Redeposition in plasma-assisted atomic layer deposition: Silicon nitride film quality ruled by the gas residence time

The requirements on the material properties and growth control of silicon nitride (SiN_x) spacer films in transistors are becoming ever more stringent as scaling of transistor structures continues. One method to deposit high-quality films with excellent control is atomic layer deposition (ALD). However, depositing SiN_x by ALD has turned out to be very challenging. In this work, it is shown that the plasma gas residence time τ is a key parameter for the deposition of SiN_x by plasma-assisted ALD and that this parameter can be linked to a so-called “redeposition effect”. This previously ignored effect, which takes place during the plasma step, is the dissociation of reaction products in the plasma and the subsequent redeposition of reaction-product fragments on the surface. For SiN_x ALD using SiH₂(NH^tBu)₂ as precursor and N₂ plasma as reactant, the gas residence time τ was found to determine both SiN_x film quality and the resulting growth per cycle. It is shown that redeposition can be minimized by using a short residence time resulting in high-quality films with a high wet-etch resistance (i.e., a wet-etch rate of 0.5 nm/min in buffered HF solution). Due to the fundamental nature of the redeposition effect, it is expected to play a role in many more plasma-assisted ALD processes.