Selective Reactive Ion Etching Research Articles

A polarization converter array based on microcavity patterned hydrophobic layer induced twisted-radial liquid crystals

We report a facile method to prepare an array of liquid-crystal polarization converters (LCPCA) based on hydrophobic microcavity arrays. The reactive ion etching (RIE) technique was chosen to treat the homeotropic polyimide (PI⊥) layers through a metal mask with a through-hole array. Under the selective RIE treatment, a hydrophobic–hydrophilic pattern is obtained due to the increase in surface energy in the etched area from ∼34.6 to ∼73.2 mN/m. The etched area forms a microcavity with a diameter of 520 μm, depth of ∼23 nm, and center distance of 690 μm, which is consistent with the dimensions of the metal mask. Since the walls of the microcavity retain hydrophobicity, the liquid-crystal (LC) molecules show a radial arrangement in the microcavity. Combining a substrate covered by a parallel rubbed homogeneous polyimide (PI∥) layer with the substrate covered by a microcavity-patterned PI⊥ layer can form a “sandwich” structure. The LC molecules in the microcavities are arranged in a twisted-radial texture, which can convert linearly polarized light to radially or azimuthally polarized light depending on the polarization direction of the incident light. The proposed method has the merits of simple steps, controllable size, and easy to fabricate large-area arrays. Our LCPCA has potential applications in tight focus, material processing, biological tissue analysis, particle capture, and optical data storage.

TiN nanobridge Josephson junctions and nanoSQUIDs on SiN-buffered Si

We report the fabrication and properties of titanium nitride (TiN) nanobridge Josephson junctions (nJJs) and nanoscale superconducting quantum interference devices (nanoSQUIDs) on SiN-buffered Si substrates. The superior corrosion resistance, large coherence length, suitable superconducting transition temperature and highly selective reactive ion etching (RIE) of TiN compared to e-beam resists and the SiN buffer layer allow for reproducible preparation and result in long-term stability of the TiN nJJs. High-resolution transmission electron microscopy reveals a columnar structure of the TiN film on an amorphous SiN buffer layer. High-resolution scanning electron microscopy reveals the variable thickness shape of the nJJs. A combination of wet etching in 20% potassium hydroxide and RIE is used for bulk nanomachining of nanoSQUID cantilevers. More than 20 oscillations of the V(B) dependence of the nanoSQUIDs with a period of ∼6 mT and hysteresis-free I(V) characteristics (CVCs) of the all-TiN nJJs are observed at 4.2 K. CVCs of the low-I c all-TiN nJJs follow theoretical predictions for dirty superconductors down to ∼10 mK, with the critical current saturated below ∼0.6 K. These results pave the way for superconducting electronics based on nJJs operating non-hysteretically at 4.2 K, as well as for all-TiN qubits operating at sub-100 mK temperatures.

Detection of SARS-CoV-2 DNA Targets Using Femtoliter Optofluidic Waveguides.

Chip-scale SARS-CoV-2 testing was demonstrated using silicon nitride (Si3N4) nanoslot fluidic waveguides to detect a tagged oligonucleotide with a coronavirus DNA sequence. The slot waveguides were fabricated using complementary metal–oxide–semiconductor (CMOS) fabrication processes, including multiscale lithography and selective reactive ion etching (RIE), forming femtoliter fluidic channels. Finite difference method (FDM) simulation was used to calculate the optical field distribution of the waveguide mode when the waveguide sensor was excited by transverse electric (TE) and transverse magnetic (TM) polarized light. For the TE polarization, a strong optical field was created in the slot region and its field intensity was 14× stronger than the evanescent sensing field from the TM polarization. The nanoscale confinement of the optical sensing field significantly enhanced the light–analyte interaction and improved the optical sensitivity. The sensitivity enhancement was experimentally demonstrated by measuring the polarization-dependent fluorescence emission from the tagged oligonucleotide. The photonic chips consisting of femtoliter Si3N4 waveguides provide a low-cost and high throughput platform for real-time virus identification, which is critical for point-of-care (PoC) diagnostic applications.

Titanium Nitride as a New Prospective Material for NanoSQUIDs and Superconducting Nanobridge Electronics.

