Uniform vertical trench etching on silicon with high aspect ratio by metal-assisted chemical etching using nanoporous catalysts

Voltage- and Metal-assisted Chemical Etching of Micro and Nano Structures in Silicon: A Comprehensive Review

Sculpting silicon at the micro and nano scales has been game-changing to mold bulk silicon properties and expand, in turn, applications of silicon beyond electronics, namely, in photonics, sensing, medicine, and mechanics, to cite a few. Voltage- and metal-assisted chemical etching (ECE and MaCE, respectively) of silicon in acidic electrolytes have emerged over other micro and nanostructuring technologies thanks to their unique etching features. ECE and MaCE have enabled the fabrication of novel structures and devices not achievable otherwise, complementing those feasible with the deep reactive ion etching (DRIE) technology, the gold standard in silicon machining. Here, a comprehensive review of ECE and MaCE for silicon micro and nano machining is provided. The chemistry and physics ruling the dissolution of silicon are dissected and similarities and differences between ECE and MaCE are discussed showing that they are the two sides of the same coin. The processes governing the anisotropic etching of designed silicon micro and nanostructures are analyzed, and the modulation of etching profile over depth is discussed. The preparation of micro- and nanostructures with tailored optical, mechanical, and thermo(electrical) properties is then addressed, and their applications in photonics, (bio)sensing, (nano)medicine, and micromechanical systems are surveyed. Eventually, ECE and MaCE are benchmarked against DRIE, and future perspectives are highlighted.

Atmospheric Gas-Phase Catalyst Etching of SiO2 for Deep Microfabrication Using HF Gas and Patterned Photoresist

Micro/nanoscale structure fabrication is an important process for designing miniaturized devices. Recently, three-dimensional (3D) integrated circuits using SiO2 via-holes interlayer filling by copper have attracted attention to extend the lifetime of Moore's law. However, the fabrication of vertical and smooth-sidewall via-hole structures on SiO2 has not been achieved using the conventional dry etching method due to the limitation of the selective etching ratio of SiO2 and hard mask materials. In this study, we developed a unique method for the deep anisotropic dry etching of SiO2 using atmospheric gas-phase HF and a patterned photoresist. The hydroxyl groups in the photoresist catalyzed the HF gas-phase dry etching of SiO2 at high-temperature conditions. Therefore, fabrication of vertical with smooth-sidewall deep microstructures was demonstrated in the photoresist-covered area on SiO2 at a processing rate of 1.3 μm/min, which is 2-3 times faster than the conventional dry etching method. Additionally, the chemical reaction pathway in the photoresist-covered area on SiO2 with HF gas was revealed via density functional theory (DFT) calculations. This simple and high-speed microfabrication process will expand the commercial application scope of next-generation microfabricated SiO2-based devices.

Fabrication of Silicon Nanowires by Metal-Assisted Chemical Etching Combined with Micro-Vibration

In this work, we design a micro-vibration platform, which combined with the traditional metal-assisted chemical etching (MaCE) to etch silicon nanowires (SiNWs). The etching mechanism of SiNWs, including in the mass-transport (MT) and charge-transport (CT) processes, was explored through the characterization of SiNW's length as a function of MaCE combined with micro-vibration conditions, such as vibration amplitude and frequency. The scanning electron microscope (SEM) experimental results indicated that the etching rate would be continuously improved with an increase in amplitude and reached its maximum at 4 μm. Further increasing amplitude reduced the etching rate and affected the morphology of the SiNWs. Adjusting the vibration frequency would result in a maximum etching rate at a frequency of 20 Hz, and increasing the frequency will not help to improve the etching effects.

Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

Metal-assisted electrochemical nanoimprinting (Mac-Imprint) scales the fabrication of micro- and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip-scale optics operating at the near-infrared spectrum. However, Mac-Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size ≈45 nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter ≈42 nm) onto silicon. In this work, roughness is reduced to sub-10 nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far-from equilibrium conditions. At this level, single-digit nanometric details such as grain-boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac-Imprint, which is argued to be twice the Debye length (i.e., 1.7 nm)-a finding that broadly applies to metal-assisted chemical etching. Last, Mac-Imprint is employed to produce single-mode rib-waveguides on pre-patterned silicon-on-insulator wafers with root-mean-square line-edge roughness less than 10 nm while providing depth uniformity (i.e., 42.9 ± 5.5 nm), and limited levels of silicon defect formation (e.g., Raman peak shift < 0.1 cm^-1 ) and sidewall scattering.

