Anisotropic Reactive Ion Etching Research Articles

Engineering of dense arrays of Vertical Si1-x Ge x nanostructures

Vertical nanostructure technologies are becoming more important for the down scaling of nanoelectronic devices such as logic transistors or memories. Such devices require dense vertical nanostructured channel arrays (VNCA) that can be fabricated through a top-down approach based on group IV materials. We present progresses on the top-down fabrication of highly anisotropic and ultra-dense Si1-x Ge x (x = 0, 0.2, 0.5) VNCAs. Dense nanowire and nanosheet patterns were optimized through high resolution lithography and transferred onto Si1-x Ge x substrates by anisotropic reactive ion etching with a fluorine chemistry. The right gas mixtures for a given Ge content resulted in perfectly vertical and dense arrays. Finally we fabricated oxide shell/SiGe core heterostructures by dry- and wet-thermal oxidation and evaluated their applicability for nanostructure size engineering, as already established for silicon nanowires. The impact of the nanostructured shape (wire or sheet), size and Ge content on the oxide growth were investigated and analysed in detail through transmission electron microscopy.

Reactive ion plasma etched surface relief gratings for low/medium/high resolution spectroscopy in astronomy

We report the fabrication of a binary-phase proof-of-concept astronomical diffraction grating embedded in a quartz substrate via reactive ion plasma etching. This grating operates at the first diffraction order within the 450 to 750 nm wavelength band. It features 1400-nm-deep, 188-nm-wide binary grooves at a 566-nm pitch, or 1767 lines/mm groove density, over a 25.4 × 25.4 mm2 area. A high depth-to-width ratio ( ∼ 8 ∶ 1 in this case) is one of the keys to near-theoretical diffraction efficiency being attained by the fabricated grating (94% at center wavelength and 70% at band edges) over a broad bandpass (>200 nm). This performance is also attributed to high-resolution micro-lithographic electron-beam patterning and anisotropic reactive ion etching process fabrication techniques. These types of binary gratings can potentially be high-throughput alternatives to Volume-Phase Holographic Gratings (VPHGs) for general spectroscopic applications. When scaled to appropriate sizes for astronomy, such gratings can serve as main or cross dispersion elements in low-, medium-, and high-resolution spectrographs not only in ground-based telescopes but also in those subject to challenging environmental conditions such as in space observatories.

Smooth Sidewalls on Crystalline Gold through Facet-Selective Anisotropic Reactive Ion Etching: Toward Low-Loss Plasmonic Devices.

Quantum plasmonics aims to harness the deeply subwavelength confinement provided by plasmonic devices to engineer more efficient interfaces to quantum systems in particular single emitters. Realizing this vision is hampered by the roughness-induced scattering and loss inherent in most nanofabricated devices. In this work, we show evidence of a reactive ion etching process to selectively etch gold along select crystalline facets. Since the etch is facet selective, the sidewalls of fabricated devices are smoother than the lithography induced line-edge roughness with the prospect of achieving atomic smoothness by further optimization of the etch chemistry. This opens up a route toward fabricating integrated plasmonic circuits that can achieve loss metrics close to fundamental bounds.

Unveiling Unintentional Fluorine Doping in TMDs During the Reactive Ion Etching: Root Cause Analysis, Physical Insights, and Solution

Layered semiconductors, such as transition metal dichalcogenides (TMDs), gained popularity due to their unique properties favoring different applications. For all practical applications, it is essential to have the ability to pattern these layered semiconductors as per the desired dimensions, which are derived from device design or circuit design. Due to its anisotropic nature, plasma or reactive ion etching (RIE) has been the most used technique for patterning bulk semiconductors or thin films. This article unveils the challenges associated with the fluorine-based (SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and CHF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) plasma etching of TMDs. Subsequently, a detailed root cause analysis is presented with the solution to the problems thereof. We have discovered: 1) high affinity of resists leading to unwanted resist residues over the TMD surface and 2) unintentional doping induced by the fluorine plasma, due to fluorine occupation at the interstitial sites near sulfur (S) defects or its occupation of an S-defect site in the TMD. This was observed despite the surface being masked by the resist. Physical insights were developed using atomic force microscopy (AFM), Raman spectroscopy, electrical characterization, field-effect mobility and transfer length method (TLM)-based contact resistance comparison, and density functional theory (DFT)-based atomistic computations. Furthermore, these insights are used to demonstrate an improved plasma recipe, which mitigates these challenges and allows realization of transistors over a patterned channel while offering characteristic similar to an intrinsic/pristine layer. Finally, the improved plasma recipe, developed for MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , is also shown to be effective for MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers.

