Sub-micron Gap Research Articles

Polarization splitter-rotator on thin film lithium niobate based on multimode interference

Polarization splitter-rotators (PSRs) are the key elements to realize on-chip polarization manipulation. Current PSRs on thin film lithium niobate (TFLN) rely on sub-micron gaps to realize mode separation, which increases the difficulties of lithography and etching. In this paper, a PSR on TFLN based on multimode interference (MMI) is demonstrated. Mode division is achieved by an MMI-based mode demultiplexer. The minimum feature size of the PSR is 1.5 µm, which can be fabricated with low-priced i-line contact aligners. Experimental results show a polarization extinction ratio (PER) > 16 dB and an insertion loss (IL) < 1.0 dB are achieved in a wavelength range of 1530-1578 nm for TE-polarized light. And a PER > 10.0 dB and an IL <2.1 dB are achieved in a wavelength range of 1530-1569 nm for TM-polarized light. This PSR could find application in the low-cost fabrication of dual-polarization TFLN-integrated photonic devices.

Experimental Study of Electrical Breakdown in MEMS/NEMS Devices With Deep Submicron Gaps

Experimental Study of Electrical Breakdown in MEMS/NEMS Devices With Deep Submicron Gaps

Two-Dimensional Titanium Carbides (Ti3C2) Mxene-Based Patterned-Electrodes for High Capacity Micro-Supercapacitors

With the fast growth of wearable & miniaturized electronic devices, internet of things (IoT), and 5G telecommunications, the need for ultrathin, lightweight, and flexible energy storage devices has expanded significantly. Micro-supercapacitors (MSCs) with electrically isolated interdigitated electrodes, have attracted great interest as major power sources for micro-electronic devices, offering high power efficiency, superior rate capability, and excellent cycle durability. MXenes, two-dimensional (2D) metal carbides and nitrides, have been considered as a promising candidate for state-of-the-art energy storage applications, due to their intriguing combinational properties such as excellent electrical conductivity, large surface area, tunable surface chemistry, and pseudo-capacity. 1 In this work, we report titanium carbides (Ti3C2) MXene-based MSCs with sub-micron gap between electrodes exhibiting high areal capacitance and superior rate capability, realized by microfabrication method based on focused ion beam (FIB) and photolithography.2 We also demonstrate a facile and reliable process for scalable manufacturing of on-chip and flexible MSCs for wearable/portable and miniaturized microelectronics. The MXene MSCs fabricated utilizing the cutting-edge nanofabrication technology in an 8-inch wafer have shown an outstanding volumetric capacitance and stable cyclic performances, suggesting the developed manufacturing process can accelerate commercialization of MXene-based energy storage devices as power sources for next-generation wearable and miniaturized microelectronics.3 [References] 1 Energy Environ. Sci. 9 2847 (2016) 2 Nano Energy 81 (2021), 105616 3 Chem. Eng. J., 450, 138456 (2022)

Integrated sub-micron vacuum gaps in semiconductor devices

We present characterization results of integrated vacuum gaps in semiconductors and report the highest breakdown field of dielectric layers ever recorded within microfabricated semiconductor devices. Difficulties associated with the characterization of vacuum gaps in the presence of high electric fields could be overcome by using cylindrical capacitors with silicon electrodes that were manufactured with standard semiconductor technology. With this approach, breakdown fields of up to 6 × 109 V/m were achieved. The vacuum gaps of 175(5) nm were significantly smaller than the mean free path of electrons within the gap such that a breakdown due to avalanche discharge was avoided. As the voltage was increased, initially a field emission current was observed that followed a Fowler–Nordheim tunneling behavior. The tunneling current started to increase at voltages about four times greater as compared to equivalent dielectric layers of silicon oxide. At higher voltages, a mechanical breakdown occurred, where the pillars that formed the central electrode of the capacitor snapped due to electrostatic forces. We provide characteristics of thin vacuum layers, which could be useful for device design in micro- and nanoelectromechanical systems as well as semiconductor devices.

