Single Lithography Step Research Articles

Plasmonics-Based Multifunctional Electrodes for Low-Power-Consumption Compact Color-Image Sensors.

High pixel density, efficient color splitting, a compact structure, superior quantum efficiency, and low power consumption are all important features for contemporary color-image sensors. In this study, we developed a surface plasmonics-based color-image sensor displaying a high photoelectric response, a microlens-free structure, and a zero-bias working voltage. Our compact sensor comprised only (i) a multifunctional electrode based on a single-layer structured aluminum (Al) film and (ii) an underlying silicon (Si) substrate. This approach significantly simplifies the device structure and fabrication processes; for example, the red, green, and blue color pixels can be prepared simultaneously in a single lithography step. Moreover, such Schottky-based plasmonic electrodes perform multiple functions, including color splitting, optical-to-electrical signal conversion, and photogenerated carrier collection for color-image detection. Our multifunctional, electrode-based device could also avoid the interference phenomenon that degrades the color-splitting spectra found in conventional color-image sensors. Furthermore, the device took advantage of the near-field surface plasmonic effect around the Al-Si junction to enhance the optical absorption of Si, resulting in a significant photoelectric current output even under low-light surroundings and zero bias voltage. These plasmonic Schottky-based color-image devices could convert a photocurrent directly into a photovoltage and provided sufficient voltage output for color-image detection even under a light intensity of only several femtowatts per square micrometer. Unlike conventional color image devices, using voltage as the output signal decreases the area of the periphery read-out circuit because it does not require a current-to-voltage conversion capacitor or its related circuit. Therefore, this strategy has great potential for direct integration with complementary metal-oxide-semiconductor (CMOS)-compatible circuit design, increasing the pixel density of imaging sensors developed using mature Si-based technology.

Comparison of Single‐Step and Two‐Step EBL T‐Gates Fabrication Techniques for InP‐Based HEMT

T-Gate fabrication processes for InP-based High electron mobility transistors (HEMTs) are described using PMMA/Al/UVIII. The single-step and two-step Electron beam lithography (EBL) methods are proposed contrastively without dielectric support layer. The opti- mal gate-foot length is 196nm for 50nm geometry path by single-step EBL technique. Since the gate-foot and gate-head are defined independently, the two-step EBL process minimizes forward scattering and enables smaller gate-foot length, which improves to be 141nm for 50nm geometry path and also 88nm for 30nm geometry path. Both EBL methods have been incorporated into InP- based HEMTs fabrication. With the gate-foot length de- creases from 196nm to 141nm, the current-gain cutoff fre- quency (fT) is improved from 125GHz to 164GHz, and also the maximum oscillation frequency (fmax )i ncreases from 305GHz to 375GHz.

Zero-bias microwave detectors based on array of nanorectifiers coupled with a dipole antenna

We report on zero-bias microwave detection using a large array of unipolar nanodevices, known as the self-switching diodes (SSDs). The large array was realized in a single lithography step without the need of interconnection layers, hence allowing for a simple and low-cost fabrication process. The SSD array was coupled with a narrowband dipole antenna with a resonant frequency of 890MHz, to form a simple rectenna (rectifying antenna). The extrinsic voltage responsivity and noise-equivalent-power (NEP) of the rectenna were ∼70V/W and ∼0.18nW/Hz1/2, respectively, measured in the far-field region at unbiased condition. Nevertheless, the estimated intrinsic voltage responsivity can achieve up to ∼5kV/W with NEP of ∼2.6pW/Hz1/2.

Filter-free, junctionless structures for color sensing.

A simple structure, efficient color splitting, sufficient output of electrical signals, and low power consumption are the important characteristics of contemporary devices for color sensing. In this study, we developed filter-free, junctionless structures that exhibited a superior photo-thermo-electrical response under a low bias voltage and a short response time in milliseconds. Although our compact sensor had a simple single-layer trench-like aluminum (Al) structure, it could perform multiple functions, including light harvesting, color-selective absorption, photo-thermo-electrical transformation, and the ability to collect photoinduced differences in electrical signals. This device exploited near-field surface plasmon resonance and cavity effects to enhance the intensity of the electric field and the color-selective absorption, ultimately resulting in significant current signals in its structured Al film. This strategy significantly simplifies not only the components of the color sensor but also its fabrication; for example, red, green, and blue color detection devices could be prepared simultaneously through a single lithography, etching, and deposition step. With its ability to provide functional filter-free, junctionless structures, this strategy has great potential for the production of devices that operate on different kinds of substrates, thereby bridging various applications of color sensing technologies.

