Copper Catalyzed Anisotropic Dissolution of Silicon in HF–HCl Solutions: Metal‐Assisted Chemical Etching Without Additional Oxidant

Metal-assisted chemical etching (MACE or MacEtch) is a versatile method for fabricating nano and micro-structured silicon (Si), which has garnered significant attention due to its potential applications in photovoltaics, sensors, and nanoelectronics. The process involves the oxidation of Si in the presence of a metal catalyst (typically noble metals like Au, Ag, or Pt) and a wet etch solution, usually comprising hydrofluoric acid (HF) and an oxidizing agent such as hydrogen peroxide (H2O2).Impressive work has already been completed in the two decades following the introduction of this method through the field of stain etching [1]. Researchers have reported anisotropic structures in silicon as high as 10,000:1 aspect ratio [2] and studied the impact of catalyst thickness [3], geometry [4], chemical ratios [5][6], and level of doping [7].In this work, we systematically tune the selectivity of the MACE process based on the geometry of desired structures, chemical ratios of HF, H2O2 and ethanol, silicon doping types, and characteristics of the metal catalyst targeting our desired metasurface optic features. We use statistical analysis such as ensemble machine learning algorithms to create an informed understanding and importance matrix for each of these variables, toward the purpose of creating refractive optical features in silicon. The important parameters in the desired final product are vertical sidewalls, 10:1 aspect ratio, minimized surface roughness in the field, an optimized geometry, and a target depth.This comprehensive statistical analysis contributes to a deeper understanding of the MACE process, offering valuable guidelines for optimizing etching conditions to achieve desired micron to nanometer structures in silicon. The findings hold promise for advancing the fabrication of silicon-based nano-devices, paving the way for novel applications in various technological fields. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525 SAND2024-04791A[1] X. Li and P. W. Bohn, "Metal-assisted chemical etching in HF/H2O2 produces porous silicon," Applied Physics Letters, vol. 77, no. 16, pp. 2572-2574, 2000, doi: 10.1063/1.1319191.[2] L. Romano et al., "Metal assisted chemical etching of silicon in the gas phase: a nanofabrication platform for X-ray optics," Nanoscale Horizons, 10.1039/C9NH00709A vol. 5, no. 5, pp. 869-879, 2020, doi: 10.1039/C9NH00709A.[3] Z. Huang et al., "Extended Arrays of Vertically Aligned Sub-10 nm Diameter [100] Si Nanowires by Metal-Assisted Chemical Etching," Nano Letters, vol. 8, no. 9, pp. 3046-3051, 2008/09/10 2008, doi: 10.1021/nl802324y.[4] P. Lianto, S.-Y. Yu, J. Wu, C. V. Thompson, and W. K. Choi, "Vertical etching with isolated catalysts in metal-assisted chemical etching of silicon," Nanoscale, vol. 4 23, pp. 7532-9, 2012. [Online]. Available: https://doi.org/10.1039/C2NR32350H.[5] C. Chartier, S. Bastide, and C. Lévy-Clément, "Metal-assisted chemical etching of silicon in HF–H2O2," Electrochimica Acta, vol. 53, no. 17, pp. 5509-5516, 2008/07/01/ 2008, doi: https://doi.org/10.1016/j.electacta.2008.03.009.[6] W. Chern et al., "Nonlithographic Patterning and Metal-Assisted Chemical Etching for Manufacturing of Tunable Light-Emitting Silicon Nanowire Arrays," Nano Letters, vol. 10, no. 5, pp. 1582-1588, 2010/05/12 2010, doi: 10.1021/nl903841a.[7] R. A. Lai, T. M. Hymel, V. K. Narasimhan, and Y. Cui, "Schottky Barrier Catalysis Mechanism in Metal-Assisted Chemical Etching of Silicon," ACS Applied Materials & Interfaces, vol. 8, no. 14, pp. 8875-8879, 2016/04/13 2016, doi: 10.1021/acsami.6b01020. Figure 1

This paper discusses a development of self-cleaning surface. Metal-assisted chemical etching (MACE) is one of the promising processes because it can produce high aspect-ratio structure on a silicon substrate which suits for hydrophobicity. Silica particles (e.g. φ1μm) were self-assembled on a substrate. After the reduction of particle size by argon ion bombardment, gold layer was deposited using the particles as a mask. As a result, gold layer that has openings with regular pitch was obtained. The substrate was then etched with mixture of hydrofluoric acid and hydrogen peroxide. The gold layer works as a catalyst and this part is selectively etched. The oxide layer of silicon is originally hydrophilic. But, it can be changed to hydrophobic by depositing hydrophobic material. Finally, self-cleaning function was examined.

