Electron Beam Lithography Patterning Research Articles

Optimization of e-beam lithography parameters for nanofabrication of sub-50 nm gold nanowires and nanogaps based on a bilayer lift-off process

Electron beam lithography (EBL) stands out as a powerful direct-write tool offering nanometer-scale patterning capability and is especially useful in low-volume R&D prototyping when coupled with pattern transfer approaches like etching or lift-off. Among pattern transfer approaches, lift-off is preferred particularly in research settings, as it is cost-effective and safe and does not require tailored wet/dry etch chemistries, fume hoods, and/or complex dry etch tools; all-in-all offering convenient, ‘undercut-free’ pattern transfer rendering it useful, especially for metallic layers and unique alloys with unknown etchant compatibility or low etch selectivity. Despite the widespread use of the lift-off technique and optical/EBL for micron to even sub-micron scales, existing reports in the literature on nanofabrication of metallic structures with critical dimension in the 10–20 nm regime with lift-off-based EBL patterning are either scattered, incomplete, or vary significantly in terms of experimental conditions, which calls for systematic process optimization. To address this issue, beyond what can be found in a typical photoresist datasheet, this paper reports a comprehensive study to calibrate EBL patterning of sub-50 nm metallic nanostructures including gold nanowires and nanogaps based on a lift-off process using bilayer polymethyl-methacrylate as the resist stack. The governing parameters in EBL, including exposure dose, soft-bake temperature, development time, developer solution, substrate type, and proximity effect are experimentally studied through more than 200 EBL runs, and optimal process conditions are determined by field emission scanning electron microscope imaging of the fabricated nanostructures reaching as small as 11 nm feature size.

Electron beam lithography and dimensional metrology for fin and nanowire devices on Ge, SiGe and GeOI substrates

Until now there is no systematic study on the effect of the substrate type on the hydrogen silsesquioxane (HSQ) electron beam lithography (EBL) patterning process. We investigate arrays of line structures with varying width and spacing, starting at 10 nm, exposed at varying dose, and developed by salty NaOH and TMAH developers on group IV semiconductor substrates. We demonstrate that the HSQ EBL process on Ge is much more limited in achieving the smallest obtainable features, having optimal uniformity and fidelity, in comparison to Si. Monte-Carlo simulations of the e-beam/substrate interactions for “pure” Si and Ge substrates, and varying content Ge/Si epitaxial layers on Si, suggest that the limitations seen are directly linked to back-scattered electron (BSE) generation. As predicted by the simulations and shown experimentally, improved fidelity and resolution of the features can be achieved by minimizing the (BSE) generation coming from the Ge contribution in the substartes. Finally, from a metrology perspective, it is demonstrated that although line patterns may appear resolved in SEM images, the variation in the brightness across neighbouring lines is a key parameter in understanding the resist clearance between lines, that will affect the next etching step for pattern transfer onto the underlying substrate. These results are important for patterning high-density line structures and nano-device engineering as required for realising state-of-the art laterally stacked group IV multi-channel field effect transistors (FETs).

High-throughput fabrication of large-scale metasurfaces using electron-beam lithography with SU-8 gratings for multilevel security printing

The field of metasurface research has rapidly developed in the past decade. Electron-beam lithography (EBL) is an excellent tool used for rapid prototyping of metasurfaces. However, Gaussian-beam EBL generally struggles with low throughput. In conjunction with the recent rise of interest in metasurfaces made of low-index dielectric materials, we propose in this study the use of a relatively unexplored chemically amplified resist, SU-8 with EBL, as a method for rapid prototyping of low-index metasurfaces. We demonstrate the use of SU-8 grating on silicon for cost-efficient fabrication of an all-dielectric multilevel security print for anti-counterfeiting purposes, which encrypt different optical information with different light illumination conditions, namely, bright-field reflection, dark-field reflection, and cross-polarized reflection. The large-scale print (1 mm2) could be exposed in a relatively short time (∼11 min) due to the ultrahigh sensitivity of the resist, while the feature size of ∼200 nm was maintained, demonstrating that SU-8 EBL resist serves as a good candidate for rapid prototyping of metasurface designs. Our results could find applications in the general area of increasing EBL patterning speed for a variety of other devices and structures.