Deep Etching of Silicon Based on Metal-Assisted Chemical Etching

A deep etching method for silicon "micro"structures was successfully developed. This wet etching process is based on metal-assisted chemical etching (MACE), which was previously mainly utilized to etch the features that have lateral dimensions of "nanometers." In this novel MACE, the critical improvement was to promote the "out-of-plane" mass transfer at the metal/Si interface with an ultrathin metal film. This enabled us to etch micrometer-wide holes, which was previously challenging due to the mass transport limitation. In addition, it was found that when ethanol was used as a solvent instead of water, the formation of porous defects was suppressed. Under the optimized etch conditions, deep (>200 μm) and vertical (>88°) holes could be carved out at a fast etch rate (>0.4 μm/min). This novel deep MACE will find utility in applications such as microelectromechanical systems (MEMS) devices or biosensors.

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching

Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

Cost-Effective Fabrication of Fractal Silicon Nanowire Arrays

Silicon nanowires (Si NWs) emerged in several application fields as a strategic element to surpass the bulk limits with a flat compatible architecture. The approaches used for the Si NW realization have a crucial impact on their final performances and their final cost. This makes the research on a novel and flexible approach for Si NW fabrication a crucial point for Si NW-based devices. In this work, the novelty is the study of the flexibility of thin film metal-assisted chemical etching (MACE) for the fabrication of Si NWs with the possibility of realizing different doped Si NWs, and even a longitudinal heterojunction p-n inside the same single wire. This point has never been reported by using thin metal film MACE. In particular, we will show how this approach permits one to obtain a high density of vertically aligned Si NWs with the same doping of the substrate and without any particular constraint on doping type and level. Fractal arrays of Si NWs can be fabricated without any type of mask thanks to the self-assembly of gold at percolative conditions. This Si NW fractal array can be used as a substrate to realize controllable artificial fractals, integrating other interesting elements with a cost-effective microelectronics compatible approach.

Dicing of composite substrate for thin film AlGaInP power LEDs by wet etching

In this paper, thin film AlGaInP LED chips with a 50 μm thick composite metal substrate (Copper-Invar-Copper; CIC) were obtained by the wet etching process. The pattern of the substrate was done by the backside of the AlGaInP LED/CIC. There was no delamination or cracking phenomenon of the LED epilayer which often occurs by laser or mechanical dicing. The chip area was 1140 μm × 1140 μm and the channel length was 360 μm. The structure of the CIC substrate was a sandwich structure and consisted of Cu as the top and bottom layers, with a thickness of 10 μm, respectively. The middle layer was Invar with a 30% to 70% ratio of Ni and Fe and a total thickness of 30 μm. The chip pattern was successfully obtained by the wet etching process. Concerning the device performance after etching, high-performance LED/CIC chips were obtained. They had a low leakage current, high output power and a low red shift phenomenon as operated at a high injected current. After the development and fabrication of the copper-based composite substrate for N-side up thin-film AlGaInP LED/CIC chips could be diced by wet etching. The superiority of wet etching process for the AlGaInP LED/CIC chips is over that of chips obtained by mechanical or laser dicing.

Replacing Metals with Oxides in Metal-Assisted Chemical Etching Enables Direct Fabrication of Silicon Nanowires by Solution Processing

Metal-assisted chemical etching (MACE) has emerged as an effective method to fabricate high aspect ratio nanostructures. This method requires a catalytic mask that is generally composed of a metal. Here, we challenge the general view that the catalyst needs to be a metal by introducing oxide-assisted chemical etching (OACE). We perform etching with metal oxides such as RuO₂ and IrO₂ by transposing materials used in electrocatalysis to nanofabrication. These oxides can be solution-processed as polymers exhibiting similar capabilities of metals for MACE. Nanopatterned oxides can be obtained by direct nanoimprint lithography or block-copolymer lithography from chemical solution on a large scale. High aspect ratio silicon nanostructures were obtained at the sub-20 nm scale exclusively by cost-effective solution processing by halving the number of fabrication steps compared to MACE. In general, OACE is expected to stimulate new fundamental research on chemical etching assisted by other materials, providing new possibilities for device fabrication.

