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Atomic layer deposition (ALD) is a vapor-phase thin-film process that grows material through repeated, alternating surface reactions. Each exposure is designed to react only with available surface sites, so repeating the cycle gives process-level control over film thickness and helps coat complex structures uniformly.
What atomic layer deposition means
ALD is a type of chemical vapor deposition (CVD). Unlike processes that expose a surface to reactants together, ALD separates the chemistry into sequential steps: one precursor modifies the surface, excess gas is removed, and a second reactant reacts with the modified surface. The sequence repeats to build a thin film. Lam Research’s ALD overview describes the technique in the context of chip manufacturing.
Its defining idea is a self-limiting surface reaction. In an ideal half-cycle, the reaction stops after the available reactive sites have been consumed. That makes cycle count a useful way to control film growth, but the resulting thickness depends on the material and process conditions.
How an ALD cycle works
A representative cycle uses two reactants and four practical stages:
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- Pulse precursor A. The first vapor enters the chamber and reacts or chemisorbs at available sites on the surface.
- Purge or remove excess precursor. Excess gas and reaction byproducts are cleared so the next reactant does not simply mix with the first in the gas phase.
- Expose the surface to reactant B. The second reactant reacts with the A-modified surface and prepares the surface for the next cycle.
- Remove excess reactant and byproducts. The chamber is cleared before the sequence begins again.
Repeated cycles grow the film. The specific precursors, temperature, exposure and purge design, growth per cycle, and resulting film properties vary by chemistry and application. Lam Research and the National Institute of Standards and Technology (NIST) semiconductor metrology program describe ALD as a process whose behavior depends on surface chemistry and process control.
Does ALD deposit exactly one atomic layer per cycle?
No. “Atomic layer” describes the scale and surface-reaction approach; it does not guarantee that each cycle deposits exactly one complete atomic layer. Lam Research explicitly cautions that deposition is not exactly one atomic layer each cycle. Growth per cycle is specific to the chemistry, surface, and operating conditions, so cycle count provides controlled growth rather than a universal fixed thickness per cycle.
For example, NIST reported a maximum growth rate of approximately 3.2 Å per cycle in a specific electron-enhanced boron nitride experiment using electron energies of 80–160 eV. That experimental result is not a typical or universal ALD rate. NIST’s 2018 boron nitride study gives the experimental context.
Why use ALD?
Conformal coverage of complex shapes
Because growth occurs through surface reactions, ALD can coat the tops, sides, and bottoms of complex features with similar thickness. This conformality is valuable for high-aspect-ratio structures and three-dimensional semiconductor designs, where a coating must reach surfaces that are difficult to cover uniformly with other approaches.
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- Low Cost - The cost of this kit is significantly less than a commercial spin coater
- Vacuum Chuck - The kit has vacuum capable chuck to mount substrates instead of double sided tape.
- Closed-Loop Digital Control - Greater control of coating process
- Dynamic Spin Range - Spin rates from 500 - 8,000 rpm can be achieved with good control
- Easy to Setup and Use
Controlled thin-film growth
Separating reactants and limiting each half-reaction to available surface sites supports precise thickness control and good wafer-scale uniformity. These characteristics make ALD useful when a thin, controlled layer is needed; they do not mean every chemistry automatically achieves the same uniformity or film quality.
Use in microelectronics and beyond
NIST identifies gate dielectrics, diffusion barriers, copper seed layers, and DRAM dielectric layers as semiconductor applications. ALD is also used to investigate materials beyond common chip layers, including boron nitride. NIST’s semiconductor metrology program and its boron nitride study illustrate that broader range.
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How ALD process variants differ
ALD variants change how reactants reach the surface or how reactions are activated. No variant is universally best; the choice depends on the target material, temperature limits, feature geometry, film properties, and desired throughput.
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| Variant | How it differs | Practical consideration |
|---|---|---|
| Thermal ALD | Uses heat to drive the surface chemistry. | Evaluate whether the process temperature suits the substrate and desired film. |
| Plasma-assisted ALD | Uses plasma species in at least one half-reaction. | May allow lower-temperature growth or a wider range of chemistries, depending on the process. |
| Temporal ALD | Alternates reactant exposures over a stationary wafer, with clearing steps between exposures. | Reactants are sequenced over time. |
| Spatial ALD | Moves the wafer through separated reactant zones. | Reactants are separated by location rather than only by timed exposure over a stationary wafer. |
These categories describe different process dimensions: thermal versus plasma-assisted concerns reaction activation, while temporal versus spatial concerns exposure architecture. The AVS ALD conference material discusses ALD mechanisms and plasma-assisted approaches. Compare specific processes by their chemistry, substrate temperature, conformality in the intended geometry, growth per cycle, film properties, and throughput; the available sources do not establish universal throughput rankings.
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- APPLICATION - The machine is designed for precise and uniform deposition of films and coatings. It can be used to deposit metal oxide film, polymer coating and metal organic film, and it is small in size and can be installed in glove box or smoky food.
- PARAMETER - Wafer size: 1mm-150mm; Speed adjustable range: 100-9999rpm; Speed resolution: 1rpm; Max Spin Time:3,000s; Time resolution: 1s
- PROGRAMMABLE - With an intuitive LCD screen and buttons, you can easily program up to 5 groups of 5-step programs.
- SAFE and REALIABLE - The hinged rotating bowl cover is designed to prevent accidental falling off of the cover; Solvent resistant transparent cover and chemical resistant rotating bowl protect your safety.
- ACCESSORIES - Package includes spin coater, oil-free vacuum pump, 10mm, 25mm and 50mm standard vacuum Spin Chuck, vacuum tube and power cable.
What limits ALD performance?
Self-limiting behavior is a process design principle, not a guarantee that every surface reaction will behave ideally. Actual results depend on the substrate surface, precursor chemistry, temperature, and exposure and purge conditions. Measuring and modeling ALD chemistry and process behavior remain technical challenges, as noted by NIST’s semiconductor metrology program.
ALD also requires repeated reactant exposures and process-specific conditions. Its control and conformality can be valuable for demanding thin-film applications, but those advantages alone do not establish that it is faster or less expensive than another deposition method.
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