Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.

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Gas-assisted focused electron beam and ion beam processing and fabrication

Field emission (FE) is based on the physical phenomenon of quantum tunneling, in which electrons are injected from the surface of materials into vacuum under the influence of an applied electric field. A variety of field emission cold cathode materials have been developed to date. In this review, we shall focus on several kinds of novel cold cathode materials that have been developed in the past decade. These include materials for microfabricated field-emitter arrays, diamond and related films, carbon nanotubes, other quasi one-dimensional nanomaterials and printable composite materials. In addition, cold cathode materials have a wide range of applications such as in flat panel displays, high-power vacuum electronic devices, microwave-generation devices, vacuum microelectronic devices and vacuum nanoelectronic devices. Applications are in consumer goods, military industries and also space technology. A comprehensive overview of the various applications is presented. Recently, recognizing the strong possibility that vacuum nanoelectronic devices using quasi one-dimensional nanomaterials, such as carbon nanotubes may emit electrons with driving voltages comparable to that of a solid-state device, there is a growing interest in novel applications of such devices. With such exciting opportunities, there is now a flurry of activities to explore applications far beyond those considered for the conventional hot cathodes that operate on thermionic emission. We shall discuss the details of a number of fascinating potential applications.

Novel cold cathode materials and applications

An extensive review is given of the results from literature on electron beam induced deposition. Electron beam induced deposition is a complex process, where many and often mutually dependent factors are involved. The process has been studied by many over many years in many different experimental setups, so it is not surprising that there is a great variety of experimental results. To come to a better understanding of the process, it is important to see to which extent the experimental results are consistent with each other and with the existing model. All results from literature were categorized by sorting the data according to the specific parameter that was varied (current density, acceleration voltage, scan patterns, etc.). Each of these parameters can have an effect on the final deposit properties, such as the physical dimensions, the composition, the morphology, or the conductivity. For each parameter-property combination, the available data are discussed and (as far as possible) interpreted. By combining models for electron scattering in a solid, two different growth regimes, and electron beam induced heating, the majority of the experimental results were explained qualitatively. This indicates that the physical processes are well understood, although quantitatively speaking the models can still be improved. The review makes clear that several major issues remain. One issue encountered when interpreting results from literature is the lack of data. Often, important parameters (such as the local precursor pressure) are not reported, which can complicate interpretation of the results. Another issue is the fact that the cross section for electron induced dissociation is unknown. In a number of cases, a correlation between the vertical growth rate and the secondary electron yield was found, which suggests that the secondary electrons dominate the dissociation rather than the primary electrons. Conclusive evidence for this hypothesis has not been found. Finally, there is a limited understanding of the mechanism of electron induced precursor dissociation. In many cases, the deposit composition is not directly dependent on the stoichiometric composition of the precursor and the electron induced decomposition paths can be very different from those expected from calculations or thermal decomposition. The dissociation mechanism is one of the key factors determining the purity of the deposits and a better understanding of this process will help develop electron beam induced deposition into a viable nanofabrication technique.

/pdf/a-critical-literature-review-of-focused-electron-beam-21cjric9ud.pdf

A critical literature review of focused electron beam induced deposition

Review: Developments in micro/nanoscale fabrication by focused ion beams

Focused electron beam induced deposition (FEBID) is a single-step, direct-write nanofabrication technique capable of writing three-dimensional metal-containing nanoscale structures on surfaces using electron-induced reactions of organometallic precursors. Currently FEBID is, however, limited in resolution due to deposition outside the area of the primary electron beam and in metal purity due to incomplete precursor decomposition. Both limitations are likely in part caused by reactions of precursor molecules with low-energy (<100 eV) secondary electrons generated by interactions of the primary beam with the substrate. These low-energy electrons are abundant both inside and outside the area of the primary electron beam and are associated with reactions causing incomplete ligand dissociation from FEBID precursors. As it is not possible to directly study the effects of secondary electrons in situ in FEBID, other means must be used to elucidate their role. In this context, gas phase studies can obtain well-resolved information on low-energy electron-induced reactions with FEBID precursors by studying isolated molecules interacting with single electrons of well-defined energy. In contrast, ultra-high vacuum surface studies on adsorbed precursor molecules can provide information on surface speciation and identify species desorbing from a substrate during electron irradiation under conditions more representative of FEBID. Comparing gas phase and surface science studies allows for insight into the primary deposition mechanisms for individual precursors; ideally, this information can be used to design future FEBID precursors and optimize deposition conditions. In this review, we give a summary of different low-energy electron-induced fragmentation processes that can be initiated by the secondary electrons generated in FEBID, specifically, dissociative electron attachment, dissociative ionization, neutral dissociation, and dipolar dissociation, emphasizing the different nature and energy dependence of each process. We then explore the value of studying these processes through comparative gas phase and surface studies for four commonly-used FEBID precursors: MeCpPtMe3, Pt(PF3)4, Co(CO)3NO, and W(CO)6. Through these case studies, it is evident that this combination of studies can provide valuable insight into potential mechanisms governing deposit formation in FEBID. Although further experiments and new approaches are needed, these studies are an important stepping-stone toward better understanding the fundamental physics behind the deposition process and establishing design criteria for optimized FEBID precursors.