Large-Scale Synthesis of Highly Uniform Silicon Nanowire Arrays Using Metal-Assisted Chemical Etching

The combination of metal-assisted chemical etching (MACE) with colloidal lithography has emerged as a simple and cost-effective approach to nanostructure silicon. It is especially efficient at synthesizing Si micro- and nanowire arrays using a catalytic metal mesh, which sinks into the silicon substrate during the etching process. The approach provides a precise control over the array geometry, without requiring expensive nanopatterning techniques. Although MACE is a high-throughput solution-based approach, achieving large-scale homogeneity can be challenging because of the instability of the metal catalyst when the experimental parameters are not set appropriately. Such instabilities can lead to metal film fracture, significantly damaging the substrate and thus compromising the nanowire array quality. Here, we report on the critical parameters that influence the stability of the metal catalyst layer for achieving large-scale homogeneous MACE: etchant composition, metal film thickness, adhesion layer thickness, nanowire diameter and pitch, metal film coverage, Si/Au/etchant interface length, and crystalline quality of the colloidal template (grain size and defects). Our results investigate the origin of the catalyst film fracture and reveal that MACE experiments should be optimized for each Si wire array geometry by keeping the etch rate below a certain threshold. We show that the Si/Au/etchant interface length also affects the etch rate and should thus be considered when optimizing the MACE experimental parameters. Finally, our results demonstrate that colloidal templates with small grain sizes (i.e., <100 μm2) can yield significant problems during the pattern transfer because of a high density of defects at the grain boundaries that negatively affects the metal film stability. As such, this work provides guidelines for the large-scale synthesis of Si micro- and nanowire arrays via MACE, relevant for both new and experienced researchers working with MACE.

Microfabrication of X-ray Optics by Metal Assisted Chemical Etching: A Review

High-aspect-ratio silicon micro- and nanostructures are technologically relevant in several applications, such as microelectronics, microelectromechanical systems, sensors, thermoelectric materials, battery anodes, solar cells, photonic devices, and X-ray optics. Microfabrication is usually achieved by dry-etch with reactive ions and KOH based wet-etch, metal assisted chemical etching (MacEtch) is emerging as a new etching technique that allows huge aspect ratio for feature size in the nanoscale. To date, a specialized review of MacEtch that considers both the fundamentals and X-ray optics applications is missing in the literature. This review aims to provide a comprehensive summary including: (i) fundamental mechanism; (ii) basics and roles to perform uniform etching in direction perpendicular to the <100> Si substrate; (iii) several examples of X-ray optics fabricated by MacEtch such as line gratings, circular gratings array, Fresnel zone plates, and other X-ray lenses; (iv) materials and methods for a full fabrication of absorbing gratings and the application in X-ray grating based interferometry; and (v) future perspectives of X-ray optics fabrication. The review provides researchers and engineers with an extensive and updated understanding of the principles and applications of MacEtch as a new technology for X-ray optics fabrication.

Metal assisted chemical etching of silicon in the gas phase: a nanofabrication platform for X-ray optics

High aspect ratio nanostructuring requires high precision pattern transfer with highly directional etching. In this work, we demonstrate the fabrication of structures with ultra-high aspect ratios (up to 10 000 : 1) in the nanoscale regime (down to 10 nm) by platinum assisted chemical etching of silicon in the gas phase. The etching gas is created by a vapour of water diluted hydrofluoric acid and a continuous air flow, which works both as an oxidizer and as a gas carrier for reactive species. The high reactivity of platinum as a catalyst and the formation of platinum silicide to improve the stability of the catalyst pattern allow a controlled etching. The method has been successfully applied to produce straight nanowires with section size in the range of 10-100 nm and length of hundreds of micrometres, and X-ray optical elements with feature sizes down to 10 nm and etching depth in the range of tens of micrometres. This work opens the possibility of a low cost etching method for stiction-sensitive nanostructures and a large range of applications where silicon high aspect ratio nanostructures and high precision of pattern transfer are required.

CMOS-Compatible Catalyst for MacEtch: Titanium Nitride-Assisted Chemical Etching in Vapor phase for High Aspect Ratio Silicon Nanostructures

