Atomic layer deposition (ALD)

Atomic layer deposition (ALD) is a technique that enables the deposition of a wide variety of thin film materials from the vapor phase. ALD holds great promise for emerging semiconductor and energy conversion technologies. As device requirements evolve toward smaller and more compact structures, ALD offers significant advantages over other deposition methods like chemical vapor deposition (CVD) and various physical vapor deposition (PVD) techniques. Its strengths include excellent conformality and precise control over material thickness and composition, which arise from the cyclic, self-saturating nature of the ALD process.

 

In 1977, Suntola and Anton introduced ALD, initially termed atomic layer epitaxy (ALE), for use in flat panel displays. As the ALE process was further refined, the capability to incorporate metals and metal oxides allowed numerous materials to be deposited in a non-epitaxial manner, leading to the rebranding of the technique as atomic layer deposition (ALD).

 

It is important to note that many ALD procedures were developed based on various CVD processes. ALD is characterized by alternating exposures of chemical precursors that react to create the desired material, often at significantly lower temperatures than their CVD counterparts.

 

The general ALD process involves continuous alternating pulses of gaseous chemical precursors that react with a substrate. These individual gas-surface reactions are referred to as "half-reactions" and represent only part of the material synthesis. During each half-reaction, the precursor is introduced into a vacuum chamber (typically maintained at less than 1 Torr) for a specified duration to ensure complete reaction with the substrate surface. This occurs through a self-limiting process that deposits no more than a monolayer on the surface. 

 

After this step, any unreacted precursor or reaction byproducts are removed using an inert carrier gas, usually nitrogen (N₂) or argon (Ar). This is followed by a pulse and purge of a counter-reactant precursor, resulting in the production of up to one layer of the desired material. The process is then cycled until the required film thickness is achieved. Typically, ALD processes are conducted at moderate temperatures (below 350°C). The temperature range in which growth saturates is known as the "ALD temperature window." Operating outside this window can lead to poor growth rates and non-ALD-type deposition, caused by factors like slow reaction kinetics or precursor condensation at low temperatures, and thermal decomposition or rapid desorption of the precursor at high temperatures. To harness the advantages of ALD, it is crucial to operate within the specified ALD temperature window for each deposition process.