Cu(In,Ga)Se2 based ultrathin solar cells the pathway from lab rigid to large scale flexible technology

The incorporation of interface passivation structures in ultrathin Cu(In,Ga)Se2 based solar cells is shown. The fabrication used an industry scalable lithography technique—nanoimprint lithography (NIL)—for a 15 × 15 cm2 dielectric layer patterning. Devices with a NIL nanopatterned dielectric layer are benchmarked against electron-beam lithography (EBL) patterning, using rigid substrates. The NIL patterned device shows similar performance to the EBL patterned device.The impact of the lithographic processes in the rigid solar cells’ performance were evaluated via X-ray Photoelectron Spectroscopy and through a Solar Cell Capacitance Simulator. The device on stainless-steel showed a slightly lower performance than the rigid approach, due to additional challenges of processing steel substrates, even though scanning transmission electron microscopy did not show clear evidence of impurity diffusion. Notwithstanding, time-resolved photoluminescence results strongly suggested elemental diffusion from the flexible substrate. Nevertheless, bending tests on the stainless-steel device demonstrated the mechanical stability of the CIGS-based device.

Dynamic Optical Grating Based on a Photomechanical Molecular Crystal

AbstractOrganic photomechanical crystals transform molecular‐scale photoisomerization events into largescale crystal shape changes. The results in this paper demonstrate that this photomechanical motion can be harnessed to reconfigure reflective elements on the crystal surface to actively control light propagation. This is accomplished by using a single organic photomechanical crystal composed of 1,2‐bis(2,4‐dimethyl‐5‐phenyl‐3‐thienyl)‐3,3,4,4,5,5‐hexafluoro‐1‐cyclopentene, which undergoes a reversible ring‐open to ring‐closed photoisomerization. Electron beam lithography patterning followed by a sublimation development step leaves periodically positioned, 120 nm deep grooves that are coated with gold metal to form a reflective grating on the crystal surface. Under UV irradiation, expansion of the bulk crystal increases the grating period by ≈5% and changes the diffraction angle of a 633 nm probe laser beam by 0.7° (first order) and 1.3° (second order). The grating period change can be reversed by visible light exposure and repeated for multiple cycles. The crystal expansion and thus the diffraction angle can also be continuously tuned by controlling the duration of light exposure. Anisotropic crystal expansion and contraction due to molecular photoisomerization enables low‐power light sources to steer light beams continuously over a range of diffraction angles.

3D structured biochip for label free determinations at the point-of-need

A novel optical label-free biosensing approach is introduced aiming to remove the current limitations of optical biosensors in their application of on-site food analysis. The proposed methodology (GRADual thin film IntErferometry, GRADE) is based on light interferometry employing an illumination source of a few nm spectral width and a 3D micro-patterned sensor surface consisting of multilevel staircase structures, fabricated through grayscale electron beam lithography and appropriate pattern transfer on thick dielectric layer. The formation of the biomolecular adlayer on the 3D structured surface sensor, is monitored with a 2D camera, without the need for broadband illumination sources and spectral analysis instrumentation. Consequently, this novel sensing approach has the potential to enable the cost-efficient monitoring of bioreactions employing a simple optical setup, a low-cost photodetector and light source, thus being an ideal platform for use at the Point-of-Need. In the present study the system is demonstrated for the detection of aflatoxin B 1 (AFB1), a very potent carcinogen detected in various food categories. • A novel optical label-free biosensing approach is introduced aiming to remove the current limitations of optical biosensors. • Based on a sensing area with a gradually-varying-thickness dielectric, fabricated through grayscale e-beam lithography. • Cost-efficient monitoring of bioreactions, ideal platform for use at the Point-of-Need.

Discrete relaxation method for hybrid e-beam and triple patterning lithography layout decomposition

Hybrid electron beam lithography (EBL) and triple patterning lithography (TPL) is an advanced technology for IC manufacture. To solve the hybrid EBL and TPL layout decomposition problem (HETLD), we propose a discrete relaxation method in this paper. Existed works hard to evaluate the quality of the obtained results theoretically. Our discrete relaxation is the first approach which can be used to evaluate the solution quality. First, we introduce an extended minimum weight dominating set coloring problem (EMDSC). We show that the EMDSC problem is a discrete relaxation of the HETLD problem, which is solved by an integer linear programming (ILP) approach. Second, we legalize the relaxation solution to a feasible solution of the initial problem by inserting stitches and assigning EBLs. Experimental results show that our method achieves the near-optimal solution.

Molecular bixbyite-like In12-oxo clusters with tunable functionalization sites for lithography patterning applications.

