Field Emission Electron Source Research Articles

Integrated multichip field emission electron source fabricated by laser-micromachining and MEMS technology

In this work, high-current field emission electron source chips were fabricated using laser-micromachining and MEMS technology. The resulting chips were combined with commercially available printed circuit boards (PCBs) to obtain a multichip electron source. By controlling the separate electron sources using an external current control circuit, we were able to divide the desired total current evenly across the individual chips deployed in the PCB-carrier. In consequence, we were able to show a decreased degradation due to the reduced current load per chip. First, a single electron source chip was measured without current regulation. A steady-state emission current of 1 mA with a high stability of ±1.3% at an extraction voltage of 250 V was observed. At this current level, a mean degradation slope of −0.7 μA/min with a nearly perfect transmission ratio of 99% ± 0.4% was determined. The measurements of a fully assembled multichip PCB-carrier electron source, using a current control circuit for regulation, showed that an even distribution of the desired total current led to a decreased degradation. This was determined by the increase in the required extraction voltage over time. For this purpose, two current levels were applied to the electron source chips of the PCB-carrier using an external current control circuit. First, 300 μA total current was evenly distributed among the individual electron source chips followed by the emission of 300 μA per electron source chip. This allows the observation of the influence of a distributed and nondistributed total current, carried by the electron source chips. Thereby, we obtained an increase in the mean degradation slope from +0.011 V/min (300 μA distributed) to +0.239 V/min (300 μA per chip), which is approximately 21 times higher. Moreover, our current control circuit improved the current stability to under 0.1% for both current levels, 300 μA distributed and 300 μA per chip.

Point field emission electron source with a magnetically focused electron beam

This paper presents a field emitter in the form of a silicon tip covered with a layer of carbon nanotubes. The emitted beam is focused with a set of two electrostatic lenses and – which is novelty in such structures – with a magnetic field. The presented approach gave very promising results. The field emitter was able to provide a high emission current (about 50 µA) and a beam with a small and homogeneous spot. Such electron sources are necessary components of many miniature MEMS and nanoelectronics devices. The presented source is dedicated especially for the use in currently developed MEMS X-ray sources and MEMS electron microscopes.

Construction of sp2/sp3 Hybrid Carbon Thin Layers on Silicon Substrates Using Nonequilibrium Excitation Reaction Fields.

The authors have developed a crystal growth process that utilizes electron beams from field emission (FE) to grow materials bottom-up by a method other than the transfer of thermal energy. In this study, highly crystalline single-walled carbon nanotubes were used as a field emission electron source. Electron beams with high resolution energy emitted from the source were irradiated onto acetylene gas as a nonequilibrium reaction field to induce acetylene dissociation. The generated carbon ions were then irradiated onto a [100] silicon substrate, resulting in the irradiation of the silicon substrate surface with graphene. Moreover, the crystal growth of sp2/sp3 hybrid carbon thin layers, which is different from the crystal structures of graphite, diamond, and diamond-like carbon, proceeded on the surface of the silicon substrate. Carbon layers on periodic crystal structures whose growth depends at least on the morphology of the substrate are formed through bridging with the binding site of the substrate. The authors have succeeded in developing a nonthermal technique of crystal bridging between different elements. The substrate on which the carbon layer is formed is not limited to silicon; other substrates with various crystal structures and periodicities are expected to be used.

Near-Monochromatic Tuneable Cryogenic Niobium Electron Field Emitter.

Creating, manipulating, and detecting coherent electrons is at the heart of future quantum microscopy and spectroscopy technologies. Leveraging and specifically altering the quantum features of an electron beam source at low temperatures can enhance its emission properties. Here, we describe electron field emission from a monocrystalline, superconducting niobium nanotip at a temperature of 5.9K. The emitted electron energy spectrum reveals an ultranarrow distribution down to 16meV due to tunable resonant tunneling field emission via localized band states at a nanoprotrusion's apex and a cutoff at the sharp low-temperature Fermi edge. This is an order of magnitude lower than for conventional field emission electron sources. The self-focusing geometry of the tip leads to emission in an angle of 3.7°, a reduced brightness of 3.8×10^{8} A/(m^{2} sr V), and a stability of hours at 4.1nA beam current and 69meV energy width. This source will decrease the impact of lens aberration and enable new modes in low-energy electron microscopy, electron energy loss spectroscopy, and high-resolution vibrational spectroscopy.

