Combination Of Electron Beam Lithography Research Articles

Development of broadband high efficiency Mid-IR gratings for high-energy ultrafast lasers

Broadband high-efficiency diffraction gratings play a crucial role in the pulse stretcher and compressor of high-energy ultrafast lasers. Nevertheless, conventional grating manufacturing techniques, including mechanical ruling and holographic recording, face challenges in creating accurate blazed groove profiles necessary for the fabrication of broadband, high-efficiency mid-infrared gratings. In this work, we utilized combined electron-beam lithography and anisotropic wet etching technology to fabricate nearly perfect blazed grooves, producing high efficiency broadband mid-infrared (IR) grating for 4.3 µm 100 femtosecond laser. Global optimization was performed to achieve a design of > 90% efficiency over spectral range of 3.6 µm – 6.6 µm. Hybrid metal-dielectric coating (Au-Al2O3) is employed and optimized to minimize absorption and to enhance diffraction efficiency and laser-induced damage threshold (LIDT). Prototype gratings undergo testing at a desired application wavelengths of 4.3 µm in a tunable range of 0.2 µm, revealing that the optimal sample achieves a diffraction efficiency of 92%, closely approaching the theoretical value of 94.2%

Machine Learning Applied to Electron Beam Lithography to Accelerate Process Optimization of a Contact Hole Layer.

Determining the lithographic process conditions with high-resolution patterning plays a crucial role in accelerating chip manufacturing. However, lithography imaging is an extremely complex nonlinear system, and obtaining suitable process conditions requires extensive experimental attempts. This severely creates a bottleneck in optimizing and controlling the lithographic process conditions. Herein, we report a process optimization solution for a contact layer of metal oxide nanoparticle photoresists by combining electron beam lithography (EBL) experiments with machine learning. In this solution, a long short-term memory (LSTM) network and a support vector machine (SVM) model are used to establish the contact hole imaging and process condition classification models, respectively. By combining SVM with the LSTM network, the process conditions that simultaneously satisfy the requirements of the contact hole width and local critical dimension uniformity tolerance can be screened. The verification results demonstrate that the horizontal and vertical contact widths predicted by the LSTM network are highly consistent with the EBL experimental results, and the classification model shows good accuracy, providing a reference for process optimization of a contact layer.

Near‐Infrared Light Trapping and Avalanche Multiplication in Silicon Epitaxial Microcrystals

AbstractThe chemical vapor deposition of silicon on a patterned silicon substrate leads to the formation of 3D microcrystals, which, due to their inclined top facets and high aspect ratio, produce a light‐trapping effect enhancing the optical absorption in the near‐infrared (NIR). In this work, it is demonstrated that Si microcrystals can form the building blocks of a new class of NIR sensitive photodetectors operating in a linear or avalanche regime. Microcrystal‐based devices are designed by coupling a 2D kinetic‐growth model with a Poisson drift‐diffusion solver and fabricated by combining electron beam lithography and low‐energy plasma‐enhanced chemical vapor deposition (LEPECVD). The optoelectronic properties of microcrystal‐based p–i–n photodiodes are investigated both theoretically and experimentally by means of finite‐difference time‐domain (FDTD) simulations and responsivity measurements. At 1000 nm wavelength, the responsivity of microcrystal‐based devices is six times higher than that of an equivalent mesa diode. Moreover, the photocurrent gains of Si microcrystals operating as an avalanche photodiode (APD), at the same wavelength, reaches 2 × 104 demonstrating the potentialities of substrate patterning, combined with epitaxial growth, for amplified photodetection applications.

Solid state dewetting of semiconductor thin films: From fundamental studies to photonic applications

Here we propose to exploit the natural instability of thin solid films, i.e. solid state dewetting, to form regular patterns of monocrystalline atomically smooth Si, Si1-xGex and Ge nanostructures that cannot be realized with conventional methods. Additionally, the solid-state dewetting dynamics is guided by pre-patterning the sample by a combination of electron-beam lithography and reactive-ion etching, obtaining precise control over number, size, shape, and relative position of the final structures. Methods and structures will be optimized towards their exploitation mainly in photonic devices application (e.g. anti-reflection coatings, colour-filters, random lasers, quantum emitters and photonic sensors).