Metal-assisted chemical etching (MacEtch) is an emerging anisotropic chemical etching technique that has been used to fabricate high aspect ratio semiconductor micro- and nanostructures. Despite its advantages in unparalleled anisotropy, simplicity, versatility, and damage-free nature, the adaptation of MacEtch for silicon (Si)-based electronic device fabrication process is hindered by the use of a gold (Au)-based metal catalyst, as Au is a detrimental deep-level impurity in Si. In this report, for the first time, we demonstrate CMOS-compatible titanium nitride (TiN)-based MacEtch of Si by establishing a true vapor-phase (VP) MacEtch approach in order to overcome TiN-MacEtch-specific challenges. Whereas inverse-MacEtch is observed using conventional liquid phase MacEtch because of the limited mass transport from the strong adhesion between TiN and Si, the true VP etch leads to forward MacEtch and produces Si nanowire arrays by engraving the TiN mesh pattern in Si. The etch rate as a function of etch temperature, solution concentration, TiN dimension, and thickness is systematically characterized to uncover the underlying nature of MacEtching using this new catalyst. VP MacEtch represents a significant step toward scalability of this disruptive technology because of the high controllability of gas phase reaction dynamics. TiN-MacEtch may also have direct implications in embedded TiN-based plasmonic semiconductor structures for photonic applications.

Resist-Free Direct Stamp Imprinting of GaAs via Metal-Assisted Chemical Etching

We introduce a method for the direct imprinting of GaAs substrates using wet-chemical stamping. The predefined patterns on the stamps etch the GaAs substrates via metal-assisted chemical etching. This is a resist-free method in which the stamp and the GaAs substrate are directly pressed together. Imprinting and etching occur concurrently until the stamp is released from the substrate. The stamp imprinting results in a three-dimensional anisotropic etching profile and does not impair the semiconductor crystallinity in the wet-chemical bath. Hole, trench, and complex patterns can be imprinted on the GaAs substrate after stamping with pillar, fin, and letter shapes. In addition, we demonstrate the formation of sub-100 nm trench patterns on GaAs through a single-step stamping process. Consecutive imprinting using a single stamp is possible, demonstrating the recyclability of the stamp, which can be used more than 10 times. The greatest benefit of this technique is the simple method of patterning by integrating the lithographic and etching processes, making this a high-throughput and low-cost technique.

Tailoring the robust superhydrophobic silicon textures with stable photodetection properties

Surface hydrophobicity of silicon with sound durability under mechanical abrasion is highly desirable for practical needs. However, the reported micro-pyramid/nanowires structures suffer from the saturation characteristics of contact angle at around 132 degree, which impede the promotions toward reaching the state of superhydrophobicity. The present study focuses on the realization of two-scale silicon hierarchical structures prepared with the facile, rapid and large-area capable chemical etching methods without the need of lithographic patterning. The designed structures, with the well combination of microscale inverted pyramids and nanowire arrays, dramatically lead to the increased wetting angle of 157.2 degree and contact-angle hysteresis of 9.4 degree. In addition, the robustness test reveals that these hierarchical textures possess the narrow contact-angle change of 4 degree responding to the varied pH values, and maintain a narrow deviation of 2 degree in wetting angle after experiencing the abrasion test. Moreover, the highly stable photodetection characteristics of such two-scale structures were identified, showing the reliable photocurrents with less than 3% of deviation under wide range of environmental humidity. By adopting a simple chemical treatment, the wetting control is demonstrated for reliable transition of superhydrophobicity and superhydrophilicity.

Interfacial Contact is Required for Metal-Assisted Plasma Etching of Silicon

For decades, fabrication of semiconductor devices has utilized well-established etching techniques to create complex nanostructures in silicon. The most common dry process is reactive ion etching which fabricates nanostructures through the selective removal of unmasked silicon. Generalized enhancements of etching have been reported with mask-enhanced etching with Al, Cr, Cu, and Ag masks, but there is a lack of reports exploring the ability of metallic films to catalytically enhance the local etching of silicon in plasmas. Here, metal-assisted plasma etching (MAPE) is performed using patterned nanometers-thick gold films to catalyze the etching of silicon in an SF6/O2 mixed plasma, selectively increasing the rate of etching by over 1000%. The catalytic enhancement of etching requires direct Si-metal interfacial contact, similar to metal-assisted chemical etching (MACE), but is different in terms of the etching mechanism. The mechanism of MAPE is explored by characterizing the degree of enhancement as a function of Au catalyst configuration and relative oxygen feed concentration, along with the catalytic activities of other common MACE metals including Ag, Pt, and Cu.

