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

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.

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A critical literature review of focused electron beam induced deposition

Focused electron-beam-induced (FEB-induced) deposition and etching are versatile, direct-write nanofabrication schemes that allow for selective deposition or removal of a variety of materials. Fundamentally, these processes are governed by an electron-induced reaction with a precursor vapor, which may either result in decomposition to a solid deposit or formation of a volatile etch by-product. The ability to induce such localized reactions by placement of a nanometer-sized focused electron probe has recently drawn considerable attention. In response, we have reviewed much of the relevant literature pertaining to both focused electron-beam-induced etching and deposition. Because these nanoscale processing techniques are still in their relative infancy, a significant amount of scientific research is being conducted to understand, and hence improve, the processes. This article summarizes the associated physics of electron-solid-vapor interactions, discusses related physical processes, and provides an introdu...

Focused, Nanoscale Electron-Beam-Induced Deposition and Etching

The creation of functional nanostructures by electron-beam-induced deposition (EBID) is becoming more widespread. The benefits of the technology include fast 'point-and-shoot' creation of three-dimensional nanostructures at predefined locations directly within a scanning electron microscope. One significant drawback to date has been the low purity level of the deposition. This has two independent causes: (1) partial or incomplete decomposition of the precursor molecule and (2) contamination from the residual chamber gas. This frequently limits the functionality of the structure, hence it is desirable to improve the decomposition and prevent the inclusion of contaminants. In this contribution we review and compare for the first time all the techniques specifically aimed at purifying the as-deposited impure EBID structures. Despite incomplete and scattered data, we observe some general trends: application of heat (during or after deposition) is usually beneficial to some extent; working in a favorable residual gas (ultra-high vacuum set-ups or plasma cleaning the chamber) is highly recommended; gas mixing approaches are extremely variable and not always reproducible between research groups; and carbon-free precursors are promising but tend to result in oxygen being the contaminant species rather than carbon. Finally we highlight a few novel approaches.

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Creating pure nanostructures from electron-beam-induced deposition using purification techniques: a technology perspective

The deposition of cobalt-containing material from dicobalt octacarbonyl using a focused electron beam is reported. The material contains between 30% and 50% (atomic) cobalt, with the balance being carbon and oxygen, and comprises nanocrystalline metallic grains embedded in a carbonaceous matrix. Arches bridging two electrodes were fabricated to allow current–voltage (I–V) measurements to be conducted. The material resistivity decreases strongly with the deposition beam current, with values as low as 45 μΩ cm being attainable. Material growth by indirect irradiation is found to give rise to material of high resistivity and can result in highly resistive arches. At low temperatures, three distinctive conduction regimes are observed, with the I–V characteristics flattening out at the origin. At 20 K step-like features suggestive of Coulomb- blockade effects were observed. Magnetic force microscope (MFM) images of the material indicates that it is ferromagnetic in nature. The application of the technique and material to fabricate MFM tips is demonstrated. The use of selective material deposits as catalyst sites for the growth of carbon nanotubes is also demonstrated.

Properties and applications of cobalt-based material produced by electron-beam-induced deposition

A new selective deposition technology using electron beam induced surface reaction has been demonstrated. Electron beam induced W and Cr patterns were deposited by using WF6, WCl6, and Cr(C6H6)2 as sources, respectively. W and Cr depositions were confirmed by analysis using Auger electron spectroscopy (AES) and x‐ray microanalysis (XMA). It was observed, with W deposition using the WF6 source, that the W pattern was deposited at below ∼50 °C substrate temperature, but that the substrate was etched at above ∼50 °C substrate temperature. The deposition rate increased with decreasing substrate temperature, and the etching rate increased with increasing substrate temperature. The deposited thickness was proportional to electron beam dose. A 0.15‐μm‐linewidth Cr pattern was deposited at 5×10−7 C/cm (3.3×10−2 C/cm2) dose by using a modified scanning electron microscope (SEM) system. These results indicate that this technology will be applicable to fabricate nanometer‐structure devices.

New selective deposition technology by electron-beam induced surface reaction

A new selective deposition technology by electron beam induced surface reaction has been demonstrated. Cr line patterns were deposited using Cr(C6H6)2 as a source. Cr deposition was confirmed by analysis using XMA and AES. The deposited thickness is proportional to electron beam dose. A 0.15 µm linewidth Cr pattern was deposited at 5×10-7 C/cm. This technology will be important for nanometer structure fabrication and new structure devices.

https://iopscience.iop.org/article/10.1143/JJAP.23.L706/pdf

Katsumi Mori

Papers

New selective deposition technology by electron-beam induced surface reaction

New Selective Deposition Technology by Electron Beam Induced Surface Reaction