Effects of O-2 addition on BCl3/Cl-2 plasma chemistry for Al etching

Effects of low level (0.5-20%) O-2 addition on BCl3/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 BCl3/Cl-2 plasma, the Al etch rate first increased and then dropped above approximate to 3% O-2 added; a transition from reactive-ion-etching (RIE) lag to inverse RIE lag occurred at an O-2 percentage of similar to 8%. Optical and mass spectrometric measurements indicated that the Cl concentration increases as O-2 is added into a BCl3/Cl-2 plasma, and that above a critical O-2 percentage (similar to 6% O-2) BxOy species are formed in the plasma through a reaction between boron chlorides and oxygen and then deposit onto the wafer surface during etching. Al etching characteristics obtained in BCl3/Cl-2/O-2 plasmas are interpreted in terms of competitive effects of increased concentrations of Cl and BxOy.