Highly Efficient Nano-Porous Polysilicon Solar Absorption Films Prepared by Silver-Induced Etching

Nano-porous polysilicon high-temperature resistant solar absorption films were prepared by a thin layer of silver nanoparticles catalytic chemical etching. The polysilicon films with average tiny grain size of approximately 30 nm were obtained by high-temperature 800 °C furnace annealing of hydrogenated amorphous silicon films that were deposited on stainless substrate by plasma-enhanced chemical vapor deposition. The uniformly distributed 19 nm sized silver nanoparticles with 8 nm interspacing deposited on poly-Si film, were controlled by thin 4 nm thickness and very slow deposition rate 0.4 nm/min of thermal evaporation. Small silver nanoparticles with short spacing catalyzes the detouring etching process inducing the nano-porous textured surface with deep threaded pores. The etching follows the trail of the grain boundaries, and takes a highly curved thread like structure. The etching stops after reaching a depth of around 1100 nm, and the rest of the bulk thickness of the film remains mostly unaffected. The structure consists of three crystal orientations (111), (220), and (331) close to the surface. This crystalline nature diminishes gradually in the bulk of the film. High absorbance of 95% was obtained due to efficient light-trapping. Hence, preparation of nano-porous polysilicon films by this simple method can effectively increase solar absorption for the receiver of the solar thermal electricity Stirling Engine.

Self-Anchored Catalyst Interface Enables Ordered Via Array Formation from Submicrometer to Millimeter Scale for Polycrystalline and Single-Crystalline Silicon

Defying text definitions of wet etching, metal-assisted chemical etching (MacEtch), a solution-based, damage-free semiconductor etching method, is directional, where the metal catalyst film sinks with the semiconductor etching front, producing 3D semiconductor structures that are complementary to the metal catalyst film pattern. The same recipe that works perfectly to produce ordered array of nanostructures for single-crystalline Si (c-Si) fails completely when applied to polycrystalline Si (poly-Si) with the same doping type and level. Another long-standing challenge for MacEtch is the difficulty of uniformly etching across feature sizes larger than a few micrometers because of the nature of lateral etching. The issue of interface control between the catalyst and the semiconductor in both lateral and vertical directions over time and over distance needs to be systematically addressed. Here, we present a self-anchored catalyst (SAC) MacEtch method, where a nanoporous catalyst film is used to produce nanowires through the pinholes, which in turn physically anchor the catalyst film from detouring as it descends. The systematic vertical etch rate study as a function of porous catalyst diameter from 200 to 900 nm shows that the SAC-MacEtch not only confines the etching direction but also enhances the etch rate due to the increased liquid access path, significantly delaying the onset of the mass-transport-limited critical diameter compared to nonporous catalyst c-Si counterpart. With this enhanced mass transport approach, vias on multistacks of poly-Si/SiO₂ are also formed with excellent vertical registry through the polystack, even though they are separated by SiO₂ which is readily removed by HF alone with no anisotropy. In addition, 320 μm square through-Si-via (TSV) arrays in 550 μm thick c-Si are realized. The ability of SAC-MacEtch to etch through poly/oxide/poly stack as well as more than half millimeter thick silicon with excellent site specificity for a wide range of feature sizes has significant implications for 2.5D/3D photonic and electronic device applications.

Hybrid black silicon solar cells textured with the interplay of copper-induced galvanic displacement

Metal-assisted chemical etching (MaCE) has been widely employed for the fabrication of regular silicon (Si) nanowire arrays. These features were originated from the directional etching of Si preferentially along <100> orientations through the catalytic assistance of metals, which could be gold, silver, platinum or palladium. In this study, the dramatic modulation of etching profiles toward pyramidal architectures was undertaken by utilizing copper as catalysts through a facile one-step etching process, which paved the exceptional way on the texturization of Si for advanced photovoltaic applications. Detailed examinations of morphological evolutions, etching kinetics and formation mechanism were performed, validating the distinct etching model on Si contributed from cycling reactions of copper deposition and dissolution under a quasi-stable balance. In addition, impacts of surface texturization on the photovoltaic performance of organic/inorganic hybrid solar cells were revealed through the spatial characterizations of voltage fluctuations upon light mapping analysis. It was found that the pyramidal textures made by copper-induced cycling reactions exhibited the sound antireflection characteristics, and further achieved the leading conversion efficiency of 10.7%, approximately 1.8 times and beyond 1.2 times greater than that of untexturized and nanowire-based solar cells, respectively.

