Why does CO₂ plasma chamber seasoning favor nanocrystalline silicon growth?

The use of a CO₂ plasma treatment (often called “seasoning”) on a hydrogenated amorphous silicon (a-Si:H) coated plasma-enhanced chemical vapor deposition chamber is known to be effective in accelerating the incubation and nucleation of hydrogenated nanocrystalline silicon (nc-Si:H) layers, which are typically grown using a plasma of silane (SiH₄) heavily diluted in H₂. Utilizing the simple diagnostic technique of optical emission spectroscopy, we show that this accelerated nucleation is primarily due to the prevention of etching and recycling of silicon (as SiH₄) from the walls by the predominantly H₂ plasma during nc-Si:H growth. In addition, this CO₂ plasma treatment results in a decreased recombination of atomic hydrogen on the walls, increasing the atomic hydrogen density in the chamber. Both of these effects act to shift the process conditions toward nc-Si:H growth. We quantify the recycling of silicon from the coated walls as a SiH₄ flow rate, giving an effective reduction of 1.8 sccm (for our reactor geometry), and show that a CO₂ plasma treatment of an a-Si:H coated chamber is equivalent to having bare metal chamber walls.