Surface chemical changes of aluminum during NF3-based plasma processing used for in situ chamber cleaning

During plasma-based in situ chamber cleaning of deposited dielectric films using NF3, a transformation of aluminum chamber parts into AlFx can take place. We studied the roles of fluorine atoms and ion bombardment in this process by employing NF3 discharges mixed with He, Ne, or Ar interacting with or without ion bombardment with sputter-deposited Al films on silicon. Aluminum oxide erosion rates and surface chemistry changes, and information on the species that evolve from the surfaces during the process, were obtained by real-time ellipsometry and mass spectrometry, respectively. Characterization of processed Al surfaces was also performed using x-ray photoelectron spectroscopy. We find that Al oxide is rapidly removed/transformed in NF3 discharges and also for C2F6/O2 rf powered discharges. We observe a complex interactive role of fluorine atoms and ion bombardment that leads to the formation of thick fluorinated Al reaction layers during exposure of Al to NF3-containing rf plasmas. For NF3-rich rf discharges, the Al fluoride layer thickness continues to grow as a function of time, without saturation. The Al fluoride layer formation can be prevented if NF3/noble gas mixtures containing a high noble gas proportion are employed instead. An increasingly thicker Al fluoride layer is produced with a greater proportion of NF3 in He/NF3, Ne/NF3, and Ar/NF3.