Nanobridge Josephson junctions and nanometer-scale superconducting quantum interference devices (nanoSQUIDs) based on titanium nitride (TiN) thin films are described. The TiN films have a room temperature resistivity of ~15 µΩ·cm, a superconducting transition temperature Tc of up to 5.3 K and a coherence length ξ(4.2 K) of ~105 nm. They were deposited using pulsed DC magnetron sputtering from a stoichiometric TiN target onto Si (100) substrates that were heated to 800 °C. Electron beam lithography and highly selective reactive ion etching were used to fabricate nanoSQUIDs with 20-nm-wide nanobridge Josephson junctions of variable thickness. X-ray and high-resolution electron microscopy studies were performed. Non-hysteretic I(V) characteristics of the nanobridges and nanoSQUIDs, as well as peak-to-peak modulations of up to 17 µV in the V(B) characteristics of the nanoSQUIDs, were measured at 4.2 K. The technology offers prospects for superconducting electronics based on nanobridge Josephson junctions operating within the framework of the Ginzburg–Landau theory at 4.2 K.

Etch selectivity during plasma-assisted etching of SiO2 and SiNx: Transitioning from reactive ion etching to atomic layer etching

Continued downscaling of semiconductor devices has placed stringent constraints on all aspects of the fabrication process including plasma-assisted anisotropic etching. To address manufacturing challenges associated with atomic-scale control, material selectivity, etch fidelity, and increasingly complex device architectures, reactive ion etching (RIE) is transitioning to plasma-assisted atomic layer etching (ALE). Even though the number of elements used in the semiconductor devices has increased several-fold over the last four decades, SiO2 and SiNx remain the most commonly used dielectric materials. In fact, fluorocarbon based, plasma-assisted ALE processes for SiO2 and SiNx have already been integrated into semiconductor manufacturing, including etching of self-aligned contacts for advanced transistors. However, several challenges remain in achieving ultrahigh etch selectivity of SiO2 over SiNx and vice versa. In this article, first, the authors provide a focused review on selective RIE of SiO2 over SiNx and contrast this with ALE. A particular focus is given to the etching mechanism, including the role of the mixing layer composition and thickness at the fluorocarbon-SiO2 interface, the F-to-C ratio in the fluorocarbon parent gas, H2 dilution, surface composition on the nonetched SiNx, ion flux and energy, Ar plasma activation duration in ALE, and chamber memory effects. Second, we discuss the reverse case of selectively etching SiNx over SiO2 with careful attention given to the role of novel hydrofluorocarbon gases and dilution of the primary feed gas with other gases such as CH4 and NO. In the second part of this review, we also discuss how novel surface chemistries are enabled by the introduction of ALE, which include selective (NH4)2SiF6 formation on the SiNx surface and selective surface prefunctionalization of SiO2 to enable ultrahigh selectivity. Through this review, the authors hope to provide the readers with an exhaustive knowledge of the selectivity mechanisms for RIE of SiO2 over SiNx and vice versa, which provides a basis for developing future highly material-selective ALE processes.

Formation of SiC Mesastructures with Gently Sloping Sidewalls by Dry Selective Etching through a Photoresist Mask

We have demonstrated that SiC mesastructures with gently sloping sidewalls form with the help of selective reactive ion etching (RIE) of silicon carbide through a photoresist mask (slanted walls are formed during simultaneous etching of SiC and the resistive mask with the edge in the shape of a sharp wedge). A simple geometrical model of etching predicts that the resultant slope of the mesastructure wall must be specified by two parameters, i.e., initial angle of the resistive wedge and the selectivity of SiC etching relative to the photoresist (ratio of etching rates of SiC and photoresist). For experiments, we used polished 4H–SiC wafers with the (0001) orientation. Photoresist regions with an edge angle of 22° were deposited using photolithography onto the Si side of the wafers. Then etching of mesastructures was performed in nitrogen trifluoride in a setup with an inductively coupled plasma. We selected RIE parameters ensuring SiC and photoresist etching with rates of 55 and 160 nm/min, respectively (etching selectivity was 1 : 3). The SiC mesastructures formed by etching have a height of 3.2 μm and gently sloping sidewalls with a slope of about 8°. This technology can be used for preparing high-voltage SiC devices with a straight bevel.

Springtail-Inspired Superamphiphobic Ordered Nanohoodoo Arrays with Quasi-Doubly Reentrant Structures.

