Fundamental advances in focused electron beam induced processing
- Publication Type:
- Thesis
- Issue Date:
- 2019
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Focused electron beam induced processing (FEBIP) is a direct-write nanofabrication technique that utilizes the electron beam of a scanning electron microscope (SEM). It encompasses the sub-techniques of electron beam induced deposition (EBID) and electron beam induced etching (EBIE). Deposition or etching is driven by electron irradiation induced decomposition of gaseous precursor molecules adsorbed to a substrate. The nature of the precursor and substrate material determine whether deposition (EBID) or etching (EBIE) occurs. EBID enables high resolution, direct-write material deposition for fabrication of arbitrary 2D or 3D nanostructures. EBIE enables direct-write etching of select materials.
One of the key advantages of EBID is the capabilities for direct-write 3D nanofabrication. However, deposition kinetics are more complex for the fabrication of 3D nanostructures, relative to the deposition of planar (0 - 2D) structures. Previously published demonstrations of complex 3D nanofabrication using EBID, have thus far been limited to the utilization of a small parameter space, namely high electron beam energies and low beam currents. A thorough experimental and theoretical investigation of 3D EBID kinetics is performed to identify the underlying factors that make 3D EBID more complex than planar EBID. Experimental results are supplemented with simulations utilizing the Monte Carlo and finite element methods. It is concluded that electron beam induced heating, typically negligible in planar EBID, is the key factor differentiating 3D EBID kinetics from their planar counterpart. Heating is shown to occur by two mechanisms, (1) thermalization of primary electrons and (2) Joule heating. The former mechanism is active during planar EBID and only becomes significant for 3D nanostructures as a result of severely restricted heat dissipation. The latter heating mechanism is expected to be unique to 3D EBID. The effects of heating upon nanostructure morphology and means of controlling the heating are demonstrated. These results should aid in the optimization of future 3D nanofabrication with EBID.
EBIE has in general, received less research attention than EBID. Influences of multiple precursor species and orientation dependent etching in single crystal materials have not been examined. The influences of these factors is determined for EBIE of single crystal diamond using a thorough experimental and theoretical investigation. Experiments are supplemented with density functional theory calculations. It is shown that EBIE of diamond using oxygen gives rise to rapid, isotropic etching, whilst the addition of hydrogen gives rise to crystographically anisotropic etching and the formation of topographic surface patterns. The etch reaction pathways are determined and etch anisotropy is caused by preferential passivation of specific crystal planes by hydrogen adsorption. It is shown that the anisotropy can be controlled by the partial pressure of hydrogen and by using a remote RF plasma source to radicalize the precursor gas. It can be used to manipulate the geometries of topographic surface patterns on diamond, as well as nano- and micro-structures fabricated by EBIE. The process can be used to fabricate perfectly symmetric structures in diamond at the nano- or meso-scale and to selectively expose {110} and {111} crystal planes. The findings constitute a comprehensive explanation of anisotropic EBIE, and advance present understanding of electron-surface interactions in general.
The major limitation of EBID is the typically low material purity obtained. Thermally driven chemical vapour deposition (CVD) from the same precursors used for EBID, is generally capable of deposition of films of far higher purity and quality. In the final chapter, electron beam induced surface chemistry patterning, is used to enable selective, patterned deposition of metallic films by thermal CVD. Three surface chemistry patterning methods, all utilizing the electron beam of a SEM are examined. The efficacy of each method for restriction of thermal CVD deposition to the patterned surfaces, is evaluated for common precursors. A continuum model of selective CVD is also presented that aids the prediction of growth parameters for optimum selectivity of thermal CVD processes in general. The results pave the way towards realization of selective CVD processes, enabled by electron beam surface chemistry patterning, that may obtain the advantages of EBID, namely high spatial resolution and applicability to substrates of arbitrary composition and geometry, without the disadvantages of low material purity.
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