Nano Hotplate Fabrication for Metal Oxide-Based Gas Sensors by Combining Electron Beam and Focused Ion Beam Lithography.

Metal oxide semiconductor (MOS) gas sensors are widely used for gas detection. Typically, the hotplate element is the key component in MOS gas sensors which provide a proper and tunable operation temperature. However, the low power efficiency of the standard hotplates greatly limits the portable application of MOS gas sensors. The miniaturization of the hotplate geometry is one of the most effective methods used to reduce its power consumption. In this work, a new method is presented, combining electron beam lithography (EBL) and focused ion beam (FIB) technologies to obtain low power consumption. EBL is used to define the low-resolution section of the electrode, and FIB technology is utilized to pattern the high-resolution part. Different Au++ ion fluences in FIBs are tested in different milling strategies. The resulting devices are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS). Furthermore, the electrical resistance of the hotplate is measured at different voltages, and the operational temperature is calculated based on the Pt temperature coefficient of resistance value. In addition, the thermal heater and electrical stability is studied at different temperatures for 110 h. Finally, the implementation of the fabricated hotplate in ZnO gas sensors is investigated using ethanol at 250 °C.

Xenon-plasma focused ion beam processing of photonic microstructures with GaAs-based quantum dots

So far, successful focused ion beam (FIB) based fabrication of photonic structures with quantum dots (QDs) has been limited to cases with above 1 µm thick cap, usually in a form of a distributed Bragg reflector of a vertical cavity, which simultaneously protects the active region from the destructive influence of the ion beam. Here, we propose optimized xenon-plasma FIB (Xe-PFIB) technology as a fast and cost-efficient solution alternative to the commonly used combination of electron beam lithography and etching. We demonstrate a 3D processing of GaAs-based photonic microstructures with InGaAs QDs emitting close to the telecom O-band for cylindrical mesas with different cap thicknesses (50-650 nm) obtained by using two approaches: (i) Xe-PFIB for both reducing the cap thickness as well as the in-plane microstructure size, and (ii) wet chemical etching for cap layer removal and subsequent Xe-PFIB for the in-plane milling. The latter appeared more efficient when judging by photoluminescence intensity. Utilizing an additional protecting layer of platinum or carbon was also tested. Eventually, we for the first time show successful FIB-based fabrication of photonic microstructures with bright emission from single QDs capped with only 200 nm layer, which indicates the prospects of this technology for processing of efficient QD-based single-photon sources for quantum communication.

Active Erbium‐Doped Silicon Nanoantenna

AbstractThe development of silicon‐based radiative nanostructures is an important step toward silicon‐based optoelectronic systems. Here an approach for the fabrication of photoluminescent Er‐doped silicon nanoantennas is demonstrated. The combination of electron beam lithography and laser annealing of a thin Er film deposited after fabrication creates an active dielectric nanoantenna operating in the standard telecommunication wavelength range (C‐band). Using the multipole decomposition method the geometrical parameters of the silicon nanoparticle when the first‐order Mie‐resonances are tuned to the erbium emission in C‐band range ( transition) is calculated. It is observed experimentally that the presence of the Mie‐resonances provides enhancement of the photoluminescence intensity up to 40% compared to the non‐resonant case. The proposed approach allows for the creation of computational optical chips compatible with existing nanofabrication technologies.

Critical dimension prediction of metal oxide nanoparticle photoresists for electron beam lithography using a recurrent neural network.

The critical dimension (CD) of lithographic patterns is the most significant indicator for evaluating the imaging performance of photoresists, and its value is seriously affected by process conditions. However, the lithographic imaging system is highly nonlinear, and extensive exposure experiments are needed to obtain the desired CD. This consumes lots of time, manpower, and cost in screening for optimal process conditions. Here, we report a combined electron beam lithography (EBL) experiment and recurrent neural network (RNN) study on the CDs of metal oxide nanoparticle photoresists, and establish a CD RNN model. Leveraging the RNN model, a process condition filter is developed to screen suitable process conditions. The experimental results demonstrate that the prediction accuracy of the CD model exceeds 93%, and the photoresist patterns under the screened process conditions can satisfy the requirements of a preset CD. This work opens up a novel perspective for accurate EBL process modeling, and provides guidance for EBL experiments.

