Two-step Electron-beam Lithography Research Articles

Hybrid Plasmonic Nanostructures for Enhanced Single-Molecule Detection Sensitivity.

Biosensing applications based on fluorescence detection often require single-molecule sensitivity in the presence of strong background signals. Plasmonic nanoantennas are particularly suitable for these tasks, as they can confine and enhance light in volumes far below the diffraction limit. The recently introduced antenna-in-box (AiB) platforms achieved high single-molecule detection sensitivity at high fluorophore concentrations by placing gold nanoantennas in a gold aperture. However, hybrid AiB platforms with alternative aperture materials such as aluminum promise superior performance by providing better background screening. Here, we report on the fabrication and optical characterization of hybrid AiBs made of gold and aluminum for enhanced single-molecule detection sensitivity. We computationally optimize the optical properties of AiBs by controlling their geometry and materials and find that hybrid nanostructures not only improve signal-to-background ratios but also provide additional excitation intensity and fluorescence enhancements. We further establish a two-step electron beam lithography process to fabricate hybrid material AiB arrays with high reproducibility and experimentally validate the higher excitation and emission enhancements of the hybrid nanostructures as compared to their gold counterpart. We foresee that biosensors based on hybrid AiBs will provide improved sensitivity beyond the capabilities of current nanophotonic sensors for a plethora of biosensing applications ranging from multicolor fluorescence detection to label-free vibrational spectroscopy.

Broadband optical Ta2O5 antennas for directional emission of light.

Highly directive antennas with the ability of shaping radiation patterns in desired directions are essential for efficient on-chip optical communication with reduced cross talk. In this paper, we design and optimize three distinct broadband traveling-wave tantalum pentoxide antennas exhibiting highly directional characteristics. Our antennas contain a director and reflector deposited on a glass substrate, which are excited by a dipole emitter placed in the feed gap between the two elements. Full-wave simulations in conjunction with global optimization provide structures with an enhanced linear directivity as high as 119 radiating in the substrate. The high directivity is a result of the interplay between two dominant TE modes and the leaky modes present in the antenna director. Furthermore, these low-loss dielectric antennas exhibit a near-unity radiation efficiency at the operational wavelength of 780 nm and maintain a broad bandwidth. Our numerical results are in good agreement with experimental measurements from the optimized antennas fabricated using a two-step electron-beam lithography, revealing the highly directive nature of our structures. We envision that our antenna designs can be conveniently adapted to other dielectric materials and prove instrumental for inter-chip optical communications and other on-chip applications.

AlGaN/GaN-Based Laterally Gated High-Electron-Mobility Transistors With Optimized Linearity

In this work, highly linear AlGaN/GaN laterally gated (or buried gate) high-electron-mobility transistors (HEMTs) are reported. The effect of gate dimensions on source-access resistance and the linearity of laterally gated devices are investigated experimentally in detail for the first time. Transistors with different gate dimensions and conventional planar devices are fabricated using two-step electron beam lithography (EBL). Current–voltage, source-access resistance, small-signal, and two-tone measurements are performed to evaluate the linearity of devices. Contrary to conventional planar HEMTs, the intrinsic transconductance of laterally gated devices monotonically increases with increasing gate voltage, showing a similar behavior as junction field-effect transistors (FETs). The source-access resistance shows a polynomial increase with the drain current, which can be reduced by decreasing the filling ratio of the buried gates. Through the optimization of these two competing factors, i.e., intrinsic transconductance and the source-access resistance, flat transconductance with high linearity is achieved experimentally. The laterally gated structure shows flat transconductance and small-signal power gain over a larger span of gate voltage that is 2.5 times higher than a planar device. Moreover, 6.9-dB improvement in output intercept point (OIP3)/ ${P}_{\text {DC}}$ is achieved. This approach can be used to improve the linearity of AlGaN/GaN HEMTs at the device level.