The skin of springtails is well-known for being able to repel water and organic liquids using their hexagonally arranged protrusions with reentrant structures. Here, a method to prepare 100 nm-sized nanohoodoo arrays with quasi-doubly reentrant structures over square centimeters through combining the nanosphere lithography and the template-protected selective reactive ion etching technique is demonstrated. The top size of the nanohoodoos, the intra-nanohoodoo distance, and the height of the nanohoodoos can be readily controlled by the plasma-etching time of the polystyrene (PS) spheres, the size of the PS spheres used, and the reactive ion etching time of silicon. The strong structural control capability allows for the study of the relationship between the nanohoodoo structure and the wetting property. Superamphiphobic nanohoodoo arrays with outstanding water/organic liquid repellent properties are finally obtained. The superamphiphobic and liquid repellent properties endow the nanohoodoo arrays with remarkable self-cleaning performance even using hot water droplets, anti-fogging performance, and the surface-enhanced Raman scattering sensitivity improvement by enriching the analyte molecules on the nanohoodoo arrays. Overall, the simple and massive production of the superamphiphobic nanohoodoo structures will push their practical application processes in diverse fields where wettability and liquid repellency need to be carefully engineered.

Flexible Bioelectronic Devices Based on Micropatterned Monolithic Carbon Fiber Mats

AbstractPolymer‐derived carbon can serve as an electrode material in multimodal neural stimulation, recording, and neurotransmitter sensing platforms. The primary challenge in its applicability in implantable, flexible neural devices is its characteristic mechanical hardness that renders it difficult to fabricate the entire device using only carbon. A microfabrication technique is introduced for patterning flexible, cloth‐like, polymer‐derived carbon fiber (CF) mats embedded in polyimide (PI), via selective reactive ion etching. This scalable, monolithic manufacturing method eliminates any joints or metal interconnects and creates electrocorticography electrode arrays based on a single CF mat. The batch‐fabricated CF/PI composite structures, with critical dimension of 12.5 µm, are tested for their mechanical, electrical, and electrochemical stability, as well as to chemicals that mimic acute postsurgery inflammatory reactions. Their recording performance is validated in rat models. Reported CF patterning process can benefit various carbon microdevices that are expected to feature flexibility, material stability, and biocompatibility.

Fabrication of self-aligned multilevel nanostructures

Multilevel three-dimensional nanostructures are essential in many integrated nanoelectronic and nanophotonic applications. With the continued shrinking of critical device dimensions, extremely precise nanoscale overlay is required between multiple individual levels of these integrated devices. Multilevel nanoimprint lithography has been proposed in the literature as a potential solution to this overlay problem. In this context, self-aligned (perfectly aligned) multilevel templates (SAMT) for multilevel nanoimprint lithography are proposed in this article. By combining nanolithography, atomic layer deposition, and highly selective reactive ion etch, SAMTs can enable the fabrication of sophisticated integrated devices. Four specific self-aligned multilevel fabrication techniques have been demonstrated that result in symmetric multilevel structures, bilaterally symmetric multilevel structures, tubular structures, and asymmetric multilevel structures, all in the sub-100nm scale. When used in conjunction with a nanoimprint lithography process, the SAMTs can enable high-throughput patterning of various nanoelectronic and nanophotonic devices using a single patterning step with perfect alignment and overlay. SAMTs further enable large area patterning, such as wafer-scale patterning and roll-to-roll patterning on flexible substrates, without compromising perfect overlay.

Ordered Micro/Nanostructures with Geometric Gradient: From Integrated Wettability "Library" to Anisotropic Wetting Surface.

Geometric gradients within ordered micro/nanostructures exhibit unique wetting properties. Well-defined and ordered microsphere arrays with geometric gradient (OMAGG) are successfully fabricated through combining colloidal lithography and inclined reactive ion etching (RIE). During the inclined RIE, the graded etching rates in vertical direction of etcher chamber are the key to generating a geometric gradient. The OMAGG can be used as an effective mask for the preparation of micro/nanostructure arrays with geometric gradient by selective RIE. Through this strategy, a well-defined wettability "library" with graded silicon cone arrays is fabricated, and the possibility of screening one desired "book" from the designated wettability "library" is demonstrated. Meanwhile, the silicon cone arrays with geometric gradient (SCAGG) can be applied to control the wetting behavior of water after being modified by hydrophilic or hydrophobic chemical groups. Based on this result, a temperature-responsive wetting substrate is fabricated by modifying poly n-isopropyl acrylamide (PNIPAM) on the SCAGG. These wettability gradients have great potential in tissue engineering, microfluidic devices, and integrated sensors.