Synthesis of high-purity and high-crystallinity single-walled carbon nanotubes homogeneously dispersed by wet process

• The highly crystalline single-walled carbon nanotubes are basic electronic elements. • It is necessary to remove contaminants in hc-SWCNTs to disperse them homogeneously. • Carbon contaminants in hc-SWCNTs disperse the shear force of hc-SWCNT bundles. • The shear force to disperse hc-SWCNTs is required to break up their aggregates. • Highly purified hc-SWCNTs could be dispersed without deteriorated chemical stability. Field emission (FE) electron sources based on high-purity and high-crystallinity single-walled carbon nanotubes (hc-SWCNTs) are promising electronic devices from the viewpoint of industrial application. However, to utilize high-purity hc-SWCNTs as a component of electronic devices, it is necessary to homogeneously disperse hc-SWCNTs using a wet process. Shear force is required to break up aggregates of hc-SWCNTs to produce a film with uniformly dispersed hc-SWCNTs, but contaminants such as fine graphite in the aggregates disperse the shear force of hc-SWCNT bundles. Therefore, it is necessary to remove these contaminants to achieve the uniform dispersion of hc-SWCNTs. In this study, we have succeeded in the highly efficient synthesis of hc-SWCNTs with few contaminants by the arc discharge method using glass-like carbon as a raw material to produce SWCNTs. It was demonstrated that hc-SWCNTs with low foreign matter contamination can be uniformly dispersed without the need for wet dispersion tools such as a homogenizer and a jet mill. Moreover, the long-term stability of the current emitted by FE devices fabricated using the dispersed solution can be obtained.

Optimization of a Field Emission Electron Source Based on Nano-Vacuum Channel Structures.

Recent discoveries in the field of nanoscale vacuum channel (NVC) structures have led to potential on-chip electron sources. However, limited research has reported on the structure or material parameters, and the superiority of a nanoscale vacuum channel in an electron source has not been adequately demonstrated. In this paper, we perform the structural optimization design of an NVC-based electron source. First, the structure parameters of a vertical NVC-based electron source are investigated. Moreover, the symmetrical NVC structure is further demonstrated to improve the emission current and effective electron efficiency. Finally, a symmetrical nano-vacuum channel structure is successfully fabricated based on simulations. The results show that the anode current exceeds 15 nA and that the effective electron efficiency exceeds 20%. Further miniaturizing the NVC structures in high integration can be utilized as an on-chip electron source, thereby, illustrating the potential in applications of electron microscopes, miniature X-ray sources and on-chip traveling wave tubes.

A stable LaB6 nanoneedle field-emission electron source for atomic resolution imaging with a transmission electron microscope

Field-induced electron emission offers the best performance point electron source for electron microscopes. Herewith we report a new cold field-emission point electron source utilizing a nanoneedle made of lanthanum hexaboride (LaB6) and its implementation in a spherical aberration-corrected transmission electron microscope (TEM). A sub-ångstrom resolution of 0.96 Å has been obtained with the TEM operated at an acceleration voltage of 60 kV and a smaller energy spread was also observed compared with the contemporary W(310) electron source. In particular, the LaB6 nanoneedle cold field-emission electron source required no thermal flashing for continuous operation of extended hours (>100 hours) while exhibiting and sustaining a high stability in emission current (<1%). We attribute the excellent performance of the LaB6 nanoneedle electron source to (i) a low work function of LaB6; (ii) excellent alignment of the nanoneedle emitter by the electron and ion dual-beam processing with nanoscale precision and (iii) robustness of the nanometric structure that suppressed mechanical vibrations of the LaB6 emitter even in a high electric field. This new LaB6 nanoneedle cold field-emission electron source enables stable high-resolution imaging in TEM and it will also benefit cryogenic electron microscopy, scanning electron microscopy, electron beam lithography, and other electron beam technologies.