Thermophotovoltaic Energy Conversion in Far-to-Near-Field Transition Regime

Recent experimental studies on near-field thermophotovoltaic (TPV) energy conversion have mainly focused on enhancing performance via photon tunneling of evanescent waves. In the sub-micron gap, however, there exist peculiar phenomena caused by the interference of propagating waves, which is seldom observed due to the dramatic increase of the radiation by evanescent waves in full spectrum range. Here, we experimentally demonstrate the oscillatory nature of near-field TPV energy conversion in the far-to-near-field transition regime (250-2600 nm), where evanescent and propagating modes are comparable due to the selective spectral response by the PV cell. Noticeably, it was possible to produce the same amount of photocurrent at different vacuum gaps of 870 and 322 nm, which is 10\% larger than the far-field value. Considering the great challenges in maintaining nanoscale vacuum gap in practical devices, this study suggests an alternative approach to the design of a TPV system that will outperform conventional far-field counterparts.

Vertical cavity capacitive transducer.

The capacitive micromachined ultrasonic transducer (CMUT) has inherent advantages, such as larger bandwidth and monolithic integration capability with electronics, when compared to the piezoelectric transducer. The most significant shortcoming of the device is the trade-off between input and output sensitivities. Adequate receive sensitivity requires an electric field intensity on the order of 105 V/m, which can be achieved by sub-micron gap heights. However, a small gap limits the device stroke and, consequently, the maximum output pressure. This paper addresses this problem by proposing a CMUT with a vertical cavity. The membrane of the device has a piston part that is surrounded by the sidewalls of a vertical cylinder formed in the substrate. The fringing electric field pulls the piston in the vertical direction; hence, the gap height remains fixed, which alleviates the hard limit on device stroke. The performance of the proposed device is compared to that of the conventional CMUT by theoretical and analytical methods, and a micro-fabrication method is devised. Additionally, a millimeter-scale device has been manufactured and tested as a proof of concept.

Расчет эффективности удвоения частоты излучения субтерагерцового гиротрона за счет решеточной нелинейности в монокристаллической пластине InP

In this work, we calculated the efficiency of second harmonic generation for sub-THz gyrotron radiation in a single-crystal InP plate embedded in a conical metal waveguide. For the values of the losses at the doubled frequency (depending on the degree of purity of the InP crystal), the dimensions of the wafer are revealed that provide the optimal effective frequency conversion. Thus, the conversion factor f –>2f can reach 5 W/kW^2 when pumped at a frequency of 263 GHz and a loss of alpha_2f = 0.1 cm^-1. The possibility of a noticeable increase in the efficiency of frequency doubling with fine tuning of the phase matching due to the selection of the size of the submicron gap between the InP plate and the plates of the metal waveguide is shown. The results obtained can be used to solve applied problems in the field of dynamic nuclear polarization spectroscopy and nuclear magnetic resonance studies.

Micromechanical Switch-Based Zero-Power Chemical Detectors for Plant Health Monitoring

In this paper, for the first time we demonstrate zero-power volatile-organic-chemical (VOC) detectors based on micromechanical switches suitable for plant health monitoring. Differently from state-of-the-art active chemical sensors, the device presented here exploits a completely passive, chemically-sensitive switch based on a bimaterial micro-cantilever and a passive switch-based readout mechanism to detect VOCs exceeding a pre-determined concentration released by unhealthy plants. When exposed to target VOCs, the polymer/metal bimaterial beam bends downward and trigger the switch due to the stress induced from the absorption of chemicals in the polymer layer. Here we show experimental demonstrations of detecting toluene, hexenol (cis-3-Hexen-1-ol, a chemical released from plants under attack by pests) and ethanol, respectively, with our fabricated prototypes. The demonstrated high sensitivity to ethanol (~8 nm/ppm ) and hexenol (~3.3 nm/ppm ) was achieved by the optimization of device geometries showing a great promise of the proposed technology to ultimately achieve 10s ppm detection limit (with a sub-micron contact gap and voltage bias) which is required for the device operation in close proximity to a plant in an open environment. [2020-0190]

Quick and Sensitive Detection of Water Using Galvanic-Coupled Arrays with a Submicron Gap for the Advanced Prediction of Dew Condensation.

