Oxygen Reactive Ion Etching Research Articles

Towards Long-Term Stable Polyimide-Based Flexible Electrical Insulation for Chronically Implanted Neural Electrodes

For chronic applications of flexible neural implants, e.g., intracortical probes, the flexible substrate material has to encapsulate the electrical conductors with a long-term stability against the saline environment of the neural tissue. The biocompatible polymer polyimide is often used for this purpose. Due to its chemical inertness, the adhesion between two polyimide layers is, however, a challenge, which can lead to delamination and, finally, to short circuits. The state-of-the-art method to improve the adhesion strength is activating the polyimide surface using oxygen reactive ion etching (O2 RIE). However, the influence of the process variations (etching time, bias power) on the long-term stability is still unclear. Therefore, we establish a test method, where the aging of a gold interdigital structure embedded in two polyimide layers and immersed in saline solution is accelerated using an elevated temperature, mechanical stress and an electrical field. A continuous measurement of a leakage current is used to define the failure state. The results show that the variation of the O2 RIE plasma process has a significant effect on the long-term stability of the test samples. Comparing the two different plasma treatments 0.5 min at 25 W and 1 min at 50 W, the long-term stability could be increased from 20.9 ± 19.1 days to 44.9 ± 18.9 days. This corresponds to more than a doubled lifetime. An ideal solution for the delamination problem is still not available; however, the study shows that the fine-tuning of the fabrication processes can improve the long-term stability of chronically implanted neural electrodes.

Fabrication of robust and reusable mold with nanostructures and its application to anti-counterfeiting surfaces based on structural colors

In this study, we report a method to fabricate molds and flexible stamps with 2D photonic crystal structures. This includes self-assembly of polystyrene particles into monolayer, oxygen reactive ion etching, thin film (chromium (Cr)) deposition, and polydimethylsiloxane replication. By tuning the thickness of Cr layer, reusable master molds with nano bumps or nano concaves could be prepared selectively. We showed that the replicated flexible stamps out of these molds exhibited structural colors. Characteristics of the colors depended on viewing angle, brightness of background and light source. And the colors even faded out when the background is white or when the stamp was bent. By using this feature, possible strategies for anti-counterfeiting applications have been suggested in this study. Since the molds are reusable and the fabrication method is simple and cost-effective, this study is expected to contribute to nano devices for industries in future.

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.

Highly Conductive Nanocrystalline Diamond Films and Electronic Metallization Scheme.

By using a methane and hydrogen process gas mixture in an appropriate hot-filament CVD process without further dopant, high electrical conductivity of over 100 S/cm has been achieved in nanocrystalline diamond films deposited on silicon single-crystalline substrates. Furthermore, it was found that an oxygen reactive-ion etching process (O-RIE) can improve the diamond film surface’s electron affinity, thus reducing the specific contact resistance. The reduction of the specific contact resistance by a factor of up to 16 was realized by the oxygen ion etching process, down to . We provide a qualitative explanation for the mechanism behind the contact resistance reduction in terms of the electron affinity of the diamond surface. With the aid of XPS, AFM, and surface wetting measurements, we confirmed that a higher surface electron affinity is responsible for the lower specific contact resistance of the oxygen-terminated nanocrystalline diamond films.

Effect of oxygen terminated surface of boron-doped diamond thin-film electrode on seawater salinity sensing

