Technology and metrology of new electronic materials and devices

Precise Surface Profiling at the Nanoscale Enabled by Deep Learning

Surface topography, or height profile, is a critical property for various micro- and nanostructured materials and devices, as well as biological systems. At the nanoscale, atomic force microscopy (AFM) is the tool of choice for surface profiling due to its capability to noninvasively map the topography of almost all types of samples. However, this method suffers from one drawback: the convolution of the nanoprobe's shape in the height profile of the samples, which is especially severe for sharp protrusion features. Here, we report a deep learning (DL) approach to overcome this limit. Adopting an image-to-image translation methodology, we use data sets of tip-convoluted and deconvoluted image pairs to train an encoder-decoder based deep convolutional neural network. The trained network successfully removes the tip convolution from AFM topographic images of various nanocorrugated surfaces and recovers the true, precise 3D height profiles of these samples.

High-precision surface profilometry on a micron-groove based on dual-comb electronically controlled optical sampling

We demonstrate an optical method for 3D profilometry of micro-nano devices with large step structures. The measurement principle is based on a dual-comb direct time-of-flight detection. An electronically controlled optical sampling (ECOPS) approach is used to improve the acquisition rate. In a proof-of-principle distance measurement experiment, the measurement precision reaches 15 nm at 4000-times averages. The method has been used to characterize the profile of a large aspect-ratio rectangular micron-groove with 10 µm width and 62.3 µm depth. By point-by-point scanning, a 3D point cloud image is obtained, and the 3D profile of the micro-structure is quantitatively reconstructed with sub-micrometer precision. The proposed high-precision, high-speed surface 3D profile measurement technology could be applied to profilometry and inspection of complex microelectronics devices in the future.

Artificial SiNz:H Synapse Crossbar Arrays with Gradual Conductive Pathway for High-Accuracy Neuromorphic Computing

Inspired by its highly efficient capability to deal with big data, the brain-like computational system has attracted a great amount of attention for its ability to outperform the von Neumann computation paradigm. As the core of the neuromorphic computing chip, an artificial synapse based on the memristor, with a high accuracy in processing images, is highly desired. We report, for the first time, that artificial synapse arrays with a high accuracy in image recognition can be obtained through the fabrication of a SiNz:H memristor with a gradient Si/N ratio. The training accuracy of SiNz:H synapse arrays for image learning can reach 93.65%. The temperature-dependent I-V characteristic reveals that the gradual Si dangling bond pathway makes the main contribution towards improving the linearity of the tunable conductance. The thinner diameter and fixed disconnection point in the gradual pathway are of benefit in enhancing the accuracy of visual identification. The artificial SiNz:H synapse arrays display stable and uniform biological functions, such as the short-term biosynaptic functions, including spike-duration-dependent plasticity, spike-number-dependent plasticity, and paired-pulse facilitation, as well as the long-term ones, such as long-term potentiation, long-term depression, and spike-time-dependent plasticity. The highly efficient visual learning capability of the artificial SiNz:H synapse with a gradual conductive pathway for neuromorphic systems hold great application potential in the age of artificial intelligence (AI).

Materials Quest for Advanced Interconnect Metallization in Integrated Circuits

Integrated circuits (ICs) are challenged to deliver historically anticipated performance improvements while increasing the cost and complexity of the technology with each generation. Front-end-of-line (FEOL) processes have provided various solutions to this predicament, whereas the back-end-of-line (BEOL) processes have taken a step back. With continuous IC scaling, the speed of the entire chip has reached a point where its performance is determined by the performance of the interconnect that bridges billions of transistors and other devices. Consequently, the demand for advanced interconnect metallization rises again, and various aspects must be considered. This review explores the quest for new materials for successfully routing nanoscale interconnects. The challenges in the interconnect structures as physical dimensions shrink are first explored. Then, various problem-solving options are considered based on the properties of materials. New materials are also introduced for barriers, such as 2D materials, self-assembled molecular layers, high-entropy alloys, and conductors, such as Co and Ru, intermetallic compounds, and MAX phases. The comprehensive discussion of each material includes state-of-the-art studies ranging from the characteristics of materials by theoretical calculation to process applications to the current interconnect structures. This review intends to provide a materials-based implementation strategy to bridge the gap between academia and industry.

Super-Resolution Fluorescence Imaging for Semiconductor Nanoscale Metrology and Inspection

The increase in the number and complexity of process levels in semiconductor production has driven the need for the development of new measurement methods that can evaluate semiconductor devices at the critical dimensions of fine patterns and simultaneously inspect nanoscale contaminants or defects. However, conventional optical inspection methods often fail to resolve device patterns or defects at the level of tens of nanometers required for device development owing to their diffraction-limited resolutions. In this study, we used the stochastic optical reconstruction microscopy (STORM) technique to image semiconductor nanostructures with feature sizes as small as 30 nm and detect individual 20 nm-diameter contaminants. STORM imaging of semiconductor nanopatterns is based on the development of a selective labeling method of fluorophores for a negative silicon oxide surface using the charge interaction of positive polyethylenimine molecules. This study demonstrates the potential of STORM for nanoscale metrology and in-line defect inspection of semiconductor integrated circuits.