We have demonstrated a highly sensitive moisture sensor that can detect water molecules, in addition to water droplets, and therefore, can predict dew condensation with high accuracy and high speed before the formation of water droplets, showing a better performance than a commercial hygrometer. Additionally, the dependence of the output response from the sensor on factors, such as the cooling rate of the sensor’s surface and the vapor pressure in the chamber, that affect the performance of the moisture sensor has been clarified. The output response showed a clear dependence on the variation in cooling rate, as well as the vapor pressure. The higher the cooling rate and vapor pressure, the higher the output response. The output response showed a linear response to the change in the above-mentioned parameters. The higher sensitivity and accuracy of the moisture sensor, as a function of the physical parameters, such as cooling rates, vapor pressure, enables the sensor to perform in advanced detection applications. The sensor can be modified to the actual target regarding the surface nature and the heat capacity of the target object, making it more suitable for wide applications.

Oxide semiconductor thin-film transistors with nano-splitting and field-surrounding channels fabricated by subwavelength photolithography

Oxide semiconductors feature high tunability of carrier concentrations under the control of electric field. In thin film transistors (TFTs), applying dual gate has been reported to be efficient in enhancing the coupling between the gate field and the channel accumulation. In this work, we demonstrate nano-splitting and field-surrounding semiconducting channels (based on InGaZnO) in TFTs, which is fabricated by facile subwavelength photolithography. In such TFTs, semiconducting channels have 200 nm gaps parallel to the drain field, are wrapped by oxide insulators and thus the gate field. The devices show enhanced performance as compared with dual-gate and single gate TFTs, exhibiting higher drain current and steeper subthreshold swing. The maximum transconductance gm is 27.9% higher than dual gate TFT and 73.1% higher than single gate TFT. According to device simulation, the improvement of the wrapping insulators device correlates with the three-dimensional accumulation of carriers and increased gate electric field near the semiconductor-dielectric interface. These surrounded-channel effects become noticeable in the device with the gap distance less than 1 μm, with gate electric field squeezing in the submicron gaps. The proposed approach offers an alternative way to take advantage of the oxide semiconductors and their application in TFTs with related circuits.

Undamaged Measurement of the Sub-Micron Diaphragm and Gap by Tri-Beam Interference

A simple, high-accuracy and non-destructive method for the measurement of diaphragm thickness and microgap width based on modulated tri-beam interference is demonstrated. With this method, a theoretical estimation error less than 0.5% for a diaphragm thickness of ∼1 μm is achievable. Several fiber-tip air bubbles with different diaphragm thicknesses (6.25, 5.0, 2.5 and 1.25 μm) were fabricated to verify our proposed measurement method. Furthermore, an improved technique was introduced by immersing the measured object into a liquid environment to simplify a four-beam interference into tri-beam one. By applying this improved technique, the diaphragm thickness of a fabricated in-fiber rectangular air bubble is measured to be about 1.47 μm, and the averaged microgap width of a standard silica capillary is measured to be about 10.07 μm, giving a corresponding measurement error only 1.27% compared with actual scanning electron microscope (SEM) results.

Electronic Modulation of Near-Field Radiative Transfer in Graphene Field Effect Heterostructures.

Manipulating heat flow in a controllable and reversible manner is a topic of fundamental and practical interest. Numerous approaches to perform thermal switching have been reported, but they typically suffer from various limitations, for instance requiring mechanical modulation of a submicron gap spacing or only operating in a narrow temperature window. Here, we report the experimental modulation of radiative heat flow by electronic gating of a graphene field effect heterostructure without any moving elements. We measure a maximum heat flux modulation of 4 ± 3% and an absolute modulation depth of 24 ± 7 mW m-2 V-1 in samples with vacuum gap distances ranging from 1 to 3 μm. The active area in the samples through which heat is transferred is ∼1 cm2, indicating the scalable nature of these structures. A clear experimental path exists to realize switching ratios as large as 100%, laying the foundation for electronic control of near-field thermal radiation using 2D materials.

