Atomic layer deposition (ALD)

Atomic layer deposition (ALD) is a true "nano" technology, allowing ultra-thin films of a few nanometres to be deposited in a precisely controlled way. The two defining characteristics of ALD - self-limiting atomic layer-by-layer growth and highly conformal coating offer many benefits in semiconductor engineering, MEMS and other nanotechnology applications.

Introducing atomic layer deposition

To learn about our FlexAL® and OpAL® ALD systems, please select from the Related Products on the right.

How does ALD work?

• ALD typically uses two or more precursors (typically liquid halide or organometallic in vapour form, but gas and solid precursors are also used), allowing a wide variety of materials to be grown. Conventional "thermal" ALD uses heat to dissociate the precursors into the reaction species. In the case of plasma ALD, one of the precursors is the plasma gas. For example:
• thermal ALD of the high-k dielectic HfO₂ typically uses precursors of TEMAH (TetrakisEthylmethylaminoHafnium) as the Hf source, and H₂O as the O source
• plasma ALD of HfO₂ also typically uses the TEMAH precursors as the Hf source, but with an O₂ plasma as the O source
• Alternate pulses of the precursors are injected into the reaction chamber, which "chemisorb" on the exposed substrate surface, with a purge step between the precursors to remove the excess. Each set of steps is called a "cycle"; each cycle grows a single atomic layer. The animation above illustrates the plasma ALD process for TiN using TiCl₄ and an H₂/N₂ plasma.
• ALD is often compared to CVD (chemical vapour deposition); the important difference is the separation of the precursor injections steps, compared to simultaneous injection in CVD. Insufficient purging between precursor steps in ALD often results in "parasitic CVD", when the linear growth of one layer per cycle is lost.
• By depositing one layer per cycle, ALD offers extreme precision in ultra-thin film growth since the number of cycles determines the number of atomic layers and therefore the precise thickness of deposited film.

The benefits of ALD

• Because the ALD process deposits precisely one atomic layer in each cycle, complete control over the deposition process is obtained at the nanometre scale
• Conformal coating can be achieved even in high aspect ratio and complex structures
• Pin-hole and particle free deposition is achieved

A wide range of materials

A very wide variety of materials is possible with ALD:

• Oxides, including HfO₂, HfSiO₂, Al₂O₃, Ta₂O₅, TiO₂, La₂O₃, SiO₂
• Nitrides, including TiN, TaN, AlN, GaN, SiN_x
• Metals, including Ru, Cu, W, Mo