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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsAtomic layer deposition (ALD) is a vapor-phase thin-film process that grows material through repeated, alternating surface reactions. Each exposure is designed to react with available surface sites and then stop, so repeating the cycle gives precise control over film thickness. Despite its name, ALD does not necessarily deposit exactly one complete atomic layer per cycle.
How does an ALD cycle work?
A typical cycle alternates two reactants, often called precursor A and reactant B. The first attaches to or reacts with the exposed surface; the second reacts with the modified surface to form the film and prepare sites for another cycle. Excess gases and reaction byproducts are removed between exposures so the reactants do not simply mix and react in the gas phase.
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- Expose the surface to precursor A. The precursor reacts or chemisorbs at available surface sites.
- Purge or remove excess precursor and byproducts. This separates the two reactant exposures.
- Expose the surface to reactant B. It reacts with the A-modified surface and regenerates sites for the next cycle.
- Remove excess reactant and byproducts. The sequence is then repeated to build the film.
Exact precursors, temperatures, exposure times, purge design, growth per cycle, and resulting film properties depend on the particular process. The sequence is the defining idea; it is not a universal recipe.
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In a suitable ALD chemistry, a half-reaction ends when the available reactive surface sites have been consumed. With those sites occupied, continuing the exposure does not keep adding material in the same way a continuously driven reaction would. The next reactant then carries the surface chemistry forward.
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This self-limiting behavior makes cycle count a useful way to control thickness. It is a process design principle, not a guarantee that every chemistry behaves ideally: the substrate surface, precursor chemistry, temperature, and exposure and purge conditions all matter. NIST identifies measurement and modeling of ALD chemistry and process behavior as continuing challenges (NIST Semiconductor Metrology Program).
Does ALD deposit one atomic layer per cycle?
No. “Atomic layer” describes the method’s near-atomic-scale control, not a promise that each cycle creates exactly one complete atomic layer. As Lam Research cautions, deposition is not exactly a single atomic layer each cycle. Growth per cycle varies with the material and process, so cycle count cannot be converted into a universal thickness without process-specific data.
For example, NIST reported a maximum growth rate of about 3.2 Å per cycle in a particular electron-enhanced boron nitride experiment using electron energies of 80–160 eV. That experimental result is specific to that chemistry and setup; it is not a typical or general ALD growth rate (NIST study of electron-enhanced ALD of boron nitride).
Why use ALD?
Because film growth is controlled by surface reactions rather than simply by how long reactants flow together, ALD can offer precise thickness control and good uniformity across a wafer. Its ability to coat tops, sides, and bottoms of complex features with similar thickness—conformality—makes it particularly useful for high-aspect-ratio and three-dimensional structures.
These properties make ALD valuable in microelectronics, where layers must be thin and cover intricate structures. NIST lists gate dielectrics, diffusion barriers, copper seed layers, and DRAM dielectrics among its semiconductor applications (NIST Semiconductor Metrology Program). ALD is not limited to a single chip layer or material family; research also covers materials such as boron nitride.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What are the main ALD process variants?
ALD processes can be distinguished by how reactants reach the surface and whether reactions use plasma activation. These are separate comparison axes: temporal or spatial describes exposure architecture, while thermal or plasma-assisted describes reaction activation.
| Process type | How it works | Practical distinction |
|---|---|---|
| Temporal ALD | Different reactants are introduced in sequence over a stationary wafer, with separation between exposures. | Reactants are separated in time. |
| Spatial ALD | The wafer moves through separate zones containing different reactants. | Reactants are separated in space rather than by alternating exposures over a stationary wafer. |
| Plasma-assisted ALD | Plasma species participate in at least one half-reaction. | Plasma activation may permit lower growth temperatures or a wider choice of chemistries, depending on the process. |
No variant is universally best. A process must be assessed for its substrate temperature, film properties, conformality in the target geometry, growth per cycle, and throughput. Those outcomes depend on the specific chemistry and equipment, and there is no single throughput ranking that applies across these variants. For an overview of ALD mechanisms and plasma-assisted approaches, see the AVS conference materials.
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How is ALD related to CVD?
ALD is a type of chemical vapor deposition (CVD), but it separates the chemistry into sequential, self-limiting surface steps. In a conventional CVD process, reactants may be present together at the surface and film growth proceeds through gas-phase or surface reactions without the same alternating exposure sequence. ALD’s separation of reactants is what enables its distinctive cycle-based control and conformal coating.
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