Self-aligned Electrodes Research Articles

Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors with a hexagonal boron nitride gate insulator

Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors with a hexagonal boron nitride gate insulator

Self-aligned single-electrode actuation of tangential and wineglass modes using PMN-PT

Considering the evolution of rotation sensing and timing applications realized in micro-electro-mechanical systems (MEMS), flexural mode resonant shapes are outperformed by bulk acoustic wave (BAW) counterparts by achieving higher frequencies with both electrostatic and piezoelectric transduction. Within the 1–30 MHz range, which hosts BAW gyroscopes and timing references, piezoelectric and electrostatic MEMS have similar transduction efficiency. Although, when designed intelligently, electrostatic transduction allows self-alignment between electrodes and the resonator for various BAW modes, misalignment is inevitable regarding piezoelectric transduction of BAW modes that require electrode patterning. In this paper transverse piezoelectric actuation of [011] oriented single crystal lead magnesium niobate–lead titanate (PMN–PT) thin film disks are shown to excite the tangential mode and family of elliptical compound resonant modes, utilizing a self-aligned and unpatterned electrode that spans the entire disk surface. The resonant mode coupling is achieved by employing a unique property of [011] PMN–PT, where the in-plane piezoelectric coefficients have opposite signs. Fabricating 1-port disk transducers, RF reflection measurements are performed that demonstrate the compound mode family shapes in the 1–30 MHz range. Independent verification of mode transduction is achieved using in-plane displacement measurements with Polytec’s laser Doppler vibrometer (LDV). While the tangential mode achieves a 40o/s dithering rate at 335 kHz resonant frequency, the n = 2 wine-glass mode achieves 11.46 nm tip displacement at 8.42 MHz resonant frequency on a radius of 60 μm disk resonator in air. A single electrode resonator that can excite both tangential and wine-glass modes with such metrics lays the foundation for a BAW MEMS gyroscope with a built-in primary calibration stage.

Highly responsive silicon-based hot-electron photodetector with self-aligned metamaterial interdigital electrodes

A sensitive silicon-based hot-electron photodetector based on a self-aligned metal–semiconductor–metal junction is developed. Nearly perfect absorption is achieved with the metamaterial optical coupling, whereas the absorption difference between the upper and lower interdigital gratings is as large as 70% near the resonant wavelength. Arising from the asymmetric photo-absorption, the measured responsivity values of the self-aligned interdigital grating devices reach 1.89 and 0.78 mA/W under zero biasing conditions at the wavelengths of 1310 and 1550 nm, respectively. These values approach the reported record photo-responsivity of hot-electron photodetectors with conventional metal–semiconductor junctions. In addition, the indication of polarity-switchable photocurrent appears due to the wavelength-dependent absorption of the upper and lower metal interdigital gratings. Our device, combining the self-aligned metamaterial interdigital electrodes with highly asymmetric absorption, shows prospects for applications in photodetection, photovoltaics, integrated optoelectronics, and optical communications.

Germanium Spherical Quantum-Dot Single-Hole Transistors With Self-Organized Tunnel Barriers and Self-Aligned Electrodes

We report the fabrication and electrical characterization of single-hole transistors (SHTs), in which a Ge spherical quantum dot (QD) weakly couples to self-aligned electrodes via self-organized tunnel barriers of Si3N4. A combination of lithographic patterning, sidewall spacers, and selfassembled growth was used for fabrication. The core experimental approach is based on the selective oxidation of poly-SiGe spacer islands located at the specially designed included-angle locations of Si3N4/Si-trenches. By adjusting processing times for conformal deposition, etch back and thermal oxidation, good tunability in the Ge QD size and its tunnel-barrier widths were controllably achieved. Each Ge QD is electrically addressable via self-aligned Si gate and reservoirs, thus offering an effective building block for implementing single-charge devices.

14GHz Schottky Diodes Using a p-Doped Organic Polymer.