Damage-Free Smooth-Sidewall InGaAs Nanopillar Array by Metal-Assisted Chemical Etching

Producing densely packed high aspect ratio In_0.53Ga_0.47As nanostructures without surface damage is critical for beyond Si-CMOS nanoelectronic and optoelectronic devices. However, conventional dry etching methods are known to produce irreversible damage to III-V compound semiconductors because of the inherent high-energy ion-driven process. In this work, we demonstrate the realization of ordered, uniform, array-based In_0.53Ga_0.47As pillars with diameters as small as 200 nm using the damage-free metal-assisted chemical etching (MacEtch) technology combined with the post-MacEtch digital etching smoothing. The etching mechanism of In_xGa_1-xAs is explored through the characterization of pillar morphology and porosity as a function of etching condition and indium composition. The etching behavior of In_0.53Ga_0.47As, in contrast to higher bandgap semiconductors (e.g., Si or GaAs), can be interpreted by a Schottky barrier height model that dictates the etching mechanism constantly in the mass transport limited regime because of the low barrier height. A broader impact of this work relates to the complete elimination of surface roughness or porosity related defects, which can be prevalent byproducts of MacEtch, by post-MacEtch digital etching. Side-by-side comparison of the midgap interface state density and flat-band capacitance hysteresis of both the unprocessed planar and MacEtched pillar In_0.53Ga_0.47As metal-oxide-semiconductor capacitors further confirms that the surface of the resultant pillars is as smooth and defect-free as before etching. MacEtch combined with digital etching offers a simple, room-temperature, and low-cost method for the formation of high-quality In_0.53Ga_0.47As nanostructures that will potentially enable large-volume production of In_0.53Ga_0.47As-based devices including three-dimensional transistors and high-efficiency infrared photodetectors.

Enhancing formation rate of highly-oriented silicon nanowire arrays with the assistance of back substrates

Facile, effective and reliable etching technique for the formation of uniform silicon (Si) nanowire arrays were realized through the incorporation of back substrates with metal-assisted chemical etching (MaCE). In comparison with conventional MaCE process, a dramatic increase of etching rates upon MaCE process could be found by employing the conductive back substrates on p-type Si, while additionally prevented the creation of nanopores from catalytic etching reaction. Examinations on the involving etching kinetics, morphologies, wetting behaviors and surface structures were performed that validated the role of back substrates upon MaCE process. It was found that the involved two pathways for the extraction of electrons within Si favored the localized oxidation of Si at Si/Ag interfaces, thereby increasing the etching rate of MaCE process. This back-substrate involved MaCE could potentially meet the practical needs for the high-yield formation of Si nanowire arrays.

Electrochemical nanoimprint lithography: when nanoimprint lithography meets metal assisted chemical etching

The functional three dimensional micro-nanostructures (3D-MNS) play crucial roles in integrated and miniaturized systems because of the excellent physical, mechanical, electric and optical properties. Nanoimprint lithography (NIL) has been versatile in the fabrication of 3D-MNS by pressing thermoplastic and photocuring resists into the imprint mold. However, direct nanoimprint on the semiconductor wafer still remains a great challenge. On the other hand, considered as a competitive fabrication method for erect high-aspect 3D-MNS, metal assisted chemical etching (MacEtch) can remove the semiconductor by spontaneous corrosion reaction at the metal/semiconductor/electrolyte 3-phase interface. Moreover, it was difficult for MacEtch to fabricate multilevel or continuously curved 3D-MNS. The question of the consequences of NIL meeting the MacEtch is yet to be answered. By employing a platinum (Pt) metalized imprint mode, we demonstrated that using electrochemical nanoimprint lithography (ECNL) it was possible to fabricate not only erect 3D-MNS, but also complex 3D-MNS with multilevel stages with continuously curved surface profiles on a gallium arsenide (GaAs) wafer. A concave microlens array with an average diameter of 58.4 μm and height of 1.5 μm was obtained on a ∼1 cm²-area GaAs wafer. An 8-phase microlens array was fabricated with a minimum stage of 57 nm and machining accuracy of 2 nm, presenting an excellent optical diffraction property. Inheriting all the advantages of both NIL and MacEtch, ECNL has prospective applications in the micro/nano-fabrications of semiconductors.

Nonlinear Etch Rate of Au-Assisted Chemical Etching of Silicon

We demonstrated time-dependent mass transport mechanisms of Au-assisted chemical etching of Si substrates. Variations in the etch rate and surface topology were correlated with catalyst features and etching duration. Nonlinear etching characteristics were associated with the formation of pinholes and whiskers. Variable rates of mass transport as a function of whisker density accounted for the nonlinear etch rates of Si. Nanopinholes on Au catalysts facilitated the vertical mass transport of reactants and byproducts, which dramatically changed the etch rate, surface topology, and porosity of Si. The suggested transport models describe the transient mass transport and the corresponding chemical reactions.

