Gas mixing method realized by back diffusion in a PECVD system with showerhead

A method of forming a microcrystalline silicon layer on a substrate in a deposition chamber, comprising:

• flowing a silane gas into a diffuser region between a backing plate and a showerhead of the deposition chamber;
• flowing the silane gas through a plurality of holes in the showerhead and into a process volume between the showerhead and a substrate support in the deposition chamber;
• igniting a plasma in the process volume;
• cycling the flow of silane gas, the cycling comprising:
• flowing the silane gas for between about 1 and 15 seconds after igniting the plasma;
• and
• back-flowing gas ions formed in the plasma through the plurality of holes in the showerhead and into the diffuser region by stopping the flow of silane gas for between about 50 and 65 seconds;
• mixing the gas ions and the silane gas in the diffuser region;
• re-flowing the gas ions and silane gas through the plurality of holes in the showerhead and into the process volume by repeating the cycle;
• and
• depositing a microcrystalline silicon layer on the substrate.