Controlling electron beam induced deposition of iron from Fe(CO)5: Inhibition of autocatalytic growth by NH3 and reactivation by electron irradiation

Abstract

Focused electron beam induced deposition (FEBID) is a versatile direct-write approach to produce nanostructures from organometallic precursor molecules. Ideally, the material is deposited only when precursors interact with and are dissociated by the impinging electrons so that the process is spatially defined by the electron beam. In reality, however, thermal surface reactions as known from chemical vapor deposition can also contribute to the dissociation of the precursors. They often produce material with higher purity but can also impair the spatial selectivity of the electron-induced deposit growth. This work aims at an approach to suppress such thermal chemistry and to re-enable it within an area defined by the electron beam. We have, thus, used a surface science approach to study the inhibition of autocatalytic growth (AG) of Fe from Fe(CO)5 by NH3 and the reactivation of AG on the surface by electron irradiation. The experiments were performed under ultrahigh vacuum conditions using thermal desorption spectrometry to characterize adsorption and reactivity of Fe(CO)5 on Fe seed layers that were prepared by dosing Fe(CO)5 during electron irradiation of the entire sample surface (referred to as EBID herein). Auger electron spectroscopy was used to monitor deposit growth and to reveal the potential inhibition of AG by NH3 as well as the reactivation of the surface by electron irradiation. The results show that adsorption of NH3 slows down AG on deposits prepared by EBID but not on Fe layers produced by AG. Electron irradiation after adsorption of NH3 reactivates the surface and thus re-establishes AG. We propose that co-injection of NH3 during FEBID from Fe(CO)5 could be a viable strategy to suppress unwanted AG contributions and, therefore, enhance the spatial control of the deposition process.

Electron-induced deposition using Fe(CO)4MA and Fe(CO)5 – effect of MA ligand and process conditions

“…The broad CO desorption signal indicates that further CO is lost from a deposit produced at 100 K as a consequence of thermal reactions when the temperature increases. The wide temperature range over which desorption occurs points to the formation of a variety of chemically different sites to which CO is bound [ 27 ]. The different shapes of the CO desorption signals obtained after irradiation at 50 and 20 eV furthermore point to structural differences between the deposits obtained at these two electron energies.…”

“…According to earlier results [ 21 , 24 – 25 ], surface mobility of the precursor at room temperature in combination with contributions of AG likely leads to aggregation and, thus, to a non-homogeneous thickness. In the extreme case that each aggregate has a thickness that exceeds the escape depth of the Ta NNN Auger electrons (referred to herein as scenario B), the attenuation of the Ta NNN signal would rather reflect the decrease of uncovered Ta substrate area than an increase of the deposit thickness [ 27 ]. In contrast, a more homogeneous deposit is anticipated at cryogenic temperature, where surface mobility is lower [ 21 ].…”

“…The increase of the Fe LMM AES intensity can be used as an alternative measure of deposit growth [ 27 ]. In the case of scenario A, signals originating from an overlayer will show a saturation behavior as the thickness increases [ 55 ].…”

“…Although such autocatalytic deposit growth is favorable with respect to deposit purity as demonstrated in the framework of area-selective deposition initiated by electron beam-induced surface activation (EBISA) [ 24 – 25 ], it can compromise spatial control by the electron beam and selectivity when aiming for 3D nanostructures [ 3 , 26 ]. Strategies to suppress autocatalytic deposit growth are thus desirable to devise FEBID processes with optimum performance [ 27 ].…”

“…Overall, electron irradiation of Fe(CO) 5 multilayers at 140 K [ 43 ], thus, yields a deposit with significantly higher content of residual C and O than reported for room-temperature FEBID processes [ 8 , 21 ]. This indicates that thermal loss of further CO from partially decarbonylated intermediates Fe x (CO) y occurs at room temperature [ 26 – 27 43 ]. Notably and similar to Fe(CO) 5 , an average of three CO ligands is desorbed from the precursors Fe 2 (CO) 9 and Fe 3 (CO) 12 when irradiated at cryogenic temperature [ 43 ].…”