Overview of ALD
Atomic layer deposition (ALD) is a thin film growth technique that offers the unique capability to coat complex, 3-dimensional objects with precise, conformal layers. In addition, ALD allows atomic-level control over the thickness and composition of the deposit.
ALD uses alternating, saturating reactions between gaseous precursor molecules and a substrate to deposit films in a layer-by-layer fashion. By repeating this reaction sequence in an ABAB… fashion, films of virtually any thickness, from atomic monolayers to micrometer dimensions, can be deposited with atomic layer precision. This alternating reaction strategy eliminates the line-of-sight or constant-exposure requirements that limit conventional methods such as physical- or chemical-vapor deposition.
As an example, consider the following binary reaction sequence for the ALD of Al2O3:
In reaction A, the substrate surface is initially covered with hydroxyl (OH) groups. The hydroxyl groups react with trimethyl aluminum (TMA) to deposit a monolayer of aluminum-methyl groups and give off methane (CH4) as a byproduct. Because TMA is inert to the methyl-terminated surface, further exposure to TMA yields no additional growth beyond one monolayer. In reaction B, this new surface is exposed to water regenerating the initial hydroxyl-terminated surface and again releasing methane. The net effect of one AB cycle is to deposit one monolayer of Al2O3 on the surface. The layer-by-layer ALD growth process allows digital film thickness control at the monolayer level.
ALD can deposit a wide variety of materials including oxides, nitrides, sulfides and metals. An integral component of the Argonne ALD research program is to invent new binary reaction sequence chemistries that extend the range of useful thin film materials.