Which processes limit purity and resolution in FEBID?

Low-energy secondary electrons (SEs) are produced abundantly by inelastic scattering in the solid of the primary keV electron beam and of backscattered electrons (BSEs) that reach areas outside of the primary beam focus. SE yields as a function of their energy typically peak well below 10 eV. SEs can thus efficiently drive precursor fragmentation via dissociative electron attachment (DEA) and this process contributes to the formation of deposit outside of the beam focus (halo formation or proximity effect). Also, DEA often leads to particularly incomplete precursor fragmentation and thus limits deposit purities as summarized by two review articles:

• Rachel M. Thorman, Ragesh Kumar T.P., D. Howard Fairbrother, Oddur Ingolfsson:
  The role of low-energy electrons in focused electron beam induced deposition: four case studies of representative precursors,
  Beilstein J. Nanotechnol. 6 (2015) 1904–1926.

• Julie A. Spencer, Samantha G. Rosenberg, Michael Barclay, Yung-Chien Wu, Lisa McElwee-White, D. Howard Fairbrother:
  Understanding the electron-stimulated surface reactions of organometallic complexes to enable design of precursors for electron beam-induced deposition,
  Appl. Phys. A 117 (2014) 1631.

Specifically tailored silane compounds with (1,1-dichloro-1-silacyclohexane) and without chlorine substituents (silacyclohexane) were synthesized to investigate processes that limit the resolution in deposit formation. In contrast the chlorine carrying precursor, silacyclohexane does not fragment noticeably via DEA. However, the observed proximity effect in FEBID is only slightly different. This gives evidence that dissociative ionization (DI) also plays a significant role in FEBID:

• Ragesh Kumar T P, Sangeetha Hari, Krishna K Damodaran, Oddur Ingólfsson, Cornelis W. Hagen:
  Electron beam induced deposition of silacyclohexane and dichlorosilacyclohexane: the role of dissociative ionization and dissociative electron attachment in the deposition process,
  Beilstein J. Nanotechnol. 8 (2017) 2376–2388.

• Oddur Ingólfsson, Ragesh T.P. Kumar, Cornelis W. Hagen, Sangeetha Hari:
  Focused Electron Beam Induced Deposition of 1,1-dichloro-1-silacyclohexane, silacyclohexane and 1,3,5-trisilacyclohexane; preliminary study on the role of low energy secondary electrons in the deposition process,
  XXIX International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC 2015), IOP Publishing, Journal of Physics: Conference Series 635 (2015) 072088.

• Elías H. Bjarnason, Benedikt Ómarsson, Nanna Rut Jónsdóttir, Ingvar Árnason,
  Oddur Ingólfsson:
  Dissociative electron attachment and dissociative ionization of 1,1-dichloro-1-silacyclohexane and silacyclohexane,
  International Journal of Mass Spectrometry 370 (2014) 39–43.

The contribution in deposit formation of SEs exiting from the underlying substrate can be diminished by suitably tailored organic adlayers. According to SEM images, these layers prevent slow electron from crossing the solid-vacuum interface presumably by changing the work function of the surface. Also, such layers suppress catalytic activity of the surface that would counteract the spatial selectivity of FEBID:

• M. Drost, F. Tu, F. Vollnhals, I. Szenti, J. Kiss, H. Marbach:
  On the Principles of Tweaking Nanostructure Fabrication via Focused Electron Beam Induced Processing Combined with Catalytic Processes,
  SMALL Methods 1 (2017) 1700095.