Metal-assisted chemical etching (MACE) is a site-selective etching process produced by a catalyst reaction at the interface between noble metal and silicon. This paper aims to make clear the applicability of sphere lithography and MACE to the fabrication of high aspect ratio Si nanostructures. The capacity to control the etched profiles and the scale extension are investigated. First, silica particles (e.g. φ1 μm) were self-assembled on a Si substrate. After the reduction of particle size via argon ion bombardment, a gold layer was deposited using the particles as a mask. The substrate was then etched with a mixture of hydrofluoric acid and hydrogen peroxide. It was found that an array of nanopillars with a regular pitch, good separation, and an aspect ratio of about 52 was produced. The effects of MACE conditions on final profiles were clarified. A limitation of this approach is the small (several millimeters) area fabricated due to the dependence on the vacuum technique (ion bombardment, Au deposition), and the size of the area limits its practical applications. Thus, Ag nanoparticles (e.g. φ150 nm) were applied. The relationship between the concentration of the Ag suspension, the Ag assembled layer, and the morphology of MACE structures was made clear. A spray method was applied to extend the deposited area of Ag particles up to φ100 mm. Finally, the effects of the cross-sectional profile on the contact angle of a water droplet were examined. By applying a high aspect ratio nanostructure on the substrate, the water contact angle increased up to 153 degrees while that without the structure is 58 degrees.

Metal-assisted chemical etch (MACE) has received more and more attention due to its low cost and simple process. Ag, Au, Pt and other noble metals can be used as catalyst in the MACE of silicon. In this paper, MACE on silicon in a hydrofluoric acid (HF) and hydrogen peroxide (H2O2) mixture with different HF-to-H2O2 molar ratio R (R = [HF]/([HF] + [H2O2])) with Fe3O4@Au core–shell magnetic nanoparticles (MNPs) as catalyst under an external magnetic field was investigated. Boron doped p-type (100) silicon wafers with resistivity of 20∼30 Ω · cm and 0.01∼0.03 Ω · cm were used as substrates, respectively. We observed that an average etching rate of 5.17 μm min−1 under an applied magnetic field while the average etching rate was 4 μm min−1 without magnetic field. The factors were investigated that affect the average etched depth in silicon substrate under magnetic field by using MACE. The relationship between the average etching depth and etching time, resistivity of silicon substrate, ratio R, and the concentration of Fe3O4@Au magnetic nanoparticles was also studied. This result can be applied to increase the etching rate of silicon to fabricate a variety of devices, where high aspect ratio silicon structure is demanded.

Until now, the best way to use metal-assisted chemical etching was to apply it to the fabrication of nanowires and nanostructures. Yet the properties of metal-assisted chemical etching in micro-scale patterns have not been the subject of much research. When the catalyst is too broad, metal-assisted chemical etching becomes difficult because of catalyst bending [1]. If micro-scale metal-assisted chemical etching is more extensively studied, it may be applied in MEMS. To understand the mechanism of micro-scale metal-assisted chemical etching, we need to examine two mechanisms for the mass transfer of solution and byproducts by defining the diffusion length, either long or short. Although a study exists that explains the two mechanisms using deposition rate [2], our study will demonstrate the two mechanisms using catalysts with varying degrees of thickness and size. The mechanism for metal-assisted chemical etching is determined by the thickness of the catalyst. For thick catalysts, a long diffusion length mechanism is used and for thinner catalysts, a short diffusion length mechanism is used. In a long diffusion length mechanism, the catalyst is thick. Since pin-holes are not generated on thick catalysts, the etching solution cannot penetrate the catalyst to dissolve silicon. Because silicon is etched from the edge of the catalyst, the points the etching solution reaches are of different lengths. As a result, this makes the etching process unstable. If etching occurs over areas that are too large, catalyst begins to bend, caused by etching from the edge of the catalyst. In a short diffusion length mechanism, the catalyst is thin; therefore, pin-holes are generated for mass transportation. Consequently, in a large area, the sample is stably etched with anisotropic properties. This means that a thin film catalyst for metal-assisted chemical etching is independent of the pattern.To understand the effects of catalyst thickness and size, Au films 10, 20, 30, and 40 nm thick were deposited by thermal evaporation on Si substrates. As metal-assisted chemical etching was best carried out with 20 nm thick catalyst, bar patterns 2, 4, 6, 8, and 10 µm wide and dot patterns of 2, 4, and 6 µm in diameter were used on 20 nm thick catalyst. Results were analyzed by scanning electron microscopy (SEM). Before deposition, native oxide was removed using a BOE solution. On 10 and 20 nm thick catalysts, metal-assisted chemical etching was successfully done by mass transfer due to pin-holes that were generated in the thin catalyst. On the other hand, on 30 and 40 nm thick catalysts, the etching rate was significantly reduced. The etching rate was unaffected by the size of the catalysts on the 20 nm thick catalyst. This shows that micro-scale metal-assisted chemical etching is independent of pattern size on thin catalysts. Acknowledgements This research was supported by the Ministry of Science, ICT and Future Planning (MSIP), Korea, under the IT Consilience Creative Program (NIPA-2014-H0201-14-1002) supervised by the National IT Industry Promotion Agency (NIPA). This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-8-0884). Reference [1] N.Geyer, B.Fuhrmann, Z.Huang, J.de Boor, H.S.Leipner and P.Werner, J Phys Chem C. 116 (2012) 13446[2] M.Zahedinejad, S.D.Farimani, M.Khaje, H.Mehrara, A.Erfanian and F.Zeinali, J Micromech. Microeng.23 (2013) 055015