Investigation of correlative parameters to evaluate EUV lithographic performance of PMMA

Investigations to evaluate the extreme ultraviolet (EUV) lithographic performance of 160 nm thick poly(methyl methacrylate) with 13.5 nm wavelength EUV light were performed using a synchrotron radiation source at Pohang Light Source-II (PLS-II). The single system enabled the determination of the sensitivity, contrast, linear absorption coefficient, critical dimension, and line edge roughness of polymer thin films through tests and measurements. The experimental findings were also compared to theoretical results and those of previously reported studies. According to the results of the dose-to-clear test and transmission measurements, the critical dimension of a line and space pattern (>50 nm) via interference lithography with 250 nm pitch grating agreed well with the results calculated using the lumped parameter model. The experimental results demonstrated that the equipment and test protocol can be used for EUV material infrastructure evaluation in academia and in industry.

Traceable Nanoscale Measurements of High Dielectric Constant by Scanning Microwave Microscopy

The importance of high dielectric constant materials in the development of high frequency nano-electronic devices is undeniable. Their polarization properties are directly dependent on the value of their relative permittivity. We report here on the nanoscale metrological quantification of the dielectric constants of two high-κ materials, lead zirconate titanate (PZT) and lead magnesium niobate-lead titanate (PMN-PT), in the GHz range using scanning microwave microscopy (SMM). We demonstrate the importance of the capacitance calibration procedure and dimensional measurements on the weight of the combined relative uncertainties. A novel approach is proposed to correct lateral dimension measurements of micro-capacitive structures using the microwave electrical signatures, especially for rough surfaces of high-κ materials. A new analytical expression is also given for the capacitance calculations, taking into account the contribution of fringing electric fields. We determine the dielectric constant values εPZT = 445 and εPMN-PT = 641 at the frequency around 3.6 GHz, with combined relative uncertainties of 3.5% and 6.9% for PZT and PMN-PT, respectively. This work provides a general description of the metrological path for a quantified measurement of high dielectric constants with well-controlled low uncertainty levels.

Coulomb barrier creation by means of electronic field emission in nanolayer capacitors

The main mechanism of energy loss in capacitors with nanoscale dielectric films is leakage currents. Using the example of Al-Al₂O₃-Al, we show that there are two main contributions, namely the cold field emission effect and the hopping conductivity through the dielectric. Our main finding is that an application of a high electric field, ∼0.6-0.7 GV m^-1, causes electrons to penetrate the dielectric. If the temperature is sufficiently low, such electrons become permanently trapped in the dielectric. To achieve a strong charging of the dielectric, the voltage needs to be high enough, so that a field emission occurs from the cathode into the dielectric. Such a strongly charged dielectric layer generates a Coulomb barrier and leads to a suppression of the leakage current. Thus, after the dielectric nanolayer of the capacitor is charged, the field emission and the hopping conductivity are both suppressed, and the hysteresis of the I-V curve disappears. The phenomenon is observed at temperatures up to ∼225 K. It would be advantageous to identify insulators in which the phenomenon of the Coulomb barriers persists even up to the room temperature, but at this time it is not known whether such dielectrics exist and/or can be designed.

Influence of the Local Scattering Environment on the Localization Precision of Single Particles

We study the fundamental limit on the localization precision for a subwavelength scatterer embedded in a strongly scattering environment, using the external degrees of freedom provided by wavefront shaping. For a weakly scattering target, the localization precision improves with the value of the local density of states at the target position. For a strongly scattering target, the localization precision depends on the dressed polarizability that includes the backaction of the environment. This numerical study provides new insights for the control of the information content of scattered light by wavefront shaping, with potential applications in sensing, imaging, and nanoscale engineering.

Implementation of PPI with Nano Amorphous Oxide Semiconductor Devices for Medical Applications

BACKGROUND

Electronic devices which mimic the functionality of biological synapses are a large step to replicate the human brain for neuromorphic computing and for numerous medical research investigations. One of the representative synaptic behaviors is paired-pulse facilitation (PPF). It has been widely investigated because it is regarded to be related to biological memory. However, plasticity behavior is only part of the human brain memory behavior.

METHODS

Here, we present a phenomenon which is opposite to PPF, i.e., paired-pulse inhibition (PPI), in nano oxide devices for the first time. The research here suggests that rather than being enhanced, the phenomena of memory loss would also be possessed by such electronic devices. The device physics mechanism behind memory loss behavior was investigated. This mechanism is sustained by historical memory and degradation manufactured by device trauma to regulate characteristically stimulated origins of artificial transmission behaviors.

RESULTS

Under the trauma of a memory device, both the signal amplitude and signal time stimulated by a pulse are lower than the first signal stimulated by a previous pulse in the PPF, representing a new scenario in the struggle for memory. In this way, more typical human brain behaviors could be simulated, including the effect of age on latency and error generation, cerebellar infarct, trauma and memory loss pharmacological actions (such as those caused by hyoscines and nitrazepam).

