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

Metal-assisted chemical etching beyond Si: applications to III-V compounds and wide-bandgap semiconductors

Metal-assisted chemical etching (MacEtch) has emerged as a versatile technique for fabricating a variety of semiconductor nanostructures. Since early investigations in 2000, research in this field has provided a deeper understanding of the underlying mechanisms of catalytic etching processes and enabled high control over etching conditions for diverse applications. In this Review, we present an overview of recent developments in the application of MacEtch to nanomanufacturing and processing of III-V based semiconductor materials and other materials beyond Si. We highlight the key findings and developments in MacEtch as applied to GaAs, GaN, InP, GaP, InGaAs, AlGaAs, InGaN, InGaP, SiC, β-Ga2O3, and Ge material systems. We further review a series of active and passive devices enabled by MacEtch, including light-emitting diodes (LEDs), field-effect transistors (FETs), optical gratings, sensors, capacitors, photodiodes, and solar cells. By reviewing demonstrated control of morphology, optimization of etch conditions, and catalyst-material combinations, we aim to distill the current understanding of beyond-Si MacEtch mechanisms and to provide a bank of reference recipes to stimulate progress in the field.

Ultrasonic-Assisted Electrochemical Nanoimprint Lithography: Forcing Mass Transfer to Enhance the Localized Etching Rate of GaAs

Electrochemical nanoimprint lithography (ECNL) has emerged as a promising technique for fabricating three-dimensional micro/nano-structures (3D-MNSs) directly on semiconductor wafers. This technique is based on a localized corrosion reaction induced by the contact potential across the metal/semiconductor boundaries. The anodic etching of semiconductor and the cathodic reduction of electron acceptors occur at the metal/semiconductor/electrolyte interface and the Pt mold surface, respectively. However, the etching rate is limited by the mass transfer of species in the ultrathin electrolyte layer between the mold and the workpiece. To overcome this challenge, we introduce the ultrasonics effect into the ECNL process to facilitate the mass exchange between the ultrathin electrolyte layer and the bulk solution, thereby improving the imprinting efficiency. Experimental investigations demonstrate a positive linear relationship between the reciprocal of the area duty ratio of the mold and the imprinting efficiency. Furthermore, the introduction of ultrasonics improves the imprinting efficiency by approximately 80 %, irrespective of the area duty ratio. The enhanced imprinting efficiency enables the fabrication of 3D-MNSs with higher aspect ratios, resulting in a stronger light trapping effect. These results indicate the prospective applications of ECNL in semiconductor functional devices, such as photoelectric detection and photovoltaics.

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.

Interfacial Interactions during Demolding in Nanoimprint Lithography

Nanoimprint lithography (NIL) is a useful technique for the fabrication of nano/micro-structured materials. This article reviews NIL in the field of demolding processes and is divided into four parts. The first part introduces the NIL technologies for pattern replication with polymer resists (e.g., thermal and UV-NIL). The second part reviews the process simulation during resist filling and demolding. The third and fourth parts discuss in detail the difficulties in demolding, particularly interfacial forces between mold (template) and resist, during NIL which limit its capability for practical commercial applications. The origins of large demolding forces (adhesion and friction forces), such as differences in the thermal expansion coefficients (CTEs) between the template and the imprinted resist, or volumetric shrinkage of the UV-curable polymer during curing, are also illustrated accordingly. The plausible solutions for easing interfacial interactions and optimizing demolding procedures, including exploring new resist materials, employing imprint mold surface modifications (e.g., ALD-assisted conformal layer covering imprint mold), and finetuning NIL process conditions, are presented. These approaches effectively reduce the interfacial demolding forces and thus lead to a lower defect rate of pattern transfer. The objective of this review is to provide insights to alleviate difficulties in demolding and to meet the stringent requirements regarding defect control for industrial manufacturing while at the same time maximizing the throughput of the nanoimprint technique.

