A recently presented chemical vapor deposition (CVD) method involves using plasma electrons as reducing agents for deposition of metals. The plasma electrons are attracted to the substrate surface by a positive substrate bias. Here, we present how a standard quartz crystal microbalance (QCM) system can be modified to allow applying a DC bias to the QCM sensor to attract plasma electrons to it and thereby also enable in situ growth monitoring during the electron-assisted CVD method. We show initial results from mass gain evolution over time during deposition of iron films using the biased QCM and how the biased QCM can be used for process development and provide insight into the surface chemistry by time-resolving the CVD method. Post-deposition analyses of the QCM crystals by cross-section electron microscopy and high-resolution x-ray photoelectron spectroscopy show that the QCM crystals are coated by an iron-containing film and thus function as substrates in the CVD process. A comparison of the areal mass density given by the QCM crystal and the areal mass density from elastic recoil detection analysis and Rutherford backscattering spectrometry was done to verify the function of the QCM setup. Time-resolved CVD experiments show that this biased QCM method holds great promise as one of the tools for understanding the surface chemistry of the newly developed CVD method.

Recently, a novel approach of depositing metallic films with chemical vapor deposition (CVD), using plasma electrons as reducing agents, has been presented and is herein referred to as e-CVD. By applying a positive substrate bias to the substrate holder, plasma electrons are drawn to the surface of the substrate, where the film growth occurs. In this work, we have characterized the electron flux at the substrate position in terms of energy and number density as well as the plasma potential and floating potential when maintaining an unbiased and a positively biased substrate. The measurements were performed using a modified radio frequency Sobolewski probe to overcome issues due to the coating of conventional electrostatic probes. The plasma was generated using a DC hollow cathode plasma discharge at various discharge powers and operated with and without precursor gas. The results show that the electron density is typically around 10(16) m(-3) and increases with plasma power. With a precursor, an increase in the substrate bias shows a trend of increasing electron density. The electron temperature does not change much without precursor gas and is found in the range of 0.3-1.1 eV. Introducing a precursor gas to the vacuum chamber shows an increase in the electron temperature to a range of 1-5 eV and with a trend of decreasing electron temperature as a function of discharge power. From the values of the plasma potential and the substrate bias potential, we were able to calculate the potential difference between the plasma and the substrate, giving us insight into what charge carriers are expected at the substrate under different process conditions.

Ferrocene [Fe(C5H5)(2) or FeCp2] is a well-known precursor molecule for iron in vapor deposition of iron containing films by, e.g., chemical vapor deposition (CVD) processes. CVD processes often use the energy in plasma discharges to decompose precursor molecules, which allows lowering the substrate temperature for deposition on sensitive materials. Herein, we studied the plasma decomposition of ferrocene in a plasma CVD reactor using in situ optical emission spectroscopy and quadrupole mass spectrometry, coupled with in silico quantum chemical modeling. We suggest a plasma chemical decomposition model under medium vacuum conditions where FeCp2 is likely to undergo neutral decomposition, detaching both Cp ligands from the iron center, followed by fragmentation via C2H2- and C3H3 to C-2, CH, H-2, and H.

We studied deposition of copper films by a pulsed electron chemical vapor deposition process using free electrons from a plasma discharge as reducing agents, with copper beta-diketonates, Cu(hfac)(2), and Cu(acac)(2) as the copper source. The mass gain per deposition cycle, as monitored by a quartz crystal microbalance sensor, suggests that pulsing allows us to access a process window with a self-limiting deposition process. X-ray photoelectron spectroscopy shows that the films are not metallic copper and that they are contaminated by carbon, oxygen, and when Cu(hfac)(2) was used, also fluorine. We speculate that the surface chemistry involves electron stimulated desorption reactions. Optical emission spectroscopy suggests redeposition of precursor fragments from plasma volume decomposition of precursor molecules desorbing during the plasma step. This redeposition limits the control of the surface chemistry during the plasma step of the deposition cycle.a