CONCLUSION

Thus, this study developed a new approach for implementing the manner in which the brain works in semiconductor devices for improving medical research.

Memristive devices based on emerging two-dimensional materials beyond graphene

With the explosion of data in the information universe and the approaching of fundamental limits in silicon-based flash memories, the exploration of new device architectures and alternative materials is necessary for next-generation memory technology. Accordingly, emerging two-dimensional (2D) material-based memristive devices have attracted increasing attention due to their unique properties and great potential in flexible and wearable devices, and even neuromorphic computing systems. Herein, we provide an overview of the recent progress on memristive devices based on 2D materials beyond graphene. The device structures and choice of active materials and electrodes materials are summarized for various types of 2D material-based memristive devices. Following the discussion and classification on the device performances and mechanisms, the challenges and perspectives on future research based on 2D materials are also presented.

Quantum size effect affecting environment assisted electron capture in quantum confinements

Ultrafast inter-Coulombic electron capture (ICEC) has been established as an important energy-transfer process in open paired-quantum-dot systems which can mediate between entrapment of free-moving electrons and release of trapped ones elsewhere by long-range electron-electron interaction within nanowires. Previous studies indicated ICEC enhancement through population and secondary decay of two-center resonance states, the latter known as inter-Coulombic decay (ICD). This study investigates the quantum-size effect of single- and double-electron states in an established model of a quasi-one-dimensional nanowire with two embedded confinement sites, represented by a pair of Gaussian wells. We analyze the ICEC related electron flux density as a function of confinement size and are able to clearly identify two distinct capture channels: a direct long-range electron-electron impulse and a conversion of kinetic energy to electron-electron correlation energy with consecutive ICD. The overlay of both channels makes ICEC extremely likely, while nanowires are a strong candidate for the next miniaturization step of integrated-circuit components.

Critical role of a double-layer configuration in solution-based unipolar resistive switching memories

Lately, resistive switching memories (ReRAM) have been attracting a lot of attention due to their possibilities of fast operation, lower power consumption and simple fabrication process and they can also be scaled to very small dimensions. However, most of these ReRAM are produced by physical methods and nowadays the industry demands more simplicity, typically associated with low cost manufacturing. As such, ReRAMs in this work are developed from a solution-based aluminum oxide (Al₂O₃) using a simple combustion synthesis process. The device performance is optimized by two-stage deposition of the Al₂O₃ film. The resistive switching properties of the bilayer devices are reproducible with a yield of 100%. The ReRAM devices show unipolar resistive switching behavior with good endurance and retention time up to 10⁵ s at 85 °C. The devices can be programmed in a multi-level cell operation mode by application of different reset voltages. Temperature analysis of various resistance states reveals a filamentary nature based on the oxygen vacancies. The optimized film was stacked between ITO and indium zinc oxide, targeting a fully transparent device for applications on transparent system-on-panel technology.

A position-controllable external stage for critical dimension measurements via low-noise atomic force microscopy

An independent external stage for low noise atomic force microscope has been developed for mid-range movements so that it aids in measurements of critical dimensions through the low-noise atomic force microscope. The maximum travel length of the external four-axes stage is 10 mm. For image scanning of the specific target region, the sample needs to be moved through two steps: coarse positioning with the external stage and fine positioning with PI XY piezo scanner. Prior to the CD measurements, we confirmed that the position errors caused by the external stage and tip stage were negligible through the reproducibility experiments. In this study, custom-designed software stored the initial position of the probe and then moved it precisely to the sample location to be measured. Subsequently, the sidewalls of an improved vertical parallel structure were measured and the repeatability and reproducibility of the CD measurements were estimated using a CDR30-EBD tip. Finally, we confirmed that tip wear could be minimized by measuring TGX1 samples with undercut structures.

Magnetism of new metastable cobalt-nitride compounds

The search for new magnetic materials with high magnetization and magnetocrystalline anisotropy is important for a wide range of applications including information and energy processing. There is only a limited number of naturally occurring magnetic compounds that are suitable. This situation stimulates an exploration of new phases that occur far from thermal-equilibrium conditions, but their stabilization is generally inhibited due to high positive formation energies. Here a nanocluster-deposition method has enabled the discovery of a set of new non-equilibrium Co-N intermetallic compounds. The experimental search was assisted by computational methods including adaptive-genetic-algorithm and electronic-structure calculations. Conventional wisdom is that the interstitial or substitutional solubility of N in Co is much lower than that in Fe and that N in Co in equilibrium alloys does not produce materials with significant magnetization and anisotropy. By contrast, our experiments identify new Co-N compounds with favorable magnetic properties including hexagonal Co3N nanoparticles with a high saturation magnetic polarization (Js = 1.28 T or 12.8 kG) and an appreciable uniaxial magnetocrystalline anisotropy (K1 = 1.01 MJ m-3 or 10.1 Mergs per cm3). This research provides a pathway for uncovering new magnetic compounds with computational efficiency beyond the existing materials database, which is significant for future technologies.