Wafer Scale Fabrication of Dense and High Aspect Ratio Sub-50 nm Nanopillars from Phase Separation of Cross-Linkable Polysiloxane/Polystyrene Blend

We demonstrated a simple and effective approach to fabricate dense and high aspect ratio sub-50 nm pillars based on phase separation of a polymer blend composed of a cross-linkable polysiloxane and polystyrene (PS). In order to obtain the phase-separated domains with nanoscale size, a liquid prepolymer of cross-linkable polysiloxane was employed as one moiety for increasing the miscibility of the polymer blend. After phase separation via spin-coating, the dispersed domains of liquid polysiloxane with sub-50 nm size could be solidified by UV exposure. The solidified polysiloxane domains took the role of etching mask for formation of high aspect ratio nanopillars by O₂ reactive ion etching (RIE). The aspect ratio of the nanopillars could be further amplified by introduction of a polymer transfer layer underneath the polymer blend film. The effects of spin speeds, the weight ratio of the polysiloxane/PS blend, and the concentration of polysiloxane/PS blend in toluene on the characters of the nanopillars were investigated. The gold-coated nanopillar arrays exhibited a high Raman scattering enhancement factor in the range of 10⁸-10⁹ with high uniformity across over the wafer scale sample. A superhydrophobic surface could be realized by coating a self-assembled monolayers (SAM) of fluoroalkyltrichlorosilane on the nanopillar arrays. Sub-50 nm silicon nanowires (SiNWs) with high aspect ratio of about 1000 were achieved by using the nanopillars as etching mask through a metal-assisted chemical etching process. They showed an ultralow reflectance of approximately 0.1% for wavelengths ranging from 200 to 800 nm.

Contact electrification induced interfacial reactions and direct electrochemical nanoimprint lithography in n-type gallium arsenate wafer

Although metal assisted chemical etching (MacEtch) has emerged as a versatile micro-nanofabrication method for semiconductors, the chemical mechanism remains ambiguous in terms of both thermodynamics and kinetics. Here we demonstrate an innovative phenomenon, i.e., the contact electrification between platinum (Pt) and an n-type gallium arsenide (100) wafer (n-GaAs) can induce interfacial redox reactions. Because of their different work functions, when the Pt electrode comes into contact with n-GaAs, electrons will move from n-GaAs to Pt and form a contact electric field at the Pt/n-GaAs junction until their electron Fermi levels (EF) become equal. In the presence of an electrolyte, the potential of the Pt/electrolyte interface will shift due to the contact electricity and induce the spontaneous reduction of MnO4- anions on the Pt surface. Because the equilibrium of contact electrification is disturbed, electrons will transfer from n-GaAs to Pt through the tunneling effect. Thus, the accumulated positive holes at the n-GaAs/electrolyte interface make n-GaAs dissolve anodically along the Pt/n-GaAs/electrolyte 3-phase interface. Based on this principle, we developed a direct electrochemical nanoimprint lithography method applicable to crystalline semiconductors.

The path of mass transfer during Au thin film-assisted chemical etching by designed surface barriers

The mass transfer during the initial etching process: Si atoms dissolve in the Au film, and then diffuse across the Au lattice, and are oxidized and etched away at the Au film/solution interface.

Electrochemical micro/nano-machining: principles and practices

Micro/nano-machining (MNM) is becoming the cutting-edge of high-tech manufacturing because of the ever increasing industrial demands for super smooth surfaces and functional three-dimensional micro/nano-structures in miniaturized and integrate devices, and electrochemistry plays an irreplaceable role in MNM.

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.

Schottky Barrier Catalysis Mechanism in Metal-Assisted Chemical Etching of Silicon

Metal-assisted chemical etching (MACE) is a versatile anisotropic etch for silicon although its mechanism is not well understood. Here we propose that the Schottky junction formed between metal and silicon plays an essential role on the distribution of holes in silicon injected from hydrogen peroxide. The proposed mechanism can be used to explain the dependence of the etching kinetics on the doping level, doping type, crystallographic surface direction, and etchant solution composition. We used the doping dependence of the reaction to fabricate a novel etch stop for the reaction.

Self-assembly nanostructured gold for high aspect ratio silicon microstructures by metal assisted chemical etching

Self-assembly Au nanostructures stabilize the catalyst during metal assisted chemical etching, improving the vertical profile of high aspect ratio Si dense micro-patterns on large area, such as diffraction gratings for X-ray phase contrast imaging.

