Electron-beam-induced deposition of materials has been known for almost 40 years
from contamination writing. It has developed into “additive lithography” with nanometer
resolution employed in scanning electron microscopes, in dedicated lithography systems,
in reducing image projection systems, and in scanning tunneling microscopes. The technique
allows deposition of nanometer- to micrometer-size structures with nanometer precision
in three dimensions without supplementary process steps such as lift-off or etching procedures.
Depending on the deposition conditions, novel compound materials are created from
organometallic precursors which form resistors with a resistivity ranging from 103 Ω· cm
to 2×10-3 Ω· cm and sustain current densities higher than
5×105 A/cm2 without damage. High-resolution transmission electron
microscope analysis of the deposits reveals a new class of nanocrystalline compound materials.
Crystals of metals or metalcarbides and oxides are immersed in a matrix of carbonaceous material.
The deposition process is compatible with conventional VLSI technology. Tips for atomic
force and scanning tunneling microscopy can be produced with radii of curvature as small
as 5 nm. Field electron emission is obtained from deposited tips starting at an extraction
voltage of 8 V and yielding 180 µ A of current at 20 V. Three-dimensional
conducting structures can be produced as sensors.
Carbon nanotube field-effect transistors with sub-20 nm long channels and on/off current ratios of >10(6) are demonstrated. Individual single-walled carbon nanotubes with diameters ranging from 0.7 to 1.1 nm grown from structured catalytic islands using chemical vapor deposition at 700 degrees C form the channels. Electron beam lithography and a combination of HSQ, calix[6]arene, and PMMA e-beam resists were used to structure the short channels and source and drain regions. The nanotube transistors display on-currents in excess of 15 microA for drain-source biases of only 0.4 V.
Two- and three-dimensional patterns and structures can be grown by electron-beam induced deposition from organic and metalorganic precursors. Using a very fine electron beam in a dedicated field emission scanning electron microscope produces nanometer size deposits which extend from surfaces to heights in the micrometer range. The material is fed to the sample through a nozzle which presents a small leakage flux to the specimen chamber. Having an image processor attached to the microscope allows two- and three-dimensional deposition of material to be controlled. Selecting special speed rates for the motion of the beam generates inclined deposits even at a 90° beam landing angle. Combining a tilted sample and the two-dimensional way of structuring yield three-dimensional structures. These nanostructures have very special characteristics with respect to resistivity and shape. Selecting dimethyl- gold-trifluoro-acetylacetonate as precursor, a current of 1 nA, and a low electron energy of 10 keV for the deposition process, resistors of 700 Ω are obtained with a specific resistivity of 10−2 Ω cm.
Electron-beam induced deposition has been shown to be capable of creating structures of nanometer size in three dimensions without supplementary process steps like lift-off or etching procedures. Tips can be produced with radii of curvature comparable to the beam diameter used for deposition, i.e., some nanometers. By choosing appropriate deposition conditions, vacuum field emitter tips yielding a current of more than one hundred micro amps at 22 V gate voltage have been fabricated. This article will discuss the decomposition process of different organometallic precursor molecules by electron impact. A Monte Carlo simulation is employed to model the scattering of the primary electrons in a tip and to compute the spatial distribution of energy left in the tip. The influence of beam energy, beam current, and precursor material on the morphology and electrical properties of the deposited material are highlighted.
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