Improving field emission performance of patterned ZnO electron emission source by optimizing array spacing

The field emission (FE) electron source has a wide range of application in the field of vacuum electronics, and the patterning of cathode emitters is one of the key technology. In this paper, a new idea of cathode structure to achieve good luminescence uniformity with the smallest emitter areas is proposed. First, the inkjet printing is applied to precisely position the patterned layer, and the branched ZnO arrays are grown by electrochemical deposition. Then the effect of array spacings on field emission performance is studied. The results show that with the increase of the array spacing, the FE performance first increases and then decreases, among which the arrays with the moderate spacing have the best FE properties, including the lowest turn-on field (Eon) of ∼0.46 V μm−1 and the most uniform luminescence. Finite element calculations using the ANSYS software were performed. Simulations are carried out to analyze the surface electric field distribution and electron movement trajectory, indicating the moderate array spacing is beneficial for FE due to the balance of the field superposition effect and shielding effect. The efficiency of pattern design helps to realize the high-performance field emission electron source, which is of great significance for flexible manufacturing and large-scale integration.

Multistage carbon nanotubes grown on foamed nickel with organic solutions as multifunctional high performance electrodes

In this study, carbon nanotubes (CNTs) are grown by easy method with organic solutions upon foamed nickel substrates. It is found that ethanol is most likely to grow higher quality CNTs than other organic solutions without deoxidation gas. The foamed nickel not only acted as substrates but also uesed as catalyst to grow CNTs and then formed good electrical and thermal contact with CNTs. Moerover, the foamed nickel has intrinsic low contact resistance with CNTs and their multistage structure also makes it as multifunctional electrode for electrical devices. With multistage and high enhancement factor, these CNTs grown on foamed nickel have better field emission properties with turn-on field at 1.4 V/μm and the threshold-field at 2.9 V/μm. With high specific surface area, it is also has high sensitive property to detect heavy metal ion such as Pb2+. The results show it has a good linear relationship exists in the range of 100–400 ppb, and the limit of detection is about 0.955 μg/L Pb2+.This easy CNTs growth method offers a potential application in making field emission electron source and other electrical electrodes.

The Design of a Reflection Electron Energy Loss Spectrometer Attachment for Low Voltage Scanning Electron Microscopy

This paper presents the design of a reflection electron energy spectrometer (REELS) attachment for low voltage scanning electron microscopy (LVSEM) applications. The design is made by carrying out a scattered electron trajectory ray paths simulation. The spectrometer attachment is small enough to fit on the specimen stage of an SEM, and aims to acquire nanoscale spatially resolved REELS information. It uses a retarding field electrostatic toroidal sector energy analyzer design, which is able to lower the kinetic energies of elastically backscattered electrons to pass energies of 10 eV or less. For the capture of 1 keV BSEs emitted in the polar angular range between 40 to 50°, direct ray-tracing simulations predict that the spectrometer attachment will have an energy resolution of around 0.4 eV at a pass energy of 10 eV, and 0.2 eV at a pass energy of 5 eV. This predicted performance will make it a suitable REELS attachment for SEMs that use field emission electron sources.