Tremendous demands for highly sensitive and stable seawater salinometers have motivated intensive research on advanced electrode materials. Boron-doped diamond (BDD) is attractive in terms of its high mechanical stability and chemical inertness, but is usually hindered by its low double-layer capacitance (Cdl) for seawater salinity detection. Here, inspired by the principle of oxygen-terminated BDD electrode endowing higher Cdl than hydrogen-terminated surface, we introduce the oxygen terminated surface by oxygen plasma or reactive ion etch (RIE), and the fabricated oxygen terminated BDD electrodes demonstrate high sensitivity and long-term stability in seawater salinity detection comparing with the hydrogen terminated BDD electrodes. Significantly, the as-fabricated O-BDD-RIE electrodes not only show remarkable enhanced response even better than the commercial platinum black electrodes but also display long-time stability which is weekly verified by continuous monitor for 90 days. The outstanding performance of the oxygen terminated BDD electrodes can be ascribed to the enhancement of C-O surface functional group on Cdl. In addition, a comprehensive analysis of effective electroactive surface area (EASA) and Cdl proves that the surface oxygen is the major factor for the improved Cdl. In summary, the excellent oxygen terminated BDD electrodes promise potential application in seawater salinity detection.

Visualization of organic/inorganic hybridization of UV-cured films with trimethylaluminum by scanning transmission electron microscopy and energy dispersive x-ray spectroscopy

The organic/inorganic hybridization of photopolymerized resist films for ultraviolet nanoimprint lithography (UV-NIL) through chemical vapor modifications of atomic layer deposition (ALD), sequential infiltration synthesis (SIS), and saturated vapor infiltration (SVI) with an inorganic precursor of trimethylaluminum (TMA) and an oxidant of water was investigated. The hybridization of the bisphenol A-based polymethacrylate resist films was compared between resin-A comprising a monomer with hydroxy groups and resin-B comprising another monomer without hydroxy groups. The elemental depth profiles by scanning transmission electron microscopy and energy dispersive x-ray spectroscopy revealed the following three things. ALD and SIS caused the hybridization of the organic resist films with inorganic alumina near the film surfaces, while SVI caused the hybridization of the resist films entirely. The hydroxy-free resin-B physically adsorbed and chemically fixed more TMA molecules than the hydroxy-containing resin-A. Although SIS progressed the entire hybridization of the resist films, different behaviors of segregation of alumina between the hydroxy-containing resin-A and hydroxy-free resin-B films were confirmed near the film surface, inside, and interface with a silicon substrate. The organic/inorganic hybridization enabled the tuning of the etching rate of the NIL resist masks with a thickness of no thicker than 20 nm in oxygen reactive ion etching often used for the removal of residual layers from imprint patterns in UV-NIL processes.

Elemental depth profiles and plasma etching rates of positive-tone electron beam resists after sequential infiltration synthesis of alumina

By scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM–EDS), we investigated the elemental depth profiles of organic electron beam resist films after the sequential infiltration synthesis (SIS) of inorganic alumina. Although a 40-nm-thick poly(methyl methacrylate) (PMMA) film was entirely hybridized with alumina, an uneven distribution was observed near the interface between the substrate and the resist as well as near the resist surface. The uneven distribution was observed around the center of a 100-nm-thick PMMA film. The thicknesses of the PMMA and CSAR62 resist films decreased almost linearly as functions of plasma etching period. The comparison of etching rate among oxygen reactive ion etching, C3F8 reactive ion beam etching (RIBE), and Ar ion beam milling suggested that the SIS treatment enhanced the etching resistance of the electron beam resists to chemical reactions rather than to ion collisions. We proposed oxygen- and Ar-assisted C3F8 RIBE for the fabrication of silica imprint molds by electron beam lithography.

Stable lotus leaf-inspired hierarchical, fluorinated polypropylene surfaces for reduced bacterial adhesion