厚膜ネガレジスト用三次元リソグラフィシミュレータの開発

Three-dimensional (3D) photoresist structures fabricated by an optical lithography have been widely used in application fields such as microfluidics, micro total analysis systems (μ-TAS), or biosensors. To date, various 3D lithography methods have been proposed. Here we study 3D thick film photolithography technique using projection lithography system for fabricating complex microstructures used in microfluidics or bio-engineering tools. In this lithography method, prediction of 3D structure profile to be fabricated which depends on the mask pattern, exposure dose and the focus depth is difficult by the existing simulation models. So, this paper reports on the simulation method for single-step negative tone thick film projection lithography. Validity of the proposed simulation approach were confirmed quantitatively by comparing with experimental results and simulated results.

Optical nonreciprocal transmission in an asymmetric silicon photonic crystal structure

An optical nonreciprocal transmission (ONT) is realized by employing the nonlinear effects in a compact asymmetric direct-coupled nanocavity-waveguide silicon photonic crystal structure with a high loaded quality factor (QL) of 42 360 and large extinction ratio exceeding 30 dB. Applying a single step lithography and successive etching, the device can realize the ONT in an individual nanocavity, alleviating the requirement to accurately control the resonance of the cavities. A maximum nonreciprocal transmission ratio of 21.1 dB as well as a working bandwidth of 280 pm in the telecommunication band are obtained at a low input power of 76.7 μW. The calculated results by employing a nonlinear coupled-mode model are in good agreement with the experiment.

Soft-mask fabrication of gallium arsenide nanomembranes for integrated quantum photonics

We report on the fabrication of quantum photonic integrated circuits based on suspended GaAs membranes. The fabrication process consists of a single lithographic step followed by inductively coupled-plasma dry etching through an electron-beam-resist mask and wet etching of a sacrificial layer. This method does not require depositing, etching, and stripping a hard mask, greatly reducing fabrication time and costs, while at the same time yielding devices of excellent structural quality. We discuss in detail the procedures for cleaning the resist residues caused by the plasma etching and present a statistical analysis of the etched feature size after each fabrication step.

Sensitivity engineering in direct contact palladium-gold nano-sandwich hydrogen sensors [Invited

In recent years, plasmonic hydrogen sensing schemes using complex hybrid Pd@Au nanostructures have attracted significant attention. However, so far, most studies have focused on investigating the sensing performance of nanosensor geometries where the constituent materials are laterally coupled. In contrast to such planar hybrid systems, which often require complex multi-step fabrication approaches, sensing devices where the materials are stacked directly on top of each other can be fabricated in a single lithography step, enabling straightforward high-throughput processing. Here, we demonstrate a novel hydrogen sensing scheme which incorporates complex hybrid plasmonic nanostructures consisting of stacked gold and palladium nanodisks. In particular, we study the influence of stacking order and geometry, experimentally and numerically, to find an optimal arrangement for a hydrogen sensor device. With an optimized sensing geometry – a stack of gold as lower and palladium as upper disk – we obtain spectral shifts as large as 30 nm at 4 vol.% H2, which is a strong improvement compared to previous indirect sensing designs. Our samples yield large absorption and scattering signals and are fabricated by low-cost hole-mask colloidal lithography and therefore yield sample sizes over areas of 1 cm2.

Angled multimode interferometer for bidirectional wavelength division (de)multiplexing.

We have demonstrated a bidirectional wavelength division (de)multiplexer (WDM) on the silicon-on-insulator platform using two 4-channel angled multimode interferometers (AMMIs) sharing the same multimode interference waveguide. An excellent match of the peak transmission wavelength of each channel between the two AMMIs was achieved. The input and output access waveguides were arranged in a configuration such that the propagation of light of one AMMI in the multimode interference waveguide suffered minimal perturbation by the input and output waveguides of the other AMMI. This type of device is ideal for the WDM system for datacom or telecom applications, e.g. an integrated optical transceiver, where the transmission wavelengths are required to match with the receiving wavelengths. The device also benefits from simple fabrication (as only a single lithography and etching step is required), improved convenience for the transceiver layout design, a reduction in tuning power and circuitry and efficient use of layout space. A low insertion loss of 3–4 dB, and low crosstalk of −15 to −20 dB, was achieved.

Geometrical optimization of microstripe arrays for microbead magnetophoresis.

