Conformality of atomic layer deposition in microchannels: impact of process parameters on the simulated thickness profile.

Unparalleled conformality is driving ever new applications for atomic layer deposition (ALD), a thin film growth method based on repeated self-terminating gas-solid reactions. In this work, we re-implemented a diffusion-reaction model from the literature to simulate the propagation of film growth in wide microchannels and used that model to explore trends in both the thickness profile as a function of process parameters and different diffusion regimes. In the model, partial pressure of the ALD reactant was analytically approximated. Simulations were made as a function of kinetic and process parameters such as the temperature, (lumped) sticking coefficient, molar mass of the ALD reactant, reactant's exposure time and pressure, total pressure, density of the grown material, and growth per cycle (GPC) of the ALD process. Increasing the molar mass and the GPC, for example, resulted in a decreasing penetration depth into the microchannel. The influence of the mass and size of the inert gas molecules on the thickness profile depended on the diffusion regime (free molecular flow vs. transition flow). The modelling was compared to a recent slope method to extract the sticking coefficient. The slope method gave systematically somewhat higher sticking coefficient values compared to the input sticking coefficient values; the potential reasons behind the observed differences are discussed.

Impact of Ions on Film Conformality and Crystallinity during Plasma-Assisted Atomic Layer Deposition of TiO2.

This work demonstrates that ions have a strong impact on the growth per cycle (GPC) and material properties during plasma-assisted atomic layer deposition (ALD) of TiO2 (titanium dioxide), even under mild plasma conditions with low-energy (<20 eV) ions. Using vertical trench nanostructures and microscopic cavity structures that locally block the flux of ions, it is observed that the impact of (low-energy) ions is an important factor for the TiO2 film conformality. Specifically, it is demonstrated that the GPC in terms of film thickness can increase by 20 to >200% under the influence of ions, which is correlated with an increase in film crystallinity and an associated strong reduction in the wet etch rate (in 30:1 buffered HF). The magnitude of the influence of ions is observed to depend on multiple parameters such as the deposition temperature, plasma exposure time, and ion energy, which may all be used to minimize or exploit this effect. For example, a relatively moderate influence of ions is observed at 200 °C when using short plasma steps and a grounded substrate, providing a low ion-energy dose of ∼1 eV nm-2 cycle-1, while a high effect is obtained when using extended plasma exposures or substrate biasing (∼100 eV nm-2 cycle-1). This work on TiO2 shows that detailed insight into the role of ions during plasma ALD is essential for precisely controlling the film conformality, material properties, and process reproducibility.

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

Atomic layer deposition (ALD) can provide nanometer-thin films with excellent conformality on demanding three-dimensional (3D) substrates. This also holds for plasma-assisted ALD, provided that the loss of reactive radicals through surface recombination is sufficiently low. In this work, we determine the surface recombination probability r of oxygen radicals during plasma ALD of SiO2 and TiO2 for substrate temperatures from 100 to ∼240 °C and plasma pressures from 12 to 130 mTorr (for SiO2). For both processes, the determined values of r are very low, i.e., ∼10-4 or lower, and decrease with temperature and pressure down to ∼10-5 within the studied ranges. Accordingly, deposition on trench structures with aspect ratios (ARs) of <200 is typically not significantly limited by recombination and obtaining excellent film conformality is relatively facile. For higher AR values, e.g., approaching 1000, the plasma time needed to reach saturation increases exponentially and becomes increasingly dependent on the process conditions and the corresponding value of r. Similar dependence on process conditions can be present for plasma ALD of other materials as well, where, in certain cases, film growth is already recombination-limited for AR values of ∼10. Radical recombination data and trends as provided by this work are valuable for optimizing plasma ALD throughput and feasibility for high-AR applications and can also serve as input for modeling of radical recombination mechanisms.