In-Plane High-Sensitivity Capacitive Accelerometer in a 3-D CMOS-Compatible Surface Micromachining Process

This paper proposes a novel design for a 3-D, high-sensitivity, single-axis lateral capacitive accelerometer. The accelerometer utilizes the entire area of the sensor for both sensing and the proof mass, which mitigates the tradeoffs often needed in conventional 2-D designs. The accelerometer structure is optimized to target the highest possible performance. Innovative Z-shaped supporting beams are introduced to limit the vertical displacement within the transducer’s submicron gap. The design was fabricated in a novel 3-D complementary metal–oxide–semiconductor-compatible surface micromachining process. This process allows the use of non-conductive materials with attractive mechanical properties to build capacitive microelectromechanical system (MEMS) devices. Polyimide is used as a core structural layer which is combined with other materials such as silicon nitride or silicon carbide to build the final sensors. The photolithography steps are limited to four, and the number of materials used is also limited to four, in order to keep the process feasible. The overall thermal budget is 300 °C, which enables above-IC MEMS integration. While the used materials provide good results, this process is not limited to them, and other materials can be used, if needed. The fabricated accelerometer measures <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$500 \times 500 \,\,\mu \text{m}^{2}$ </tex-math></inline-formula> and achieves 58 fF/g sensitivity in a ±4 g range in an open-loop system, yielding a competitive sensitivity per unit area. [2018-0125]

Submicron gap reduction of micro-resonator based on porous silica ridge waveguides manufactured by standard photolithographic process

In this paper, we demonstrate a new method to obtain a low gap between the access waveguide and the racetrack cavity of a micro-resonator based on porous silica material and using standard photolithographic process. The photolithographic process is first carried out on porous silicon layers to obtain porous silicon ridge waveguides that constitutes the racetrack micro-resonator. Then the full oxidation of the micro-resonator is performed. Because of the volume expansion of silicon that occurs during its full oxidation, the gap is reduced. Lower gaps and then lower coupling distances can be obtained using a low cost photolithographic process compared to e-beam technology. This original method proposed in this paper allows to improve the miniaturization of such porous silica micro-resonators which is of great interest for integrated optical biosensor application.

Novel fabrication of extremely high aspect ratio and straight nanogap and array nanogap electrodes

We reported a reproducible, simple and novel method for fabricating electrodes with high aspect ratio and highly straight nanometer size gap. The gap size could be controlled in a wide range between several nm up to several hundreds of nm, although in this work our individual gap size was 172 nm. In this method a single nanofiber was aligned as a sacrificial structure using a secondary electrical field and its diameter lowered through thermal treatment to obtain smallest gap size. Then, after deposition of blanket metal, nanofiber was removed obtaining nano scale gap. We predict the application of these simple fabricated gaps for diverse fields. The expensive, time consuming, none-reproducible and low accuracy fabrication methods can supersede by this cost effective, facile and fast method. It can provide mass production for precisely positioned nanoscale gaps, ignoring electrode and substrate materials. The electrodes hold the advantage of maximum straightness comparing to other similar nano fabrication methods. Fabricated gaps have large aspect ratio (in the order of 106). Also array gaps with average gap size of 400 nm were fabricated. Same size gaps could be fabricated in a row by this method. Different parameters in alignment were taken into account and studied. The process shows great potential for facile and batch fabrication of parallel submicron gaps for diverse applications including gas sensing, bio-sensing and nanofluidic systems.

Wafer‐level vacuum‐encapsulated silicon resonators with arc‐welded electrodes

This work presents wafer-level vacuum-encapsulated silicon resonators that utilise movable electrodes and arc welding in order to achieve deep sub-micron transduction gaps. The devices are fabricated using micro-electro-mechanical systems (MEMS) integrated design for inertial sensors (MIDIS) process, a commercial pure-play MEMS process, offered by Teledyne DALSA Semiconductor Inc.. The default minimum transduction gap in the MIDIS process is 1.5 μm. Here, the work introduces a technique to permanently reduce the transduction gap of the resonator using localised arc welding to a designed width of ∼200 nm. The prototype Lamé mode resonators are encapsulated in an ultra-clean 10 mTorr vacuum cavity that ensures long-term stability. The quality factor was measured to be 1.37 million at a resonance frequency of 6.89 MHz. With the narrower gap, the motional resistance of the resonators is reduced by a factor of ten times.

