Lesson 13.4Micro & nanoChapter 13 · Micro- and nanofabrication

Atomic layer deposition (ALD)

Coats a surface one atomic layer per cycle with alternating gases

Atomic layer deposition (ALD) grows a film one layer of atoms at a time by letting two gases take turns over a heated surface. Each gas can only react with what the previous one left behind, so every round adds exactly one thin layer, on every surface it can reach. It makes the ultra-thin insulating layers inside modern chips and protective coatings only a few nanometers thick.

Photo: CINT, Los Alamos, Public domain, via Wikimedia Commons

One ALD cycle in four panels, plus an even coating in a trenchEach panel shows a tiny piece of the surface; circled numbers match the steps below. The bottom drawing shows why ALD matters: an even film inside a deep trench.

How it works

Pulse gas A. The wafer or part sits in a small chamber, heated to typically 150–350 °C under low pressure. A short puff of the first precursor (a reactive gas that carries the element to deposit), here trimethylaluminum (TMA), bonds to every available spot on the surface. Once every spot is taken, it stops: the reaction is self-limiting.
Purge. Nitrogen or argon sweeps out the leftover gas, so the two precursors never meet in the open space.
Pulse gas B. A puff of the second precursor, here water vapor, reacts with the layer left by gas A and turns it into aluminum oxide. Again it stops by itself when the layer is complete.
Purge again. The byproducts and leftover water are swept out. One cycle is done, and the film is about 0.1 nm thicker: roughly a tenth of a nanometer per cycle.
Repeat. The cycle runs hundreds of times; you choose the thickness simply by counting cycles. A 10 nm film needs roughly 100 cycles, typically taking several minutes to an hour.

Key facts

Materials deposited: Oxides (aluminum oxide, hafnium oxide, titanium oxide), nitrides (titanium nitride), some metals; on silicon, glass, metals, polymers and powders
Typical parts: Gate insulators and capacitor layers in chips, moisture barriers, coated powders and catalysts
Part size: From nanoparticles to 300 mm wafers and batches of wafers or parts
Precision: Thickness set to a fraction of a nanometer by counting cycles; even inside holes many times deeper than wide
Surface finish: Copies the surface below it exactly, without adding roughness
Batch size it suits: Single wafers to large batches; high-volume chip production
Tooling and machine cost: No part-specific tooling; lab reactors from roughly $100,000, production tools in the millions
Speed: Slow: typically about 0.1 nm per cycle and a few seconds per cycle, so most films stay under about 100 nm

Strengths and limits

Strengths

Thickness controlled to a fraction of a nanometer
Perfectly even film, even in deep, narrow holes
Dense, pinhole-free films at fairly low temperature
Coats complex 3D shapes and powders

Limits

Very slow: thick films are impractical
Precursor gases can be toxic, flammable or costly
Only some materials have a suitable pair of gases
Films can trap impurities from the precursors

Where it's used

MicroelectronicsHafnium-oxide gate insulators in processors, capacitor layers in memory chips
Displays and flexible electronicsThin barriers that keep moisture out of OLED screens
EnergyProtective coatings on battery materials, layers in some high-efficiency solar cells
MEMS and sensorsInsulating and protective coatings on tiny moving parts
Medical and opticsBiocompatible coatings, anti-reflection layers

How to recognize it

You can't see it. ALD films are typically a few to a few tens of nanometers thick, thousands of times thinner than a hair. They hide inside the chips of your phone and computer and in the moisture barrier of some OLED screens.
No color, no texture. A film this thin usually leaves no visible trace; at most, thicker films can shift the color of a surface slightly, like a soap bubble.
Only a microscope tells. On an electron-microscope cross-section, an ALD film shows as a band of perfectly even thickness that follows every corner and trench.

Compare with

CVD coating11.9feeds both gases at once, so it grows films much faster but less evenly in deep holes.Molecular beam epitaxy13.3also works one atomic layer at a time, but with beams in ultra-high vacuum and to continue a crystal; it only coats surfaces facing the beams.PVD coating11.8sprays material in straight lines, so hidden walls get little or no coating.

Watch it

HYCOAT ITN · A clear beginner explanation of the ALD cycle

Asianometry · The history of ALD and why chipmakers depend on it

Applied Science · A home-built ALD reactor and a real coated result

Inrfucirvine · A real cleanroom ALD tool being loaded and run

See real photos

Wikipedia: Atomic layer deposition — schematics of the pulse–purge cycle and of spatial ALD
Wikimedia Commons photo search

Remember

Two gases take turns; each turn stops by itself once the surface is covered.
One cycle adds about a tenth of a nanometer, so thickness is set by counting cycles.
The film is equally thick everywhere, even deep inside tiny holes.