Hybrid Metal-Semiconductor Nanostructure for Ultrahigh Optical Absorption and Low Electrical Resistance at Optoelectronic Interfaces

Engineered optoelectronic surfaces must control both the flow of light and the flow of electrons at an interface; however, nanostructures for photon and electron management have typically been studied and optimized separately. In this work, we unify these concepts in a new hybrid metal-semiconductor surface that offers both strong light absorption and high electrical conductivity. We use metal-assisted chemical etching to nanostructure the surface of a silicon wafer, creating an array of silicon nanopillars protruding through holes in a gold film. When coated with a silicon nitride anti-reflection layer, we observe broad-band absorption of up to 97% in this structure, which is remarkable considering that metal covers 60% of the top surface. We use optical simulations to show that Mie-like resonances in the nanopillars funnel light around the metal layer and into the substrate, rendering the metal nearly transparent to the incoming light. Our results show that, across a wide parameter space, hybrid metal-semiconductor surfaces with absorption above 90% and sheet resistance below 20 Ω/□ are realizable, suggesting a new paradigm wherein transparent electrodes and photon management textures are designed and fabricated together to create high-performance optoelectronic interfaces.

Deep etching of single- and polycrystalline silicon with high speed, high aspect ratio, high uniformity, and 3D complexity by electric bias-attenuated metal-assisted chemical etching (EMaCE)

In this work, a novel wet silicon (Si) etching method, electric bias-attenuated metal-assisted chemical etching (EMaCE), is demonstrated to be readily available for three-dimensional (3D) electronic integration, microelectromechinal systems, and a broad range of 3D electronic components with low cost. On the basis of the traditional metal-assisted chemical etching process, an electric bias was applied to the Si substrate in EMaCE. The 3D geometry of the etching profile was effectively controlled by the bias in a real-time manner. The reported method successfully fabricated an array of over 10 000 vertical holes with diameters of 28 μm on 1 cm(2) silicon chips at a rate of up to 11 μm/min. The sidewall roughness was kept below 50 nm, and a high aspect ratio of over 10:1 was achieved. The 3D geometry could be attenuated by the variable applied bias in real time. Vertical deep etching was realized on (100)-, (111)-Si, and polycrystalline Si substrates. Complex features with lateral dimensions of 0.8-500 μm were also fabricated with submicron accuracy.

Contact resistivity decrease at a metal/semiconductor interface by a solid-to-liquid phase transitional metallo-organic silver

We present a new approach to ensure the low contact resistivity of a silver paste at a metal/semiconductor interface over a broad range of peak firing temperatures by using a solid-to-liquid phase transitional metallo-organic silver, that is, silver neodecanoate. Silver nanoclusters, thermally derived from silver neodecanoate, are readily dissolved into the melt of metal oxide glass frit even at low temperatures, at which point the molten metal oxide glass frit lacks the dissociation capability of bulk silver into Ag(+) ions. In the presence of O(2-) ions in the melt of metal oxide glass frit, the redox reaction from Ag(+) to Ag(0) augments the noble-metal-assisted etching capability to remove the passivation layer of silicon nitride. Moreover, during the cooling stage, the nucleated silver atoms enrich the content of silver nanocolloids in the solidified metal oxide glass layer. The resulting contact resistivity of silver paste with silver neodecanoate at the metal/semiconductor interface thus remains low-between 4.12 and 16.08 mΩ cm(2)-whereas without silver neodecanoate, the paste exhibits a contact resistivity between 2.61 and 72.38 mΩ cm(2) in the range of peak firing temperatures from 750 to 810 °C. The advantage of using silver neodecanoate in silver paste becomes evident in that contact resistivity remains low over the broad range of peak firing temperatures, thus providing greater flexibility with respect to the firing temperature required in silicon solar cell applications.

Review of nanostructured devices for thermoelectric applications

A big research effort is currently dedicated to the development of thermoelectric devices capable of a direct thermal-to-electrical energy conversion, aiming at efficiencies as high as possible. These devices are very attractive for many applications in the fields of energy recovery and green energy harvesting. In this paper, after a quick summary of the fundamental principles of thermoelectricity, the main characteristics of materials needed for high efficiency thermoelectric conversion will be discussed, and a quick review of the most promising materials currently under development will be given. This review paper will put a particular emphasis on nanostructured silicon, which represents a valid compromise between good thermoelectric properties on one side and material availability, sustainability, technological feasibility on the other side. The most important bottom-up and top-down nanofabrication techniques for large area silicon nanowire arrays, to be used for high efficiency thermoelectric devices, will be presented and discussed.