Atomic Layer Epitaxy

Atomic layer deposition (ALD), historically termed atomic layer epitaxy (ALE), refers to a highly controlled thin-film growth method in which alternating monolayers of reactants are deposited onto a substrate. The method yields films that are characteristically uniform, conformal and aligned with the crystal lattice of the underlying material. Widely employed in semiconductor fabrication, ALD enables the controlled growth of nanometre-scale films through sequences of self-limiting chemical reactions.
The development of ALD is closely associated with research undertaken in Finland during the 1970s, when the technique was conceived as a means of producing high-quality thin films for early flat-panel display technologies. Over the subsequent decades, its precision and compatibility with increasingly miniaturised device structures contributed to its global adoption in microelectronics and related industries.

Background and historical development

The technique was first devised in 1974 by Tuomo Suntola while working at the Instrumentarium company in Finland. A patent for the process was granted in 1976. Suntola initially sought to create reliable thin films of zinc sulphide suitable for electroluminescent display panels. His approach relied on exploiting surface chemical reactions that proceed only until all available adsorption sites are occupied, thereby forming a single monolayer per reaction step.
From these early applications in display technology, ALD expanded rapidly as a versatile thin-film method. Its accuracy in controlling film thickness to the atomic scale made it attractive for the emerging semiconductor industry. The ability to deposit ultrathin, defect-minimised layers proved crucial in sustaining the continued miniaturisation of integrated circuits, often linked to Moore’s law. In recognition of these contributions, Suntola received the Millennium Technology Prize in 2018.

Principles and mechanism of deposition

ALD differs fundamentally from basic chemical vapour deposition (CVD) processes. Rather than exposing the substrate to all chemical reactants simultaneously, ALD employs a sequential pulsing strategy in which complementary precursors are introduced alternately into a reaction chamber. Typical precursor pairs include trimethylaluminium and water for the production of aluminium oxide films.
The process relies on a saturating chemisorption mechanism:

• In the first pulse, the initial precursor adsorbs on the substrate surface. Adsorption continues only until all reactive sites are occupied.
• A purge or “dead time” follows, flushing out unreacted precursor and reaction by-products.
• A second precursor is introduced, reacting with the surface-bound species to complete one atomic layer of the desired compound.
• A subsequent purge removes excess reactant before the cycle restarts.

Because each half-reaction stops automatically once the surface is saturated, the amount of material deposited per cycle is fixed and reproducible. This self-limiting behaviour enables film thickness to be governed by the number of deposition cycles rather than by reaction duration, providing exceptional uniformity even over complex three-dimensional structures.

Technical features and operational advantages

ALD exhibits several notable characteristics that distinguish it from alternative thin-film deposition techniques:

• Monolayer-by-monolayer control: Each reaction cycle ideally yields one monolayer, facilitating precise tuning of film thickness from a few angstroms to several tens of nanometres.
• Excellent conformality: Saturating surface reactions allow uniform coating of substrates featuring high aspect-ratio structures, deep trenches or intricate geometries.
• Low defect density: Thin films produced by ALD typically exhibit reduced pinhole formation and enhanced uniformity compared with many traditional deposition methods.
• Temperature flexibility: ALD can often be conducted at relatively low temperatures, broadening its applicability to temperature-sensitive materials.
• Chemical versatility: A wide range of oxides, nitrides, sulphides, metals and hybrid materials can be produced, depending on precursor chemistry.

These features make ALD attractive for modern semiconductor nodes where precise material interfaces and ultrathin dielectric layers are essential.

Applications in semiconductor and materials technology

The adoption of ALD has been pivotal in numerous technological domains. In semiconductor manufacturing, it is used extensively for depositing high-k dielectric materials, gate oxides and barrier layers within transistors and interconnects. It contributes to the reliable fabrication of capacitors, tunnelling structures and passivation coatings in memory devices.
Beyond electronics, ALD supports applications in:

• Optoelectronics: producing transparent conductive layers, optical coatings and films for display devices.
• Energy technologies: fabricating protective layers in lithium-ion batteries, catalysts and thin-film photovoltaics.
• Nanotechnology: functionalising nanoporous materials and forming controlled nanoscale interfaces for sensors and advanced composites.