Flexible and Stretchable Bio-Integrated Electronics Based on Carbon Nanotube and Graphene

Scientific and engineering progress associated with increased interest in healthcare monitoring, therapy, and human-machine interfaces has rapidly accelerated the development of bio-integrated multifunctional devices. Recently, compensation for the cons of existing materials on electronics for health care systems has been provided by carbon-based nanomaterials. Due to their excellent mechanical and electrical properties, these materials provide benefits such as improved flexibility and stretchability for conformal integration with the soft, curvilinear surfaces of human tissues or organs, while maintaining their own unique functions. This review summarizes the most recent advanced biomedical devices and technologies based on two most popular carbon based materials, carbon nanotubes (CNTs) and graphene. In the beginning, we discuss the biocompatibility of CNTs and graphene by examining their cytotoxicity and/or detrimental effects on the human body for application to bioelectronics. Then, we scrutinize the various types of flexible and/or stretchable substrates that are integrated with CNTs and graphene for the construction of high-quality active electrode arrays and sensors. The convergence of these carbon-based materials and bioelectronics ensures scalability and cooperativity in various fields. Finally, future works with challenges are presented in bio-integrated electronic applications with these carbon-based materials.

Interband Transitions in Monolayer and Few-Layer WSe2 Probed Using Photoexcited Charge Collection Spectroscopy

Transition-metal dichalcogenides are currently under rigorous investigation because of their distinct layer-dependent physical properties originating from the corresponding evolution of the band structure. Here, we report the highly resolved probing of layer-dependent band structure evolution for WSe₂ using photoexcited charge collection spectroscopy (PECCS). Monolayer, few-layer, and multilayer WSe₂ can be probed in top-gate field-effect transistor platforms, and their interband transitions are efficiently observed. Our theoretical calculations show a great coincidence with the PECCS results, proving that the indirect Γ-K and Γ-Λ transitions as well as the direct K-K transition are clearly resolved in multilayer WSe₂ by PECCS.

Photoelectrochemical modulation of neuronal activity with free-standing coaxial silicon nanowires

Optical methods for modulating cellular behaviour are promising for both fundamental and clinical applications. However, most available methods are either mechanically invasive, require genetic manipulation of target cells or cannot provide subcellular specificity. Here, we address all these issues by showing optical neuromodulation with free-standing coaxial p-type/intrinsic/n-type silicon nanowires. We reveal the presence of atomic gold on the nanowire surfaces, likely due to gold diffusion during the material growth. To evaluate how surface gold impacts the photoelectrochemical properties of single nanowires, we used modified quartz pipettes from a patch clamp and recorded sustained cathodic photocurrents from single nanowires. We show that these currents can elicit action potentials in primary rat dorsal root ganglion neurons through a primarily atomic gold-enhanced photoelectrochemical process.

Changing molecular conjugation with a phenazine acceptor for improvement of small molecule-based organic electronic memory performance

Two small molecules with electron-accepting azobenzene or phenazine blocks, were synthesized. The experimental findings suggest that the molecule with larger conjugation may promote the memory performance by an enhanced strong charge transfer effect.

Metrology for the next generation of semiconductor devices

The semiconductor industry continues to produce ever smaller devices that are ever more complex in shape and contain ever more types of materials. The ultimate sizes and functionality of these new devices will be affected by fundamental and engineering limits such as heat dissipation, carrier mobility and fault tolerance thresholds. At present, it is unclear which are the best measurement methods needed to evaluate the nanometre-scale features of such devices and how the fundamental limits will affect the required metrology. Here, we review state-of-the-art dimensional metrology methods for integrated circuits, considering the advantages, limitations and potential improvements of the various approaches. We describe how integrated circuit device design and industry requirements will affect lithography options and consequently metrology requirements. We also discuss potentially powerful emerging technologies and highlight measurement problems that at present have no obvious solution.

The switching of strong spin wave beams in patterned garnet films

The application of spin waves in communication with information encoded in amplitude and phase could replace or enhance existing microelectronic and microwave devices with significantly decreased energy consumption. Spin waves (SW) are usually transported in a magnetic material shaped to act as a waveguide. However, the implementation of SW transport and switching in plane homogeneous magnetic films and running as a narrow beam with a small divergence angle still present a challenge. We propose a realization of a strong SW switchers based on a patterned yttrium iron garnet (YIG) film that could serve as a magnonic fundamental building block. Our concept relies on the creation of a narrow beam of relatively short-wavelength SW by effect of a total non-reflection, found to be tied to refraction on the decreasing internal magnetic field, near a line of antidots at YIG. Nonreciprocal SW excitation by a microstrip antenna is used for controlling the direction of the signal flow. We demonstrate unique features of the propagation of microwave-excited SW beams, provide insight into their physics and discuss their potential applications in high-frequency devices.

