It can be noted that the QCM signal drops substantially during the plasma step. This behaviour has been observed before and is ascribed to drawing an electron current through the QCM crystal, causing rapid resistive heating of the QCM sensor. Control experiments without Cu(hfac)2 dosing also exhibit a steep drop in the QCM signal during plasma exposure followed by a return to a stable baseline (Fig. ), which shows that this perturbation of the QCM signal is not associated with a mass change. The increase in mass evolution after plasma exposure suggests an additional mass gain within the chamber. This could be due to the redeposition of species leaving the reactor walls and the substrate surface, entering the plasma volume, being decomposed and deposited back on to the substrate. It should here be noted that the chamber walls are not heated and that some precursor condensation on them thus is expected. To study this, we used OES to identify the excited species in the plasma volume. Fig. shows the emission spectrum of plasma measured during a plasma pulse in the time-resolved e-CVD deposition cycle. The plasma mainly shows emission lines from excited argon atoms, with notable spectral lines of neutral argon (Ar (I)) at wavelengths of 696. and ionized argon (Ar (II)) at wavelengths of 399.4 nm, 433.9 nm. Additionally, the observed emission spectra contain spectral lines of neutral copper (Cu (I)) at 389.30, 405.03, 448.76, 450.96, 457.45, 464.91, 467.24 and 470.91 nm, and singly ionized copper (Cu (II)) emission at 385.33, 393.15 and 459.78 nm. Doubly ionized copper (Cu 2+ , Cu (III)) exhibit emission at 435.28 and 441.41 nm. Furthermore, optical emission from fluorine radicals (F (I)) is observed at 685.6, 703.7, and 712.8 nm, alongside emission from excited oxygen (O (I)) at 777.2 nm and 844.7 nm. The OES thus suggests substantial levels of Cu(hfac)2 being decomposed in the plasma in spite of the prior purge phase, which explains the increase in the mass evolution after the plasma pulse, observed in Fig. .