Effects of O₂ Addition on BCl₃/Cl₂ Plasma Chemistry for Al Etching

Effects of low level (0.5–20%) O 2 addition on BCl 3 /Cl 2 plasma chemistry have been investigated using several diagnostic tools: optical emission spectroscopy, microwave interferometry, and mass spectrometry. Experiments were performed using a magnetically enhanced planar 13.56-MHz rf plasma reactor, where aluminum etching was also performed using samples masked with a photoresist pattern of lines and spaces. As O 2 was added into a BCl 3 /Cl 2 plasma, the Al etch rate first increased and then dropped above ≈3% O 2 added; a transition from reactive-ion-etching (RIE) lag to inverse RIE lag occurred at an O 2 percentage of ∼8%. Optical and mass spectrometric measurements indicated that the Cl concentration increases as O 2 is added into a BCl 3 /Cl 2 plasma, and that above a critical O 2 percentage (∼6% O 2 ) B x O y species are formed in the plasma through a reaction between boron chlorides and oxygen and then deposit onto the wafer surface during etching. The Al etching characteristics obtained in BCl 3 /Cl 2 /O 2 plasmas are interpreted in terms of competitive effects of increased concentrations of Cl and B x O y .