Manipulation of magnetic beads plays an increasingly important role in molecular diagnostics. Magnetophoresis is a promising technique for selective transportation of magnetic beads in lab-on-a-chip systems. We investigate periodic arrays of exchange-biased permalloy microstripes fabricated using a single lithography step. Magnetic beads can be continuously moved across such arrays by combining the spatially periodic magnetic field from microstripes with a rotating external magnetic field. By measuring and modeling the magnetophoresis properties of thirteen different stripe designs, we study the effect of the stripe geometry on the magnetophoretic transport properties of the magnetic microbeads between the stripes. We show that a symmetric geometry with equal width of and spacing between the microstripes facilitates faster transportation and that the optimal period of the periodic stripe array is approximately three times the height of the bead center over the microstripes.

Terahertz emission based on large‐area photoconductive emitters illuminated via beam interference

The use of beam interference in combination with a large-area photoconductive emitter for the generation of pulsed terahertz (THz) radiation is presented. An interference pattern with a period twice that of the THz emitter is generated with a transmission phase grating, placed directly in front of the photoconductive emitter. This way, efficient THz generation is achieved with a single metallisation layer and a single lithography step in the fabrication technology of the THz emitter.

In Situ Regeneration of Si-based ARROW-B Surface Plasmon Resonance Biosensors.

Si-based antiresonant reflecting optical waveguide type B (ARROW-B) surface plasmon resonance (SPR) biosensors allow label-free high-sensitivity detection of biomolecular interactions in real time. The ARROW-B waveguide, which has a thick guiding layer, provides efficient coupling with a single-mode fiber. The Si-based ARROW-B SPR biosensors were fabricated by using the standard semiconductor fabrication processes with a single-step lithography. A fluid flow system was designed to transport samples or analytes. The waveguide consists of propagation and SPR sensing regions. The propagation regions in the front and rear of the SPR sensing region have a symmetric cladding structure to isolate them from environmental changes. A high-index O-ring is used to seal the liquid flow channel. The intensity interrogation method was used to characterize the sensors. The sensitivity of the biosensors was 3.0 × 103 µW/RIU (refractive index unit) with a resolution of 6.2 × 10−5 RIU. An in situ regeneration process was designed to make the sensors reusable and eliminate re-alignment of the optical measurement system. The regeneration was realized using ammonia-hydrogen peroxide mixture solutions to remove molecules bound on the sensor surface, such as self-assembled 11-mercapto-1undecanoic acid and bovine serum albumin. SPR was used to monitor the regeneration processes. The experimental results show that the sensing response did not significantly change after the sensor was reused more than 10 times. In situ regenerations of the sensors were achieved.

RF characterization of unipolar nanorectifiers at zero bias

A novel unipolar nanodevice, the self-switching diode (SSD), has recently shown promising properties as an ultrahigh-speed detector by exploiting its nonlinear diode-like behavior and its inherently low parasitic capacitance. In this report, two large SSD arrays, each has ∼1000 SSDs connected in parallel within the fingers of an inter-digital structure, were characterized based on coplanar measurement in the microwave region using a network analyzer. These SSD arrays were realized in a single lithography step, hence allowing for a simple, fast and low-cost fabrication process. Analysis on rectification properties of SSD is presented and compared with the measurement results. Despite of the large impedance mismatch between SSD array and measurement system, the device was able to rectify microwave signal with extrinsic responsivity achieved up to 400 V/W at unbiased condition. The estimated noise-equivalent power was ∼41 pW/Hz1/2, comparable to other diode-based rectifiers at high speed.

Degeneracy Breaking of Wood’s Anomaly for Enhanced Refractive Index Sensing

We introduce an ultrasensitive detection technique for refractive index (RI) sensing based on an array of nanometer scale slot-antennas milled in a thin gold layer using a single lithographic step. Our experimental figures of merit (FOMs) of 140–210 in the telecom wavelength range approach the fundamental limit for standard propagating SPR sensors (∼250).The underlying mechanism enabling this is the combination of a narrowband resonance of the slot-antennas with degeneracy breaking of Wood’s anomaly under slightly nonperpendicular illumination. In addition, we explore the sensitivity of the device to the depth of the analyte layer. This concept can be easily tuned to any desired wavelength and RI range by modifying the slot dimensions and the array spacing, thus rendering it highly useful for numerous sensing applications.

