Nanofabrication Applications Research Articles

(Invited) Plasma Immersion Ion Implantation: Technology, Modelling, and Applications for Nanofabrication and 2D Materials

Plasma Immersion Ion Implantation (PIII) is an ion implantation technology in which the target to be implanted (i.e. a semiconductor wafer) is immersed in a plasma, and implanted with positive ions by the application of a high-voltage negative-bias pulse to the target. Because of the high plasma density achievable in modern plasma sources, large fluences can be implanted across large wafer areas, without beam scanning. PIII is well suited for a variety of nanofabrication applications. Because of its compatibility with ordinary silicon CMOS device processing, high fluence PIII is a versatile method for in situ fabrication of nanocrystals for integrated circuit applications. This talk will review the physics of energetic ions implanted into solid materials, and illustrate how post-implant thermal treatment can be used to form nanocrystals in the ion-implanted region. A number of potential applications of PIII as a materials synthesis technique will be discussed.

Poly(styrene)-block-Maltoheptaose Films for Sub-10 nm Pattern Transfer: Implications for Transistor Fabrication.

Sequential infiltration synthesis (SIS) into poly(styrene)-block-maltoheptaose (PS-b-MH) block copolymer using vapors of trimethyl aluminum and water was used to prepare nanostructured surface layers. Prior to the infiltration, the PS-b-MH had been self-assembled into 12 nm pattern periodicity. Scanning electron microscopy indicated that horizontal alumina-like cylinders of 4.9 nm diameter were formed after eight infiltration cycles, while vertical cylinders were 1.3 nm larger. Using homopolymer hydroxyl-terminated poly(styrene) (PS–OH) and MH films, specular neutron reflectometry revealed a preferential reaction of precursors in the MH compared to PS–OH. The infiltration depth into the maltoheptaose homopolymer film was found to be 2.0 nm after the first couple of cycles. It reached 2.5 nm after eight infiltration cycles, and the alumina incorporation within this infiltrated layer corresponded to 23 vol % Al2O3. The alumina-like material, resulting from PS-b-MH infiltration, was used as an etch mask to transfer the sub-10 nm pattern into the underlying silicon substrate, to an aspect ratio of approximately 2:1. These results demonstrate the potential of exploiting SIS into carbohydrate-based polymers for nanofabrication and high pattern density applications, such as transistor devices.

All-optical motion control of metal nanoparticles powered by propulsion forces tailored in 3D trajectories

Increasing interest has been drawn to optical manipulation of metal (plasmonic) nanoparticles due to their unique response on electromagnetic radiation, prompting numerous applications in nanofabrication, photonics, sensing, etc. The familiar point-like laser tweezers rely on the exclusive use of optical confinement forces that allow stable trapping of a single metal nanoparticle in 3D. Simultaneous all-optical (contactless) confinement and motion control of single and multiple metal nanoparticles is one of the major challenges to be overcome. This article reports and provides guidance on mastering a sophisticated manipulation technique harnessing confinement and propulsion forces, enabling simultaneous all-optical confinement and motion control of nanoparticles along 3D trajectories. As an example, for the first time to our knowledge, programmable transport of gold and silver nanospheres with a radius of 50 and 30 nm, respectively, along 3D trajectories tailored on demand, is experimentally demonstrated. It has been achieved by an independent design of both types of optical forces in a single-beam laser trap in the form of a reconfigurable 3D curve. The controlled motion of multiple nanoparticles, far away from chamber walls, allows studying induced electrodynamic interactions between them, such as plasmonic coupling, observed in the presented experiments. The independent control of optical confinement and propulsion forces provides enhanced flexibility to manipulate matter with light, paving the way to new applications involving the formation, sorting, delivery, and assembling of nanostructures.

