In situ plasma pre-treatments of Silicon for ALD of Al2O3

International audienceIn situ plasma pre-treatments of Silicon for ALD of Al2O3P. Dubreuil1, E. Scheid1LAAS-CNRS – Toulouse (FR)One common drawback of the Atomic Layer Deposition (ALD) is the phenomenon of discrete nucleation on the initial surface, which lead to a non-continuous growth of very thin layers. For instance, on conventionally cleaned silicon, it is impossible to grow a continuous layer of Al2O3 thinner than 3nm by ALD using Trimethylaluminum (TMA) and water (H2O) [1]. In this work, we study the influence of in-situ plasma treatments in an ALD equipment just before Al2O3. First part studies the creation of an interfacial layer by nitridation/oxidation of the silicon using an Inductively Coupled Plasma of N2, NH3 and/or O2. We show that it allows in any case a strong enhancement of the density of the nucleation site, leading to an apparent linear kinetic of deposition versus the number of ALD cycles. Material characterizations at the atomic scale allow the comparison of the Si3N4, SiON and SiO2 interfaces and their combination with the deposited Al2O3. They emphasize the importance of in-situ processing, in order to avoid oxidation in air. Second part studies the effect of the in-situ cleaning/etching of the silicon by SF6 or H2 plasmas. In both cases, we show that pressure, plasma power and time parameters lead to a strong decrease of the nucleation density. For instance, with SF6 plasma, the deposition kinetic exhibits a delay of growth of about 60 cycles, instead of about 5 in case of conventionally cleaned silicon. This phenomenon can be simulated by a decrease of the nucleation density by a factor of 200, i.e. a relative density of nuclei of about 2.5x 10-5. AFM characterizations confirm the existence of these nuclei, which are revealed by performing hundreds of ALD cycles. Also the deposition kinetic of Al2O3 in the first cycles is a marker of the silicon surface cleanness, and that in-situ plasma etching can be useful for the optimization of this property. The combination of in-situ plasma cleaning and plasma nitridation of silicon seems then a promising solution for the realization of