Atomic Layer Deposition

Atomic layer deposition (ALD) is a thin-film fabrication technique based on sequential, self-limiting surface reactions carried out in the gas phase. It is regarded as a subclass of chemical vapour deposition (CVD), yet differs fundamentally from CVD through its alternating and non-overlapping exposure of precursors. The method enables precise control over film thickness, exceptional conformality on complex surfaces and atomic-scale accuracy, making it indispensable in contemporary semiconductor manufacturing and nanomaterials synthesis.

Principles and Operation

ALD involves exposing a substrate to gaseous precursors in a time-separated sequence. In a typical cycle, precursor A is first pulsed into the reactor, where its molecules adsorb onto the surface and react in a self-limiting manner: only available reactive sites participate, ensuring a fixed, saturating amount of material is deposited. After unreacted precursor and by-products are purged, precursor B is introduced, reacting similarly with the modified surface. This completes one ALD cycle.
Because each exposure saturates independently, the film growth per cycle is determined by surface chemistry rather than gas-phase reaction rates. By adjusting the number of cycles, it is possible to grow films with sub-nanometre precision over large and geometrically intricate substrates. The technique therefore yields highly uniform, conformal coatings and is central to device scaling in accordance with Moore’s law.

Applications and Research Landscape

ALD is now a mainstream technology in semiconductor processing, widely used for high-κ dielectrics, barrier layers, and interface engineering. Beyond microelectronics, ALD is essential in catalysis, energy storage, protective coatings and optical materials. Its versatility has fostered rapid expansion in research, with hundreds of ALD chemistries published. Several authoritative reviews, including those by Puurunen, Miikkulainen, Knoops and Mackus, summarise the expanding field.
Related techniques include:

• Molecular Layer Deposition (MLD), which uses organic precursors to create polymeric or hybrid organic–inorganic films.
• Sequential Infiltration Synthesis (SIS) or vapour phase infiltration (VPI), which modifies polymers through alternating precursor exposures and is widely used in nanolithography and hybrid material fabrication.

Historical Development

Early conceptual roots of ALD emerged in the Soviet Union during the 1960s. At the Leningrad Technological Institute, Valentin Aleskovsky and Stanislav Koltsov developed Molecular Layering based on the “framework hypothesis” introduced in 1952. Their experiments involved sequential reactions of metal chlorides with water on silica and other substrates. By 1965 the term Molecular Layering had been adopted, and the principles were laid out comprehensively in Koltsov’s 1971 doctoral work.
In Finland, the modern form of ALD was devised in 1974 by Tuomo Suntola for thin-film electroluminescent (TFEL) displays. Suntola named the technique atomic layer epitaxy (ALE) and patented its use across more than twenty countries. The shift from high-vacuum to inert-gas reactors enabled the use of compounds such as ZnCl₂, H₂S and H₂O, greatly broadening its industrial feasibility. The first large-scale demonstration of ALE-fabricated TFEL displays occurred in 1983 at Helsinki-Vantaa Airport.
Throughout the 1980s and 1990s, ALE formed the manufacturing backbone for TFEL displays, while research at Finnish universities refined the technique. In 1987 Suntola founded Microchemistry Ltd to develop ALE for broader applications. By the late 1990s, Microchemistry’s expertise led to its acquisition by ASM International, establishing ASM Microchemistry as the primary commercial supplier of ALD reactors. The 2000s saw the emergence of Beneq and Picosun in Finland, expanding industrial access to ALD systems.
The adoption of ALD in mainstream semiconductor manufacturing—particularly for DRAM and advanced logic devices—became a turning point. Micron Technology’s development of high-dielectric films via ALD around the 90-nm technology node, and Intel’s implementation of ALD for high-κ gate dielectrics at the 45-nm node, cemented ALD’s technological significance.
Historical clarification of the parallel Soviet and Finnish developments—Molecular Layering and Atomic Layer Epitaxy—has been aided by the Virtual Project on the History of ALD (VPHA) established in 2013. By the early 2000s the term atomic layer deposition had gained global acceptance, having been proposed by Markku Leskelä in analogy with chemical vapour deposition.

Surface Reaction Mechanisms

In ideal ALD, each precursor reacts only with a finite set of surface functional groups. When all reactive sites are consumed, no further film growth occurs until the next precursor arrives. Thus:

• Growth is self-limiting, ensuring reproducibility and uniformity.
• Precursors must be sufficiently reactive but stable under processing conditions.
• Reactions must avoid gas-phase interactions, necessitating purge steps.

Departures from ideality—such as steric hindrance, precursor decomposition or insufficient saturation—can reduce conformality or alter growth rates. Understanding these mechanisms is crucial for designing reliable ALD chemistries.
A typical ALD reaction sequence includes:

1. Adsorption of precursor A onto reactive surface groups, forming a chemisorbed layer.
2. Purge to remove excess precursor and reaction by-products.
3. Adsorption of precursor B, reacting with the modified surface to complete the monolayer.
4. Purge, returning the system to its initial state for the next cycle.

This sequence enables digital control over film thickness, with each cycle contributing typically between 0.5 and 2 Å.

Industrial and Technological Impact

ALD’s conformality allows it to coat high-aspect-ratio microstructures, making it central to:

• Advanced node transistor architectures,
• DRAM capacitors,
• Through-silicon vias,
• Nanoporous catalysts,
• Protective and barrier layers in energy devices.