Detection of silicon-containing etch products during SiN x etching in a hydrogen fluoride plasma using quadrupole mass spectrometry

We have designed and fabricated a two-stage differentially pumped setup for a quadrupole mass spectrometer to identify stable etch products generated during etching of SiNx in an HF plasma under an applied bias voltage of −240 V to the substrate. In this setup, the etch products can be directly collected into a sampling cone with a 1 mm aperture at a distance of ∼3 mm from the sample surface, which is shorter than the mean free path in the HF plasma at 10 mTorr. We show that SiF4 is the primary Si-containing etch product. It is unlikely that SiF2 is an etch product during SiNx etching at the conditions employed in this study. We also detected small fractions of SiH2F2 and SiHF3 released from the SiNx surface during etching. Since the electron impact ionization cross sections for SiH2F2 and SiHF3 are not available in the literature, we could not estimate their relative abundance compared to SiF4. We were unable to conclusively detect NH3 as the primary N-containing etch product due to background H2O in the chamber, which has a cracking pattern that overlaps with NH3 at mass-to-charge ratios of 16 and 17.