Metrology of DNA arrays by super-resolution microscopy

Recent results in the assembly of DNA into structures and arrays with nanoscale features and patterns have opened the possibility of using DNA for sub-10 nm lithographic patterning of semiconductor devices. Super-resolution microscopy is being actively developed for DNA-based imaging and is compatible with inline optical metrology techniques for high volume manufacturing. Here, we combine DNA tile assembly with state-dependent super-resolution microscopy to introduce crystal-PAINT as a novel approach for metrology of DNA arrays. Using this approach, we demonstrate optical imaging and characterization of DNA arrays revealing grain boundaries and the temperature dependence of array quality. For finite arrays, analysis of crystal-PAINT images provides further quantitative information of array properties. This metrology approach enables defect detection and classification and facilitates statistical analysis of self-assembled DNA nanostructures.

Switching of Resistive Memory Behavior from Binary to Ternary Logic via Alteration of Substituent Positioning on the Subphthalocyanine Core

Two new axially or peripherally functionalized subphthalocyanines with the decoration of donor-acceptor substituents have been successfully synthesized, characterized and employed in the application of resistive memory device via solution-processable technique. Axially substituted subphthalocyanine shows ternary resistive memory behavior with well-separated current ratios of 1:10⁶:10⁸ between "OFF", "ON1" and "ON2" states, while only binary logic is observed for peripherally substituted subphthalocyanine. Computational studies show the presence of two well-separated charge transfer states in the axially substituted subphthalocyanine, while the charge transfer processes between the peripheral substituents and the subphthalocyanine core are found to be very close in energy. This work has demonstrated the impact of the substituent positioning on the subphthalocyanine-based memory device performance, providing a new research dimension for the future design and development of multistate organic resistive memory.

Robust overlay metrology with differential Mueller matrix calculus

Overlay control is of vital importance to good device performances in semiconductor manufacturing. In this work, the differential Mueller matrix calculus is introduced to investigate the Mueller matrices of double-patterned gratings with overlay displacements, which helps to reveal six elementary optical properties hidden in the Mueller matrices. We find and demonstrate that, among these six elementary optical properties, the linear birefringence and dichroism, LB' and LD', along the ± 45° axes show a linear response to the overlay displacement and are zero when the overlay displacement is absent at any conical mounting. Although the elements from the two 2 × 2 off-diagonal blocks of the Mueller matrix have a similar property to LB' and LD', as reported in the literature, we demonstrate that it is only valid at a special conical mounting with the plane of incidence parallel to grating lines. The better property of LB' and LD' than the Mueller matrix elements of the off-diagonal blocks in the presence of overlay displacement verifies them to be a more robust indicator for the diffraction-based overlay metrology.

In Situ Patterning of Ultrasharp Dopant Profiles in Silicon

We develop a method for patterning a buried two-dimensional electron gas (2DEG) in silicon using low kinetic energy electron stimulated desorption (LEESD) of a monohydride resist mask. A buried 2DEG forms as a result of placing a dense and narrow profile of phosphorus dopants beneath the silicon surface; a so-called δ-layer. Such 2D dopant profiles have previously been studied theoretically, and by angle-resolved photoemission spectroscopy, and have been shown to host a 2DEG with properties desirable for atomic-scale devices and quantum computation applications. Here we outline a patterning method based on low kinetic energy electron beam lithography, combined with in situ characterization, and demonstrate the formation of patterned features with dopant concentrations sufficient to create localized 2DEG states.

Amplitude and phase beam shaping for highest sensitivity in sidewall angle detection

In integrated circuits manufacturing, specific structures are used as tools to evaluate the quality of the lithographic process, and the shape of these structures is often described by a few parameters, of which in particular the sidewall angle suffers from considerable inaccuracies. Using scalar diffraction theory, we investigate whether a properly shaped cylindrically focused probing beam could increase the ability to detect tiny changes in this angle in the case of a cliff-like structure, modeled as a phase object. This paper describes the theoretical formulation used to calculate the optimized beam and compares its performance with the case of a focused plane wave.

Demonstration of a robust magnonic spin wave interferometer

Magnonics is an emerging field dealing with ultralow power consumption logic circuits, in which the flow of spin waves, rather than electric charges, transmits and processes information. Waves, including spin waves, excel at encoding information via their phase using interference. This enables a number of inputs to be processed in one device, which offers the promise of multi-input multi-output logic gates. To realize such an integrated device, it is essential to demonstrate spin wave interferometers using spatially isotropic spin waves with high operational stability. However, spin wave reflection at the waveguide edge has previously limited the stability of interfering waves, precluding the use of isotropic spin waves, i.e., forward volume waves. Here, a spin wave absorber is demonstrated comprising a yttrium iron garnet waveguide partially covered by gold. This device is shown experimentally to be a robust spin wave interferometer using the forward volume mode, with a large ON/OFF isolation value of 13.7 dB even in magnetic fields over 30 Oe.