Polypropylene (PP) is used in a wide variety of medical components, but is susceptible to bacteria surface colonization and biofilm formation, which lead to infections and inflammations. In this study, we report on the micro-/nanostructuring and surface functionalization of PP substrates through various oxygen and fluorine reactive ion etching (RIE) treatments and their effects on wettability and bacteria adhesion. We found that oxygen treatment creates a hydrophilic surface that reduces bacteria adhesion by 68.7% compared to the control, but additional nanostructuring reduces the surface's anti-biofouling properties due to increased microscale roughness and air pockets that reduce the effectiveness of the liquid barrier. We demonstrate that a fluorine etch chemistry may be utilized to create lotus leaf-inspired, low surface energy, hierarchical microstructure/nanofibrils in PP. Due to the low surface energy and hierarchical morphology, the surface exhibits lotus-leaf wetting (high contact angle ~155° and low contact angle hysteresis < 10°) where water droplets easily roll off the surface in contrast to other PP samples. The lotus leaf-inspired hierarchical, fluorinated surfaces exhibit a 99.6% reduction of E. coli cell adhesion compared to untreated PP. These surfaces demonstrate water contact angle stability over a week in contrast to hydrophilic samples, where the contact angle degrades after just a few days. These new surfaces may help reduce the spread of infections from various plastic medical components without the need for the loading of antibacterial agents that eventually deplete from the surface.

Selective anisotropic etching of GaN over AlGaN for very thin films

Selective etching of gallium nitride (GaN) over aluminum gallium nitride (AlxGa1-xN) with inductively coupled plasma and reactive ion etching (RIE) was examined using only chlorine and oxygen gasses. Etch selectivity was heavily influenced by the amount of oxygen present during etching and was slightly influenced by RIE power. Surface roughness was also influenced heavily by the oxygen flow and RIE power which is important for local and across-wafer uniformity. Etch rates were intentionally minimized for use for highly controlled etching of very thin GaN and Al0.25Ga0.75N epitaxial layers. Maximum tested etch rates for GaN and Al0.25Ga0.75N were 200 and 15 Å/min, respectively, and maximum selectivity between GaN and Al0.25Ga0.75N achieved was at least 68.5 to 1. Above a certain oxygen flow, the etch rate of both GaN and Al0.25Ga0.75N drop so drastically that it was impractical to obtain the etch rate and selectivity in a timely manner. Optimum selectivity was obtained with a low oxygen flow to inhibit Al0.25Ga0.75N etching while steadily etching GaN. Although Al0.25Ga0.75N acts as an etch stop with excellent selectivity, significant over-etching can still cause damage to the underlying layers through ion bombardment. This damage can be predicted through an extrapolation of collected experimental data points for a target a specific epitaxial sheet resistance. This allows sufficient over etch to maximize process margin while minimizing epitaxial damage.

Development of UV-Curable Resins Suitable for Reverse-Tone Lithography for Au Metamaterials Using a Print-and-Imprint Method

Abstract We developed a UV-curable resin (NL-SU1) suitable for screen printing with laser-drilled polyimide masks and reverse-tone nanoimprint lithography. The viscosity of the UV-curable resin composed of two bisphenol A-based monomers was adjusted to 11.0 Pa·s for the screen printing process. It was determined by photo-differential scanning calorimetry that photoinitiator Irgacure 369 was suitable for high methacrylate consumption in UV curing. The UV-curable resin after curing could be used as a top-coated resist layer on another imprinted resist layer because of its sufficient contrast in oxygen reactive ion etching and argon ion milling. We demonstrated a method for reverse-tone lithography in a print–and-imprint method to fabricate 20-nm-thick and 50-nm-linewide Au split-ring resonator arrays.

Fabrication of block copolymer templates by using dually responsive photoresist bottom layers

A simple and novel method was developed to fabricate a nanoporous template by using a bilayer system composed of a silicon-containing block copolymer top layer and a dually responsive photoresist bottom layer. The dually responsive bottom layer became cross-linked by heating. The cross-linked polymer was mechanically hard and insoluble in common solvents. The block copolymer pattern was transferred to the bottom layer by oxygen reactive ion etching. High-aspect-ratio nanopore arrays were generated. The cross-linked bottom layer became soluble again by chain scission upon exposure to UV light. Therefore, the template with a highly orientated array of cylindrical nanopores could be easily removed by dipping in an aqueous base solution. This versatile template of nanoporous structures could be used to create freestanding 1D nanostructures of a variety of functional materials.

