Atomic layer deposition

An Atomic Layer Deposition (ALD) process is a vapor-phase, thin-film deposition method driven by the chemical saturation of surfaces, allowing for conformal coatings of surfaces with very high aspect ratios. The reason for this uniform coating lies in the saturative chemisorption of sequential cycles of precursor vapors.

Principles

A repetitive sequence of four steps described below, so called cycle, achieves the coating of the surface.

Cycle sequence

Initial state: The surface of the material is terminated with hydroxyl groups (formed during contact with air).

1. The first precursor is introduced: The precursor molecules react with the surface hydroxyles forming a single saturated monolayer.
2. Evacuation: The byproducts of the reaction (methane - CH₃) and the unreacted precursor molecules are pumped out of the chamber
3. Water (H₂0) is introduced: Water (H₂0) is pulsed into the reactor. This will remove the CH₃ groups and create oxide bridges, and passivate the surface with hydroxyles.
4. Evacuation: The byproducts of the reaction (methane - CH₃) and the unreacted precursor molecules are pumped out of the chamber.

Final state: A single oxide layer has been formed and surface hydroxyl groups are used again to perform another cycle.

A simplistic visualisation of a binary reaction sequence is illustrated below.

Schematic representation of ALD using self-limiting surface chemistry and an AB binary reaction sequence.‎

Notes

• During the coating procedure, N₂ is used as a carrier gas in a user-determined flow, and a scroll pump attached to the exhaust continuously pumps the byproducts from the reactor.
• The ratio of the oxide thickness over the growth cycles gives the growth per cycle (GPC)

TMA example

To enrich the simplistic visualisation given above, an example of an ALD cycle to form a layer of AlO_x is given in this section.

The single-cycle sequence is divided based on earlier descriptions.

The four steps for the deposition of one layer of AlO_x. Illustrations are taken from the Savannah manual

The initial and final state of the surface in an ALD cycle. Both surfaces have hydroxyl groups that are susceptible to a new ALD cycle

An ALD cycle in Savannah

The following image is a part of the "playlist" in the ALD2 system corresponding to the deposition sequence.

An example of a playlist from the ALD2 tool corresponding to the deposition sequence. In the green square colour, the four ALD steps of a cycle are included

1. pulse: An electrical signal turns on the precursor valve, introducing precursor A
2. wait: The reaction products and the non-reacted molecules of precursor A are pumped out of the system for a set time
3. pulse: An electrical signal turns on the precursor valve, introducing precursor B (water)
4. wait: The reaction products and the non-reacted molecules of precursor B (water) are pumped out of the system for a set time
5. goto: The loop command. It defines the line that the cycle will restart as well as the amount of cycles that you desire.