The low carrier mobility of organic semiconductors and the high parasitic resistance and capacitance often encountered in conventional organic Schottky diodes hinder their deployment in emerging radio frequency (RF) electronics. Here, these limitations are overcome by combining self-aligned asymmetric nanogap electrodes (≈25nm) produced by adhesion lithography, with a high mobility organic semiconductor, and RF Schottky diodes able to operate in the 5G frequency spectrum are demonstrated. C16 IDT-BT is used, as the high hole mobility polymer, and the impact of p-doping on the diode performance is studied. Pristine C16 IDT-BT-based diodes exhibit maximum intrinsic and extrinsic cutoff frequencies (fC ) of >100and 6GHz, respectively. This extraordinary performance is attributed to the planar nature of the nanogap channel and the diode's small junction capacitance (<2pF). Doping of C16 IDT-BT with the molecular p-dopant C60 F48 improves the diode's performance further by reducing the series resistance resulting to intrinsic and extrinsic fC of >100and ≈14GHz respectively, while the DC output voltage of an RF rectifier circuit increases by a tenfold. Ourwork highlights the importance of the planar nanogap architecture and paves the way for the use of organic Schottky diodes in large-area RF electronics of the future.

Germanium Quantum-Dot Array with Self-Aligned Electrodes for Quantum Electronic Devices.

Semiconductor-based quantum registers require scalable quantum-dots (QDs) to be accurately located in close proximity to and independently addressable by external electrodes. Si-based QD qubits have been realized in various lithographically-defined Si/SiGe heterostructures and validated only for milli-Kelvin temperature operation. QD qubits have recently been explored in germanium (Ge) materials systems that are envisaged to operate at higher temperatures, relax lithographic-fabrication requirements, and scale up to large quantum systems. We report the unique scalability and tunability of Ge spherical-shaped QDs that are controllably located, closely coupled between each another, and self-aligned with control electrodes, using a coordinated combination of lithographic patterning and self-assembled growth. The core experimental design is based on the thermal oxidation of poly-SiGe spacer islands located at each sidewall corner or included-angle location of Si3N4/Si-ridges with specially designed fanout structures. Multiple Ge QDs with good tunability in QD sizes and self-aligned electrodes were controllably achieved. Spherical-shaped Ge QDs are closely coupled to each other via coupling barriers of Si3N4 spacer layers/c-Si that are electrically tunable via self-aligned poly-Si or polycide electrodes. Our ability to place size-tunable spherical Ge QDs at any desired location, therefore, offers a large parameter space within which to design novel quantum electronic devices.

Enhanced Surface-Emitting Vertical InGaN Light-Emitting Diodes by Self-Aligned Sidewall Electrodes and Optical Cavity Length Modulation

A technique to enhance the light extraction of InGaN-based micro-light-emitting diodes (LEDs) using an etch-back and a self-aligned sidewall electrode structure was demonstrated. By removing the sa...

Fabrication of Silicon Microtips with Integrated Electrodes

This paper describes a simple method to fabricate silicon microtips with integrated self-aligned polarization electrodes for development of MEMS and electron field emission devices. The method is based on the wet bulk micromachining of the silicon substrate in KOH solutions and utilizes low stress PECVD SiOxNy films obtained at low temperatures (320°C) as structural material for both mechanical support and electrical insulation of the electrodes. For the electrodes sputtered Cr films were utilized. The microtip formation process was studied by optical and electronic microscopy to analyze tip geometry and the characteristic etch rate of the different stages during the tips formation. The fabricated devices were matrixes with different numbers of microtips, each with a typical height of 52 μm and with diameter at the apex below 1 μm. In the best case, the distance between the apex of the tips and the metallic electrode was lower than 5 μm. The results also show that the low stress SiOxNy film is essential to attain the necessary flatness required by the process.