Thermal Probe Maskless Lithography for 27.5 nm Half-Pitch Si Technology

Thermal scanning probe lithography is used for creating lithographic patterns with 27.5 nm half-pitch line density in a 50 nm thick high carbon content organic resist on a Si substrate. The as-written patterns in the poly phthaladehyde thermal resist layer have a depth of 8 nm, and they are transformed into high-aspect ratio binary patterns in the high carbon content resist using a SiO2 hard-mask layer with a thickness of merely 4 nm and a sequence of selective reactive ion etching steps. Using this process, a line-edge roughness after transfer of 2.7 nm (3σ) has been achieved. The patterns have also been transferred into 50 nm deep structures in the Si substrate with excellent conformal accuracy. The demonstrated process capabilities in terms of feature density and line-edge roughness are in accordance with today's requirements for maskless lithography, for example for the fabrication of extreme ultraviolet (EUV) masks.

Monolithic integration of Pb(Zr,Ti)O3 thin film based resonators using a complete dry microfabrication process

Monolithic fabrication of lead zirconate titanate [Pb(Zr,Ti)O3 or PZT] based thin film resonant devices such as microcantilevers, Lamb wave and bulk acoustic wave resonators are demonstrated. High-performance PZT thin films with a thickness of 2.6 μm are prepared on a silicon on insulator wafer by a sputtering deposition process. A highly selective reactive ion etching process is employed for micro-patterning of PZT, platinum electrodes, and SiO2 insulation layer. Self-actuation of the PZT microcantilevers is demonstrated and the frequency response is characterized using a laser Doppler vibrometer. The frequency response of the Lamb wave resonator is evaluated by measuring its transmission characteristic using a network analyzer. For a Lamb wave resonator with a length of 240 μm and an interdigital period of 80 μm, the 1st order and 2nd resonance frequencies are 15.3 and 41.8 MHz, respectively.

Fabrication of Two-Depth Apodized Grating by Selective Etching of InP in CH4/H2 Reactive Ion Etching

We fabricated the basic structure of an apodized grating with two depths of 120 and 200 nm for InP in one etching process during reactive ion etching (RIE) with methane and hydrogen. In the process, we first performed selective RIE, in which InP windows of the grating were etched in a region with a thick layer but not etched with polymer deposition in a region without the thick layer. Next, we performed oxygen plasma exposure to remove the polymer and a second RIE for the windows of the gratings both with and without the thick layers.

Fabrication of cone-shaped boron doped diamond and gold nanoelectrodes forAFM–SECM

We demonstrate a reliable microfabrication process for a combined atomic force microscopy(AFM) and scanning electrochemical microscopy (SECM) measurement tool. Integratedcone-shaped sensors with boron doped diamond (BDD) or gold (Au) electrodes werefabricated from commercially available AFM probes. The sensor formation process is basedon mature semiconductor processing techniques, including focused ion beam (FIB)machining, and highly selective reactive ion etching (RIE). The fabrication approachpreserves the geometry of the original AFM tips resulting in well reproduciblenanoscaled sensors. The feasibility and functionality of the fully featured tipsare demonstrated by cyclic voltammetry, showing good agreement between themeasured and calculated currents of the cone-shaped AFM–SECM electrodes.

High density submicron features using a laser pattern generator and double patterning

Line and space patterns have been fabricated in Si with periods as small as 1.3μm using a double patterning technique and an optical direct write tool. The pattern density of a single lithography step is shown to be limited by the proximity effect and that gratings with periods smaller than about 2.0μm are beyond the resolution of the 405nm wavelength laser direct write tool employed in these experiments. The lithography-etch–lithography-etch technique, which includes a highly selective reactive ion etch step, provides a robust way to create 4mm long gratings of 1.5μm period. Variations in this two step lithography process limit the practical minimum pitch for long gratings to about 1.4μm.

