In terms of representative chemistry, consider an ALD process for aluminum oxide. A pulse of aluminum alkyl compound, in this case trimethylaluminum, is introduced to the process chamber. The untreated substrate has been prepared prior to the ALD process so that it has a well-ordered covering of hydroxyls on the surface and the aluminum alkyl reacts with hydroxyls that coat the surface forming an Al-O bond and losing a CH4 group through reaction between the CH3 ligand and the surface OH group (Step 1). CH4 is a gas; it is pumped away and any residual in the chamber is removed using a fast inert gas purge (Step 2). The surface is now coated with Al-CH3 and the reactant, in this case water, is introduced to the chamber (Step 3). It reacts with the Al-CH3 bonds, generating more CH4 gas, a bridging Al-O-Al and an Al-OH bond. The residual CH4 is removed from the system using pump/purge (Step 4). The bridged Al-O-Al becomes part of the growing film and the Al-OH at the film surface presents a new, hydroxyl coated surface that is ready for the ALD process to start all over again.
ALD processing thus requires a very demanding and precise combination of effective precursor delivery and control with process and tool monitoring. The ALD process consists of many cycles of short cycle-time steps employing multiple precursors delivered as very small, tightly controlled gas pulses. The key advantage of ALD processing is the fact that it produces perfectly uniform films over large area substrates and perfect three dimensional conformality in the film. As well, the controlled monolayer-by-monolayer growth allows the user precise control over film thickness. With the proper selection of precursor and reactants, time-at-temperature burdens can be kept very low in ALD processing. The primary disadvantage of the process is its relatively slow deposition rate, but this is not proving to be a serious impediment to its use for devices having nanometer scale feature sizes.
ALD processes can be either thermally driven or plasma enhanced. Plasma enhanced ALD processes using direct plasma, remote plasma, and combined direct/remote plasma have been demonstrated. They are normally carried out using single-wafer cluster tool equipment configurations. Figure 2 shows schematic depictions of different kinds of ALD reactors.
Thin Film Deposition