Fabrication of a MEMS Micromirror Based on Bulk Silicon Micromachining Combined With Grayscale Lithography

A 1D MEMS (Micro-Electro-Mechanical Systems) mirror for LiDAR applications, based on vertically asymmetric comb-drive electrostatic actuators, is presented in this work employing a novel fabrication process. This novel micromachining process combines typical SOI-based bulk micromachining and grayscale lithography, enabling the fabrication of combs actuators with asymmetric heights using a single lithography step in the active layer. With this technique, the fabrication process is simplified, and the overall costs are reduced since the number of required lithography steps decrease. The fabricated mirrors present self-aligned electrodes with a $2.8~\mu \text{m}$ gap and asymmetric heights of the movable and the fixed electrodes of 20 $\mu \text{m}$ and 50 $\mu \text{m}$ , respectively. These asymmetric actuators are an essential feature for the operation mode of this device, enabling both in resonant and static mode operation. A mirror field of view (FOV) of 54° at 838 Hz was achieved under low-pressure, when resonantly operated, and a FOV of 0.8° in the static mode.

Self-aligned microfluidic contactless dielectrophoresis device fabricated by single-layer imprinting on cyclic olefin copolymer.

The trapping and deflection of biological cells by dielectrophoresis (DEP) at field non-uniformities in a microfluidic device is often conducted in a contactless dielectrophoresis (cDEP) mode, wherein the electrode channel is in a different layer than the sample channel, so that field penetration through the interceding barrier causes DEP above critical cut-off frequencies. In this manner, through physical separation of the electrode and sample channels, it is possible to spatially modulate electric fields with no electrode-induced damage to biological cells in the sample channel. However, since this device requires interlayer alignment of the electrode to sample channel and needs to maintain a thin interceding barrier (~ 15μm) over the entire length over which DEP is needed (~ 1cm), variations in alignment and microstructure fidelity cause wide variations in cDEP trapping level and frequency response across devices. We present a strategy to eliminate interlayer alignment by fabricating self-aligned electrode and sample channels, simultaneously with the interceding barrier layer (14-μm width and 50-μm depth), using a single-layer imprint and bond process on cyclic olefin copolymer. Specifically, by designing support structures, we preserve fidelity of the high aspect ratio insulating posts in the sample channel and the interceding barrier between the sample and electrode channels over the entire device footprint (~ 1cm). The device operation is validated based on impedance measurements to quantify field penetration through the interceding barrier and by DEP trapping measurements. The presented fabrication strategy can eventually improve cDEP device manufacturing protocols to enable more reproducible DEP performance. Graphical abstract.

(Keynote) Gate-All-Around Nanowire & Nanosheet FETs for Advanced, Ultra-Scaled Technologies

As two-dimensional CMOS scaling is reaching its physical limits, maintaining profitable node-to-node cost-per-transistor gains requires increasingly constraining design restrictions. To help preserve the power, performance, area, and cost (PPAC) logic roadmap several innovations are being explored, such as cell height reduction by decreasing the number of metal tracks. This allows compensating for the recent trend of a more modest pitch scaling, but it also requires fin depopulation from two fins to one fin per finFET transistor for ultra-scaled cells with reduced number of metal tracks [1,2], which impacts the device performance. As such, in order to keep the scaling gains, introduction of novel device architectures is needed, with gate-all-around (GAA) nanowire (NW) or nanosheet (NS) FETs [2-8] being regarded as the most promising finFET replacements for advanced nodes (Fig. 1). These devices provide better electrostatics allowing further gate length (Lgate) scaling, with NS offering higher current drivability (ION) per layout footprint than NW thanks to larger effective widths (Weff). Furthermore, by having the option to vary the NS Weff on the same chip/wafer, this enables additional design flexibility, typically targeting wider NS for high performance computing and narrower NS for low power applications [5,7,8]. At the same time, vertically stacking several NS enables further boosting ION at the expense of an increase in parasitic capacitance. Minimizing the latter is crucial for delivering both optimal performance and power efficiency. That can be achieved by reducing the vertical distance between the stacked lateral NS as demonstrated in Fig. 2 and via the implementation of inner spacers in-between them [2,6]. Both features introduce considerable challenges for device fabrication. However, it still shares many of the building blocks of a finFET flow thus allowing a smoother technological transition for manufacturing as compared to other device architectures. In the case of the NS vertical pitch, its shrinkage requires scaled Si/SiGe-multilayer stacks growth and thinner gate stacks that can be implemented with the guarantee of similar gate control on all NS surfaces. That has been demonstrated for a stack with an alternative, thinner (<2.5nm) n-type effective work function (EWF) metal layer yielding tight, low threshold voltage values (VT < 0.2V - NMOS), while showing improved noise and reliability behavior [9].Another GAA device architecture, with vertical NW or NS (VNW/VNS) and vertically defined Lgate, has been shown to have the potential to enable denser and more energy efficient SRAMs and MRAMs when used as the cell transistors or selector, respectively [10-13]. In the overall scaling roadmap, the latter type of memory has been gaining increased momentum as an alternative for enabling large, ultra-high-density last-level caches for systems with reduced area and energy. Overall, GAA VNW/VNS FETs represent a more disruptive technological transition, moving from 2D to 3D layouts. Nevertheless, they can also open new scaling paths in the third dimension. In this context, the feasibility of co-integrating on the same wafer and in a cost-effective way finFETs (or GAA lateral NS FETs) for high performance logic and GAA VNS FETs as MRAM selector is evaluated by process simulations, experimentally validated for each type of devices (Fig. 3). In this scheme, cost adders were minimized by sharing the maximum number of critical process steps for both FETs. A replacement metal gate process, standardly used in finFETs and lateral NS FETs can be also advantageous for VNS FETs as it can allow shrinkage of the channel cross-section for improved electrostatics (without impacting the bottom/top S/D [14]), while also enabling new techniques for stress-induced mobility enhancement [12]. Specific for VNS FETs, a self-aligned gate electrode can be easily implemented by relying on the different oxidation kinetics of Si and SiGe and using SiGe/Si/SiGe instead of Si-only pillars.