Indium oxides have been widely applied in many technological areas, but their utilization in lithography has not been developed. Herein, we illustrated a family of unprecedented In12-oxo clusters with a general formula [In12(μ4-O)4(μ2-OH)2(OCH2CH2NHCH2CH2O)8(OR)4X4]X2 (where X = Cl or Br; R = CH3, C6H4NO2 or C6H4F), which not only present the largest size record in the family of indium-oxo clusters (InOCs), but also feature the first molecular model of bixbyite-type In2O3. Moreover, through the labile coordination sites of the robust diethanolamine-stabilized In12-oxo core, these InOCs can be accurately functionalized with different halides and alcohol or phenol derivatives, producing tunable solubility. Based on the high solution stability as confirmed by ESI-MS analysis, homogeneous films can be fabricated using these In12-oxo clusters by the spin-coating method, which can be further used for electron beam lithography (EBL) patterning studies. Accordingly, the above structural regulations have significantly influenced their corresponding film quality and patterning performance, with bromide or p-nitrophenol functionalized In12-oxo clusters displaying better performance of sub-50 nm lines. Thus, the here developed bixbyite-type In12-oxo cluster starts the research on indium-based patterning materials and provides a new platform for future lithography radiation mechanism studies.

Photonic integration of uniform GaAs nanowires in hexagonal and honeycomb lattice for broadband optical absorption

We present an experimental approach toward the realization of GaAs nanowires in the form of square, hexagonal, and honeycomb lattices for photonic integration toward enhanced optical properties. We have carried out a design and fabrication process on GaAs wafers using electron beam lithography patterning, reactive ion etching for hard mask removal, and inductively coupled plasma etching of the material. The resulting photonic crystals are analyzed by field emission scanning electron microscopy. Nanowire array designs in a square, hexagonal, and honeycomb lattice with a variable height of nanowires have been studied. Using finite-difference time-domain simulation, we can derive the comparative optical absorption properties of these nanowire arrays. A very high broadband absorbance of >94% over the 400 nm–1000 nm wavelength range is studied for hexagonal and honeycomb arrays, while a square lattice array shows only a maximum of 85% absorption. We report a minimum of 2% reflectance, or 98% optical absorbance, over 450 nm–700 nm and over a wide angle of 45° through hexagonal and honeycomb lattice integration in GaAs. These results will have potential applications toward broadband optical absorption or light trapping in solar energy harvesting.

Nano illumination microscopy: a technique based on scanning with an array of individually addressable nanoLEDs.

In lensless microscopy, spatial resolution is usually provided by the pixel density of current digital cameras, which are reaching a hard-to-surpass pixel size / resolution limit over 1 µm. As an alternative, the dependence of the resolving power can be moved from the detector to the light sources, offering a new kind of lensless microscopy setups. The use of continuously scaled-down Light-Emitting Diode (LED) arrays to scan the sample allows resolutions on order of the LED size, giving rise to compact and low-cost microscopes without mechanical scanners or optical accessories. In this paper, we present the operation principle of this new approach to lensless microscopy, with simulations that demonstrate the possibility to use it for super-resolution, as well as a first prototype. This proof-of-concept setup integrates an 8 × 8 array of LEDs, each 5 × 5 μm2 pixel size and 10 μm pitch, and an optical detector. We characterize the system using Electron-Beam Lithography (EBL) pattern. Our prototype validates the imaging principle and opens the way to improve resolution by further miniaturizing the light sources.

Fabrication of highly dense arrays of nanocrystalline diamond nanopillars with integrated silicon-vacancy color centers during the growth

Highly dense arrays of diamond nanopillars have been fabricated using nanocrystalline diamond films (NCD) as the starting material. The fabrication process consisted of electron beam lithography (EBL), aluminum mask deposition and inductively coupled O2 plasma reactive ion etching. The EBL pattern fidelity was enhanced by proximity corrections and dose variations. Optical characterizations of the arrays indicated the incorporation of silicon-vacancy centers during NCD growth as well as enhanced fluorescence and photoluminescence intensities in the well-developed pillar arrays. Transferring this fabrication method to monocrystalline diamond, such dense arrays of diamond nanopillars could be applied in quantum photonics as emitter arrays or photonic crystals upon integration of color centers.

Use of monocrystalline gold flakes for gap plasmon-based metasurfaces operating in the visible

Gap plasmon-based optical metasurfaces have been extensively used for demonstration of flat optical elements with various functionalities efficiently operating at near-infrared and telecom wavelengths. Extending their operation to the visible is however impeded by the progressively increased plasmon absorption for shorter wavelengths. We investigate the possibility to improve the performance of gap plasmon-based metasurfaces in the visible by employing monocrystalline gold flakes as substrates instead of evaporated polycrystalline gold films, while using the electron-beam lithography patterning of the evaporated thin gold films for fabrication of top gold nanobricks, which define gap-plasmon resonator elements of the metasurfaces. We demonstrate that the efficiency can be improved by modest but noticeable amount of ≈5% if all other configuration parameters are preserved.