Development and Analysis of Software for the Numerical Simulation of Field Emission Electron Sources

Development and Analysis of Software for the Numerical Simulation of Field Emission Electron Sources

Silicon Field Emission Electron Source With Individually Controllable Single Emitters

An electron source with an array of individually controllable single tips allows the observation of each emitter current simultaneously as well as the measurement of the integral current. Furthermore, the initial activation process of the individual tips and current distribution of the source can be examined. The Si cathode consists of an array of 2×2 conically shaped emitter structures on a borosilicate glass substrate. The emitters were fabricated by laser micromachining and wet etching. Integral field emission (FE) measurements were performed in a diode configuration in a vacuum chamber at pressures of about 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> mbar with a self-aligned Si extraction grid, due to adjustment pins. The total emission current was regulated to a predefined value (4.0, 10, 20, 40, and 50 μA) by an external regulating circuit and recorded individually during the measurement. The onset voltage for a current of 1 nA varied between 320 and 430 V. The voltage conversion factor is in the range of 7.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 1.3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Constant current measurements over a longer period of time (1800 values with a sample rate of 0.5 Hz) were performed and show a total current fluctuation less than 0.6% due to the regulation circuit. Despite the stable total current in regulated operation, it was observed that the individual emitters fluctuate up to 75% for low (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> A) and 5% for high (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> A) currents.

Analytical Cryo-Scanning Electron Microscopy of Hydrated Polymers and Microgels.

Despite the fact that scanning electron microscopes (SEM) coupled with energy-dispersive X-ray microanalysis (EDS) has been commercially available for more than a half-century, SEM/EDS continues to develop and open new opportunities to study the morphology of advanced materials. This is particularly true in applications to hydrated soft matter. Developments in field-emission electron sources that enable low-voltage imaging of uncoated polymers, silicon-drift detectors that enable high-efficiency collection of X-rays characteristic of light elements, and cryogenic methods to effectively cryo-fix hydrated samples have opened new opportunities to apply techniques relatively well established in hard-materials applications to challenging new problems involving synthetic polymers. We have applied cryo-SEM imaging and spatially resolved EDS to collect new information characterizing polyelectrolyte microgels. These are charged gel particles with dimensions in the range of 0.1-100 μm. Perhaps most notable is the fact that the high hydration levels-the samples are mostly water-allow robust calibration curves to be generated using frozen-hydrated buffers with known salt and/or hydrocarbon compositions. Such calibration curves enable quantitative composition measurements in the low-concentration extremes associated with high-swelling hydrogels. We use an experimentally derived carbon calibration curve to determine the microgel swell ratio, Q. The swell ratio, arguably, is the single most important gel characteristic because it is directly related to the mesh size of the networked polymer, which in turn determines many of the gel's mechanical and transport properties. While Q can be experimentally measured in macroscopic gels based on weight measurements in the dry and hydrated states, it is very difficult to measure in a microgel, and the fact that EDS in a cryo-SEM can determine Q from a single X-ray spectrum is significant. Furthermore, because of the electrostatic charge distributed along the polymer chains, the presence and concentration of counter-ions play a critical role in polyelectrolyte systems. While conceptually understood for decades, experimental measurements of counter-ion concentrations have been largely limited to a relatively small set of materials that involve macroscopic samples. By developing calibration curves from frozen-hydrated buffer of known ionic strength, we measure the concentration of Na counter-ions in microgels of poly(acrylic acid) (PAA) with a limit of detection of ∼0.014 M. Such measurements may help resolve some long-standing questions in polyelectrolyte science concerning counter-ion condensation. Even in the absence of a calibration curve, we show that spatially resolved X-ray spectroscopy can map the spatial distribution of a cationic oligopeptide complexed within a hydrated PAA microgel because of the nitrogen fingerprint that, albeit at very low concentration, is unique to the peptide. We look specifically at the case of a microgel with a so-called core-shell structure, where, again, the underlying polyelectrolyte science responsible for core-shell formation remains incompletely understood. These examples highlight how a modern cryo-SEM can be exploited to quantitatively characterize hydrated soft matter. The approach is almost certain to continue its development and impact as the base of experienced practitioners, the accessibility to well-configured microscopes, and the abundance of challenging problems involving hydrated soft matter all continue to grow.