Germanium quantum dot infrared photodetectors addressed by self-aligned silicon nanowire electrodes

Germanium quantum dots (GeQDs), addressed by self-aligned and epitaxial silicon nanowires (SiNWs) as electrodes, represent the most fundamental and the smallest units that can be integrated into Si optoelectronics for 1550 nm wavelength detection. In this work, individual GeQD photodetectors have been fabricated based on a low temperature self-condensation of uniform amorphous Si (a-Si)/a-Ge bilayers at 300 °C, led by rolling indium (In) droplets. Remarkably, the diameter of the GeQD nodes can be independently controlled to achieve wider GeQDs for maximizing infrared absorption with narrower SiNW electrodes to ensure a high quality Ge/Si hetero-epitaxial connection. Importantly, these hetero GeQD/SiNW photodetectors can be deployed into predesigned locations for scalable device fabrication. The photodetectors demonstrate a responsivity of 1.5 mA W−1 and a photoconductive gain exceeding 102 to the communication wavelength signals, which are related to the beneficial type-II Ge/Si alignment, gradient Ge/Si epitaxial transition and a larger QD/NW diameter ratio. These results indicate a new approach to batch-fabricate and integrate GeQDs for ultra-compact Si-compatible photodetection and imaging applications.

Polarisation-independent liquid crystal lens array with additional dielectric films over self-aligned dual hole-patterned electrodes

ABSTRACT This study demonstrates a type of polarisation-independent liquid crystal (LC) lens arrays that are composed of two orthogonally homogeneous LC layers and self-aligned dual hole-patterned array electrodes. Additional dielectric optical adhesive films coated on patterned electrode surfaces are used to prevent the occurrence of disclination line. The completed LC lens array shows available minimum focal lengths of 12.82 and 13.35 mm at 7 Vrms with respect to individual aluminium (Al) and indium tin oxide (ITO) hole-patterned array electrodes. The LC lens array exhibits the characteristics of electrically tunable focuses, no disclination line, and polarisation-independence.

Fabrication of a high-density emitter array for electrospray thrusters using field emitter array process

To improve the thrust density of electrospray thrusters using ionic liquids as the propellant, we have fabricated a high-density emitter array utilizing the fabrication technique of a field emitter array (FEA) or gated nano electron sources. The density of the emitter array can reach approximately 4 million cm−2, which is four orders of magnitude higher than that of conventional electrospray thrusters. Performing deep reactive ion etching from the back side of the same structure as FEAs and removing their Ni cones by dilute nitric acid, we have successfully fabricated sub-μm scale capillary emitters with self-aligned extractor electrodes and ionic liquid reservoirs behind them. Current measurements were also conducted with EMI-BF4 and EMI-DCA as the propellant. Although the ion was not extracted with EMI-BF4 owing to its electrochemical reaction with the electrodes, both positive and negative ion currents were detected using EMI-DCA, where the current density exceeded that of conventional electrospray thrusters.