Neon and helium focused ion beam etching of resist patterns

Helium ion microscopy has attracted many applications in imaging, nanofabrication and analysis. One important field of study in nanofabrication using ion beam is the milling or etching of materials using a helium or neon focused ion beam (FIB), with and without chemical gas assistance. In particular, the neon FIB has a relatively high sputtering rate with a lower probability of swelling and less re-deposition issues compared to a helium FIB. Here, both neon and helium FIB etchings are investigated for milling and repairing electron-beam lithography (EBL) defined hydrogen silsesquioxane (HSQ) and polymethyl methacrylate (PMMA) resist patterns. Different dosages of neon FIB etching result in distinct etching profiles. Using the appropriate doses, arrays of uniform gap with aspect ratio more than 20 can be achieved on HSQ nanostructures. The neon FIB etching has a resolution of 20 nm on HSQ patterns. With XeF2 assistance, neon FIB etching can be enhanced for etching depth by a factor of ∼1.2. Whereas, helium FIB can also etch thick HSQ patterns, with much lower etch rates. But with XeF2 assistance, helium FIB etching depth can be enhanced significantly by a factor of around 5. Furthermore, both helium and neon FIB etching methods have been employed to selectively remove residual particles in deep and narrow trenches without affecting the resist patterns. The chemical analysis of these residual particle composition and resist patterns can be also performed using helium ion microscopy coupled with secondary ion mass spectrometry (SIMS) using neon FIB. Besides, a neon FIB can also effectively etch PMMA patterns which are commonly used in nanofabrication and the unwanted connections can be etched away.

Cryogenic etching for pattern transfer into silicon of Mix-and-Match structured resist layers

A Mix-and-Match lithography method for a high-resolution, high-precision and cost effective lithography tool using DLW and FE-SPL was developed and successfully realized. The pattern transfer from the photoresist to the silicon substrate is done by so-called “cryogenic etching”. It means that the substrate is cooled down to cryogenic temperatures. In contrast to etching processes at standard room temperature, the cryogenic temperatures (below −100 °C) enable a highly anisotropic etching process. The difference between etching at room temperature compared to cryogenic etching is carried out in this work. The advantages of the etching process are highlighted for the pattern transfer from Mix-and-Match-structured samples. Therefore, the used photoresist mr-P 1201LIL (microresist technologies GmbH) has been examined concerning its silicon-to-resist selectivity which could be determined to be 6:1 for the applied etching recipe. Using cryogenic etching, we are now able to transfer the Mix-and-Match-structured patterns into silicon with appreciable high selectivities. This opens a novel pathway for the manufacturing of quantum devices on large wafers.The paper discusses current research results based on the TU Ilmenau Nanopositioning and Nanomeasuring Machines (NPMM) including the novel application of nanofabrication. First, the basic setup and the resulting benefits of the NPMMs for measuring and fabrication in a working volume of up to 200 mm × 200 mm × 25 mm while abiding nanometer accuracy is described. This is in contradiction to state-of-the-art AFM scanners, which have a limited working range of appr. 100 μm × 100 μm. Next, the principle and the results of different nanofabrication technologies are shown. These include Scanning Probe Lithography (SPL), Direct Laser Writing (DLW) and UV-nanoimprint lithography (NIL). Last, efforts for further improving the feature placement accuracy of the NPMMs as well as attempts to combine several fabrication technologies to improve their throughput are touched on.

Supercritical lens array in a centimeter scale patterned with maskless UV lithography.

Microlens arrays (MLAs) are widely used in optical imaging, dense wavelength division multiplexing, optical switching, and microstructure patterning, etc. However, the light modulation capability for both the conventional refractive-type MLA and planar diffractive-type MLA is still staying at the diffraction-limited scale. Here we propose and experimentally demonstrate a high numerical aperture (NA) supercritical lens (SCL) array which could achieve a sub-diffraction-limited focal spot lattice in the far field. The intensity distribution for all the focal spots has good uniformity with the lateral size around ${0.45}\lambda {\rm /NA}$0.45λ/NA (0.75X Airy unit). The elementary unit in the SCL array composes a series of concentric belts with a feature size in micrometer scale. By utilizing an ultrafast ultraviolet lithography technique, a centimeter scale SCL array could be successfully patterned within 10 mins. Our results may provide possibilities for the applications in optical nanofabrication, super-resolution imaging, and ultrafine optical manipulation.