Highly confined gap surface plasmon modes in metal strip–gap–film configurations

Plasmonic waveguides supporting gap surface plasmon (GSP) modes localized in a deep subwavelength dielectric gap between a metal strip and underlay film are investigated numerically and experimentally at telecom wavelengths. Particularly, we demonstrate that GSP waveguides with only the top metal layer structured to form the waveguide, including nanoantenna couplers optimized for normal incident Gaussian beam excitation, can be fabricated in a single lithography step leading to 15% experimental coupling efficiency of GSP modes with ∼10 μm propagation length when the gap size is 10 times smaller than the free-space wavelength.

Optical Metasurfaces and Prospect of Their Applications Including Fiber Optics

Metasurfaces have emerged in the recent years as a platform to design subwavelength-thick optical components (“flat optics”), which can be used to implement any optical function (beam deflection, focusing, waveplates, etc). These flat optical components can be fabricated using a single lithographic step. The approach is particularly suited for patterning nonconventional substrates, such as semiconductor laser facets and optical fiber facets. In this paper, we review recent applications of metasurfaces to flat optical devices, including their use in semiconductor lasers and fiber optics. Metasurfaces make it possible to design all properties of light (amplitude, phase, and polarization), which enable us to build a large variety of flat optical components, including planar lenses, quarter-wave plates, optical vortex plates, holograms for vector beam generation, and ultrathin perfect absorbers and color coatings. We also review flat collimating lenses integrated on the facets of mid-infrared and far-infrared (terahertz) quantum cascade lasers, and novel techniques to create large arrays of nanostructures on fiber facets.

On-chip grating coupler array on the SOI platform for fan-in/fan-out of MCFs with low insertion loss and crosstalk.

We report the design and fabrication of a compact multi-core fiber fan-in/fan-out using a grating coupler array on the SOI platform. The grating couplers are fully-etched, enabling the whole circuit to be fabricated in a single lithography and etching step. Thanks to the apodized design for the grating couplers and the introduction of an aluminum reflective mirror, a highest coupling efficiency of -3.8 dB with 3 dB coupling bandwidth of 48 nm and 1.5 dB bandwidth covering the whole C band, together with crosstalk lower than -32 dB are demonstrated.

Seeded growth of highly crystalline molybdenum disulphide monolayers at controlled locations

Monolayer transition metal dichalcogenides are materials with an atomic structure complementary to graphene but diverse properties, including direct energy bandgaps, which makes them intriguing candidates for optoelectronic devices. Various approaches have been demonstrated for the growth of molybdenum disulphide (MoS2) on insulating substrates, but to date, growth of isolated crystalline flakes has been demonstrated at random locations only. Here we use patterned seeds of molybdenum source material to grow flakes of MoS2 at predetermined locations with micrometre-scale resolution. MoS2 flakes are predominantly monolayers with high material quality, as confirmed by atomic force microscopy, transmission electron microscopy and Raman and photoluminescence spectroscopy. As the monolayer flakes are isolated at predetermined locations, transistor fabrication requires only a single lithographic step. Device measurements exhibit carrier mobility and on/off ratio that exceed 10 cm(2) V(-1) s(-1) and 10(6), respectively. The technique provides a path for in-depth physical analysis of monolayer MoS2 and fabrication of MoS2-based integrated circuits.

Simple and low cost integration of highly conductive three-dimensional electrodes in microfluidic devices.

This work presents a fast, simple, and cost-effective technique for fabricating and integrating highly conductive 3D microelectrodes into microfluidic devices. The 3D electrodes are made of low cost, commercially available conductive adhesive and carbon powder. The device can be fabricated by a single-step soft lithography and controllable injections of a conductive composite into microchannels. Functioning of the microfluidic device with 3D electrodes was demonstrated through DEP particle switching as an example for a wide range of microfluidic applications.

MEMS-Based Conjugate Surfaces Flexure Hinge

This paper presents a new concept flexure hinge for MEMS applications and reveals how to design, construct, and experimentally test. This hinge combines a curved beam, as a flexible element, and a pair of conjugate surfaces, whose contact depends on load conditions. The geometry is conceived in such a way that minimum stress conditions are maintained within the flexible beam. A comparison of the new design with the other kind of revolute and flexible joints is presented. Then, the static behavior of the hinge is analyzed by means of a theoretical approach, based on continuum mechanics, and the results are compared to those obtained by means of finite element analysis (FEA) simulation. A silicon hinge prototype is also presented and the construction process, based on single step lithography and reactive ion etching (RIE) technology, is discussed. Finally, a crucial in–SEM experiment is performed and the experimental results are interpreted through the theoretical models.