Highly enhanced field emission properties of a carbon nanotube cathode on a titanium alloy substrate modified by alkali

Field emission electron sources with low turn-on electric fields, high emission current density, and good stability are of paramount importance in widespread applications, and in particular vacuum nanoelectronic equipment. Herein, an alkali modification approach was rationally designed to tailor the morphology of the commonly used compact and planar titanium alloy substrate. As expected, the modified substrate exhibited an interconnected porous network morphology, which offers remarkably strengthened adhesion with the screen-printed carbon nanotube cathode atop. As a result, the devices delivered comprehensively improved performance. Specifically, in the continuous driving mode, the turn-on electric field was as low as 0.79 V/μm, while the emission current of 85.13 mA/cm2 was relatively high. In parallel, in the pulse driving mode, the emission current density reached 1.266 A/cm2 under an electric field of 6.1 V/μm. Encouragingly, the cathode exhibited rather decent emission stability even under continuous operation for hours. This work provides a practical strategy for further enhancement of the field emission performance of carbon nanotube cathodes and highlights the significance of engineering titanium alloy substrates in future development.

Electron transmission through suspended graphene membranes measured with a low-voltage gated Si field emitter array

We experimentally demonstrate the transmission of electrons through different number (1, 2, and 5) of suspended graphene layers at electron energies between 20 and 250 eV. Electrons with initial energies lower than 40 eV are generated using silicon field emitter arrays with 1 μm pitch, and accelerated towards the graphene layers supported by a silicon nitride grid biased at voltages from −20 to 200 V. We measured significant increase in current collected at the anode with the presence of graphene, which is attributed to the possible generation of secondary electrons by primary electrons impinging on the graphene membrane. Highest output current was recorded with monolayer graphene at approximately 90 eV, with up to 1.7 times the incident current. The transparency of graphene to low-energy electrons and its impermeability to gas molecules could enable low-voltage field emission electron sources, which often require ultra-high vacuum, to operate in a relatively poor vacuum environment.

Quantitative Backscattered Electron Imaging of Bone Using a Thermionic or a Field Emission Electron Source

Quantitative backscattered electron imaging is an established method to map mineral content distributions in bone and to determine the bone mineralization density distribution (BMDD). The method we applied was initially validated for a scanning electron microscope (SEM) equipped with a tungsten hairpin cathode (thermionic electron emission) under strongly defined settings of SEM parameters. For several reasons, it would be interesting to migrate the technique to a SEM with a field emission electron source (FE-SEM), which, however, would require to work with different SEM parameter settings as have been validated for DSM 962. The FE-SEM has a much better spatial resolution based on an electron source size in the order of several 100 nanometers, corresponding to an about 10^5 to 10^6 times smaller source area compared to thermionic sources. In the present work, we compare BMDD between these two types of instruments in order to further validate the methodology. We show that a transition to higher pixel resolution (1.76, 0.88, and 0.57 μm) results in shifts of the BMDD peak and BMDD width to higher values. Further the inter-device reproducibility of the mean calcium content shows a difference of up to 1 wt% Ca, while the technical variance of each device can be reduced to pm 0.17 wt% Ca. Bearing in mind that shifts in calcium levels due to diseases, e.g., high turnover osteoporosis, are often in the range of 1 wt% Ca, both the bone samples of the patients as well as the control samples have to be measured on the same SEM device. Therefore, we also constructed new reference BMDD curves for adults to be used for FE-SEM data comparison.

