Electron density measurements in a slot antenna microwave plasmasource by means of the plasma oscillation method

Abstract

The absolute electron density in the downstream region of a 2.45 GHz microwave plasma source has been measured by the plasma oscillation method. In the plasma oscillation method a weak electron beam injected into the plasma excites electrostatic electron waves oscillating at the electron plasma frequency, which is proportional to the square root of the electron density. The plasma source is a slot antenna (SLAN) type, that is the microwave power is coupled from a ring resonator through equidistantly positioned resonant coupling slots into the plasma chamber which is made of quartz. The results of the plasma oscillation method are compared with the results of single and double Langmuir probe (LP) measurements. In Ar and Ar:O2 mixtures reasonable agreement between the two diagnostic methods is found. However, in depositing plasmas traditional LPs cannot be used with confidence. We studied Ar:O2 gas mixtures with hexamethyldisiloxane (HMDSO) added downstream for the deposition of quartz-like films. We found that even a small amount of HMDSO in the gas mixture leads to erroneous LP measurements due to probe surface coating, while the plasma oscillation method is still applicable here.