High performance 33.7 GHz surface acoustic wave nanotransducers based on AlScN/diamond/Si layered structures

Surface acoustic wave (SAW) devices are essential devices for communication and sensing, but usually have an operation frequency limit well below 20 GHz due to the constraints of material properties and fabrication capability. By using an AlScN/diamond layered structure with a high electromechanical coupling coefficient K2 and our proposed two-step exposure electron beam lithography (EBL) process for ultra-fine patterns, we have fabricated SAW devices with resonant frequency up to 33.7 GHz in the Ka-band, the highest one ever reported for SAW devices electrically excited by interdigital transducers (IDTs). Combined with finite element analysis, we identified that series resonances are fundamental and high order Rayleigh modes, and K2 are in the range of 1.21%–2.32%, 200% higher compared to those of traditional AlN/diamond-based SAW devices. The high order modes become stronger and dominant, particularly suitable for the development of ultrahigh frequency SAW devices and applications. In addition, the proposed EBL process showed its superb capability to make ultra-fine IDTs down to the nano-scale with excellent smooth edges and uniform patterns, suitable for ultrahigh frequency SAW development.

Disorder-Enabled Pure Chirality in Bilayer Plasmonic Metasurfaces

We experimentally study the effect of rotational disorder at the unit-cell level on the optical response of chiral bilayer plasmonic metasurfaces consisting of asymmetric dimers of gold nanoantennas. The structures are fabricated based on a two-step electron-beam lithography process in combination with a precision alignment procedure. For a comprehensive characterization of the chiral optical response of the fabricated bilayer metasurfaces, we combine two different measurement strategies, namely, direct measurement of the circular transmission using circularly polarized incident light and measurement of the Jones matrix using interferometric spectroscopy with linearly polarized incident light. This allows us both to do a comprehensive analysis of the polarization dependent properties and to retrieve the effective polarization eigenstates of the metasurface in the ordered and disordered regime directly from experimental data. Our results demonstrate that rotational disorder provides a route toward chiral plasmonic metasurfaces with orthogonal, purely circular eigenstates, leading to circular dichroism and optical activity without linear birefringence. Our experimental findings are additionally complemented and verified by numerical simulations for the ordered case.

Multipolar Coupling in Hybrid Metal–Dielectric Metasurfaces

We study functional hybrid metasurfaces consisting of metal–dielectric nanoantennas that direct light from an incident plane wave or from localized light sources into a preferential direction. The directionality is obtained by carefully balancing the multipolar contributions to the scattering response from the constituents of the metasurface. The hybrid nanoantennas are composed of a plasmonic gold nanorod acting as a feed element and a silicon nanodisk acting as a director element. In order to experimentally realize this design, we have developed a two-step electron-beam lithography process in combination with a precision alignment step. The optical response of the fabricated sample is measured and reveals distinct signatures of coupling between the plasmonic and the dielectric nanoantenna elements that ultimately leads to unidirectional radiation of light.

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.

Advanced in-situ electron-beam lithography for deterministic nanophotonic device processing.

We report on an advanced in-situ electron-beam lithography technique based on high-resolution cathodoluminescence (CL) spectroscopy at low temperatures. The technique has been developed for the deterministic fabrication and quantitative evaluation of nanophotonic structures. It is of particular interest for the realization and optimization of non-classical light sources which require the pre-selection of single quantum dots (QDs) with very specific emission features. The two-step electron-beam lithography process comprises (a) the detailed optical study and selection of target QDs by means of CL-spectroscopy and (b) the precise retrieval of the locations and integration of target QDs into lithographically defined nanostructures. Our technology platform allows for a detailed pre-process determination of important optical and quantum optical properties of the QDs, such as the emission energies of excitonic complexes, the excitonic fine-structure splitting, the carrier dynamics, and the quantum nature of emission. In addition, it enables a direct and precise comparison of the optical properties of a single QD before and after integration which is very beneficial for the quantitative evaluation of cavity-enhanced quantum devices.

