AVS 69 Session AP+PS+TF-WeM: Plasma Deposition and ALD Processes for Coatings and Thin Films

Conformality is a fundamental characteristic of atomic layer deposition (ALD) thin film growth technique. “Conformal” film refers to a film that covers all surfaces of a complex three-dimensional substrate with everywhere the same thickness and properties. ALD - invented independently by two groups in 1960s and 1970s - has since late 1990s been transformational in semiconductor technology. Apart from semiconductors, conformal ALD films find applications and interest in widely varied fields such as microelectromechanical systems, pharmaceutical powder processing, optical coatings, battery technologies and heterogeneous catalysts.

Conformality follows directly from the “ideal ALD” principles: growth of material through the use of repeated separate self-terminating (i.e., saturating and irreversible) gas-solid reactions of at least two compatible reactants on a solid surface. Obtaining conformality in practice is not self-evident, however. Reasons for deviation from conformality are multiple, ranging from mass transport limitations to slow reaction kinetics and various deviations from ideal ALD (e.g., by-product reactivity or a continuous chemical vapor deposition (CVD) component through reactant decomposition or insufficient purging). Incomplete conformality can also be intentional: a saturation profile inside a feature can be exposed, to enable an analysis of kinetic parameters of the reactions.

Uniform ALD infilling of macroscopic nanoporous solids with aspect ratio greater than 10⁴ can require precursor dose times on the order of 10³ seconds, at least four orders of magnitude longer than typical dose times for ALD on wafers. For ALD processes based on relatively stable precursors and straightforward chemistries, such as ALD of Al₂O₃ from trimethylaluminum (TMA) and H₂O, very long doses are generally harmless, but for some ALD processes, precursor decomposition and other side reactions are significant concerns. For example, at a deposition temperature of 180 °C, which has been chosen previously for ZnO ALD infilling to ensure rapid removal of H₂O from nanopores during purging,¹ diethylzinc (DEZ) decomposition² and surface Zn deethylation³ may be substantial on the ~10³ s timescale.

In this work, we investigate the potential of temperature cycling as a route toward rapid yet controlled infilling of macroscopic nanoporous solids. We infill Al₂O₃ nanoparticle compacts of ~1.5 mm thickness and ~100 nm pore size (aspect ratio > 10⁴) with ZnO using DEZ and H₂O as precursors, and we cycle the substrate temperature between ~160 °C (during H₂O purges) and ~120 °C (during all other steps). DEZ infiltration is accomplished via static dosing, wherein DEZ vapor is held in the ALD chamber for >10³ seconds with the pump valve closed so that, in principle, diffusion/reaction and saturation are observable as a rise (due to byproducts) and flattening of the ALD chamber pressure, respectively. The 120/160 °C cyclical-temperature process produces clear saturation signals in the pressure trace, whereas fixed-temperature processes carried out entirely at 120 or 160 °C do not, apparently due to incomplete H₂O removal at 120 °C and DEZ decomposition and/or other side reactions at 160 °C. We use a variety of characterization techniques, including SEM/EDS, XRD, and electrical conductivity measurements, to assess the uniformity and purity of the ZnO infills.

1. A. Cendejas, D. Moher, and E. Thimsen, J. Vac. Sci. Technol. A 39, 012406 (2021).
2. J. D. Ferguson, A. W. Weimer, and S. M. George, J. Vac. Sci. Technol. A 23, 118 (2005).
3. T. Weckman and K. Laasonen, J. Phys. Chem. C 122, 7685 (2018).

This work investigates the temporal evolution of a high-power impulse magnetron sputtering (HiPIMS), with a positive cathode reversal, discharge by using the Hiden ANALYTICAL PSM probe that can measure the ion energies as well as perform charge to mass (q/m) measurements. This work builds off other diagnostic work done on the chamber such as time resolved electron energy distribution functions from our time resolved Langmuir probe technique, previous work with measuring ion energy distribution functions using the SEMION probe, as well as preliminary measurements taken with the PSM probe. This work was performed on several different target materials such as Ag, Ti, ZnTe, and W. The diagnostic capabilities of the PSM allows for differentiation between the working gas and target material ions which enables us to calculate the ratio of target ions for a given condition. Previous work we have done has shown that at early stages of the positive cathode reversal there is an elevated population of metal ions and that the overall fraction of metal to working gas fraction is at its highest. Time and energy resolved mass spec data was collected on this system for a variety of conditions such as pulse lengths, pressures, and target material with the objective of developing a better understanding for the energetics at play. Additionally, by running the PSM with the filament on and a properly set repelling voltage it was possible to get a sense of the neutral energies as well. This required a deconvolution of the energy that the neutrals gain from the ionization from the filament. The goal was to use the results of those experiments to build a framework of understanding and to use that information to deposit better films, such as increasing the hardness of TiN, reducing the resistivity of N doped ZnTe, improving the crystallinity of W films or improving the optical properties of a Ag thin film.