DNA-Guided Assemblies toward Nanoelectronic Applications.

Programmable DNA-guided self-assembly of nanoscale functional materials is of great interest in the context of nanofabrication or nanoelectronics. Attributed to its unique sequence programmability and precisely defined dimension, DNA has been used as a template for constructing ordered one-, two-, and three-dimensional architectures with organic, inorganic, and polymeric building blocks via various self-assembly strategies. Moreover, the accessible integration capability of DNA with diverse electronics-related materials propels the field of DNA-guided assemblies toward nanoelectronic applications. In this review, we outline the development in the area of DNA-guided assemblies that are constructed from carbon nanotubes, fullerenes, polymers, metals, metal oxides, and minerals using various strategies, with the focus on the bottom-up DNA-guided conducting, semiconducting, and insulating nanomaterials applied on nanoelectronics. We also review some bottom-up and top-down hybrid methods that precisely immobilize functional DNA-guided materials on substrates with high throughput. Finally, we describe the challenges in nanofabrication and potential applications of DNA-guided assemblies in nanoelectronics.

Plasmonic interference lithography by coupling the bulk plasmon polariton mode and the waveguide mode

Plasmonic lithography based on surface plasmon polariton has been proven to breaking the diffraction limit and deliver the super-resolution patterns. However, most previously reported studies suffer from the low energy efficiency and subwavelength excitation grating that obstructs the application in nanofabrication. In this work, a special plasmonic lithography prototype is proposed based on the coupling of the bulk plasmon polariton mode squeezed through the hyperbolic metamaterial (HMM) and the waveguide mode supported in the coupling layer/HMM/photoresist sandwich structure. The results demonstrate that periodic patterns with strong lithography light intensity (>220%) over the whole photoresist layer compared with incident optical intensity, high aspect ratios (1.5:1) and a half-pitch of 57.5 nm can be generated, under the interference of the fourth-order diffracted light of grating. The lithography linewidth can reach 1/16 of the mask period of 920 nm and ~1/8 of the wavelength of the 436 nm illumination light. This design of period reduction makes the device fabrication much easier, costless and even exhibits good tolerance to the roughness of the multilayer. In addition, theoretical analyses performed are widely applicable to systems working at other ultraviolet wavelengths including 248, 365 and 405 nm.

Optical nanomanipulation on solid substrates via optothermally-gated photon nudging

Constructing colloidal particles into functional nanostructures, materials, and devices is a promising yet challenging direction. Many optical techniques have been developed to trap, manipulate, assemble, and print colloidal particles from aqueous solutions into desired configurations on solid substrates. However, these techniques operated in liquid environments generally suffer from pattern collapses, Brownian motion, and challenges that come with reconfigurable assembly. Here, we develop an all-optical technique, termed optothermally-gated photon nudging (OPN), for the versatile manipulation and dynamic patterning of a variety of colloidal particles on a solid substrate at nanoscale accuracy. OPN takes advantage of a thin surfactant layer to optothermally modulate the particle-substrate interaction, which enables the manipulation of colloidal particles on solid substrates with optical scattering force. Along with in situ optical spectroscopy, our non-invasive and contactless nanomanipulation technique will find various applications in nanofabrication, nanophotonics, nanoelectronics, and colloidal sciences.

External-energy-assisted nanomachining with low-stiffness atomic force microscopy probes

Thermal atomic force microscope (AFM) nanolithography enables many nanofabrication applications, but it requires special thermal probes. In this paper, we developed a vibration-assisted sample heating approach to implement the nanomachining of poly(methyl methacrylate) (PMMA) with a regular AFM probe with low stiffness (~0.16 N/m). The joule heating softens the PMMA, which lowers the normal force. The in-plane vibration with high frequency was applied to reduce the adhesion force between the AFM tip and the PMMA with elevated temperature, which enhances nanomachining performances. By using this approach, uniform nanopatterns with controllable dimensions were fabricated on PMMA films with small setpoint force.

Detecting nanometric displacements with optical ruler metrology.