Silicon nanowire arrays fabricated by metal-assisted wet chemical etching have emerged as a promising architecture for solar energy harvesting applications. Here we investigate the dynamics and transport properties of photoexcited carriers in nanowires derived from an intrinsic silicon wafer using the terahertz (THz) time-domain spectroscopy. Both the dynamics and the pump fluence dependence of the photoinduced complex conductivity spectra up to several THz were measured. The photoinduced conductivity spectra follow a Lorentz dependence, arising from surface plasmon resonances in nanowires. The carrier lifetime was observed to approach 0.7 ns, which is limited primarily by surface trapping. The intrinsic carrier mobility was found to be ~1000 cm(2)/(V · s). Compared to other silicon nanostructures, these relative high values observed for both the carrier lifetime and mobility are the consequences of high crystallinity and surface quality of the nanowires fabricated by the metal-assisted wet chemical etching method.

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.

Aims: This work uses the MACE method to synthesize SiNWs- NiNPs/NiONPs to degrade organic pollutants by photocatalysis. Background: Photocatalytic degradation has been applied as an attractive solution to remove several organic pollutants. Heterostructured nanomaterials have become an interesting platform for investigation. Metal-assisted chemical etching (MACE) stands out as a promising technique because it is simple, low cost, and fast. Objective: Attain the degradation of methyl orange (MO) in the presence of silicon nanowires (SiNWs) in heterojunction with Nickel/Nickel Oxide nanoparticles (NiNPs-NiONPs). Methods: SiNWs were synthesized by metal (Ag) assisted chemical etching (MACE) of monocrystalline silicon wafers. NiNPs were non-electrolytically deposited on the SiNWs (electroless method). The morphology of the SiNWs- NiNPs/NiONPs was observed by SEM. Results: Heterogeneous photocatalytic degradation of methyl orange (C14H14N3NaO3S) in an aqueous solution at a concentration of 20 ppm had an efficiency of 66.5% after 180 min under UV irradiation. The MO degradation percentage was determined using UV-visible spectrophotometry. Conclusion: The SiNWs-NiNPs/NiONPs were obtained composed mainly of Si covered by SiO2 decorated on the tips with Ni (II) in the form of NiO and a small amount of nickel metal. The removal efficiency obtained at 180 min of light exposure was 66.5%. After the photocatalysis tests, further oxidation of the NiNPS into NiONPS, was attributed to the reactive oxygen species in the aqueous medium based on the changes of the oxygen and Ni2p3/2 peaks by XPS. Other: Through XPS, the oxidation state of the SiNWs- NiNPs/NiONPs was analyzed.

