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.

Electrochemical local etching of silicon in etchant vapor.

Direct machining and imprinting of Si are beneficial for simplifying the fabrication of microelectromechanical systems, nanoelectromechanical systems, optical devices, and fin field-effect transistors, and for reducing process costs. Electrochemical micromachining has been introduced for highly doped Si, but complex structures cannot be imprinted directly. With chemical imprinting, complex nano/micropatterns can be imprinted even on low-doped Si, but the physical contact can damage the templates. In this study, we demonstrated an electrochemical local etching (ELE) method for fabricating nano/micrometer structures on semiconductors in a noncontact manner. Polygon tips were prepared as templates on highly doped n-type Si via etching in KOH. A constant space is maintained between the template and the target Si using a gap layer to prevent damage and contamination. In the etchant vapor, the voltage bias between the template and the target Si leads to condensation of the etchant. Because the etching region is localized by the condensation of the etchant, even low-doped semiconductors can be imprinted in submicrometer patterns in a single step. When the etchant condensation is suppressed, the etching area is reduced and the resolution is increased, allowing direct imprinting of the polygonal submicrometer pattern. ELE has the potential to produce complex nano/micrometer structures in a single step without photoresists and physical contact.

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.

Subwavelength photocathodes via metal-assisted chemical etching of GaAs for solar hydrogen generation.

MacEtch allows subwavelength-structured (SWS) texturing on the GaAs surface without compromising crystallinity. The current density increases greatly, which is directly due to the reduction in the reflectance. Photons absorbed under reduced light reflectance are less affected by the charge recombination arising from crystal defects. The catalytic metal remaining after MacEtch serves as a catalyst for water splitting and increases the open-circuit potentials of the SWS GaAs photocathodes. The SWS GaAs not only amplifies the absorption of light, but also improves the collection of deeply generated photons at long wavelengths. The solar-weighted reflectance (SWR) of SWS GaAs is 6.6%, which was much lower than the 39.0% of bare GaAs. The light-limited photocurrent density (LLPC) increased by approximately 90% and the tafel slope improved as etching progressed. The external quantum efficiency was as high as 80%, especially at long wavelengths, after MacEtch. SWS GaAs photocathodes fabricated using MacEtch significantly reduce reflectance and recombination loss, thereby improving the key performance of PEC for hydrogen production. This technology can fully utilize the high absorption rate and carrier mobility of GaAs and is applicable to various photoelectric conversion device performance enhancements.

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.

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.

Nanostructured GaAs solar cells via metal-assisted chemical etching of emitter layers.

GaAs solar cells with nanostructured emitter layers were fabricated via metal-assisted chemical etching. Au nanoparticles produced via thermal treatment of Au thin films were used as etch catalysts to texture an emitter surface with nanohole structures. Epi-wafers with emitter layers 0.5, 1.0, and 1.5 um in thickness were directly textured and a window layer removal process was performed before metal catalyst deposition. A nanohole-textured emitter layer provides effective light trapping capabilities, reducing the surface reflection of a textured solar cell by 11.0%. However, because the nanostructures have high surface area to volume ratios and large numbers of defects, various photovoltaic properties were diminished by high recombination losses. Thus, we have studied the application of nanohole structures to GaAs emitter solar cells and investigated the cells' antireflection and photovoltaic properties as a function of the nanohole structure and emitter thickness. Due to decreased surface reflection and improved shunt resistance, the solar cell efficiency increased from 4.25% for non-textured solar cells to 7.15% for solar cells textured for 5 min.

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.