Graphene-Promoted Adhesion-Reduced Expansion of Discontinuous Palladium Nanowires upon Hydrogenation.

Monitoring conductivity changes of discontinuous palladium (Pd) nanostructures upon hydrogenation is becoming one of the most promising approaches toward hydrogen sensing. Development of sensors in this type has long been impeded due to strong ubiquitous interfacial adhesion which could distinctly restrict Pd expansion so as to hinder the closing of a nanogap. Herein, graphene underlayers were applied in the fabrication of nanogap-based hydrogen sensors to promote the lateral expansion of a Pd nanowire upon hydrogenation by reducing the adhesion between the metal and the substrate. In order to clarify details as well as mechanisms underlaid of graphene-enhanced Pd expansion, nanowire samples with serial lengths (6-48 μm) and gaps (0-260 nm in width) were controllably prepared on single-layer graphene (SLG), double-layer graphene (DLG), and quadruple-layer graphene (QLG, DLG × 2) via the combination of electron beam lithography (EBL) and electron beam deposition (EBD) technology. Response features and intrinsic analysis in physical sense of the graphene-based discontinuous Pd circuits upon hydrogen were established, in light of which the effects of underlayers on Pd expansion and on nanogap closing process were investigated. Such graphene-promoted expansion was demonstrated through the achievement of the closure of a large gap threshold (Gt) up to 260 nm as well as the systematical investigation of its influence on the sensing performance.

Nanoscale Patterning of In Vitro Neuronal Circuits.

Methods for patterning neurons in vitro have gradually improved and are used to investigate questions that are difficult to address in or ex vivo. Though these techniques guide axons between groups of neurons, multiscale control of neuronal connectivity, from circuits to synapses, is yet to be achieved in vitro. As studying neuronal circuits with synaptic resolution in vivo poses significant challenges, we present an in vitro alternative to validate biophysical and computational models. In this work we use a combination of electron beam lithography and photolithography to create polydimethylsiloxane (PDMS) structures with features ranging from 150 nm to a few millimeters. Leveraging the difference between average axon and dendritic spine diameters, we restrict axon growth while allowing spines to pass through nanochannels to guide synapse formation between small groups of neurons (i.e., nodes). We show this technique can be used to generate large numbers of isolated feed-forward circuits where connections between nodes are restricted to regions connected by nanochannels. Using a genetically encoded calcium indicator in combination with fluorescently tagged postsynaptic protein, PSD-95, we demonstrate functional synapses can form in this region.

Miniaturized Sulfite-Based Gold Bath for Controlled Electroplating of Zone Plate Nanostructures.

X-ray zone plates made from gold are common optical components used in X-ray imaging experiments. These nanostructures are normally fabricated using a combination of electron-beam lithography and gold electroplating with cyanide gold baths. In this study, we present a gold electroplating process in a miniaturized gold-suplphite bath. The miniaturization is enabled by on-chip reference plating areas with well defined sizes, offering a reliable way to control the height of the structures by carefully choosing the plating time at a given current density in accordance with a calibration curve. Fabricated gold zone plates were successfully used in X-ray imaging experiments with synchrotron radiation. Although gold electroplating of nanostructures is a well-established method, details about the actual process are often missing in the literature. Therefore, we think that our detailed descriptions and explanations will be helpful for other researchers that would like to fabricate similar structures.