Nanometer-Thick Crystalline Carbon Films Having a Spinel Structure Grown on ZnO Substrates: Implications for New Ceramic–Carbon Composition

I developed a bottom-up process of crystal growth using a field emission (FE) electron beam without transfer of heat energy. In this study, highly crystalline single-walled carbon nanotubes were used as the FE electron source. Acetylene was irradiated with an electron beam of high-resolution energy emitted from the electron source. Then, zinc oxide (ZnO) was irradiated with the carbon-based ions dissociated from the acetylene and electron beam, which formed a nonequilibrium excitation reaction field. As a result, a crystalline carbon thin film with a spinel-like structure different from the structures of graphite and diamond was grown on the ZnO surface. It is considered that the carbon film can be formed on substrates with a periodic crystal structure, not only ZnO. I confirmed that a carbon film with a periodic crystal structure independent of the crystal structure of the underlying substrate was grown, which bridged with the substrate. Thus, I have established a technique of crystal bridging between a ceramic and carbon for the first time to the best of our knowledge.

Si Nanowire With Integrated Space-Charge- Limited Conducted Schottky Junction for Enhancing Field Electron Emission and Its Gated Devices

Integrated field emission electron sources with low driving voltage are desired for micro/nano vacuum electronics. Herein, we demonstrate the enhanced and stable electron emission from crystalline gold-silver (Au-Ag) alloy nanoparticle capped Si nanowires (SiNWs) and the achieved gated electron sources with low driving voltage (<; 50 V). Thermally induced self-assembly hemispherical (50-80 nm) crystalline Au-Ag nanoparticles were prepared and used as masks in plasma etching to form vertically aligned high aspect ratio (~14) SiNWs. The nanoparticles were preserved after the etching, forming in situ integrated Au-Ag/Si junctions with each individual SiNWs. The SiNWs with capped nanoparticles possess superior field emission current density (45.80 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and current stability (fluctuation ~6%). A self-aligned process was developed to fabricate gated field emission devices, using the Au-Ag nanoparticle capped SiNWs as cathode. The gated devices showed field emission at low gate voltage (69.52-μA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 42.1 V). Experimental and numerical simulation studies demonstrated the Au-Ag/Si contact behaved as space-charge-limited (SCL) Schottky conduction. The idea behind the enhancing and stable emission is that the SCL-Schottky junction undergoes a process of negative feedback and thus suppressed the current and shot-noise from the outstanding SiNWs. Accordingly, more SiNWs were activated and contributed stable and higher field emission current.

Nanoscale silicon field emitter arrays with self-aligned extractor and focus gates

Out-of-plane focusing is essential for electron beam collimation in gated field emission sources. The focus electrode redirects electrons emitted by the tip with a wide angle towards the central axis, resulting a small focal spot at the anode. Here, we demonstrate for the first time, very high density (108 emitters/cm2) arrays of double-gated field emission electron sources with self-aligned apertures and integrated nanowire current limiters. Release of the emitters after fabrication required the combination of a highly selective dry-etch and an isotropic wet-etch to avoid the loss of the insulator between the two gates. The aperture diameters are ∼360 nm and ∼570 nm for the extractor gate and focus gate, respectively. The turn-on voltage was low (15-20) V and anode currents of 400 nA were measured at 25 V. We compared devices with different extractor gate thicknesses resulting from planarization non-uniformity, and demonstrate the influence of the focus gate on anode current. The focal spot size was measured, using a low energy phosphor screen, to be around 700 µm for a 500 µm device when the ratio was 0.35.

Highly enhanced field emission from copper oxide nanoparticle decorated vertically aligned carbon nanotubes: Role of interfacial electronic structure

Field emission (FE) properties of Copper (3 nm and 10 nm) decorated vertically aligned carbon nanotubes (VACNTs) grown on Si substrate before and after oxidation are investigated. The current density is found to be the maximum (20 mA/cm2) for 3 nm thick Cu decorated VACNTs after oxidation. The variation in conventionally monitored FE parameters like work function and field enhancement factor cannot explain the experimentally determined high FE current density and its variation. A critical analysis of the electronic structure reveals that the ratio of CuO/Cu2O on VACNTs plays an important role in controlling the FE current density. The highly enhanced FE current density and its temporal stability of 3 nm Cu decorated VACNTs after oxidation indicate that it may be used as a potential material for a FE electron source in future vacuum microelectronic devices.