Improving adhesion strength between layers of an implantable parylene-C electrode

Parylene-C (PC) is one of the most frequently used materials for flexible implantable electrode. Its interface with metal or other PC layer is critical in PC–metal-PC “sandwich” implantable neural electrodes in relation to their long-term reliability. In this study, we aim to develop a method to improve metal-PC and PC–PC adhesion in PC-based neural electrodes. “Nano-forest” structure, composed of nanopillar array is fabricated on the surface of bottom PC by oxygen reactive ion etching (RIE). The contribution of this layer of nano-structure to mechanical adhesion and sealing property are evaluated respectively. The results verify that for PC based “sandwich” electrode, not only the mechanical adhesion among three layers, but also the sealing ability between PC layers are significantly improved.

A simple maskless process for the fabrication of vertically aligned high density hematite and graphene/magnetite nanowires

The fabrication of vertically aligned iron oxide nanowire arrays using a simple maskless etching approach.

Durability to oxygen reactive ion etching enhanced by addition of synthesized bis(trimethylsilyl)phenyl-containing (meth)acrylates in ultraviolet nanoimprint lithography

We investigated the selection of bis(trimethylsilyl)phenyl-containing (meth)acrylates as additives to improve the durability to oxygen reactive ion etching (O2 RIE) of sub-50 nm imprint resist patterns suitable for bubble-defect-free UV nanoimprinting with a readily condensable gas. 2,5-Bis(2-acryloyloxyethoxy)-1,4-bis(trimethylsilyl)benzene, which has a diacrylate chemical structure similar to that of glycerol 1,3-diglycerolate diacrylate used as a base monomer, and 3-(2-methacryloyloxyethoxy)-1-(hydroxylethoxy)-2-propoxy-3,5-bis(trimethylsilyl)benzene, which has a hydroxy group similar to the base monomer, were synthesized taking into consideration the Ohnishi and ring parameters, and the oxidization of the trimethylsilyl moiety to inorganic species during O2 RIE. The addition of the latter liquid additive to the base monomer decreased etching rate owing to the good miscibility of the additive in the base monomer, while the addition of the former crystalline additive caused phase separation after UV nanoimprinting. The latter additive worked as a compatibilizer to the former additive, which is preferred for etching durability improvement. The coexistence of the additives enabled the fabrication of a 45 nm line-and-space resist pattern by UV nanoimprinting, and its residual layer could be removed by O2 RIE.

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

Amphiphilic liquid‐crystalline 4‐miktoarm star copolymers with a siloxane junction leading to cylindrically nanostructured templates for a siloxane‐based nanodot array

ABSTRACTWell‐defined A3B‐, A2B2‐, and AB3‐type 4‐miktoarm star copolymers (Mn = 10,500–16,200, Mw/Mn = 1.16–1.18) consisting of poly(ethylene oxide) (PEO) and polymethacrylate bearing an azobenzene mesogen (PMA(Az)) as the arms and cyclotetrasiloxane as the core unit were synthesized using a combined route composed of a thiol‐ene click reaction and atom transfer radical polymerization. Microphase‐separated structures of the star copolymers in thin films with a thickness of approximately 100 nm were investigated by GISAXS and TEM. The A3B‐type star‐(PEO)3[PMA(Az)]1 copolymer formed a more highly ordered PEO cylinder array with perpendicular alignment in the PMA(Az) matrix than that of the corresponding linear‐type block copolymer. The center‐to‐center distance of the PEO cylinders and the cylinder diameter were 13 and 4 nm, respectively. The highly ordered star‐(PEO)3[PMA(Az)]1 thin film was directly transferred to a siloxane‐based nanodot array by oxygen reactive ion etching. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1175–1188

Controllable strain recovery of shape memory polystyrene to achieve superhydrophobicity with tunable adhesion

