Hydrogenated amorphous and nanocrystalline silicon films manufactured by plasma deposition techniques are used widely in electronic and optoelectronic devices. The crystalline fraction and grain size of these films determines electronic and optical properties; the nanocrystal nucleation mechanism, which dictates the final film structure, is governed by the interactions between the hydrogen atoms of the plasma and the solid silicon matrix. Fundamental understanding of these interactions is important for optimizing the film structure and properties. Here we report the mechanism of hydrogen-induced crystallization of hydrogenated amorphous silicon films during post-deposition treatment with an H(2) (or D(2)) plasma. Using molecular-dynamics simulations and infrared spectroscopy, we show that crystallization is mediated by the insertion of H atoms into strained Si-Si bonds as the atoms diffuse through the film. This chemically driven mechanism may be operative in other covalently bonded materials, where the presence of hydrogen leads to disorder-to-order transitions.

The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5–10 years.

Microphase segregated block copolymer melts and solids have long garnered significant scientific interest due to their ability to spontaneously form periodic morphologies at controllable length scales. [1][2][3] Their utility as stand-alone nanostructured functional materials [4][5][6][7][8] or as templates for the fabrication of hierarchical solids [9][10][11][12] is well documented.However, their deployment in large scale applications has often been limited by the cost and scalability of the living polymerization techniques necessary for the precise control of molecular parameters required to achieve well defined morphologies. Here, we report that well ordered, processible, and functional polymer melts with periodic nanostructures can be obtained in bulk quantity by simple blending of commercially available triblock copolymer surfactants with a series of commodity homopolymers that selectively associate with one of blocks through hydrogen bonding. While the neat surfactants are disordered in the melt, scattering measurements indicate that the blends undergo a disorder-to-order transition to yield stable microphase separated structures. Moreover, the functional groups present in the homopolymers provide a convenient handle for subsequent templating schemes including phase selective chemistries and depositions, chemical modifications, and binding of active dopants including nanoparticles. The results are general, and suggest a low cost, high volume strategy to produce ordered spherical, cylindrical, or lamellar microphase separated polymer melts for commercial use.

We have elucidated the reaction mechanism and the role of the reactive intermediates in the atomic layer deposition (ALD) of aluminum oxide from trimethyl aluminum in conjunction with O(3) and an O(2) plasma. In situ attenuated total reflection Fourier transform infrared spectroscopy data show that both -OH groups and carbonates are formed on the surface during the oxidation cycle. These carbonates, once formed on the surface, are stable to prolonged O(3) exposure in the same cycle. However, in the case of plasma-assisted ALD, the carbonates decompose upon prolonged O(2) plasma exposure via a series reaction kinetics of the type, A (CH(3)) --> B (carbonates) --> C (Al(2)O(3)). The ratio of -OH groups to carbonates on the surface strongly depends on the oxidizing agent, and also the duration of the oxidation cycle in plasma-assisted ALD. However, in both O(3) and O(2) plasma cycles, carbonates are a small fraction of the total number of reactive sites compared to the hydroxyl groups.

A plasma-enhanced atomic layer deposition (ALD) process was developed for the growth of SiNx thin films using Si2Cl6 and NH3 plasma. At substrate temperatures ≤400 °C, we show that this ALD process leads to films with >95% conformality over high aspect ratio nanostructures with a growth per cycle of ∼1.2 Å. The film growth mechanism was studied using in situ attenuated total reflection Fourier transform infrared spectroscopy. Our data show that on the SiNx growth surface, Si2Cl6 reacts with surface -NH2 groups to form surface -NH species, which are incorporated into the growing film. In the subsequent half cycle, radicals generated in the NH3 plasma abstract surface Cl atoms, and restore an NHx (x = 1,2)-terminated surface. Surface Si-N-Si bonds are also primarily formed during the NH3 plasma half-cycle. The infrared data and Rutherford backscattering combined with hydrogen forward scattering shows that the films contain ∼23% H atoms primarily incorporated as -NH groups.

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We have investigated the surface reaction mechanisms during the atomic layer deposition (ALD) of TiO2 using titanium tetraisopropoxide (TTIP) and ozone using in situ attenuated total reflection Fourier transform infrared spectroscopy. The ozone reaction mechanism is fundamentally different than water-based ALD of metal oxides, where −OH groups are the reactive sites for metal alkoxides. In ozone-based ALD, the isopropoxy-ligand-terminated surface, generated after TTIP exposure, reacts with ozone, forming different surface carbonates. Upon subsequent TTIP exposure, the isopropoxy ligands chemisorb on the surface carbonates, releasing CO2. The growth rate, ∼0.52 Å/cycle, is almost independent of the substrate temperature between 150 and 250 °C.

The fabrication of next-generation semiconductor devices has created a need for low-temperature (≤400 °C) deposition of highly-conformal (>95%) SiO2, SiNx, and SiC films on high-aspect-ratio nanostructures. To enable the growth of these Si-based dielectric films, semiconductor manufacturers are transitioning from chemical vapor deposition to atomic layer deposition (ALD). Currently, SiO2 films deposited using ALD are already being integrated into semiconductor device manufacturing. However, substantial processing challenges remain for the complete integration of SiNx films deposited by ALD, and there are no known processes for ALD of SiC at temperatures that are compatible with semiconductor device manufacturing. In this focused review, the authors look at the status of thermal and plasma-assisted ALD of these three Si-based dielectric films. For SiO2 ALD, since low-temperature processes that deposit high-quality films are known, the authors focus primarily on the identification of surface reaction mechanisms using chlorosilane and aminosilane precursors, as this provides a foundation for the ALD of SiNx and SiC, two material systems where substantial processing challenges still exist. Using an understanding of the surface reaction mechanisms, the authors describe the underlying reasons for the processing challenges during ALD of SiNx and SiC and suggest methodologies for process improvement. While both thermal and plasma-assisted SiNx ALD processes have been reported in the literature, the thermal NH3-based ALD processes require processing temperatures >500 °C and large NH3 doses. On the other hand, plasma-assisted SiNx ALD processes suffer from nonuniform film properties or low conformality when deposited on high-aspect-ratio nanostructures. In the SiNx section, the authors provide a broad overview of the currently known thermal and plasma-assisted SiNx ALD processes using chlorosilane, trisilylamine, and aminosilane precursors, describe the process shortcomings, and review the literature on precursor reaction pathways. The authors close this section with suggestions for improving the film properties and conformality. In the case of SiC, the authors first outline the limitations of previously reported SiC ALD processes and highlight that unlike SiO2 and SiNx plasma-assisted ALD, no straightforward pathway for low-temperature plasma-assisted growth is currently apparent. The authors speculate that low-temperature ALD of SiC may require the design of completely new precursors. Finally, they summarize the progress made in the ALD of C-containing SiNx and SiO2 films, which may provide many of the benefits of SiC ALD in semiconductor manufacturing. In closing, through this review, the authors hope to provide the readers with a comprehensive knowledge of the surface reactions mechanisms during ALD of Si-based dielectrics, which would provide a foundation for future precursor and process development.