Chemical carving lithography with scanning catalytic probes

This study introduces a new chemical carving technique as an alternative to existing lithography and etching techniques. Chemical carving incorporates the concept of scanning probe lithography and metal-assisted chemical etching (MaCE). A catalyst-coated probe mechanically scans a Si substrate in a solution, and the Si is chemically etched into the shape of the probes, forming pre-defined 3D patterns. A metal catalyst is used to oxidize the Si, and the silicon oxide formed is etched in the solution; this local MaCE reaction takes place continuously on the Si substrate in the scanning direction of probes. Polymer resist patterning for subsequent etching is not required; instead, scanning probes pattern the oxidation mask directly and chemical etching of Si occurs concurrently. A prototype that drives the probe with an actuator was used to analyze various aspects of the etching profiles based on the scanning speeds and sizes of the probe used. This technique suggests the possibility of forming arbitrary structures because the carving trajectory is formed according to the scan direction of the probes.

Anodic Imprint Lithography: Direct Imprinting of Single Crystalline GaAs with Anodic Stamp

Anodic imprint lithography patterns the GaAs substrate electrochemically by applying a voltage through a predefined anodic stamp. This newly devised technique performs anodic etching in a stamping manner. Stamps that serve as anodic electrodes are fabricated precisely, and the patterns can be imprinted continuously on GaAs substrates. The anodic current locally oxidizes the GaAs through the metal attached to the stamp, and the GaAs oxides are subsequently removed by an acid in the solution. The process is simplified because the metal catalyst is not left on the substrate and the use of an oxidizing agent is not required. Anodic imprint lithography integrates the lithography and etching steps without the use of a polymer resist. Predefined anodic stamps with fin, pillar, and mesh arrays clearly imprinted trenches, holes, and embossed disk arrays on the GaAs substrates, respectively. Anodic imprints replace photons and electrons in conventional lithography with electrochemical stamping, which can simplify existing techniques that are highly complex for extreme nanopatterning.

Photoelectric effect accelerated electrochemical corrosion and nanoimprint processes on gallium arsenide wafers

Here we report photoelectric-effect-enhanced interfacial charge transfer reactions.

Here we report photoelectric-effect-enhanced interfacial charge transfer reactions. The electrochemical corrosion rate of n-type gallium arsenide (n-GaAs) induced by the contact potential at platinum (Pt) and GaAs boundaries can be accelerated by the photoelectric effect of n-GaAs. When a GaAs wafer is illuminated with a xenon light source, the electrons in the valence band of GaAs will be excited to the conduction band and then move to the Pt boundaries due to the different work functions of the two materials. This results in an enhanced contact electric field as well as an enlarged Pt/GaAs contact potential. Consequently, in the presence of electrolyte solution, the polarizations of both the Pt/solution interface and the GaAs/solution interface at the Pt/GaAs/solution 3-phase boundary are enhanced. If the accumulated electrons on the Pt side are removed by electron acceptors in the solution, anodic corrosion of GaAs will be accelerated strictly along the Pt/GaAs/solution 3-phase boundary. This photo-enhanced electrochemical phenomenon can increase the corrosion rate of GaAs and accelerate the process of electrochemical nanoimprint lithography (ECNL) on GaAs. The method opens an innovative, highly efficient, low-cost nanoimprint technique performed directly on semiconductors, and it has prospective applications in the semiconductor industry.

Electrochemical nanoimprinting of silicon

The indirect nature of existing parallel micromachining strategies that combine sacrificial templates with top-down processes to etch 3D micro- and nanostructures inherently produces poor out-of-plane patterning fidelity. Here, the patterning fidelity of our process is measured for microscale curvilinear 3D objects to be less than 20 nm in rms averaged over features as wide as 10 µm. These results are attributed to increased pathways for diffusion, which increase the kinetics of the anodic reaction. Using this approach, arrays of nanotextured silicon lenses are deterministically imprinted to illustrate Mac-Imprint’s ability to directly pattern hierarchical micro- and nanostructures and enable fabrication of biomimetic optical designs on silicon.

Scalable nanomanufacturing enables the commercialization of nanotechnology, particularly in applications such as nanophotonics, silicon photonics, photovoltaics, and biosensing. Nanoimprinting lithography (NIL) was the first scalable process to introduce 3D nanopatterning of polymeric films. Despite efforts to extend NIL’s library of patternable media, imprinting of inorganic semiconductors has been plagued by concomitant generation of crystallography defects during imprinting. Here, we use an electrochemical nanoimprinting process—called Mac-Imprint—for directly patterning electronic-grade silicon with 3D microscale features. It is shown that stamps made of mesoporous metal catalysts allow for imprinting electronic-grade silicon without the concomitant generation of porous silicon damage while introducing mesoscale roughness. Unlike most NIL processes, Mac-Imprint does not rely on plastic deformation, and thus, it allows for replicating hard and brittle materials, such as silicon, from a reusable polymeric mold, which can be manufactured by almost any existing microfabrication technique.