High-power laser-patterning graphene oxide: A new approach to making arbitrarily-shaped self-aligned electrodes

We demonstrate the fabrication of self-aligned laser-reduced graphene oxide patterns with a spatial resolution/laser spot size ratio of 1/10, lower than anything reported before using laser-reduction. Laser light modifies graphene oxide (GO) by removing the oxygen-containing groups turning GO into a more graphene-like nanomaterial. Our method is based on high laser power density used for the reduction of GO that results in ablation of the GO film. This enabled us to remove the laser spot illuminated area while inducing the selective graphene oxide reduction at the periphery of the laser spot achieving resistivity of 1.6·10−5 Ω m, as low as values previously reported for other rGO. Therefore, we can exploit laser-induced reduction at high laser power density to pattern GO films with conductive dimensions that are a fraction of the laser spot size. This innovative method is scalable, inexpensive, and straightforward, allowing conductive circuits on arbitrary, flexible, and transparent substrates for applications in lightweight electronics and wearables.

Biophysical phenotyping of single cells using a differential multiconstriction microfluidic device with self-aligned 3D electrodes

Precise measurement of mechanical and electrical properties of single cells can yield useful information on the physiological and pathological state of cells. In this work, we develop a differential multiconstriction microfluidic device with self-aligned 3D electrodes to simultaneously characterize the deformability, electrical impedance and relaxation index of single cells at a high throughput manner (>430 cell/min). Cells are pressure-driven to flow through a series of sequential microfluidic constrictions, during which deformability, electrical impedance and relaxation index of single cells are extracted simultaneously from impedance spectroscopy measurements. Mechanical and electrical phenotyping of untreated, Cytochalasin B treated and N-Ethylmaleimide treated MCF-7 breast cancer cells demonstrate the ability of our system to distinguish different cell populations purely based on these biophysical properties. In addition, we quantify the classification of different cell types using a back propagation neural network. The trained neural network yields the classification accuracy of 87.8% (electrical impedance), 70.1% (deformability), 42.7% (relaxation index) and 93.3% (combination of electrical impedance, deformability and relaxation index) with high sensitivity (93.3%) and specificity (93.3%) for the test group. Furthermore, we have demonstrated the cell classification of a cell mixture using the presented biophysical phenotyping technique with the trained neural network, which is in quantitative agreement with the flow cytometric analysis using fluorescent labels. The developed concurrent electrical and mechanical phenotyping provide great potential for high-throughput and label-free single cell analysis.

Laser printer patterned sacrificed layer for arbitrary design and scalable fabrication of the all-solid-state interdigitated in-planar hydrous ruthenium oxide flexible micro supercapacitors

We demonstrate the successful fabrication of all-solid-state flexible micro-supercapacitors (μSCs) on commercial polyethylene terephthalate (PET) sheet using a commercial laser printer. The interdigitated in-planar electrodes of the μSC are fabricated by commercial toner cartridge and their self-aligned stacking configurations are made by depositing Pt thin films and hydrous ruthenium oxide (RuO·xH2O, hRuO2) layers on the PET sheets using a post-low-temperature annealing process. The μSC cells can be combined in parallel or series by an arbitrary printing process operated on a personal computer, thereby avoiding unnecessary external connections and improving the capacitive behavior. The fabricated hRuO2-based μSCs exhibit excellent electrochemical-energy storage, along with ultrahigh volumetric power density (73460 mW cm−3) and energy density (24.9 mWh cm−3). This facile and simplified strategy constructs μSCs with tailored self-aligned electrodes of arbitrary design, excellent mechanical stability, and high flexibility. Therefore, it offers a high-productivity, low-cost, photolithography-free microfabrication of μSCs for microscale supercapacitor applications.

