Direct Electron Beam Writing of Silver-Based Nanostructures

Abstract

Direct writing utilizing a focused electron beam constitutes an interesting alternative to resist-based techniques, as it allows for precise and flexible growth onto any conductive substrate in a single-step process. One important challenge, however, is the identification of appropriate precursors which allow for deposition of the material of choice, e.g., for envisaged applications in nano-optics. In this regard the coinage metal silver is of particular interest since it shows a relatively high plasma frequency and, thus, excellent plasmonic properties in the visible range. By utilizing the precursor compound AgOMeBu, direct writing of silver-based nanostructures via local electron beam induced deposition could be realized for the first time. Interestingly, the silver deposition was strongly dependent on electron dose; at low doses of 30 nC/μm a dominant formation of pure silver crystals was observed, while at higher electron doses around 10 nC/μm large carbon contents were measured. A scheme for the enhanced silver deposition under low electron fluxes by an electronic activation of precursor dissociation below thermal CVD temperature is proposed and validated using material characterization techniques. Finally, the knowledge gained was employed to fabricate well-defined two-dimensional deposits with maximized silver content approaching 75 at. %, which was achieved by proper adjustment of the deposition parameters. The corresponding deposits consist of plasmonically active silver crystallites and demonstrate a pronounced Raman signal enhancement of the carbonaceous matrix.

Gas-assisted silver deposition with a focused electron beam

“…The I D / I G ratio of 0.67, which corresponds to a semi-graphitic matrix [ 23 ], can explain the decreased resistivity of our deposits compared to Endrino’s results [ 22 ], since the graphitic structure of carbon allows better conductivity. Furthermore, the strong Raman signal intensity arises from the surface enhanced Raman from the silver crystals as previously reported by various groups [ 13 , 24 ]. We assume that the line conductivity could be further improved by purification of the deposit, thus reducing the amount of carbon.…”

“…It was previously observed for FEBID dots fabricated with a different Ag(I) carboxylate precursor (silver dimethylbutyrate) that the silver content within the PE beam area was lower than in the halo [ 13 ]. This was explained by the presence of two different regimes within and outside the PE beam area.…”

“…For that, the precursor should ideally be volatile at room or slightly elevated temperatures, evaporate without decomposition, and be susceptible towards electron-induced dissociation resulting in the desired compound [ 11 – 12 ]. Recently, a silver precursor for gas phase FEBID was reported [ 13 ]. Silver FEBID was also realized in an encapsulated liquid phase [ 14 ].…”

Mechanical Properties of 3D Nanostructures Obtained by Focused Electron/Ion Beam-Induced Deposition: A Review

“…This coincides with the experimental finding that gas-assisted ion and electron beam deposit materials often contain all the constitutional atoms of the precursor molecule [18,19]; see also Table 1 for the deposit composition of mechanically investigated materials. However, there are exceptions, and high purity material of cobalt, iron, gold, and silver can be deposited directly [20][21][22][23][24][25]. Furthermore, for these and other metals, current research activities successfully concentrate on novel precursor design and in situ or post-growth purification techniques [26][27][28][29][30][31][32][33][34] and their simulation [35].…”

Towards the third dimension in direct electron beam writing of silver

“…In an extreme case, this even hinders the evolution of nanostructures. Instead, a pronounced halo of a diameter in the micrometer range occurs [ 9 ].…”