Selective dry etching of UV-nanoimprinted resin passivation masks for area selective atomic layer deposition of aluminum oxide

To demonstrate area selective atomic layer deposition (ALD) using UV-nanoimprinted resin patterns as physical passivation masks, we investigated the removal of UV-cured resin films subjected to sequential mutual doses of trimethylaluminum (TMA) and H2O by using dry etching procedures. On the basis of the removal of a residual layer characteristic from imprint resin patterns by anisotropic oxygen reactive ion etching (O2 RIE), oxidatively etched UV-cured films were modified with TMA and H2O in an ALD-like cyclic manner. Atomic force microscopy and time-of-flight secondary ion mass spectrometry [time-of-flight secondary ion mass spectrometry (TOF-SIMS)] analysis suggested that the combination of physical Ar ion milling and subsequent chemical O2 RIE enabled the elimination of oxidized UV-cured resin masks modified with 5-cycle TMA doses. By contrast, Ar ion milling or O2 RIE left organic or inorganic residues on silicon surfaces, respectively. A TMA-modified hybridized resin layer was etched by physical Ar ion milling; subsequently, the organic residual resin layer was removed by chemical O2 RIE in the case of 5-cycle modification with TMA. The mapping image of Al+ visualized by TOF-SIMS suggested that line patterns of aluminum oxide were left selectively on unmasked silicon substrate surfaces by site-selective dry etching of TMA-modified imprint resin passivation masks with 500 nm linewidth.

Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching.

Nanostructuring allows altering of the electronic and photonic properties of two-dimensional (2D) materials. The efficiency, flexibility, and convenience of top-down lithography processes are, however, compromised by nanometer-scale edge roughness and resolution variability issues, which especially affect the performance of 2D materials. Here, we study how dry anisotropic etching of multilayer 2D materials with sulfur hexafluoride (SF6) may overcome some of these issues, showing results for hexagonal boron nitride (hBN), tungsten disulfide (WS2), tungsten diselenide (WSe2), molybdenum disulfide (MoS2), and molybdenum ditelluride (MoTe2). Scanning electron microscopy and transmission electron microscopy reveal that etching leads to anisotropic hexagonal features in the studied transition metal dichalcogenides, with the relative degree of anisotropy ranked as: WS2 > WSe2 > MoTe2 ∼ MoS2. Etched holes are terminated by zigzag edges while etched dots (protrusions) are terminated by armchair edges. This can be explained by Wulff constructions, taking the relative stabilities of the edges and the AA' stacking order into account. Patterns in WS2 are transferred to an underlying graphite layer, demonstrating a possible use for creating sub-10 nm features. In contrast, multilayer hBN exhibits no lateral anisotropy but shows consistent vertical etch angles, independent of crystal orientation. Using an hBN crystal as the base, ultrasharp corners can be created in lithographic patterns, which are then transferred to a graphite crystal underneath. We find that the anisotropic SF6 reactive ion etching process makes it possible to downsize nanostructures and obtain smooth edges, sharp corners, and feature sizes significantly below the resolution limit of electron beam lithography. The nanostructured 2D materials can be used themselves or as etch masks to pattern other nanomaterials.

실리콘 레이저 소결에서 은 나노 입자의 도핑 효과에 관한 연구

Silicon is used as an essential material in the electronic and energy industries owing to its high accessibility and inherent semiconducting properties. Typically, silicon devices are produced by top-down chemical etchings, such as anisotropic etching and reactive ion etching, to make micropatterns and surface textures. In addition, doping using thermal diffusion or ion implantation is also required to optimize electrical characteristics. We propose a one-step method for fabricating multi-scale silicon layer based on the concurrent interaction of doping of silver nanoparticles during laser-induced sintering of silicon nanocrystals. The silver-doped silicon patterns were characterized by a uniform distribution and the concentration of dopants was easily adjustable by controlling the relative amount of silver nanoparticles. We expect that this method will contribute to fabricating multi-scale silicon semiconductors without using complicated chemical and vacuum processes.

