Extraction of negative ions from pulsed electronegative inductively coupled plasmas having a radio-frequency substrate bias

Abstract

Pulsed electronegative plasmas are promising candidates for reducing charge buildup during microelectronics fabrication by extracting negative ions into features. By modulating power in inductively coupled plasmas (ICPs), the plasma potential collapses during the power-off period, thereby allowing negative ions to be extracted. In principle, application of a radio-frequency (rf) substrate bias should accelerate these ions into features. In practice, this goal is not always achieved due to the unfavorable dynamics of the plasma potential. We computationally investigated the extraction of negative ions in the afterglow of pulsed ICPs having rf substrate biases sustained in Ar/Cl2 gas mixtures. We found that the extraction of negative ions is optimized by delaying the transition to a capacitive heating mode in the afterglow, which can be achieved by the addition of Ar to Cl2 plasmas. Increasing the bias voltage causes a capacitive heating mode to begin earlier, which prevents negative ions from being extracted. To circumvent this effect, schemes were investigated in which the rf bias is applied for only a portion of the pulse period. At high rf frequencies (≈10 MHz), ions striking the substrate have only thermal energies due to the majority of the applied bias being dropped across the bulk plasma. At lower frequencies (≈2 MHz), negative ions with 2–25 eV energy were extracted with an anisotropic angular distribution due to more favorable sheath formation.