Thickness-dependent charge transport in few-layer MoS₂ field-effect transistors

Molybdenum disulfide (MoS2) is currently under intensive study because of its exceptional optical and electrical properties in few-layer form. However, how charge transport mechanisms vary with the number of layers in MoS2 flakes remains unclear. Here, exfoliated flakes of MoS2 with various thicknesses were successfully fabricated into field-effect transistors (FETs) to measure the thickness and temperature dependences of electrical mobility. For these MoS2 FETs, measurements at both 295 K and 77 K revealed the maximum mobility for layer thicknesses between 5 layers (∼3.6 nm) and 10 layers (∼7 nm), with ∼70 cm(2) V(-1) s(-1) measured for 5 layer devices at 295 K. Temperature-dependent mobility measurements revealed that the mobility rises with increasing temperature to a maximum. This maximum occurs at increasing temperature with increasing layer thickness, possibly due to strong Coulomb scattering from charge impurities or weakened electron-phonon interactions for thicker devices. Temperature-dependent conductivity measurements for different gate voltages revealed a metal-to-insulator transition for devices thinner than 10 layers, which may enable new memory and switching applications. This study advances the understanding of fundamental charge transport mechanisms in few-layer MoS2, and indicates the promise of few-layer transition metal dichalcogenides as candidates for potential optoelectronic applications.

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

This review presents recent progress on charge transport properties, carrier scattering mechanisms, and carrier mobility engineering of two-dimensional transition metal chalcogenides.

Integration of High-k Oxide on MoS2 by Using Ozone Pretreatment for High-Performance MoS2 Top-Gated Transistor with Thickness-Dependent Carrier Scattering Investigation

A top-gated MoS2 transistor with 6 nm thick HfO2 is fabricated using an ozone pretreatment. The influence to the top-gated mobility brought about by the deposition of HfO2 is studied statistically, for the first time. The top-gated mobility is suppressed by the deposition of HfO2 , and multilayered samples are less susceptible than monolayer ones.

A Solution-Processable Donor-Acceptor Compound Containing Boron(III) Centers for Small-Molecule-Based High-Performance Ternary Electronic Memory Devices

A novel small-molecule boron(III)-containing donor-acceptor compound has been synthesized and employed in the fabrication of solution-processable electronic resistive memory devices. High ternary memory performances with low turn-on (V(Th1)=2.0 V) and distinct threshold voltages (V(Th2)=3.3 V), small reading bias (1.0 V), and long retention time (>10(4) seconds) with a large ON/OFF ratio of each state (current ratio of "OFF", "ON1", and "ON2"=1:10(3):10(6)) have been demonstrated, suggestive of its potential application in high-density data storage. The present design strategy provides new insight in the future design of memory devices with multi-level transition states.

Atomic force microscope caliper for critical dimension measurements of micro and nanostructures through sidewall scanning

A novel atomic force microscope (AFM) dual-probe caliper for critical dimension (CD) metrology has been developed. The caliper is equipped with two facing tilted optical fiber probes (OFPs) wherein each can be used independently to scan either sidewall of micro and nanostructures. The OFP tip with length up to 500 μm (aspect ratio 10:1, apex diameter ⩾10 nm) has unique features of scanning deep trenches and imaging sidewalls of relatively high steps with exclusive profiling possibilities. The caliper arms-OFPs can be accurately aligned with a well calibrated opening distance. The line width, line edge roughness, line width roughness, groove width and CD angles can be measured through serial scan of adjacent or opposite sidewalls with each probe. Capabilities of the presented AFM caliper have been validated through experimental CD measurement results of comb microstructures and AFM calibration grating TGZ3.

Nanoscale Buckling of Ultrathin Low-k Dielectric Lines during Hard-Mask Patterning

Commonly known in macroscale mechanics, buckling phenomena are now also encountered in the nanoscale world as revealed in today's cutting-edge fabrication of microelectronics. The description of nanoscale buckling requires precise dimensional and elastic moduli measurements, as well as a thorough understanding of the relationships between stresses in the system and the ensuing morphologies. Here, we analyze quantitatively the buckling mechanics of organosilicate fins that are capped with hard masks in the process of lithographic formation of deep interconnects. We propose an analytical model that quantitatively describes the morphologies of the buckled fins generated by residual stresses in the hard mask. Using measurements of mechanical properties and geometric characteristics, we have verified the predictions of the analytical model for structures with various degrees of buckling, thus putting forth a framework for guiding the design of future nanoscale interconnect architectures.

Transformers: the changing phases of low-dimensional vanadium oxide bronzes

In this feature article, we explore the electronic and structural phase transformations of ternary vanadium oxides with the composition M_xV₂O₅where M is an intercalated cation.

Bipolar electric-field enhanced trapping and detrapping of mobile donors in BiFeO3 memristors

Pulsed laser deposited Au-BFO-Pt/Ti/Sapphire MIM structures offer excellent bipolar resistive switching performance, including electroforming free, long retention time at 358 K, and highly stable endurance. Here we develop a model on modifiable Schottky barrier heights and elucidate the physical origin underlying resistive switching in BiFeO3 memristors containing mobile oxygen vacancies. Increased switching speed is possible by applying a large amplitude writing pulse as the resistive switching is tunable by both the amplitude and length of the writing pulse. The local resistive switching has been investigated by conductive atomic force microscopy and exhibits the capability of down-scaling the resistive switching cell to the grain size.