Fabrication of a hydrophobic three-dimensional hybrid structure using an UV-curable electron beam resist

Biomimetic techniques are being studied for use in many fields. The hydrophobic surface is among the biomimetic features used in many industrial products, such as liquid-crystal display monitors and windows. The surface has a fine hybrid three-dimensional (3D) structure, comprising micro- and nanoscale structures that can be seen on the surfaces of the lotus leaf and rose petal. Many methods of fabricating 3D hybrid structures have been developed. In this study, a new fabrication method, employing UV nanoimprint lithography (UV-NIL) and electron beam lithography (EBL), was developed to prepare a polymeric hydrophobic surface. When employing this method, EBL forms a nanoscale pattern directly on the microscale pattern generated by UV-NIL. A UV-curable electron-beam resist polymer was prepared with a positive-tone EBL pattern on a UV-NIL pattern. The fabrication of a hydrophobic structure was demonstrated.

Stencil mask using ultra-violet-curable positive-tone electron beam resist

Stencil masks are used in fabrication processes such as photolithography for circuit patterning and micro-electromechanical systems devices. A stencil mask has micro- and nano-scale through-hole patterns on a substrate, and the metal or Si through-patterns are chemically etched along electron beam (EB) lithographic patterns on a resist. However, the electron beam lithography (EBL) and chemical etching processes are complex. Therefore, in this study, a simple and high-throughput fabrication process is proposed. In this process, an ultra-violet (UV)-curable positive-type EB resist was prepared, which can be used to prepare a polymer substrate that can be lithographed by an EB. The micro-scale through-hole patterns were created on a resist substrate with a thickness of approximately 5 μm. Cr deposition was demonstrated on an Si wafer using a fabricated stencil mask. The deposited Cr thin film traced the EBL pattern on the wafer through the mask. The usefulness of the method was demonstrated by preparing a polymer stencil mask.

Ultra-narrow band perfect metamaterial absorber based on dielectric-metal periodic configuration

The ultra-narrow band perfect metamaterial absorber (PMA) based on dielectric-metal periodic configuration for near infrared spectral region is proposed. The PMA consists of a dielectric resonant structure and a metal substrate. Two design examples show that the proposed PMAs are able to exhibit nonpolarizing absorption peaks at normal and oblique incidences respectively. Moreover, the absorption spectra of the PMA for oblique incidence are insensitive to the incident angle under TE polarized light illumination. But this PMA exhibits a large spectral shift by rotating the angle of incidence under TM polarized light illumination. Then a PMA sample for oblique incidence is fabricated through thin-film deposition and electron beam lithography patterning. Finally, the reflection spectra of the fabricated PMA sample are measured. The absorption spectra obtained by subtracting the measured reflection spectra from incident spectra are consistent with the theory. Thus the novel characteristics of the ultra-narrow band PMA based on dielectric-metal periodic configuration are experimentally demonstrated.

CALDER: The Second-Generation Light Detectors

The main aim of the cryogenic wide-area light detectors with excellent resolution project is the development of cryogenic light detectors with large active area (~50 mm × 50 mm) and noise energy resolution smaller than 20-eV RMS. Such detectors will be used to discriminate the background in next generation large-mass bolometric experiments, such as cryogenic underground observatory for rare events. In this paper, we present the fabrication process of the phonon-mediated kinetic inductance detectors (KIDs). In the first part of the project, Al KIDs have been developed. Thin film Al (40 nm) were evaporated on high quality, high resistivity (>10 kΩ·cm) Si(1 0 0) wafers using a high vacuum electron beam evaporator. Detectors were patterned by direct-write Electron Beam Lithography (EBL) using positive tone resist AR-P 669.06. To improve the energy resolution of our detector, superconductors with higher kinetic inductance, such as the substoichiometric titanium nitride (TiNx), were developed. TiNx is deposited with reactive dc magnetron sputtering. Thus, the fabrication process is subtractive and consists of EBL patterning through negative tone resist AR-N 7700 and SF6 etch using a Deep Reactive Ion Etching - Inductively Coupled Plasma. Critical temperature of TiNx samples was measured using the 4-point probe geometry.