We introduce the optical ruler, an electromagnetic analog of a physical ruler, for nanoscale displacement metrology. The optical ruler is a complex electromagnetic field in which singularities serve as the marks on the scale. It is created by the diffraction of light on a metasurface, with singularity marks then revealed by high-magnification interferometric observation. Using a Pancharatnam-Berry phase metasurface, we demonstrate a displacement resolving power of better than 1 nanometer (λ/800, where λ is the wavelength of light) at a wavelength of 800 nanometers. We argue that a resolving power of ~λ/4000, the typical size of an atom, may be achievable. An optical ruler with dimensions of only a few tens of micrometers offers applications in nanometrology, nanomonitoring, and nanofabrication, particularly in the demanding and confined environment of future smart manufacturing tools.

Autonomous dynamic control of DNA nanostructure self-assembly.

Biological cells routinely reconfigure their shape using dynamic signalling and regulatory networks that direct self-assembly processes in time and space, through molecular components that sense, process and transmit information from the environment. A similar strategy could be used to enable life-like behaviours in synthetic materials. Nucleic acid nanotechnology offers a promising route towards this goal through a variety of sensors, logic and dynamic components and self-assembling structures. Here, by harnessing both dynamic and structural DNA nanotechnology, we demonstrate dynamic control of the self-assembly of DNA nanotubes-a well-known class of programmable DNA nanostructures. Nanotube assembly and disassembly is controlled with minimal synthetic gene systems, including an autonomous molecular oscillator. We use a coarse-grained computational model to capture nanotube length distribution dynamics in response to inputs from nucleic acid circuits. We hope that these results may find use for the development of responsive nucleic acid materials, with potential applications in biomaterials science, nanofabrication and drug delivery.

Color‐Tunable Eu3+ and Tb3+ Co‐Doped Nanophosphors Synthesis by Plasma‐Assisted Method

AbstractWith increasing health consciousness, Y2O3‐based rare earth nanophosphors are considered as promising luminescent complexes for bio‐applications. In the present study, an atmospheric pressure plasma‐electrochemical technique is demonstrated for the synthesis of Eu3+/Tb3+ single‐doped or co‐doped Y2O3 nanophosphors from merely an aqueous solution of the corresponding rare earth nitrite salts. Systematic experiments were performed to prepare (Y1‐x‐yEuxTby)2O3 nanophosphors of various Tb3+ and Eu3+ ratios (x:y = 1:0, 2:1, 1:1, 1:2, and 0:1), with the ultimate goal to achieve the colour tunability by simply adjusting the dopant compositions. Results indicated successfulness synthesis of crystalline Eu/Tb single‐doped and co‐doped Y2O3 nanophosphors with Tb3+ and Eu3+ ions being uniformly incorporated into the Y2O3 host matrix. The generated products showed apparent downshift behaviour under ultraviolet irradiation, and characteristic spectral excitation and emission bands were detected by the photoluminescence measurement. Furthermore, by adjusting the relative composition ratios of the terbium and europium ions, the emission colours were shown to be regulated to a large extent. The demonstrated process can be characterized as simple, versatile and environmentally‐friendly, featuring great flexibility in colour tunability, and therefore can present a considerable interest for emerging nanofabrication applications.

Electric Field Induced Molecular Assemblies Showing Different Nanostructures and Distinct Emission Colors

AbstractApplication of external stimuli in self‐assembly processes would offer greater degrees of freedom to regulate the supramolecular nanostructures and functions of self‐assembling molecules. In particular, the utilization of electric field to control molecular self‐assembly is of fundamental significance, and it contributes to the development of applications in nanofabrication and optoelectronic fields. Here, the self‐assembly of an anionic platinum complex ([Pt(tfmpy)(CN)2]−Bu4N+, tfmpy = 2‐(4‐(trifluoromethyl)phenyl)pyridine) is studied in the absence or presence of an electric field. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images demonstrate an interesting morphological transformation from rod‐like to flower‐shaped nanoaggregate structures. For rod‐like nanostructure, selected area electron diffraction (SAED) and powder X‐ray diffraction (PXRD) analysis suggest that the Bu4N+ cations are squeezed between adjacent platinum(II) complex anions, forming alternating layers of two ions. In addition, SAED result suggests that the flower‐shaped nanoaggregate is constructed by a layer‐by‐layer packing through the formation of Pt⋅⋅⋅Pt and π–π stacking interactions. Importantly, confocal fluorescence imaging shows that these two different stable assemblies possess distinct emission colors and lifetimes. This unique feature might be useful in various optoelectronic applications including data recording, anti‐counterfeiting, smart windows, etc.