The article deals with the synthesis of silicon nanowires by the method of metal-assisted chemical etching (MACE). The purpose of this work is to use in light sensors silicon nanowires that synthesized by the MACE. To achieve this goal, it was necessary to synthesize silicon nanowires by the MACE and to study their structural features, to obtain diode structures based on silicon nanowires and to study their sensitivity to visible radiation, to establish the influence of technological parameters of the MSHT process on the sensitivity of photo sensors and sensors. Today, nanostructures are obtained by the method of reactive ion etching, electrochemical etching and plasma implantation. However, most of these methods require sophisticated process equipment. Metal-assisted chemical etching (MACE) method is very promising for nanostructures creation. Two-stage MACE was used to make the samples. In the first stage, silver nanoparticles were deposited on the surface of single crystalline silicon. In the second stage, the samples were etched in a solution of water, hydrogen peroxide and hydrofluoric acid and, as a consequence, a structured surface was obtained. It should be noted that the samples were treated at different times of each of the stages of MACE. Based on the obtained samples, diode light sensors with different photoelectric parameters were synthesized. For the sensors obtained, the sensitivity was calculated in the photodiode and photogenerator modes, and the volt-ampere, luxury-ampere characteristics and dependences of the current density on the illumination density were constructed. structured silicon has a mesh-like appearance, where the dark regions are gaps (pores) and the light ones are silicon (pore walls, nanowires). First, compare the samples with different deposition times. It is seen that as the deposition time of the particles increases from 10 to 40 s, a more developed silicon surface is formed. Also, with the duration of deposition 40s on the silicon surface there are needle-like formations, which according to the results in the work are silver dendrites. All the above characteristics were analyzed for the effect of the deposition time of silver particles t1 and etching time t2 on them. Studies have shown that the optimum technological conditions of metal assisted chemical etching of silicon for using in light sensors are the following conditions: the deposition time of silver particles 40c, etching time 30 min, the content of hydrogen peroxide in the second solution of 0.8 ml. This mode of synthesis of the structuring of silicon substrates provides maximum photosensitivity ratios both in the photodiode (1.53 mA / lmV) and in the photogenerator (304 mA / W) modes. The influence of the concentration of H2O2 used in the second stage is crucial only for the photocurrent, which is not critical, and the influence of the AgNO3 concentration in the first stage of MACE does not have a significant effect on the parameters.

In this work, we introduce a maskless, resist-free rapid prototyping method to fabricate three-dimensional structures using electron beam induced deposition (EBID) of amorphous carbon (aC) from a residual hydrocarbon precursor in combination with metal-assisted chemical etching (MaCE) of silicon. We demonstrate that EBID-made patterned aC coating, with thickness of even a few nanometers, acts as a negative "mask" for the etching process and is sufficient for localized termination of the MaCE of silicon. Optimal aC deposition settings and gold film thickness for fabrication of high-aspect-ratio nanoscale 3D silicon structures are determined. The speed necessary for optimal aC feature deposition is found to be comparable to the writing speed of standard Electron Beam Lithography and the MaCE etching rate is found to be comparable to standard deep reactive ion etching (DRIE) rate.