https://pages.jh.edu/chem/fairbr/Publications/Gas%20Phase%20EBID%20review.pdf

The role of low-energy electrons in focused electron beam induced deposition: four case studies of representative precursors.

Niobium-gated field-emitter arrays with Pt tips were fabricated using focused ion and electron beams. A promising approach, based on a two-step etch process in a Nb/SiO2/Si structure, has been implemented for the suppression of beam-induced damage and contamination in the processed area during the production of the gate openings. Only the top Nb layer was removed for gate openings by physical sputtering using the focused ion beam. The underlying SiO2 was subsequently removed by wet etching. Deposition of Pt pillars into these gate openings using electron-beam-induced chemical reaction resulted in field emission at an applied gate bias of 50 V even without any thermal annealing process.

Maskless fabrication of field-emitter array by focused ion and electron beam

A dual beam system consisting of focused ion and electron beams was used for manufacturing of Nb-gated silicon field emitter arrays. Gate opening was produced either by reactive focused ion beam etching of both the niobium and the silicon dioxide layer or by physical sputtering of the niobium layer by focused ion beam and subsequent wet etching of the underlying silicon dioxide layer. Platinum tips were deposited into the gate opening using an electron beam induced chemical reaction. Prototype devices, which exhibit field emission, were produced successfully. Several process parameters such as ion dose, beam diameter, and etch duration were systematically varied to identify the optimum condition for the fabrication of field emitter arrays.

Field emitter array fabricated using focused ion and electron beam induced reaction

The effect of laser irradiation on the electron emission efficiency of field emitter arrays has been investigated as a function of wavelength using different harmonics of a diode-pumped Nd:YLF laser. While irradiation in the infrared or visible range did not induce any observable change in the emission behavior, ultraviolet laser irradiation, even at a lower fluence compared to infrared or visible, resulted in a significant increase of the emission efficiency, revealing that the photodecomposition of organic contaminants is the main mechanism for cleaning. During laser irradiation, the emission current increases gradually, approaching a saturation level as the surface is cleaned.

Laser cleaning of field emitter arrays for enhanced electron emission

Metal-gated Pt field emitter arrays (FEAs) have been manufactured using a dual beam system consisting of focused ion and electron beams. The gate opening in Nb or Au gate layers was produced by physical sputtering using a focused ion beam and subsequent wet etching of the underlying silicon dioxide layer. Deposition of a platinum tip into the gate opening using electron-beam-induced chemical reaction resulted in field emission without any thermal annealing process. After wet etching an enlargement of the gate opening in Nb-gated FEAs was observed due to the beam-induced enhanced etching. In contrast, the smaller gate opening in Au-gated FEAs was easily fabricated because of the suppression of the enhanced etching. Consequently, the gate opening was controlled by selection of the gate metal material. The turn-on voltage for field emission decreased by decreasing the diameter of the gate opening.

Fabrication process of field emitter arrays using focused ion and electron beam induced reaction

A diode-pumped Q-switched Nd:YLF laser was used for the cleaning of microfabricated silicon field emitter arrays. The laser wavelength was systematically varied from infrared to visible and ultraviolet, while the emission current was measured simultaneously. Infrared (λ=1047 nm) or visible (λ=523 nm) laser irradiation did not alter the emission behavior. Ultraviolet laser irradiation at λ=349 nm, on the other hand, resulted in a significant increase in the emission current due to cleaning of the emitter tip surface. Numerical simulation of the laser induced temperature rise at the silicon surface and the wavelength dependence of the cleaning efficiency indicate that photochemical decomposition, requiring energetic photons, is the main mechanism for the removal of the contaminants. However, if higher energetic photons at λ=262 nm (fourth harmonic) are used, a temporary decrease in emission current during laser irradiation was observed. A simultaneous decrease of the vacuum level in the test chamber indicat...

O. Yavas

Papers

Maskless fabrication of field-emitter array by focused ion and electron beam

Field emitter array fabricated using focused ion and electron beam induced reaction

Laser cleaning of field emitter arrays for enhanced electron emission

Fabrication process of field emitter arrays using focused ion and electron beam induced reaction

Improvement of electron emission of silicon field emitter arrays by pulsed laser cleaning