Design and Fabrication of the Second-Generation KID-Based Light Detectors of CALDER

The goal of the cryogenic wide-area light detectors with excellent resolution project is the development of light detectors with large active area and noise energy resolution smaller than 20 eV RMS using phonon-mediated kinetic inductance detectors (KIDs). The detectors are developed to improve the background suppression in large-mass bolometric experiments such as CUORE, via the double readout of the light and the heat released by particles interacting in the bolometers. In this work we present the fabrication process, starting from the silicon wafer arriving to the single chip. In the first part of the project, we designed and fabricated KID detectors using aluminum. Detectors are designed by means of state-of-the-art software for electromagnetic analysis (SONNET). The Al thin films (40 nm) are evaporated on high-quality, high-resistivity (> 10 kΩ cm) Si(100) substrates using an electron beam evaporator in a HV chamber. Detectors are patterned in direct-write mode, using electron beam lithography (EBL), positive tone resist poly-methyl methacrylate and lift-off process. Finally, the chip is diced into 20 × 20 mm2 chips and assembled in a holder OFHC (oxygen-free high conductivity) copper using PTFE support. To increase the energy resolution of our detectors, we are changing the superconductor to sub-stoichiometric TiN (TiNx) deposited by means of DC magnetron sputtering. We are optimizing its deposition by means of DC magnetron reactive sputtering. For this kind of material, the fabrication process is subtractive and consists of EBL patterning through negative tone resist AR-N 7700 and deep reactive ion etching. Critical temperature of TiNx samples was measured in a dedicated cryostat.

Strain relaxation of germanium-tin (GeSn) fins

Strain relaxation of biaxially strained Ge1-xSnx layer when it is patterned into Ge1-xSnx fin structures is studied. Ge1-xSnx-on-insulator (GeSnOI) substrate was realized using a direct wafer bonding (DWB) technique and Ge1-xSnx fin structures were formed by electron beam lithography (EBL) patterning and dry etching. The strain in the Ge1-xSnx fins having fin widths (WFin) ranging from 1 μm down to 80 nm was characterized using micro-Raman spectroscopy. Raman measurements show that the strain relaxation increases with decreasing WFin. Finite element (FE) simulation shows that the strain component in the transverse direction relaxes with decreasing WFin, while the strain component along the fin direction remains unchanged. For various Ge1-xSnx fin widths, transverse strain relaxation was further extracted using micro-Raman spectroscopy, which is consistent with the simulation results.

Lithographic performance of ZEP520A and mr-PosEBR resists exposed by electron beam and extreme ultraviolet lithography

Pattern transfer by deep anisotropic etch is a well-established technique for fabrication of nanoscale devices and structures. For this technique to be effective, the resist material plays a key role and must have a high resolution, reasonable sensitivity, and high etch selectivity against the conventional silicon substrate or underlayer film. In this work, the lithographic performance of two high etch resistance materials was evaluated: ZEP520A (Nippon Zeon Co.) and mr-PosEBR (micro resist technology GmbH). Both materials are positive tone, polymer-based, and nonchemically amplified resists. Two exposure techniques were used: electron beam lithography (EBL) and extreme ultraviolet (EUV) lithography. These resists were originally designed for EBL patterning, where high quality patterning at sub-100 nm resolution was previously demonstrated. In the scope of this work, the authors also aim to validate their extendibility to EUV for high resolution and large area patterning. For this purpose, the same EBL process conditions were employed at EUV. The figures of merit, i.e., dose to clear, dose to size, and resolution, were obtained, and these results are discussed systematically. It was found that both materials are very fast at EUV (dose to clear lower than 12 mJ/cm2) and are capable of resolving dense lines/space arrays with a resolution of 25 nm half-pitch. The quality of patterns was also very good, and the sidewall roughness was below 6 nm. Interestingly, the general-purpose process used for EBL can be extended straightforwardly to EUV lithography with comparably high quality and yield. Our findings open new possibilities for lithographers who wish to devise novel fabrication schemes exploiting EUV for fabrication of nanostructures by deep etch pattern transfer.

Electron beam lithography using fixed beam moving stage

Large area patterns with small submicron features are difficult to write using conventional electron beam lithography (EBL) methods. This would be more challenging especially if the patterns have large lateral aspect ratios such as waveguide tapers. Conventionally, the patterning area is divided into smaller write fields and the stage moves in between various write fields. Precise stage movement is necessary to reduce stitching errors. However, even the most accurate laser interferometer based control systems are prone to stochastic thermal drifts. In this paper, new methods of EBL patterning are explored using the stitch free method of writing and overcoming the conventional time constraints for writing large area patterns. Further, the methods presented are suited for writing structures with micron and nanosized features in the same pattern.