Fabrication and characterization of annular magnetic nanostructures

Large arrays of permalloy (Ni81Fe19) annular structures were fabricated using a self-aligned patterning process based on ion-beam proximity lithography (IBPL), where a broad beam of energetic He ions is shaped into billions of ion beamlet by a stencil mask to pattern electron beam sensitive resist. IBPL was used to form an array of circular openings in poly(methyl methacrylate) (PMMA) resist, followed by a tone reversal process to form circular pillars in an underlying polymethylglutarimide (PMGI) layer. The PMGI pillars were conformally coated with silicon oxide, which was followed by anisotropic reactive ion etch (RIE) to form silicon oxide rings, which were transferred into the underlying sputter deposited permalloy thin film by ion milling. This fabrication approach was used to make 6 x 6 mm2 arrays of rings on a constant pitch of 670 nm with the outer diameters varied between 350 nm and 450 nm with a fixed inner diameter of 150 nm. Three unique samples that were fabricated and characterized using a vibrating sample magnetometer. The measured M-H loops showed the switching from an onion to a vortex and back to an onion state and are in good agreement with micromagnetic simulations and previously published data.

Gold thickness impact on the enhancement of SERS detection in low-cost Au/Si nanosensors

In this paper, we present a study on the optimization of Au/Si surface-enhanced Raman scattering sensors for ultrasensitive chemical detection. Au/Si nanosensors are carried out at the wafer scale with a low-cost and quick fabrication method composed of an anisotropic reactive ion etching followed by an electron beam evaporation of a gold layer. For this investigation, the thickness of the gold layer varied from 10 to 50 nm, and thiophenol molecules were chosen as model molecules of which the concentration varied from $$10^{-12}$$ to $$10^{-3}$$ M. The studied optimization exploits the effect of the gold layer thickness on the detection limit of thiophenol molecules for a fixed geometry of Si nanopillar. For all the cases investigated here, detection limits between $$9\times 10^{-12}$$ and $$3\times 10^{-8}$$ M are experimentally found for the detection of thiophenol molecules with the Au/Si nanopillars. Moreover, the best detection limits are achieved with the thickest gold layer. Finally, a 3D FDTD-based numerical analysis confirmed the experimental results.

Integrated 0–30 V switching driver circuit fabricated by mesa isolation postprocess of standard 5 V CMOS LSI for MEMS actuator applications

A 30 V switching circuit with standard CMOS foundry-made transistors on a silicon-on-insulator (SOI) wafer is proposed.The key fabrication process is mesa isolation postprocessing that physically separates a series-connected transistor's body so that we can apply independent substrate (body) voltages to each transistor. The process is a combination of anisotropic and isotropic deep reactive ion etching (DRIE) with a single mask. In the experiment, CMOS transistors were fabricated on a 9-µm-thick 6-in. SOI wafer in an LSI foundry company, and the postprocess was carried out in a MEMS cleanroom. In this paper, full integration with gate-voltage-level shift circuits and dynamic driving of an electrostatic comb-drive MEMS device with series-connected transistors are presented.

橋架け構造により高い擦り耐性を実現するガラスナノピラーの作製

In this study, we proposed bridged glass nanopillar structures with high scratch resistance. The glass nanopillars were joined each other with bridged structures. The bridged glass nanopillars were fabricated using anisotropic talbot photolithography and reactive ion etching. AFM and FE-SEM analyses showed that the bridged glass nanopillar structures were successfully fabricated by photolithography and dry etching. Scratch resistance of the bridged glass nanopillers improved seven times stronger than that of the nanopillers without bridged structures in friction test using a flannel cloth. Furthermore, bridged glass nanopillars were not collapsed through high stress friction test using a steel wool. Additionally, self-cleaning effect of the bridged glass nanopillars with superhydrophilic behavior were successfully demonstrated by spraying water droplets. We expect that the proposed bridged glass nanopillar structures will be a highly promising technology for self-cleaning glass.

Anisotropic Oxygen Reactive Ion Etching for Removing Residual Layers from 45 nm-width Imprint Patterns

Anisotropic Oxygen Reactive Ion Etching for Removing Residual Layers from 45 nm-width Imprint Patterns

Fabrication of artificial graphene in a GaAs quantum heterostructure

The unusual electronic properties of graphene, which are a direct consequence of its two-dimensional honeycomb lattice, have attracted a great deal of attention in recent years. Creation of artificial lattices that re-create graphene's honeycomb topology, known as artificial graphene, can facilitate the investigation of graphenelike phenomena, such as the existence of massless Dirac fermions, in a tunable system. In this work, the authors present the fabrication of artificial graphene in an ultrahigh quality GaAs/AlGaAs quantum well, with lattice period as small as 50 nm, the smallest reported so far for this type of system. Electron-beam lithography is used to define an etch mask with honeycomb geometry on the surface of the sample, and different methodologies are compared and discussed. An optimized anisotropic reactive ion etching process is developed to transfer the pattern into the AlGaAs layer and create the artificial graphene. The achievement of such high-resolution artificial graphene should allow the observation for the first time of massless Dirac fermions in an engineered semiconductor.