Metal-assisted chemical etching is a simple and low-cost etching process for semiconductor surfaces. There have been many reports on Si surfaces loaded with noble metals in HF solutions with oxidants such as H2O2.[1,2] Noble metals catalyze the reduction of the oxidant (H2O2), resulting in enhanced oxidation of the Si surface around the loaded metals. Because the oxide is immediately dissolved in the HF solution, selective etching of the Si surface occurs. This etching mode is used to form a variety of three-dimensional nanostructures not only on Si but also on other semiconductor surfaces.We have applied this etching mode to the machining of a Ge surface in O2-containing water. So far, we revealed fundamental etching properties such as pore formation and patterning in this system with noble metals (Pt and Ag),[3] and recently, we demonstrated Pt-assisted chemical flattening.[4,5] In this scheme, a catalyst plate comprising a soft elastomer coated with a sputtered Pt film, and a Ge wafer were brought into contact and rotated independently in the same plane in water. The processed Ge surface includes few protrusions with a lateral size on the order of 10 nm, which is probably caused by the selective removal of protrusions from the Ge surface by the catalytic activity of Pt.However, a problem in this system is the use of noble metals as catalysts. After metal-assisted chemical etching, residual metals on a semiconductor surface have to be removed. For example, aqua regia is effective for dissolving Pt. However, such a strong oxidative solution causes severe damage to a Ge surface. To solve this issue, graphene can be used as a substitute for noble metals to achieve catalyst-assisted chemical etching.[6] In this talk, we discuss the etching properties of a p-type Ge(100) surface loaded with reduced graphene oxide (RGO) in O2-containing water. In order to obtain the RGO, a graphene oxide (GO) ink, used as a starting material, was either heated at 900°C in Ar ambient for 10 min or immersed in a solution comprising the GO ink, hydrazine and N,N-dimethylformamide.[7] Then we deposited the obtained RGO on a Ge surface in the form of aggregated particles or dispersed flakes. After immersing the samples into water exposed to air, we found that the Ge surface was preferentially etched around the loaded RGO. The etching rate as well as the etched morphology was revealed by atomic force microscopy observations, and the etching mechanism is proposed. These findings show the possibility of using RGO as a catalyst to enhance the chemical etching of a Ge surface in water. [1] Z. Huang, N. Geyer, P. Werner, J. de Boor and J. Gösele, Adv. Mat., vol. 23, no. 2, pp. 285-308 (2011).[2] X. Li, Current Opinion in Solid State and Materials Science, vol. 16, pp. 71-81 (2012).[3] T. Kawase, A. Mura, K. Nishitani, Y. Kawai, K. Kawai, J. Uchikoshi, M. Morita and K. Arima, J. Appl. Phys., vol. 111, no. 12, pp. 126102 1-3 (2012).[4] T. Kawase, Y. Saito, A. Mura, T. Okamoto, K. Kawai, Y. Sano, M. Morita, K. Yamauchi, and K. Arima, ChemElectroChem, vol. 2, no. 11, pp. 1656-1659 (2015).[5] T. Kawase, A. Mura, Y. Saito, T. Okamoto, K. Kawai, Y. Sano, K. Yamauchi, M. Morita, and K. Arima, ECS Transactions, vol. 75, no. 1, pp. 107-112 (2016).[6] J. Kim, D.H. Lee, J.H. Kim and S.-H. Choi, ACS Appl. Mat. and Inter., vol. 7, no. 43, pp. 24242-24246 (2015).[7] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, A. Velamakanni and R.S. Ruoff, Nano Lett., vol. 9, no. 4, pp. 1593-1597 (2009).

Fabrication of three-dimensional GaAs antireflective structures by metal-assisted chemical etching

Binuclear metal complexes. XLVI[1]. Electronic and electrochemical properties of copper(II)M(II) binuclear complexes of N,N′-bis(5-t-butylsalicylidene)alkanediamines

Five new dinuclear Cu(II) complexes were designed and synthesized, using hypocrellin B, a naturally occurring photosensitizer that has received extensive studies as promising photodynamic therapy (PDT) agent, as bridging ligand, and five kinds of diimine ligands, including 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), and dipyrido[3,2-a:2',3'-c]phenazene (dppz), as terminal ligands, respectively. The Cu(2+)-HB complexes exhibit improved water solubility, enhanced absorptivity in the phototherapeutic window of 600-900 nm, and increased binding affinity toward dsDNA than their parent HB. The biologically accessible redox potential of Cu(II)/Cu(I) couple renders the five Cu(2+)-HB complexes chemical nuclease activities in the presence of reducing agent such as ascorbic acid. Moreover, the readily available redox potential of Cu(II)/Cu(I) couple switches the photodynamic activity from type II mechanism (singlet oxygen mechanism) for HB to type I mechanism (radical mechanism) for the Cu(2+)-HB complexes. Of the five Cu(2+)-HB complexes, complex 3-5 with terminal diimine ligands of tmp, dpq, and dppz, respectively, can photocleave supercoiled pBR322 DNA more efficiently than HB. These findings open a new avenue for the development of the HB derivatives with higher photodynamic activity and better clinical applicability.

Systematic research on an n-GaAs substrate through a combination of colloidal crystal templating, electroless plating/two-step catalyzation and subsequent metal-assisted chemical etching was carried out. Using self-organized polystyrene spheres as a mask, metal particles, i.e., Cu, Ag and Pd, were selectively deposited at sites resulting in the formation of metal honeycomb patterns on GaAs. Ordered GaAs convex arrays were fabricated by the chemical etching of GaAs originating from the honeycomb-patterned Ag and Pd metals, which acted as etching catalysts, whereas the effectiveness of Cu as a catalyst in metal-assisted etching was not confirmed. Each metal catalyst resulted in the formation of a characteristic etched structure and etching depth, and thus, etching rate. In addition, different anisotropic etching structures were obtained by Ag-assisted etching for (100)- and (111)-oriented substrates. The crystal-face orientation as well as the metal used affects the rate of metal-assisted chemical etching and the morphology obtained.