MEMS optical interferometry-based pressure sensor using elastomer nanosheet developed by dry transfer technique

We developed an elastomer-based Fabry–Perot interferometer with a submicron gap between a freestanding thin film and a substrate by a dry transfer technique. A newly developed elastomeric nanosheet using a polystyrene–polybutadiene–polystyrene triblock copolymer (SBS) provides a low Young’s modulus of 40 MPa, a large elastic strain of 38%, and high adhesiveness. A freestanding SBS nanosheet can be formed by a dry transfer technique without vacuum and high-temperature processes owing to the high adhesiveness of SBS nanosheets. With the pressure change, the freestanding nanosheet was found to deform with good adhesion between the dry transferred SBS and the substrate.

Ultrabroadband super-Planckian radiative heat transfer with artificial continuum cavity states in patterned hyperbolic metamaterials

Localized cavity resonances due to nanostructures at material surfaces can greatly enhance radiative heat transfer (RHT) between two closely placed bodies owing to stretching of cavity states in momentum space beyond light line. Based on such understanding, we numerically demonstrate the possibility of ultra-broadband super-Planckian RHT between two plates patterned with trapezoidal-shaped hyperbolic metamaterial (HMM) arrays. The phenomenon is rooted not only in HMM's high effective index for creating sub-wavelength resonators, but also its extremely anisotropic isofrequency contour. The two properties enable one to create photonic bands with a high spectral density to populate a desired thermal radiation window. At sub-micron gap sizes between such two plates, the artificial continuum states extend outside light cone, tremendously increasing overall RHT. Our study reveals that structured HMM offers unprecedented potential in achieving a controllable super-Planckian radiative heat transfer for thermal management at nanoscale.

Resonant pitch and roll silicon gyroscopes with sub-micron-gap slanted electrodes: Breaking the barrier toward high-performance monolithic inertial measurement units

This paper presents the design, fabrication, and characterization of a novel high quality factor (Q) resonant pitch/roll gyroscope implemented in a 40 μm (100) silicon-on-insulator (SOI) substrate without using the deep reactive-ion etching (DRIE) process. The featured silicon gyroscope has a mode-matched operating frequency of 200 kHz and is the first out-of-plane pitch/roll gyroscope with electrostatic quadrature tuning capability to fully compensate for fabrication non-idealities and variation in SOI thickness. The quadrature tuning is enabled by slanted electrodes with sub-micron capacitive gaps along the (111) plane created by an anisotropic wet etching. The quadrature cancellation enables a 20-fold improvement in the scale factor for a typical fabricated device. Noise measurement of quadrature-cancelled mode-matched devices shows an angle random walk (ARW) of 0.63° √h−1 and a bias instability of 37.7° h−1, partially limited by the noise of the interface electronics. The elimination of silicon DRIE in the anisotropically wet-etched gyroscope improves the gyroscope robustness against the process variation and reduces the fabrication costs. The use of a slanted electrode for quadrature tuning demonstrates an effective path to reach high-performance in future pitch and roll gyroscope designs for the implementation of single-chip high-precision inertial measurement units (IMUs).

Dynamics of hydrogenated amorphous silicon flexural resonators for enhanced performance

Hydrogenated amorphous silicon thin-film flexural resonators with sub-micron actuation gaps are fabricated by surface micromachining on glass substrates. Experimentally, the resonators are electrostatically actuated and their motion is optically detected. Three different configurations for the electrostatic excitation force are used to study the dynamics of the resonators. In the first case, a dc voltage (Vdc) is added to an ac voltage with variable excitation frequency (Vac(ω)) and harmonic, superharmonic, and subharmonic resonances of different orders are observed. The second case consists on mixing the dc voltage (Vdc) with an ac voltage applied at a fixed frequency of twice the natural frequency of the resonator (V(2ω0)). High-amplitude parametric resonance is excited at the natural frequency of the system, ω0. This configuration allows a separation between the frequencies of the excitation and the mechanical motion. Finally, in the third case, the dc voltage (Vdc) is combined with both ac voltages, Vac(ω) and V(2ω0), and parametric resonance is excited and emerges from the fundamental harmonic resonance peak. The single-degree-of-freedom equation of motion is modeled and discussed for each case. The nonlinearity inherent to the electrostatic force is responsible for modulating the spring constant of the system at different frequencies, giving rise to parametric resonance. These equations of motion are simulated in the time and frequency domains, providing a consistent explanation of the experimentally observed phenomena. A wide variety of possible resonance modes with different characteristics can be used advantageously in MEMS device design.