Traceable GISAXS measurements for pitch determination of a 25 nm self-assembled polymer grating

The feature sizes of only a few nanometres in modern nanotechnology and next-generation microelectronics continually increase the demand for suitable nanometrology tools. Grazing-incidence small-angle X-ray scattering (GISAXS) is a versatile technique to measure lateral and vertical sizes in the nanometre range, but the traceability of the obtained parameters, which is a prerequisite for any metrological measurement, has not been demonstrated so far. In this work, the first traceable GISAXS measurements, demonstrated with a self-assembled block copolymer grating structure with a nominal pitch of 25 nm, are reported. The different uncertainty contributions to the obtained pitch value of 24.83 (9) nm are discussed individually. The main uncertainty contribution results from the sample–detector distance and the pixel size measurement, whereas the intrinsic asymmetry of the scattering features is of minor relevance for the investigated grating structure. The uncertainty analysis provides a basis for the evaluation of the uncertainty of GISAXS data in a more general context, for example in numerical data modeling.

Reconstruction of sub-wavelength features and nano-positioning of gratings using coherent Fourier scatterometry

Optical scatterometry is the state of art optical inspection technique for quality control in lithographic process. As such, any boost in its performance carries very relevant potential in semiconductor industry. Recently we have shown that coherent Fourier scatterometry (CFS) can lead to a notably improved sensitivity in the reconstruction of the geometry of printed gratings. In this work, we report on implementation of a CFS instrument, which confirms the predicted performances. The system, although currently operating at a relatively low numerical aperture (NA = 0.4) and long wavelength (633 nm) allows already the reconstruction of the grating parameters with nanometer accuracy, which is comparable to that of AFM and SEM measurements on the same sample, used as reference measurements. Additionally, 1 nm accuracy in lateral positioning has been demonstrated, corresponding to 0.08% of the pitch of the grating used in the actual experiment.

Tunnel conductivity switching in a single nanoparticle-based nano floating gate memory

Nanoparticles (NPs) embedded in a conductive or insulating matrix play a key role in memristors and in flash memory devices. However, the role of proximity to the interface of isolated NPs has never been directly observed nor fully understood. Here we show that a reversible local switching in tunnel conductivity can be achieved by applying an appropriate voltage pulse using the tip of a scanning tunnelling microscope on NPs embedded in a TiO₂ matrix. The resistive switching occurs in the TiO₂ matrix in correlation to the NPs that are in proximity of the surface and it is spatially confined to the single NP size. The tunnel conductivity is increased by more than one order of magnitude. The results are rationalized by a model that include the charge of NPs that work as a nano floating gate inducing local band bending that facilitates charge tunnelling and by the formation and redistribution of oxygen vacancies that concentrate in proximity of the charged NPs. Our study demonstrates the switching in tunnel conductivity in single NP and provides useful information for the understanding mechanism or resistive switching.

Current transport mechanism at metal-semiconductor nanoscale interfaces based on ultrahigh density arrays of p-type NiO nano-pillars

The present work focuses on a qualitative analysis of localised I-V characteristics based on the nanostructure morphology of highly dense arrays of p-type NiO nano-pillars (NiO-NPs). Vertically aligned NiO-NPs have been grown on different substrates by using a glancing angle deposition (GLAD) technique. The preferred orientation of as grown NiO-NPs was controlled by the deposition pressure. The NiO-NPs displayed a polar surface with a microscopic dipole moment along the (111) plane (Tasker's type III). Consequently, the crystal plane dependent surface electron accumulation layer and the lattice disorder at the grain boundary interface showed a non-uniform current distribution throughout the sample surface, demonstrated by a conducting AFM technique (c-AFM). The variation in I-V for different points in a single current distribution grain (CD-grain) has been attributed to the variation of Schottky barrier height (SBH) at the metal-semiconductor (M-S) interface. Furthermore, we observed that the strain produced during the NiO-NPs growth can modulate the SBH. Inbound strain acts as an external field to influence the local electric field at the M-S interface causing a variation in SBH with the NPs orientation. This paper shows that vertical arrays of NiO-NPs are potential candidates for nanoscale devices because they have a great impact on the local current transport mechanism due to its nanostructure morphology.

Spin wave nonreciprocity for logic device applications

The utilization of spin waves as eigenmodes of the magnetization dynamics for information processing and communication has been widely explored recently due to its high operational speed with low power consumption and possible applications for quantum computations. Previous proposals of spin wave Mach-Zehnder devices were based on the spin wave phase, a delicate entity which can be easily disrupted. Here, we propose a complete logic system based on the spin wave amplitude utilizing the nonreciprocal spin wave behavior excited by microstrip antennas. The experimental data reveal that the nonreciprocity of magnetostatic surface spin wave can be tuned by the bias magnetic field. Furthermore, engineering of the device structure could result in a high nonreciprocity factor for spin wave logic applications.

