Tuesday, July 7, 2015

Redeposition effects in plasma-assisted atomic layer deposition

Here it is - a very important study published by Oxford Instruments and TU Eindhoven on "redeposition effects" in plasma-assisted atomic layer deposition - not to be confused with "CVD effects" or "thermal decomposition of precursor effects" or other disturbing effects make the life as a ALD process guy non-conformal.

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

Harm C. M. Knoops, K. de Peuter and W. M. M. Kessels
Appl. Phys. Lett. 107, 014102 (2015); http://dx.doi.org/10.1063/1.4926366

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 t Bu)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.

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