Atomic Layer Deposition

Steps in an ALD process
Figure 1. Steps in an ALD process.
Atomic layer deposition is similar to LPCVD except that the chemical process is broken down into steps that isolate different adsorption and reaction steps to have self-limiting reactions. The process employs separate pulses of precursors and reactants that pass sequentially through the process chamber. Figure 1 illustrates the ALD process. With the substrate in the process chamber and under high vacuum, an initial precursor is introduced into the chamber. The molecular character of the precursor is such that it will form a chemically-bound monolayer on the substrate surface (Step 1). Any layers beyond the monolayer are only bound by physisorption forces which are weak enough to allow any precursor other than that in the monolayer to be pumped away under high vacuum. Once the monolayer is present on the substrate, the chamber is re-evacuated and purged to remove any excess precursor (Step 2). Next, a reactant is introduced to the process chamber (Step 3). It reacts with the monolayer material to form the desired compound on the substrate surface (Step 4). By-products of this reaction are pumped away.

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.

Different kinds of single wafer ALD reactor configurations
Figure 2. Different kinds of single wafer ALD reactor configurations.

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