Meaning
Gas-phase deposition process used to grow conformal thin films of materials on a substrate through alternating, self-limiting chemical reactions driven by thermal energy. In semiconductor fabrication, thermal ALD provides atomic-scale thickness control and perfect step coverage on high-aspect-ratio structures. The process relies on the sequentially pulsed introduction of gaseous precursors into a reaction chamber.
This cycle is repeated to build the film layer by layer.
Reaction Mechanism
The cycle starts with the injection of the first precursor gas, which chemisorbs on the active sites of the substrate surface until it reaches saturation. A purge step follows, using an inert gas like nitrogen or argon to remove any unreacted precursor and gaseous byproducts. The second precursor is then introduced, reacting with the chemisorbed layer to form the desired material while releasing volatile byproducts.
A final purge completes the cycle, leaving a single monolayer of material.
Process Parameter
Reaction temperatures typically run between 150 and 400 degrees Celsius, which is high enough to activate the chemical reactions but low enough to prevent thermal decomposition of the precursors. The chamber pressure must be carefully controlled, usually held at a few torr during the deposition steps. Purge times are optimized to balance the deposition rate against the risk of CVD-like reactions that would degrade the film uniformity.
These factors must be adjusted for each unique combination of precursors and substrates.
Quality Inspection
Manufacturing units in China use spectroscopic ellipsometry and transmission electron microscopy to verify the thickness and uniformity of the deposited layers. If the film exhibits poor step coverage or surface roughness, process engineers adjust the precursor pulse and purge times in the recipe. The high uniformity of the thermal ALD process is necessary for producing the narrow channels in advanced memory and logic chips.
Fabrication plants compile these deposition logs to prove that the process meets the strict quality requirements of the chip designers.
