Metal-assisted chemical etching of silicon and the behavior of nanoscale silicon materials as Li-ion battery anodes

This review outlines the developments and recent progress in metal-assisted chemical etching of silicon, summarizing a variety of fundamental and innovative processes and etching methods that form a wide range of nanoscale silicon structures. The use of silicon as an anode for Li-ion batteries is also reviewed, where factors such as film thickness, doping, alloying, and their response to reversible lithiation processes are summarized and discussed with respect to battery cell performance. Recent advances in improving the performance of silicon-based anodes in Li-ion batteries are also discussed. The use of a variety of nanostructured silicon structures formed by many different methods as Li-ion battery anodes is outlined, focusing in particular on the influence of mass loading, core-shell structure, conductive additives, and other parameters. The influence of porosity, dopant type, and doping level on the electrochemical response and cell performance of the silicon anodes are detailed based on recent findings. Perspectives on the future of silicon and related materials, and their compositional and structural modifications for energy storage via several electrochemical mechanisms, are also provided.

Catalyst feature independent metal-assisted chemical etching of silicon

We demonstrate metal-assisted chemical etching of Si substrates with consistent etching rates for a wide range of metal catalysts of dots and stripes in meshes and solid arrays.

High-Density Silicon Nanowires Prepared via a Two-Step Template Method

High density ordered Si nanowire arrays can be fabricated from a Fe2O3 template annealed from polystyrene (PS) microsphere layers via a metal-assisted chemical etching method. The metal mesh films, containing position- and density-defined pores that determine the position and density of the remaining structures after etching, are extremely important for achieving high quality Si nanowires. By adding a structural inversion process, a Au metal mesh with arrays of high density nanopores is devised as a catalyst for metal-assisted chemical etching of silicon. The density of Si nanowires can be increased to two times that of the single-layer PS microspheres and further to three times when a double layer of PS microspheres is introduced. The two-step template method for the preparation of high-density Si nanowires shows great potential in the fields of nanofabrication and nanoelectronics.

Effect of catalyst shape on etching orientation in metal-assisted chemical etching of silicon

Silicon nanowires with vertical, slanting and zigzag architectures have been fabricated by metal-assisted chemical etching of silicon wafers (n-Si(100), n-Si(111) and n-Si(110)).