3D Patterning of Si by Contact Etching With Nanoporous Metals

Nanoporous gold and platinum electrodes are used to pattern n-type silicon by contact etching at the macroscopic scale. This type of electrode has the advantage of forming nanocontacts between silicon, the metal and the electrolyte as in classical metal assisted chemical etching while ensuring electrolyte transport to and from the interface through the electrode. Nanoporous gold electrodes with two types of nanostructures, fine and coarse (average ligament widths of ~30 and 100 nm, respectively) have been elaborated and tested. Patterns consisting in networks of square-based pyramids (10 × 10 μm² base × 7 μm height) and U-shaped lines (2, 5, and 10 μm width × 10 μm height × 4 μm interspacing) are imprinted by both electrochemical and chemical (HF-H₂O₂) contact etching. A complete pattern transfer of pyramids is achieved with coarse nanoporous gold in both contact etching modes, at a rate of ~0.35 μm min⁻¹. Under the same etching conditions, U-shaped line were only partially imprinted. The surface state after imprinting presents various defects such as craters, pores or porous silicon. Small walls are sometimes obtained due to imprinting of the details of the coarse gold nanostructure. We establish that np-Au electrodes can be turned into “np-Pt” electrodes by simply sputtering a thin platinum layer (5 nm) on the etching (catalytic) side of the electrode. Imprinting with np Au/Pt slightly improves the pattern transfer resolution. 2D numerical simulations of the valence band modulation at the Au/Si/electrolyte interfaces are carried out to explain the localized aspect of contact etching of n-type silicon with gold and platinum and the different surface state obtained after patterning. They show that n-type silicon in contact with gold or platinum is in inversion regime, with holes under the metal (within 3 nm). Etching under moderate anodic polarization corresponds to a quasi 2D hole transfer over a few nanometers in the inversion layer between adjacent metal and electrolyte contacts and is therefore very localized around metal contacts.

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.

Chemistry and engineering of cyclodextrins for molecular imaging

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides bearing a basket-shaped topology with an "inner-outer" amphiphilic character. The abundance of hydroxyl groups enables CDs to be functionalized with multiple targeting ligands and imaging elements. The imaging time, and the payload of different imaging elements, can be tuned by taking advantage of the commercial availability of CDs with different sizes of the cavity. This review aims to offer an outlook of the chemistry and engineering of CDs for the development of molecular probes. Complexation thermodynamics of CDs, and the corresponding implications for probe design, are also presented with examples demonstrating the structural and physiochemical roles played by CDs in the full ambit of molecular imaging. We hope that this review not only offers a synopsis of the current development of CD-based molecular probes, but can also facilitate translation of the incremental advancements from the laboratory to real biomedical applications by illuminating opportunities and challenges for future research.

Chemical Imprinting of Crystalline Silicon with Catalytic Metal Stamp in Etch Bath

Conventional lithography using photons and electrons continues to evolve to scale down three-dimensional nanoscale patterns, but the complexity of technology and equipment is increasing due to diffraction and scattering problems. Physical contact lithography methods, such as nanoimprint and soft lithography, have been developed as an alternative technique. These techniques imprint predefined structures on a stamp to the polymer resist and use the polymer resist as a mask to dry etch the nanostructure on the substrate. In this study, we introduce a method of chemically imprinting crystalline silicon (Si) with a catalytic stamp to enable the direct etching of the Si without using a polymer mask. A metal catalyst is deposited on the predefined structure of the stamp. The stamp physically contacts the Si in the etching bath, and metal-assisted chemical etching occurs on the semiconductor surface. Since the metal catalyst is mounted on a stamp, it can be used repeatedly. This is a technology that combines conventional lithography and etching without using a polymer resist. This technology not only produced nano/microscale arrays of circular and square holes and trench structures but also successfully produced complex eagle-shaped structures that contained such structures.

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.

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.