Superconducting Nano Wire Circuits Fabricated using a Focused Helium Beam

Electrical circuits fabricated from high-transition temperature superconductors (HTS) are very difficult to pattern due to the lack of a reliable etching process. Chemical etching can be used for large features, but undercutting limits the feature size to tens of microns. Dry etching is required for smaller features, but there is no anisotropic reactive ion etch for these materials. Therefore dry etching must be done with isotropic argon ion milling. The ion milling process generates excess heat and unfortunately oxide superconductors are sensitive to high temperature. Overheating the material by ion milling causes it to deoxygenate which turns the superconductor into an insulator. Therefore to prevent overheating the critical dimension for argon ion milling is typically limited to a few microns.

Wicking nanopillar arrays with dual roughness for selective transport and fluorescence measurements.

Silicon nanopillars are important building elements for innovative nanoscale systems with unique optical, wetting, and chemical separation functionalities. However, technologies for creating expansive pillars arrays on the submicron scale are often complex and with practical time, cost, and method limitations. Herein we demonstrate the rapid fabrication of nanopillar arrays using the thermal dewetting of Pt films with thicknesses in the range from 5 to 19 nm followed by anisotropic reactive ion etching (RIE) of the substrate materials. A second level of roughness on the sub-30 nm scale is added by overcoating the silicon nanopillars with a conformal layer of porous silicon oxide (PSO) using room temperature plasma enhanced chemical vapor deposition (PECVD). This technique produced environmentally conscious, economically feasible, expansive nanopillar arrays with a production pathway scalable to industrial demands. The arrays were systematically analyzed for size, density, and variability of the pillar dimensions. We show that these stochastic arrays exhibit rapid wicking of various fluids and, when functionalized with a physiosorbed layer of silicone oil, act as a superhydrophobic surface. We also demonstrate high brightness fluorescence and selective transport of model dye compounds on surfaces of the implemented nanopillar arrays with two-tier roughness. The demonstrated combination of functionalities creates a platform with attributes inherently important for advanced separations and chemical analysis.

M‐shaped Grating by Nanoimprinting: A Replicable, Large‐Area, Highly Active Plasmonic Surface‐Enhanced Raman Scattering Substrate with Nanogaps

Plasmonic nanostructures separated by nanogaps enable strong electromagnetic-field confinement on the nanoscale for enhancing light-matter interactions, which are in great demand in many applications such as surface-enhanced Raman scattering (SERS). A simple M-shaped nanograting with narrow V-shaped grooves is proposed. Both theoretical and experimental studies reveal that the electromagnetic field on the surface of the M grating can be pronouncedly enhanced over that of a grating without such grooves, due to field localization in the nanogaps formed by the narrow V grooves. A technique based on room-temperature nanoimprinting lithography and anisotropic reactive-ion etching is developed to fabricate this device, which is cost-effective, reliable, and suitable for fabricating large-area nanostructures. As a demonstration of the potential application of this device, the M grating is used as a SERS substrate for probing Rhodamine 6G molecules. Experimentally, an average SERS enhancement factor as high as 5×10⁸ has been achieved, which verifies the greatly enhanced light-matter interaction on the surface of the M grating over that of traditional SERS surfaces.

Pore-size reduction protocol for SiN membrane nanopore using the thermal reflow in nanoimprinting for nanobio-based sensing

Micro- and nano-fabrication methods facilitate the use of nanostructures for the separation of collections of particles and nanobio-based optical and electrochemical sensing. We have presented an easy and simple nanopore size reduction method of a low-stressed silicon nitride (SiN) membrane nanosieve (100×100 μm2) using a nanoimprinting method based on a natural thermal reflow of the contact imprinting polymer, possibly maintaining compatibility with complementary metal-oxide semiconductor integrated circuit processes. The nanopore pattern size of this nanosieve membrane was precisely patterned by a nanoimprinting process using an electron beam patterned silicon master, to about 30-nm diameter. By employing mainly an electron beam resist reflow phenomena after a nanoimprinting process and anisotropic reactive ion etch, the etch holes' size was fabricated to be the same with nanopatterns on the polymer. The contact imprinting master can be used continually for the generation of nanopore patterns simply and easily. It can endure harsh conditions like high temperature up to 800°C, and it is inert to many aggressive and strong chemicals. Also, this would be a low-cost, simple, and easy fabrication method for the precise and reliable size-reduction control of nanopores for mass production of nanobio sensors or chips.