Atmospheric pressure plasma directed assembly during photoresist removal: A new route to micro and nano pattern formation

We propose a new route for pattern formation based on atmospheric pressure plasma directed assembly during photoresist removal. Atmospheric plasma etching of AZ5214E resist coated on Silicon leads to the formation of periodic, oxygen-plasma resistant residuals on the underlying substrate. The O2 content in the He/O2 gas feed was found to play significant role on the size and density of the formed structures. Fourier analysis of the spatial morphology of surfaces confirmed quantitatively the approximate periodicity and its dependence on O2 content. XPS analysis revealed the formation of sulfur-containing compounds coming from DNQ molecules of AZ5214E, which organize to etching-resistant molecules due to the synergistic effect of etching and radiation from the radio frequency (RF) discharge. Finally, we applied anisotropic plasma etching based on a pulsed-gas alternating plasma process in low pressure to transfer the atmospheric plasma induced pattern on the underlying Silicon substrate fabricating quasi-ordered Si pillars. The results show the potential of atmospheric plasma directed assembly for uniform, large-area and open-air pattern definition for application in modern nanofabrication.

Automated tuning of high-order waveforms for picoliter resolution jetting of rheologically challenging materials

Drop on demand piezoelectric jetting has become an important tool for direct patterning as well as adaptive material deposition in micro- and nano-fabrication applications. Key performance and reliability metrics for jetting include lowest dispense volume, mean volume accuracy, volume variance, and drop position accuracy. Using physics-based models that accurately describe drop formation is challenging as these models require several jet parameters that can vary due to manufacturing tolerances from one jet to the next, and are unavailable non-destructively. Such manufacturing variations are particularly important when pushing the limits of volume resolution while achieving other performance and reliability targets. Current practice involves ad-hoc manual recalibration of the actuation waveform to obtain reliable jetting of a variety of materials, which prevents the use of waveforms defined by large numbers of parameters. In order to achieve the above mentioned metrics, a promising approach is to explore high order waveforms; however, such waveforms are unsuited for ad-hoc manual techniques. Here, we propose an automatic waveform tuning method that: (i) circumvents the need for complex forward models by estimating drop volume and velocity measurements via computer vision; (ii) uses genetic algorithm based stochastic optimization to tune the waveform; (iii) using a real-time sensing based on image processing that has been benchmarked against an International Electrotechnical Commission (IEC) standard for imaging-based measurement of droplet volume and (iv) enables exploration of high order waveforms (defined by up to 10 parameters) to maximize the performance/reliability of a given combination of jetting system and material. The experimental results presented in this paper show that with such methods, fluids considered rheologically impossible to jet in the literature can be jetted reliably with impressive performance metrics such as drop volumes of 1.1 pL achieved for ethyl acetate. The drop radii can also be reduced to as low as 25.6% of the jet aperture radius while achieving drop velocity sufficient to maintain position accuracy for ethyl acetate.

Optical-Force-Dominated Directional Reshaping of Au Nanodisks in Al-Au Heterodimers.

The optical reshaping of metallic nanostructures typically requires intense laser pulses to first approach or achieve melting, followed by surface-tension-dominated reshaping, transforming the original nanostructures into more spherical morphologies. Here, we report the directional optical reshaping of the Au nanodisk of an Al-Au heterodimer in the illuminated junction of an atomic force microscope (AFM). Both the heightening and the repositioning of the Au nanodisk component are induced, reducing the gap between the two nanodisks. There are three contributors to this process: the photothermal softening of the Au lattice, the optical force applied to the Au nanodisk by the Al nanodisk, and the optical force from the nearby AFM tip. The asymmetric reshaping of the heterodimer is observable structurally, through electron microscopic imaging, and through changes in the heterodimer optical response. This optical-force-directed shape manipulation may have potential applications in nanofabrication, optically induced nanomanufacturing, sensing, and quality control.

