Analysis of a transient region during the initial stage of atomic layer deposition

In atomic layer deposition (ALD), it is well known that a linear relationship exists between the deposited film thickness and the number of deposition cycles, which is due to its inherent characteristics of self-limited surface reaction between reactants. However, during the initial stage of ALD, the outermost surface is gradually converted from pre-deposited substrates into an actual film as ALD of the film proceeds. Therefore, a transient region should exist, which causes a nonlinear dependence of film thickness on the number of deposition cycles, because the characteristics of the surface adsorption of reactants is dependent on the exposed film surface. To estimate the accurate film thickness, especially for film thickness less than 10 nm, we propose a simple analytical kinetic model in the transient region. The experimental results of TiN–ALD performed on the ${\mathrm{SiO}}_2$ substrate are consistent with the existence of the transient region. Furthermore, it has been found that the probability of adsorption of a reactant to predeposited ${\mathrm{SiO}}_2$ is considerably lower than that to TiN films.