Evaluation of the Electrical Asymmetry Effect by spectroscopic measurements of capacitively coupled discharges and silicon thin film depositions

A capacitively coupled hydrogen diluted silane discharge operated at pressures above 500 Pa and driven by a combination of 13.56 MHz and 27.12 MHz is investigated by infrared laser absorption spectroscopy. Adjusting the phase angle between the two applied frequencies enables the control of the discharge symmetry via the Electrical Asymmetry Effect (EAE). The absorption measurements prove that the degree of dissociation of SiH4 and, thus, the radical flux remains approximately constant, independently of the phase angle. Furthermore, the results show that the DC self-bias can be adjusted by tuning the phase, so that the sheath voltages change. Thus, the ion properties at the electrodes can be controlled via the EAE. This has an effect on the deposition of thin films, which are grown on glass substrates at various phase angles and analyzed using Raman spectrometry: Reducing the ion energy at the substrate leads to a decrease of the amorphous fraction in the predominantly microcrystalline film.