Voltage-Control Spintronics Memory With a Self-Aligned Heavy-Metal Electrode

Voltage-control spintronics memory (VoCSM) is a spintronics-based memory that uses the voltage-control-magnetic-anisotropy (VCMA) effect as a selection method and the spin-Hall effect as a write method, and is a candidate for future nonvolatile main memory because of its advantages such as high density, low energy consumption, and robustness against read disturbance. In this paper, we demonstrate VoCSM with a self-aligned heavy-metal electrode, which is the most desirable in mass production and can expect the lowest critical current ( $I_{C})$ due to its narrow electrode structure. We obtain the value of $I_{C}$ of around $400~\mu \text{A}$ for a magnetic tunnel junction of size $35~{\mathrm{ nm}} \times 245$ nm by using a self-aligned heavy-metal electrode. The value is lowered to one-third that of the conventional VoCSM structure and is comparable with that of spin-transfer torque writing. Moreover, $I_{C}$ decreased due to the VCMA effect and reached $260~\mu \text{A}$ for $\text{V}_{\mathrm {MTJ}} = -0.8$ V. We also found that the diffusion spin current from the lateral region, which is the concern matter of this structure, did not contribute very much to the switching due to the extremely short spin diffusion length of the heavy-metal electrode. It indicates that VoCSM with a self-aligned heavy-metal electrode may lead to the production of high-density and low-energy-consumption memory.

An electrostatic ion pump with nanostructured Si field emission electron source and Ti particle collectors for supporting an ultra-high vacuum in miniaturized atom interferometry systems

We report a field emission-based, magnetic-less ion pump architecture for helping maintain a high vacuum within a small chamber that is compatible with miniaturized cold-atom interferometry systems. A nanostructured silicon field emitter array, with each nano-sharp tip surrounded by a self-aligned proximal gate electrode, is used to generate a surplus of electrons that cause impact ionization of gas molecules. A two-stage cylindrical electron collector, made of titanium, is used to increase the travel distance of the electrons, augmenting the ionization probability; gas ionization is subsequently followed by gettering of the ions by a negatively charged, annular-shaped titanium electrode. A proof-of-concept pump prototype was characterized using a 25 cm3 stainless steel vacuum chamber backed up by an external turbomolecular pump, a diaphragm pump, and a standard ion pump. Pumping action was observed with the electrostatic pump operating alone after an initial rapid rise of the chamber pressure due to electron/ion scrubbing. In addition, running the electrostatic pump in combination with the standard ion pump results in a lower vacuum level compared to the vacuum level produced by the standard ion pump acting alone. A proposed reduced-order model accurately predicts the functional dependence of the pressure versus time data and provides a good estimate of the characteristic pumping time constant inferred from the experiments.

Electrostatic tuning in linear regime for 25.9-MHz micromechanical resonator with 120-nm capacitive gap

In this paper, electrostatic tuning in the linear regime for a high-frequency single-crystal silicon resonator is investigated. An electrostatic actuated I-shaped bulk acoustic resonator (IBAR) is fabricated in our experiment owing to its advantage of a large electrode area. The single-crystal silicon resonator is actuated by electrostatic force. The electrostatic force is improved by reducing the capacitive gap to 120 nm. The fabrication of the capacitive gap is based on the process for a high-aspect-ratio self-aligned stepped polysilicon electrode for a single-crystal silicon microstructure. The effective stiffness of an IBAR is softened by increasing the bias voltage, which is applied between the polysilicon electrodes and the resonant structure. For the tuning frequency in the linear regime, the ac actuation voltage is small to avoid electrical nonlinearity. The resonant frequency of the IBAR is measured to be 25.9 MHz in a vacuum. The temperature coefficient of frequency (TCF) of our device is approximately −14 ppm/ °C, which is attained by boron doping. When the resonator operates in the linear regime, frequency tuning can achieve 1833 ppm over 130 °C by changing the bias voltage from 2 to 23 V at room temperature. The temperature drift of the full operating range of a high-frequency resonator is compensated by electrostatic tuning for the first time.