On the Verification of EEDFs in Plasmas with Silane using Optical Emission Spectroscopy

Abstract

We measured the electron density and electron energy distribution function (EEDF) of plasmas in our reactor which is intended for silicon oxide and nitride deposition. Langmuir- probe measurements showed that the EEDF of Ar plasma could largely be described by the Maxwell-Boltzmann (MB) distribution function, but it also contained a fraction (~0.5 %) of fast electrons in the energy range between 20 and 40 eV, strongly deviating from the MB distribution. We also measured relative mean electron temperatures (kTe) by optical emission spectroscopy (OES) which were calibrated by the absolute Langmuir-probe measurements. The kTe as measured by OES in Ar plasma decreased from 1.7 eV at 1.1 Pa to 1.4 eV at 12 Pa, while Langmuir-probe measurements showed a decrease from 1.7 eV to 0.8 eV. This difference is caused by the OES method, which is especially sensitive to the fraction of fast electrons in the plasma. OES can be used instead of Langmuir-probe measurements when depositing plasmas are used. Combining both methods, we demonstrated that EEDFs as measured by the Langmuir probe in Ar-N2, and Ar-N2O plasmas, resemble EEDFs in plasmas with small additions of silane, provided that (a) precursor fractions in plasma are small (SiH4 {less than or equal to} 0.8 % and N2O {less than or equal to} 15 %), and (b) total pressure does not exceed 3.6 Pa (27 mTorr). As such, the measured EEDF without silane can be used as input for chemical modeling and optimization of deposition processes in plasmas containing silane.