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Electron-beam-induced deposition (EBID) is a process of decomposing gaseous molecules by an electron beam leading to deposition of non-volatile fragments onto a nearby substrate. The electron beam is usually provided by a scanning electron microscope, which results in high spatial accuracy (potentially below one nanometer) and the possibility to produce free-standing, three-dimensional structures.


Process

The focused electron beam of a scanning electron microscope (SEM) or
scanning transmission electron microscope A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is tÉ›mor ›sti:i:É›m As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing ...
(STEM) is commonly used. Another method is ion-beam-induced deposition (IBID), where a
focused ion beam Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a s ...
is applied instead. Precursor materials are typically liquid or solid and gasified prior to deposition, usually through vaporization or sublimation, and introduced, at accurately controlled rate, into the high-vacuum chamber of the electron microscope. Alternatively, solid precursors can be sublimated by the electron beam itself. When deposition occurs at a high temperature or involves corrosive gases, a specially designed deposition chamber is used; it is isolated from the microscope, and the beam is introduced into it through a micrometre-sized orifice. The small orifice size maintains differential pressure in the microscope (vacuum) and deposition chamber (no vacuum). Such deposition mode has been used for EBID of diamond. In the presence of the precursor gas, the electron beam is scanned over the substrate, resulting in deposition of material. The scanning is usually computer-controlled. The deposition rate depends on a variety of processing parameters, such as the partial precursor pressure, substrate temperature, electron beam parameters, applied current density, etc. It usually is in the order of 10 nm/s.


Deposition mechanism

Primary electron energies in SEMs or STEMs are usually between 10 and 300 keV, where reactions induced by electron impact, i.e. precursor dissociation, have a relatively low cross section. The majority of decomposition occurs via low energy electron impact: either by low energy secondary electrons, which cross the substrate-vacuum interface and contribute to the total current density, or inelastically scattered (backscattered) electrons.


Spatial resolution

Primary S(T)EM electrons can be focused into spots as small as ~0.045 nm. While the smallest structures deposited so far by EBID are point deposits of ~0.7 nm diameter., deposits usually have a larger lateral size than the beam spot size. The reason are the so-called proximity effects, meaning that secondary, backscattered and forward scattered (if the beam dwells on already deposited material) electrons contribute to the deposition. As these electrons can leave the substrate up to several microns away from the point of impact of the electron beam (depending on its energy), material deposition is not necessarily confined to the irradiated spot. To overcome this problem, compensation algorithms can be applied, which is typical for electron beam lithography.


Materials and precursors

As of 2008 the range of materials deposited by EBID included Al, Au, amorphous carbon, diamond, Co, Cr, Cu, Fe, GaAs, GaN, Ge, Mo, Nb, Ni, Os, Pd, Pt, Rh, Ru, Re, Si, Si3N4, SiOx, TiOx, W, and was being expanded. The limiting factor is the availability of appropriate precursors, gaseous or having a low sublimation temperature. The most popular precursors for deposition of elemental solids are
metal carbonyl Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe ch ...
s of Me(CO)x structure or
metallocenes A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are the metallocene de ...
. They are easily available, however, due to incorporation of carbon atoms from the CO ligands, deposits often exhibit a low metal content. Metal-halogen complexes ( WF6, etc.) result in cleaner deposition, but are more difficult to handle as they are toxic and corrosive. Compound materials are deposited from specially crafted, exotic gases, e.g. D2GaN3 for GaN.


Advantages

*Very flexible regarding deposit shape and composition; the electron beam is lithographically controlled and a multitude of potential precursors is available *Lateral size of the produced structures and accuracy of deposition are unprecedented *The deposited material can be characterized using the electron microscopy techniques (
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,
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, EDS,
electron diffraction Electron diffraction refers to the bending of electron beams around atomic structures. This behaviour, typical for waves, is applicable to electrons due to the wave–particle duality stating that electrons behave as both particles and waves. Si ...
) during or right after deposition. In situ electrical and optical characterization is also possible.


Disadvantages

*Serial material deposition and low deposition rates in general limit throughput and thus mass production *Controlling the elemental or chemical deposit composition is still a major challenge, as the precursor decomposition pathways are mostly unknown *Proximity effects can lead to unintended structure broadening


Ion-beam-induced deposition

Ion-beam-induced deposition (IBID) is very similar to EBID with the major difference that
focused ion beam Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a s ...
, usually 30 keV Ga+, is used instead of the electron beam. In both techniques, it is not the primary beam, but secondary electrons which cause the deposition. IBID has the following disadvantages as compared to EBID: *Angular spread of secondary electrons is larger in IBID thus resulting in lower spatial resolution. *Ga+ ions introduce additional contamination and radiation damage to the deposited structure, which is important for electronic applications. *Deposition occurs in a
focused ion beam Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a s ...
(FIB) setup, which strongly limits characterization of the deposit during or right after the deposition. Only SEM-like imaging using secondary electrons is possible, and even that imaging is restricted to short observations due to sample damaging by the Ga+ beam. The use of a dual beam instrument, that combines a FIB and an SEM in one, circumvents this limitation. The advantages of IBID are: *Much higher deposition rate *Higher purity


Shapes

Nanostructures of virtually any 3-dimensional shape can be deposited using computer-controlled scanning of electron beam. Only the starting point has to be attached to the substrate, the rest of the structure can be free standing. The achieved shapes and devices are remarkable: *World smallest magnet *Fractal nanotrees *Nanoloops (potential nano
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device) *Superconducting nanowires IBID human growth.jpg, Snapshots of growing a doll-like nanostructure by IBID IBID bacteriophage.jpg, A model of
bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacteri ...
grown by IBID IBID tower.jpg, A model of
Leaning Tower of Pisa The Leaning Tower of Pisa ( it, torre pendente di Pisa), or simply, the Tower of Pisa (''torre di Pisa'' ), is the ''bell tower, campanile'', or freestanding bell tower, of Pisa Cathedral. It is known for its nearly four-degree lean, the result ...
grown by IBID EBIDloop.JPG, Letter Φ grown by EBID


See also

*
Electron microscopy An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
*
Focused ion beam Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a s ...
*
Metal carbonyl Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe ch ...
*
Metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...
*
Organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
* Scanning electron microscope *
Scanning transmission electron microscopy A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is tÉ›mor ›sti:i:É›m As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing ...
*
Transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...
* Researcher :
Lisa McElwee-White Lisa McElwee-White is currently the Colonel Allen R. and Margaret G. Crow Professor of Chemistry at the University of Florida. Career Lisa McElwee-White received her B.S. degree in Chemistry from the University of Kansas in 1979, and complet ...


References


External links

* "Nanofabrication: Fundamentals and Applications" Ed.: Ampere A. Tseng, World Scientific Publishing Company (March 4, 2008), , {{ISBN, 978-981-270-076-6
K. Molhave: "Tools for in-situ manipulations and characterization of nanostructures", PhD thesis, Technical University of Denmark, 2004
Electron beam Electron microscopy Nanotechnology