Batch ALD: Characteristics, comparison with single wafer ALD, and examples

Abstract

A comparison is made between single-wafer and (vertical) batch systems for Atomic Layer Deposition (ALD). Various characteristic times that play a role in such systems are compared. Many of these characteristic times are substantially shorter in single wafer systems. In case wafers with a high density of deep trenches are processed in a batch system, the required length of the precursor pulse could be as long as 10–60 s, depending on the specific chemistry of the process. This time is largely determined by the time it takes to saturate the very large surface of the trenched substrates. Despite that, batch systems with load sizes of 50–100 wafers are superior in terms of overall throughput. Two classes of ALD processes are distinguished, ‘normal’ ALD, in which the active precursors are separated by inert gas purges, and partial ALD in which one of the ALD steps is replaced by a pulsed CVD step. In the first category two examples are discussed, Al 2O 3 and HfO 2 ALD, characterized by short and long surface saturation times, respectively. In the second category (partial) ALD of TiN and Si 3N 4 are discussed. First a thin TiN layer is deposited through a short CVD pulse; in the second, true ALD pulse NH 3 reacts with the TiN surface to remove any remaining Cl. It is demonstrated that this procedure combines a high overall growth rate with a low sheet resistance. In the Si 3N 4 process the first pulse is a CVD process in which a thin layer of amorphous Si is deposited. In the second (ALD) pulse N and/or NH2 radicals generated by a remote plasma are used to convert the Si into Si 3N 4. The paper concludes with a short discussion on new types of carbide and nitride films that have potential to be deposited in batch type ALD systems.