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.