Linear and nonlinear optical properties of dewetted SiGe islands

We propose to exploit the natural mechanical instability of thin solid films to form regular patterns of monocrystalline atomically smooth silicon and germanium nanostructures that cannot be realized with conventional methods. The solid-state dewetting dynamics is guided by pre-patterning the sample by a combination of electron-beam lithography and reactive-ion etching, obtaining precise control over number, size, shape, and relative position of the final Si1-xGex structures. Here we describe our progress in the spectroscopic investigation of individual dewetted Si1-xGex nanoislands: in the linear regime, bright Mie-type localized resonances are detected in the visible spectral range, with a spectral position that can be tuned by modifying the size of the nanoparticles. In the non-linear regime, instead, sizable third-harmonic generation is observed at the level of single islands. We believe that these results will be pivotal to a novel approach in spectral filtering, sensing and structural color with all-dielectric photonic devices.

High Power 980-nm Broad-Area Lasers with Distributed Bragg Reflection Grating

High-power and highly stable 980 nm semiconductor lasers are widely used in fiber laser pumping and instrumentation applications. We report on a distributed Bragg reflector (DBR) semiconductor laser manufactured on the waveguide structure of an asymmetric large optical cavity. Wavelength stabilization is achieved using a 500-µm-long DBR with a sixth-order Bragg grating, which is fabricated using a combination of electron beam lithography and inductively coupled plasma etching. When the device injection current is 22 A, the output power ranges up to 16 W, the slope efficiency is 0.73 W/A, and a spectral linewidth of 1.4 nm is achieved.

Ligand Nanocluster Array Enables Artificial-Intelligence-Based Detection of Hidden Features in T-Cell Architecture.

Protein patterning has emerged as a powerful means to interrogate adhering cells. However, the tools to apply a sub-micrometer periodic stimulus and the analysis of the response are still being standardized. We propose a technique combining electron beam lithography and surface functionalization to fabricate nanopatterns compatible with advanced imaging. The repetitive pattern enables a deep-learning algorithm to reveal that T cells organize their membrane and actin network differently depending upon whether the ligands are clustered or homogeneously distributed, an effect invisible to the unassisted human eye even after extensive image analysis. This fabrication and analysis toolbox should be useful, both together and separately, for exploring general correlation between a spatially structured subcellular stimulation and a subtle cellular response.

Nanometer-scale ordered arrangement of diamond nanoparticles on substrates via electrostatic deposition

We demonstrate a nanometer-scale ordered arrangement of diamond nanoparticles (DNPs) on SiO2 surfaces via the technique of electrostatic deposition. Using the lift-off process combining electron beam lithography and surface functionalization with 3-aminopropyltriethoxysilane, the negative zeta potential of SiO2 surfaces are locally inversed to positive. Consequently, the DNPs are deposited on the limited SiO2 surface areas with ≥100 nm pitch line-and-space and dot array patterns where the electrostatic attractive interaction works. Since this approach does not rely on any specific material property other than zeta potential, it would be applicable to any material combination of charged nanoparticles and substrates.

Fabrication Process for Deep Submicron SQUID Circuits with Three Independent Niobium Layers.

We present a fabrication technology for nanoscale superconducting quantum interference devices (SQUIDs) with overdamped superconductor-normal metal-superconductor (SNS) trilayer Nb/HfTi/Nb Josephson junctions. A combination of electron-beam lithography with chemical-mechanical polishing and magnetron sputtering on thermally oxidized Si wafers is used to produce direct current SQUIDs with 100-nm-lateral dimensions for Nb lines and junctions. We extended the process from originally two to three independent Nb layers. This extension offers the possibility to realize superconducting vias to all Nb layers without the HfTi barrier, and hence to increase the density and complexity of circuit structures. We present results on the yield of this process and measurements of SQUID characteristics.

Optical properties of niobium nitride plasmonic nanoantennas for the near- and mid-infrared spectral range

Investigating new materials plays a very important role for advancing the field of nanofabrication and nanoplasmonics. Even though niobium nitride (NbN) is mainly known for its superconducting properties when fabricating superconducting nanowire single-photon detectors, we demonstrate that it is also a material for plasmonic nanoantenna applications. In this work we measure physical properties of thin NbN films, such as permittivity and superconductivity, and demonstrate the feasibility and tuning of the plasmonic nanoantenna resonance throughout the near- and mid-infrared spectral range. Therefore, we fabricate NbN structures, using electron beam lithography in combination with Ar ion-beam etching. Additionally, we determine the refractory properties of the NbN nanoantennas, namely their high temperature stability. We find that they are stable up to 500°C under ambient conditions. These aspects make them attractive for a multitude of plasmonic applications ranging from refractory emitters/absorbers to candidates for plasmonically improved superconducting single-photon detectors.

