# Mechanisms controlling silicon nitride growth by plasma-enhanced atomic layer deposition

• K. de Peuter

Student thesis: Master

### Abstract

As the quest for increasing the number of transistors per chip continues, transistor designs are shifted to new materials. To fulfill the requirements for further performance increase and downscaling, high-? metal gate (HKMG) structures have been implemented as gate dielectrics. Within these transistors, sidewall spacers have the important task of protecting the HKMG structure and determining the spacing between the contacts in the transistor. Therefore, the spacer needs to have a high resistance against processing steps and a precise thickness. Silicon nitride is the most commonly used material for spacers, due to its high chemical inertness. The deposition of these films has to happen at low temperature, and the resulting films have to be conformal: the thickness of the film should not vary along the sidewall of the transistor and follow the topology of the structures very well. Conventional processes that deposit silicon nitride do not fulfill these requirements.Plasma-Enhanced Atomic Layer Deposition is known to provide thin films with a low thermal budget, excellent conformality, and high quality. Although this technique has already been demonstrated for deposition of silicon nitride, an in-depth study of the mechanisms behind this process is necessary for further improvement. This thesis deals with several of these mechanisms involved in growth and material properties of SiNx films grown with Bis(t-butylamino)silane (BTBAS) as precursor and a N2 plasma. The growth mechanism was investigated by experiments using various reactants, complemented with simulations. As measurement for inertness, wet etch resistance in hydrogen fluoride was determined for a series of SiNx films.The growth experiments showed that a N-2 plasma is important in the growth of silicon nitride using BTBAS. The growth using a N-2 plasma was significantly higher than the growth using a NH3 or N-2-H2 plasma, which caused inhibition of the film surface. This inhibition could be reduced by exposing an inhibited surface to again a N-2 plasma. DFT simulations showed that this inhibition is occurring because the BTBAS precursor needs undercoordinated sites to adsorb, in contrast to other precursors that need N-H terminates surfaces. The NH3 and H radicals that are present in H-containing plasmas can passivate these undercoordinated sites, disabling adsorption. Exposing a surface to a N-2 plasma creates undercoordinated sites, that enable adsorption.By using the BTBAS/N-2 plasma ALD process at deposition temperatures of 300-500 °C, low etch rates of 2 nm/min in buffered HF can be achieved. It was shown that the limiting factor in material quality is caused by redeposition of reaction products, which result in impurities in the film. This effect can be reduced by decreasing the residence time t of particles in the plasma. A lower $\tau$ reduces the chance for products to dissociate and redeposit, achieving higher material purity and better etch resistance. Controlling the residence time allows to go to higher deposition pressures, which is favorable for film conformality. Excellent conformality was achieved in aspect ratios up to 3 whilst retaining low etch rates. The high inertness and conformality of these films are promising for usage of the BTBAS/ N- process in spacer applications.
Date of Award 31 Mar 2014 English Harm C.M. Knoops (Supervisor 1), C.K. Ande (Supervisor 2) & W.M.M. (Erwin) Kessels (Supervisor 2)

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