Thickness-dependent interfacial Coulomb scattering in atomically thin field-effect transistors

Two-dimensional semiconductors are structurally ideal channel materials for the ultimate atomic electronics after silicon era. A long-standing puzzle is the low carrier mobility (μ) in them as compared with corresponding bulk structures, which constitutes the main hurdle for realizing high-performance devices. To address this issue, we perform a combined experimental and theoretical study on atomically thin MoS2 field effect transistors with varying the number of MoS2 layers (NLs). Experimentally, an intimate μ-NL relation is observed with a 10-fold degradation in μ for extremely thinned monolayer channels. To accurately describe the carrier scattering process and shed light on the origin of the thinning-induced mobility degradation, a generalized Coulomb scattering model is developed with strictly considering device configurative conditions, that is, asymmetric dielectric environments and lopsided carrier distribution. We reveal that the carrier scattering from interfacial Coulomb impurities (e.g., chemical residues, gaseous adsorbates, and surface dangling bonds) is greatly intensified in extremely thinned channels, resulting from shortened interaction distance between impurities and carriers. Such a pronounced factor may surpass lattice phonons and serve as dominant scatterers. This understanding offers new insight into the thickness induced scattering intensity, highlights the critical role of surface quality in electrical transport, and would lead to rational performance improvement strategies for future atomic electronics.

Exploring the limits of N-type ultra-shallow junction formation

Low resistivity, near-surface doping in silicon represents a formidable challenge for both the microelectronics industry and future quantum electronic devices. Here we employ an ultra-high vacuum strategy to create highly abrupt doping profiles in silicon, which we characterize in situ using a four point probe scanning tunnelling microscope. Using a small molecule gaseous dopant source (PH3) which densely packs on a reconstructed silicon surface, followed by encapsulation in epitaxial silicon, we form highly conductive dopant sheets with subnanometer control of the depth profiles. This approach allows us to test the limits of ultra-shallow junction formation, with room temperature resistivities of 780 Ω/□ at an encapsulation depth of 4.3 nm, increasing to 23 kΩ/□ at an encapsulation depth of only 0.5 nm. We show that this depth-dependent resistivity can be accounted for by a combination of dopant segregation and surface scattering.

Monte Carlo simulation of CD-SEM images for linewidth and critical dimension metrology

In semiconductor industry, strict critical dimension control by using a critical dimension scanning electron microscope (CD-SEM) is an extremely urgent task in near-term years. A Monte Carlo simulation model for study of CD-SEM image has been established, which is based on using Mott's cross section for electron elastic scattering and the full Penn dielectric function formalism for electron inelastic scattering and the associated secondary electron (SE) production. In this work, a systematic calculation of CD-SEM line-scan profiles and 2D images of trapezoidal Si lines has been performed by taking into account different experimental factors including electron beam condition (primary energy, probe size), line geometry (width, height, foot/corner rounding, sidewall angle, and roughness), material properties, and SE signal detection. The influences of these factors to the critical dimension metrology are investigated, leading to build a future comprehensive model-based library.

On-demand nanodevice with electrical and neuromorphic multifunction realized by local ion migration

A potential route to extend Moore's law beyond the physical limits of existing materials and device architectures is to achieve nanotechnology breakthroughs in materials and device concepts. Here, we discuss an on-demand WO(3-x)-based nanoionic device where electrical and neuromorphic multifunctions are realized through externally induced local migration of oxygen ions. The device is found to possess a wide range of time scales of memorization, resistance switching, and rectification varying from volatile to permanent in a single device, and these can furthermore be realizable in both two- or three-terminal systems. The gradually changing volatile and nonvolatile resistance states are experimentally demonstrated to mimic the human brain's forgetting process for short-term memory and long-term memory.We propose this nanoionic device with its on-demand electrical and neuromorphic multifunction has a unique paradigm shifting potential for the fabrication of configurable circuits, analog memories, digital-neural fused networks, and more in one device architecture.

Strain-gated piezotronic transistors based on vertical zinc oxide nanowires

Strain-gated piezotronic transistors have been fabricated using vertically aligned ZnO nanowires (NWs), which were grown on GaN/sapphire substrates using a vapor-liquid-solid process. The gate electrode of the transistor is replaced by the internal crystal potential generated by strain, and the control over the transported current is at the interface between the nanowire and the top or bottom electrode. The current-voltage characteristics of the devices were studied using conductive atomic force microscopy, and the results show that the current flowing through the ZnO NWs can be tuned/gated by the mechanical force applied to the NWs. This phenomenon was attributed to the piezoelectric tuning of the Schottky barrier at the Au-ZnO junction, known as the piezotronic effect. Our study demonstrates the possibility of using Au droplet capped ZnO NWs as a transistor array for mapping local strain. More importantly, our design gives the possibility of fabricating an array of transistors using individual vertical nanowires that can be controlled independently by applying mechanical force/pressure over the top. Such a structure is likely to have important applications in high-resolution mapping of strain/force/pressure.