Hybrid nanoantennas for directional emission enhancement

Plasmonic and dielectric nanoparticles offer complementary strengths regarding their use as optical antenna elements. While plasmonic nanoparticles are well-known to provide strong decay rate enhancement for localized emitters, all-dielectric nanoparticles can enable high directivity combined with low losses. Here, we suggest a hybrid metal-dielectric nanoantenna consisting of a gold nanorod and a silicon nanodisk, which combines all these advantages. Our numerical analysis reveals a giant enhancement of directional emission together with simultaneously high radiation efficiency (exceeding 70%). The suggested hybrid nanoantenna has a subwavelength footprint, and all parameters and materials are chosen to be compatible with fabrication by two-step electron-beam lithography.

Tandem array of nanoelectronic readers embedded coplanar to a fluidic nanochannel for correlated single biopolymer analysis.

We have developed a two-step electron-beam lithography process to fabricate a tandem array of three pairs of tip-like gold nanoelectronic detectors with electrode gap size as small as 9 nm, embedded in a coplanar fashion to 60 nm deep, 100 nm wide, and up to 150 μm long nanochannels coupled to a world-micro-nanofluidic interface for easy sample introduction. Experimental tests with a sealed device using DNA-protein complexes demonstrate the coplanarity of the nanoelectrodes to the nanochannel surface. Further, this device could improve transverse current detection by correlated time-of-flight measurements of translocating samples, and serve as an autocalibrated velocimeter and nanoscale tandem Coulter counters for single molecule analysis of heterogeneous samples.

Femtoliter Droplet Handling in Nanofluidic Channels: A Laplace Nanovalve

Analytical technologies of ultrasmall volume liquid, in particular femtoliter to attoliter liquid, is essential for single-cell and single-molecule analysis, which is becoming highly important in biology and medical diagnosis. Nanofluidic chips will be a powerful tool to realize chemical processes for such a small volume sample. However, a technical challenge exists in fluidic control, which is femtoliter to attoliter liquid generation in air and handling for further chemical analysis. Integrating mechanical valves fabricated by MEMS (microelectric mechanical systems) technology into nanofluidic channels is difficult. Here, we propose a nonmechanical valve, which is a Laplace nanovalve. For this purpose, a nanopillar array was embedded in a nanochannel using a two-step electron beam lithography and dry-etching process. The nanostructure allowed precise wettability patterning with a resolution below 100 nm, which was difficult by photochemical wettability patterning due to the optical diffraction. The basic principle of the Laplace nanovalve was verified, and a 1.7 fL droplet (water in air) was successfully generated and handled for the first time.

Controlled addressing of quantum dots by nanowire plasmons

We demonstrate optical near field coupling of small quantum dot (QD) ensembles and surface plasmons propagating along a silver nanowire. The nanowire fabrication and the aligned QD deposition close to one nanowire end rely on a two-step electron beam lithography procedure. We observe both the addressing of QDs by plasmons and the excitation of plasmonic nanowire modes by QDs. We use the fluorescence signals to quantify the QD/plasmon coupling and show that part of the plasmon-induced QD fluorescence couples back to plasmonic modes.

Back Cover: Plasmonic antennas, positioning, and coupling of individual quantum systems (Phys. Status Solidi B 4/2012)

Daniel Dregely et al. (see their Feature Article on pp. 666–677) investigated plasmonic Yagi-Uda antennas and measured experimentally the modes supported by the antenna geometry by means of optical near-field microscopy technique. By expanding the single antenna to a two-dimensional antenna array with the antenna axes pointing out of the substrate plane, superior directive properties were obtained compared to the single antenna. The authors present three different methods to couple quantum systems to plasmonic antennas. Two-step electron beam lithography allows for reliable positioning of core-shell quantum dots and nitrogen-vacancy centers in nanodiamonds with a sub-10 nm accuracy. The hybrid system of directive plasmonic antennas and quantum emitters can serve as an efficient single photon source. It is suitable for high-speed information transfer at optical frequencies on the nanoscale for future applications.

