Plasma diagnostics of a SF₆radiofrequency discharge used for the etching of silicon

Abstract

The neutral molecules and the positive and negative ions issuing from a radio-frequency discharge have been identified and their concentrations measured by means of mass spectrometry and a Langmuir probe, as a function of several parameters: pressure, gas flow rate, RF power, SF6-Ar mixture, and the silicon area to be etched. The concentration of SF4 produced in the reactor is correlated to the silicon area submitted to the discharge. The major positive ions are SF3+ and SF5+, the negative ions are SF3-, SF5-, F- and S2F7-. The electron and ion density measurements, obtained from Langmuir probe characteristics, give evidence of electron attachment. The rates of ionisation attachment and electron impact dissociation reactions have been estimated from these measurements. The kinetic scheme obtained shows that there are three sources for the atomic fluorine: attachment, ionisation and dissociation from SF6 and SF4 molecules.