Electron-enhanced atomic layer deposition (EE-ALD) was utilized for the growth of TiO₂, TiN, and TiCN films at T < 70 °C.Three Ti-based films were grown using sequential exposures of tetrakis(dimethylamido) titanium (TDMAT) precursor and electrons together with a continuous reactive background gas (RBG) (Figure 1). The electrons accelerated across a grid bias of 100 V desorb surface species by electron stimulated desorption.The electrons also dissociate the RBG as they travel to the substrate.The RBGs utilized to tune the film composition were oxygen (O₂) for TiO₂ and ammonia (NH₃) for TiN and TiCN.

TiO₂ EE-ALD was performed at T < 70 °C using TDMAT together with an O₂ RBG at ~1 mTorr. O₂ is believed to be dissociated by the electron beam, creating O radicals that add oxygen to form TiO₂ and remove N and C from the TDMAT precursor. XPS showed no detectable C or N in the TiO₂ films and a slightly O-rich stoichiometry. Oxygen-rich TiO₂ films may be more photochemically active due to bandgap narrowing and could be useful in photocatalysis.

TiN EE-ALD was conducted at T < 70°C using TDMAT together with an NH₃ RBG at a pressure of ~1 mTorr. NH₃ is believed to be dissociated by the electron beam, liberating H and NH_x radicals that facilitate Ti nitridation and C removal from the film. Electron exposures for 20 seconds led to the rapid nucleation of TiN films with purities of > 96% and ultralow resistivities of < 120 µΩ-cm. These films may be useful as diffusion barriers in backend interconnects.

Incomplete C removal resulted from shorter electron exposures during TiN EE-ALD using TDMAT with NH₃ RBG.This carbon residual provides a pathway for the deposition of TiCN, an important amorphous ternary nitride diffusion barrier.TiCN EE-ALD film growth and properties were explored for electron exposure lengths from 20 seconds to 0.5 seconds.Shorter electron exposures led to an increase of the C:Ti ratio from ~0.03 to ~0.33 as measured by x-ray photoelectron spectroscopy (XPS) (Figure 2).Shorter electron exposures also produced a decrease in film density from ~5.3 g/cm³ to ~3.3 g/cm³.In addition, shorter electron exposures yielded an increase in the film resistivity from < 120 µΩ-cm to ~2000 µΩ-cm as measured by ex situ spectroscopic ellipsometry (SE).Consequently, the enhanced diffusion barrier properties of TiCN EE-ALD films will be accompanied by higher film resistivities and lower film densities than TiN EE-ALD films.

Our research presents a non-violent approach to remove H⁺/Cl^- pairs, a common reaction byproduct, after depositing chlorosilane-type Si precursors (such as dichlorosilane, SiH₂Cl₂) on a SiN surface. While previous studies have suggested the use of H radicals in NH₃ and CH₃NH₂ plasma to remove Cl impurities, these methods have limitations in high-aspect-ratio structures. The recombination rate of radicals may be too fast to reach the bottom of the trench, resulting in the accumulation of Cl and possibly the formation of a salt layer that inhibits further film growth. Through first-principles calculations, we propose an alternative solution by introducing polar molecules, such as NH₃, N₂H₄, CH₃NH₂, and (CH₃)₂NH. When these polar molecules are dosed onto the surface, they can first abstract the H⁺ from the H⁺/Cl^- pair to form a complex with Cl^-. Adjacent polar molecules then solvate the complex, forming a stable "microsolvation cluster." This cluster effectively captures Cl^- while weakening the interactions between Cl^- and surface amine groups, providing a pathway for HCl desorption with a lower energy barrier. Our calculations of free energy surface show that when the surface is saturated with CH₃NH₂ molecules, the energy barrier for HCl desorption can be significantly reduced at 300K. Based on these promising findings, we propose a novel and non-aggressive atomic layer deposition (ALD) scheme to grow SiN at low temperatures while achieving a higher growth rate.