In this work, different superhydrophobic structures were fabricated on the surfaces of polystyrene (PS) blocks, using oxygen reactive ion etch (ORIE) and controllable strain recovery. The PS used here is a thermal shape memory polymer (SMP). Microhills are first generated on the surface of a PS block by ORIE. The PS block is then heated up, which triggers the strain recovery. During this process of recovering its original shape, the PS block gradually reduces its lateral dimensions, while increasing its thickness. Using different recovery temperatures, the surface morphologies can be controlled, which provides an approach to adjust surface wetting properties, including adhesion.At a temperature of 148 °C, microhills deformed to high-aspect-ratio nanowires. The corresponding PS surface has wetting properties similar to those of a lotus surface. The wetting is in Cassie–Baxter state, and a water drop is easy to get off from this surface. When the recovery temperature is increased to 162 °C, microwrinkles appear on the PS surface due to the different stiffnesses between the oxygen-treated top layer and the underlying PS substrate. These microwrinkles, together with nanowires located on their tops, form hybrid micro/nanostructures on the PS surface. The corresponding surface has wetting properties similar to those of a rose petal. The wetting is in a mixed state of Wenzel and Cassie–Baxter. A water droplet is stuck on the surface, although its apparent contact angle is as high as 166° ± 2°.

Resist-free antireflective nanostructured film fabricated by thermal-NIL.

Resist-free antireflective (AR) nanostructured films are directly fabricated on polycarbonate (PC) film using thermal-nanoimprint lithography (T-NIL) and the moth-eye shape of AR nanostructure is elaborately optimized with different oxygen reactive ion etching conditions. Anodic aluminum oxide (AAO) templates are directly used as master molds of T-NIL for preparation of AR nanostructures on PC film without an additional T-NIL resist. AR nanostructures are well arranged with a period of about 200 nm and diameter of about 150 nm, which corresponds to those of the AAO template mold. The moth-eye AR nanostructures exhibit the average reflectance of 2% in wavelength range from 400 to 800 nm. From the results, highly enhanced AR properties with simple direct imprinting on PC film demonstrate the potential for panel application in the field of flat display, touch screen, and solar cells.

Singular Sheet Etching of Graphene with Oxygen Plasma

This paper reports a simple and controllable post-synthesis method for engineering the number of graphene layers based on oxygen plasma etching. Singular sheet etching (SSE) of graphene was achieved with the optimum process duration of 38 seconds. As a demonstration of this SSE process, monolayer graphene films were produced from bilayer graphenes. Experimental investigations verified that the oxygen plasma etching removes a single layer graphene sheet in an anisotropic fashion rather than anisotropic mode. In addition, etching via the oxygen plasma at the ground electrodes introduced fewer defects to the bottom graphene layer compared with the conventional oxygen reactive ion etching using the powered electrodes. Such defects can further be reduced with an effective annealing treatment in an argon environment at 900–1000°C. These results demonstrate that our developed SSE method has enabled a microelectronics manufacturing compatible way for single sheet precision subtraction of graphene layers and a potential technique for producing large size graphenes with high yield from multilayer graphite materials.

Singular Sheet Etching of Graphene with Oxygen Plasma

Abstract This paper reports a simple and controllable post-synthesis method for engineering the number of graphene layers based on oxygen plasma etching. Singular sheet etching (SSE) of graphene was achieved with the optimum process duration of 38 seconds. As a demonstration of this SSE process, monolayer graphene films were produced from bilayer graphenes. Experimental investigations verified that the oxygen plasma etching removes a single layer graphene sheet in an anisotropic fashion rather than anisotropic mode. In addition, etching via the oxygen plasma at the ground electrodes introduced fewer defects to the bottom graphene layer compared with the conventional oxygen reactive ion etching using the powered electrodes. Such defects can further be reduced with an effective annealing treatment in an argon environment at 900–1000°C. These results demonstrate that our developed SSE method has enabled a microelectronics manufacturing compatible way for single sheet precision subtraction of graphene layers and a potential technique for producing large size graphenes with high yield from multilayer graphite materials.