Simultaneous Measurement of Bend Direction and Degree of Curvature using Micromachined Grating Fibers

We report the design and fabrication of a novel fiber optic bend sensor capable of measuring the bend direction and degree of curvature simultaneously. The sensor has a serial arrangement of two fiber Bragg gratings (FBGs) with two different reflection center wavelengths and similar asymmetrical cross sections with different etching faces on an independent single-mode fiber. These cross sections were fabricated by anisotropic reactive ion etching (RIE) using CF4 plasma. The reflection center wavelengths of the two FBGs shifted sinusoidally with a phase difference of π/2 as a function of the bend direction under constant bend feeder displacement. Because it was shown that the reflection center wavelength shifted linearly as a function of the degree of curvature of an FBG with an asymmetrical cross section in our previous experiment, this sensor could be used to detect both the bend direction and the curvature from a pair of central reflection wavelength shifts for two FBGs. These experimental results were compared with theoretical values calculated from a strain of the core in an etched optical fiber.

Asymmetric Wettability of Nanostructures Directs Leidenfrost Droplets

Leidenfrost phenomena on nano- and microstructured surfaces are of great importance for increasing control over heat transfer in high power density systems utilizing boiling phenomena. They also provide an elegant means to direct droplet motion in a variety of recently emerging fluidic systems. Here, we report the fabrication and characterization of tilted nanopillar arrays (TNPAs) that exhibit directional Leidenfrost water droplets under dynamic conditions, namely on impact with Weber numbers ≥40 at T ≥ 325 °C. The directionality for these droplets is opposite to the direction previously exhibited by macro- and microscale Leidenfrost ratchets where movement against the tilt of the ratchet was observed. The batch fabrication of the TNPAs was achieved by glancing-angle anisotropic reactive ion etching of a thermally dewet platinum mask, with mean pillar diameters of 100 nm and heights of 200-500 nm. In contrast to previously implemented macro- and microscopic Leidenfrost ratchets, our TNPAs induce no preferential directional movement of Leidenfrost droplets under conditions approaching steady-state film boiling, suggesting that the observed droplet directionality is not a result of the widely accepted mechanism of asymmetric vapor flow. Using high-speed imaging, phase diagrams were constructed for the boiling behavior upon impact for droplets falling onto TNPAs, straight nanopillar arrays, and smooth silicon surfaces. The asymmetric impact and directional trajectory of droplets was exclusive to the TNPAs for impacts corresponding to the transition boiling regime, linking asymmetric surface wettability to preferential directionality of dynamic Leidenfrost droplets on nanostructured surfaces.

Lithography-free approach to highly efficient, scalable SERS substrates based on disordered clusters of disc-on-pillar structures

We present a lithography-free technological strategy that enables fabrication of large area substrates for surface-enhanced Raman spectroscopy (SERS) with excellent performance in the red to NIR spectral range. Our approach takes advantage of metal dewetting as a facile means to create stochastic arrays of circular patterns suitable for subsequent fabrication of plasmonic disc-on-pillar (DOP) structures using a combination of anisotropic reactive ion etching (RIE) and thin film deposition. Consistent with our previous studies of individual DOP structures, pillar height which, in turn, is defined by the RIE processing time, has a dramatic effect on the SERS performance of stochastic arrays of DOP structures. Our computational analysis of model DOP systems confirms the strong effect of the pillar height and also explains the broadband sensitivity of the implemented SERS substrates. Our Raman mapping data combined with SEM structural analysis of the substrates exposed to benzenethiol solutions indicates that clustering of shorter DOP structures and bundling of taller ones is a likely mechanism contributing to higher SERS activity. Nonetheless, bundled DOP structures appeared to be consistently less SERS-active than vertically aligned clusters of DOPs with optimized parameters. The latter are characterized by average SERS enhancement factors above 107.