Film Conformality and Extracted Recombination Probabilities of O Atoms during Plasma-Assisted Atomic Layer Deposition of SiO₂, TiO₂, Al₂O₃, and HfO₂

Abstract

Surface recombination of plasma radicals is generally considered to limit film conformality during plasma-assisted atomic layer deposition (ALD). Here, we experimentally studied film penetration into high-aspect-ratio structures and demonstrated that it can give direct information on the recombination probability r of plasma radicals on the growth surface. This is shown for recombination of oxygen (O) atoms on SiO2, TiO2, Al2O3, and HfO2 where a strong material dependence has been observed. Using extended plasma exposures, films of SiO2 and TiO2 penetrated extremely deep up to an aspect ratio (AR) of ∼900, and similar surface recombination probabilities of r = (6 ± 2) × 10–5 and (7 ± 4) × 10–5 were determined for these processes. Growth of Al2O3 and HfO2 was conformal up to depths corresponding to ARs of ∼80 and ∼40, with r estimated at (1–10) × 10–3 and (0.1–10) × 10–2, respectively. Such quantitative insight into surface recombination, as provided by our method, is essential for modeling radical-surface interaction and understanding for which materials and conditions conformal film growth is feasible by plasma-assisted ALD.

TiO2–SiO2 nanocomposite thin films deposited by direct liquid injection of colloidal solution in an O2/HMDSO low-pressure plasma

“…This is because TiO2 NPs are located at different locations across the thickness of the SiO2 matrix (85 +/-6 nm) and the PECVD process used in this study provides highly conformal plasma deposition. [41][42][43][44][45] Therefore, the deeper the particle is located inside the matrix, the larger is the size of the structure observed by SEM and AFM on the topmost surface [15,46]. From the high-magnification and highly-localized analysis obtained both from TEM and SEM/AFM, the plasma-deposited TiO2-SiO2 NC thin films indicated a rather homogenous spatial distribution.…”

Oxygen Recombination Probability Data for Plasma-Assisted Atomic Layer Deposition of SiO2 and TiO2

“…As a result of the steady-state exponential decay in radical density, it takes exponentially more time to reach a certain coverage at a further distance z into the trench. Correspondingly, as described in more detail in our previous work, 5 the penetration depth of the deposited film increases logarithmically with the plasma exposure time according to Here, the half-thickness-penetration-depth PD 50% is defined as the depth at which the film thickness has decreased to 50% of its value at the entrance of the high-AR structure, 10 which in our model corresponds to the depth at which 50% coverage (i.e., θ = 1/2) has been reached. Equation 6 indicates that the slope of PD 50% / h versus ln( t ) is only dependent on r .…”

“…In our previous work, 5 we derived a direct relation between r and the AR up to which film growth is achieved by plasma ALD. This relation ( eq 6 ) is used here to determine values of r under different process conditions.…”

“…As a result, the surface recombination probability r of these radicals generally limits the aspect ratio (AR) up to which the conformal film growth by plasma ALD is feasible. 4 , 5 …”

“…The values of r are determined using the (recombination-limited) penetration depths of the deposited films into extremely high-AR trench structures, as described in our previous work. 5 Moreover, we discuss the impact of r on the minimum plasma time needed to reach saturation on a high-AR structure. We focus on acquiring values of r to build a database that is valuable for practical applications in industry and to provide input for fundamental insight into the underlying radical–surface interaction.…”