The controlled co-delivery of reactive and energetic plasma species during plasma-enhanced atomic layer deposition (PEALD) enables the growth of epitaxial layers at significantly reduced temperatures which are prohibitive to other methods. However, this capability is challenged by the complexity which arises from the reliance on plasma-surface interactions, and it is thus necessary to understand the influence of the plasma properties on the growth kinetics and resultant film properties. Among the III-nitride binary compounds, indium nitride (InN) is particularly well-suited for the investigation of the roles of reactive and energetic plasma species, as high-quality crystalline films can be achieved using trimethylindium (TMI) and a relatively simple N₂/Ar plasma rather than N₂/Ar/H₂ or NH₃/Ar plasmas which generate greater varieties of species. This was explored in recent studies of InN PEALD on gallium nitride (GaN) using in situ synchrotron x-ray scattering, which revealed that the growth mode is correlated with the relative density of atomic N, while coarsening behavior is influenced by ion flux.[1]

In this work, epitaxial InN films are grown by PEALD on GaN (0001) at approximately 320 °C using TMI and N₂/Ar plasma within various regimes of plasma species generation in order to investigate the influence on the resultant film properties. Optical emission spectroscopy and Langmuir probe measurements are used to correlate the production of atomic N and ions with the N₂ and Ar gas flows into the inductively coupled plasma (ICP) source. The InN films are characterized by atomic force microscopy (AFM), x-ray reflectivity (XRR), high-resolution x-ray diffraction (HRXRD), in-plane grazing incidence diffraction (IP-GID), synchrotron grazing incidence wide-angle x-ray scattering (GIWAXS), and x-ray photoelectron spectroscopy (XPS). The films are found to exhibit wurtzite phase and sixfold rotational symmetry with a clear epitaxial relationship to the GaN. Low fluxes of atomic N are found to promote larger domains, increased crystalline order, and smoother morphology compared to films grown with high atomic N fluxes. For the high atomic N flux condition, increasing ion flux is found to promote a very rough morphology containing large cluster-like features and decreased in-plane crystalline order, but increased out-of-plane crystalline order and a reduction in mosaic twist.

[1] J. M. Woodward et al., J. Vac. Sci. Technol. A 40, 062405 (2022)

Deposition of patterned coatings to generate hybrid surface properties often require a multi-step process, such as the use of masks or lithography [1]. We proposed recently a simple scalable method for the deposition of patterned coatings (morphological and chemical contrasts) [2]. As a case study, the deposition of propargyl methacrylate (PMA) based-coatings was realized, as, due to its structure (one double and one triple bond), this molecule allows very fast deposition, and can lead to hydrophobic coatings, without the need of fluorinated atoms. Moreover, we showed that, depending on the deposition conditions, one could obtain hybrid hydrophilic/hydrophobic patterns.

To test the flexibility and the limits of the method, we now investigate further this route and try to understand better the effects of the gap between the electrodes and of the pulsed/non pulsed injection of the power. The deposited coating was characterized using µ-XPS, FTIR and contact angle measurements, whereas the discharge was characterized using a high intensity camera, electrical measurements (oscilloscope and Rogowski coil), and mass spectrometry. Due to the DBD configuration, the coating exhibits two distinct areas : the spot (S) area, corresponding to the coating deposited under the filament position, and the between spot (BS) area, corresponding to the coating deposited outside the filament position.

Filaments are immobilized thanks to beads fixed to one of the dielectrics, reducing locally the gap and therefore igniting specifically the discharge at these locations. Here the gap was fixed at 2, 2.5 and 3 mm. it is shown (using a high intensity camera, and electrical measurements) that an increase in gap leads to more energetic discharges inside the filaments. As shown by XPS, this leads to a lower preservation of the precursor structure for the “S”area coating. In parallel, due to higher voltages needed to light up the discharge, surface discharges take place between the filaments leading to subsequent deposition also between the spots located under the filaments.

Pulsing the power (in the ms range) leads to more localized filaments, to a global change in the plasma behavior and to a change in the coating chemistry [3] .

References :

[1]. A. Demaude et al., Langmuir, 2019, 35 (30), 9677–9683

[2] A. Demaude et al., Advanced Science. 2022, 9 (15), 2200237