Electron beam-induced deposition

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 (STEM) is commonly used. Another method is ion-beam-induced deposition (IBID), where a focused ion beam 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 carbonyls of Me(CO)x structure or metallocenes. 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.
Ion-beam-induced deposition.
Ion-beam-induced deposition (IBID) is very similar to EBID with the major difference that focused ion beam, 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:
The advantages of IBID are:
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: 