Cation-Activated Avidity for Rapid Reconfiguration of DNA Nanodevices.

The ability to design and control DNA nanodevices with programmed conformational changes has established a foundation for molecular-scale robotics with applications in nanomanufacturing, drug delivery, and controlling enzymatic reactions. The most commonly used approach for actuating these devices, DNA binding and strand displacement, allows devices to respond to molecules in solution, but this approach is limited to response times of minutes or greater. Recent advances have enabled electrical and magnetic control of DNA structures with sub-second response times, but these methods utilize external components with additional fabrication requirements. Here, we present a simple and broadly applicable actuation method based on the avidity of many weak base-pairing interactions that respond to changes in local ionic conditions to drive large-scale conformational transitions in devices on sub-second time scales. To demonstrate such ion-mediated actuation, we modified a DNA origami hinge with short, weakly complementary single-stranded DNA overhangs, whose hybridization is sensitive to cation concentrations in solution. We triggered conformational changes with several different types of ions including mono-, di-, and trivalent ions and also illustrated the ability to engineer the actuation response with design parameters such as number and length of DNA overhangs and hinge torsional stiffness. We developed a statistical mechanical model that agrees with experimental data, enabling effective interpretation and future design of ion-induced actuation. Single-molecule Förster resonance energy-transfer measurements revealed that closing and opening transitions occur on the millisecond time scale, and these transitions can be repeated with time resolution on the scale of one second. Our results advance capabilities for rapid control of DNA nanodevices, expand the range of triggering mechanisms, and demonstrate DNA nanomachines with tunable analog responses to the local environment.

Integrated Metal–Air Battery and Selective Electrolytic Leaching Cell for the Preparation of Nanoporous Metals

Electrolytic cells are used in various electrochemical applications to drive nonspontaneous chemical reactions through electrical energy supply. A typical application in nanofabrication involves the synthesis of nanoporous materials by selective electrolytic leaching, during which an electrical bias voltage is used to drive the removal of sacrificial elements. Here we report on an electrolytic cell configuration that can do the selective leaching work without the need of external electrical energy, taking advantage of two phenomena: On one hand, during the “spontaneous” discharge of a metal–air battery, materials are removed from the anode; on the other hand, during nanofabrication by “nonspontaneous” selective leaching, sacrificial elements are removed from the starting material. We integrate these two processes in such a way that both “battery discharging” and “selective leaching” occur simultaneously. Here we have demonstrated the concept through the fabrication of nanoporous Au by selective leaching of Ag from Au/Ag parent materials in a nonoxidizing aqueous solution, a process that absolutely requires an electrical energy to drive the removal of Ag. In our integrated cell configuration, no external electrical energy was used. This new concept can be extended to the fabrication of a broad range of nanoporous materials under mild conditions, including the use of pure water as electrolyte as demonstrated in this work.

Ultrathin Copper Nanowire Synthesis with Tunable Morphology Using Organic Amines for Transparent Conductors

High-quality monodispersed copper nanowires with an ultrathin diameter of 13.5 nm, lengths up to 30 μm (aspect ratio >104) were successfully synthesized by a facile and controllable hydrothermal reduction procedure. The synthesis utilized glucose in the presence of hexadecylamine (HDA) and octadecylamine (ODA) as the capping agents. The copper decahedra nanoparticles with a low-surface-energy {111} plane formed pentatwinned one-dimensional nanowires, which was exactly verified by selected-area electron diffraction. Furthermore, the diameter and relative film conductivity of copper nanowires are sensitive to the HDA/ODA molar ratio. The conductor film made of the high-quality and ultrathin copper nanowires shows high transmittance and low resistance (83.83%, 61 Ω/□), exhibiting great potential in the applications of nanofabrication, transparent and flexible conductors, organic light-emitting diodes, and more.