Etching mechanisms of Si and SiO2 in fluorocarbon ICP plasmas: analysis of the plasma by mass spectrometry, Langmuir probe and optical emission spectroscopy

In this paper, we analyse, by the use of different plasma diagnostics, appearance potential mass spectrometry (APMS), optical emission spectroscopy (OES) and Langmuir probe measurements, a commercialized ICP source devoted to the etching of SiO2 using a Si mask. First, the influence of the gas composition (C2F6 mixed with H2 or CH4) and the residence time (varying gas flow rate) on the etching rates and selectivity is studied to optimize the process. Second, in order to improve the understanding of the etching mechanisms, the plasma is characterized according to the previous discharge conditions. We point out the presence of plasma instability due to the electronegative character of the fluorocarbon gas used. To determine the ion flux (Φi) which is an essential parameter for oxide etching, Langmuir probe measurements have been associated with a plot of the bias power versus bias voltage (Pbias(Ei)). Absolute concentrations of CFx (x = 1-3), CH3 and CHF2 radicals have been determined by APMS and the atomic fluorine concentration has been sampled by OES using argon actinometry. The techniques employed for concentration determinations are largely discussed. Finally, we compare the evolutions of the etch rates and the evolutions of the different plasma species with experimental conditions