Plasma-assisted atomic layer deposition: an in situ diagnostic study

Erik Langereis
Abstract & Cover

The use of in situ spectroscopic ellipsometry (SE) during a variety of atomic layer deposition (ALD) processes was exemplified in this work, clearly demonstrating the merits of the combination of in situ SE and ALD. In situ SE yields accurate information on the film thickness and the growth rate per cycle and it was shown that this information can be used to determine ALD saturation curves, to study initial film growth (nucleation effects), and even to probe half-cycles with a submonolayer sensitivity with respect to the surface chemical species. These aspects are all very relevant for ALD film growth, both in terms of fundamental understanding and in terms of process optimization and control. Additionally, it was shown that in situ SE yields also important information on the material properties during the ALD process when the film is still being deposited. Besides the optical properties such as the refractive index and the optical band gap, also insight into the electrical properties of conductive (metal nitride) films can be obtained non-intrusively. The latter aspect can be used to study the influence of film thickness and ALD growth condition (e.g., the setting of precursor/reactant dosing, deposition temperature, etc) on the electrical properties of the films. Furthermore, the crystalline phase of the thin film materials can be established from the in situ SE measurements and phase transitions can even be probed during the ALD growth process. As is generally valid for ellipsometry measurements, the validity and the accuracy of the information extracted from the measurements depends critically on the interpretation of the data in terms of optical modeling. Corroboration of the results by other (ex situ) techniques is therefore key, especially when addressing absolute values of physical quantities. The situation is mitigated when mainly addressing specific trends in the data such that in situ SE is particularly useful during ALD film growth for process monitoring and control. The value of SE in monitoring the film growth has recently been recognized by ALD tool manufacturers and has led to the development of commercial ALD reactors with integrated in situ spectroscopic ellipsometry capability. It is, therefore, anticipated that in situ SE has a bright application prospect in the field of ALD. Finally we want to note that in situ SE cannot only be used to study, optimize and design ALD processes. The combination of in situ SE and ALD is also very powerful for fundamental studies of physical and chem[1]ical effects related to ultrathin films. The precise growth control and the relatively high level of understanding of the surface chemistry associated with ALD can, e.g., be exploited in studies of the optical properties of surface layers of chemical species or in studies of (quantum) size effects in ultrathin semiconductor and metal films.

Source of Information
Harm Knoops
Eindhoven University of Technology
(Eindhoven, Netherlands)
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