Circular dichroism of spatially complementary chiral nanostructures

Circular dichroism (CD) is widely used in biology, medicine, and physics. Three-dimensional (3D) chiral structures have been extensively studied because of their ability to produce significant CD effects. Previously reported 3D chiral structures are limited due to the complexity of fabrication processes and CD mechanisms. Here, spatially complementary chiral nanostructure (SCCN) arrays, which comprise bottom silver films with zigzag-shaped nanoslit and top complementary silver zigzag-shaped nanowires, are theoretically and experimentally shown to provide the CD effect. SCCN arrays are prepared experimentally by combining electron beam lithography (EBL) with normal electron beam deposition (NEBD) method and by utilizing EBL and NEBD only once. Numerical results demonstrate that localized surface plasmon excited on top complementary silver zigzag-shaped nanowires and surface plasmon polariton excited on bottom silver films with zigzag-shaped nanoslit result in the CD effect of SCCN arrays. In addition, the CD effect can be tuned by changing the width of the top complementary silver zigzag-shaped nanowires. Such type of chiral nanostructures has easy tunability, simple fabrication, and a better understanding of chiral optical response, which provides a new design for spatially chiral optoelectronic devices.

Deterministically fabricated quantum dot single-photon source emitting indistinguishable photons in the telecom O-band

In this work, we develop and study single-photon sources based on InGaAs quantum dots (QDs) emitting in the telecom O-band. Quantum devices are fabricated using in situ electron beam lithography in combination with thermocompression bonding to realize a backside gold mirror. Our structures are based on InGaAs/GaAs heterostructures, where the QD emission is redshifted toward the telecom O-band at 1.3 μm via a strain-reducing layer. QDs pre-selected by cathodoluminescence mapping are embedded into mesa structures with a backside gold mirror for enhanced photon-extraction efficiency. Photon-autocorrelation measurements under pulsed non-resonant wetting-layer excitation are performed at temperatures up to 40 K, showing pure single-photon emission, which makes the devices compatible with stand-alone operation using Stirling cryocoolers. Using pulsed p-shell excitation, we realize single-photon emission with a high multi-photon suppression of g(2)(0) = 0.027 ± 0.005, an as-measured two-photon interference visibility of (12 ± 4)%, a post-selected visibility of (96 ± 10)%, and an associated coherence time of (212 ± 25) ps. Moreover, the structures show an extraction efficiency of ∼5%, which is comparable to values expected from numeric simulations of this photonic structure. Further improvements of our devices will enable implementations of quantum communication via optical fibers.

Space charge-limited current transport in thin films of alkyl-functionalized silicon nanocrystals

We describe the fabrication and electrical characterization of all-silicon electrode devices to study the electronic properties of thin films of silicon nanocrystals (SiNCs). Planar, highly doped Si electrodes with contact separation of 200 nm were fabricated from silicon-on-insulator substrates, by combination of electron beam lithography and reactive ion etching. The gaps between the electrodes of height 110 nm were filled with thin-films of hexyl functionalized SiNCs (diameter 3 nm) from colloidal dispersions, via a pressure-transducing PDMS (polydimethylsiloxane) membrane. This novel approach allowed the formation of homogeneous SiNC films with precise control of their thickness in the range of 15–90 nm, practically without any voids or cracks. The measured conductance of the highly resistive SiNC films at high bias voltages up to 60 V scaled approximately linearly with gap width (5–50 μm) and gap filling height, with little device-to-device variance. We attribute the observed, pronounced hysteretic current–voltage (I–V) characteristics to space-charge-limited current transport, which—after about twenty cycles—eventually blocks the current almost completely. We propose our all-silicon device scheme and gap filling methodology as a platform to investigate charge transport in novel hybrid materials at the nanoscale, in particular in the high resistivity regime.