Development of magnetic nanoactuator systems

This paper presents the development of mechanically active arrays, which consist of free-standing Ti beam cantilevers with an integrated NiFe cube with critical dimensions of 100nm allowing for control by an external magnetic field gradient. A process flow is investigated based on two-step electron-beam lithography and reactive ion etching combined with magnetron sputtering of the functional magnetic nanostructures. A sacrificial layer technology is developed for dry etching of Ti anisotropically and Si isotropically at the same time. The saturation magnetization of the deposited NiFe is 0.78T giving rise of magneto-static forces in order of pN. AFM and MFM measurements confirm the size and position of the magnetic cubes.

Patterning concept for sculptured nanostructures with arbitrary periods

An approach to deposit sculptured nanostructures with arbitrary interstructure spacing is presented. Based on a combination of glancing angle deposition with a preceding two-step electron beam lithography substrate patterning, the concept allows the deposition of nanostructures on artificial seeds at any predetermined place on the substrate. Due to the glancing angle deposition principle, with the help of an appropriate substrate rotation during deposition, those structures can additionally be shaped into nearly arbitrary morphologies. To demonstrate the feasibility of the proposed method, separated sculptured nanostructures of silicon with interstructure spacings of (10–20) μm were assembled.

Process for 20nm T gate on Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epilayer using two-step lithography and zigzag foot

After metallization, a 20nm T gate with a straight foot is not mechanically stable because the support given by the foot is too weak. We have proposed a zigzag gate foot to enhance mechanical support and developed a process using two-step electron beam lithography and zigzag foot shape to fabricate 20nm T gates for high performance Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs modulation-doped field-effect transistors. Two-step lithography reduces electron forward scattering by defining the foot on a thin (40nm) bottom layer of polymethyl methacrylate at the second step, the T-gate head having been developed at the first step. Adopting a low temperature development technique for the second step reduces the detrimental effect of head exposure on foot definition. With this process, stand-alone 20nm zigzag T gates have been successfully fabricated on an Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epitaxial wafer using a 20keV electron beam. With a higher-voltage electron beam, this process can be used to fabricate sub-20-nm T gates.

Rectifying Effect in Boron Nanowire Devices

It has been found that Ni forms ohmic contacts and Ti forms Schottky-barrier contacts to boron nanowires (BNWs). Using two-step electron-beam lithography, Ni and Ti electrodes are subsequently attached onto the ends of a single BNW. As a result, a nanoscale rectifier is created using a BNW.

Fabrication of ultra-short T gates by a two-step electron beam lithography process

Fabrication of ultra-short T gates by a two-step electron beam lithography process

Fabrication of ultra-short T gates by a two-step electron beam lithography process

This paper proposes a two-step electron beam lithography process, i.e., exposing feet and heads in two separate steps for ultra-short T gates, using a PMMA/UNIII resist stack separated by a thin lift off resist (LOR) layer. Results from both experiments and Monte Carlo simulations show that the two-step lithography process has advantages over the traditional one-step process of being able to pattern shorter feet with a much thicker top layer. The optimum thickness of the LOR for patterning T shape profiles in the PMMA/UVIII resist stack has been found to be about 20 nm.

Diffusion-cooled superconducting hot electron bolometer heterodyne mixer between 630 and 820 GHz

We report on heterodyne mixing experiments between 630-820 GHz using a diffusion-cooled superconducting hot electron bolometer at intermediate frequencies from 1-2 GHz. The niobium bolometer which is cooled by diffusion of the hot electrons into a normal conductor has dimensions of 0.3 /spl mu/m/spl times/0.15 /spl mu/m and was fabricated at KOSMA with a two-step electron beam lithography process. The film thickness of the device is 30 nm with T/sub c/ of 6.1 K and /spl Delta/T/sub c/ of 0.7 K. The mixing experiments are performed with a tunerless waveguide mixer block previously used for very low-noise SIS receivers. At a physical temperature of 2.2 K, receiver noise temperatures at 666 GHz are 1500 K at 1.0 GHz intermediate frequency, increasing rapidly above 1.5 GHz. The dissipated local oscillator power is evaluated to be 65-90 nW.