Metalized Nanotube Tips Improve Through Thickness Thermal Conductivity in Adhesive Joints

The through-thickness thermal conductivity in conventional adhesive joints (of approximately 0.3 W/m-K) fails to meet the thermal load transfer requirement in numerous applications to enable lean manufacturing and improve system reliability to thermal load. Carbon nanotubes are known to possess extremely high thermal conductivity along the longitudinal axis. According to molecular dynamics simulations, the value can be as high as 3500 W/m-K at room temperature for multi-walled carbon nanotubes (MWCNT). Meanwhile, the transverse thermal conductivity perpendicular to the longitudinal axis of the MWCNTs is known to be relatively low, approximately 10-15 W/m-K. Existing studies of mixing the MWCNTs in polymers for adhesive joints only achieved minimal enhancement in the thermal conductivity and failed to satisfy the thermal property requirement for the adhesive joints. In order to properly utilize the superior axial thermal conductivity of the MWCNTs, vertically aligned MWCNTs have been used in this study and incorporated in the adhesive joint configuration. Analytical parametric study was conducted to identify critical parameters that affect the overall thermal conductivity of the joint and to provide guidelines for the process development. The process development involved growing the vertically aligned MWCNTs on silicon wafers. The aligned nanotube array was partially infused with epoxy adhesive. Selective reactive ion etching of the epoxy revealed the nanotube tips. In order to reduce the impedance mismatch and phonon scattering at the interface between the nanotube tips and the adherends, gold was thermally evaporated on the nanotube tips. The measured thermal conductivity of the adhesive joint device incorporating the MWCNTs was 262 W/m-K, which is significantly larger compared to that of less than 1 W/m-K without the MWCNTs.

Reactive ion etching of FePt using inductively coupled plasma

We propose a reactive ion etching (RIE) process of an L1 0-FePt film which is expected as one of the promising materials for the perpendicular magnetic recording media. The etching was carried out using an inductively coupled plasma (ICP) RIE system and an etching gas combination of CH 4/O 2/NH 3 was employed. The L1 0-FePt films were deposited on (1 0 0)-oriented MgO substrates using a magnetron sputtering system. The etching masks of Ti were patterned on the FePt films lithographically. The etch rates of ∼16 and ∼0 nm/min were obtained for the FePt film and the Ti mask, respectively. The atomic force microscopy (AFM) analyses provided the average roughness ( R a) value of 0.95 nm for the etched FePt surface, that is, a very flat etched surface was obtained. Those results show that the highly selective RIE process of L1 0-FePt was successfully realized in the present study.

Preparation of thin GaAs suspended membranes for gas micro-sensors using plasma etching

The GaAs/AlGaAs heterostructure layer system grown by MBE on GaAs substrate was designed to be used for micro-mechanical structure fabrication. In the first step, double-side aligned photolithography is carried out to define the etching masks on both sides of the substrate. After this, highly selective reactive ion etching (SRIE) of GaAs from the front side is used to determine the lateral dimensions of the membrane structure. The vertical dimension is defined by deep backside SRIE through a 300 μm thick GaAs substrate to the AlGaAs etch stop layer, hence the structure thickness is precisely determined by the thickness of MBE-grown GaAs layer over this etch stop layer. The last step is selective etching of the AlGaAs etch stop layer. The thermal resistance value of the membrane structure as high as 21 K/mW is achieved.

Enhanced voltage-current characteristics of GaN nanowires treated by a selective reactive ion etching

In characterizing the electrical properties of individual NWs (nanowires), the amorphous oxide layer on the surface of NWs is known to limit the electrical conductivity owing to the contact barriers between metal electrodes and NWs. To remove the native oxide layer, a systematic reactive ion etching (RIE) was performed, resulting in a gradual decrease of the diameters of NWs. Voltage-current characteristics of the GaN NW devices treated by tuning the RIE process were improved as reflected by a 1000 times increase in conductance, which was in turn attributed to the removal of the thick (d∼3.5nm) contact barrier formed by the native oxide layer.

Fabrication of nonspherical colloidal particles via reactive ion etching of surface-patterned colloidal crystals

We have created multilayered nonspherical colloidal particles by reactive ion etching (RIE) of surface-patterned colloidal crystals. The effectiveness of the Au patterns as etching masks in defining and controlling the particle morphologies depend on their relative dimensions to the particles and the RIE conditions. As a result, nonspherical particles with shape and size more diversiform and controllable than those obtained via RIE of pure colloidal crystals were fabricated. This approach can be applied to a wide of particles (ceramic, metal, semiconductor) as long as the selective RIE between the masks and particles is afforded, which definitely provides a facile and controllable route to produce nonspherical particles of various morphologies. Furthermore, we demonstrated that the patterned particles could be used as templates for fabricating novel nanostructures.