Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Plasma-assisted atomic layer deposition (ALD) is an energy-enhanced method for the synthesis of ultra-thin films with A-level resolution in which a plasma is employed during one step of the cyclic deposition process. The use of plasma species as reactants allows for more freedom in processing conditions and for a wider range of material properties compared with the conventional thermally-driven ALD method. Due to the continuous miniaturization in the microelectronics industry and the increasing relevance of ultra-thin films in many other applications, the deposition method has rapidly gained popularity in recent years, as is apparent from the increased number of articles published on the topic and plasma-assisted ALD reactors installed. To address the main differences between plasma-assisted ALD and thermal ALD, some basic aspects related to processing plasmas are presented in this review article. The plasma species and their role in the surface chemistry are addressed and different equipment configurations, including radical-enhanced ALD, direct plasma ALD, and remote plasma ALD, are described. The benefits and challenges provided by the use of a plasma step are presented and it is shown that the use of a plasma leads to a wider choice in material properties, substrate temperature, choice of precursors, and processing conditions, but that the processing can also be compromised by reduced film conformality and plasma damage. Finally, several reported emerging applications of plasma-assisted ALD are reviewed. It is expected that the merits offered by plasma-assisted ALD will further increase the interest of equipment manufacturers for developing industrial-scale deposition configurations such that the method will find its use in several manufacturing applications.

/pdf/plasma-assisted-atomic-layer-deposition-basics-opportunities-1iqbq8cz04.pdf

Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges

The site-specific engineering of colloidal surfaces has provided a powerful approach to pushing the boundaries of today's materials research. The resulting surface-anisotropic and patchy particles have become the center of vital research areas, ranging from the need for large-scale fabrication techniques to exploring new applications of these materials. This Review summarizes patchy particle fabrication techniques, including but not limited to particle and nanosphere lithography and glancing-angle deposition. The variety of existing patchy particle fabrication techniques is revealed and the need for a scalable approach to high-volume patchy particle production is identified. Ongoing modeling efforts describing patchy particle interactions and properties are reviewed as potential predictive tools. Research endeavors that deal with the directed assembly of patchy particles in electric and magnetic fields, as well as with supraparticular assembly through chemical interactions, are discussed. The Review is concluded with a note on the future application of patchy particles as phoretic motors.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/marc.200900614

Fabrication, assembly, and application of patchy particles.

The field of plasma etching is reviewed. Plasma etching, a revolutionary extension of the technique of physical sputtering, was introduced to integrated circuit manufacturing as early as the mid 1960s and more widely in the early 1970s, in an effort to reduce liquid waste disposal in manufacturing and achieve selectivities that were difficult to obtain with wet chemistry. Quickly, the ability to anisotropically etch silicon, aluminum, and silicon dioxide in plasmas became the breakthrough that allowed the features in integrated circuits to continue to shrink over the next 40 years. Some of this early history is reviewed, and a discussion of the evolution in plasma reactor design is included. Some basic principles related to plasma etching such as evaporation rates and Langmuir–Hinshelwood adsorption are introduced. Etching mechanisms of selected materials, silicon, silicon dioxide, and low dielectric-constant materials are discussed in detail. A detailed treatment is presented of applications in current silicon integrated circuit fabrication. Finally, some predictions are offered for future needs and advances in plasma etching for silicon and nonsilicon-based devices.

/pdf/plasma-etching-yesterday-today-and-tomorrow-4a8eftp390.pdf

Plasma etching: Yesterday, today, and tomorrow

Perspectives on oblique angle deposition of thin films: From fundamentals to devices

We report the melting of nanorod arrays of copper at temperatures much lower than its bulk melting point (1083°C). The Cu nanorods were produced by an oblique angle sputter deposition technique through a physical self-assembly mechanism due to the shadowing effect. The as-deposited nanorods were ∼2300nm in length, ∼100nm in diameter, and separated from each other with gaps varying between ∼10 and ∼30nm. The melting process was investigated through the analysis of scanning electron microscopy, transmission electron microscopy, and x-ray diffraction measurements. Start of premelting (or surface melting) has been observed to occur at annealing temperature ∼400°C under vacuum (10−6Torr). As the temperature was raised the arrays of Cu nanorods started to coalesce and formed a dense continuous film with a (111) texture at ∼550°C. The results of this work may be useful for low temperature soldering applications.

/pdf/low-temperature-melting-of-copper-nanorod-arrays-2w6yvsoi0j.pdf

Low temperature melting of copper nanorod arrays

Amorphous silicon nanocolumns, square nanospirals, and multilayer spiral/column rods are fabricated on bare Si substrates and monolayer colloid substrates by glancing angle deposition. The grown films are studied by scanning electron microscopy and transmission electron microscopy. The size of the deposited Si columns and spirals increases with the size of colloid particles for fixed incident angle of deposition flux. The feasibility of fabricating separated, well-ordered square spirals provides a cost effective and simple way to fabricate photonic crystals.

FABRICATION OF Si NANOCOLUMNS AND Si SQUARE SPIRALS ON SELF-ASSEMBLED MONOLAYER COLLOID SUBSTRATES

A vast array of Cu nanorods with a diameter of 10-20 nm grown by an oblique angle deposition technique was utilized as an adhesive layer for bonding 200 mm Si wafers at low temperatures. The focus ion beam/scanning electron microscope images illustrate that the Cu nanorod array underwent coalescence readily upon a bonding temperature at 200°C. Upon 400°C, a dense Cu bonding layer with homogeneous structure was achieved. A fully dense bonding structure was also obtained upon a lower bonding temperature at 300°C followed by a postannealing at 400°C in a reducing ambient.

Low Temperature Wafer Bonding by Copper Nanorod Array

http://www.ualr.edu/txkarabacak/wp-content/uploads/2010/01/JA-25.pdf

Phase transformation of single crystal β-tungsten nanorods at elevated temperatures

Plasma-assisted atomic layer deposition (ALD) of Cu, via Cu I I (tmhd) 2 (tmhd = tetramethyl-3,5-heptanedionate) and an inductively coupled hydrogen plasma, is shown on metallic and dielectric surfaces. Nonselective deposition was achieved on SiO 2 , Au, and TaN x in a temperature range between 60 and 400°C. Deposition was self-limiting from ∼90 to 250°C. A novel method to determine self-limiting behavior of the first half-reaction is presented; it is determined by pulsing the precursor once, for a long time, and the resulting growth is measured by Rutherford backscattering spectrometry. Further, saturation curves for plasma-assisted ALD of each half-reaction and as a function of purging time were also determined. In contrast, thermal ALD via Cu I I (tmhd) 2 and H 2 was attempted and was very slow within the self-limiting temperature range. These experiments were undertaken on all the metallic and dielectric surfaces studied here including a plasma-assisted atomic layer deposited Cu seed.

https://iopscience.iop.org/article/10.1149/1.1850340/pdf

Pei-I Wang

Papers

Low temperature melting of copper nanorod arrays

FABRICATION OF Si NANOCOLUMNS AND Si SQUARE SPIRALS ON SELF-ASSEMBLED MONOLAYER COLLOID SUBSTRATES

Low Temperature Wafer Bonding by Copper Nanorod Array

Phase transformation of single crystal β-tungsten nanorods at elevated temperatures

Inductively Coupled Hydrogen Plasma-Assisted Cu ALD on Metallic and Dielectric Surfaces